A major reason for the failure of crack growth rate versus applied stress intensity factor range correlation under smoothly varying small scale yielding loadings at a given (negative) stress ratio is dissimilarities in the crack closure stress field. It is essential to assess the relevance and accuracy of crack closure measurement before coming to a conclusion that crack closure phenomenon is unable to explain the trends in growth rate in a given experiment [ 1]. While establishing the presence/absence of crack closure in a fatigue crack growth experiment is relatively straightforward, no general method has yet evolved to measure the effect of crack closure on the crack driving force. Load-crack mouth opening displacement (CMOD) records during fatigue cycling are routinely used for estimating crack closure. A fundamental issue is the relevance of the load level at which crack closure begins (or crack is completely open) in estimating the correct crack driving force. Elber [2] hypothesizes that crack extension does not occur while the crack is closed and considers the stress range over which the crack is fully open as the effective range for crack growth. Stress ratio effect (mainly in the positive range) and many other aspects of variable amplitude crack growth are successfully explained on this basis. Newman [3] provides an analytical justification for Elber's hypothesis by demonstrating, in a finite element simulation of fatigue crack growth, that the strain ahead of the crack tip does not vary much below the level of crack opening stress. However, in a study by Ohta et al. [4] of crack tip strains by moire fringe multiplication technique, it is shown that the strain ahead of the crack tip continues to vary well beyond the crack closure stress determined by a displacement gauge. Evidence abounds that cracks continue to grow in mode II while closed in a mode I sense in near threshold crack growth tests. On the other hand it is also reported [1] that crack growth rates are severely retarded even while the crack tip remains propped open due to asperity contact elsewhere in the wake. This discussion suggests that the load level, marking the beginning of the crack closure process (or the end of crack opening process), can not in general, be used to estimate the correct crack driving force.There is a strong need for establishing 'benchmark' crack closure/opening stress values which lead to the correct crack driving force. These benchmark crack closure values can then be used to assess other indirect crack closure measurement techniques. In experiments where crack growth rate versus externally applied crack driving force correlation gets affected only due to crack Int Journ of Fracture 53 (1992) R54 closure one can determine the true crack opening stress values purely by rationalising fatigue crack growth rate data in the absence and in the presence of crack closure. This way benchmark crack closure values can be established from crack growth rate considerations. Accuracy in estimation of crack opening stress lev...
Linear Elastic Fracture Mechanics, LEFM, principles are successfully used to characterize the behavior of flawed structures and components under monotonic loading. Crack length increment, da, correlates with maximum stress intensity factor, K , when the monotonic plastic zone size is small. Cracks can grow under cyclic loading even at stress levels lower than that required for crack extension under static loading. Pads et al. [1,2] established the correlation between crack growth rate, da/dN, and stress intensity factor range, delta-K, of the load cycle. Aplication of LEFM principles to crack extension under monotonic cyclic loading can be seen as: loading response correlating restriction parameter parameter Monotonic da Kmax Monotonic plastic zone size small Cyclic da/dN delta-K ? A small monotonic plastic zone implies a still smaller cyclic plastic zone. Therefore Small Scale Monotonic Yielding (SSMY) becomes a 'sufficient' condition for a valid application of LEFM parameters to fatigue crack growth. However, fatigue crack growth is essentially a cyclic process in the low and intermediate growth rate regimes and hence depends on the cyclic variation of strain rather than on maximum strain ahead of the crack tip. Rice [3] demonstrated, for perfectly plastic as well as for strain hardening material with stable hysteresis loop, that the strain range ahead of the crack tip depends on the applied stress range and is relatively insensitive to the maximum applied stress. It is, therefore, reasonable to expect that applied delta-K can represent fatigue crack growth rate in the low and intermediate growth rate regimes even though K is not defined due to large scale tensile yielding. There is, therefore, a need to reassess the 'necessary' Small Scale Yielding (SSY) condition since the small monotonic plastic zone requirement is too restrictive. Rice [3] and Johnson and Paris [4] pointed out that under net section yielding the unloading deformation at the crack tip can be elastic provided the amplitude of loading is small. Hence Int Journ of Fracture 52 (1991) R4 delta-K can still represent fatigue crack growth rates. Frost, Pook and Denton [5] found that delta-K described growth rate even under net section yielding in some materials in high stress ratio tests. Most literature reviews like Suresh and Ritchie's [6] and Brown's [7], still maintain that monotonic plastic zone be small for obtaining da/dN -delta-K correlation under fatigue loading. Brown et al. [8], in their critical comparison of parameters for high strain fatigue crack growth maintain that LEFM is applicable only for stress range less than one third the flow stress value. For stress values beyond this limit they have evaluated delta-J, delta-CTOD, Crack Tip Opening Displacement, and cyclic plastic zone size as possible crack growth rate correlating parameters. However, their crack growth rate data under uni-axial, equi-biaxial and torsional loading at R=-1.0 do not rule out effective stress intensity factor range as an appropriate correlating parameter. Raj...
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