QUANTITATIVE NON-DESTRUCTIVE EVALUATION OF CRACK LENGTH AND STRESS INTENSITY FACTOR IN FATIGUED AUSTENITIC STAINLESS STEEL

The present study proposes a new non-destructive method not only for estimating length of fatigue cracks but for quantitatively evaluating maximum values of stress intensity factor applied to plate specimens of an austenitic stainless steel SUS304 (equivalent of AISI304) fatigued at room temperature. The method makes use of plasticity-induced martensitic phase transformation that takes place in the plasticity wake regions around growing fatigue cracks. Distributions of the volume fraction of a' martensitic phase around fatigue cracks were measured with ferrite scope. The results were compared with the distributions of strain measured by the digital image correlation method and with those of vertical magnetic flux density Bz above the fatigue cracks in plate specimens magnetized by a strong elctromagnet of 0.5 T. It was revealed that the distance between the points where Bz reached the maximum and minimum values Bzmax and Bzmin had good linear correlations with crack length 2a irrespective of the stress ratio R. The Bzmax and Bzmin values also showed good linear relations with the maximum values of the stress intensity factor &lax applied. The relations are also independent of the R ratio. These results imply that not only crack length but also maximum values of the applied stress intensity factor can be quantitatively evaluated in an electromagnetic non-destructive way.