Study On The Effects Affecting Local Stress And Failure Strength Of Composite Mechanically Fastened Joints

Mechanically fastened joints are one of the most important parts of composite structures design.Studying the effects affecting joints’ stress and failure strength by numerical simulation can optimizethe design as well as raise the efficiency and reduce the cost considerably. Since involving contact,damage and many other complicated problems of non-isotropic material, currently the analysis ofcomposite mechanical joints is still an open ended issue. The main contents in this master thesis aresummarized as follows:1. A simplified three-dimensional finite element model of single-bolt joint was built. The stressdistribution along the joint hole was investigated. The effect of stacking sequences, toleranceclearances and friction coefficients between bolt-hole contact surfaces on the stress distributionalong the hole was studied.2. A three-dimensional finite element model of single fastener and double-lap joints was built.Hashin failure criteria and the improved Camanho stiffness degradation model were employed tosimulate the failure process of joint parts under tensile loads. The results reveal that ply ratioshave significant effect on failure strength of single fastener and double-lap joints while width-to-diameter ratios have little effect.3. A three-dimensional finite element model of multiple fasteners and single-lap joints was built tocalculate the load distribution among fasteners. The influence of tie plate stiffness and toleranceclearances on the load distribution was studied. Based on this, the failure process and strength ofthe multiple fasteners under tensile loads were predicted. Besides, the effect of toleranceclearances and friction coefficients on failure strength in multiple fasteners was also investigated.4. A novel kind of laminate element made of block units containing multiple layers was proposed.It is considered that the in-plane strain of each layer varies along the thickness direction and theinfluence of damage in each layer on the performance of the element can be reflected. Under thecondition of meeting the project accuracy requirements, the model made of these novel elementscan greatly reduce the computation time as well as eliminate ‘chatter’ phenomenon in thenumerical simulation of contact problems and improve the convergence performance.All the numerical simulation was finished based on ABAQUS together with user definedsubroutines (UMAT).