Investigating failure in composite stiffener run-outs

Abstract

The aim of this thesis is to improve the understanding of failure initiation and propagation, in stractural composites featuring geometrical discontinuities. In particular, this study focuses on stiffener run-outs as these are particularly significant in the aerospace industry. The improved understanding achieved in this project results in a direct comparison of the performance of different stiffener run-out configurations, and contributes towards validating the applicability of failure models to representative structural components. In this work, the use of a compliant web design for improved damage tolerance in stiffener run-outs is investigated. Three different configurations were compared to establish the merits of a compliant design: a baseline configuration, a configuration with optimised tapering and a compliant configuration. The performance of these configurations, in terms of strength and damage tolerance, was compared numerically using a parametric finite element analysis. The energy release rates for debonding and delamination, for different crack lengths across the specimen width, were used for this comparison. The three configurations were subsequently manufactured and tested. The manufacturing process used in this study led to sound skin-stiffener run-outs whose design was validated against a numerical study. In order to monitor the failure process, Acoustic Emission (AE) equipment and Digital Image Correlation (DIC) were used. AE data recorded during skin-stiffener run-out compression tests proved useful to analyse the failure processes which take place in these specimens. The predicted failure loads, based on the energy release rates, showed good accuracy, particularly when the distribution of energy release rate across the width of the specimen was taken into account. It was shown that the compliant configuration failed by debonding and showed improved damage tolerance compared to the baseline and tapered stiffener run-outs. It can be concluded that the variation of the energy release rates across the width should be considered when the stiffener run-outs are designed. The results further show that, in the design of skin-stiffener run-outs, it is important to consider the possibility of failure modes other than debonding, and that compliant termination schemes offer the possibility of improved damage tolerance.