Fracture Toughness/Young's Modulus Correlation for Low‐Density Fibrous Silica Bodies

Fracture toughness and static Young's modulus were measured for the low‐density silica fiber materials used as tiles in the thermal protection system of the Space Shuttle. The fracture behavior was found to be in excellent agreement with a previously formulated micromechanical model and allowed both (density) classes of tile material to be correlated to a single function. A similar correlation was also found between strength and Young's modulus, which is the basis of a nondestructive evaluation test for these materials. It was shown that the value of Young's modulus determined from a dynamic test can be substantially greater than that determined in a static mechanical test. This effect must be taken into account in the correlation. Finally, it was also determined that these materials have significant variations in K c , both within and between production units, so that the strength variability in these materials is dependent on both fracture toughness and flaw‐size variations.