Ultrasonic characterization and inspection of open cell foams

The nondestructive characterization of the material properties and the detection of hidden discontinuities in a highly porous, open cell, carbon foam using ultrasonics are presented. The propagation of ultrasonic waves in carbon foam specimens and their relationship with the elastic properties of the material are studied through theoretical modeling and laboratory experiments. The geometric and the overall elastic properties of the material are determined from microscopic data and wave propagation experiments performed under dry and fluid coupling conditions. A broadband, hybrid system based on laser generation and air-coupled detection of ultrasound is first employed for primary wave (P-wave) characterization. Significant discontinuities within the specimen are then detected using a narrowband, air-coupled ultrasonic setup as well as an automated fluid-coupled real-time intelligent ultrasonic setup. The use of the discrete wavelet transform is investigated as an effective tool for signal denoising in air-coupled ultrasonic probing of the foams. A simple, one-dimensional model, based on a periodic spring-mass system and a previously developed homogenization theory, is used to predict the wave speed.