Quantitative Analysis of Fatigue Fractographs for Some Heat Treated Steels

Abstract

Microfractographic aspects of fatigue fracture were quantitatively studied in relation to the strength level of materials and stress ratio. The specimens used were heat-treated low carbon and low alloy steels with 4 levels of Vickers hardness (H_V 140, 330, 430, and 480). Fatigue tests were conducted in the low- and high-cycle regions under axial load conditions (stress ratio R=-1 and 0).
The striation pattern was observed for all the materials and its area fraction tended to become maximum at a certain value of stress intensity factor, ΔK. The upper limit ofΔK at which the striation disappeared increased with an increase of the Vickers hardness of materials.
The dimple was found simultaneously with the striation and its area fraction increased with an increase of ΔK. It was shown that the average dimple size for different materials fell on a single linear line when plotted as a function of the cyclic plastic zone size at the crack tip on a log scale.
The intergranular fracture was also observed among striation patterns except for the material of H_V 330. The area fraction was decreased with increasing ΔK for the materials of H_V 140 and H_V 430. The applicability of these quantitative microfractographic data for failure analysis was pointed out.