Chowdhury, Khairul Alam (2007-09). Damage initiation, progression and failure of polymer matrix composites due to manufacturing induced defects. Doctoral Dissertation.
Thesis
In polymer matrix composites (PMCs) manufacturing processes can induce de- fects, e.g., voids, fiber misalignment, irregular fiber distribution in the cross-section and broken fibers. The effects of such defects can be beneficial or deleterious de- pending on whether they cause failure suppression or enhancement by localized de- formation processes e.g., crazing, shear yielding and fiber-matrix debonding. In this study, a computational approach is formulated and implemented to develop solu- tions for general boundary-value problems for PMC microstructures that accounts for micromechanics-based constitutive relations including fine scale mechanisms of material failure. The defects considered are voids, and the microstructure is explic- itly represented by a distribution of fibers and voids embedded in a polymer matrix. Fiber is modeled as a linearly elastic material while the polymer matrix is mod- eled as an elastic-viscoplastic material. Two distinct models for the matrix behavior are implemented: (i) DruckerAcA?A?Prager type Bodner model that accounts for rate and pressure-sensitivity, and (ii) improved macromolecular constitutive model that also accounts for temperature dependence, small-strain softening and large-strain harden- ing. Damage is simulated by the Gearing-Anand craze model as a reference model and by a new micromechanical craze model, developed to account for craze initiation, growth and breakdown. Critical dilatational energy density criterion is utilized to predict fiber-matrix debonding through cavitation induced matrix cracking. An extensive parametric study is conducted in which the roles of void shape, size and distribution relative to fiber in determining damage initiation and evolution are investigated under imposed temperature and strain rate conditions. Results show there are significant effects of voids on microstructural damage as well as on the overall deformational and failure response of composites.
