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

Delamination growth due to face sheet/core debonding is a major concern due to its inherent weakness in sandwich composites which can be exacerbated due to the presence of flaws. In this research work The primary objective of this research was to characterize the delamination crack growth behavior in E-glass face sheet/polyurethane foam core sandwich composite with pre-existing initial delamination crack at a face sheet/core interface under static and fatigue for mode I and mixed mode loading. For mode I static loading two types of delamination experiments, namely T-peel test and wedge test were implemented to evaluate fracture toughness in polyurethane foam core sandwich composites. It is shown that both tests can provide reliable values for mode I fracture toughness. Two analytical solutions for models (double cantilever beam (DCB) specimen and tilted debond specimen (TDS) test) based on elastic foundation analysis are modified and validated for mode I loading. For mixed mode static loading, mixed mode bending (MMB) specimen was employed to evaluate energy release rate, compliance and global mode ratio for composite under investigation. It was observed that energy release rate, compliance and global mode ratio increases with increase in crack length. For fatigue life is investigated for constant displacement amplitude for mode I and mixed mode loading using single cantilever beam (SCB) specimen and mixed mode bending (MMB) specimen, respectively. The fatigue test results are in terms of crack growth rate versus energy release rate and Paris law constants are estimated to predict failure of polyurethane foam core sandwich composite. Decreasing crack growth rate as a function of fatigue cycles was attributed to stress relaxation at delamination crack tip. The effect of lever arm distance (c) for mixed mode loading is investigated. It is shown that both compliance and energy release increases with an increase in the lever arm distance (c) and global mode ratio decreases and mode I become more dominant. The polyurethane foam core sandwich composites results are new. The energy release rate and Paris law constants for a growing crack are low for polyurethane foam core sandwich composites as compare to PVC core sandwich composites. Finally, finite element analyses are conducted to validate the experimental results. The results obtained from experiments and finite element analysis showed good agreement.

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