Date of Award

12-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Aeronautics and Astronautics

First Advisor

Anthony N. Palazotto, PhD

Abstract

Composite materials are strong, lightweight, and stiff making them desirable in aerospace applications. However, a practical issue arises with composites in that they behave unpredictably in bolted joints, where damage and cracks are often initiated. This research investigated a solution to correcting the problem with composite bolted joints. A novel hybrid composite material was developed, where thin stainless steel foils were placed between and in place of preimpregnated composite plies during the cure cycle to reinforce stress concentrations in bolted joints. This novel composite was compared to control samples experimentally in quasi-static monotonic loading in double shear configuration in 9-ply and 18-ply layups. It was also investigated in quasi-static loading in single shear configuration using 18-ply samples in both protruding head and countersunk head configurations. Progressive failure samples were compared to data to explain which phenomenon in the material caused certain features in experimental curves. The final goal of the experimental effort was to perform an initial cycle fatigue comparison between the novel hybrid and control materials. The parallel research effort explored creating finite element models that could correctly represent and predict the behavior of this hybrid system. This was the first effort employing numerical failure criterion alongside a rigorous experimentation across multiple configurations. Hybridizing the composite material increased yield by as much as 25% and increased ultimate load capacity as much as 42%. The finite element models employed Hashin failure criteria and proved the ability to predict the yield load capacity to within 6.5% error.

AFIT Designator

AFIT-ENY-DS-20-D-057

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