RICERCA ITALIANA     

Numerical models for the analysis of Rolling Contact Fatigue (RCF) life

Politecnico di Bari

Abstract

The research plan will be developed on various themes, coordinated by Proff. Demelio and Ciavarella, and as a continuation of activities undertaken at PoliBA in the last 5 years, also in collaboration with excellence centres in Europe such as Sheffield, Ecole Polytechnique, Leicester, etc.

The research program of Prof. Demelio will examine the mechanism due to defect and inclusions subsurface. This will study the stress concentration with semi-analytical methods extending the work of Greenwood, and validated with more sophisticated methods due to Kelly et al., as well as a FEM investigation.

A second part of the research plan will be an experimental plan, which will allow to consider existing correlations between fatigue limits under standard tension/compression or rotating bending, with that under RCF, with and without a calibrated hole in the specimen, with typical rail steels. The experimental plan will make use of the testing machine developed at DIMeG.

A second part of the research plan will be devoted to the analysis of the plastic deformation damage mechanism, in terms of residual stresses, as well as standard multiaxial fatigue criteria, and finally of ratchetting plastic deformations.

Principal Investigator

Giuseppe Pompeo DEMELIO Politecnico di BARI

Research Objectives

Rolling contact fatigue affects various important engineering applications, such as rolling bearings, gears and rail-wheel contacts. In the latter case, failure may be of interest for both the wheel and the rail, without having particularly evident alerting signs and producing serious consequences in terms of damage, casualities and injuries.
The three mentioned applications can be reconducted to surface fatigue, but are tradizionally studied and treated in different manners and with different approaches, neglecting a deeper understanding of the damage phenomena at the origin of such differences.
There is, therefore, the necessity of having a better and more reliable predicting tool, as a consequence of scientific understanding of the various damage phenomena and consenting of better distillating differencies or analogies between the various applications.
In the case of rail-wheel contact, we have isolated in the literature a few major failure mechanisms, i.e.: surface fatigue (ratchetting and wear), sub-surface fatigue (high cycle fatigue), and fatigue from subsurface defects.
In the former case, it is not possible to neglect large incremental and cyclic plastic deformations, leading to failure (crack initiation) or wear, of low-cycle fatigue. In sub-surface fatigue, at depth around 3-10 mm, the predictive models refer to standard multiaxial fatigue criteria. Finally, for subsurface fatigue originated from defects or inclusions >>>

First Results

Research unit coordinated by Prof. Demelio:

1) Diagrams of stress concentrations in presence of the hole/inclusion as a function of loading conditions, geometry, materials combinations

2) Determination of stress cycles (possibly multiaxial) for hole/inclusion subsurface

3) Atzori-Lazzarin diagram for Hertzian loading and crack or e cricca or hole/inclusion subsurface

Research unit coordinated by Prof. Ciavarella:

1) Residual stresses for perfect plasticity model (in particular, hydrostatic component as required for DangVan's criterion)
2) Same as 1, but linear kinematic,
3) Same as 1, but Bower's non-linear kinematic
4) Same as 1, but with Chaboche model

Multiaxial fatigue parameters
5) DangVan's criterion
6) Papadopoulos' criterion
7) Susmel-Lazzarin's criterion
8) Comparison will older criteria

Geometries
1) 2D Hertzian contact (Carter type tractions)
2) Axisymmetric or 3D Hertzian contact (‘lemon like' tractions)

Loads (Q is tangential load, P is normal load):
1) Pure rolling,
2) rolling with Q/P>0 (traction), up to full sliding Q/P=f;
3) rolling with Q/P<0 (braking), up to full sliding Q/P=-f;Experimental plan
1) fatigue test results on rail steel specimen with calibrated drilled hole
2) RCF test results

Overall c >>>

Timescale

24 months

National and international background

Rolling Contact Fatigue (RCF) is one of the most complex areas of fatigue, at the boundary between fatigue, crack propagation and wear, with presently a lack of true quantitative models. The development of rail materials has been constant from the early 1820s to date from iron to Bessemer steel, and finally to the modern pearlitic rail steels of high carbon content (0.5%), and low level of phosphorus and other impurities (however, for switch and other crossing components under more severe operational conditions, bainitic and Hadfield's manganese steels are used). These metallurgical improvements have solved many technical problems and permitted the progress in the railways, but have largely progressed independently on RCF understanding, Indeed, despite the critical nature of the components involved, there is lack of extensive prototypical testing with respect to, for example, car manufacturing and aeronautical industries where ultimately fatigue testing in service is conducted on a large number of vehicles (Smith, [1]).
As surface RCF fatigue and wear seem related by the common mechanism of RF (Ratchetting Failure), i.e. the progressive cumulative process of shear strain increase up to very large values, it is to be expected that, as in wear, many factors other than standard and more easily established material properties affect the performance of a given system and make difficult to make a priori even rough quantitative estimates. Generally, in wear empirical models >>>