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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Geometric and Scale Effects on Energy absorption of Structural Composites

Author: ZHANG, Chi Chi

Year: 2011

Supervisor(s): Paul Hogg

The challenge faced by structural designers is becoming increasingly difficult as the imposed design criteria of energy absorbing structures requires weight reduction of structures without compromising cost and crushing performance. The current research is thus aimed at investigating the energy absorption of fibre reinforced composites measured as a function of geometry and scale within weight-critical structures. At the first stage, an innovative structure composed of four intersecting composite plates was tested. It was found that the structural stability played a crucial role in this intersecting structure. In order to avoid generating buckling failure before turning to a progressive crushing regime, Finite Element Method (FEM) was used on composite structures as a technical tool. At the second stage, three geometric structures containing corrugated composite laminates and possessing better structural stability were designed and examined. To increase the interlaminar fracture toughness properties of composite materials, through-thickness stitching methods were introduced. Fracture toughness (Mode-I and Mode-II) and flexure tests were performed on composite materials for comparing the effectiveness of different crushing mechanisms. Fracture toughness results presented a significant improvement of using stitching methods on Mode-I properties, while slight reduction on Mode-II properties was also detected. They also indicated the flexural properties of structural composites can significantly affect their energy absorption capabilities. At the final stage, six different factors including resin type, fibre architecture, crushing speed and stitching parameters were scaled in several levels in a modified geometric structure. An optimization approach based on Taguchi methods was utilised in order to statistically determine the relationship and assist in evaluating the contribution of each factor on crushing properties. It showed that by selecting the combinations of these factors with correct levels, the energy absorbed can be improved remarkably. It found that the crushing performance of this structural composite was mainly dominated by resin and fibre architecture, which contributed 71% capability of energy absorption. The other 29% capability was dominated by trigger, beam web length, edge stitching density and the crushing speed.