Date of Award

Summer 1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical & Aerospace Engineering

Program/Concentration

Engineering Mechanics

Committee Director

Ram Prabhakaren

Committee Director

John H. Crews

Committee Member

Chuh Mei

Committee Member

J. Mark Dorrepaal

Committee Member

Stephen Cupschalk

Abstract

A combined analytical and experimental study is conducted to determine the effects of clearance and bearing-bypass loading for mechanically fastened joints in composites. A simple method of analysis is developed to account for the nonlinear effects of bolt-hole clearance. The nonlinear load-contact variations for clearance-fit fasteners are also measured using specially instrumented fasteners. For a quasi-isotropic graphite/epoxy laminate, results show that the contact arc as well as the peak stresses around the hole and their locations are strongly influenced by the clearance. After a slight initial nonlinearity, the peak stresses vary linearly with applied load. The typical clearance levels are shown to have only a minor influence on the overall joint stiffness.

Quasi-isotropic graphite/epoxy laminates (T300/5208) are tested under combined bearing and bypass loading to study failure modes and strengths. Radiographs are made after damage onset and after ultimate load to examine the failure modes. Also the laminate stresses near the bolt-hole are calculated for each test condition, and then used with appropriate failure criteria to analyze the test data. The tension data show a linear interaction for combined bearing and bypass loading with damage developing in the net-tension mode and growing to failure in the same mode. Failure modes are more complex in compression. The compression bearing-bypass strengths for damage-onset show an unexpected interaction involving the bearing mode. Compressive bypass loads reduce the bearing strength by decreasing the bolt-hole contact arc and thus increasing the severity of the bearing loads. Bearing damage-onset, for compressive bearing-bypass loads, causes a weakening of the offset-section leading to ultimate failure in the offset compreson-bearing mode. Damage-onset is predicted reasonably well using the peak stresses at the hole. Strength predictions indicate that damage corresponding to ultimate strength is governed by the maximum stress near the hole.

This investigation will help improve the basic understanding of composite bolted joints and lead to better structural design procedures.

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DOI

10.25777/vcsf-3m71

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