An Orthotropic Augmented Finite Element Method (A-FEM) for High-Fidelity Progressive Damage Analyses of Laminated Composites

This research focuses on extending a recently developed augmented finite element method (A-FEM) to account for the complicated progressive damage processes in laminated composites, which are of orthotropic nature and typically develop multiple types of cracking systems including intra-ply matrix/fiber splitting, fiber rupture in tension and/or kinking in compression, and inter-ply delamination. The orthotropic A-FEM represents all these major damage modes with improved nonlinear cohesive zone models (CZMs), while explicitly considering asymmetric tension- and compression-responses. A rigorous verification and validation process demonstrates that the developed orthotropic A-FEM can adequately account for the initiation and propagation of various types cracks and their coupled evolution under complex stress environments, without need for additional degrees of freedom. A-FEM predictions of progressive damage processes in several multidirectional notched and un-notched laminates, including the initiation of multiple cracks and their nonlinearly coupled progression with delaminations all the way up to the final, catastrophic failure, are in excellent agreement with experimental measurements and observations. This research also focused on resolving an essential ambiguity regarding the nonlinear shear stress-strain relation and its critical effects on the progressive failure of composite laminates. A careful comparison between the simulated results of pure elastic shear and nonlinear shear, coupled with experimentally observed surface cracking development, it shows that the sub-micron crazing is predominantly responsible for the composite shear nonlinearity, because, it is not possible to recover the shear nonlinearity no matter how many microcracks generate. Further, the combined use of (experimentally measured) mode II toughness, peak shear strength, and the entire shear nonlinear curve can reproduce not only the shear nonlinearity but also the limited multiple cracking in the specimen as observed in experiments through DIC measurements.