DETECTION OF POROSITY IN GLASS-CERAMIC MATRIX COMPOSITES USING AN ULTRASONIC MULTIPLE-GATE C-SCAN TECHNIQUE

Ceramic matrix composite (CMC) plates consisting of silicon carbide fi bers in a barium magnesium aluminosilicate matrix (SiC/BMAS) were obta ined for mechanical and thermal properties characterization. Each plat e had dimensions of 150 x 150 x 3 mm (6 x 6 x 0.12 in.) from which mec hanical test specimens, each 150 min (6 in.) long, were to be cut. To ensure that the material runs properly consolidated and free of porosi ty the plates were inspected using an ultrasonic multiple-gate C-scan technique previously developed for graphite epoxy composites. This tec hnique allowed the placement of multiple peak-detection gates between the front and back surface echoes on the A-scan signal. Because each g ate recorded tire amplitude variation for a very narrow time-of-flight range, the frequent fluctuations in signal amplitude due to the inhom ogeneity of the material affected one or two gates at times, while the other gates remained sensitive to small amplitude signals from defect s. The increased sensitivity allowed the detection of very small mater ial defects such as porosity. Using this technique for each plate reve aled an isolated area of manufacturing abnormalities, presumed to be p orosity, near the center of one plate. Based on the C-scan information , the pattern for cutting out the mechanical test specimens was altere d and the region containing the abnormalities was sectioned, polished, and optically inspected. Optical microscopy clearly showed extensive porosity and a region of poor consolidation in the matrix material at the depth indicated by the C-scans. Details of the multiple-gate ultra sonic C-scan technique, results of the ultrasonic evaluation, and dest ructive analysis are discussed.