Statistical moments of backscattered ultrasound in porous fiber reinforced composites

Statistical moments are important tools for understanding wave propagation in random media and are applied here to backscatter measurements in porous, fiber-reinforced polyimide composites. High temperature resins, such as PMR-15, used in load critical components are much more prone to void production than traditional epoxies. Porosity was induced in thin laminates by cure cycle perturbations and quantified destructively as to void content (0.3 to 8.1%) and radius distribution (10 to 250 /spl mu/m). Measurements of the statistical moments of the scattered field were made using a single, large aperture transducer. Backscatter signals were acquired and used to calculate the rectified ensemble average, and the point signal-to-noise ratio. The lateral and axial full-width half-maximum (FWHM) were extracted from a two-dimensional auto-covariance of the field and compared to the limiting values set by the diffraction pattern and bandwidth of the measurement transducer. Results confirm that the statistical behavior of the echo amplitudes from porous fabric laminates follow predictions based on circular Gaussian statistics for void contents in the range 2 to 5%. Deviations from these predictions occur for void contents outside this range primarily due to changes in the void distribution. These results are important to the development of better void content measurement techniques and are particularly crucial when imaging specularly reflecting defects embedded in a porous volume because spatial and contrast resolution are limited by speckle and the image signal-to-noise.