“…doi:10. 1016/j.ijsolstr.2005.11.006 of imperfect interfaces and cracks (Solodov et al, 1993;Hirose and Achenbach, 1993;Rudenko and Vu, 1994;Fassbender and Arnold, 1996;Nazarov and Sutin, 1997;Hirsekorn, 2001;Chen et al, 2001;Donskoy et al, 2001;Solodov and Korshak, 2002;Pecorari, 2003;Gusev et al, 2003;Biwa et al, 2004). Especially when a crack is closed, it may remain undetected by the linear ultrasonic techniques (Rokhlin and Kim, 2003) even though it can be excited ultrasonically to generate measurable second and higher order harmonic signals (Buck et al, 1978).…”