ISSN:
1089-7550
Source:
AIP Digital Archive
Topics:
Physics
Notes:
The influence of surface roughness on the detection of subsurface scatterers using phase-coherent, ultrasonic immersion transducers is reported. Experiments were conducted with phase-coherent transducers, immersed in a liquid bath, that insonified samples with roughened surfaces. Measurements were made of: (1) the signal-to-noise ratio for a subsurface void when viewed through a rough surface; (2) the frequency-dependent loss (between 2 and 20 MHz) that was induced in the transmission and reflection coefficients of the coherent beam for flat aluminum plates with root-mean-square roughness varying between 5 and 50 μm; and (3) the effects of surface roughness on the material noise. The following results were found. First, surface roughness greatly degraded the signal-to-noise ratio (S/N) for some typical inspection geometries. Second, after appropriate normalization, the transmission and reflection coefficients for the coherent beam were found to be nearly universal functions of the angle of incidence, except near the critical angles. Large reductions (several orders of magnitude) in the phase-coherent signal S resulted for some typical inspection setups. The noise, on the other hand, remained nearly unchanged for the same setups. The degradation in S/N was attributed to the randomization of the phase of the coherently transmitted and reflected beams. Both the sensitivity of the signal and the relative insensitivity of the backscatter noise are explained in terms of the phase-screen approximation. This simple approximation is also used to explain the nearly universal form of the normalized transmission and reflection coefficients, and to predict that the backscatter noise may be either slightly enhanced or slightly decreased by surface roughness.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.353366
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