Formation Of Non-propagating Cracks Research Articles

Fatigue notch sensitivity and the notch size effect

It has long been known that the fatigue notch factor K F, given by σ w0/ σ w, where σ w0 is smooth specimen fatigue strength ( R = −1) and σ w is the fatigue strength of a notch-containing specimen, is less than the theoretical stress concentration factor, K T. Empirical rules have been proposed by Peterson and by Kuhn and Hardrath for the estimation of K F which involve the notch radius and a material constant. El Haddad, Topper and Smith provided one means for understanding the factors determining K F. The present paper provides an alternative approach based upon a consideration of the development of fatigue crack closure. The effect of crack closure on the formation of non-propagating cracks is also discussed.

On crack closure and the notch size effect in fatigue

A method for determining the fatigue notch-size-effect is presented based upon the development of closure in the wake of a newly formed crack growing from a notch. A comparison with experimental results is in accord with predictions. The effect of crack closure on the formation of non-propagating cracks at notches and on notch sensitivity is also discussed.

The mechanism of the formation of non-propagating fatigue cracks

AbstractNon-propagating cracks were formed in sharply notched iron specimens stressed for 107 cycles or more at a stress slightly below the notch fatigue limit. These cracks extended markedly at the retest with the same stress after annealing at 973 K. Tangled dislocations of extremely high density, developed ahead of the cracks, were almost annealed out during the annealing. Cyclic strain-hardening due to these dislocations seemed to relate to the formation of non-propagating cracks in iron specimens. On the other hand, such cracks were not observed in stainless steel showing a high strain-hardening character. From these results as well as the fact that these cracks can be formed in very sharply-notched specimens only, it is concluded that the formation of non-propagating cracks in machined notches is influenced by three factors: (1) the amount of cyclic strain-hardening, (2) the rate of cyclic strain-hardening, and (3) the stress field associated with notch sharpness.

Effect of tempering temperature on notch sensitivity of quenched steels

1. The variation of the notch sensitivity with the notch radius differs for quenched steels tempered at high and low temperatures. With a large notch radius the notch sensitivity of steel tempered at high temperature is close to unity and decreases sharply with decreasing notch radius. The notch sensitivity of steels tempered at low temperature remains low both with sharp and blunt notches. 2. The reason for the sharp reduction of the notch sensitivity with a small notch radius after tempering at high temperature is the formation of nonpropagating cracks. 3. After low-temperature tempering the second-order residual stresses favor nucleation of a fatigue crack at some distance from the surface, which in the presence of a stress gradient in the notch leads to an increase of the fatigue limit of notched samples and lower sensitivity to stress concentrations.

Non-propagating fatigue cracks in an aluminium-1/2% magnesium alloy

Non-propagating fatigue cracks have been observed at the roots of sharp notches in specimens of a single-phase Al-1/2% Mg alloy. The rate of fatigue crack growth, and the extent of cyclic deformation on either side of the crack, decrease with increase in length of crack. The process of formation of non-propagating cracks can be repeated in the same specimen by means of intermediate anneals. The results are discussed in terms of the cyclic strain-hardening of the material.