Correlation of acoustic emission with stress intensity factor and plastic zone size for notched tensile specimens of AISI type 304 stainless steel

Correlation of acoustic emission (AE) total counts N with stress intensity factor K and plastic zone size r_p has been examined for data generated during tensile deformation of notched specimens of nuclear and commercial grade AISI type 304 stainless steel. The variation of total AE counts with stress intensity factor in log-log scale has indicated that N is related to K as N=AK^m where A and m are constants but the magnitudes of A and m are different at low and high K regimes. Both the steels indicate higher values of m up to macroyielding than those obtained from analysis of AE data between macroyielding and the stress corresponding to K _max values in the experiments. The magnitudes of m were found to be higher for the commercial grade steel than that for the nuclear grade one and dependent on thickness. Analysis of AE data has also indicated that the relationship between N and r _p can be expressed by the equation N=αr^β_p. The value of the constant β was experimentally found to be 1.3 via microhardness measurements. This value is higher than the theoretically assumed value of β∼ 1in the literature. The value of m in the equation N=AKm can be obtained from the value of the exponent β of the relationship N=α r^β as m=2β and such values of m can be correlated to the directly estimated values of the exponent in the N-K relationship. The amounts of strain induced α‘ martensite formed at notch tips in the two steels were found to increase with increasing applied stress below the nominal yield strength of unnotched specimens.