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.
marine propeller shaft pdf 14
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. 1. 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. 2. 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.
The chemical composition of the shaft sample, analyzed by optical emission spectrometry, is presented in Table 1. 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. [1]. This high-alloy stainless steel offers exceptional corrosion resistance in combination to high strength and toughness.
Fatigue crack in the present case is clearly initiated from shaft circumference and propagated to the interior forming a characteristic macroscopic smooth surface pattern. Surface topography displays evidence of clearly defined beach marks, which evolved as concentric rings (Fig. 4). Beach marks or crack arrest marks constitute crack front progression marks that formed due to intermittent loading and/or development of compressive stress state ahead of the crack tip. Surface flaws, such as pits, grooves, dents etc., serve as crack initiation sites since they cause increase of local stress concentration factor (K t). Ratchet marks are also another fatigue characteristic feature (see right part of Fig. 3a), indicating the presence of multiple crack initiation sites and high stress concentration, see also Ref. [5]. Furthermore, triangular shape ratchet areas may also indicate the occurrence of complex stress state including torsional loading mode.
Secondary electron imaging (SEI) micrographs showed at a higher magnification the morphology of the various fracture zones on the shaft fracture surface (Fig. 5). The fatigue zone revealed mainly smeared areas from friction between crack mating surfaces with extensive secondary cracking (Fig. 5a, b). Moreover, poorly resolved striations were shown in isolated areas as depicted in Fig. 5c. Fatigue striations, a surface feature of fatigue crack propagation zone, constitute a microscopic fingerprint of the operation of cyclic loading. Striation pattern is formed by blunting and re-sharpening of the crack tip during stress cycling. Mean striation spacing corresponds to the average advancement of fatigue crack per load cycle. General aspects of fatigue fractographic features and the corresponding formation mechanism are also reported in Ref. [10].
Fatigue crack initiation area was viewed under low power stereomicroscope and under SEM (Fig. 6). The fatigue origin is located on a dent/pit coming most probably from mechanical damage during operation or handling/maintenance (Fig. 6a). The recess was filled with a wedge-shaped hard scale salt deposit (matched approximately to CaCO3) analyzed by means of energy dispersive x-ray spectrometry (EDS), see Fig. 6b, c and d. Selected area elemental analysis indicated severe and relatively uniform surface contamination resulted from hard scale deposits (carbonate related) that might address shaft journal cooling and operation issues. No other structural abnormality or foreign body inclusion that could be connected to the failure was identified during the fractographic study.
System prognostics: Inspection for potential excessive clearance and shaft misalignment could be proposed at the commissioning and installation stage. The option of proper analysis of system kinematics for resonant vibration frequencies could be further investigated. Moreover, the evaluation of residual stress level could be also a useful indicator for fatigue life assessment.
The engine must be designed and constructed to function throughout its normal operating range of crankshaft rotational speeds and engine powers without inducing excessive stress in any of the engine parts because of vibration and without imparting excessive vibration forces to the aircraft structure.
The operation test must include the testing found necessary by the Administrator to demonstrate backfire characteristics, starting, idling, acceleration, overspeeding, functioning of propeller and ignition, and any other operational characteristic of the engine. If the engine incorporates a multispeed supercharger drive, the design and construction must allow the supercharger to be shifted from operation at the lower speed ratio to the higher and the power appropriate to the manifold pressure and speed settings for rated maximum continuous power at the higher supercharger speed ratio must be obtainable within five seconds.
By a procedure approved by the FAA, operating limitations must be established which specify the maximum allowable number of flight cycles for each engine life-limited part. Engine life-limited parts are rotor and major static structural parts whose primary failure is likely to result in a hazardous engine effect. Typically, engine life-limited parts include, but are not limited to disks, spacers, hubs, shafts, high-pressure casings, and non-redundant mount components. For the purposes of this section, a hazardous engine effect is any of the conditions listed in 33.75 of this part. The applicant will establish the integrity of each engine life-limited part by:
If the engine is designed to operate with a propeller, the following tests must be made with a representative propeller installed by either including the tests in the endurance run or otherwise performing them in a manner acceptable to the Administrator:
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