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Case Study: Failure Analysis of Diesel Engine Crankshaft
Three pieces of a broken crank shaft of a 1300 HP V-12 twin-turbo charged, inter-cooled Man Diesel Engine from a 61 Ft Yacht were submitted for failure analysis. Two main bearings and two rod bearing halves were also submitted.
ITS© Figure 1: Crankshaft in as received condition
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Key Words Car, Engine, Diesel, Shaft, Failure Analysis, Bearing Alloy Steel Circumstances leading to failure Visual Examination of General Physical Area
Excessive stress and failure of the welds.
The shaft had fractured on a connecting rod journal. The fracture extended from a fillet to a diagonally opposite fillet and further extended through the wall, thus leaving the crankshaft in three pieces. The middle piece that hd a complete fracture face was totally black in color. Fine fracture features in the projecting zones were obliterated because of rubbing with the mating surfaces was observed. Photographs submitted with the crankshaft showed that two pistons had chipped edges. One connecting rod had blackening too. Both of the journal bearing rods had worn, blackened, squeezed out and detached from the base. The base of one of the bearings had heat tinting while the other still maintained its original blue.
ITS© Figure 2: The broken middle piece.
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ITS© Figure 3: Fracture surface on the broken middle piece.
ITS© Figure 4: The journal bearing pad halves
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ITS© Figure 5: The journal bearing base halves.
One cylinder sleeve showed a crack starting from the edge. No obvious damage was observed to the cylinder heads at the incident site though the possibility of valve damage was not ruled out. Six push rods were also bent.
ITS© Figure 6: Damaged pistons and connecting rods.
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ITS© Figure 7: Sectioning location on broken middle piece of the crankshaft.
Figure 8: Scoring at surface of broken journal
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Chemical A chemical analysis conducted on the crank shaft journal Analysis Element Carbon Manganese Phosphorus Sulphur Silicon Chromium Nickel Molybdenum Copper Vanadium
Crankshaft Percent 0.37 1.24 0.008 0.056 0.30 <0.06 <0.06 0.03 0.01 0.13
SAE 1536 Percent 0.30/0.37 1.20/1.50 0.040 Max 0.050 Max 0.15/0.30 -----------
The shaft is fabricated from an SAE 1536 carbon steel, the requirements of which are shown above for comparison only since no requirements are specified.
Macro The cut surface of section 2 showed a crack connecting the fracture surface to the fillet of the examination rod journal.
ITS ©
Figure 9 :Showing crack connecting the fracture surface to the fillet of the rod journal.
Macro The Cut surface of section 2 revealed a crack connecting the fracture surface to the fillet of the examination connecting rod. A section of end rod journal was cut and prepared for examination. The polished sample showed that the journal surface had been hardened and the hardened layer had a thickness of approximately 0.14 inch. The micro structure of the cross section below the hardened layer showed ferrite envelops at the former austenite grain boundaries with generally spheroids cementite matrix. Partially transformed pearlite could be observed at some locations.
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A Rockwell hardness survey made on the cross section using the 30N scale yielded the following obtained: Distance from outer surface(in) Hardness Rockwell C Equivalent
0.02 30N82 C65
0.05 30N691/2 C51
0.075 30N68 C50
0.115 30N681/2 C37
Core 1 30N44 C23
Core 2 30N45 C24
A location below the fillet where the direction of the fracture changes and appeared to be the © origin was selected for microscopic examination. Microscopic cracksITSwere observed in this region. The cracks were found to be intergranular. The microstructure was found to be essentially similar to what was observed in the specimen taken from the unaffected journal, above. There was no massive inclusion or lack of homogeneity observed.
Scanning The fracture surface edges at section 3 and 6 were examined under Scanning Electron Electron Microscope. There was no indication of any fatigue markings observed. Microscopy
Figure 10: Microstructure below the fillet of the fractured journal showing a trans-granular crack. 275 Magnification
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Figure 11: Fracture Surface at location 3. 930X Magnification
Figure 12: Fracture surface at location 6 showing metal flow close to surface at location 6. 1420X Magnification.
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Figure 13: Microstructure under the hard case of a journal away from the fractured journal. 100X Magnification.
Figure 14: EDS of Black Film
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Figure 15: EDS at the dark globular feature of the microstructure.
Discussion The blackening of the journal surface indicates that the surface had been over heated. Since the surfaces of other journals were unaffected, it was clear that the heating was localized and for a short duration. This ruled out general starvation of oil arising from low oil level. There were no significant changes in the microstructure suggesting that temperatures and time were lower than when such a change would place. Scanning electron microscopic examination indicated that the fracture did not take place by fatigue. Microscopic examination at the location below the filler which would be the likely origin of failure showed no in-homogeneity, massive inclusions or other discontinuities. Scoring on the surface of the broken journal suggests contact between the pad and the journal surface with hard a foreign object like a metallic fragment, harder than the journal surface. The bearing pads are softer and are an unlikely cause of the scoring. Such a hard fragment would normally get imbedded in the pad. The frictional forces may detach the pad in such a case, as observed by discoloration of one of the bases, and block the lubrication hole in the shaft, eventually depriving the journal of oil supply. Such an event will manifest itself by interactive features such as localizes heating, softening, wear and squeezing out of the bearing metal and imbalance of the shaft. The unexpected combination of bending and torsion stresses thus produced exceeded the design limits of the shaft which gave way along its weakest plane, the diagonal distance between the fillets. The damage on the small end of the connecting rods was consequential to fracture.
Conclusion From above observations and discussion it is concluded that the failure of the crankshaft occurred because of a combination of bending and torsional stressed beyond design. The stresses were caused by imbalance of the shaft due to localized heating. Ingress of foreign matter could be the cause of the observed localized heating.
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