Study On Quasi-static Indentation And Residual Compressive Properties Of Delaminated Carbon Fiber/bismaleimide Composite Laminates

Carbon fiber reinforced bismaleimide composite laminates have been widely used in aircraft structures, like wing skin panels, hatch plates, and so on, due to their outstanding mechanical and temperature resistance properties. However, matrix cracks, interlaminar delamination and fiber cracks can be easily induced in composite laminates when subjected to impact loading, which leads to the large degradation of load carrying capacity of composite laminates. Therefore, in order to avoid catastrophic accidents, it is essentially important to carry out the damage assessment on aircraft composite structures. However, damage assessment aimed at aircraft composites is badly insufficient currently. Carbon fiber/bismaleimide composite laminates are riveted to the metal frame of aircraft, hence the boundary condition of composite laminates is four edges clamped. Based on the real application condition, in this study, carbon fiber/bismaleimide composite laminates were manufactured to understand the damage caused by quasi-indentation under four edges clamped and their residual compressive properties, which was a part of work for damage assessment on low-velocity impact. Meanwhile, delaminated composite laminates were also manufactured by pre-inserting PTFE films and the effects of delamination on quasi-static indentation damage and residual compressive properties of composite laminates were studied. The main conclusions of this study are listed below:Firstly, eigen contact forces and dent depths obtained from single loading and multiple loading respectively were compared. The results showed that it was feasible to study the quasi-static indentation damage by multiple loading. Meanwhile, quasi-static indentation under different boundary conditions, simply supported and four edges clamped, were conducted. The results illustrated that there were many differences between quasi-indentation damage under different boundary conditions, including propagation mode of delamination, eigen contact force, dent depth-indentation force curve and damage width-indentation force curve. Thus, the differences of quasi-indentation damage caused by different boundary conditions can not be ignored.Secondly, the effects of delamination depth and size on quasi-static indentation damage under four edges clamped were evaluated with the assistance of nondestructive testing. The results showed that eigen contact force and eigen dent depth decreased with the reducing of pre-inserted delamination depth. Compared with damage depth and damage width, eigen contact fore and eigen dent depth could reflect the effects of delamination on damage resistance of composite laminates better. Therefore, eigen contact force and eigen dent depth were more reasonable to evaluate the damage resistance of composite laminates.Finally, the effects of delamination on residual compressive strength, effective compressive modulus and maximum compressive strain of composite laminates were discussed respectively. The results showed that residual compressive strength exhibited a more obvious and regular evolution with the change of pre-inserted delamination depth and size when compared to effective compressive modulus and maximum compressive strain. Specifically, with the decreasing of delamination depth(before middle plane), the residual compressive strength of delaminated laminates increased regularly. In despite of the intact samples or delaminated samples, the residual compressive strength reduced by about 5% with the appearance of delamination while the residual compressive strength decreased to around 50% with the propagation of delamination. Therefore, it was more proper to select residual compressive strength as the parameter to evaluate the damage tolerance of composite laminates.