Nondestructive Evaluation of Zirconium Phosphate Bonded Silicon Nitride Radomes

The performance advances of radar-guided missiles have created a need for radome materials with improved strength, toughness, and thermal shock capabilities. Zirconium phosphate bonded silicon nitride (Zr-PBSN), which has a low and thermally stable dielectric constant, high rain erosion resistance and a low-cost processing method, has been developed for radome applications in advanced tactical missiles. Pressureless sintering reduces processing costs, but is untried for radome manufacturing. The tendency for catastrophic failure requires that each radome fabricated with this material/method be inspected for defects prior to use. Visible, thermographic and ultrasonic nondestructive evaluation (NDE) methods have been tested with Zr-PBSN discs containing fabricated flaws likely to be present in a radome.

Ultrasonic C-scanning using a 0.25" diameter, 15 MHz focused transducer with a pulse-echo configuration was clearly superior at detecting cracks, delaminations, impurities, voids and porosity variation. A method for determining local porosity via the longitudinal elastic wave velocity was developed and can be incorporated into an ultrasonic scanning system. A system that uses a computer to perform all motion control, data acquisition, and data manipulation, but requiring a skilled operator for scan setup and interpretation of the data has been proposed.