STATISTICAL MOMENTS OF BACKSCATTERED ULTRASOUND IN POROUS FIBER-REINFORCED COMPOSITES

Statistical moments are important tools for understanding wave propaga tion in random media and are applied here to backscatter measurements in porous, fiber-reinforced polyimide composites. High temperature res ins, such as PMR-15, used in load critical components are much more pr one to void production than traditional epoxies. Porosity was induced in thin laminates by cure cycle perturbations and quantified destructi vely as to void content (0.3 to 8.1%) and radius distribution (10 to 2 50 mu m). Measurements of the statistical moments of the scattered hel d were made using a single, large aperture transducer. Backscatter sig nals were acquired and used to calculate the rectified ensemble averag e, and the point signal-to-noise ratio. The lateral and axial full-wid th half-maximum (FWHM) were extracted from 8 two-dimensional auto-cova riance of the field and compared to the limiting values set by the dif fraction pattern and bandwidth of the measurement transducer. Results confirm that the statistical behavior of the echo amplitudes from poro us fabric laminates follow predictions based on circular Gaussian stat istics for void contents in the range 2 to 5%. Deviations from these p redictions 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 part icularly crucial when imaging specularly reflecting defects embedded i n a porous volume because spatial and contrast resolution are limited by speckle and the image signal-to-noise.