M250 grade maraging steel is being used as casing material for large solid rocket motors, due to its good fracture toughness and high strength to weight ratio compared to other steels. Essentially the inert weights of a launch vehicle have to be maintained at bear minimum in order to gain a corresponding advantage in payload. The fracture mechanics based design of casing thickness is around 7.8 mm with low margin of safety. The important considerations for such design include the initial size of the flaw, critical flaw size and sub-critical flaw growth characteristics. For M250 maraging steel casing the critical flaw size has been worked out to be 5 x 2 mm tight crack equal to 50% reflection from AMS 'G' notch (2.5 mm long, 1.3 mm deep, 0.26 mm width) which has to be detected by NDT with 90% probability at 95% confidence level. Ultrasonic testing is the only suitable NDT method sensitive for detection of planar defects like cracks. As the end use of the component is a single shot affair, UT plays a significant role in detecting and evaluating all critical flaws. In case of sub-critical flaws, it is required not only to size the flaw but also characterize it for taking right decisions. As the conventional methods of flaw sizing are inadequate to address the critical flaw parameter, signal processing to analyse the wave characters of flaw echo is applied in certain cases. While presenting a typical case, relevant emerging techniques for flaw analysis are outlined in this paper.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 2, pages 505 - 508 Publisher:Ashgate Publishing Company
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