LASER GENERATION AND DETECTION FOR SURFACE WAVE INTERACTION WITH DIFFERENT DEFECT GEOMETRIES

The enhancement of surface waves in the near-field of a defect has been reported by several authors. It has been demonstrated that the interaction between the incident Rayleigh wave with the reflected Rayleigh wave, plus the mode-converted surface skimming longitudinal wave, explains the significant increase in signal amplitude that is encountered as a detection point approaches a smooth machined defect inclined normal to the surface. However, this is not a typical defect geometry. For example, rolling contact fatigue in rails grows at an angle to the surface, and stress corrosion cracking grows as branched defects. We present results of experimental measurements on machined slots with varied geometries, including defects which are normal or inclined to the surface, and show the effect of branched defect geometries on the wave propagation and signal enhancement. We use laser generation and detection, and compare results with finite element method (FEM) models. We also investigate frequency enhancements for angled and branched defects, to highlight further potential measurement techniques when using scanning laser detection.