Da. Stubbs et Lp. Zawada, DETECTION OF POROSITY IN GLASS-CERAMIC MATRIX COMPOSITES USING AN ULTRASONIC MULTIPLE-GATE C-SCAN TECHNIQUE, Materials evaluation, 54(7), 1996, pp. 827-831
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.