Crack-Closure Effects on the Growth of Small Surface Cracks in Titanium-Aluminum Alloys

Abstract

Among other factors, the reported failure of linear elastic fracture mechanics to correlate the growth rates of large and very small fatigue cracks has been attributed to microstructural and crack closure effects. These two factors have been examined in experiments on three titanium-rich Ti-Al alloys, which display significant differences in plastic slip character ranging from homogeneous wavy slip to extremely heterogeneous planar slip. The variation in slip character has a dramatic effect on the level of roughness induced crack closure, which in turn, strongly influences the growth rate behavior of large fatigue cracks. In the present project the behavior of small fatigue surface cracks in these materials was investigated using two specialized experimental techniques. A computer-controlled photomicroscopic system was employed to monitor surface crack length directly, and an automated laser interferometric technique was used independently to determine crack length and to measure the development of crack closure. Although slip character has a strong influence on the propagation of large cracks, the present findings indicate that there is a minimal effect of slip character on the growth of small fatigue cracks. Direct crack opening displacement measurements indicate that this is largely because of a transient development of crack closure in the small cracks. For the materials tested, the crack depth required to fully develop crack closure ranged from 5 to 13 times the mean distance between grain boundaries. Above this crack size, data from small surface cracks and large cracks in C(T) specimens were well consolidated by the linear-elastic parameter ΔK.