Access Type

Open Access Dissertation

Date of Award

January 2015

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Golam M. Newaz

Abstract

The aim of this research work was to investigate the effect of impact damage on in-plane buckling and compression-compression fatigue behavior for a new sandwich structure made from E-glass/epoxy face sheets over end-grain balsa wood core. Low velocity impact tests were carried out using a drop-weight impact tower by impacting the sandwich beam at the center with energy level slightly higher than threshold energy level of 8.8 J. Edge-wise compression static tests were conducted for impacted and non-impacted samples to address energy absorption characteristics of these composites. Analytical and experimental investigations were carried out to measure critical buckling loads and study the response and failure modes of debonded composite sandwich beams under compressive loads. These composite sandwich beams with local delamination caused by low velocity impact were utilized to evaluate the compression fatigue performance. Compression-Compression fatigue tests were conducted for specimens with and without impact damage. Compressive residual strengths were obtained and the growth of delamination was monitored during fatigue tests. Although fatigue performance was adversely affected due to the presence of impact induced damage, it was observed that delamination growth does not occur in fatigue for in-plane stress levels below 40% of compression-after-impact (CAI) values for this class of sandwich composites. Results showed that there was significant degradation of fatigue life due to impact damage in relation to undamaged composite. Also, it was observed that any appreciable stiffness loss in fatigue does not occur below 50% CAI value. The combined damage consisting of delamination, core shear and skin failure was found to be the dominant failure mode under compression fatigue. The finite element analysis (ABAQUS) was utilized to predict the interfacial stress and stress distribution along thickness of the undamaged composite beam. The normal compressive stress distribution along thickness was plotted. The results showed very good agreement for facing and core stress values obtained by the analytical and numerical solutions. The predicted interfacial stress value was found to be between the facing and core stresses. These micromechanics results provide a clear understanding of the local behavior and how they influence the overall composite behavior. A unique contribution of the thesis work is compressive fatigue response characteristics of glass fiber sandwich composites subjected to lateral impact. These results are likely to be integrated into design of lightweight decks in automotive and truck applications.

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