Failure criteria for some polyurethane propellants

Abstract Experimental and theoretical studies were made on failure criteria applicable to simple case‐bonded hollow cylindrical grains. The minimum strain at break at −75°F. for uniaxial tensile test was found to be at least twice the hoop strain at −75°F. of a thermally cycled, case‐bonded hollow cylinder of propellant when no failures were observed, whereas, when the minimum strain at break was equal to or less than the hoop strain of the cycled cylinder, the grain always cracked. For uniaxial tension, where the rate of elongation and temperature are controlled variables, the failure elongation varies from specimen to specimen and from batch to batch. A study of batch variability of the uniaxial test showed that a test at any test temperature from −40 to 180°F. could be used to characterize batch quality. The correlation of breaking strain at different rates and temperatures with the use of the WLF technique on one batch was found to be improved if an additional vertical shift factor was added to account for the lowering of the maximum failure elongation for temperatures below 0°F. However, by using a diagonal shift with rate, a graph of failure elongation versus temperature shows lower variability than does the WLF analysis.