Failure Analysis And Strength Prediction Of Composite Bolted Joints

The design of composite material joints is the key step of aircraft structural design. In order to reduce testing cost and guarantee structural safety, developing numerical simulation methods which can be used to predict failure and strength of composite joints is of great significance; For the purpose of maximum playing the superior performance of composite material so that which can be extensively used in main load-bearing aircraft parts, it has important practical meaning and reference value for engineering applications to explore methods and measures of improving composite joint efficiency.This paper carried on research on above mentioned. As to bolted joints which are typically used in aircraft structural components, firstly three dimensional parameterized progressive damage model of single bolt-jointed composite laminate was developed by APDL(ANSYS Parameter Design Language) based on progressive damage theory and the technology platform of finite element analysis software ANSYS. The model took both nonlinear contact behavior and bolt tightening into consideration, by which failures and strength of nine groups of single bolt joints of different layer sequences and geometric dimensions in reference literature were predicted. The results obtained agreed well with experimental ones, which certified the validity of the developed model. The damage accumulative rules of composite laminates in above joints were analyzed detailed by showing the damage extension processes of laminate layers in above joints simulated from the initial damages to the final failure occurred to the joints. And effects of layer sequences of laminates, joint dimensions and layer numbers on joining strength were compared and summed up.Then for the problem of unequal pin load distribution in multiple bolt-jointed composite laminate, the pin load distribution was calculated and analyzed. The methods and steps of calculating pin load ratios using the contact analysis function of ANSYS were described in detail. The pin load distribution of multiple-bolt beveled-lap composite joints was researched. Pin load ratios in two and three bolts beveled-lap joints were calculated and which were compared with that of traditional equal-thickness lap joints. The results show that the two lap types of joints are both that the bolt away from the loading end of laminates carries less load and the one near the loading end carries more, but the pin load distribution of beveled-lap joints used for calculating is all superior to that of corresponding equal-thickness lap joints; In the meantime, effect of bolt diameter on pin load distribution was also researched, results show that reducing the diameter of bolt near the loading end is good for improving the pin load ratio, whereas no matter increasing or reducing the diameter of bolt away from the loading end, it is helpless for optimizing pin load distribution but even has opposite effect.Finally, based on developed progressive damage model of single-bolt composite joints before, models for predicting strength of multiple-bolt joints were established, by which strength of two and three equal diameter bolts composite joints in both types of beveled-lap and equal-thickness lap was calculated and the joint efficiency of the two lap types was compared and analyzed. Results show that both the weigh and load efficiency of beveled-lap joints used is higher than the corresponding equal-thickness lap joints, that is. the former have the better joining efficiency. The conclusion is obtained that multiple-bolt beveled-lap joints will have higher joining efficiency than corresponding equal-thickness lap ones if only the former keep pin load distribution superior to the latter when loaded.