Inspection of Inhomogeneities in Carbon/Phenolic Matrix Composite Materials Using NDE Techniques

Abstract

Carbon matrix composite (CMC) materials consist of carbon fibers embedded in a carbonaceous matrix. Due to low density, high thermal conductivity and excellent mechanical properties at elevated temperatures, the CMCs are applied to aircraft brake disks. The properties of CMC are dependent on the manufacturing methods used for production and fiber arrangement. It is desirable to perform nondestructive evaluation of material properties and part homogeneity for structural integrity of CMC brake disks. In this paper several ultrasonic techniques were applied to carbon matrix composites for the evaluation of spatial variations in material properties which are attributable to the manufacturing process. For a carbon matrix composite manufactured by chemical vapor infiltration (CVI) method, the spatial variation of ultrasonic velocity was found to be influenced by the densification behavior in CVI process. Ultrasonic velocity and attenuation depend on a density variation of materials. Low frequency through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. Also a motorized system was adopted to measure ultrasonic velocity at a local point in CMC materials under the same coupling conditions. Manual results were comparable with those obtained by the motorized system using dry-coupling ultrasonics and through transmission method in immersion. Pulse-echo C-scans were used to image near-surface material property anomalies during CVI process.