Fatigue limit

About: Fatigue limit is a research topic. Over the lifetime, 20489 publications have been published within this topic receiving 305744 citations. The topic is also known as: endurance limit & fatigue strength.

Mean Stress Effects in Stress-Life and Strain-Life Fatigue

Abstract: Various approaches to estimating mean stress effects on stress-life and strain-life behavior are compared with test data for engineering metals. The modified Goodman equation with the ultimate tensile strength is found to be highly inaccurate, and the similar expression of Morrow using the true fracture strength is a considerable improvement. However, the Morrow expression employing the fatigue strength coefficient ′ σ f may be grossly non-conservative for metals other than steels. The Smith, Watson, and Topper (SWT) method is a reasonable choice that avoids the above difficulties. Another option is the Walker approach, with an adjustable exponent γ that may be fitted to test data, allowing superior accuracy. Handling mean stress effects for strainlife curves is also discussed, including the issue of mathematical consistency with mean stress equations expressed in terms of stress. A new and mathematically consistent method for incorporating the Walker approach into strainlife curves is developed. With γ = 0.5, this result gives a new strain-based interpretation of the SWT method.

Fracture mechanics and notch sensitivity

Abstract: A diagram valid for the analysis of the fatigue limit of cracks and notches centred in an infinite plate was recently proposed by the authors of the present work with the aim to make explicit the bridging at the fatigue limit between defect sensitivity (correlated to the length parameter a 0 , according to El Haddad-Topper-Smith's definition) and notch sensitivity (correlated to a*, where a* is a particular notch depth corresponding to the intersection between the ΔK th and Δσ 0 /K t curves). The expression a* = K 2 t .a 0 being valid, defect sensitivity and notch sensitivity were seen as two sides of the same medal. Such a diagram is now extended to finite size components by simply introducing the shape factor a commonly used in fracture mechanics. The obtained critical defect size is termed a D , which is a material and geometry dependent parameter, in order to distinguish it from a 0 , which is a material parameter. As a consequence the critical notch depth aN is introduced, such that. aN = K 2 t .a D This results in the proposal of a 'universal' diagram able to summarize experimental data related to different materials, geometry and loading conditions. The diagram, the validity of which is checked by means of several results available in the literature, is applied both to the interpretation of the scale effect and to the surface finishing effect.

Statistical analysis of defects for fatigue strength prediction and quality control of materials

Abstract: A wide range of studies and experimental evidence have shown that the lower bound of fatigue properties can be correctly predicted by considering the maximum occurring defect size. The estimate of this dimension can be done by analysing the defect sizes using the statistics of extremes. The scope of this paper is to discuss and investigate the two key points in a successful application of this technique: the first is the choice of statistical method for the analysis of data; the second is the knowledge of the minimum number of defects needed to obtain a good estimate of extreme defects. The results obtained in this study allow one to formulate a procedure for estimating the extreme defects with a precision suitable for fatigue strength prediction.

Fatigue deformation of TiAl base alloys

Abstract: The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti3Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 106 cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation.