Materials Science Forum Vols. 830-831

Abstract: Rolling contact bearings often referred as antifriction bearings are found in many applications where rotating components are involved. Extreme operating conditions, counterfeit or inferior quality bearings, inadequate lubrication, etc often lead to bearing failures. Each rolling contact bearing failure leaves traces of evidences on what caused its failure. Failure analysis of one such bearing is the subject of this paper.One of the engine ground tests was aborted due to the failure of a self-aligning deep groove ball bearing used in turbo-pump of a liquid rocket engine. Series of engine ground tests are conducted as a part of injector acceptance for a liquid rocket engine. The bearing failure occurred in the seventh engine test of one of such series of ground tests. The bearing mentioned here is made of SAE 52100 steel. Post-test hardware inspection revealed that the bearing and the dynamic seals on either side of the bearing are in failed condition. Inner race of bearing was found blackish in appearance with indentation and corrosion marks. Localized black spots were observed in outer race. Bearing balls were found in deformed condition. The paper highlights the failure analysis of the bearing, root cause of the bearing failure and recommendations to avoid such failures.

Abstract: Austenitic stainless steel AISI 316L is used for components of cryogenic engine in satellite launch vehicles due to its better mechanical properties at low temperatures. In one such application, AISI 316L stainless steel bellows are used in electro-pneumatic command valve of a cryogenic engine. This valve employs a hydro-formed bellow of 0.14mm thickness as an actuator element. When one of the electro-pneumatic command valve was vibrated without pressure, crack was noticed at the inner diameter of the bellow. Detailed metallurgical analysis indicated that the cracking to be due to fatigue.

Abstract: Maraging steels have attractive combination of strength and toughness making them ideal choice for a number of structural applications. In one such application, this steel was used to fabricate shear bolts in an interstage of a satellite launch vehicle. Four numbers of these shear bolts were found to fail exactly at the head-shank interface under assembly condition. The failure has occurred when the structure was assembled with bolts to the specified torque of 43N-m. Detailed investigations revealed that the fasteners were under assembly loads and exposed to marine environment. The cracks initiated at the corrosion pits located at the head-shank interface propagated inwards in a transgranular mode. Fractography revealed brittle transgranular features with cleavage facets and corrosion products on the fracture surface. The EDS analysis at the corrosion pits near the fracture edge indicated the presence of chlorine. Based on detailed metallurgical analysis, it was concluded that the shear bolts failed due to the “Stress Corrosion Cracking”.

Abstract: In this research, the analysis has been made amongst the different 'theories of failure' for composite materials. All theories which include bi-axial strength envelopes and stress–strain curves for a range of multi-directional laminates, loaded under uni-axial or bi-axial conditions.The predictions and experimental data have been analyzed, to identify the strengths and weakness, together with a ranking of the overall effectiveness of each theory, which has been made by the AHP-MCDM tool to facilitate the reader to select the best theory for given design situation.