Keywords: reliability analysis, fatigue, optimization, failure probability, soil tillage machines, soil tillage forces.

In a reliability-based design optimization (RBDO) approach, it is important to take into account the cyclic loading on soil tillage machines, in order to better control the fatigue failure. Therefore, a new approach for intergating the fatigue life performance into the RBDO approach for the desgin of soil tillage machines, is developed and presented in this paper.

The proposed approach uses sequential optimization and the reliability assessment (SORA) method for solving the RBDO problem due to its efficiency and accuracy [1]. The fatigue limit state (FLS), which represents the reliability constraint of the SORA method, is determined using the method of Soderberg, based on the fact that the applied stress on soil tillage machines is primarily within the elastic range. This approach allows one to consider both the variability in the global tillage force and the cyclic variability in the local tillage force. The variability in the global tillage force, derived from the variations in the tillage system parameters (soil engineering properties, tool design parameters and operational conditions), is modeled using the methodology proposed by Abo Al-kheer et al. [2]. The variability in the local tillage force is modeled with the assumption that the local tillage force is a percentage of the global tillage force.

This approach is applied to the design of a chisel plough shank. The results show that the percent change of the objective function is increased by 2.13%, comparing the results of the classical SORA method, that uses the strength limit state (SLS) as a reliability constraint. However, this approach improves the fatigue life performance and reduces the ratio between the objective function and the number of cycles to failure, significantly. The ratio between the objective function and the number of cycles for the SORA-based SLS is equal to 14.94 mm³/cycle, while the value of this ratio for the SORA-based FLS is equal to 2.32 mm³/cycle. It is concluded that integrating the fatigue limit state into the SORA method guarantees the required reliability level and assures the fatigue life. Furthermore, this leads to the reduction of the portion of the volume for a fatigue cycle.

1

Y. Aoues, A. Chateauneuf, "Benchmark study of numerical methods for reliability-based design optimization", Structural and Multidisciplinary Optimization, 41, 277-294, 2010. doi:10.1007/s00158-009-0412-2

2

A. Abo Al-kheer, M. G. Kharmanda, A. El-hami, "Probabilistic analysis of soil tillage forces", in "Proceedings of the 18th Triennial ISTRO Conference", Izmir, Turkey, T5-003-1/T5-003-15, 2009.

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