ENHANCED ULTRASONIC-DETECTION OF FATIGUE CRACKS BY LASER-INDUCED CRACK CLOSURE

Fatigue cracks are usually initiated by small geometrical irregulariti es or material inhomogeneities that give rise to sharp local stress co ncentrations. In the early stages of fatigue, small cracks often remai n hidden from conventional ultrasonic detection by stronger scattering from the very same structural imperfection that produced them in the first place. A new experimental method was developed to selectively in crease the sensitivity of ultrasonic echographic techniques for such h idden fatigue cracks by exploiting one of their most characteristic fe atures, their susceptibility for closure under compressive stress. The rmo-optical modulation by pulsed infrared laser irradiation was introd uced to produce a temporary compressive thermal stress on the surface of the specimen. The resulting dynamic closure of microcracks was dete cted by a high-frequency ultrasonic surface wave technique. It is demo nstrated that this method can be used to effectively distinguish fatig ue cracks from other structural imperfections present in the material. (C) 1998 American Institute of Physics.