Investigation of Hydrotest Failure of Thiokol Chemical Corporation 260-Inch-Diameter SL-1 Motor Case
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CLEVELAND OH LEWIS RESEARCH CENTER
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The Thiokol Chemical Corporation 260-inch-diameter SL-1 motor case failed during hydrotest at a pressure of 542 pounds per square inch, which was about 56 percent of proof pressure. The motor case was constructed of 250 grade maraging steel plate joined mostly by submerged arc automatic welding. An investigation of the failure was conducted by a Committee composed of members from Industry and Government. Investigation showed that the failure originated from a defect that was not detected by nondestructive testing techniques prior to aging. Nondestructive testing was not conducted between aging and hydro-test. The defect causing the failure was in the heat-affected zone of a longitudinal submerged arc weld on the cylindrical section of the motor case. The area where the defect was located had been repaired by a manual gas-tungsten-arc TIG weld. The defect, which was submerged within the vessel wall and oriented longitudinally, was approximately 1.4 inches long and has a width of about 0.10 inch. Four other undetected defects of significant size were discovered after the motor case failure. All of these defects were also located beneath manual TIG weld repairs. One of these defects, which was in the same longitudinal weld as the defect causing the motor case failure, was involved in the fracture as a secondary origin. The other defects were discovered during reinspection of all welds by nondestructive testing techniques after hydro- burst.