7 - Progressive damage and failure

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This chapter introduces micromechanics-based progressive damage simulations, wherein local failures occur and accumulate as damage progresses through a composite material, resulting in a nonlinear stress-strain response. In contrast, Chapter 4 considered only damage initiation, without any damage propagation. A subvolume elimination method is employed, where subcell or constituent material stiffnesses are instantaneously reduced to a very low value once a specified failure criterion is reached within the subvolume. For laminates, this is done at the microscale within each ply. The pathological mesh dependence introduced by this method (when used within HFGMC or finite-element analysis), is illustrated and discussed. Although significant portions of the MATLAB code from previous chapters are preserved, due to the added complexities associated with progressive damage, new driver scripts that include the ability to apply loading incrementally and perform iterations within a given increment are required. Because of the nonlinearity present in progressive damage, which is controlled by the local fields and their redistribution as damage progresses, the impact of the chosen micromechanics theory, local failure criterion, and microstructural representation is significantly amplified compared to linear elastic analysis. Consequently, these influences are studied in detail for PMC and CMC composite materials and laminates through example problems. It is demonstrated that HFGMC, with its normal-shear coupling and accurate local fields, enables capture of the details of damage progression, which are particularly influential for disordered (random) microstructures. Of course, since detailed progressive damage simulations are computationally expensive, maximizing computational efficiency is highly desirable. If only damage initiation is of interest, then the margin of safety predictions from Chapter 4 can be employed without the need for more computationally expensive progressive damage analysis. Furthermore, in fiber-dominated situations in PMCs, the present chapter demonstrates that simpler, more efficient, micromechanics theories like MOC and MT are sufficient to predict the composite ultimate strength. This is because the effect of local fields is suppressed in fiber-dominated situations. Finally, for CMCs, it is shown that the progressive damage behavior is highly influenced by damage initiation location (i.e., interface vs. matrix), which is controlled by their relative strengths.