Fracture toughness and critical crack size tests were conducted on a variety of fracture specimens that were machined from actual Space Shuttle solid rocket motor (SRM) D6AC steel cylindrical case segments. Plane-strain fracture toughness tests were conducted at four temperatures ranging from −30‡C to 55‡C. Average values ranged from a low of 87.6 MPa-m1/2 at −30 ‡C to a high of 120.3 MPa-m1/2 at room temperature. Fracture tests were also conducted on seven different crack configurations that simulate details in the SRM field and factory joint locations. These configurations included: surface and through cracks, corner and through cracks at an open hole, and through and corner cracks at a pin-loaded hole. Elastic stress-intensity factors at failure, KIe, were found to be constant within ± 10% for the range of crack and specimen sizes tested. The fatigue pre-cracking stress levels were found to have a strong influence on KIe. An effective elastic fracture toughness, accounting for crack-closure effects, was found to be nearly constant for a wide range of fatigue pre-cracking stress-intensity factor levels.
Fatigue tests were also conducted on simulated SRM case-segment joint leak-check port specimens to determine crack initiation locations, fatigue-crack growth and critical crack sizes. Eddy-current probes, identical to those used in actual hardware, were used to monitor crack growth. Specimens were cycled at either proof-test load or flight-load levels at various temperatures until failure. Critical crack sizes obtained were greater than 25 mm in length and required more than 2,700 cycles to grow from a damage-tolerant crack size (1 mm) to failure.