PSI - Issue 38

Ronald Schrank et al. / Procedia Structural Integrity 38 (2022) 30–39 Ronald Schrank / Structural Integrity Procedia 00 (2021) 000 – 000

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4.3. Sensitivity analysis

The impact of the current 5 independent load channels to the total fatigue damage at the relevant hotspots is investigated in order to identify further negligible load channels. Successive excluding of single load channels shows a low impact of to the total fatigue damage. Knuckle loads , / with limited impact cannot be clearly assessed to be negligible and will be kept. Resulting from the sensitivity analysis, 4 independent load channels remain for further processing. 4.4. Damage equivalent loads Damage equivalent load amplitudes (with constant load amplitude) will be calculated for each of the 4 independent load channels. They will be used as start values for the upcoming iterative “try and error” procedure and are calculated: where: - cycle number for equivalent load amplitude ; e. g. = 100000 - load amplitudes from time-history ( ) of projected reference loads - slope of S/N-line - number of load amplitudes Load amplitudes are calculated from time-history ( ) of projected reference loads using the rainflow counting method. Load mean values are ignored. See Radaj et al (2007) for fundamental basics in fatigue theory.

Fig. 9. Time-history of projected reference load, damping strut (approx. 15000 repetitions)

Note: Any correlation and interaction of the 4 independent load channels at reference will be lost by damage equivalent loads definition. This can affect hotspot fatigue damages and has to be compensated by otherwise load modifications for the upcom ing iterative “try and error” procedure. 4.5. “Try and error” Based on the previously found damage equivalent load amplitudes for the 4 independent load channels further modifications will be done using an iterative “try and error” method. Best matching between the fatigue damages at the relevant hotspots and the reference damages of the full test is the objective of this final step of manual fatigue test simplification. Modifications of load amplitude level could be supplemented by changes of load correlation (phase inversion), adding constant load (mean load) or further modifications of the (constant) load directions. Detailed proceeding depends from the current situation and cannot be generalized. In general, many tries / iterations are necessary to obtain a sufficient matching of damages of full test and simplified test. This approach is limited to a few number of free parameters. Otherwise, monitoring of influences isn´t possible anymore and the overview will be lost.