SENSITIVITY OF MATRIX-DOMINATED FAILURE MODES TO CARBON/EPOXY CONSTITUENT PROPERTIES VIA COMPUTATIONAL MICROMECHANICS

A micromechanical approach can promote understanding of composite material failure mechanisms and increase our knowledge of the key parameters that control intralaminar damage. Such knowledge is vital in order to develop mesoscale (ply level) damage models of composite materials. Classical micromechanics can predict the elastic properties of a lamina, but capturing intralaminar failure modes needs finite-element based approaches that incorporate the modeling of damage for each of the constituents. The response of micromechanical models of unidirectional composite plies is affected by many factors; among them there are: the randomness of fibers distribution, the constitutive and failure models of each constituent and the mechanical characteristics of the matrix-fiber interface layer (interphase). Sensitivity studies to volume fraction, to fiber distribution and to the explicit simulation of the interphase do exist in recent literature. Yet, the influence on failure stresses and modes of constituent properties and damage parameters is still an open issue. The authors present a sensitivity study of micromechanically computed strengths to the most relevant constituent parameters which affect matrix dominated failure modes (transverse traction, transverse compression and shear). For this purpose finite-element (FE) Representative Volume Elements (RVEs) are generated in SIMULIA-ABAQUS by means of a dedicated Python script. The script applies multiple sets of boundary conditions in order to guarantee the periodicity of the microstructure and to reproduce the different load cases. Global RVE mechanical responses, i.e. averaged stresses vs. strains, are obtained by homogenization. The size of the RVEs is selected so that the homogenized stress fluctuations are vanishingly small for any random distribution of fibers generated using the probability density function that, according to recent studies, produces the smallest FE model for RVEs. The typical non-linear behavior of epoxies is reproduced by means of a pressure-dependent elasto-plastic constitutive relation, characterized by an exponential hardening followed by a linear softening law. The interphase is modeled using ABAQUS cohesive elements modified through an ad-hoc traction-separation constitutive model. Both matrix and interphase constitutive laws are implemented via user-defined material routines (UMAT). The systematic comparison of the mechanical responses, in terms of homogenized stress vs. strain curves, is used in order to discover correlations between the constituents properties and lamina failure stresses for modes of interest. The insight gained with the proposed approach will also constitute the basis for the development of specific tests aimed at determining the most relevant micromechanical parameters whose measure is a major challenge for current experimental procedures.