Frequency Effects in High-Frequency Pulse- Echo Ultrasonic Imaging of Titanium Metal Matrix Composites

Inspection of titanium matrix composites (TiMMCs) is required during manufacture, to ensure that defects are found before the material is incorporated into complex structures. The manufacturing process involves lay-up of fibres followed by Hot Isostatic Pressing. Individual fibres must remain in alignment and with precise separations in order to maintain the mechanical properties of the material. Finding defects at an early stage would considerably reduce production costs. We previously reported the use of high frequency ultrasound in pulse-echo mode for experimental investigations of single layer long-fibre silicon-carbide reinforced titanium structures. Individual fibres 100 μm diameter could be imaged in the frequency range 35-80 MHz. The present work extends the technique to multi-layer TiMMC structures. C-scan images were produced in which individual fibre layers have been resolved, and titanium matrix-rich regions have been identified. Using full-waveform capture and post-capture analysis, it was found that the frequency spectrum of the reflected pulse varies with the depth in the structure from which it has been reflected. Such effects are expected where the wavelength is close to the fibre spacing. The sensitivity of the frequency spectrum to these “phononic crystal” effects in the TiMMC can be used to determine the quality of the structure.