Evaluation of nondestructive testing techniques for the space shuttle nonmetallic thermal protection system

ABSTRACTA program was conducted to evaluate nondestructive analysistechniques for the detection of defects in rigidized surface insu­lation (a candidate material for the Space Shuttle thermal protec­tion system). Uncoated, coated, and coated and bonded sampleswith internal defects (voids, cracks, delaminations, densityvariations, and moisture content), coating defects (holes, cracks,thickness variations, and loss of adhesion), and bondline defects(voids and unbonds) were inspected by x-rayradiography, acous­tic, microwave, high-frequencyultrasonic, beta backscatter,thermal, holographic, and visual techniques. Additional uncoatedsamples with density variations were inspected using x-rayradi­ography and microwave reflection techniques and subsequentlytested to determine their tensile strength. The detectability ofeach type of defect was determined for each technique (whenapplicable). A possible relationship between microwave reflectionmeasurements (or x-ray-radiographydensity measurements) andthe tensile strength was established. A possible approach forin-processinspection using a combination of x-rayradiography,acoustic, microwave, and holographic techniques wasrecommended.