SYMPOSIA PAPER Published: 01 January 1995

Fracture Toughness and Critical Crack Sizes for the Space Shuttle Solid Rocket Motor D6AC Steel Case


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.

Author Information

Newman, JC
NASA Langley Research Center, Hampton, VA
Bland, JD
NASA Marshall Space Flight Center, Huntsville, AL
Berry, RF
NASA Langley Research Center, Hampton, VA
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Developed by Committee: E08
Pages: 799–821
DOI: 10.1520/STP16418S
ISBN-EB: 978-0-8031-5311-0
ISBN-13: 978-0-8031-1996-3