Behaviour of composite solid propellant under biaxial tensile loading

Biaxial tensile tests were carried out on cruciform specimens of composite solid propellants. Digital Image Correlation (DIC) was used to measure the strain in the gage section. Experiments were conducted at displacement rates of 1, 50 and 1000 mm/min (Y direction) to study the effect of the displacement rate on the material response. Two displacement rate ratios of 1:1 and 0.5:1 (X:Y directions) are considered to understand the influence of biaxiality of loading on the mechanical response. The biaxial experiments were also conducted at temperatures of 20 degrees C and 55 degrees C to understand the effect of temperature on the material response. The stress-strain response under biaxial loading was observed to be non-linear and dependent on the displacement rate. The slope of the stress-strain response in the linear region and the yield stress increased with an increase in the displacement rate for all the loading situations. There was no significant change observed in the slope of the stress-strain response in the linear region and yield stress with variation in temperature from 20 to 55 degrees C. However, a significant decrease in failure strain was observed with an increase in temperature from 20 degrees C to 55 degrees C. The mechanical response under equi-biaxial (displacement rate ratio - X:Y of 1:1) loading is compared with the uniaxial loading responses. The yield stress was observed to be 25%-47% higher during equi-biaxial tests as compared to uniaxial tests. However, the failure strain was 50%-70% lower in the equi-biaxial loading compared to uniaxial loading. The fractography and elemental mapping using Energy Dispersive Spectroscopy (EDS) of the fractured surfaces were carried out. A large number of cavities which are bigger in size and depth were observed on the fractured surface of the uniaxially loaded specimens as compared to equi-biaxially loaded specimens.