AbUmct-S&ess and strain Md of a propagating fatigue crack and the resulting crack opening and &sing behavior were anaiyscd. It was found that a propq&g fatigue crack was closed at ten& external Ioads due to the cyclic&y induced residual stresses. Strain range v&e AC, in the vicinity of the crack tip was found to be close& r&ted with the ellective stress intensity factor range AK* which was determined w the b&s of the analytical crack opening and closing behavior at its tip. Application of this analysis to the non-prupagating fatigue crack probkm and tbc f&igue crack -ion @terns under variable stress amplitude conditions revealed that both Aa, and AK, were essential parameters governing fatigue crack growth rate.
ONE OF the most remarkable successes in recent fatigue studies may be in the application of the linear fracture mechanics concept to the fatigue crack propagation problems. Good correlation between crack propagation rate and range of stress intensity factor has generally been established for various materials and is widely used in many fields of engineering applications. However, it is known that this correlation sometimes conflicts in some details with ex~~men~ly observed phenomena of fatigue crack propagation. Some examples of disagreement may be found in the non-propagating fatigue crack problem and also in acceleration and retardation of fatigue crack propagation rate under variable stress amplitude conditions.
According to recent advances in the linear fracture mechanics, it is probable that the stress intensity factor of a crack generated at a notch or a stress raiser always increases, or at least does not decrease with increasing crack kngth [1,2]. If a unique mon~~ous relation is assumed between crack pro~fion rate -and range of stress intensity factor, the crack generated at a stress raiser should always be propagated acceleratingly until fracture. Contrary to this supposition, non-propagating fatigue cracks are surely observed experimentally at sharp notches under certain conditions.
The present authors studied this non-propagating fatigue crack problem by making elastic-plastic stress and strain analysis of a ~~ff~~on~~ CM& with t~oriozis ~~~~~~, using the finite element method f3,4]. By combining this analytical results with a critical strain range fracture criterion at a certain distance ahead of the crack, the experimental results of the non-propagating fatigue crack by Frost and Dugdale[5] were quantitatively well interpreted [3,4]. Fukuhara[6] also obtained similar results, although the analysis was completely elastic. However, the behavior of crack opening and closure predicted by these analyses [4,7] was quite different from that expected from the experimental observations [8].
Recently Elber [9] pointed out that closure of a propagating fatigue crack occurred under zero to tension loading and that the rate of crack propagation under variable amplitude loading was well related with the effective range of stress intensity factor, which was determined on the basis of the real behavior of crack tip opening. From previous work [3,4], it has been known that the analysis of tire stuttinary crock cannot predict this type of crack behavior.
In this paper, therefore, the el~~c-plastic analysis of a prop~~g fatigue crack is Con~Gted under cyclic loading, using the finite element method. Referring to this analytical results, the non-propagating fatigue crack problem and the fatigue crack propagation rate under variable stress amplitude loading will be discussed in terms of the eff cctive stress intensity factor and the strain range of several finite elements in the vicinity of the crack tip, both of which may reflect more or less the real opening and closure behavior of the propagating fatigue crack.