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The 2D truss shown in the figure below, subjected to static displacement histories, was used in OpenSees to determine the material stress-strain response, see Figure 1. The length and cross sectional area of the truss element were defined as unity so that the material stress is equal to the truss axial force (stress=Force/Area) and the material strain is equal to the bar elongation (strain=Elongation/Length).
To maintain the integrity of the system subjected to imposed loads when a material (mainly Concrete01 in tension) reaches a zero-tangent situation, the model actually consisted of two parallel trusses connected to the same nodes. One truss was made up of the material being considered, the other was an elastic truss with unit stiffness. The material stresses were obtained by recording the element forces rather than the applied forces, and the material strains were obtained by recording the nodal displacement at the free dof (elements in parallel have the same deformation and different forces, depending on relative stiffnesses).
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