Composites Today

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Simulation method that can accurately predict damage progression in carbon fibre composites 

Researchers at Sophia University, Japan have developed a new numerical simulation method using the quasi-three-dimensional extended finite element method (XFEM) that can accurately predict the process of damage progression and ultimate strength of carbon fibre reinforced plastic (CFRP). When the analysis using this method was performed, results equivalent to those of actual strength tests were obtained, suggesting the high validity of this method. Further development of this research, which enables the acquisition of highly accurate material strength data through numerical simulation, is expected to lead to shortening of the design process and reduction of development costs.

Lightweight and tough CFRP laminated materials are widely used in various applications such as precision equipment, industrial machinery, medical supplies, and sporting goods. Because it is possible to reduce weight while maintaining strength, if the application of CFRP laminated materials expands, especially in automobiles, ships, and aircraft, which are the cornerstones of logistics, it is expected that greater fuel efficiency will be possible than before. In order to expand the use and versatility of CFRP, it is necessary to meticulously carry out the prototyping process to collect detailed strength test data of CFRP members and the design process to determine the actual dimensions and layout. Therefore, one of the issues was that these processes took a lot of time and cost. In order to solve this problem, methods have been explored to reproduce the prototyping and design processes through numerical simulation.

The finite element method (FEM) has been widely applied as a conventional damage propagation analysis method for CFRP laminated structures. In this method, it is necessary to introduce a damage model that can express the typical damage modes of CFRP, such as delamination, matrix cracking (cracking of the base material that holds the fibres together), and fibre breakage. However, with these methods, it was necessary to divide the elements along cracks such as matrix cracks, making model creation difficult.

In this study, researchers developed a quasi-3D XFEM with 8-node hexahedral elements and applied it to damage propagation analysis of CFRP laminates. They succeeded in improving the efficiency and accuracy of the simulation by modelling multiple matrix cracks independently for each element. The researchers also found that the convergence and accuracy of the simulation can be improved by introducing an appropriate damage model such as the zig-zag cohesion model (ZCZM) or the zig-zag enhanced cohesion model (ZECZM). Furthermore, they compared the strength test data obtained from experiments in the Open Hole Tension test (OHT test), the Quasi-static Indentation test (QSI test), and the Compression-After-Impact (CAI) test, and demonstrated that data closer to the experimental values ​​could be obtained than the conventional method.

Therefore, in order to appropriately model the damage of CFRP laminates, this research group developed a method for damage propagation analysis using quasi-three-dimensional XFEM, in which delamination is an interface element used in normal FEM, and matrix cracking is modelled as an approximation function of a discontinuous displacement field by XFEM. Then, we evaluated the validity of this analysis method by comparing it with strength data obtained from actual OHT, QSI, and CAI tests.

The results of this research were published online in the international academic journal “Composite Structures” on April 13, 2023.

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