Addis Tessema

Date of Award

Spring 2019

Document Type

Open Access Dissertation


Mechanical Engineering

First Advisor

Addis Kidane


Understanding the mechanics of Fiber Reinforced Composites (FRC) that guide the design and optimization of laminate structures, have attracted numerous researchers. For the last five decades, various analytical and numerical models have been developed to understand the damage and failure mechanisms in FRC. For long, Classical laminate plate theory (CLPT) has majorly been used as a theoretical guide in the design of composite structures. The CLPT framework is developed based on the consideration of an infinitesimal wide plate that depicts a planar stress condition. Thus, as it considers only the planar stresses, the out of plane interlaminar stresses are absent from the CLPT formulation. However, for a laminate with finite width or geometrical discontinuity a complex stress condition (which includes interlaminar stresses) is expected on the free surface of the discontinuities. In such, researcher has developed models that considered a three-dimensional stress condition, and these models have revealed the great relevance of understanding the stress condition at the free-edge on the design and failure prediction of composite structures. Nevertheless, there is still a need for further experimental investigations on the free-edge stress/strains localization and their association with the damage initiated at the free-edge.

In this study a noble experimental technique is developed and applied to study the strain localization at free-edge of composite laminate under a monotonic tensile load. The technique is used to evaluate the local strain variation across the layers of the laminate free-edge along with the applied uni-axial tensile load. The developed experimental technique has incorporated high magnification optical system for digital image correlation (DIC) in which the full field displacement and strain are measured in-situ at micro-scale. Further, based on the DIC results, the initiation and growth of cracks from the free-edge can be captured, thus, the influence of the strain localization on the formation of damage can be investigated. In overall, this dissertation is grouped in to four categories to address the various issues raised regarding free-edge effect.

In the first section, a single a quasi-isotropic (±45/ 90/ 0)s laminate is considered and the strain localization and progressive damage formation at the free-edge of the laminate is investigate. In the second part, four different laminates with varying laminas stacking arrangement are fabricated, and the influence of the relative location of laminas on the interlaminar strain localization and damage formation is studied. In the third part, by taking one stacking arrangement with different fiber angle of the off-axis plies, the effect of ply’s orientation on the local intra-lamina and interlaminar strain localization and damage formation is studied. In the last section, investigation is conducted to understand the explicit correlation between local damage, material residual stiffness degradation and loss in thermal conductivity of carbon fiber composite.