Variability of large-crack fatigue-crack-growth thresholds in structural alloys
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I. INTRODUCTION
LARGE cracks in metals subjected to cyclic loading generally exhibit a threshold stress intensity range (Kth) below which fatigue crack growth (FCG) ceases to occur. The large-crack threshold value is an important material parameter in life-prediction methods based on fracture mechanics, since the FCG life, most of which is spent in the near-threshold regime, is very sensitive to the critical stressintensity range where crack extension begins.[1,2] Unfortunately, the Kth value depends on a large number of factors including microstructure, loading history, temperature, frequency, crack size, and, among others, environment.[3–8] Furthermore, the value of Kth shows substantial variations due to material variability and testing methods. For example, the Kth value of structural alloys shows large variations with the stress ratio (R), which is the ratio of the minimum to maximum stress in a fatigue cycle.[9] The origins of the stress-ratio dependence and the corresponding large variations are not entirely understood and are a subject of recent debates. Studies of fatigue mechanisms in the near-threshold regime have revealed the presence of extrinsic and intrinsic processes[10] that can alter significantly the threshold value at which FCG ceases to occur. Extrinsic fatigue mechanisms are those that affect the driving force, i.e., K; they usually operate in the crack wake and include crack closure mechanisms due to plastic wake,[11–17] crack deflection and branching,[18,19] fracture-surface asperity and roughness,[18,19] as well as oxide wedging.[20,21] In contrast, intrinsic fatigue mechanisms are those that usually operate at or ahead of the crack tip and reflect the material’s resistance to cyclic deformation and fatigue failure in the crack-tip region; for
KWAI S. CHAN, Institute Scientist, is with the Materials Engineering Department, Southwest Research Institute, San Antonio, TX 78238-5166. Contact e-mail: [email protected] Manuscript submitted December 10, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
example, they include the emission of dislocations from the crack tip or the to-and-fro motion of dislocations in a cellular network formed ahead of the crack tip. The high sensitivity of the large-crack fatigue threshold to the R ratio, which is illustrated in Figure 1(a) for Ti-6Al4V,[22–26] has often been explained on the basis of crack closure mechanisms that are present in the crack wake.[3,4,10–17,27–29] However, numerous studies of crack-closure measurements have showed that at low R ratios (R 0.5), fatigue cracks are closed at loads that are higher than the minimum load in a fatigue cycle. Nonetheless, the validity of the concept of crack closure and the corresponding explanation of the stress-ratio effect in FCG have been questioned in recent years by Vasudevan et al.,[31] who presented a fatigue analysis that was purported to show that plasticity-induced and asperity-induced crack closure either do not exist or are negligible, such that crack closure cannot be the mechanism responsib
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