Detection of the onset of fatigue crack growth in rail steels using acoustic emission

Acoustic emission has held promise for quantitative evaluation of the extent of crack growth in metallic materials during fracture and fatigue. A correlation between acoustic emission and the stress intensity factor was derived which allows prediction of the stress intensity factor (K or ΔK) from acoustic emission measurements. At very low values of ΔK, corresponding to the onset of fatigue crack growth, it was expected that acoustic emission would detect this event. The experimental procedure to verify this assumption involved high cycle fatigue of four-point bending specimens of a number of rail steels. The fatigue testing was stopped whenever the first indication of acoustic emission activity, manifested by a measurable ΔK, occurred. This was followed by optical and scanning electron microscopy to detect the sources of cracking in the steels. The results show that, in the rail steels, cracks grow near inclusions. Crack lengths as small as 0.009 mm were detected and the success rate in the detection of the onset of crack growth was 100%.