Computational micromechanical stress-strain and damage analysis of UD CFRP composite ply under Uni-axial and bi-axial quasi-static loads

Abstract

Keywords: Polymer Matrix Composites (PMCs); Computational micromechanics; Finite Element Methods (FEM); fiber-matrix interface; Cohesive Zone Modelling (CZM); Drucker-Prager plasticity model; ductile damage criterion; stress tri-axiality; micro-voids. Attributed to the excellent specific strength and stiffness combined with the expanded design space, fiber-reinforced Polymer Matrix Composites (PMCs) are extensively utilized in various advanced structural applications. Among the available various fiber architectures such as unidirectional (UD), woven, braided, etc., UD laminated composites are the primary candidate materials for manufacturing the load-bearing composite structures. Besides, depending on the prevailing complicated load paths, in general, composite structures are manufactured using UD laminates consisting of multidirectional plies. Even though UD composite laminates exhibit superior mechanical properties in the fiber direction, the transverse and the through-thickness (matrix-dominated) directions are the weak links in these structural materials. In particular, early micro-cracks initiated in the transverse or off-axis plies cause inter-laminar delamination that eventually leads to structural failure. Based on the damage-resistant design philosophy, to prevent microcracking to occur, large factors of safety are used for structural design purposes. Knowing that the microcracking in transverse plies and the consequent intra-ply fracture is the root cause for structural failure, the current thesis is aimed to investigate the effect of various micro-scale parameters on the macroscopic stress-strain and damage behavior of a UD composite ply, especially, in the matrix-dominated directions. For the aforesaid purpose, detailed computational micromechanics studies are conducted. In order to accomplish the computational micromechanical analysis, high fidelity three-dimensional Representative Volume Element (RVE) models are created based on the fiber distribution observed in an actual microscopic image of a UD composite ply. Apart from having the random fiber distribution, the generated RVE model has distinctly modeled fibers, fiber-matrix interface, and matrix material. Assuming that the generated RVE geometrical model is taken from an infinitely periodic microstructure, to obtain the periodic stress-strain and damage profiles, the Periodic Boundary Conditions (PBC) are applied to the RVE Finite Element (FE) model. Further, to capture the individual micro constituent's damage initiation, propagation, and interaction, the Cohesive Zone Modelling (CZM) methodology is used for fiber-matrix interfaces. To capture the plastic deformation followed by fracture, the matrix material stress-strain, and failure behavior is modeled using the linear Drucker-Prager plasticity model combined with a ductile damage criterion. For the micro-scale FE analysis and subsequent experimental validation purposes, a thermo-set carbon-epoxy (IM7/8552) UD composite is taken as a reference material system. Based on the conducted detailed computational micromechanical studies, the major contribution from the current thesis work to the literature are as follows: i) thoroughly validated computational micromechanical methodology under various load cases; ii) considering the limitations in the available experimental techniques for estimating the fiber-matrix interface properties, the current research work proposes a coupled experimental - numerical strategy to predict the Mode I interface properties; iii) under various uni-axial and biaxial load conditions, micro scale parameters that cause the damage initiation and propagation is identified; iv) detailed stress-strain and damage analysis lead to a conclusion that contrast to the state-of-the-art computational micromechanics methodology, in-situ matrix stress strain behavior should be used for predicting the transverse compression and shear stress-strain behavior of a UD composite ply; v) further, the current research work highlighted the necessity for using the stochastic fiber-matrix interface properties for predicting the upper and lower bounds of the experimentally observed macroscopic stress-strain variation, and vi) finally, a small percent of micro-voids (< 1.5% void volume fraction) shows a detrimental effect on the macroscopic strength of the UD composite ply. To extend the presented quasi-static computational micromechanical methodology for predicting the fatigue life of a UD composite ply under fiber perpendicular loads, appropriate material models for capturing the fatigue damage evolution of the fiber-matrix interface, and the matrix material are implemented into the commercial FE software Abaqus using user-defined material models. The implemented interface fatigue damage model is validated by comparing the predicted crack growth rate vs. normalized energy release rate curve to the experimental curves under individual and mixed-mode load cases. Similarly, the predicted Stress vs. Number of cycles (S-N) curve for the epoxy matrix is compared to the experimental S-N curve.