PSI - Issue 3

46 J.L. González et al. / Procedia Structural Integrity 3 (2017) 41–47 6 Author name / Structural Integrity Procedia 00 (2017) 000–000 hardness bolt than that of the other bolts and applied a greater tightening torque. This factors were combined to reduce the fatigue life of the failed bolt and to produce the prematurely fail. 3.3 Failure s quence Figure 5 shows a cross‐sectional sketch of an engine cylinder, where it is observed that the bolts hold the crown to the piston and prevent it from falling into the combustion chamber of the cylinder. According to the collected evidence it is determined that the sequence of the failure was the following: 1. The fatigue failure of the bolt 3 starts practically from the start‐up of the engine, causing a progressive cracking during the operation of the engine, until the crack reached a sufficient size for the bolt to break. 2. When breaking bolt 3, the load had to be supported by the other three bolts, which represents an increase of at least 8.33% of the load on each bolt; overloading the remaining bolts deforms them and causes the cylinder crown to loosen and start moving, generating additional loads on the bolts, causing the bolt 2 to fail. This failure occurs after a few cycles of engine rotation. 3. When the bolt 2 fails, the increase of the load on the two remaining bolts is 50%, which is well above its mechanical resistance, so that it finally breaks by overloading in tension. 4. When the cylinder crown bolts are broken, it loosens and locks the free movement of the piston‐rod system, but as the crankshaft continues to rotate by the thrust of the other 15 cylinders, the connecting rod strikes against the piston and against the piston. Monoblock, bending and breaking, until finally breaking the cylinder. 5. Restricted movement of the connecting rod and piston of the cylinder overload the systems of the rest of the engine, which causes the activation of the alarms and the stop of the engine.

Fig. 5. Design drawing of the cylinder of the diesel engine, where the crown (102) and the bolts holding it (103) are observed. Note that if the bolts 103 are broken, the crown 102 is released.