Thesis Defense: Zhiye Li, “Micromechanical Studies of Glass Fiber Reinforced Epoxy Matrix Composites Undergoing Deformation and Damage at High Strain Rates”

August 22, 2018





Doctoral Candidate

Zhiye Li

Friday, August 31, 2018


Latrobe 106

“Micromechanical Studies of Glass Fiber Reinforced Epoxy Matrix Composites Undergoing Deformation and Damage at High Strain Rates

This study develops an experimentally calibrated and validated 3D finite element model for simulating strain-rate dependent deformation and damage behavior in representative volume elements of S-glass fiber reinforced epoxy matrix composites. The fiber and matrix phases in the model are assumed to be elastic with their interfaces represented by potential-based and non-potential, rate-dependent cohesive zone models. Damage, leading to failure, in the fiber and matrix phases is modeled by a rate-dependent non-local scalar CDM model. The interface and damage models are calibrated using experimental results available in the literature, as well as from those conducted in this work. A limited number of tests are conducted with a cruciform specimen that is fabricated to characterize interfacial damage behavior. Validation studies are subsequently conducted by comparing results of FEM simulations with cruciform and from micro-droplet experiments. Sensitivity analysis are conducted to investigate the effect of mesh, material parameters and strain rate on the evolution of damage. Furthermore, their effects on partitions of the overall energy are also explored. Finally the paper examines the effect of microstructural morphology on the evolution of damage and its path.

Apart from exploring the damage mechanism, this study examines the effectiveness of periodic boundary conditions (PBCs), when applied to heterogeneous representative volume elements (RVEs) subjected to high strain-rate loading conditions. Even for a periodic multi-phase microstructure, the local stress and strain responses in the RVE under conditions of high strain-rate are not periodic. Stress waves propagate in the microstructure and interact with heterogeneities, resulting in reflection and transmission at the interfaces. To mitigate the limitations of PBCs, space and time dependent boundary conditions (STBCs), derived from analytical solutions to the 1D wave propagation problem, are proposed in this paper. This results in significant increase in the efficiency of the RVE analysis since it is not necessary to include larger RVEs. The paper introduces analytical solutions of the longitudinal and shear wave equations for elastic two-phase materials under time dependent boundary conditions. Subsequently a 3D composite RVE problem, is solved to investigate the efficacy of STBC. Results show that STBCs significantly improve the accuracy over PBCs for the same RVE. From this analysis, a strain-rate ≥ 10E5 per second is considered to be suitable transition point from periodic to space-time dependent boundary conditions for heterogeneous elastic composites.

In the end, this study proves that effective material properties are also influenced by stress wave propagation and strain rate. The relation of wavefront motion and macroscopic stiffness of RVE, and strain rate effect of the composites material properties are investigated. Various simulations at different strain rates are conducted to show that the homogenized models proposed in this study have considerable accuracy.

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