Introduction
One Eighty was assigned to investigate the root cause of a failure which occurred within the main engine of a large fishing vessel. The component which initially broke, and subsequently caused a series of catastrophic failures, was identified to be the lower connecting stud on the big end of one of the connecting rods. The image below shows the location of this stud within the con-rod sub-assembly.
Figure 1: Example of connecting rod, big end and studs. The red arrow indicates the stud believed to have failed first.
The physical evidence of the failure was examined and the metallurgical and mechanical properties of the failed stud were tested and compared with those of a reference stud which was fit for purpose throughout its lifetime.
Investigation
Visual Inspection
Visual inspection of the fracture surface of the failed stud showed clear signs of fatigue crack initiation and propagation, followed by fast fracture. Fatigue failure occurs as a result of small but repeated stresses which cause progressive cracking. A crack will grow outward, leaving striations or beach-marks evident on the failure surface, until the intact material can no longer withstand the conditions and final fast fracture occurs. Figure 2 below shows Zone I where crack initiation occurred, Zone II where crack propagation occurred and where beach-marks are evident, and Zone III where fast brittle fracture occurred.
Figure 2: Failure surface of the bottom stud on the con-rod big end and the zones of fatigue failure
Dye Penetrant Testing
As the crack was observed to have initiated at the root of a thread, all the studs from the other units were inspected by means of die penetrant testing to check for similar defects.
Dye penetrant testing is a non-destructive testing technique which involves first covering a surface with a low surface tension liquid dye. The dye fills the cavities of any surface-breaking discontinuities. The surface is then wiped clean before a white developer is applied. Coloured stains will then make the location of surface defects evident as the dye is drawn into the developer by capillary action.
The dye penetrant test indicated that no cracks were evident on any of the other con-rod studs. This proved that the failure of the bottom stud was an isolated incident and not a result of a global problem inherent in all similar engines.
Figure 3: Connecting rod stud showing no indications after dye penetrant testing. This picture is
representative of results on all other tested studs
Dimensional Testing
The failed stud showed the presence of local necking just below the taper leading to the thread. For this reason, the diameters of the failed stud, the reference stud and all other collected studs, were measured and compared.
Figure 4:
Connecting rod failed bottom stud showing the points of diameter measurement and area of necking
The fact that local necking began, means that the stud was strained past its yield strength and therefore it can be concluded that, at some point during the failure sequence, the stud was subjected to a stress greater than its material strength.
It was a possibility that the straining which resulted in necking may have been produced during the installation of the stud. The fitting process involves bolt tensioning where the fastener is stretched axially with a hydraulic piston to an achieve preload in the stud and nut assembly. Further investigation however, concluded that the stud could not have been overstretched during installation. Thus, the necking must have occurred during the failure sequence.
Mechanical and Compositional Analysis
Vickers hardness testing, round tensile testing, microstructural analysis and a spectrographic (compositional) analysis were all performed to compare the failed stud with the reference stud. The results showed no differences in chemical, mechanical or metallurgical properties which could have caused the failure under investigation.
Examination of the Failed Studs Thread and Nut
The lower stud of the con-rod’s big end was identified to have been the component to fail first, but it was evident that the upper stud also failed catastrophically at some stage in the sequence. However, as the investigation developed, it became clear that the upper stud played a role in the initial failure. The evidence which supports this was found when examining the upper stud’s nut and thread. Figure 5 below shows how the nut has mushroomed. This plastic deformation around the nut’s mating surface indicates that the nut worked loose during operation. Most often this is a result of insufficient tightening of the fastener subsystem.
Figure 5: Mushrooming on Upper Stud Nut
Further indication that the nut worked loose during operation is provided by the flattened length of thread evident on the stud just below the mushroomed nut. As Figure 6 shows more clearly, the stud’s thread contained within the nut shows no sign damage while the exposed thread just beneath it is severely deformed.
Figure 6: Upper Stud’s Deformed Thread (Red Arrow) and Undamaged Thread Protected by Mushroomed Nut
Figure 2: SEM image showing cracks propagating from the interior of the pipe
Sequence of Failure
With all the evidence of the investigation thoroughly examined, it was possible to determine the sequence of events which lead to the catastrophic failure of the fishing vessel’s main engine. This sequence is described below:
- The upper stud and nut of the connecting rod in question, was insufficiently tightened during installation.
- The upper stud and nut assembly worked loose during operation.
- The lower stud became overloaded which caused necking.
- Fatigue cracking initiated in the lower stud’s threaded section.
- The crack propagated through the stud’s cross section over time.
- Catastrophic failure of lower stud occurred.
- Catastrophic failure of the connecting rod’s big end occurred.
- Catastrophic failure of main engine occurred.
