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 fatig ue. A correlation between acoustic emission and the stress intensity f actor was derived which allows prediction of the stress intensity fact or (K or Delta K) from acoustic emission measurements. At very low val ues of Delta K, corresponding to the onset of fatigue crack growth, it was expected that acoustic emission would detect this event. The expe rimental procedure to verify this assumption involved high cycle fatig ue of four-point bending specimens of a number of rail steels. The fat igue testing was stopped whenever the first indication of acoustic emi ssion activity, manifested by a measurable Delta K, occurred. This was followed by optical and scanning electron microscopy to detect the so urces of cracking in the steels. The results show that, in the rail st eels, 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 cr ack growth was 100%.