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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a) Empty vehicle

b) Loaded vehicle

Fig. 6. Two-parameter and one-parameter stress spectra – empty vehicle and loaded vehicle – estimated hot-spot stresses.

3. Fatigue life prediction

3.1. Fatigue damage calculation

The Palmgren-Miner rule was applied in this case study. According to this hypothesis, the fatigue limit state is achieved when the following condition is fulfilled: ∑ = 1 , (4) where is the number of cycles with stress range ∆ and is number of cycles to failure at the same stress range ∆ . The calculation considered the measured stress-time histories (the stress spectra analysed for the estimated hot spot stresses) and estimated bi-linear fatigue life curve. The main feature of a stress cycle that affects the fatigue damage of welded structural details is the stress range. According to the recommendation in the above-mentioned standard, the influence of the mean stress of each cycle was not considered.

3.2. Fatigue life predictions for various service conditions

The data for modelling service conditions was obtained under two loading states: empty vehicle ride and fully loaded vehicle ride, as detailed in section 2.2. Service fatigue life of the structural detail could be determined for any combination of these two loading states. Table 2 gives fatigue life predictions for several types of service conditions. They are based on the mean fatigue curve as described in section 2.1. Therefore, the values indicate 50% probability of survival, i.e. 50% probability of fatigue failure of the structural detail.