The prediction of implement fatigue life

Abstract Estimates of implement fatigue life are needed to avoid failure without excessive weight and cost. Life under complex loading can be predicted by computer analysis of the elasto-plastic stress-strain behaviour in conjunction with a cumulative damage law. It is necessary to know (1) the service loading and duty cycles, (2) relationships between load and strain and (3) the effect of these strains on fatigue life. Research in all three areas is described, using a rotary cultivator as an example. Load history data from the rotary cultivator have been obtained in a range of short, severe tests and during long term farm use. Fatigue lives predicted from these data in the long term tests were an order of magnitude higher than in the short, severe tests. The effect on fatigue life of varying operating parameters was also determined. Load/strain relationship were found for the rotary cultivator by finite element analysis. The locations of critical, highly stressed areas did not change greatly with different loading conditions, and were confirmed by the initiation of fatigue cracks during laboratory tests. The finite element mesh used was not fine enough to give accurate predictions of stress magnitudes. Predicted maximum stress in the most critical area was approximately 20% too low. Strain data recorded or estimated during complete test programmes allowed predicted and actual lives to be compared for two fatigue failures. Good agreement was obtained under appropriate assumptions, but the predictions showed the importance of fixing strain gauges in the correct positions and assigning the correct weld class to the joint. A fracture mechanics analysis gave insight into the design parameters affecting crack growth, but depended too heavily on assumed geometrical factors and initial flaw size to be of much value for agricultural engineering problems.