Nondestructive Evaluation of Low Density Fibrous Ceramics

In order to ensure a minimum strength for the low density fibrous tiles to be used in the thermal protection system of the Space Shuttle an NDE test that uses sonic velocity measurements to predict strength has been developed. The empirical correlation between strength and sonic velocity which is the basis of this test, is shown to be consistent with a previously developed micro-mechanical model. The model is reviewed and is shown to describe the fracture behavior of these fibrous materials regardless of their density or testing direction. Measurement of the density and sonic velocity in these materials allows Young’s modulus to be calculated and as low modulus materials tend to have low fracture toughness and strength, they can be eliminated by this test.Appropriate accept/reject criteria can be developed statistically from the strength or fracture toughness correlations but for use in design, other factors such as the size-dependence of strength, the stress fields encountered during flight, and the property variations within tiles should also be incorporated.