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The results show that the steel shaft failed due to intergranular stress cracking
In this paper, we present a failure analysis methodology of a structural component using a conventional 14-step failure analysis approach. This failure analysis methodology focused on observation, information gathering, preliminary visual examination and record keeping, nondestructive testing, mechanical testing, selecting/preservation of fracture surfaces, macroscopic examinations, microscopic examinations, metallography, failure mechanism determination, chemical analysis, mechanical failure analysis, testing under simulated service conditions, and final analysis and report. The application of this methodology is demonstrated in the failure analysis of a mixer unit shaft made of AISI 304 stainless steel. Using this failure analysis approach, we pinpointed the primary mode of failure and developed a means of circumventing this type of failure in the future. The results show that the steel shaft failed due to intergranular stress cracking (sensitization during welding) at the heat affected zones (weld plugs).Fracture failure analysis of an agitator shaft in a large vessel is investigated in the present work. This analysis methodology focused on fracture surface examination and finite element method (FEM) simulation using Abaqus software for stress analysis. The results show that the steel shaft failed due to inadequate fillet radius size and more importantly marking defects originated during machining on the shaft. In addition, after visual investigation of the fracture surface, it is concluded that fracture occurred due to torsional–bending fatigue during operation.A fractured in-service ship-propeller shaft (50.8 mm, i.e., 2-inches nominal diameter) was examined to determine the causes of failure and to recommend preventive measures to minimize the risk of recurrence.
The findings of the failure analysis investigation suggest strongly that the shaft failed due to rotating bending fatigue initiated from the surface and close to the keyway area. The origin is located on a surface flaw (recess or dent) of approximately 100 μm depth, which could have probably being caused either during installation, operation, or maintenance. In addition, scoring lines formed due to friction-related processes and found on the journal surface were considered as stress raisers acting as potential sites for fatigue crack initiation. Careful review of the shaft service conditions and the implementation of suitable inspection procedures adapted to the vessel planned maintenance are recommended as necessary corrective actions for failure prevention.A single part of the broken shaft (50.8 mm; i.e., 2-inches nominal diameter), along with one being still in operation, is shown in Fig.The matched piece (2nd half) of the fractured shaft was not available (probably sank after fracture). Both shafts were driven by a dual heavy duty ship engine (2 × 720 HP), transmitting rotational motion to the propellers, and they have been in service for almost 14 years. Shaft fracture, during navigation, led to significant loss of engine power and temporary loss of vessel stability, without any additional safety-related consequence. A simplified drawing that shows a general layout of the shaft and the related components along with the fracture location is shown in Fig. The above incident led to the activation of a failure analysis procedure in order to evaluate the cause of failure and recommend preventive measures to minimize the risk of recurrence.
Macrofractographic evaluation was performed using a stereomicroscope. Chemical analysis for steel grade identification was conducted using optical emission spectrometry. Hardness testing was performed using a universal hardness tester employing standard Rockwell C technique according to ASTM E-18 and Vickers hardness technique under 5 kg-force applied load according BS EN ISO 6507-1 standard. In addition, high-magnification fractographic observations were conducted on ultrasonically cleaned specimens, using a scanning electron microscope with a secondary electron detector for topographic evaluation and an energy dispersive x-ray spectrometer for elemental analysis.The chemical composition of the shaft sample, analyzed by optical emission spectrometry, is presented in Table . The material composition matches to the special high-alloy stainless steel grade, which is almost equivalent to AISI XM-19/UNS S20910 standard steel grade (austenitic steel), see Ref.
This high-alloy stainless steel offers exceptional corrosion resistance in combination to high strength and toughness.Hardness distribution (cross-sectional) obtained by employing Rockwell C and Vickers testing techniques is shown in Table. Vickers and HRC measurements are also in close agreement to the values obtained from standard hardness conversion tables. Ultimate tensile strength level, estimated from hardness, varied within ca. 1000–1200 MPa (~145 to 174 ksi).Signs of distortion due to potential shaft misalignment were not identified during rotation. Macrofractographic investigation showed clear evidence of the occurrence of fatigue failure mechanism (Figs and ). No visible shaft distortion and essentially flat fracture surface indicated the operation fatigue caused under complex rotational/in-plane/reversed bending loading mode. Industrial machine elements and engine components, subjected to various complex stress states, such as transmission of rotational motion or behaving as cantilever beams sustaining high bending moments, suffered from torsional overload and fatigue failures.