Multiaxial Mechanical Behavior And Fatigue Life Prediction Of Composite Laminates

In addition to the excellent properties of composite materials such as fatigue resistance and corrosion resistance,laminated composites also have the advantages of designability of lay-up,simple processing technology and low material cost.Laminated composites have been widely used in aviation and aerospace.However,the research on static behavior and fatigue characteristics of laminated composites under complex loads is still very limited.In this paper,the multiaxial mechanical behavior and fatigue life prediction model under combined tensile-torsional loading are systematically studied for laminated composites.The main research work and conclusions of this dissertation are summarized as follows.(1)The experimental investigation of unidirectional composites has been conducted at different off-axis angles.The static tension and tension-tesion fatigue tests were carried out at different off-axis angles,and the corresponding mechanical properties were obtained.In addition,the static tension and compression tests were also carried out to obtain the mechanical properties of matrix.(2)The experimental investigation of laminated composites has been conducted under tension-torsion biaxial loading.For laminated composites in the form of [45/70/0/-70/-45],multiaxial static strength tests were carried out at different biaxial stress ratios,and the corresponding mechanical properties were obtained.Multiaxial fatigue tests were carried out at different biaxial stress ratios and different phase angles,the effects of biaxial stress ratio and phase angle on fatigue life have been analyzed.(3)The theoretical investigation on the stiffness of unit cell with fibers distributed randomly has been performed.It is assumed that composite materials are composed of matrix and randomly distributed fiber monofilaments.The hexagonal uniform distribution was used as the initial distribution of fiber monofilaments to construct the unit cell.And fiber monofilaments were not allowed to cross the boundary of the unit cell in the through-thckness direction.Simulating a fully elastic collision of a smooth sphere,the unit cell with fibers distributed randomly has been established.By introducing periodic boundary conditions,the mechanical properties of composite materials can be predicted by the unit cell with fibers distributed randomly.And the predicted values agree well with the related test results of carbon fiber reinforced composite and glass fiber reinforced composite,which verifies the validity of the model.(4)A macro-meso multiaxial static stiffness prediction model for laminated composites has been established,which is based on the unit cell with fibers distributed randomly.Firstly,the unit cell of composite laminates which is composed of several layers was established.Secondly,mesh the unit cell of composite laminates.By introducing periodic boundary conditions,the stiffness properties of composite materials can be predicted by the unit cell of composite laminates.And the validity of the model has been verified by the experimental stiffness results of tensile test of unidirectional composites at different off-axis angles and the experimental stiffness results of laminated composites at different biaxial stress ratios.(5)A macro-meso multiaxial static strength prediction model for composite laminates has been established.Based on inclusion theory and the theory of elastoplastic mechanics,elastic mechanical analysis of stress field of infinite matrix and fiber which contained in the infinite matrix were carried out.One energy-type failure criterion was proposed for matrix,and the range of application of Hashin failure criteria has been extended to fiber monofilament.According to the theory of gradual damage propagation,combining the failure criteria of the unit cell of composite laminates,the macro-meso multiaxial static strength prediction model has been established.And the validity of the model has been verified by the experimental strength results of tensile test of unidirectional composites at different off-axis angles and the experimental strength results of laminated composites at different biaxial stress ratios.(6)A macro-meso multiaxial fatigue life prediction model for laminated composites has been established,which is based on the macro-meso multiaxial static strength prediction model for composite laminates.For the fiber and the matrix,corresponding fatigue damage parameters which considering the effects of stress,strain and loading path were proposed respectively.Combined with damage analysis of composite laminates under multiaxial static loading,the macro-meso multiaxial fatigue life prediction model has been established.And the validity of the model has been verified by the experimental results of tension-tesion fatigue tests of unidirectional composites at different off-axis angles and the multiaxial experimental results of laminated composites at different biaxial stress ratios and different phase angles.