Author

Vahid Tavaf

Date of Award

Fall 2019

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Sourav Banerjee

Abstract

The objectives of this research are to develop a comprehensive method to quantify the material degradation of composite materials in the presence of distributed damages and predict the failure in the defected composites using the updated effective material properties. Defects can grow in composites at the macro scale caused by the microscale voids, matrix cracks, fiber breakages starting from manufacturing processes to the high and low cycle fatigue loads, high temperature, and high humidity during operation. Hence, to predict a more realistic failure model, it is necessary to consider the repercussions of degraded materials. The proposed research work is divided into two major parts

Effect of distributed defects on effective material properties of the composites is required for the progressive failure models. Although the degradation of the effective material properties due to the presence of the lower scale damages is well investigated, how each material coefficients should be degraded in a progressive failure model is still a dilemma. The percentage of defects, the shape of the defects, and their stochastic distribution may affect the individual material coefficients in a unique way and may not be uniform across the constitutive matrix. Therefore, to find how the individual material coefficient in a constitutive matrix changes due to the presence of the voids and fiber breakage, all material coefficients in a constitutive matrix were studied. The representative volume element of a fiber-matrix composite was studied with appropriate boundary conditions and respective material coefficients were calculated. It was found that the local gradients of the degradation curve obtained for each material coefficient are not linear with the increasing percentage of degradation and not uniform for all material coefficients. The different shapes and locations of the defects with constant percentage of defect were found to have an effect on the material coefficients.

The objective of this study was to investigate the effect of multiple hole orientations on the strength of composite materials and the interaction of crack paths as well. Generally, holes may be created in the composite structures due to the assembling or the joining of parts. A high stress gradient may occur at the vicinity of their edge as a result of holes. The stress gradient around a hole is not only dependent on material constants, but also fiber direction and hole orientations. This complex gradient stress may affect strength of structures and damage propagations. Hence, to fulfill the requirements, peridynamic method was employed to predict strain energy, crack paths, and tensile strength of the composite materials in the presence of multiple holes orientations with different fiber directions.

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