Fretting fatigue under variable amplitude loading and the role of contact geometry
Problem Little research has taken place to investigate fretting fatigue under conditions of variable amplitude loading (VAL) and no research using a complex loading spectra representative of reality. The effect of cycles below the endurance limit has yet to be established for fretting fatigue within such a spectrum. These cycles are of particular concern owing to the large number of these cycles present in comparison to other cycles within the spectrum. Any effects of VAL on local conditions within the contact had also yet to be established. Solution This project attacked the problem of fretting fatigue under conditions of VAL on two fronts. Firstly a vigorous VAL testing program (reconstructed from in-service data) was employed to investigate the effect of VAL on life and damage in general. The relative importance of cycles within the spectrum, particularly those below the endurance limit, with regards to life was investigated. Secondly to establish the effect of VAL within the contact region finite element modelling (FEM) was performed. Single and three level loading histories were applied to the model in order to establish the effect of VAL locally within the contact and offer explanations to the experimental observations. A series of damage prediction parameters including Ruiz and strain life initiation parameters were assessed for their ability to predict such behaviour. A methodology for predicting fretting fatigue life and damage has also been developed during this project. Two contact geometries were tested: a cylindrical Hertzian contact and the rounded punch contact. Conclusions Cycles of amplitude below the constant amplitude fretting fatigue endurance limit are nondamaging within a VAL spectrum. This was primarily attributed to the cycles below the endurance limit having a unique location of damage that other cycles do not influence. More sites of crack initiation were observed in samples that had experienced VAL than those tested under conditions of constant amplitude loading (CAL). The multiple sites of initiation were attributed to the point of maximum damage changing location during VAL. The size of the slip region of a cycle was found to decrease post overload as was the magnitude of Ruiz predicted damage. Strain life parameters also predicted a beneficial effect of an overload on the predicted lives of following CAL cycles. Miner predictions of life were conservative due to the assumptions made by the parameter. Miner sums damage over the entire contact region, essentially attributing that damage to a single location. Miner therefore does not take into account the changing location of maximum damage and the effects of load order and interaction. An alternative methodology for predicting fretting fatigue life during VAL has been presented that has been shown to be more accurate than traditional Miner and can account for unique features within the VAL spectrum e.g. training flights. Both Miner and the VAL methodology have shown that it is the small amplitude cycles within a spectrum that are the most damaging.