BEAM PROFILES IN WOOD FROM CRITICALLY REFRACTED LONGITUDINAL-WAVE PROBES

Critically refracted longitudinal wave techniques have the potential t o overcome some of the limitations typically encountered with ultrason ic inspection of wood materials. This paper presents the results of a continuing beam profile study for two custom designed, pitch-catch L-C R probes (200 kHz and 1.25 MHz). Samples of clear, straight-grained So uthern Pine and Douglas Fir dimension lumber were examined. The highes t signal amplitude was observed very near to the surface of the wood s pecimens (along the grain direction) with rapid attenuation occurring below the surface (at oblique angles to the grain direction). Some att enuation was attributed to growth ring effects, while most attenuation was attributed to grain angle effects. Significant increases in trans mission frequency can be realized when using L-CR waves, leading to gr eater sensitivity to material properties. L-CR waves show great promis e for the inspection of a variety of wood materials, particularly reco nstituted wood products where attenuation is a persistent problem.