Fatigue crack propagation experiments on a Ti-6A1-4V alloy were carried out within the framework of linear elastic fracture mechanics. Crack propagation results encompassing a wide range of fatigue crack growth rates, Δa/ΔN, and stress intensity range, ΔK, were plotted in the conventional manner (that is, log-log plots of Δa/ΔN as a function of ΔK) for studies conducted in room temperature air and dry argon environments. Corresponding fatigue striations formed in both environments were measured and compared with macroscopic crack extension rates.
Examination of very low fatigue crack propagation rates at low ΔK levels disclosed a stress intensity threshold level below which Mode I fatigue crack propagation became insignificant.
In a limited investigation at very low fatigue crack growth rates, the fatigue threshold levels were observed to decrease with increasing stress ratio, R (ratio of σmin/σmax= Kmin/Kmax). The influence of stress ratio also was observed at ΔK levels above the threshold; cyclic crack growth rates increased for the increased values of R. However, for the ranges of ΔK, R, and growth rate considered in this work, the R ratio influence appeared most pronounced at the lowest growth rates and lessened with increasing ΔK.
Comparable fatigue crack growth tests (R = 0.33) conducted on the Ti-6A1-4V alloy in air and inert dry argon at room temperature yielded almost identical results for cyclic crack growth rates encompassing those associated with the fatigue threshold (Δa/ΔN ≃5 × 10-9 in./cycle) upward past 10-5 in./cycle. Within this range, the fracture surface morphology changed from a cleavage-like appearance at threshold levels to a typically striated and dimpled appearance at higher growth rates.
No significant difference in fatigue crack growth rate behavior was reported for Ti-6A1-4V specimens tested in either the STA (solution treated and aged) or STA + annealed heat treated condition.
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