Research On Prediction Of Damage Failure And Fatigue Life For Composite Bolted Joints

Mechanically fastened joints of composite materials are mainly components in aircraft structures, which need to be subjected to static loadings and fatigue loadings during working. The prediction of damage failure and fatigue life is one of questions that designers always pay attention to. In order to adequately utilize potential of composites, the method of predicting damage failure and fatigue life which should be fit to engineering is an important research project now, which is also one of primary questions which should be in urgent need of being solved.A progressive damage method of predicting the static strength and the fatigue life of mechanically fastened joints in fibre-reinforced plastics is established in the paper, which is based on the theory of unidirectional ply. The equations of virtual work are deduced to analyze stress-strain states under static and fatigue loading conditions. Nonlinear shear stress-strain behavior of a unidirectional ply, in the 1-2 and 3-1 planes, is considered in the analysis of three dimensional stresses for the laminate of mechanically fastened joints. The residual strength degradation model and the residual stiffness degradation model based on the degradation of stiffness of the composite lamina subjected to fatigue loadings are deduced and established by coupling the normalized fatigue life method. The three-dimensional Hashin-type failure criteria are introduced into the method to detect damage for diverse damage modes. The criteria of the structure catastrophe and the sudden material property degradation rules including the correlation among four basic damage mechanisms are also established. A software module of progressive damage analyses for bolted composite laminates is compiled, which is convenient for the application in engineering.An experimental program which includes lots of unidirectional laminae is conducted using T300/BMP-316 material for researching the static and fatigue behaviour of the unidirectional ply. The experimental research for mechanical behaviour of a unidirectional lamina is to set up expressions of the normalized fatigue life, the residual strength degradation and the residual stiffness degradation of a unidirectional ply in the longitudinal, transverse and in-plane-shear directions, which also verifies the fact of nonlinear shear stress-strain behavior of a unidirectional ply in the 1-2 planes. A series of static and fatigue experiments which include lots of specimens are performed on bolted composite laminates that include different sizes, different lay orientations and different torques. Dye-Enhanced X-Radiography is used to check the progressive damage process as evaluation techniques, which can be made nondestructively.Finally, the ultimate strength and fatigue life of composite bolted joints in static tension and...