Fatigue assessment with special respect to size effects and material behaviour within local strain-based approaches

Paper #:
  • 2017-01-0356

  • 2017-03-28
In vehicle design and engineering, the fatigue of materials is a size-dependent phenomenon, which occurs in every safety relevant component. An inaccurate fatigue assessment, neglecting relevant influencing factors, may therefore either lead to considerable safety risks or to a significant oversizing of the component. Due to the microscopic and macroscopic structure and the related deformation and fatigue mechanisms, the fatigue life estimation requires an understanding of the cyclic material behaviour as well as the damage mechanisms of materials on different scales. In this respect, local strain-based fatigue design concepts are advantageous for estimating the fatigue properties of components with arbitrary size and geometry, because the applicable material models allow an implementation of a realistic cyclic material behaviour and a relation to different damage mechanisms in the elastic as well as the elastic-plastic load regime. As recent research indicates, an additional consideration of size effects may enhance the accuracy of fatigue design regarding the influence of component geometry and load conditions on the fatigue life of components, accounting for the actual local stress-strain state as well as statistical effects of the component size on fatigue life. Considering complex load time histories, the transient material behaviour has to be taken into consideration, since the microstructure as well as the local material properties of engineering materials changes continuously, even during macroscopically elastic deformation. Assuming a cyclic stabilization will in this case result in a misinterpretation of the local stress-strain state and the related fatigue damage, although the necessary data for a more detailed description of the material behaviour can be obtained from fatigue test results without additional effort. The presented work shows, how the implementation of appropriate fatigue damage and material models and size effects will reduce numerical as well as experimental effort, while enhancing the component fatigue strength and reducing safety margins.
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