Papers by Keyword: Fatigue Damage Evolution

Abstract: The high cycle fatigue tests for smooth specimens of TC25 titanium alloy under different stress ratios are carried out on a MTS 809 Material Test Machine at a given maximum stress level of 917MPa at ambient temperature, the high cycle fatigue lifetimes for such alloy are measured, and the effects of stress amplitude and mean stress on high cycle fatigue life are analyzed. The initial resistance is measured at the two ends of smooth specimen of TC25 titanium alloy, every a certain cycles, the fatigue test is interrupted, and the current resistance values at various fatigue cycles are measured. The ratio of resistance change is adopted to characterize the fatigue damage evolution in TC25 titanium alloy, and a modified Chaboche damage model is applied to derive the fatigue damage evolution equation. The results show that the theoretical calculated values agree well with the test data, which indicates that the modified Chaboche damage model can precisely describe the accumulated damage in TC25 titanium alloy at high cycle fatigue under unaxial loading. Finally, the high cycle fatigue lifetimes for TC25 titanium alloy specimens at different strain hardening rates are tested at a given stress ratio of 0.1, the effect of strain hardening on fatigue life is investigated based on a microstructure analysis on TC25 titanium alloy, and an expression between fatigue life and strain hardening rate is derived

Abstract: Damage mechanics has been applied to describe the cyclic behaviour of glass fibre-polyester and carbon fibre-epoxy composites with different lay-ups under various loading conditions. Damage evolution was determined by continually monitoring fatigue modulus degradation and measuring the crack density. These methods complemented each other. They showed that the damage could be separated into two stages. Damage evolved rapidly for the first 10% of life, followed by a more gradual and linear accumulation for the remainder of life. In general, the transition from the first to the second stage indicated a change from transverse matrix cracking to fibre-matrix debonding and coalescence. Damage mechanics was applied to the fatigue modulus changes that occurred in the stress-strain hysteresis loops, monitored throughout life. A two-stage model was applied to express damage evolution using the modulus- and crack-based damage parameters. This model successfully described cyclic damage evolution for different lay ups of the PMCs. The significance of which was that the amount of fatigue damage for any stress level at the end of the initial stage could be used to accurately predict fatigue life and construct a stress-life diagram for the given composite

Abstract: Using damage mechanics, cyclic damage evolution has been described and evaluated in a non-crimped glass epoxy fabric composite. A fundamental fatigue study has been carried out by progressively monitoring the fatigue damage modulus and crack density throughout the life of an [0,+45,90,-45]2 (antisymmetric) laminate cycled at a stress ratio R (minimum stress/maximum stress) of 0.1. Development of damage can be separated into two main stages. Initially, damage increases very quickly during the first 10% of life (Stage I). Afterwhich, it increases more slowly at a relatively constant rate to failure (Stage II). The changes in the fatigue modulus and crack density both show the same behaviour. A large amount of damage in the form of transverse matrix cracks develops during the first cycle. These then remain constant throughout life. By contrast, the number of shear matrix cracks increase continually. The crack density is cycle, not stress dependent. This behaviour is reflected by changes in the fatigue modulus. Using damage mechanics, a representative equation has been applied to express the progressive evolution of damage. The significance of which is that the amount of fatigue damage at the end of Stage I for any stress level can be used to predict fatigue life and the stress-life diagram for the laminate.