Bond quality inspection for nonhomogeneous highly attenuating heat shield blocks

A NASA effort is underway to develop a nondestructive inspection method to evaluate the bonding condition for heat shield blocks that are bonded to a composite substrate. The initial effort is reported in this paper. The goal is to demonstrate a proof of concept for a reliable detection method using a single-sided scan capability applied from the outside of the heat shield blocks. Two representative samples with known defects are made available for the initial proof of concept; the results of one sample are reported here. The two specimens had been scanned using various nondestructive testing methods including immersion ultrasound, handheld ultrasound, air-coupled ultrasound, microwave, Terahertz, computed radiography, and shearography. These efforts were performed by industry experts and academia. All were limited in their detectability of the known flawed regions in the specimens and suffered high levels of uncertainty in the good regions. Most nondestructive evaluation methods are sensitive to detecting changes in the material property. This means that an air pocket is easy to detect in an unbounded interface but when the air pocket is absent and the two surfaces are in intimate contact (kissing bond) most methods fails to detect the difference. Electromagnetic methods (radiography, thermography, microwave, Terahertz) will not detect a change in material property. Shearography is not successful in attempting to detect changes in stiffness through the highly damping thick heat shield layer. As a mechanical wave, ultrasound is the primary method that may be able to detect a kissing bond. However, it was quickly recognized that ultrasound has to overcome a number of difficult challenges related to the nonhomogeneous and highly damping nature of the thermal shield material. Inhomogeneity causes pronounced changes in signal strength that make the distinction between good and bad areas very difficult. At the same time, scatter and attenuation become detrimental to high frequency ultrasonic signals, which are necessary to resolve echoes returning from the bond line. Conversely, low frequency sound waves provide poor spatial and temporal resolution due to the broader wavelengths associated with the signal.