Investigation Of Stitched Carbon Fiber Reinforced Plastics Laminates Subjected To Side-impact And Compression After Impact

Carbon fiber reinforced plastics(CFRP)has gradually replaced the traditional metal materials and has been widely used in various aerospace structures because of its excellent properties such as lightweight and high-strength.However,the application of CFRP in aircraft structure is mostly the external shell,so it is very vulnerable to the impact of foreign objects in the process of aircraft operation or maintenance.These impacts usually come from different angles with different energies,which may cause serious damage to the composite aircraft structure and result in a significant reduction in its residual strength.Therefore,it is urgent to explore the damage laws of CFRP under different impact conditions,and try to improve its impact resistance and damage tolerance,so as to improve the flight safety of aircraft.At present,the research on the forward impact properties of composite laminates has been relatively mature at home and abroad,so this paper combined experiments and finite element simulation to focus on exploring the side-impact and residual compression performance of CFRP laminates.At the same time,the interlaminar stitching process was introduced to explore the effect of sutures on the side-impact properties of CFRP.The specific research work is as follows:Firstly,stitched and unstitched CFRP laminates with three different layups and two different stitching densities were prepared by the VARTM technique.The sideimpact experiments of 2,5,10,and 15 J energies were carried out in order to explore the effect of stitching on the side-impact performance of CFRP samples and compare the performance differences of stitched laminates with different suture densities,layups,and impact energies.Then the damage of CFRP laminates after side-impact was detected by three methods: visual observation,ultrasonic C-scanning and MicroComputed Tomography,and the influence of the above factors on structural impact damage was studied.After that,the compression experiments were conducted on the unimpacted and impacted laminates to compare their differences in compressive load,compression displacement,and compression failure modes of each group of samples.And the residual compressive strength after side-impact was calculated by the formula to study the damage tolerance of CFRP laminates.In addition,the side-impact model of stitched and unstitched CFRP laminates was established and the side-impact process of them was simulated by the finite element simulation software Abaqus.In this way,the damage evolution regulars of laminates which are not easy to observe in the experiment could be analyzed,and the reliability of the model has been verified by the experimental results.Based on the experiment and simulation results,it can be found that the introduction of stitching can effectively improve the side-impact resistance and damage tolerance of CFRP laminates.Compared with the unstitched laminates,the peak impact contact force of the laminates with a suture density of 15 mm×15 mm increased by 4.35%-12.43%,and the increase of residual compressive strength after side-impact was up to 13.38%.There was a semielliptical delamination area near the side-impact point on the CFRP panel.The main compression failure mode after side-impact of the unstitched laminates was delamination propagation while the stitched laminates was global buckling.The maximum impact contact force of CFRP laminates obtained by simulation and experiment were relatively close,and the maximum error of them was 22.40%.The intra-layer damage form of CFRP laminates after side impact is mainly fiber compression,which expands along the fiber direction in each layer.