Date of Award

2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Shivakumar, Kunigal

Abstract

Edge delamination in composite laminates with adjacent layers oriented at different fiber angles is a major failure mode because of the existence of high interlaminar stresses and poor interlaminar properties. Mitigation of edge stresses poses a challenge even to date. This research provides a detailed analysis and a potential approach to solve this problem in a carbon/epoxy composite laminate. Two extreme laminates of stacking sequence (0n/90n)s and (+45n/-45n)s subjected to separately applied tensile and thermal loading were considered. These problems have been treated in the literature as a mathematical or bare interface model, wherein the material properties jumped between the adjacent layers of different fiber orientations. A microscopic analysis of laminate cross section showed that the interface was not really bare but there was a thin resin layer of thickness of about 5.0% of the ply thickness. This realization completely changed the modeling and potential modification of the interphase. The region between the plies was represented by a resin layer interphase. A three-dimensional composite finite element (FE) analysis was performed using ANSYS version 12 code. The FE modeling and analysis were verified with the literature for both (0/90)s and (+45/- 45)s laminates for axial tensile loading as well as temperature change. The resin interphase layer with thicknesses of 2.5%, 5.0% and 7.5% of the ply thickness were modeled using three different material properties representing: elastic (brittle epoxy), elastic-plastic (toughened epoxy) and non-linear (interleaved polymer nanofiber composite). As the layer thickness became zero, the bare interface results were recovered. Then, for non-linear resin layer the edge stresses reduced indicating that the interleaving of interphase region had a potential to mitigate edge stresses and thus the edge delamination failure.

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