Abstract
Recent studies on fatigue crack growth of an alpha 2 titanium aluminide,Ti-24A1-11Nb intermetallic at room temperature [1-3] indicate a strong dependence of fatigue crack growth rate on the stress ratio, R (ratio of minimum stress to maximum stress) . Figure 1 shows the variation of da/dN with stress intensity factor range, AK for different stress ratios, both in SE(T) and C(T) specimens [1] . It can be seen that the threshold SIF-range decreases considerably with increase in stress ratio . Dependence of crack growth rate on effective stress intensity range, AK Cf K118 -K , should consolidate data of this type into a single curve, where K is the stress intensity corresponding to the load at which the crack opens fully during the loading part of a cycle . It is assumed here, for simplicity, that the opening and closing loads are the same. It was suggested earlier by the authors [1] that an argument can be made for the existence of a mean stress effect on the crack growth rate in this material, based upon the closure levels required to produce equivalent driving force, AK A,, at different values of R. From the data of Fig. 1 at R=0.1 and R=0.7, crack closure and AKtf arguments would require a K, value of approximately 5 MPaVm for the R=0.1 data to consolidate with R=0 .7 data, assuming the latter had no closure. However, load-displacement measurements made near the crack tip using a precise laser interferometry system [4] have revealed relatively small closure load levels (K,/K. not exceeding 0.4 for R=0.1) for constant amplitude fatigue in this material. Thus, the substantial difference in the rate of crack growth at any given AK for R=0.5 and R=0.7 cannot be explained solely on the basis of crack closure . It is therefore considered necessary to include the effect of mean stress, in addition to crack closure, in expressing the fatigue crack growth rate as :
Published Version
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