Efficient Algorithms for Composite Structural Analysis with Different Tension-Compression Material Properties

Composites materials may exhibit a remarkable difference in material properties depending on whether the state of stress is tension or compression. This tension-compression discontinuity in material properties often leads to a nonconvergent finite element solution. Two approaches, an explicit algorithm and an implicit algorithm, are presented in this paper to eliminate this convergence problem. In the explicit algorithm, the material properties are selected based on the sign of the components of the strain tensor from the previous increment while retaining the abrupt change in stiffness from tension to compression. The time step is repeated a few times until the stress tensor and the stiffness matrix are determined to be converged. In the implicit algorithm, a new mechanical constitutive law is developed that replaces the tension-compression discontinuity by a transition region. This produces immediate convergence, the time step does not need to be repeated, and it is time step independent. The efficiency of the algorithms are demonstrated for heat resistant composite materials. Examples are given to demonstrate that the algorithms produce fast convergence, but more importantly to demonstrate how the stress results vary significantly when analyses are conducted with tension material properties only, compression material properties only, and tension-compression material properties.