Launch vehicle boosters are fabricated out of high strength metallic such as M-250 maraging steel. Quite often these hardwares are fabricated well ahead of their actual use some times running into few years. Long storage of fracture prone metallic pressure chambers pose problems associated with Stress Corrosion Cracking (SCC) in the weldments. The residual stress already present in the weldments trigger corrosion induced cracks. The paper presents details of SCC induced degradations encountered by an M-250 hardware stored for over several years. While many of the induced cracks in the case are part - through, few of these are unfortunately through - cracks giving rise to leaks during the hydro test. Although it is easily possible to identify the surface cracks and through - cracks using techniques like Dye Penetrant (DP), it is difficult to tackle the cracks if they are concealed\buried. An estimation of the size of the cracks is to be attempted through NDT techniques like radiography or ultrasonic testing (UT). Nevertheless then criticality can be assessed by means of advanced techniques like Acoustic Emission (AE). This paper gives an insight into the NDT evaluation done on the aforesaid badly corroded hardware using state-of-art NDT techniques. Some efforts to correlate the evaluation done by UT and AE is presented. It is seen that whenever there is branching of the cracks, it is difficult or impossible to size it no matter what the technique is used. Nevertheless an active technique like AE plays the vital role in estimating the severity of part - through or through - cracks especially when the severity is governed by such factors as orientation of cracks and the stress field. The studies presented in the paper presents certain interesting features brought out by the advanced NDT techniques. Efforts were also made to assess the severity of cracks through theoretical estimation of burst pressures. Correlation of theoretical estimation with AE in assessing the safety margin was also attempted. The paper brings out the above estimation for various types of joints having degree of bending and membrane stresses.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2417 - 2422 Publisher:Ashgate Publishing Company
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