Fatigue Crack Propagation in Biaxial Stress Fields

Abstract

Abstract--Biaxial tension compression fatigue tests were conducted with cruciform shaped specimens in a closed-loop servo hydraulic testing machine. The effects of static and cyclic non-singular stresses acting parallel to the crack plane on the crack growth rate are discussed based on the experimental observations 0 1 crack opening behaviour and fractography. Those non-singular stresses did affect the growth rate significantly under certain conditions. The range of crack-tip opening displacement was found to be a better parameter in correlating the growth rate than the stress intensity range or its elfective range. The rate tended to increase with increasing non-singular stress which is correlated to the opening displacement range. This tendency was explained by the shift of fracture mechanisms to a more brittle type due to a higher elevation of hydrostatic stress near the crack tip for the case of a larger non-singular stress term.

On the mechanism of fatigue crack propagation in ductile metallic materials

“…In the author's opinion for each ductile metal or alloy there exists a unique relation between ΔCTOD and da / dN as long as the static fracture does not dominate the processes in front of the crack tip. There are materials, where there exists a simple linear relation between da / dN and ΔCTOD as shown in examples in this paper; however, there may be materials with a progressive increase of da / dN with increasing ΔCTOD, 55 the crack tip blunting angle and the ratio of the part of CTOD, which generates the new fracture surface and the part of CTOD, which is caused by a deformation of the crack flanks in the vicinity of the crack tip may cause a deviation from the linear relation between ΔCTOD and da / dN .…”

The Effect of Stress State Characteristics on the Surface Fatigue Cracks Growth Rate Taking into Account Plastic Deformations

“…Therefore, a stress biaxiality factor in a pipe wall λ= σ ln /σ hp can be more and less than zero; for the majority of spherical tank wall points (for example, in the point A) under an excessive pressure the biaxiality factor is The formula for the assessment of the fatigue crack growth rate under biaxial loading was suggested in the paper [20]. This formula was developed based on the well-proved Paris formula and looks like n I da dN C 1 k K , (1) where x y is the load biaxiality factor defined as the relationship of stresses parallel to the crack plane…”

“…From early papers, Tanaka et al [1] carried out the most serious research examining the fatigue crack growth rate under biaxial loadings. Biaxial fatigue tests were conducted on cross-shaped samples by a hydraulic test machine.…”

Fatigue Crack Growth Under Biaxial Loading

“…Linear elastic fracture mechanics predicts that the component of stress parallel to the crack does not contribute to the singular terms and thus has no effect on fatigue crack growth. However, experimental evidence has provided conflicting indications; the effect of an applied static or cyclic tensile stress parallel to the crack is sometimes considered to increase, decrease or have no effect on the growth rate [1][2][3][4][5][6][7][8][9][10]. Similar conflicting observations were reported for the application of a compressive stress parallel to the crack [4-113. One of the factors responsible for these anomalies is that nominal stress for each arm of a cruciform specimen are used for the correlation of experimental data.…”

“…For a cruciform specimen, the nominal stress in the x-direction could introduce a local stress in the y-direction, and vice versa. When local stresses, such as those at the centre of an uncracked specimen, were used for correlation purposes, the effect of different biaxial stress ratios on crack growth rate was found to be small [4,6,7,10]. However, it must be noted that even with the so-called local stresses they are reference stresses only.…”