Podbury, Matthew John; (2002) A finite element study of the stress distribution around mechanical fasteners in composite laminates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A major concern when designing structures with polymer based composite laminates is the stress concentration developed around mechanically fastened joints. As composite laminates behave anisotropically, the design procedures available for metallic joints are not applicable, and hence a comprehensive guide for designing with these materials must be developed. Experimental evaluation of joint strength is the most accurate, however, to provide a range of data for every joint configuration is both time consuming and very expensive. With the advent of powerful computers at relatively low cost and more sophisticated software tools, numerical methods have become more desirable in predicting the stress distribution and with appropriate failure criteria can provide accurate strength prediction. In this study, commercially available finite element software was used to perform a three-dimensional stress analysis on mechanically fastened composite laminate double-lap joints. To enable accurate ply and inter-laminar stress prediction, a replica technique was adopted, whereby the material properties in each element were oriented according to the stacking sequence used. The model was developed so that the bolt assembly could be simulated accurately, by creating a mesh for each individual component, allowing contact to be modelled at every interface. A bolt preload was provided by applying an appropriate temperature drop to a beam element within the bolt shank. The initial study concentrated on a single bolt composite laminate double lap joint, whereby the effects of clamping preload, bolt/hole clearance, bolt elasticity, laminate elasticity and stacking sequence, on the stress distribution in the vicinity of the fastener were analysed. The investigation was then continued by varying the outer diameter and stiffness of the washer and subsequently using these results to develop a multi-fastener model. The results showed good agreement with previously published work and provide engineers with valuable guidance when designing mechanically fastened double lap joints of this type.

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