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A mechanism-based progressive failure analyses (PFA) approach is developed for fiber reinforced composite laminates. Each ply of the laminate is modeled as a nonlinear elastic degrading lamina in a state of plane stress according to Schapery theory (ST). In this theory, each lamina degrades as characterized through laboratory scale experiments. In the fiber direction, elastic behavior prevails, however, in the present work, the phenomenon of fiber microbuckling, which is responsible for the sudden degradation of the axial lamina properties under compression, is explicitly accounted for by allowing the fiber rotation at a material point to be a variable in the problem. The latter is motivated by experimental and numerical simulations that show that local fiber rotations in conjunction with a continuously degrading matrix are responsible for the onset of fiber microbuckling leading to kink banding. These features are built into a user defined material subroutine that is implemented through the commercial finite element (FE) software ABAQUS in conjunction with classical lamination theory (CLT) that considers a laminate as a collection of perfectly bonded lamina (Herakovich, C.T., 1998. Mechanics of Fibrous Composites. Wiley, New York). The present model, thus, disbands the notion of a fixed compressive strength, and instead uses the mechanics of the failure process to provide the in situ compression strength of a material point in a lamina, the latter being dictated strongly by the current local stress state, the current state of the lamina transverse material properties and the local fiber rotation. The inputs to the present work are laboratory scale, coupon level test data that provide information on the lamina transverse property degradation (i.e. appropriate, measured, strain-stress relations of the lamina transverse properties), the elastic lamina orthotropic properties, the ultimate tensile strength of the lamina in the fiber direction, the stacking sequence of the laminate and the geometry of the structural panel. The validity of the approach advocated is demonstrated through numerical simulations of the response of two composite structural panels that are loaded to complete failure. A flat, 24-ply unstiffened panel with a cutout subjected to in-plane shear loading, and a double notched 70-ply unstiffened stitched panel subjected to axial compression are selected for study. The predictions of the simulations are compared against experimental data. Good agreement between the present PFA and the experimental data are reported.

In this paper, a mechanism-based lamina level modeling approach is used as the basis for developing a macroscopic (lamina level) model to capture the mechanisms of kink banding. Laminae are modeled as inelastic degrading homogenized layers in a state of plane stress according to Schapery Theory (ST). However, the principal orthotropic material axes are allowed to rotate as a function of deformation. In ST, each lamina degrades as characterized through laboratory scale experiments. In the fiber direction, elastic behavior prevails; however, in this work, the phenomenon of fiber microbuckling leading to kink banding, which is responsible for the sudden degradation of the axial lamina properties under compression, is explicitly accounted for by allowing the fiber rotation at a material point to be a variable in the problem. These features are built into a user-defined material subroutine that is implemented through the commercial finite element (FE) software ABAQUS. Thus, in this model we eschew the notion of a fixed compressive strength of a lamina and instead use the mechanics of the failure process to provide the in situ compression strength of a material point in a lamina, the latter being dictated strongly by the current local stress state, the current state of the lamina transverse material properties, and the local fiber rotation. The inputs to this model are laboratory scale, coupon level test data (at the lamina level) that provide information on the lamina transverse property degradation (that is, appropriate, measured, strain-stress relations of the lamina transverse properties), the elastic lamina orthotropic properties and the geometry of the lamina. The validity of the approach advocated is demonstrated through numerical simulations of unidirectional lamina with initial fiber imperfections. The predictions of the simulations reported in this paper are compared against previously reported results from micromechanical analyses. Good agreement between the present macroscopic modeling approach and the previous micromechanical observations are reported.

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