Subharmonic phased array for crack evaluation (SPACE)

Crack depth is one of the important factors determining material strength. Hence, the accurate measurement of crack depth is essential to ensure the reliability of aged structures and manufactured products. If cracks are open, crack depth can be measured by ultrasonics because ultrasound is strongly scattered by the crack tip (Fig. 10.1a) [1]. However, if cracks are closed because of compression residual stress [2, 3] and/or oxide debris generated between the crack faces [4], ultrasonic testing can result in the underestimation (Fig. 10.1b) or nondetection (Fig. 10.1c) of cracks since ultrasound penetrates through closed cracks. Subharmonic phased array for crack evaluation (SPACE) is a novel imaging method for measuring closed-crack depths [5, 6]. SPACE uses the subharmonics generated by short-burst waves and a phased array algorithm with frequency filtering. It enables the precise measurement of closed-crack depths. This chapter starts from fundamental aspects of subharmonic generation at closed cracks. It then describes the principle of SPACE and its application to several types of closed crack.