PSI - Issue 3

J.L. González et al. / Procedia Structural Integrity 3 (2017) 41–47 Author name / Structur l Integrity Procedia 00 (2017) 000–000

44

Fig. 3. Fracture surface of the cylinder crown bolt # 3 failed in the diesel engine, seen in the SEM showing fatigue grooves.

The metallographic examination and the chemical composition of the failed bolts, including the failed bolt, showed no alterations that could be related to the failure and were similar in the bolts without failure, however, the hardness of the bolt that failed by fatigue (328 HBN) Was between 5 and 8% less than in the other bolts (344 HBN).

3.1 Results and discussion 3.1 Analysis of failure causes.

According to the results of metallography and chemical analysis, the four cylinder crown bolts, including fatigue failure, meet the specifications for a 4140 T steel and their characteristics are similar to those of bolt. Likewise, non destructive tests performed in field, and Eddy current tests performed during engine inspection, do not show the presence of cracks or damages prior to failure, therefore it is ruled out that the cause of fatigue Was that the bolt had defects or that there had been an incorrect operation or maintenance that had generated service damage, leaving as the only possibility that the bolt was subjected to high stresses since its installation, in combination with that the material had a resistance to the Minor fatigue. The results of the hardness tests showed that the faulty bolt had a 5% hardness lower than the other bolts, so the fatigue limit (which is approximately 0.5UTS and the UTS is proportional to the hardness) of the bolt 3 had a fatigue limit of 76.85 ksi, which is slightly lower than the rest of the bolts, which is 79.4 ksi. Therefore, in order to verify the possible cause of the fatigue of the cylinder bolt 3 it is necessary to determine the amplitude of stress to which it was subjected and to compare that value with the fatigue limit of the material. The value of the stress to which the bolt 3 of the cylinder of the diesel engine failed can be calculated from the fracture mechanics, as follows: The fracture mechanics predicts that a cracked body ruptures when the stress intensity factor (K I ) is equal to or greater than the flat strain fracture toughness (K IC ) of the material [Gonzales (2013)]. The equation for determining the K I of the bolt is found in Annex C of the API 579-1 / ASME FFS-1 Fitness for Service 2007 standard and is reproduced below: The mode I stress intensity factor   I m m b b K M M a      (1)

where:

2 3 4 0.926 1.771 26.412 78.481 87.911         

(2)

m M