The systematic study of the variation in tensile properties with the extent of loading in highly filled systems has received little attention even though it is important to a variety of industries. An investigation was therefore made of the mechanical properties of a polyurethan rubber loaded with uniform glass spheres. Systems containing up to 62.5 vol.% fillers were investigated in tension on a strain rate range of 0.074 to 1850 min.−1 and temperatures of −50 to +80°C. The mechanical behavior at low extensions was found to be represented by the generalized Maxwell model. The Williams‐Landel‐Ferry relationship was satisfactorily applied to the reduction of the data. Good agreement was obtained between the glassy‐state transition temperature calculated from the reduced tensile data and from experimental differential thermal analysis results. The increase in the modulus with increasing filler content was satisfactorily described by an empirical modification of the Eilers relationship. The application of the reduced tensile data to the calculation of the stress relaxation modulus is also shown. Reasonable agreement was obtained between the calculated and experimental data.
Experimental studies of propellant specimen and motor behavior during failure provide insight into the probable mechanisms of failure in a composite propellant system. At low rates and high temperatures, failure results from at least two successive processes: first, a dewetting or breaking of the binder to oxidizer bonds, and second, a tearing in the binder structure initiated at points of stress concentration such as regions of binder-oxidizer dewetting. Stress-strain analysis of a motor shows the potential failure points, and a failure analysis indicates the required mechanical properties to withstand failure. Laboratory testing has given a method of classifying propellant behavior, an estimate of material failure variability, and a method of correlating the uniaxial with biaxial tensile properties and with rate and temperature of test. Uniaxial tensile properties are obtained more conveniently by laboratory measurements, but biaxial properties appear to be related more directly to inner bore grain cracking. Good correlations were demonstrated between motor failures in thermal cycling and the failure properties of the propellant from laboratory tests.
An improved instrument has been designed and constructed for the measurement of the linear decomposition and sublimation rates of solid substances. This instrument, which is provided with automatic recording devices, eliminates many of the shortcomings of the earlier prototypes and makes possible the determination of linear rates with a precision of 1%. Important new features include (1) an integral heating element and thermocouple assembly; (2) stainless-steel and Plexiglas construction throughout; (3) heavy, solid-copper electrical-conductor supports for the heating element; and (4) a precision position-sensing potentiometer. The use of cut strands of Plexiglas 55 is recommended for periodic standardization and calibration of the instrument.
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