Modelling methods for calculating fatigue stresses from simulated tire loads

The purpose of this work is to determine the most practical way to determine the stresses in the drill rig from simulated tire loads. The tire loads are measured from rigid body driving simulation of the rig and the stress analysis is done in frequency and time domain with FEA. The stress analysis is done in the time domain with a mode superposition method based transient analysis and the frequency domain analysis is done as a random vibration analysis. The fatigue life calculation is done in time and frequency domain as well. The cycle calculation for the fatigue life calculation in time domain is done with the rainflow method and the frequency domain calculation is done with the Dirlik method. Because the analysis is done in two different software, first the rigid body simulation and then the flexible body stress analysis, possibilities of flexible multibody simulation software is tested, where the multibody simulation and the stress analysis could be combined. All of the analyses were done with the mode-superposition method. The fatigue life results from the different analyses were compared to previous fatigue life calculation method that SANDVIK has used, the unit force method. Modal based transient analysis gave the most similar results both in time and frequency fatigue life calculation. The random vibration analysis gave differing results. The results could be perfected after field testing of the rig. This way the calculation model and its mode shapes could be determined more accurately. The flexible multibody simulation software was not ideal in this application. The wheel-to-ground contact did not work properly and there were other features that did not behave correctly. In the end the mode based transient analysis is suggested to be used in order to get the stress results from simulated tire forces. Random vibration analysis is also suggested but in the analysis the calculation model should be calibrated with the data obtained from field testing first. The rainflow method and the Dirlik method are both good ways to calculate the cycles from stress data