Ti-6Al-4V titanium alloy small crack growth behavior and life prediction of
|School||Beijing Institute of Aeronautical Materials|
|Course||Engineering , Solid Mechanics|
|Keywords||Expansion behavior Small cracks Life Prediction Titanium alloy Constant amplitude loading Crack closure Crack growth rate Fatigue life Heat-treated condition Fatigue life|
In this paper, two types of heat treatment state of Ti-6Al-4V titanium alloy forgings fatigue crack initiation and propagation behavior. These two heat treated state of the material has a similar lamellar structure, different α slice width. The research work mainly includes: (i) the lamellar structure of the small crack growth behavior of crack growth acceleration, deceleration, such as crack propagation path. (Ii) the use of stress intensity factor range (△ K) constant amplitude load R = 0 and -1 long crack and small crack growth rate (da / dN) association, in order to compare the different stress than under two materials whether has a small crack effect. (Iii) the difference of the two materials lamellar width of the small crack growth behavior; (iv) based life prediction model for crack closure theory and FASTRAN II state two heat treated Ti-6Al-4V alloy constant amplitude loading and Mini-TWIST spectrum loading under the fatigue life and the small crack growth rate calculation and prediction, to explore life issues on a reasonable estimate of the Ti-6Al-4V alloy. Through test and analysis of the calculation, the following main conclusions: the two materials, alpha lamellar cluster boundary and the original beta grain boundaries α relatively small crack propagation hindering role in the size of the two adjacent α lamellar cluster direction , two α bundling difference angle is large, small cracks extended stagnation longer small crack propagation across the boundary after direction change is also large. This effect in the larger grain size of the material was more obvious. Constant amplitude loading R = 0 and -1 two materials showed a small crack effect, R = -1 small crack effect is particularly evident with negative stress than under the crack closure mechanism about. Small crack propagation, the two materials exhibit different stress ratio difference. R = 0 under α lamellae width difference expansion of the early small crack (less than 200μm). R = -1 small crack growth behavior of the two materials, there is little difference, indicating that this stress ratio, the less impact on small crack growth behavior differences in the width of the film strip. Constant amplitude loading R = -1, the fatigue life of the two materials in a lower stress level dispersion of smaller grain size material exhibits more obvious. SEM fracture analysis found dispersed fatigue life under low stress level summary reasons and crack source location different. Using two materials SENT specimen R ~ 1 constant amplitude loads and Mini1WIST spectrum contained under total fatigue life and crack propagation rate calculation and prediction software based on the crack closure model theory FASTRAN 11. The results show that in most cases, beginning from the initial flaw size of the hypothetical material calculated and predicted values ??agree well with the experimental results, except for the titanium alloy material with a smaller width of the sheet layer, the stress level (180MPa) Prediction accuracy is poor, a certain extent this is caused due to the low stress levels result in fatigue life of the dispersion is too large. Constant amplitude loading R - 1 and 0, the predictive value of the small crack growth rate in the test data dispersed with forecast results to verify the effect of small cracks.