PSI - Issue 5

Miloslav Kepka et al. / Procedia Structural Integrity 5 (2017) 1409–1416 Miloslav Kepka et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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routes and computational predictions of service fatigue life of important structural details and vehicle components. The methodology was presented in professional literature on several occasions in the past (Kepka, 1995) as well as more recently (Kepka, 2015). It was employed successfully for developing a range of Škoda trolleybuses and buses. Its success is evidenced by the fact that these vehicles have developed practically no fatigue cracks in service or only very rare ones. The defects were repaired immediately, the causes were identified and eliminated by taking appropriate steps. Lessons learned from these rare fatigue failures are always valuable to designers as well as to researchers who develop fatigue life prediction methods. The same applies to earlier incidents (whose disclosure used to be prohibited) the documentation and information on which is trustworthy, though perhaps incomplete in some cases. The case study described here is a similar example of investigation efforts pursued by the Regional Technological Institute (a research centre affiliated with the Faculty of Mechanical Engineering of the University of West Bohemia), often in collaboration with the above-named Research and Testing Institute Plzen (Kepka, 2016). Here, a detailed examination was undertaken on a welded structural detail in a rear axle housing of Škoda trolleybuses operated in San Francisco in the United States. Fatigue cracks have occurred in the welds between the rear axle housing and short brackets. Attached to these brackets were rear suspension beams and radius rods that transmit internal forces (ensure stability) between the vehicle body and undercarriage. The arrangement is illustrated in the sketch in Fig. 1, described in the text below and shown in the photograph in Fig. 4.

Fig. 1. Rear axle configuration.

Nomenclature ∆ stress range cycles to failure 1 stress range intercept 1 first fatigue strength exponent 2 second fatigue strength exponent number of standard deviations number of cycles with stress range ∆ number of cycles to failure at stress range of ∆ 1 fatigue transition point