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This paper presents that the prediction model of fatigue crack initiation life of TC4 titanium alloy,and the feasibility of the model in predicting the fatigue crack initiation life of TC4 titanium alloy is verified by experiments. Based on the micro dislocation model of crack initiation and Tanaka mura’s formula of crack life,considering the surface roughness,a finite element model for fatigue crack initiation of TC4 titanium alloy is proposed,and the validity of the simulation model is verified by experiments. The results show that the crack initiation forms of the model are different under different load levels. At high stress levels,the model has a large number of independent microcracks in addition to the initiation and propagation of the main cracks,with a large crack density. At low stress levels,the model has a small number of independent microcracks,with a small crack density.

Stitched composite material not only suppresses delamination expansion, but also improves inter-laminar toughness and compressive characteristics after impact by introducing stitch in the thickness direction. Equivalent hole method simulates the initial damage of stitched plate. The finite element analysis method with damage evolution process based on threedimensional Hashin criterion with 3D solid element modelling laminates,3D bar element modelling stitch simulated the stitched composite laminates damage evolution effectively. The calculation results were in good agreement with the experimental data.

This article presents an experiment and simulation study on the effect of Z-pin reinforced on the pull-off and sheat carrying capacity of fiber placement composite T-joints,we used CMT5504 to conduct the pull-off test meanwhile we used MTS to conduct the shear test. we studied Z-pin reinforced And non-reinforced test-pieces,drawing Load-displacement curves.The pull-off test results donate that Z-pin can effectively improved the pull-off carrying capacity of T-joints and slow the decrease of the load significantly after pick loading compared to the non-reinforced T-joints. The failure location is between stiffener and interlayer. The shear test results donate that Z-pin can effectively improve the shear carrying capacity of T-joints. The failure location of Z-pin reinforced test is between skin and interlayer and the failure location of non-reinforced test is bwtween stiffener and interlayer. On this basis,we created pull-off and shear model of T-joint specimens. The interface failure was simulated with cohesive zone model,and the effect of Z-pin was simulated with nonlinear springs inserted into the interface. The numerical results agree well with the test data and shows that the model is effective.

A 3D model of composite laminates with bolted joint is created by using the software ABAQUS. By introducing the 3D Hashin failure criteria and considering the tightening torque,the partition and the global degradation models were established. With this method,the failure process and the linking strength of single-lap,single-bolt joints were calculated. The results shows that the load-displacement curve by using global degradation model doesn’t degrade enough and the computing results are more accurate by using the partition degradation model. With the increase of thickness,the linking strength of bolted single-lap composite joints decrease all the time and the speed of degradation gets slower and slower. It means that with the increase of thickness,the lifting bearing capacity of structure reduces. It also indicates that the ultimate strength of laminate is enhanced with appropriate tightening torque and excessive tightening torque impairs the load carrying capacity of the structure.