Because of their high-temperature strength properties, silicon carbide/silicon carbide (SiC/SiC) composite materials targeted for use in turbine components were developed at the NASA Glenn Research Center. Impact damage that occurs in hostile combustion environments reduces the strength properties of components made from these materials. Therefore, it was necessary to develop nondestructive evaluation (NDE) methods to detect and characterize impact damage in SiC/SiC composite materials for material development and future inspection purposes. The complex nature of the composite weave and the amount of porosity in ceramic matrix composites makes investigation with ultrasonic methods extremely challenging. Conventional x-ray methods are not able to detect planar defects, such as delaminations. Although micro-focused computed tomography has been useful in detecting damage depth in small SiC/SiC composite samples, it has significant limitations with respect to inspection time, costs, and sample size. All these limitations necessitated the research and improvement of other inspection methods, such as pulsed thermography, for SiC/SiC composite materials.
Therefore, impact damage in 2.4-mm-thick SiC/SiC composite materials was investigated and characterized at Glenn with pulsed thermography(ref. 1). Pulsed thermography is an NDE technique that monitors the thermal response of a material over time. The surface of a component is heated with an instantaneous, uniform pulse of heat. As heat flows toward the back side of the material, disruptions in heat flow, due to subsurface discontinuities, result in localized surface temperature variations that are detected with an infrared camera.
In order to examine impact damage, researchers impacted SiC/SiC coupons with 1.59-mm-diameter steel spheres at velocities from 115 to 400 m/sec. Damage that went undetected optically was detected with pulsed thermography at impact velocities as low as 220 m/sec. A flat-bottom-hole thermal standard was fabricated to determine defect resolution limits and impact damage depth in these materials. Holes with hole-diameter-to-depth aspect ratios above 2.5 were detectable. Although detectable in the thermal image sequence, the edges of flat-bottom holes with depths 1.93 mm from the interrogation surface were not readily resolved because of their proximity to the back surface. The information acquired from the thermal standard was compared with thermal data acquired from an impacted SiC/SiC sample with known damage to determine the impact damage depth.
An illustration of how the depth of damage analysis can be applied follows. The figure shows a thermal image of an impacted sample with the cooling curves for an averaged 13- by 13-pixel area reference region and a delaminated 1- by 1-pixel area. The sample, impacted at 300 m/sec, had clearly identifiable damaged and undamaged regions in the thermal image. As shown on the cooling curve, the time of departure from the reference region is 0.031 sec. According to an equation derived from the analysis of the SiC/SiC standard (ref. 1), this time corresponds to a depth of 2.03 mm from the surface, which has an 8.5-percent error when compared with the depth of the delamination of 1.87 mm, as measured from a computed tomography image.
Cooling behavior for an impacted SiC/SiC sample with td representing the time of departure from the reference region, where the reference region represents an average 13- by 13-pixel area and the damaged region is a 1- by 1-pixel region.
In addition to impact damage, these results can be utilized to characterize manufacturing defects in samples and components made from SiC/SiC materials with thicknesses of approximately 2.4 mm. This research represents a portion of a larger effort to develop and improve pulsed thermography for the detection and characterization of impact damage in uncoated and environmental-barrier-coated SiC/SiC composite materials. Research of impact damage in environmental-barrier-coated SiC/SiC composite materials with pulsed thermography is ongoing.
Last updated: October 6, 2006
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