Phase field modeling of the defect evolution and failure
The plastic recovery processes in ultrafine and nano grained metals and the yield criteria and failure mechanisms in polymer matrix composite are the two major topics in this work. In the first part of the work, a phase field dislocation dynamics (PFDD) approach is introduced, which tracks the evolution of the dislocations in ultrafine and nano grained metals and takes into account the elastic interaction between dislocations, obstacles and the applied resolved shear stress on a single slip plane. Two phenomena, the reverse plastic strain during cyclic loading and plastic strain recovery upon unloading, are studied. One major finding of our simulations is that these two plastic recovery processes are related to the formation of dislocation structures during loading, and additional grain size inhomogeneity will increase the amount of plastic strain recovered. In the second part of the work, a phase field damage model (PFDM) is presented to study the onset of yielding and crack propagation in polymer matrix composite. The effect of two damage parameters, the fracture toughness Gc and crack length scale parameter l0, are first investigated. The former is shown to determine the energy needed during crack propagation and the latter is observed to control the crack nucleation process. Moreover, two asymmetric damage models are compared regarding their yield surfaces and it is found that the model of Miehe et al. leads to a linear pressure modified von Mises relation. Next, the PFDM reveals that the yield criterion in amorphous polymers should be described in terms of local stress and strains fields and cannot be extended directly from applied stress field values. Furthermore, it is demonstrated that the same damage model can be used to study the failure under shear yielding and crazing conditions. And if local defects in the samples such as voids are included explicitly in the simulations, the PFDM is able to explain the breakdown of the pressure modified von Mises relation during crazing, which is due to the local stress concentration resulted from defects. Lastly, the PFDM captures several fracture mechanisms during the Mode I interlaminar fracture in a particle toughened interlayer, which qualitatively agree with experimental observations. It sheds light upon the underlying reasons for different crack patterns, including the influence of the particle stiffness, particle-matrix interfacial toughness, lamina-interlayer interfacial toughness and spatial particle distribution. The effective toughness of the interlaminar region under the condition of perfect bonding between the lamina and interlayer is also studied, which predicts the effect of particle properties and interfacial toughness on the toughness behavior of the interlayer in agreement with experimental results.
Koslowski, Purdue University.
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