!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Module.- LIB_XBEAM Henrik Hesse. 07/01/2011 - Last Update 07/01/2011
!
!-> Language: FORTRAN90, Free format.
!
!-> Description.-
!
!   Compute element matrices for rigid-body components of the 3-noded geometrically-exact beam element.
!
!-> Subroutines.-
!
!    -xbeam_mrs:        Compute tangent mass matrix.
!    -xbeam_kmass:      Compute tangent mass stiffness matrix
!    -xbeam_cgyr:       Compute tangent gyroscopic damping matrix.
!    -xbeam_kgyr:       Compute tangent gyroscopic stiffness matrix.
!    -xbeam_fgyr:       Compute discrete gyroscopic forces.
!
!    -xbeam_mrr:        Compute reference system mass matrix.
!    -xbeam_crr:        Compute reference system damping matrix.
!
!    -xbeam_fext:       Compute influence coefficients for force vector.
!
!    -xbeam_rot:        Definition of rotation operator in Euler quaternions
!    -xbeam_quadskew:   See Shearer and Cesnik (2007)
!
!    -xbeam_lapack_inv: Computes the inverse of a matrix
!    -xbeam_2norm:      Computes the 2norm of a vector
!
!-> Remarks.-
!
!  1) The degrees of freedom in the element are the displacements and the
!     Cartesian Rotation Vector (CRV) at each of the nodes)
!
!  2) The element can have 2 or 3 nodes.
!
!-> Modifications.-
! 20120317 A.Da Ronch Conditional compilation added (NOLAPACK)
! 201509xx S. Maraniello - Added terms for spherical joints
!                        - Added testing module
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
module lib_xbeam
 use, intrinsic     :: iso_c_binding
 implicit none

! Shared variables within the module.
 integer,private,parameter:: MaxNodCB3=3               ! Max number of nodes per element is 3.
 real(8),private,parameter,dimension(3,3):: Unit= &    ! Unit matrix.
&         reshape((/1.d0,0.d0,0.d0,0.d0,1.d0,0.d0,0.d0,0.d0,1.d0/),(/3,3/))

 contains

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_MRS
!
!-> Description:
!
!    Compute the tangent mass matrix for rigid equation.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_mrs (NumNodesElem,r0,Ri,ElemMass,Mass,NumGauss)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)   :: NumNodesElem      ! Number of nodes in the element.
  real(8),intent(in)   :: r0       (:,:)    ! Initial position/orientation of grid points.
  real(8),intent(in)   :: Ri       (:,:)    ! Current position/orientation of grid points.
  real(8),intent(in)   :: ElemMass (:,:)    ! Inertial properties in the element.
  real(8),intent(inout):: Mass     (:,:)    ! Tangent mass matrix.
  integer,intent(in)   :: NumGauss          ! Number of Gauss points in the element.

! Local variables.
  integer :: i,j                   ! Counters.
  integer :: iGauss                ! Counter on the Gaussian points.
  real(8) :: Jacobian              ! ds/deta, with eta the nondimensional arclength.
  real(8) :: CoordGauss (NumGauss) ! Coords of Gauss points.
  real(8) :: WeightGauss(NumGauss) ! Coords of Gauss points.
  real(8) :: ShapeFun(MaxNodCB3)    ! Shape functions in a gauss point.
  real(8) :: ShapeDer(MaxNodCB3)    ! Derivatives of ShapeFun in a gauss point.

  real(8) :: Ri_g (6)              ! Displacement/rotation at Gauss point.
  real(8) :: dr0_g(6)              ! Derivative of r0_g.
  real(8) :: CaB(3,3)              ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)              ! Tangential operator.
  real(8) :: Yp(6,6),D(6,6)        ! Ypsilon and D.
  real(8) :: ARC(6,6)              ! A_{RC}
  real(8) :: DTYpN(6,6*MaxNodCB3)  ! Strain matrix operator.
  real(8) :: N (6,6*MaxNodCB3)     ! Element shape function matrix

! Define Gauss points and loop on them.
  ! allocate (CoordGauss(NumGauss))
  ! allocate (WeightGauss(NumGauss))
  call fem_1d_gauss_val (NumGauss,CoordGauss,WeightGauss)

  do iGauss=1,NumGauss
! Obtain the shape functions and their derivatives.
    call fem_1d_shapefun (NumNodesElem,CoordGauss(iGauss),ShapeFun,ShapeDer)

! Global coordinates at the gauss points and element Jacobian.
    do i=1,6
      dr0_g(i)= dot_product (ShapeDer(1:NumNodesElem),r0(1:NumNodesElem,i))
    end do
    Jacobian =sqrt(dot_product (dr0_g(1:3),dr0_g(1:3)))
    ! Jacobian =sqrt(dot_product (dr0_g,dr0_g))

! Compute the current position vector and rotations.
    do i=1,6
      Ri_g(i) = dot_product (ShapeFun(1:NumNodesElem),Ri(1:NumNodesElem,i))
    end do

! Compute element shape function.
    N =0.d0
    do i=1,6
      do j=1,NumNodesElem
        N (i,i+(j-1)*6)= ShapeFun(j)
      end do
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CaB= transpose(rotvect_psi2mat(Ri_g(4:6)))
    Rot= rotvect_psi2rot  (Ri_g(4:6))

!Operators for the tangent mass matrix.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ri_g(1:3)),CaB)
    ARC(4:6,4:6)=CaB

    D=0.d0
    D(1:3,1:3)=CaB
    D(4:6,4:6)=Unit

    Yp=0.d0
    Yp(1:3,1:3)=Unit
    Yp(4:6,4:6)=Rot

    DTYpN=matmul(matmul(transpose(D),Yp),N)

! Compute mass tangent stiffness.
    Mass= Mass + WeightGauss(iGauss)*Jacobian*matmul(ARC,matmul(ElemMass,DTYpN))
  end do
  ! deallocate (CoordGauss,WeightGauss)
 end subroutine xbeam_mrs

 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_RBMRS
!
!-> Description:
!
!    Compute the lumped mass contributions to the tangent mass matrix for
!    rigid equation.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!   2) The lumped masses are defined by the inertia tensor of the nonstructural masses
!      in the section of the node, and expressed with respect to the local B frame.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_rbmrs (NumNodesElem,r0,Ri,NodalMass,Mass)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)   :: NumNodesElem      ! Number of nodes in the element.
  real(8),intent(in)   :: r0       (:,:)    ! Initial position/orientation of grid points.
  real(8),intent(in)   :: Ri       (:,:)    ! Current position/orientation of grid points.
  real(8),intent(in)   :: NodalMass(:,:,:)  ! Inertia tensor of lumped masses at element nodes.
  real(8),intent(inout):: Mass     (:,:)    ! Tangent mass matrix.

! Local variables.
  integer :: i,iNode               ! Counters.
  real(8) :: CaB(3,3)              ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)              ! Tangential operator.
  real(8) :: ARC(6,6)              ! A_{RC}
  real(8) :: Yp(6,6),D(6,6)        ! Ypsilon and D.
  real(8) :: DTYpN(6,6*MaxNodCB3)  ! Strain matrix operator.
  real(8) :: N (6,6*MaxNodCB3)     ! Element shape function matrix

! Loop in the nodes.
  do iNode=1,NumNodesElem
    ! Compute element shape function.
    N =0.d0
    do i=1,6
      N(i,i+(iNode-1)*6)= 1.d0
    end do
! Compute the nodal coordinate transformation matrix and rotational operator.
    CaB= transpose(rotvect_psi2mat(Ri(iNode,4:6)))
    Rot= rotvect_psi2rot(Ri(iNode,4:6))

!Operators for the tangent mass matrix.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ri(iNode, 1:3)),CaB)
    ARC(4:6,4:6)=CaB

    D=0.d0
    D(1:3,1:3)=CaB
    D(4:6,4:6)=Unit

    Yp=0.d0
    Yp(1:3,1:3)=Unit
    Yp(4:6,4:6)=Rot

    DTYpN=matmul(matmul(transpose(D),Yp),N)

! Compute mass tangent stiffness.
    Mass= Mass + matmul(ARC,matmul(NodalMass(iNode,:,:),DTYpN))
  end do
 end subroutine xbeam_rbmrs

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_KMASS
!
!-> Description:
!
!    Compute tangent damping matrix for motions of reference system.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_kmass (NumNodesElem,r0,Ri,RiDDot,VrelDot,ElemMass,Kmass,NumGauss)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDDot   (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: VrelDot  (6)     ! Linear/angular acceleration of reference frame.
  real(8),intent(in)    :: ElemMass (6,6)   ! Inertia properties in the element.
  real(8),intent(inout) :: Kmass     (:,:)   ! Tangent gyroscopic damping matrix.
  integer,intent(in)    :: NumGauss         ! Number of Gauss points in the element.

! Local variables
  integer :: i,j                    ! Counters.
  integer :: iGauss                 ! Counter on the Gaussian points.
  real(8) :: Jacobian               ! ds/deta, with eta the nondimensional arclength.
  real(8) :: ShapeFun(MaxNodCB3)    ! Shape functions in a gauss point.
  real(8) :: ShapeDer(MaxNodCB3)    ! Derivatives of ShapeFun in a gauss point.
  real(8) :: CoordGauss (NumGauss) ! Coords of Gauss points.
  real(8) :: WeightGauss(NumGauss) ! Coords of Gauss points.

  real(8) :: dr0_g (6)              ! Spacial derivative of r0 at Gauss point.
  real(8) :: Ra(3),RaDDot(3)        ! Displacement and its derivative at Gauss point.
  real(8) :: Psi(3),PsiDDot(3)      ! Rotation vector and its derivative at Gauss point.
  real(8) :: ddq(6)                 ! state of system

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)               ! Tangential operator.
  real(8) :: Yp(6,6),D(6,6)         ! Ypsilon and D.
  real(8) :: ARC(6,6)               ! A_{RC} matrix
  real(8) :: N (6,6*MaxNodCB3)      ! Element shape function matrix.

  real(8) :: MDtYddq(6),dDtYddq(6,6),dARCdqMDtYddq(6,6)                  ! Derivatives d^2(.)/d(q)^2 computed at Gauss points.
  real(8) :: MARCtVrelDot(6),dARCdqMARCtVrelDot(6,6),dARCtVrelDot(6,6)   ! Derivatives d(.)/d(Vrel) computed at Gauss points.

! Define Gauss points and loop on them.
  call fem_1d_gauss_val (NumGauss,CoordGauss,WeightGauss)

  do iGauss=1,NumGauss

! Obtain the shape functions and their derivatives.
    call fem_1d_shapefun (NumNodesElem,CoordGauss(iGauss),ShapeFun,ShapeDer)

! Global coordinates and Jacobian at current Gauss point.
    do i=1,6
      dr0_g(i)= dot_product (ShapeDer(1:NumNodesElem),r0(1:NumNodesElem,i))
    end do
    Jacobian =sqrt(dot_product (dr0_g(1:3),dr0_g(1:3)))

! Compute the current position vector and rotations, and their derivatives.
    do i=1,3
      Ra     (i)=dot_product (ShapeFun(1:NumNodesElem),Ri   (1:NumNodesElem,i))
      RaDDot (i)=dot_product (ShapeFun(1:NumNodesElem),RiDDot(1:NumNodesElem,i))
      Psi    (i)=dot_product (ShapeFun(1:NumNodesElem),Ri   (1:NumNodesElem,i+3))
      PsiDDot(i)=dot_product (ShapeFun(1:NumNodesElem),RiDDot(1:NumNodesElem,i+3))

    end do

    ddq(1:3)= RaDDot
    ddq(4:6)= PsiDDot

! Compute element shape functions.
    N =0.d0
    do i=1,6
      do j=1,NumNodesElem
        N (i,i+(j-1)*6)= ShapeFun(j)
      end do
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat  (Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot  (Psi)

!Operators in d(Mtddq)/dq.
    D=0.0d0
    D(1:3,1:3)=CaB
    D(4:6,4:6)=Unit

    Yp=0.0d0
    Yp(1:3,1:3)=Unit
    Yp(4:6,4:6)=Rot

    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    MDtYddq= matmul(ElemMass,matmul(transpose(D),matmul(Yp,ddq)))

    dARCdqMDtYddq(1:3,1:3)= 0.d0
    dARCdqMDtYddq(1:3,4:6)= -matmul(CaB,matmul(rot_skew(MDtYddq(1:3)),Rot))
    dARCdqMDtYddq(4:6,1:3)= -rot_skew(matmul(CaB,MDtYddq(1:3)))
    dARCdqMDtYddq(4:6,4:6)= -matmul(CaB,matmul(rot_skew(MDtYddq(4:6)),Rot))    &
&                           -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(MDtYddq(1:3)),Rot)))

    dDtYddq= 0.d0
    dDtYddq(1:3,4:6)= matmul(CBa,matmul(rot_skew(ddq(1:3)),transpose(Rot)))
    dDtYddq(4:6,4:6)= rotvect_a1 (Psi,ddq(4:6))

! Compute d(Mtddq)/dq part in Kmass
    Kmass= Kmass + WeightGauss(iGauss) * Jacobian                                 &
&                  * matmul((dARCdqMDtYddq + matmul(ARC,matmul(ElemMass,dDtYddq))),N)

! Additional operators in d(madVrel)/dq
    MARCtVrelDot=matmul(ElemMass,matmul(transpose(ARC),VrelDot))

    dARCdqMARCtVrelDot(1:3,1:3)= 0.d0
    dARCdqMARCtVrelDot(1:3,4:6)= -matmul(CaB,matmul(rot_skew(MARCtVrelDot(1:3)),Rot))
    dARCdqMARCtVrelDot(4:6,1:3)= -rot_skew(matmul(CaB,MARCtVrelDot(1:3)))
    dARCdqMARCtVrelDot(4:6,4:6)= -matmul(CaB,matmul(rot_skew(MARCtVrelDot(4:6)),Rot))    &
&                                -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(MARCtVrelDot(1:3)),Rot)))

    dARCtVrelDot(1:3,1:3)= matmul(CBa,rot_skew(VrelDot(4:6)))
    dARCtVrelDot(1:3,4:6)= matmul(CBa,matmul((rot_skew(VrelDot(1:3))+rot_skew(matmul(rot_skew(VrelDot(4:6)),Ra))),transpose(Rot)))
    dARCtVrelDot(4:6,1:3)= 0.d0
    dARCtVrelDot(4:6,4:6)= matmul(CBa,matmul(rot_skew(ddq(4:6)),transpose(Rot)))

! Compute d(madVrel)/dq and add to Kmass
    Kmass= Kmass + WeightGauss(iGauss) * Jacobian                                 &
&                  * matmul((dARCdqMARCtVrelDot + matmul(ARC,matmul(ElemMass,dARCtVrelDot))),N)

  end do
 end subroutine xbeam_kmass

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_RBKMASS
!
!-> Description:
!
!    Compute the lumped mass contributions to the tangent damping matrix
!    for motions of reference system.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!   2) The lumped masses are defined by the inertia tensor of the nonstructural masses
!      in the section of the node, and expressed with respect to the local B frame.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_rbkmass (NumNodesElem,r0,Ri,RiDDot,VrelDot,NodalMass,Kmass)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)   :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)   :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)   :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)   :: RiDDot   (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)   :: VrelDot  (6)     ! Linear/angular acceleration of reference frame.
  real(8),intent(in)   :: NodalMass(:,:,:) ! Inertia tensor of lumped masses at element nodes.
  real(8),intent(inout):: Kmass     (:,:)  ! Tangent gyroscopic damping matrix.

! Local variables
  integer :: i,iNode               ! Counters.
  real(8) :: Ra(3),RaDDot(3)        ! Displacement and its derivative at Gauss point.
  real(8) :: Psi(3),PsiDDot(3)      ! Rotation vector and its derivative at Gauss point.
  real(8) :: ddq(6)                 ! state of system

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)               ! Tangential operator.
  real(8) :: Yp(6,6),D(6,6)         ! Ypsilon and D.
  real(8) :: ARC(6,6)               ! A_{RC} matrix
  real(8) :: N (6,6*MaxNodCB3)      ! Element shape function matrix.

  real(8) :: MDtYddq(6),dDtYddq(6,6),dARCdqMDtYddq(6,6)                 ! Derivatives d^2(.)/d(q)^2 computed at Gauss points.
  real(8) :: MARCtVrelDot(6),dARCdqMARCtVrelDot(6,6),dARCtVrelDot(6,6)  ! Derivatives d(.)/d(Vrel) computed at Gauss points.

! Loop in the nodes.
  do iNode=1,NumNodesElem

 ! Compute the current position vector and rotations, and their derivatives.
		Ra = Ri(iNode,1:3)
		RaDDot = RiDDot(iNode,1:3)
		Psi = Ri (iNode,4:6)
		PsiDDot= RiDDot(iNode,4:6)

    ddq(1:3)= RaDDot
    ddq(4:6)= PsiDDot

! Compute element shape functions at the corrent node.
    N =0.d0
    do i=1,6
      N (i,i+(iNode-1)*6)= 1.d0
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat  (Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot  (Psi)

!Operators in d(Mtddq)/dq.
    D=0.0d0
    D(1:3,1:3)=CaB
    D(4:6,4:6)=Unit

    Yp=0.0d0
    Yp(1:3,1:3)=Unit
    Yp(4:6,4:6)=Rot

    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    MDtYddq= matmul(NodalMass(iNode,:,:),matmul(transpose(D),matmul(Yp,ddq)))

    dARCdqMDtYddq(1:3,1:3)= 0.d0
    dARCdqMDtYddq(1:3,4:6)= -matmul(CaB,matmul(rot_skew(MDtYddq(1:3)),Rot))
    dARCdqMDtYddq(4:6,1:3)= -rot_skew(matmul(CaB,MDtYddq(1:3)))
    dARCdqMDtYddq(4:6,4:6)= -matmul(CaB,matmul(rot_skew(MDtYddq(4:6)),Rot))    &
&                           -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(MDtYddq(1:3)),Rot)))

    dDtYddq= 0.d0
    dDtYddq(1:3,4:6)= matmul(CBa,matmul(rot_skew(ddq(1:3)),transpose(Rot)))
    dDtYddq(4:6,4:6)= rotvect_a1 (Psi,ddq(4:6))

! Compute d(Mtddq)/dq part in Kmass
    Kmass= Kmass + matmul((dARCdqMDtYddq + matmul(ARC,matmul(NodalMass(iNode,:,:),dDtYddq))),N)

! Additional operators in d(madVrel)/dq
    MARCtVrelDot=matmul(NodalMass(iNode,:,:),matmul(transpose(ARC),VrelDot))

    dARCdqMARCtVrelDot(1:3,1:3)= 0.d0
    dARCdqMARCtVrelDot(1:3,4:6)= -matmul(CaB,matmul(rot_skew(MARCtVrelDot(1:3)),Rot))
    dARCdqMARCtVrelDot(4:6,1:3)= -rot_skew(matmul(CaB,MARCtVrelDot(1:3)))
    dARCdqMARCtVrelDot(4:6,4:6)= -matmul(CaB,matmul(rot_skew(MARCtVrelDot(4:6)),Rot))    &
&                                -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(MARCtVrelDot(1:3)),Rot)))

    dARCtVrelDot(1:3,1:3)= matmul(CBa,rot_skew(VrelDot(4:6)))
    dARCtVrelDot(1:3,4:6)= matmul(CBa,matmul((rot_skew(VrelDot(1:3))+rot_skew(matmul(rot_skew(VrelDot(4:6)),Ra))),transpose(Rot)))
    dARCtVrelDot(4:6,1:3)= 0.d0
    dARCtVrelDot(4:6,4:6)= matmul(CBa,matmul(rot_skew(ddq(4:6)),transpose(Rot)))

! Compute d(madVrel)/dq and add to Kmass
    Kmass= Kmass + matmul((dARCdqMARCtVrelDot + matmul(ARC,matmul(NodalMass(iNode,:,:),dARCtVrelDot))),N)

  end do
 end subroutine xbeam_rbkmass

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_KGYR
!
!-> Description:
!
!    Compute tangent gyroscopic stiffness matrix
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!   2) The contribution due to the derivative of the mass matrix with the
!      degrees of freedom are included (thus the dependency with the
!      accelerations).
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_kgyr (NumNodesElem,r0,Ri,RiDot,RiDDot,Vrel,VrelDot, &
&                                  ElemMass,Kgyr,NumGauss)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: RiDDot   (:,:)   ! Second time derivative of Ri.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: VrelDot  (6)     ! Currnet time derivative of Vrel.
  real(8),intent(in)    :: ElemMass (6,6)   ! Inertia properties in the element.
  real(8),intent(inout) :: Kgyr     (:,:)   ! Tangent gyroscopic stiffness matrix.
  integer,intent(in)    :: NumGauss         ! Number of Gauss points in the element.

! Local variables
  integer :: i,j                    ! Counters.
  integer :: iGauss                 ! Counter on the Gaussian points.
  real(8) :: Jacobian               ! ds/deta, with eta the nondimensional arclength.
  real(8) :: ShapeFun(MaxNodCB3)    ! Shape functions in a gauss point.
  real(8) :: ShapeDer(MaxNodCB3)    ! Derivatives of ShapeFun in a gauss point.
  real(8) :: CoordGauss (NumGauss) ! Coords of Gauss points.
  real(8) :: WeightGauss(NumGauss) ! Coords of Gauss points.

  real(8) :: dr0_g (6)              ! Spacial derivative of r0 at Gauss point.
  real(8) :: Ra(3),Psi(3)           ! Displacement and Rotation vector at Gauss point.
  real(8) :: RaDot(3),PsiDot(3)     ! Time derivatives of Ra and Psi.
  real(8) :: RaDDot(3)              ! Time derivatives of RaDot.
  real(8) :: RotPsiDot(3)           ! Rot*PsiDot.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame.
  real(8) :: VBdot(3)               ! Time derivatives of VB.
  real(8) :: Vgyr(6)                ! Vectors with angular velocities.
  real(8) :: Phat(6),Pgyr(6)        ! Vectors with linear/angular momenta

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3),RotDot(3,3)   ! Tangential operator and time derivative.
  real(8) :: ARC(6,6),AWRC(6,6)     ! A_{RC} and A_{wRC}.
  real(8) :: dARCdqPgyr(6,6),dAWRCdqPhat(6,6)
  real(8) :: dVgyrdq  (6,6)         ! d(Vdot)/dq at Gauss point.
  real(8) :: dVdq     (6,6)         ! dV/dq at Gauss point.
  real(8) :: N (6,6*MaxNodCB3)      ! Element shape function matrix.

! Define Gauss points and loop on them.
  call fem_1d_gauss_val (NumGauss,CoordGauss,WeightGauss)

  do iGauss=1,NumGauss

! Obtain the shape functions and their derivatives.
    call fem_1d_shapefun (NumNodesElem,CoordGauss(iGauss),ShapeFun,ShapeDer)

! Global coordinates and Jacobian at current Gauss point.
    do i=1,6
      dr0_g(i)= dot_product (ShapeDer(1:NumNodesElem),r0(1:NumNodesElem,i))
    end do
    Jacobian =sqrt(dot_product (dr0_g(1:3),dr0_g(1:3)))

! Compute the current position vector and rotations, and their derivatives.
    do i=1,3
      Ra     (i)=dot_product (ShapeFun(1:NumNodesElem),Ri    (1:NumNodesElem,i))
      RaDot  (i)=dot_product (ShapeFun(1:NumNodesElem),RiDot (1:NumNodesElem,i))
      RaDDot (i)=dot_product (ShapeFun(1:NumNodesElem),RiDDot(1:NumNodesElem,i))
      Psi    (i)=dot_product (ShapeFun(1:NumNodesElem),Ri    (1:NumNodesElem,i+3))
      PsiDot (i)=dot_product (ShapeFun(1:NumNodesElem),RiDot (1:NumNodesElem,i+3))
    end do

! Compute element shape functions.
    N =0.d0
    do i=1,6
      do j=1,NumNodesElem
        N (i,i+(j-1)*6)= ShapeFun(j)
      end do
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat(Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot(Psi)
    RotDot=rotvect_drotdpsib(Psi,PsiDot)
    RotPsiDot=matmul(Rot,PsiDot)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= RotPsiDot + matmul(CBa,Vrel(4:6))

    Phat= matmul(ElemMass(:,1:3),VB)+matmul(ElemMass(:,4:6),WB)

    VBdot= matmul(CBa,RaDDot+rot_cross(Vrel(4:6),RaDot)+rot_cross(VrelDot(4:6),Ra)+VrelDot(1:3)) &
&          + rot_cross(VB,RotPsiDot)

    Vgyr(1:3)= matmul(CBa,matmul(rot_skew(Vrel(4:6)),RaDot)) + matmul(rot_skew(VB),matmul(Rot,PsiDot))
    Vgyr(4:6)= matmul(RotDot+matmul(rot_skew(WB),Rot),PsiDot)

    Pgyr= matmul(ElemMass,Vgyr)

! Operators in Kgyr from the derivatives of velocities.
    dVdq(1:3,1:3)= matmul(CBa,rot_skew(Vrel(4:6)))
    dVdq(1:3,4:6)= matmul(rot_skew(VB),Rot)
    dVdq(4:6,1:3)= 0.0d0
    dVdq(4:6,4:6)= RotDot + matmul(rot_skew(WB),Rot)

    dVgyrdq(1:3,1:3)= matmul(CBa,rot_skew(VrelDot(4:6))) &
&                   - matmul(rot_skew(RotPsiDot),matmul(CBa,rot_skew(Vrel(4:6))))
    dVgyrdq(1:3,4:6)= matmul(rot_skew(VBdot),Rot) + matmul(rot_skew(VB),RotDot)
    dVgyrdq(4:6,1:3)= 0.d0
    dVgyrdq(4:6,4:6)= rotvect_b1(Psi,PsiDot,PsiDot) + matmul(rot_skew(WB),rotvect_a1(Psi,PsiDot))    &
&                     - matmul(rot_skew(RotPsiDot),RotDot+matmul(rot_skew(WB),Rot))

! Operators in Kgyr from the derivatives of matrices ARC and AWRC.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    dARCdqPgyr= 0.d0
    dARCdqPgyr(1:3,4:6)= -matmul(CaB,matmul(rot_skew(Pgyr(1:3)),Rot))
    dARCdqPgyr(4:6,1:3)= -matmul(CaB,matmul(rot_skew(Pgyr(1:3)),CBa))
    dARCdqPgyr(4:6,4:6)= -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(Pgyr(1:3)),Rot)))   &
&                        -matmul(CaB,matmul(rot_skew(Pgyr(4:6)),Rot))

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    dAWRCdqPhat= 0.d0
    dAWRCdqPhat(1:3,4:6)= -matmul(CaB,matmul(rot_skew(Phat(1:3)),RotDot)+matmul(rot_skew(WB),matmul(rot_skew(Phat(1:3)),Rot)))
    dAWRCdqPhat(4:6,1:3)= -matmul(CaB,matmul(matmul(rot_skew(WB),rot_skew(Phat(1:3))),CBa))          &
&                         +matmul(CaB,matmul(rot_skew(Phat(1:3)),matmul(rot_skew(WB),CBa)-matmul(CBa,rot_skew(Vrel(4:6)))))

    dAWRCdqPhat(4:6,4:6)= -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(Phat(1:3)),RotDot)                                   &
&                                                         +matmul(rot_skew(WB),matmul(rot_skew(Phat(1:3)),Rot))))              &
&                         -matmul(CaB,matmul(matmul(rot_skew(VB),rot_skew(Phat(1:3))),Rot))                                    &
&                         -matmul(CaB,matmul(rot_skew(Phat(4:6)),RotDot)+matmul(rot_skew(WB),matmul(rot_skew(Phat(4:6)),Rot)))

! Add the contribution of the current Gauss point to inertial stiffness matrix.
    Kgyr= Kgyr + WeightGauss(iGauss) * Jacobian &
&         * matmul((dARCdqPgyr + matmul(ARC,matmul(ElemMass,dVgyrdq)) + dAWRCdqPhat + matmul(AWRC,matmul(ElemMass,dVdq))),N)

  end do
 end subroutine xbeam_kgyr

 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_RBKGYR
!
!-> Description:
!
!    Compute the lumped mass contributions to the tangent gyroscopic
!    stiffness matrix
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!   2) The contribution due to the derivative of the mass matrix with the
!      degrees of freedom are included (thus the dependency with the
!      accelerations).
!   3) The contribution due to the derivative of the mass matrix with the
!      degrees of freedom are included (thus the dependency with the
!      accelerations).
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_rbkgyr (NumNodesElem,r0,Ri,RiDot,RiDDot,Vrel,VrelDot, &
&                         NodalMass,Kgyr)
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: RiDDot   (:,:)   ! Second time derivative of Ri.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: VrelDot  (6)     ! Currnet time derivative of Vrel.
  real(8),intent(in)    :: NodalMass(:,:,:) ! Inertia tensor of lumped masses at element nodes.
  real(8),intent(inout) :: Kgyr     (:,:)   ! Tangent gyroscopic stiffness matrix.

! Local variables
  integer :: i,iNode                ! Counters.
  real(8) :: Ra(3),Psi(3)           ! Displacement and Rotation vector at Gauss point.
  real(8) :: RaDot(3),PsiDot(3)     ! Time derivatives of Ra and Psi.
  real(8) :: RaDDot(3)              ! Time derivatives of RaDot.
  real(8) :: RotPsiDot(3)           ! Rot*PsiDot.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame.
  real(8) :: VBdot(3)               ! Time derivatives of VB.
  real(8) :: Vgyr(6)                ! Vectors with linear/angular velocities.
  real(8) :: Phat(6),Pgyr(6)        ! Vectors with linear/angular momenta

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3),RotDot(3,3)   ! Tangential operator and time derivative.
  real(8) :: ARC(6,6),AWRC(6,6)     ! A_{RC} and A_{wRC}.
  real(8) :: dARCdqPgyr(6,6),dAWRCdqPhat(6,6)
  real(8) :: dVgyrdq  (6,6)         ! d(Vdot)/dq at Gauss point.
  real(8) :: dVdq     (6,6)         ! dV/dq at Gauss point.
  real(8) :: N (6,6*MaxNodCB3)      ! Element shape function matrix.

! Loop in the element ndoes.
  do iNode=1,NumNodesElem

! Compute the current position vector and rotations, and their derivatives.
	Ra     = Ri    (iNode,1:3)
	RaDot  = RiDot (iNode,1:3)
	RaDDot = RiDDot(iNode,1:3)
	Psi    = Ri    (iNode,4:6)
	PsiDot = RiDot (iNode,4:6)

! Compute element shape functions at the corrent node.
    N =0.d0
    do i=1,6
      N (i,i+(iNode-1)*6)= 1.d0
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat(Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot(Psi)
    RotDot=rotvect_drotdpsib(Psi,PsiDot)
    RotPsiDot=matmul(Rot,PsiDot)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= RotPsiDot + matmul(CBa,Vrel(4:6))

    Phat= matmul(NodalMass(iNode,:,1:3),VB)+matmul(NodalMass(iNode,:,4:6),WB)

    VBdot= matmul(CBa,RaDDot+rot_cross(Vrel(4:6),RaDot)+rot_cross(VrelDot(4:6),Ra)+VrelDot(1:3)) &
&          + rot_cross(VB,RotPsiDot)

    Vgyr(1:3)= matmul(CBa,matmul(rot_skew(Vrel(4:6)),RaDot)) + matmul(rot_skew(VB),matmul(Rot,PsiDot))
    Vgyr(4:6)= matmul(RotDot+matmul(rot_skew(WB),Rot),PsiDot)

    Pgyr= matmul(NodalMass(iNode,:,:),Vgyr)

! Operators in Kgyr from the derivatives of velocities.
    dVdq(1:3,1:3)= matmul(CBa,rot_skew(Vrel(4:6)))
    dVdq(1:3,4:6)= matmul(rot_skew(VB),Rot)
    dVdq(4:6,1:3)= 0.0d0
    dVdq(4:6,4:6)= RotDot + matmul(rot_skew(WB),Rot)

    dVgyrdq(1:3,1:3)= matmul(CBa,rot_skew(VrelDot(4:6))) &
&                   - matmul(rot_skew(RotPsiDot),matmul(CBa,rot_skew(Vrel(4:6))))
    dVgyrdq(1:3,4:6)= matmul(rot_skew(VBdot),Rot) + matmul(rot_skew(VB),RotDot)
    dVgyrdq(4:6,1:3)= 0.d0
    dVgyrdq(4:6,4:6)= rotvect_b1(Psi,PsiDot,PsiDot) + matmul(rot_skew(WB),rotvect_a1(Psi,PsiDot))    &
&                     - matmul(rot_skew(RotPsiDot),RotDot+matmul(rot_skew(WB),Rot))

! Operators in Kgyr from the derivatives of matrices ARC and AWRC.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    dARCdqPgyr= 0.d0
    dARCdqPgyr(1:3,4:6)= -matmul(CaB,matmul(rot_skew(Pgyr(1:3)),Rot))
    dARCdqPgyr(4:6,1:3)= -matmul(CaB,matmul(rot_skew(Pgyr(1:3)),CBa))
    dARCdqPgyr(4:6,4:6)= -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(Pgyr(1:3)),Rot)))   &
&                        -matmul(CaB,matmul(rot_skew(Pgyr(4:6)),Rot))

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    dAWRCdqPhat= 0.d0
    dAWRCdqPhat(1:3,4:6)= -matmul(CaB,matmul(rot_skew(Phat(1:3)),RotDot)+matmul(rot_skew(WB),matmul(rot_skew(Phat(1:3)),Rot)))
    dAWRCdqPhat(4:6,1:3)= -matmul(CaB,matmul(matmul(rot_skew(WB),rot_skew(Phat(1:3))),CBa))          &
&                         +matmul(CaB,matmul(rot_skew(Phat(1:3)),matmul(rot_skew(WB),CBa)-matmul(CBa,rot_skew(Vrel(4:6)))))

    dAWRCdqPhat(4:6,4:6)= -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(Phat(1:3)),RotDot)                                   &
&                                                         +matmul(rot_skew(WB),matmul(rot_skew(Phat(1:3)),Rot))))              &
&                         -matmul(CaB,matmul(matmul(rot_skew(VB),rot_skew(Phat(1:3))),Rot))                                    &
&                         -matmul(CaB,matmul(rot_skew(Phat(4:6)),RotDot)+matmul(rot_skew(WB),matmul(rot_skew(Phat(4:6)),Rot)))

! Add the contribution of the current Gauss point to inertial stiffness matrix.
    Kgyr= Kgyr + matmul((dARCdqPgyr + matmul(ARC, matmul(NodalMass(iNode,:,:),dVgyrdq)) &
&                     + dAWRCdqPhat + matmul(AWRC,matmul(NodalMass(iNode,:,:),dVdq))),N)

  end do
 end subroutine xbeam_rbkgyr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_CGYR
!
!-> Description:
!
!    Compute tangent gyroscopic damping matrix
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_cgyr (NumNodesElem,r0,Ri,RiDot,Vrel,ElemMass,Cgyr,NumGauss)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: ElemMass (6,6)   ! Inertia properties in the element.
  real(8),intent(inout) :: Cgyr     (:,:)   ! Tangent gyroscopic damping matrix.
  integer,intent(in)    :: NumGauss         ! Number of Gauss points in the element.

! Local variables
  integer :: i,j                    ! Counters.
  integer :: iGauss                 ! Counter on the Gaussian points.
  real(8) :: Jacobian               ! ds/deta, with eta the nondimensional arclength.
  real(8) :: ShapeFun(MaxNodCB3)    ! Shape functions in a gauss point.
  real(8) :: ShapeDer(MaxNodCB3)    ! Derivatives of ShapeFun in a gauss point.
  real(8) :: CoordGauss (NumGauss) ! Coords of Gauss points.
  real(8) :: WeightGauss(NumGauss) ! Coords of Gauss points.

  real(8) :: dr0_g (6)              ! Spacial derivative of r0 at Gauss point.
  real(8) :: Ra(3),RaDot(3)         ! Displacement and its derivative at Gauss point.
  real(8) :: Psi(3),PsiDot(3)       ! Rotation vector and its derivative at Gauss point.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame..
  real(8) :: PB(3),HB(3)            ! Linear and angular momenta in the material frame..

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)               ! Tangential operator.
  real(8) :: dVgyrdqdot(6,6)        ! d(Vdot)/d(qdot) at Gauss point.
  real(8) :: dVdqdot   (6,6)        ! dV/d(qdot) at Gauss point.
  real(8) :: ARC(6,6),AWRC(6,6),dAWRCdqdotPhat(6,6)
  real(8) :: N (6,6*MaxNodCB3)      ! Element shape function matrix.

! Define Gauss points and loop on them.
  call fem_1d_gauss_val (NumGauss,CoordGauss,WeightGauss)

  do iGauss=1,NumGauss

! Obtain the shape functions and their derivatives.
    call fem_1d_shapefun (NumNodesElem,CoordGauss(iGauss),ShapeFun,ShapeDer)

! Global coordinates and Jacobian at current Gauss point.
    do i=1,6
      dr0_g(i)= dot_product (ShapeDer(1:NumNodesElem),r0(1:NumNodesElem,i))
    end do
    Jacobian =sqrt(dot_product (dr0_g(1:3),dr0_g(1:3)))

! Compute the current position vector and rotations, and their derivatives.
    do i=1,3
      Ra    (i)=dot_product (ShapeFun(1:NumNodesElem),Ri   (1:NumNodesElem,i))
      RaDot (i)=dot_product (ShapeFun(1:NumNodesElem),RiDot(1:NumNodesElem,i))
      Psi   (i)=dot_product (ShapeFun(1:NumNodesElem),Ri   (1:NumNodesElem,i+3))
      PsiDot(i)=dot_product (ShapeFun(1:NumNodesElem),RiDot(1:NumNodesElem,i+3))
    end do

! Compute element shape functions.
    N =0.d0
    do i=1,6
      do j=1,NumNodesElem
        N (i,i+(j-1)*6)= ShapeFun(j)
      end do
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat  (Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot  (Psi)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= matmul(Rot,PsiDot) + matmul(CBa,Vrel(4:6))

    PB= matmul(ElemMass(1:3,1:3),VB)+matmul(ElemMass(1:3,4:6),WB)
    HB= matmul(ElemMass(4:6,1:3),VB)+matmul(ElemMass(4:6,4:6),WB)

!Operators in Cgyr.
    dVdqdot=0.0d0
    dVdqdot(1:3,1:3)= CBa
    dVdqdot(4:6,4:6)= Rot

    dVgyrdqdot(1:3,1:3)= 2.d0*matmul(CBa,rot_skew(Vrel(4:6)))-matmul(rot_skew(WB),CBa)
    dVgyrdqdot(1:3,4:6)= matmul(rot_skew(VB),Rot)
    dVgyrdqdot(4:6,1:3)= 0.d0
    dVgyrdqdot(4:6,4:6)= 2.d0*rotvect_drotdpsib(Psi,PsiDot)+matmul(rot_skew(WB),Rot)

    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    dAWRCdqdotPhat(1:3,1:3)= 0.d0
    dAWRCdqdotPhat(1:3,4:6)= -matmul(CaB,matmul(rot_skew(PB),Rot))
    dAWRCdqdotPhat(4:6,1:3)= -rot_skew(matmul(CaB,PB))
    dAWRCdqdotPhat(4:6,4:6)= -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(PB),Rot)))   &
&                            -matmul(CaB,matmul(rot_skew(HB),Rot))

! Compute strain matrix operator and material tangent stiffness.
    Cgyr= Cgyr + WeightGauss(iGauss) * Jacobian   &
&                * matmul((matmul(ARC,matmul(ElemMass,dVgyrdqdot)) + dAWRCdqdotPhat + matmul(AWRC,matmul(ElemMass,dVdqdot))),N)

  end do
 end subroutine xbeam_cgyr

 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_RBCGYR
!
!-> Description:
!
!    Compute the lumped mass contributions to the tangent gyroscopic damping
!    matrix
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_rbcgyr (NumNodesElem,r0,Ri,RiDot,Vrel,NodalMass,Cgyr)
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (NumNodesElem,6)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (NumNodesElem,6)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (NumNodesElem,6)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: NodalMass(NumNodesElem,6,6) ! Inertia tensor of lumped masses at element nodes.
  real(8),intent(inout) :: Cgyr     (6,6*MaxNodCB3)   ! Tangent gyroscopic damping matrix.

! Local variables
  integer :: i,iNode                ! Counters.
  real(8) :: Ra(3),RaDot(3)         ! Displacement and its derivative at Gauss point.
  real(8) :: Psi(3),PsiDot(3)       ! Rotation vector and its derivative at Gauss point.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame..
  real(8) :: PB(3),HB(3)            ! Linear and angular momenta in the material frame..

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)               ! Tangential operator.
  real(8) :: dVgyrdqdot(6,6)        ! d(Vdot)/d(qdot) at Gauss point.
  real(8) :: dVdqdot   (6,6)        ! dV/d(qdot) at Gauss point.
  real(8) :: ARC(6,6),AWRC(6,6),dAWRCdqdotPhat(6,6)
  real(8) :: N (6,6*MaxNodCB3)      ! Element shape function matrix.

! Loop in the element ndoes.
  do iNode=1,NumNodesElem
! Compute the current position vector and rotations, and their derivatives.
    Ra     = Ri    (iNode,1:3)
    RaDot  = RiDot (iNode,1:3)
    Psi    = Ri    (iNode,4:6)
    PsiDot = RiDot (iNode,4:6)

! Compute element shape functions at the corrent node.
    N =0.d0
    do i=1,6
      N (i,i+(iNode-1)*6)= 1.d0
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat  (Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot  (Psi)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= matmul(Rot,PsiDot) + matmul(CBa,Vrel(4:6))

    PB= matmul(NodalMass(iNode,1:3,1:3),VB) + matmul(NodalMass(iNode,1:3,4:6),WB)
    HB= matmul(NodalMass(iNode,4:6,1:3),VB) + matmul(NodalMass(iNode,4:6,4:6),WB)

!Operators in Cgyr.
    dVdqdot=0.0d0
    dVdqdot(1:3,1:3)= CBa
    dVdqdot(4:6,4:6)= Rot

    dVgyrdqdot(1:3,1:3)= 2.d0*matmul(CBa,rot_skew(Vrel(4:6)))-matmul(rot_skew(WB),CBa)
    dVgyrdqdot(1:3,4:6)= matmul(rot_skew(VB),Rot)
    dVgyrdqdot(4:6,1:3)= 0.d0
    dVgyrdqdot(4:6,4:6)= 2.d0*rotvect_drotdpsib(Psi,PsiDot)+matmul(rot_skew(WB),Rot)

    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    dAWRCdqdotPhat(1:3,1:3)= 0.d0
    dAWRCdqdotPhat(1:3,4:6)= -matmul(CaB,matmul(rot_skew(PB),Rot))
    dAWRCdqdotPhat(4:6,1:3)= -rot_skew(matmul(CaB,PB))
    dAWRCdqdotPhat(4:6,4:6)= -matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(PB),Rot)))   &
&                            -matmul(CaB,matmul(rot_skew(HB),Rot))

! Compute strain matrix operator and material tangent stiffness.
    Cgyr= Cgyr + matmul(&
                        (&
                         matmul(&
                                ARC,&
                                matmul(NodalMass(iNode,:,:),&
                                       dVgyrdqdot)&
                                ) + dAWRCdqdotPhat + matmul(AWRC,&
                                                            matmul(&
                                                                  NodalMass(iNode,:,:),&
                                                                  dVdqdot)&
                                                           )&
                        ),&
                        N)

  end do
 end subroutine xbeam_rbcgyr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine xbeam_FGYR
!
!-> Description:
!
!    Compute generalized gyroscopic forces.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_fgyr (NumNodesElem,r0,Ri,RiDot,Vrel,ElemMass,Qgyr,NumGauss)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: ElemMass (6,6)   ! Inertia properties in the element.
  real(8),intent(inout) :: Qgyr     (:)     ! Vector of discrete gyroscopic forces.
  integer,intent(in)    :: NumGauss         ! Number of Gauss points in the element.

! Local variables
  integer :: i                      ! Counters.
  integer :: iGauss                 ! Counter on the Gaussian points.
  real(8) :: Jacobian               ! ds/deta, with eta the nondimensional arclength.
  real(8) :: ShapeFun(MaxNodCB3)    ! Shape functions in a gauss point.
  real(8) :: ShapeDer(MaxNodCB3)    ! Derivatives of ShapeFun in a gauss point.
  real(8) :: CoordGauss (NumGauss) ! Coords of Gauss points.
  real(8) :: WeightGauss(NumGauss) ! Coords of Gauss points.

  real(8) :: dr0_g (6)              ! Spacial derivative of r0 at Gauss point.
  real(8) :: Ra(3),Psi(3)           ! Displacement and Rotation vector at Gauss point.
  real(8) :: RaDot(3),PsiDot(3)     ! Time derivatives of Ra and Psi.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame.
  real(8)  ::VBgyr(3),WBgyr(3)      ! Time derivatives of VB and WB.
  real(8) :: Vhat(6),Vhatgyr(6)     ! Vectors with linear/angular velocities.

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3),RotDot(3,3)   ! Tangential operator and time derivative.
  real(8) :: ARC(6,6),AWRC(6,6)     ! A_{RC} and A_{wRC} matrices.

! Define Gauss points and loop on them.
  call fem_1d_gauss_val (NumGauss,CoordGauss,WeightGauss)

  do iGauss=1,NumGauss

! Obtain the shape functions and their derivatives.
    call fem_1d_shapefun (NumNodesElem,CoordGauss(iGauss),ShapeFun,ShapeDer)

! Global coordinates and Jacobian at current Gauss point.
    do i=1,6
      dr0_g(i)= dot_product (ShapeDer(1:NumNodesElem),r0(1:NumNodesElem,i))
    end do
    Jacobian =sqrt(dot_product (dr0_g(1:3),dr0_g(1:3)))

! Compute the current position vector and rotations, and their derivatives.
    do i=1,3
      Ra     (i)=dot_product (ShapeFun(1:NumNodesElem),Ri    (1:NumNodesElem,i))
      RaDot  (i)=dot_product (ShapeFun(1:NumNodesElem),RiDot (1:NumNodesElem,i))
      Psi    (i)=dot_product (ShapeFun(1:NumNodesElem),Ri    (1:NumNodesElem,i+3))
      PsiDot (i)=dot_product (ShapeFun(1:NumNodesElem),RiDot (1:NumNodesElem,i+3))
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa    = rotvect_psi2mat(Psi)
    CaB    = transpose(CBa)
    Rot    = rotvect_psi2rot(Psi)
    RotDot = rotvect_drotdpsib(Psi,PsiDot)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= matmul(Rot,PsiDot) + matmul(CBa,Vrel(4:6))

    VBgyr= matmul(CBa,matmul(rot_skew(Vrel(4:6)),RaDot)) + matmul(rot_skew(VB),matmul(Rot,PsiDot))
    WBgyr= matmul((RotDot+matmul(rot_skew(WB),Rot)),PsiDot)

! Operators in Qgyr
    ARC=0.d0
    ARC(1:3,1:3) = CaB
    ARC(4:6,1:3) = matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6) = CaB

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    Vhat(1:3)=VB
    Vhat(4:6)=WB

    Vhatgyr(1:3)=VBgyr
    Vhatgyr(4:6)=WBgyr

! Add the contribution of the current Gauss point to inertial force.
    Qgyr= Qgyr + WeightGauss(iGauss) * Jacobian * (matmul(ARC,matmul(ElemMass,Vhatgyr)) + matmul(AWRC,matmul(ElemMass,Vhat)))

  end do
 end subroutine xbeam_fgyr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine xbeam_RBFGYR
!
!-> Description:
!
!    Lumped mass contribution to the generalized gyroscopic forces.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_rbfgyr (NumNodesElem,r0,Ri,RiDot,Vrel,NodalMass,Qgyr)
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: NodalMass(:,:,:) ! Inertia tensor of lumped masses at element nodes.
  real(8),intent(inout) :: Qgyr     (:)     ! Vector of discrete gyroscopic forces.

! Local variables
  integer :: iNode                  ! Counters.
  real(8) :: Ra(3),Psi(3)           ! Displacement and Rotation vector at Gauss point.
  real(8) :: RaDot(3),PsiDot(3)     ! Time derivatives of Ra and Psi.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame.
  real(8)  ::VBgyr(3),WBgyr(3)      ! Time derivatives of VB and WB.
  real(8) :: Vhat(6),Vhatgyr(6)     ! Vectors with linear/angular velocities.

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3),RotDot(3,3)   ! Tangential operator and time derivative.
  real(8) :: ARC(6,6),AWRC(6,6)     ! A_{RC} and A_{wRC} matrices.

! Loop in the element ndoes.
  do iNode=1,NumNodesElem

! Compute the current position vector and rotations, and their derivatives.
	Ra     = Ri    (iNode,1:3)
	RaDot  = RiDot (iNode,1:3)
	Psi    = Ri    (iNode,4:6)
	PsiDot = RiDot (iNode,4:6)

! Compute the current coordinate transformation matrix and rotational operator.
    CBa    = rotvect_psi2mat(Psi)
    CaB    = transpose(CBa)
    Rot    = rotvect_psi2rot(Psi)
    RotDot = rotvect_drotdpsib(Psi,PsiDot)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= matmul(Rot,PsiDot) + matmul(CBa,Vrel(4:6))

    VBgyr= matmul(CBa,matmul(rot_skew(Vrel(4:6)),RaDot)) + matmul(rot_skew(VB),matmul(Rot,PsiDot))
    WBgyr= matmul((RotDot+matmul(rot_skew(WB),Rot)),PsiDot)

! Operators in Qgyr
    ARC=0.d0
    ARC(1:3,1:3) = CaB
    ARC(4:6,1:3) = matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6) = CaB

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    Vhat(1:3)=VB
    Vhat(4:6)=WB

    Vhatgyr(1:3)=VBgyr
    Vhatgyr(4:6)=WBgyr

! Add the contribution of the current Gauss point to inertial force.
    Qgyr= Qgyr +(matmul(ARC, matmul(NodalMass(iNode,:,:),Vhatgyr)) &
&              + matmul(AWRC,matmul(NodalMass(iNode,:,:),Vhat)))
  end do
 end subroutine xbeam_rbfgyr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine xbeam_mrr
!
!-> Description:
!
!    Compute the mass matrix corresponding to the reference system accelerations.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_mrr (NumNodesElem,r0,Ri,ElemMass,MRR,NumGauss)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)   :: NumNodesElem      ! Number of nodes in the element.
  real(8),intent(in)   :: r0       (:,:)    ! Initial position/orientation of grid points.
  real(8),intent(in)   :: Ri       (:,:)    ! Current position/orientation of grid points.
  real(8),intent(in)   :: ElemMass (:,:)    ! Inertial properties in the element.
  real(8),intent(inout):: MRR     (:,:)     ! Tangent mass matrix.
  integer,intent(in)   :: NumGauss          ! Number of Gauss points in the element.

! Local variables.
  integer :: i                          ! Counters.
  integer :: iGauss                     ! Counter on the Gaussian points.
  real(8) :: Jacobian                   ! ds/deta, with eta the nondimensional arclength.
  real(8) :: CoordGauss (NumGauss) ! Coords of Gauss points.
  real(8) :: WeightGauss(NumGauss) ! Coords of Gauss points.
  real(8) :: ShapeFun(MaxNodCB3)        ! Shape functions in a gauss point.
  real(8) :: ShapeDer(MaxNodCB3)        ! Derivatives of ShapeFun in a gauss point.

  real(8) :: Ri_g (6)              ! Displacement/rotation at Gauss point.
  real(8) :: dr0_g(6)              ! Derivative of r0_g.
  real(8) :: CaB(3,3)              ! Coordinate transformation matrix.
  real(8) :: ARC(6,6)              ! A_{RC} matrix

! Define Gauss points and loop on them.
  call fem_1d_gauss_val (NumGauss,CoordGauss,WeightGauss)

  do iGauss=1,NumGauss
! Obtain the shape functions and their derivatives.
    call fem_1d_shapefun (NumNodesElem,CoordGauss(iGauss),ShapeFun,ShapeDer)

! Global coordinates at the gauss points and element Jacobian.
    do i=1,6
      dr0_g(i)= dot_product (ShapeDer(1:NumNodesElem),r0(1:NumNodesElem,i))
    end do
    Jacobian =sqrt(dot_product (dr0_g(1:3),dr0_g(1:3)))

! Compute the current position vector and rotations.
    do i=1,6
      Ri_g(i) = dot_product (ShapeFun(1:NumNodesElem),Ri(1:NumNodesElem,i))
    end do

! Compute the current coordinate transformation matrix.
    CaB= transpose(rotvect_psi2mat(Ri_g(4:6)))

!Operators for the reference mass matrix.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ri_g(1:3)),CaB)
    ARC(4:6,4:6)=CaB

! Compute reference mass matrix.
    MRR= MRR + WeightGauss(iGauss)*Jacobian*matmul(ARC,matmul(ElemMass,transpose(ARC)))
  end do
 end subroutine xbeam_mrr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_RBMRR
!
!-> Description:
!
!    Compute the lumped mass contribution to the mass matrix corresponding
!    to the reference system accelerations.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_rbmrr (NumNodesElem,r0,Ri,NodalMass,MRR)
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)   :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)   :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)   :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)   :: NodalMass(:,:,:) ! Inertia tensor of lumped masses at element nodes.
  real(8),intent(inout):: MRR     (:,:)    ! Tangent mass matrix.

! Local variables.
  integer :: iNode                 ! Counters.
  real(8) :: CaB(3,3)              ! Coordinate transformation matrix.
  real(8) :: ARC(6,6)              ! A_{RC} matrix

! Loop in the element ndoes.
  do iNode=1,NumNodesElem

! Compute the current coordinate transformation matrix.
    CaB= transpose(rotvect_psi2mat(Ri(iNode,4:6)))

!Operators for the reference mass matrix.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ri(iNode,1:3)),CaB)
    ARC(4:6,4:6)=CaB

! Compute reference mass matrix.
    MRR= MRR + matmul(ARC,matmul(NodalMass(iNode,:,:),transpose(ARC)))
  end do
  return
 end subroutine xbeam_rbmrr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_CRR
!
!-> Description:
!
!    Compute tangent damping matrix CRR for motions of reference system.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_crr (NumNodesElem,r0,Ri,RiDot,Vrel,ElemMass,CRR,NumGauss)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: ElemMass (6,6)   ! Inertia properties in the element.
  real(8),intent(inout) :: CRR     (:,:)    ! Tangent gyroscopic damping matrix.
  integer,intent(in)    :: NumGauss         ! Number of Gauss points in the element.

! Local variables
  integer :: i                      ! Counters.
  integer :: iGauss                 ! Counter on the Gaussian points.
  real(8) :: Jacobian               ! ds/deta, with eta the nondimensional arclength.
  real(8) :: ShapeFun(MaxNodCB3)    ! Shape functions in a gauss point.
  real(8) :: ShapeDer(MaxNodCB3)    ! Derivatives of ShapeFun in a gauss point.
  real(8) :: CoordGauss (NumGauss) ! Coords of Gauss points.
  real(8) :: WeightGauss(NumGauss) ! Coords of Gauss points.

  real(8) :: dr0_g (6)              ! Spacial derivative of r0 at Gauss point.
  real(8) :: Ra(3),RaDot(3)         ! Displacement and its derivative at Gauss point.
  real(8) :: Psi(3),PsiDot(3)       ! Rotation vector and its derivative at Gauss point.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame..
  real(8) :: PB(3),HB(3)            ! Linear and angular momenta in the material frame..

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)               ! Tangential operator.
  real(8) :: ARC(6,6),AWRC(6,6)
  real(8) :: dAWRCdvrelPhat(6,6)
  real(8) :: dVgyrdvrel(6,6)        ! d(Vdot)/d(vrel) at Gauss point.
  real(8) :: dVdvrel   (6,6)        ! dV/d(vrel) at Gauss point.

! Define Gauss points and loop on them.
  call fem_1d_gauss_val (NumGauss,CoordGauss,WeightGauss)

  do iGauss=1,NumGauss

! Obtain the shape functions and their derivatives.
    call fem_1d_shapefun (NumNodesElem,CoordGauss(iGauss),ShapeFun,ShapeDer)

! Global coordinates and Jacobian at current Gauss point.
    do i=1,6
      dr0_g(i)= dot_product (ShapeDer(1:NumNodesElem),r0(1:NumNodesElem,i))
    end do
    Jacobian =sqrt(dot_product (dr0_g(1:3),dr0_g(1:3)))

! Compute the current position vector and rotations, and their derivatives.
    do i=1,3
      Ra    (i)=dot_product (ShapeFun(1:NumNodesElem),Ri   (1:NumNodesElem,i))
      RaDot (i)=dot_product (ShapeFun(1:NumNodesElem),RiDot(1:NumNodesElem,i))
      Psi   (i)=dot_product (ShapeFun(1:NumNodesElem),Ri   (1:NumNodesElem,i+3))
      PsiDot(i)=dot_product (ShapeFun(1:NumNodesElem),RiDot(1:NumNodesElem,i+3))
    end do

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat  (Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot  (Psi)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= matmul(Rot,PsiDot) + matmul(CBa,Vrel(4:6))

    PB= matmul(ElemMass(1:3,1:3),VB)+matmul(ElemMass(1:3,4:6),WB)
    HB= matmul(ElemMass(4:6,1:3),VB)+matmul(ElemMass(4:6,4:6),WB)

!Operators in CRR.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    dVdvrel=transpose(ARC)

    dVgyrdvrel(1:3,1:3)= -matmul(rot_skew(WB),CBa) + matmul(CBa,rot_skew(Vrel(4:6)))
    dVgyrdvrel(1:3,4:6)= -matmul(CBa,rot_skew(Radot))+matmul(rot_skew(WB),matmul(CBa,rot_skew(Ra))) &
&                        -matmul(CBa,matmul(rot_skew(Vrel(4:6)),rot_skew(Ra)))
    dVgyrdvrel(4:6,1:3)=  0.d0
    dVgyrdvrel(4:6,4:6)= -matmul(rot_skew(WB),CBa) + matmul(CBa,rot_skew(Vrel(4:6)))

    dAWRCdvrelPhat=0.d0
    dAWRCdvrelPhat(1:3,4:6)= - matmul(CaB,matmul(rot_skew(PB),CBa))
    dAWRCdvrelPhat(4:6,1:3)= - matmul(CaB,matmul(rot_skew(PB),CBa))
    dAWRCdvrelPhat(4:6,4:6)= - matmul(CaB,matmul(rot_skew(HB),CBa)) &
&                            + matmul(matmul(CaB,matmul(rot_skew(PB),CBa)),rot_skew(Ra)) &
&                            - matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(PB),CBa)))

! Compute strain matrix operator and material tangent stiffness.
    CRR= CRR + WeightGauss(iGauss) * Jacobian                                   &
&              * (matmul(ARC,matmul(ElemMass,dVgyrdvrel)) + dAWRCdvrelPhat + matmul(AWRC,matmul(ElemMass,dVdvrel)))

  end do
 end subroutine xbeam_crr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine XBEAM_RBCRR
!
!-> Description:
!
!    Compute the lumped mass contribution to the tangent damping matrix
!    CRR for motions of reference system.
!
!-> Remarks.-
!
!   1) New values are added to those already in the matrix.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_rbcrr (NumNodesElem,r0,Ri,RiDot,Vrel,NodalMass,CRR)
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)    :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)    :: r0       (:,:)   ! Initial position/orientation of grid points.
  real(8),intent(in)    :: Ri       (:,:)   ! Current position/orientation of grid points.
  real(8),intent(in)    :: RiDot    (:,:)   ! Current time derivative of position/CRV of points.
  real(8),intent(in)    :: Vrel     (6)     ! Linear/angular velocity of reference frame.
  real(8),intent(in)    :: NodalMass(:,:,:) ! Inertia tensor of lumped masses at element nodes.
  real(8),intent(inout) :: CRR     (:,:)    ! Tangent gyroscopic damping matrix.

! Local variables
  integer :: iNode                  ! Counters.
  real(8) :: Ra(3),RaDot(3)         ! Displacement and its derivative at Gauss point.
  real(8) :: Psi(3),PsiDot(3)       ! Rotation vector and its derivative at Gauss point.
  real(8) :: VB(3),WB(3)            ! Linear and angular velocity in the material frame..
  real(8) :: PB(3),HB(3)            ! Linear and angular momenta in the material frame..

  real(8) :: CBa(3,3),CaB(3,3)      ! Coordinate transformation matrix.
  real(8) :: Rot(3,3)               ! Tangential operator.
  real(8) :: ARC(6,6),AWRC(6,6)
  real(8) :: dAWRCdvrelPhat(6,6)
  real(8) :: dVgyrdvrel(6,6)        ! d(Vdot)/d(vrel) at Gauss point.
  real(8) :: dVdvrel   (6,6)        ! dV/d(vrel) at Gauss point.

! Loop in the element ndoes.
  do iNode=1,NumNodesElem

! Compute the current position vector and rotations, and their derivatives.
	Ra     = Ri    (iNode,1:3)
	RaDot  = RiDot (iNode,1:3)
	Psi    = Ri    (iNode,4:6)
	PsiDot = RiDot (iNode,4:6)

! Compute the current coordinate transformation matrix and rotational operator.
    CBa= rotvect_psi2mat  (Psi)
    CaB= transpose(CBa)
    Rot= rotvect_psi2rot  (Psi)

! Compute the current linear and angular velocity.
    VB= matmul(CBa,RaDot+matmul(rot_skew(Vrel(4:6)),Ra)+Vrel(1:3))
    WB= matmul(Rot,PsiDot) + matmul(CBa,Vrel(4:6))

    PB= matmul(NodalMass(iNode,1:3,1:3),VB) + matmul(NodalMass(iNode,1:3,4:6),WB)
    HB= matmul(NodalMass(iNode,4:6,1:3),VB) + matmul(NodalMass(iNode,4:6,4:6),WB)

!Operators in CRR.
    ARC=0.d0
    ARC(1:3,1:3)=CaB
    ARC(4:6,1:3)=matmul(rot_skew(Ra),CaB)
    ARC(4:6,4:6)=CaB

    AWRC=0.d0
    AWRC(1:3,1:3) = matmul(CaB,rot_skew(WB))
    AWRC(4:6,1:3) = matmul(rot_skew(Ra),matmul(CaB,rot_skew(WB))) + matmul(CaB,rot_skew(VB))
    AWRC(4:6,4:6) = matmul(CaB,rot_skew(WB))

    dVdvrel=transpose(ARC)

    dVgyrdvrel(1:3,1:3)= -matmul(rot_skew(WB),CBa) + matmul(CBa,rot_skew(Vrel(4:6)))
    dVgyrdvrel(1:3,4:6)= -matmul(CBa,rot_skew(Radot))+matmul(rot_skew(WB),matmul(CBa,rot_skew(Ra))) &
&                        -matmul(CBa,matmul(rot_skew(Vrel(4:6)),rot_skew(Ra)))
    dVgyrdvrel(4:6,1:3)=  0.d0
    dVgyrdvrel(4:6,4:6)= -matmul(rot_skew(WB),CBa) + matmul(CBa,rot_skew(Vrel(4:6)))

    dAWRCdvrelPhat=0.d0
    dAWRCdvrelPhat(1:3,4:6)= - matmul(CaB,matmul(rot_skew(PB),CBa))
    dAWRCdvrelPhat(4:6,1:3)= - matmul(CaB,matmul(rot_skew(PB),CBa))
    dAWRCdvrelPhat(4:6,4:6)= - matmul(CaB,matmul(rot_skew(HB),CBa)) &
&                            + matmul(matmul(CaB,matmul(rot_skew(PB),CBa)),rot_skew(Ra)) &
&                            - matmul(rot_skew(Ra),matmul(CaB,matmul(rot_skew(PB),CBa)))

    CRR= CRR +(matmul(ARC, matmul(NodalMass(iNode,:,:),dVgyrdvrel)) + dAWRCdvrelPhat      &
&            + matmul(AWRC,matmul(NodalMass(iNode,:,:),dVdvrel)))

  end do
  return
 end subroutine xbeam_rbcrr

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!-> Subroutine xbeam_FEXT
!
!-> Description:
!
!    Matrix of influence coefficients for external forces.
!
!-> Remarks.-
!
!  1) Follower forces are assumed to be given in the local deformed frame (B frame).
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 subroutine xbeam_fext (NumNodesElem,Ri,Flags,Fmat,FollowerForce,FollowerForceRig,Cao)
  use lib_fem
  use lib_rot
  use lib_rotvect

! I/O Variables.
  integer,intent(in)        :: NumNodesElem     ! Number of nodes in the element.
  real(8),intent(in)        :: Ri    (:,:)      ! Current position/orientation of grid points.
  logical,intent(in)        :: Flags (:)        ! Identify master nodes.
  real(8),intent(inout)     :: Fmat  (:,:)      ! Forces/moments on the element nodes.
  logical(c_bool),intent(in)        :: FollowerForce    ! =T if follower forces.
  logical(c_bool),intent(in)        :: FollowerForceRig ! =T if follower force in the body-fixed frame.
  real(8),intent(in)        :: Cao   (:,:)      ! Rotation operator from inertial frame to reference frame

! Local variables.
  integer :: iNode                 ! Counter on the element nodes.
  real(8) :: CaB(3,3)              ! Transformation matrix from B to a
  real(8) :: ARC(6,6)              ! Operators to project the forces on the dofs.

!--> Case of follower forces in the body-fixed frame.
  if (FollowerForceRig) then
    !--> Case of follower forces on the structure.
    if (FollowerForce) then
      ! Loop in the element nodes.
      do iNode=1,NumNodesElem
         if (Flags(iNode)) then
           ! Operators to project the forces on the dofs.
           CaB= transpose(rotvect_psi2mat(Ri(iNode,4:6)))
           ARC = 0.d0
           ARC(1:3,1:3)= CaB
           ARC(4:6,1:3)= matmul(rot_skew(Ri(iNode,1:3)),CaB)
           ARC(4:6,4:6)= CaB                                            !rotvect_psi2rot(Ri(iNode,4:6))
        !    ARC(4:6,4:6)= matmul(rotvect_psi2rot(Ri(iNode,4:6)), CaB)

           ! Compute influence coefficients for follower forces.
           Fmat(:,6*(iNode-1)+1:6*iNode)= Fmat(:,6*(iNode-1)+1:6*iNode) + ARC
         end if
      end do

    !--> Case of dead forces on the structure but following the (a) frame.
    else
      do iNode=1,NumNodesElem
         if (Flags(iNode)) then
           ! Operators to project the forces on the dofs.
           ARC = 0.d0
           ARC(1:3,1:3)= Unit
           ARC(4:6,1:3)= rot_skew(Ri(iNode,1:3))
           ARC(4:6,4:6)= Unit

           ! Compute influence coefficients for follower forces.
           Fmat(:,6*(iNode-1)+1:6*iNode)= Fmat(:,6*(iNode-1)+1:6*iNode) + ARC
         end if
      end do
    end if

  !--> Case of dead forces in the inertial frame. Not possible to have structural follower forces and rigid-body dead loads.
  else
    do iNode=1,NumNodesElem
       if (Flags(iNode)) then
         ! Operators to project the forces on the dofs.
         ARC = 0.d0
         ARC(1:3,1:3)= Cao
         ARC(4:6,1:3)= matmul(rot_skew(Ri(iNode,1:3)),Cao)
         ARC(4:6,4:6)= Cao

         ! Compute influence coefficients for follower forces.
         Fmat(:,6*(iNode-1)+1:6*iNode)= Fmat(:,6*(iNode-1)+1:6*iNode) + ARC
       end if
    end do
  end if

  return
 end subroutine xbeam_fext

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!->Function XBEAM_ROT
!
!->Description.-
!
!   Definition of rotation operator in Euler quaternions
!   See Aircraft Control and Simulation by Stevens, Lewis
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 function xbeam_Rot (q)
     use lib_rot
! I/O Variables.
  real(8),  intent(inout):: q(:)           ! Quaternion
  real(8)             :: xbeam_Rot(3,3)

  q = q/sqrt(dot_product(q, q))
  xbeam_Rot = 0.d0

  xbeam_Rot(1,1) = q(1)*q(1) + q(2)*q(2) - q(3)*q(3) - q(4)*q(4)
  xbeam_Rot(2,2) = q(1)*q(1) - q(2)*q(2) + q(3)*q(3) - q(4)*q(4)
  xbeam_Rot(3,3) = q(1)*q(1) - q(2)*q(2) - q(3)*q(3) + q(4)*q(4)

  xbeam_Rot(1,2) = 2.0d0*(q(2)*q(3) + q(1)*q(4))
  xbeam_Rot(2,1) = 2.0d0*(q(2)*q(3) - q(1)*q(4))

  xbeam_Rot(1,3) = 2.0d0*(q(2)*q(4) - q(1)*q(3))
  xbeam_Rot(3,1) = 2.0d0*(q(2)*q(4) + q(1)*q(3))

  xbeam_Rot(2,3) = 2.0d0*(q(3)*q(4) + q(1)*q(2))
  xbeam_Rot(3,2) = 2.0d0*(q(3)*q(4) - q(1)*q(2))

  return
 end function xbeam_Rot

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!->Function XBEAM_QUADSKEW
!
!->Description.-
!
!  Quaternion ODE to compute orientation of body-fixed frame a.
!  See Shearer and Cesnik (2007) for definition
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 function xbeam_QuadSkew (w)

! I/O Variables.
  real(8),  intent(in):: w(:)           ! Angular velocity
  real(8)             :: xbeam_QuadSkew(4,4)

  xbeam_QuadSkew = reshape((/0.d0,w(1),w(2),w(3),-w(1),0.d0,-w(3),w(2),-w(2),w(3),0.d0,-w(1),-w(3),-w(2),w(1),0.d0/),   &
&                          (/4,4/), ORDER = (/ 2, 1 /))
  return
 end function xbeam_QuadSkew

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!->Function XBEAM_DROT_DQ
!
!->Description.-
!
!   Given the rotation operator in Euler quaternions provided by XBEAM_ROT, Rga,
!   (See Aircraft Control and Simulation by Stevens, Lewis or Cesnik 2007),
!   computes the 3rd order tensor C:
!
!   Cijk = dR_ij/dq_k
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

function xbeam_dRot_dq (q)

    ! I/O Variables.
    real(8),  intent(inout):: q(:)           ! Quaternion
    real(8)                :: xbeam_dRot_dq(3,3,4)
    real(8)                :: C(3,3,4)

    C=0.d0

    !dCdq1
    ![  2*q(1),  2*q(4), -2*q(3)],
    ![ -2*q(4),  2*q(1),  2*q(2)],
    ![  2*q(3), -2*q(2),  2*q(1)]
    C(1,:,1) = [  2*q(1),  2*q(4), -2*q(3)]
    C(2,:,1) = [ -2*q(4),  2*q(1),  2*q(2)]
    C(3,:,1) = [  2*q(3), -2*q(2),  2*q(1)]

    !dCdq2
    ![ 2*q(2),  2*q(3),  2*q(4)],
    ![ 2*q(3), -2*q(2),  2*q(1)],
    ![ 2*q(4), -2*q(1), -2*q(2)]
    C(1,:,2) = [ 2*q(2),  2*q(3),  2*q(4)]
    C(2,:,2) = [ 2*q(3), -2*q(2),  2*q(1)]
    C(3,:,2) = [ 2*q(4), -2*q(1), -2*q(2)]

    !dCdq3
    ![ -2*q(3), 2*q(2), -2*q(1)],
    ![  2*q(2), 2*q(3),  2*q(4)],
    ![  2*q(1), 2*q(4), -2*q(3)]
    C(1,:,3)=[ -2*q(3), 2*q(2), -2*q(1)]
    C(2,:,3)=[  2*q(2), 2*q(3),  2*q(4)]
    C(3,:,3)=[  2*q(1), 2*q(4), -2*q(3)]

    !dCdq4
    ![ -2*q4,  2*q1, 2*q2],
    ![ -2*q1, -2*q4, 2*q3],
    ![  2*q2,  2*q3, 2*q4]])
    C(1,:,4)=[ -2*q(4),  2*q(1), 2*q(2)]
    C(2,:,4)=[ -2*q(1), -2*q(4), 2*q(3)]
    C(3,:,4)=[  2*q(2),  2*q(3), 2*q(4)]

    xbeam_dRot_dq=C

    return

end function xbeam_dRot_dq

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!->Function XBEAM_D2ROT_DQ2
!
!->Description.-
!
!   Given the rotation operator in Euler quaternions provided by XBEAM_ROT, Rga,
!   (See Aircraft Control and Simulation by Stevens, Lewis or Cesnik 2007),
!   computes the 4rd order tensor C:
!
!   Cijkl = ddR_ij/dqk_ql = constant
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

subroutine xbeam_d2Rot_dq2 (C)

    ! I/O Variables.
    real(8)                :: q(4)      ! Quaternion
    real(8), intent(inout) :: C(3,3,4,4)
    real(8)                :: dCxq1(3,3,4),dCxq2(3,3,4),dCxq3(3,3,4),dCxq4(3,3,4)

    C=0.d0

    ! utility matrices
    q=0.d0
    q(1)=1.d0
    dCxq1 = xbeam_dRot_dq(q)
    q=0.d0
    q(2)=1.d0
    dCxq2 = xbeam_dRot_dq(q)
    q=0.d0
    q(3)=1.d0
    dCxq3 = xbeam_dRot_dq(q)
    q=0.d0
    q(4)=1.d0
    dCxq4 = xbeam_dRot_dq(q)

    !C_dqi_dq1
    C(:,:,1,1)=dCxq1(:,:,1)
    C(:,:,2,1)=dCxq1(:,:,2)
    C(:,:,3,1)=dCxq1(:,:,3)
    C(:,:,4,1)=dCxq1(:,:,4)

    !C_dqi_dq2
    C(:,:,1,2)=dCxq2(:,:,1)
    C(:,:,2,2)=dCxq2(:,:,2)
    C(:,:,3,2)=dCxq2(:,:,3)
    C(:,:,4,2)=dCxq2(:,:,4)

    !C_dqi_dq3
    C(:,:,1,3)=dCxq3(:,:,1)
    C(:,:,2,3)=dCxq3(:,:,2)
    C(:,:,3,3)=dCxq3(:,:,3)
    C(:,:,4,3)=dCxq3(:,:,4)

    !C_dqi_dq4
    C(:,:,1,4)=dCxq4(:,:,1)
    C(:,:,2,4)=dCxq4(:,:,2)
    C(:,:,3,4)=dCxq4(:,:,3)
    C(:,:,4,4)=dCxq4(:,:,4)

    ! dC1x
    !C(:,:,1,1) = [ [2, 0, 0], [0, 2, 0], [0, 0, 2]  ]
    !C(:,:,1,2) = [ [0, 0, 0], [0, 0, 2], [0, -2, 0] ]
    !C(:,:,1,3) = [ [0, 0, -2],[0, 0,  0],[2, 0,  0] ]
    !C(:,:,1,4) = [ [0, 2, 0], [-2, 0, 0],[ 0, 0, 0] ]

    ! dC2x
    !C(:,:,2,1) = [ [0,  0, 0], [0,  0, 2], [0, -2, 0]]
    !C(:,:,2,2) = [ [2,  0,  0], [0, -2,  0], [0,  0, -2]]
    !C(:,:,2,3) = [ [0, 2, 0], [2, 0, 0], [0, 0, 0] ]
    !C(:,:,2,4) = [ [0, 0, 2], [0, 0, 0], [2, 0, 0]]

    return

end subroutine xbeam_d2Rot_dq2

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!->Function XBEAM_SPH_origin
!
!->Description.-
!
! For given orientation (q) and angular velocities/accelerations, the routine
! returns the position, velocity and acceleration of the FoR A origin
! such that the node where the spherical joint is applied has zero displacement,
! velocity and acceleration.
!
! Note that the routine assumes that for q=(1,0,0,0) the FoR A and G are parallel!
! Also, q s the defined by the rotation that transofrms G into A (or, conversely,
! the rotation that bring A from its initial configuration to its current one).
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

subroutine xbeam_sph_origin (q,w,dw,possph0,Rsph0,posA,velA,accA)

    ! I/O Variables.
    real(8), intent(inout):: q(4)          ! Quaternion
    real(8), intent(in)   :: w(3), dw(3)   ! FoR A angular velocity/acceleration
    real(8), intent(inout):: Rsph0(3), possph0(3)      ! position of spherical joint in FoR A (constant) and G (for q=(1,0,0,0))
    real(8), intent(out)  :: posA(3), velA(3), accA(3) ! pos/vel/acc of FoR A origin when having spherical joints

    ! Internal
    real(8)   :: dq(4), ddq(4)     ! Quaternion I and II time derivatives
    real(8)   :: Om(4,4), dOm(4,4) ! Omega matrix for quaternion time derivative
                                   ! and its time derivative

    real(8)   :: dC(3,3,4), ddC(3,3,4,4) ! Tensors of rotational operator
    real(8)   :: Ctot(3,3)               ! matrix for sums
    integer   :: ii,jj

    ! skew symmetric matrices for quaterion
    Om =xbeam_QuadSkew(w)
    dOm=xbeam_QuadSkew(dw)

    ! compute quaternion derivatives
    dq  = -0.5_8 * matmul(Om,q)
    ddq = -0.5_8 * ( matmul(dOm,q) + matmul(Om,dq) )

    ! compute tensors
    call xbeam_d2Rot_dq2 (ddC)
    dC=xbeam_dRot_dq(q)

    ! quaternion acceleration term
    Ctot=0.d0
    do ii=1,4
        Ctot = Ctot + dC(:,:,ii)*ddq(ii)
    end do

    ! quaternion velocities term
    do ii=1,4
        do jj=1,4
            Ctot = Ctot + ddC(:,:,ii,jj)*dq(ii)*dq(jj)
        end do
    end do

    ! compute acceleration
    accA=0.d0
    accA = -matmul(transpose(Ctot),Rsph0)

    ! compute velocity
    Ctot=0.d0
     do ii=1,4
        Ctot = Ctot + dC(:,:,ii)*dq(ii)
    end do
    velA=-matmul(transpose(Ctot),Rsph0)

    ! compute position
    posA = possph0 - matmul(transpose(xbeam_Rot(q)),Rsph0)

    return

end subroutine xbeam_sph_origin

    ! -------------------------------------------------------------------------
    !
    ! Build quaternion using definition
    !
    !--------------------------------------------------------------------------
    subroutine build_quat(a,nvec,qvec)

        real(8), intent(out) :: qvec(4)  ! Quaternion
        real(8), intent(in)  :: nvec(3)  ! normal vector
        real(8)  :: a        ! angle of rotation

        qvec=0.d0
        qvec(1)  =cos(0.5_8*a)
        qvec(2:4)=sin(0.5_8*a)*nvec

    end subroutine build_quat

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
!->Subroutines XBEAM_2NORM
!
!->Description.-
!
!   computes the 2norm of a vector
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 function xbeam_2norm (V)
! I/O Variables.
  real(8),  intent(in):: V(:)           ! Vector
  real(8)             :: xbeam_2norm

! Local variables.
  integer            :: i

  xbeam_2norm = 0.d0
  do i=1,size(V)
      xbeam_2norm = xbeam_2norm + V(i)*V(i)
  end do
  xbeam_2norm = sqrt(xbeam_2norm)

  return
 end function xbeam_2norm

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
end module lib_xbeam

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! S. Maraniello 15 Sep 2015
!
! Testing module
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
module lib_xbeam_test

 use lib_xbeam
 implicit none

    real(8),parameter:: pi=3.14159265358979

    real(8)  :: q(4)          ! Quaternion
    real(8)  :: w(3), dw(3)   ! FoR A angular velocity/acceleration
    real(8)  :: Rsph0(3), possph0(3)      ! position of spherical joint in FoR A (constant) and G (at t=0 or for q=(1,0,0,0))
    real(8)  :: posA(3), velA(3), accA(3) ! pos/vel/acc of FoR A origin when having spherical joints
    real(8)  :: nv(3) ! normal veoctor to help defining quaternions
    real(8)  :: alpha, alpha02 ! angle of rotation

    integer   :: ii,jj

 contains

    ! --------------------------------------------------------------------------
    ! Test build quaternion
    subroutine build_quat_test

        print *, '--------------------------------------------- Test build_quat'
        alpha=pi/3.d0
        print *, 'alpha=', alpha

        do ii=1,3
            nv=0.d0
            nv(ii)=1.d0
            call build_quat(alpha,nv,q)
            print*, 'nv=', nv
            print*, 'quat=', q
        end do

    end subroutine build_quat_test

    ! --------------------------------------------------------------------------
    subroutine xbeam_sph_origin_test

        print *, '--------------------------------------- Test xbeam_sph_origin'

        Rsph0=0.d0; w=0.d0; dw=0.d0
        possph0=0.d0; Rsph0=0.d0;
        posA=0.d0; velA=0.d0; accA=0.d0;

        ! -------------------------------------------------------------- case 01
        print *, '--- case 01'
        Rsph0(1)=2.0
        alpha=pi/6.d0
        nv=0.d0;
        nv(2)=1.d0
        call build_quat(alpha,nv,q)
        ! get initial position in FoR G

        ! get FoR A origin position
        call xbeam_sph_origin(q,w,dw,Rsph0,Rsph0,posA,velA,accA)

        print *, 'posA', posA
        print *, 'posA expected=(0.26794,0,1)'

        ! -------------------------------------------------------------- case 02
        print *, '--- case 02'
        Rsph0(1)=2.d0;
        alpha= pi/2.d0
        print *, 'alpha=', alpha, 'Rsph0=', Rsph0

        do ii=1,3
            ! define rotation (build quaternion)
            nv=0.d0
            nv(ii)=1.d0
            call build_quat(alpha,nv,q)
            ! get initial position in FoR G

            ! get FoR A origin position
            call xbeam_sph_origin(q,w,dw,Rsph0,Rsph0,posA,velA,accA)
            print *, 'posA', posA
        end do

        ! -------------------------------------------------------------- case 03
        print *, '--- case 03'
        Rsph0(1:2)=2.d0;
        print *, 'Rsph0=', Rsph0
        !alpha02= pi/6.d0
        !print *, 'alpha IC=', alpha02, 'Rsph0=', Rsph0

        nv=0.d0; nv(2)=1.d0;
        call build_quat(alpha02,nv,q)
        ! get initial position in FoR G

        alpha=pi/2.d0
        print *, 'alpha rotation = ', alpha
        do ii=1,3
            ! define rotation (build quaternion)
            nv=0.d0
            nv(ii)=1.d0
            call build_quat(alpha,nv,q)
            ! get FoR A origin position
            call xbeam_sph_origin(q,w,dw,Rsph0,Rsph0,posA,velA,accA)
            print *, 'posA', posA
        end do

        ! -------------------------------------------------------------- case 04
        print *, '--- case 04'
        print *, 'Test posA when pospsh0 != Rsph0 (FoR A and G parallel but not coincident)'
        Rsph0=0.d0; Rsph0(1:2)=2.d0;
        possph0=0.d0; possph0(1)=5.d0; possph0(2)=3.d0;
        alpha=pi/2.d0

        print *, 'alpha rotation = ', alpha
        print *, 'Rsph0= ', Rsph0
        print *, 'posph0=', possph0

        do ii=1,3
            ! define rotation (build quaternion)
            nv=0.d0
            nv(ii)=1.d0
            call build_quat(alpha,nv,q)
             ! get FoR A origin position
            call xbeam_sph_origin(q,w,dw,possph0,Rsph0,posA,velA,accA)
            print *, 'posA', posA
        end do

        ! -------------------------------------------------------------- case 05
        print *, '--- case 05'
        print *, 'Test acceleration due to angular velocities only!'

        ! assign a random initial position (last of case 04)
        Rsph0=0.d0; Rsph0(1)=1.d0; Rsph0(2)=2.d0;!Rsph0(1:2)=2.d0;
        possph0=0.d0; possph0(1)=5.d0; possph0(2)=3.d0;
        alpha=pi/2.d0
        call xbeam_sph_origin(q,w,dw,possph0,Rsph0,posA,velA,accA)

        print *, 'alpha rotation = ', alpha
        print *, 'Rsph0= ', Rsph0
        print *, 'posph0=', possph0
        print *, 'posA', posA

        do ii=1,3
            ! define angular velocity
            w=0.d0; w(ii)=5.d0
            ! check posA is always the same
            call xbeam_sph_origin(q,w,dw,possph0,Rsph0,posA,velA,accA)
            print *, 'w   ', w
            print *, 'velA', velA
            print *, 'accA', accA
        end do
        ! neutral axis case
        w=0.d0; w(1)=5.d0; w(2)=10.d0;
        call xbeam_sph_origin(q,w,dw,possph0,Rsph0,posA,velA,accA)
        print *, 'w   ', w
        print *, 'velA', velA
        print *, 'accA', accA

        ! -------------------------------------------------------------- case 06
        print *, '--- case 06'
        print *, 'Test acceleration due to angular velocities and accelerations'
        print *, '(when acting in the same direction)'

        ! assign a random initial position (last of case 04)
        Rsph0=0.d0; Rsph0(1)=1.d0; Rsph0(2)=2.d0;!Rsph0(1:2)=2.d0;
        possph0=0.d0; possph0(1)=5.d0; possph0(2)=3.d0;
        alpha=pi/2.d0
        call xbeam_sph_origin(q,w,dw,possph0,Rsph0,posA,velA,accA)

        print *, 'alpha rotation = ', alpha
        print *, 'Rsph0= ', Rsph0
        print *, 'posph0=', possph0
        print *, 'posA', posA

        do ii=1,3
            ! define angular velocity/accelerations
            w=0.d0; w(ii)=5.d0
            dw=0.d0; dw(ii)=3.d0
            ! check posA is always the same
            call xbeam_sph_origin(q,w,dw,possph0,Rsph0,posA,velA,accA)
            print *, 'w   ', w
            print *, 'dw   ', dw
            print *, 'velA', velA
            print *, 'accA', accA
        end do
        ! neutral axis case
        w=0.d0; w(1)=5.d0; w(2)=10.d0;
        dw=0.d0; dw(1)=5.d0; dw(2)=10.d0;
        call xbeam_sph_origin(q,w,dw,possph0,Rsph0,posA,velA,accA)
        print *, 'w   ', w
        print *, 'dw   ', dw
        print *, 'velA', velA
        print *, 'accA', accA

         ! --- case 01
         ! posA  0.26794920700158609        0.0000000000000000        1.0000000252367827
         ! posA expected=(0.26794,0,1)
         ! --- case 02
         ! alpha=   1.5707963705062866      Rsph0=   2.0000000000000000        0.0000000000000000        0.0000000000000000
         ! posA   0.0000000000000000        0.0000000000000000        0.0000000000000000
         ! posA   2.0000000874227801        0.0000000000000000        1.9999999999999980
         ! posA   2.0000000874227801       -1.9999999999999980        0.0000000000000000
         ! --- case 03
         ! Rsph0=   2.0000000000000000        2.0000000000000000        0.0000000000000000
         ! alpha rotation =    1.5707963705062866
         ! posA   0.0000000000000000        2.0000000874227801       -1.9999999999999980
         ! posA   2.0000000874227801        0.0000000000000000        1.9999999999999980
         ! posA   4.0000000874227783       8.74227821245909809E-008   0.0000000000000000
         ! --- case 04
         !alpha rotation =    1.5707963705062866
         !Rsph0=    2.0000000000000000        2.0000000000000000        0.0000000000000000
         !posph0=   5.0000000000000000        3.0000000000000000        0.0000000000000000
         !posA   3.0000000000000000        3.0000000874227801       -1.9999999999999980
         !posA   5.0000000874227801        1.0000000000000000        1.9999999999999980
         !posA   7.0000000874227783        1.0000000874227821        0.0000000000000000
         !--- case 05
         !Test velocity!
         !alpha rotation =    1.5707963705062866
         !Rsph0=    1.0000000000000000        2.0000000000000000        0.0000000000000000
         !posph0=   5.0000000000000000        3.0000000000000000        0.0000000000000000
         !posA   7.0000000437113883        2.0000000874227810        0.0000000000000000
         !w      5.0000000000000000        0.0000000000000000        0.0000000000000000
         !velA  -0.0000000000000000       -0.0000000000000000       -10.000000000000000
         !accA  -49.999999999999957      -2.18556950493109525E-006  -0.0000000000000000
         !w      0.0000000000000000        5.0000000000000000        0.0000000000000000
         !velA  -0.0000000000000000       -0.0000000000000000        5.0000000000000000
         !accA -1.09278475246554763E-006   24.999999999999979       -0.0000000000000000
         !w      0.0000000000000000        0.0000000000000000        5.0000000000000000
         !velA   4.9999995628860949        10.000000218556941       -0.0000000000000000
         !accA  -50.000001092784707        24.999997814430472       -0.0000000000000000
         !w      5.0000000000000000        10.000000000000000        0.0000000000000000
         !velA  -0.0000000000000000       -0.0000000000000000       1.77635683940025046E-015
         !accA  7.10542735760100186E-015  -0.0000000000000000       -0.0000000000000000
         !--- case 06
         !Test acceleration due to angular velocities and accelerations
         !(when acting in the same direction)
         !alpha rotation =    1.5707963705062866
         !Rsph0=    1.0000000000000000        2.0000000000000000        0.0000000000000000
         !posph0=   5.0000000000000000        3.0000000000000000        0.0000000000000000
         !posA   7.0000000437113883        2.0000000874227810        0.0000000000000000
         !w      5.0000000000000000        0.0000000000000000        0.0000000000000000
         !dw      3.0000000000000000        0.0000000000000000        0.0000000000000000
         !velA  -0.0000000000000000       -0.0000000000000000       -10.000000000000000
         !accA  -49.999999999999957      -2.18556950493109525E-006  -6.0000000000000000
         !w      0.0000000000000000        5.0000000000000000        0.0000000000000000
         !dw      0.0000000000000000        3.0000000000000000        0.0000000000000000
         !velA  -0.0000000000000000       -0.0000000000000000        5.0000000000000000
         !accA -1.09278475246554763E-006   24.999999999999979        3.0000000000000000
         !w      0.0000000000000000        0.0000000000000000        5.0000000000000000
         !dw      0.0000000000000000        0.0000000000000000        3.0000000000000000
         !velA   4.9999995628860949        10.000000218556941       -0.0000000000000000
         !accA  -47.000001355053044        30.999997945564630       -0.0000000000000000
         !w      5.0000000000000000        10.000000000000000        0.0000000000000000
         !dw      5.0000000000000000        10.000000000000000        0.0000000000000000
         !velA  -0.0000000000000000       -0.0000000000000000       1.77635683940025046E-015
         !accA  7.10542735760100186E-015  -0.0000000000000000       1.77635683940025046E-015

    end subroutine xbeam_sph_origin_test

 end module lib_xbeam_test