섬유강화 열가소성 복합재료의 변형률 속도의 영향을 고려한 3차원 점진파괴 모델 개발에 관한 연구

Abstract

The glass fiber reinforced thermoplastic (GFRP) has been increasingly employed for impact-induced automotive parts because it not only has the excellent impact performance, but also a low material cost and recyclability compared with thermoset materials. Furthermore, lower production costs and higher production rates can now be guaranteed for large volume markets, such as the transportation and automotive markets. In order to develop lightweight GFRP automotive components with enhanced reliability, it is important to predict the damage behaviors under the impact load, because an impact can result in a significant amount of damage in composite structures with various failure modes. For these reasons, the effects of strain rates on the mechanical behavior of GFRP must be investigated in depth due to the fact that the significant strain rates from ten to several hundred per second can be induced in the automobile crashworthy event. In the field of fiber reinforced composites, many new studies have been reported to understand in-depth the strain rate dependent failure behavior. Nevertheless, most of the work published regarding dynamic mechanical experiments and application of the strain rate dependent failure model to impact modeling has focused on fiber reinforced thermoset based composites. Because the thermoplastic composites experience different damage behavior compared to thermoset composites due to the contrasting mechanical behavior of the matrix, there is a need of understanding in the strain rate effects on the mechanical behavior of GFRP. This can be the basis of constitutive equation model for the GFRP which enable the true characterization of their strain rate dependent mechanical properties and simulation of their damage behavior under the impact loading. On the other words, the reliable material model for GFRP is required and it needs to be based on an experimental investigation and have the ability to consider the effect of strain rate on the mechanical behaviors. Additionally, because the composite structures typically have a thickness on the order of few millimeters, the majority of studies mostly report on modeling the progressive failure in thin shell composites. However, this planar assumption is obviously unacceptable in most impact modeling because it can involve the boundary conditions with a nontrivial out-of-plane stress response. Also, because the matrix delamination failure mode is severely governed by the out-of-plane stress between adjacent layers, incorporation of all stress tensor components is essential for the accurate failure analysis of composite structures in the impact problem. In this study, the strain rate dependent three-dimensional (3D) progressive failure model was developed to accurately describe the nonlinear failure properties of GFRP under the impact load. Firstly, the effects of strain rate on the mechanical behavior of GFRP was investigated experimentally by using the split Hopkinson bar equipment. The dynamic tensile, compressive and bias-extension shear tests were performed with respect to the strain rate and the failure modes in fracture surfaces were analyzed by using the scanning electron microscopy. Then, the strain rate dependent 3D progressive model was developed based on the experimental study by using the different damage evolution laws and property degradation models for each available failure mode. The developed progressive failure model was implemented into the commercial finite element analysis software LS-Dyna. In conclusion, it was found that the proposed failure model can be used to predict not only damage progression and impact behavior, but also interlaminar delamination. It is expected that the newly devised material modeling technique would be widely used to design the various fiber reinforced thermoplastic structures.