In situ monitoring of crack growth in mild steel under closure conditions using a piezotransducer array

This paper reports on an experimental study into the application of surface -mounted piezotransducer elements to the detection of remote crack growth u nder loading conditions promoting severe closure. Crack growth is inferred from measurements of the acoustic power transmitted through lamb modes acro ss wave paths bridging the crack. It is shown that the proposed arrangement is able to detect sub-millimetre crack growth despite the presence of clos ure, although variations in the degree of closure were found to cause large sporadic fluctuations in the transmitted acoustic power. Although informat ive of crack-state, these episodes could impair the proper interpretation o f the sensor response data, highlighting the need for a cautious and robust approach to the application of the method. In the present implementation, the piezoceramic elements are placed in regions exposed to cyclic stress, r aising the issue of element durability. More importantly, no evidence of pi ezoceramic degradation was found in the course of extended testing at worki ng stress levels, bolstering confidence with respect to the application of these elements to long-term in situ health monitoring.