The effect of slip ratio on the rolling contact fatigue property of railway wheel steel
Highlights
► Rolling contact fatigue tests for wheel steel are conducted. ► Fatigue strength decreases with increasing slip ratio. ► Water penetration inside cracks is considered in finite element analysis. ► Stress intensity factor of a branched crack from analysis increased with slip ratio. ► The analysis result can explain the experimental result.
Introduction
Shelling is one of the typical rolling contact fatigue (RCF) failures of railway wheels. Rejection of a wheel due to shelling is known to increase drastically under heavy haul operation in winter [1], [2], [3]. For understanding these phenomena, a fluid penetration model has been used.
A fluid penetration model [4] of RCF crack propagation had been proposed and validated by many researchers [5], [6], [7], [8], [9], [10]. The model has been applied to evaluation of failures on mechanical elements subjected to rolling contacts, like gears, bearings, rails, and wheels. According to this model, a large vertical load due to heavy haul operation and continuous water lubrication due to heavy snow in winter accelerate the RCF damage and cause shelling earlier than anticipated.
On the other hand, creepage between wheels and rails due to rolling radius difference in curves is inevitable on railway tracks. Therefore, there are tangential stresses and slip phenomena on wheel treads. Eventually, initiation of RCF cracks, being the origin of shelling, is accelerated by creepage. It is usually observed that RCF cracks on wheel treads causing shelling grow perpendicular to creepage direction [11]. Consequently, the slip ratio due to creepage between wheels and rails during curving is an influencing factor in shelling. The effect of slip ratio had often been discussed using the traction coefficient and the shakedown map [12], [13], [14]. However, the relationship between the map and the fluid penetration model has not yet been entirely cleared from the point of view of the RCF mechanism.
The objective of the present paper is to evaluate the rolling contact fatigue (RCF) property of the railway wheel steel. The effect of the slip ratio on the RCF fatigue strength of the railway wheel steel was evaluated. RCF tests were conducted using two cylindrical contact specimens under water lubrication at a slip ratio of 0.0–1.0%. The range of slip ratio is determined to assume the creepage between wheel and rail during curving obtained from multibody dynamics simulation taking dimensional data of bogies, wheels, rails and tracks into account. As a result, it was found that the traction coefficient increased with an increase in the slip ratio, and the fatigue strength decreased simultaneously. The results were evaluated by the shakedown map and Hirakawa’s RCF fatigue map. Experimental fatigue limits could be expressed more precisely by the criterion of Hirakawa’s RCF fatigue map than that expressed by the well-known shakedown map.
Stress intensity factors (SIFs) of the cracks produced by the RCF test were calculated by finite element (FE) analysis where the effect of water pressure due to the penetration of water into the cracks is taken into account. Two peaks of the maximum tangential SIF occurred during one cycle of rolling contact. The direction of the crack propagation was estimated by the maximum tangential stress criteria. The results of the RCF test showed that the cracks were initiated at the surface, propagated obliquely in the depth direction and then branched into two directions. One was towards the surface, and the other was towards the depth.
These two crack directions were inspected experimentally, and compared with the directions estimated from the SIF obtained by the FE analysis. The SIF of a branched crack propagation towards the surface increases with the increase in slip ratio. This result corresponded qualitatively to the fact that the fatigue strength decreases with an increase in the slip ratio. It is therefore concluded that the effect of slip ratio on the RCF property is dominated by the SIF of crack propagation towards the surface after branching.
Section snippets
Material and specimens
The material used in the present paper is AAR class C wheel steel [15]. Table 1 shows the chemical composition of the material. Fig. 1 shows the configuration of the two types of the test specimens. One is corresponding to the wheel with 120 mm in diameter and the other is 178–190 mm in diameter corresponding to the rail.
Wheel specimens were taken from a rim part of a full-scale wheel. The rail specimens were cut from a quench-tempered billet of the wheel steel after forging. Table 2 shows the
Evaluation by shakedown map and Hirakawa’s RCF map
The experimental results described in Section 2 were evaluated by a shakedown map and Hirakawa’s RCF map. The shakedown map was proposed to evaluate the RCF property by four criteria, that is, elastic limit, elastic shakedown, plastic shakedown and ratcheting by Johnson [12]. Hirakawa’s RCF map was proposed by Hirakawa [16], and it used fatigue limit as a criterion. Here, the criterion was assumed to be shear fatigue strength τw expressed as follows:where ke is the shear
FE modelling of RCF test
Fig. 11 shows the FE models for the RCF tests. The 2-D plane strain models were used. The wheel specimen was modelled by a semi-infinite flat plate and the rail specimen was modelled by a cylinder. Infinite elements were applied for the wheel specimen of the flat plate model. Therefore, it was possible to conduct an accurate analysis using the flat plate model with a fewer number of elements compared with the cylinder model. Due to the above character of the modelling, the flat plate model is
Mechanism of shelling formation
The mechanism of shelling formation is summarized in Fig. 19. First, after a crack propagates to 0.8 mm in depth, the crack branches in two main directions. Because the SIF for crack propagation in the depth direction is large, the branched crack propagates in that direction. Because the SIF for crack propagation towards the surface is comparatively small, the branched crack propagates slowly towards the surface. Next, the SIF for crack propagation in the depth direction decreases with crack
Conclusion
In order to clarify the effect of the slip ratio on the rolling contact fatigue property of railway wheel steel, rolling contact fatigue tests using two cylindrical specimen and FE analysis were conducted. As a result, the following results were found:
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Fatigue strength obtained by RCF tests decreased with the increase in the slip ratio, where the traction coefficient increased with the slip ratio.
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The above results were evaluated by a shakedown map and Hirakawa’s RCF map. Experimental plots were
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