PSI - Issue 3

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

45

5

2 3 4 1.04 3.64 16.86 32.59 28.41         

(3)

b M

2 o a R

(4)

 

In equation 1, the term b  is zero and the term m  corresponds to the ultimate stress in the bar (which is the value sought), therefore, substituting K I for K IC and entering the value of a = 0.22 in (5.6 mm) measured just before the failure and corresponding to the length of the smooth portion of the fracture, as shown in Figure 4, equation 1 is solved for m  :

IC K M a 

(5)





m

m

The K IC value for SAE 4140 T steel quenched in oil and tempered at 540 ° C is 66 MPa√m (60 ksi√plg) [Wahab (2014)] and M m is 1.38 (calculated value). Thus, by replacing values, a maximum stress m  = 52.3 ksi is obtained on the bolt that failed due to fatigue. The next step is to calculate the nominal stress, ie the stress caused by the specified tightening torque which is the stress that theoretically should have the bolt failed. The nominal stress depends on the diameter and pitch of the bolt. According to engine manufacturer's information, the tightening torque is 40 lb-ft and the bolt diameter is 0.5 in., with 15-stride pitch. To perform the calculation of the stress in the bolt, an available-to public application was used [website (2017)], obtaining a value of a  = 32.3 ksi. Observing and comparing the above values, it is observed that the fatigue bolt underwent greater stresses (52.3 ksi) and has a lower fatigue strength (76.85) than the non-fatigued ones, which were subjected to less stress (32.3 ksi) and had a higher fatigue strength (79.4 ksi), therefore it is shown that the number 3 bolt of the diesel engine crown was a candidate to fail prematurely.

Fig. 4. Extension of the fatigue fracture in the failed bolt.

3.2 Contributing factors Since there are no records of prolonged stops of the diesel engine, or repairs made by the operator of the engine, it is ruled out that the operator had made any manipulation of the bolts and any other internal part of the engine. On the other hand, it is known that the engine manufacturer is who manufactures and sells the crown bolts and therefore, the cause of the failure is attributable to the manufacturer, who installed a less‐