ULTRASONICS IN AN ALUMINUM FOAM

Open celled foamed aluminum typically consists of an irregular frame o f solid struts with typical lengths of the order of millimeters and th icknesses of a fraction of a millimeter. Total aluminum volume fractio ns are of the order of 10%. Quoted values of static modulus and mass d ensity allow one to infer long wavelength(much greater than strut leng th) ultrasonic wave speeds. These predictions are compared with the ve locities observed in 100 kHz transient signals. At higher frequencies, where wavelengths are comparable with strut lengths, ultrasound can b e transported only diffusively. Wave speeds are unmeasurable and of li ttle meaning anyway. We therefore also report on ultrasonic measuremen ts appropriate for the high-frequency regime. These include diffusivit y, absorption and modal density. Recent literature has reported measur ements of diffusivities and absorption in other materials, particularl y in metallic polycrystals. Measurements of modal densities are new. ( C) 1998 Elsevier Science B.V.