Development of a power-law crack growth model for a rocket motor propellant exhibiting nonlinear viscoelastic behavior

The focus of this work is the examination of the risk posed by the presence of a shear crack at the inhibitor bondline in a common rocket motor design using fracture mechanics. Cracks in propellant increase the available surface area for combustion, which may cause failure through over pressurization of the case. Although prediction of the onset of crack growth is important; prediction of the rate of crack growth is critical in the present application. A successful motor firing may still be achieved if the burning rate of the propellant exceeds the rate of the crack growth.; The objective of this research is to develop a procedure to determine instantaneous crack lengths from test data so that coefficients for a power-law crack growth model could be determined. The power-law model relates effective crack speed to effective stress intensity factors. Once the crack growth power-law model is fit, conclusions regarding the effects of pressure and presence of a bondline on the resistance to crack growth were made.; In many cases, such as when an environmental chamber is used, the crack length in fracture specimens cannot be directly observed, and therefore, an indirect method for determining crack length is needed. In the study described here, a series of Oblique Tension/Shear (OTS) fracture specimens were tested in tension. Samples were extracted from bulk and bondline sections of a dissected rocket motor propellant grain. An approach was developed to extract the softening effects due to distributed damage from the fracture test data such that softening related only to macro-crack growth remains for use in determination of instantaneous crack lengths. Excellent agreement was achieved between the predicted crack lengths and crack lengths extracted from video when the latter was available.