| Popis: |
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 degrees , 45 degrees , 65 degrees , 70 degrees , 75 degrees , 80 degrees , 85 degrees , 90 degrees . 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 degrees to 90 degrees , 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. > |
