Application of acoustic emission testing in failure analysis: case studies

Abstract

The paper is primarily concerned with the detection of hydrogen embrittlement in Failure Analysis by employing acoustic emission (AE), monitoring techniques. This has been demonstrated on (1) a high carbon steel which failed by splitting during the drawing and handling opera¬tions, and (2) a pipeline steel which failed catastrophically while in ser¬vice. The results show that the presence of hydrogen in a high carbon steel can be effectively determined by monitoring AE during delayed, cracking tests. It has been observed that the AE counts rise slowly with increasing time in the initial stage of embrittlement, whereas they rise rapidly in the later stage prior to fracture. It has also been found that the initial embrittlement phase is characterized by low amplitude signals whereas the rapid crack growth is marked by high amplitude signals. The initial period of low amplitude signals is caused by the diffusion of hy¬drogen to the maximum triaxially stressed region causing microvoid to form. As the microvoid starts linking up giving rise to some critical crack size the amplitude of emitted signals increases. The results of the tensile tests that were conducted on pipe line steel indicates that the presence of notch in tensile test sample leads to a significant change in the AE activ¬ity. It shows that the counts per event in the notched specimen is higher than that in smooth wherever these is significant hydrogen pick up.