Use of acoustic emission and ultrasonic techniques for monitoring crack initiation/growth during ratcheting studies on 304LN stainless steel straight pipe

• Acoustic emission and ultrasonic study of crack initiation/growth during ratcheting.
• Straight pipe specimen of 304LN stainless steel under reversed four point bending.
• Acoustic emission revealed crack initiation between 826 and 845 cycles.
• Ultrasonic showed the crack after 1000 and final rupture occurred after 1203 cycles.
• Superior sensitivity of acoustic emission for early detection of fatigue cracks.

This paper describes the use of acoustic emission (AE) and ultrasonic techniques for monitoring crack initiation/growth during ratcheting studies on a straight pipe made of 304LN austenitic stainless steel under reversed four point bending. Generation of higher AE activity and AE events with higher peak amplitudes accompanied by AE cluster formation in a localized region between 826 and 845 cycles revealed the initiation of crack during the ratcheting test. Ultrasonic examination by using multiple skip technique conducted periodically at regular intervals of 25 cycles confirmed the formation of the crack only after 1000 cycles. Final rupture of the pipe occurred by through-thickness cracks after 1203 and 1225 cycles at two locations. In the range of cycles where the cracks grew, AE increased with the accumulated cycles, indicating good correlation between crack growth and AE. Acoustic emission was correlated with ballooning. While the ballooning rate decreases with cycles, count rate for different sensors increase. This has been attributed to the superposition of the generation of AE due to crack growth which increases continuously to that generated by circumferential change in strain. The superior sensitivity and reliability of acoustic emission for early detection of fatigue cracks during ratcheting test in pipe specimen is demonstrated.