In the present paper, the theory of critical distances (TCD) is reformulated in order to make it suitable for predicting fatigue lifetime of notched components in the medium-cycle fatigue regime. This extension of the TCD takes as its starting point the idea that the material characteristic length, L, changes as the number of cycles to failure, N f , changes. In order to define the L versus N f relationship two different strategies were investigated. Initially, we attempted to determine it by using the L values calculated considering material properties defined at the two extremes, namely static failure and the fatigue limit. This strategy, though correct from a philosophical point of view, contained some problems in its practical application. We subsequently attempted to determine the L versus N f relationship by means of two calibration fatigue curves; (one generated by testing plain specimens and the second one generated by testing notched specimens). This second strategy was found to be much more simple to apply to practical problems, resulting in estimations characterized by a higher accuracy. The reliability of the devised method was systematically checked by using experimental results generated by testing notched specimens of low-carbon steel containing different geometrical features and tested using various loading types, stress ratios and specimen thicknesses. The accuracy of the method was further verified by using several data sets taken from the literature. Our method was seen to be successful giving predictions falling always within the scatter band of the data from the parent material. These results are very interesting, especially considering that the TCD is very easy to use because it requires only a linear-elastic stress analysis.

A novel formulation of the theory of critical distances to estimate lifetime of notched components in the medium-cycle fatigue regime

High strength low alloy (HSLA) steels, used in a wide variety of applications as structural components are subjected to cyclic loading during their service lives. Understanding the cyclic deformation behavior of HSLA steels is of importance, since it affects the fatigue life of components. This paper combines experiments with finite element based simulations to develop a crystal plasticity model for prediction of the cyclic deformation behavior of HSLA-50 steels. The experiments involve orientation imaging microscopy (OIM) for microstructural characterization and mechanical testing under uniaxial and stress-strain controlled cyclic loading. The computational models incorporate crystallographic orientation distributions from the OIM data. The crystal plasticity model for bcc materials uses a thermally activated energy theory for plastic flow, sell and latent hardening, kinematic hardening, as well as yield point phenomena. Material parameters are calibrated from experiments using a genetic algorithm based minimization process. The computational model is validated with experiments on stress and strain controlled cyclic loading. The effect of grain orientation distributions and overall loading conditions on the evolution of microstructural stresses and strains are investigated.

Modeling Cyclic Deformation of HSLA Steels Using Crystal Plasticity

A practical method for statistical analysis of strain–life fatigue data

The degree of interplay among variables in dental implant treatment presents a challenge to randomized clinical trials attempting to answer questions in a timely, unbiased, and economically feasibl...

A critical perspective on mechanical testing of implants and prostheses

In this research work, the influence of higher surface hardness on the fretting fatigue life of hard anodized aerospace AL7075-T6 alloy was investigated. An optimization of the parameters of hard anodized Al7075-T6 alloy to obtain higher surface hardness was presented. Confirmation test was carried out to show the improvement after using the best parameter combination attained from the optimization process. The result shows that hard anodized coating (surface with hardness of 360 HV and thickness of around 17 μm) can significantly improve fretting fatigue life of specimens at low stress, while at high stress, the extent of fretting fatigue life decreases. Hard anodized coating with higher thickness (29 mm) and high surface hardness (393 HV) decreased the fretting fatigue life of specimens at stresses beyond 200 MPa, this mainly attributed to the coating brittleness and micro-cracking.

Ming-Wei Lu

Papers

Evaluation of the staircase and the accelerated test methods for fatigue limit distributions