An investigation of the relationship between contrast and azimuthal angle for imaging porosity in graphite/epoxy composites with ultrasonic polar backscatter

The relationship between contrast and azimuthal angle in the ultrasonic polar backscatter technique was investigated, with the working hypothesis that the contrast between porous and pore-free regions should decrease as the azimuthal angle of insonification nears perpendicularity to any fiber orientation. Porosity was simulated by the introduction of spherical hollow-carbon inclusions, uniformly distributed, in a localized region of a 16-ply uniaxial graphite/epoxy composite. Two-dimensional integrated backscatter images were acquired for azimuthal angles (measured relative to the fiber axis) of 0°, 45°, 65°, 70°, 75°, 80°, 85°, 90°. For each azimuthal angle 169 sites in a pore-free region and 169 sites in the `porous´ region (containing 2% volume fraction of `porosity´) were averaged to obtain a spatially averaged integrated backscatter value for each region. As the azimuthal angle of insonification was varied from 0° to 90°, the contrast decreased monotonically from a value of 9.1 dB to a value of 0.8 dB. The results indicate the feasibility of this approach for determining the upper limits of detectability of porosity in composites of increasingly complex fiber orientations