Source code for pyfem.elements.Spring

# SPDX-License-Identifier: MIT
# Copyright (c) 2011–2026 Joris J.C. Remmers

from .Element import Element
from pyfem.util.transformations import toElementCoordinates, toGlobalCoordinates

from numpy import zeros, eye, array



class Spring ( Element ):

  #Number of dofs per element
  dofTypes = ['u','v']

  def __init__ ( self, elnodes , props ):
    Element.__init__( self, elnodes , props )



  def getTangentStiffness ( self, elemdat ):
    
    #Compute the current state vector

    a  = toElementCoordinates( elemdat.state  , elemdat.coords )
    Da = toElementCoordinates( elemdat.Dstate , elemdat.coords )

    #Compute the elongation of the spring
    elong = a[2]-a[0] 

    #Compute the force in the spring
    Fs = elong * elemdat.props.k

    #Compute the element internal force vector in the element coordinate system
    elFint = array([-Fs,0.,Fs,0])

    #Determine the element tangent stiffness in the element coordinate system
    elKbar = zeros( (4,4) )

    elKbar[:2,:2] =  elemdat.props.k*eye(2)
    elKbar[:2,2:] = -elemdat.props.k*eye(2)

    elKbar[2:,:2] = elKbar[:2,2:]
    elKbar[2:,2:] = elKbar[:2,:2]

    #Rotate element tangent stiffness to the global coordinate system
    elemdat.stiff = toGlobalCoordinates( elKbar, elemdat.coords )
    elemdat.fint = toGlobalCoordinates( elFint, elemdat.coords )

  
#------------------------------------------------------------------



  def getInternalForce ( self, elemdat ):

    #Compute the current state vector

    a  = toElementCoordinates( elemdat.state  , elemdat.coords )
    Da = toElementCoordinates( elemdat.Dstate , elemdat.coords )

    #Compute the elongation of the spring
    elong = a[2]-a[0] 

    #Compute the force in the spring
    Fs = elong * elemdat.props.k

    #Compute the element internal force vector in the element coordinate system
    elFint = array([-Fs,0.,Fs,0])

    #Rotate element fint to the global coordinate system
    elemdat.fint = toGlobalCoordinates( elFint, elemdat.coords )