Microstructural damage and fracture processes in a composite solid rocket propellant

A study has been made of the microstructural damage and fracture processes associated with the fracture-toughness behavior of a polymer-matrix composite solid rocket-propellant material. Specifically, nonlinear-elastic fracture-mechanics tests were performed, as a function of displacement rate and temperature (-54 to 71 degree C), on center-cracked sheet test samples to determine fracture toughness in the form of J-integral resistance curves and viscoelastic resistance curves for the inert propellant H-24; in addition, in situ video imaging was employed to characterize the deformation and interaction between microstructural features and the crack-path morphology. It was found that at the lowest temperatures the increased polymer matrix strength resulted in enhanced cavitation and particle delamination, leading to larger crack-tip fracture process zones and hence to higher fracture toughness. At the high temperatures, the weaker polymer matrix was seen to tear readily and to advance the crack before significant particle delamination or cavitation would occur; this mechanism resulted in small fracture process zones and accounts for the decrease in fracture toughness with increasing temperature. The relationships between the toughness and fracture process in the material are discussed in terms of characterizing parameters for microstructural damage.