Ultrasonic transducers for inspecting porous materials

Three transducers have been designed and tested to determine their applicability for inspection of porous materials at 100 kHz. An electrodynamic transducer consisting of many turns of wire wrapped on a low acoustic impedance slab and placed in a static magnetic field was found to have an impedance which is still a factor of 20 greater than that of porous materials. Results obtained with a highly damped, multi-element lead niobate cylindrical transducer indicate that it is unsuited to pulse-echo studies of porous materials. An array transducer comprised of 25 lead zirconate titanate extensional mode transducers in parallel with both mechanical and electrical damping gave promising results, indicating the possibility of detecting both in-plane cracks and bonding defects.