Abstract
Recent advances in harmonic imaging of industrial materials are reviewed. The conventional ultrasonic testing, which is based on acoustic impedance mismatch between contact materials, fails to detect kissing bonds, micro damage and micro defects within a bond layer of dissimilar materials. To the contrary, the nonlinear ultrasonic method, in particular higher harmonic method, makes use of higher harmonics excited at disordered structures mentioned above under incidence of large amplitude of tone-burst waves of a fixed frequency. This review is composed of three parts; key concept of nonlinear ultrasonics, instruments for higher harmonic imaging and harmonic images of kissing bonds, nonmetallic inclusions in metal, creep damage, microcracks in SiC/Ti composite, and weld boundaries. The higher harmonic imaging could be indispensable technique for quality assurance of bonded interfaces of dissimilar materials in near future.
