Fatigue Damage Modelling Andanalysis For Fiber Reinforced Composite Materials

Fiber-reinforced composite materials are widely applied in aeronautics, astronautics and other fields due to their excellent mechanical performances. Researches about the mechanical properties of composite materials under static loading have been developed a lot. Then the fatigue study of composite materials started to be payed significant attention, which was covered by static loading researches before. The fatigue damage mechanisms of fiber-reinforced composites are much more complicated than those of metal materials, fatigue behaviour and failure modes of composites are different between a variety of materials and internal constituents as well. The anistropic fiber-reinforced composite materials are composed of fiber, matrix and interface. Four basic damage modes grow gradually under cyclic loading, including matrix microcracking, fibers/matrix debonding, delamination and fiber fracture. Meanwhile, the couple interactions of basic damage modes play an important part as well. Therefore, it is significant for theoretical and engineering researchers to study the fatigue damage mechanisms and to predict fatigue life of fiber-reinforced composite laminates accurately. In this paper, the fatigue damage mechanisms of the fiber-reinforced composites were thoroughly discussed based on the theory of continuum damage mechanics and the fatigue life was predicted as well. The main works are sum up as followed.1. The fatigue damage mechanisms of fiber-reinforced unidirectional composites were studied on the basis of continuum damage mechanics and Ladevèze theory. The damage modes of unidirectional ply were divided into: longitudinal fiber breakage, transverse matrix microcracking and shear fiber/matrix debonding in plane. Constitutive equations were established, considering the couple mechanisms of transverse damage and shear damage. According to thermodynamics and Gibbs free potential energy, the general form of thermodynamic driving force under multiaxial cyclic loading were established, the thermodynamic driving forces under pure transverse and shear cyclic loading were acquired as well. In the framework of irreversible thermodynamics, three fatigue damage evolution equations of couple damage modes were established. Fatigue tests of 0o, 90o and 45o GFRP unidirectional laminates in literature were used to identify parameters of damage evolution equations, respectively. Failure criterion and numerical methods for the fatigue damage model were constructed. The process of fatigue damage growth was discussed and fatigue life was predicted. According to the good agreement between the predicting results and fatigue tests of off-axial GFRP unidirectional laminates, the new fatigue damage model has been testified strongly.2. A unified fatigue strength measure was explored, coping with effects of both mean-stress and fiber-orientation on off-axis fatigue behavior of unidirectional composites over a range of stress ratios on the basis of continuum damage mechanics. Firstly, the effects of empirical fatigue strength ratio and non-dimensional effective stress on fatigue behavior of unidirectional polymer matrix composites were studied. Then a modified form of empirical fatigue strength ratio was established and tested on the off-axis fatigue behaviors of unidirectional composites under different loading conditions. The modified non-dimensional effective stress was established and discussed as well, on the basis of Tsai–Hill static failure criterion. A new fatigue predicting model with effects of fiber orientation and stress ratio has been formulated by the modified non-dimensional effective stress. Considering three damage modes in elementary ply, parameters of the model were obtained under R =0 for unidirectional glass/epoxy composites and under R =0.1 for unidirectional carbon/epoxy composites, respectively. Finally, the validity of the new fatigue predicting model was evaluated by comparisons with off-axis fatigue tests of literature under R =0.5. The results show that the new fatigue predicting model can adequately describe the off-axis S-N relationships for both unidirectional glass/epoxy and carbon/epoxy composite laminates under constant-amplitude cyclic loading with non-negative mean stresses.3. The effect law of various loading modes on fatigue behaviors of glass fiber reinforced composite(GFRP) laminates was discussed thoroughly. An effective phenomenological fatigue predicting model was established based on resudial stiffness model, taking account of the effects of stress ratio, loading frequency as well as fiber orientation on the fatigue behavior of GFRP laminates. Parameters of the model were fitted out according to fatigue tests of uniaxial tension loading R =0.1, uniaxial compression loading R =10 and uniaxial tension loading with frequency f =1Hz. The fatigue life of GFRP laminates of the same system with arbiturary lay up under other loading modes was predicted and compared with corresponding fatigue tests of literature. The results show a good agreement between predictions and experiment data for a wide range of GFRP laminates, which is useful to assist and decrease the amount of fatigue tests.