Cumulative Fatigue Damage Numerical Analysis For Composite Laminates

Fatigue failure is one of general failure modes for materials and structures in engineering. Fatigue limit for metal materials is 40% ~ 50% of their tensile strength. But as for carbon fiber/epoxy composite material, its failure limit is 90% of its tensile strength. Though fatigue performance of composite materials is better than metal materials, it is inevitable to have the fatigue problems for composite materials. But the fatigue-crack-propagation analysis method for metal materials may be not suitable to analyze fatigue delamination of composite laminates. There are two main fatigue failure modes for laminates, failure between layers and interfacial debonding. These failure modes always occurs relatively lower fatigue loadings. Therefore, classical fracture mechanics cannot solve fatigue problem of laminated composite materials owing to their nonlinearity. At the same time, there are many factors such as variety of layup, thickness and loading states etc, to influence laminated composite materials’ property. Thus it is not economical to conduct the fatigue delamination experiment for laminates. But it is an effective way to study the laminates fatigue problems by using numerical method, which becomes more powerful with the development of computer techniques and identifies the technology maturity in the engineering.In this thesis, combing with fracture mechanics and damage mechanics, the fatigue delamination and fatigue life for composites laminates are studied by numerical method.Firstly, the paper introduces the discrete element constitutive law for composites delamination under monotonic and cyclic loading is introducing, which is a bilinear constitutive law of discrete element. Spring element is used to describe the static damage in monotonic loading, and an exponential function is adopted to describe fatigue cumulative damage rate for laminates under cyclic loading. The computation process implemented in commercial finite element software ABAQUS is provided. A subroutine UEL is developed to implement fatigue cumulative damage model for composite laminates, which can lay theoretical foundation for analyzing fatigue delamination of laminated composites later.Secondly, the crack propagation of laminated composite under monotonic loadin g is simulated by using discrete element, where three situations are taken into account, such as double cantilever beam, end notch fracture test and mixed mode bending. The delamination evolution of composite laminates is analyzed, and it is validated by comparing with VCCT results. Also the influence of the two factors including spring stiffness and element size is investigated. Results show that the numerical results obtained by the discrete element model have a little sensitivity to these factors.Thirdly, the crack propagation of double cantilever beam, end notch fracture test and mixed mode bending for laminated composite under cyclic loading is also studied by using discrete element. The crack growth rate of the initial crack is obtained. In addition, the relationship between the fatigue crack growth rate and the energy release rate of the fatigue crack for laminates under the four loading cases is established, which are in good agreement with the laboratory data in literatures.Finally, a virtual crack closure technique is established to predict the fatigue life of the composite materials. And fatigue life of double cantilever beam and end notch fracture test of laminated composites is predicted.