Numerical and experimental study on composite structures under a crushing load
thesisposted on 01.03.2017, 02:06 by Chiu, Ngai Sum Louis
Crashworthiness of aerostructures is one of the most important factors in their design. Energy absorbing structures within the airframe can contribute substantially to crash protection. The move towards fibre reinforced composite materials, due to their improved performance, presents both opportunities and challenges to the design of energy absorbing structures. The multitude of damage mechanisms of composite structures increase energy absorption capability while making analysis significantly more difficult than those of traditional metallic structures. Currently, the design process of composite structures is heavily dependent on expensive and time consuming physical testing, which can be reduced by the use of validated virtual testing capability. Unfortunately, currently available numerical models have not been able to meet this demand. The aim of this study is to develop and validate a comprehensive, predictive material damage model for the analysis of composite laminate structures under a crushing load, which can be used for virtual testing within the design process for energy absorbing composite structures. A robust damage model, to simulate the response of composite structures subjected to a crushing load, was developed to overcome limitations in commercially available composite damage models. This work makes a significant contribution to the analysis of damage in composite structures by the improved modelling of: • The nonlinear material response, • The unloading and load reversal behaviour, • Physically based damage mechanisms, • Damage mode interactions, and • The characteristic length. The superior predictive capability of this material damage model has been demonstrated in this work. This model only requires physically measurable parameters as input and artificial manipulation of these parameters is not necessary. The model was implemented into Abaqus/Explicit as a VUMAT subroutine and validated against theoretical expectations. An experimental program was conducted to probe rate dependent behaviour and to provide data for subsequent validation work. A set of tulip-triggered cylindrical tube specimens were tested at a range of nominal strain rate between 2×10^(-4) to 100 s^(-1). Overall, the material response was found to be rate independent. Consistent force responses and damage morphologies were observed for all the strain rates investigated. The standard deviation of the results for each strain rate was less than 8%, demonstrating that good reliability was achieved in the experiments. The effectiveness of the material damage model was assessed using experimental data obtained from experimental testing as well as the literature. Results showed that the model can accurately predict the force response of a range of different crushed composite structures. The simulated energy absorption of each test case was within 4% to 12% of the experimental value. The experimentally observed post-crush deformation and the damage morphologies were also accurately predicted by the model. The results have demonstrated the effectiveness of this new model in predicting the crush response of composite structures. This model can be applied in industry to reduce costly and time consuming physical testing during the design process of components, thus reducing cycle time and/or increasing performance of the end product.