EVALUATION OF THE INFLUENCE OF SURFACE CONDITIONS ON WHEEL-RAIL CONTACT PERFORMANCES

Transcription

1 EVALUATION OF THE INFLUENCE OF SURFACE CONDITIONS ON WHEEL-RAIL CONTACT PERFORMANCES Abstract M.Pau, F. Aymerich, F. Ginesu Department of Mechanical Engineering, University of Cagliari Piazza d Armi, Cagliari, ITALY pau@dimeca.unica.it M. Ishida, H. Chen Track Dynamics Laboratory Railway Dynamics Research Division Railway Technical Research Institute Hikari-cho, Kokubunji-shi Tokyo , JAPAN mako@rtri.or.jp For many years, the contact between wheel and rail was studied treating both components as smooth. Only with the advances in tribology, have scientists become aware of the importance of the substantial effect of surface roughness in contact-related phenomena such as adhesion, wear, surface fatigue and so on. Thus, over the last 30 years, a number of theoretical models and numerical methods have been devised to simulate the contact between two rough surfaces. Although these studies can provide information about parameters such as real contact area (RCA) and local pressure distribution, the underlying hypotheses are often quite remote from reality. This suggests the use of experimental techniques that facilitate the independent verification of scientific observations. Recently the authors have proposed an ultrasound-based method for studying wheel-rail contact. This technique, originally developed in the early 70s, allows to relate the reflection of ultrasonic waves from an incomplete contact interface with size and shape of nominal contact area, RCA, contact pressure and contact stiffness. During the investigation described here, we analysed the influence of surface roughness on real contact area in two different wheel-rail systems. These consisted of a 500 mm diameter non-flanged and a 860 mm diameter flanged wheel and a common rail currently employed by Japan Railway Companies Group tracks (0.7% carbon steel with 800MPa tensile strength). The surface of the materials was rubbed with different grades of sandpaper, obtaining several levels of uniform surface roughness. The specimens were then loaded until a maximum Hertzian pressure of about 700 MPa was achieved in the contact ellipse. The wheel-rail contact was tested, for every applied load, with the ultrasonic system and the reflection data were post-processed with a theoretical model developed by Krolikowski et al.,

2 in order to obtain the RCA for each roughness value. The results show an impressive increase in RCA with decreasing roughness, with values up to 7 times higher passing from the highest to the lowest RMS roughness value. These findings, combined with the current knowledge of contact phenomena, may improve accuracy in evaluating wheel-rail contact performance, thus helping railway engineers to better predict the adhesion and local stresses of wheel-rail contact, and to gain a deeper understanding of vehicle/track interaction. 1 Introduction Although it is universally known that most wheel-rail contact studies are based on Hertz s theory, over the last twenty years an increasing number of attempts have been made to overcome some limitations intrinsic in the fundamental assumption underlying Hertzian analysis. Thus a series of non-hertzian effects have been identified and examined. These are all fairly computation-intensive and are known to strongly affect the performance of the wheelrail system. In brief, besides the well known effects due to the presence of friction in the contact region (not taken into account in Hertz s theory) the main differences observed in applied engineering and not considered by Hertz include: plastic deformations residual stresses additional effects due to materials surface roughness. Concerning the last point, which is also the central issue addressed by this investigation, the pioneering work on the role of surface texture in wheel-rail contact performance was conducted by Burton Paul [1]. Exploiting the earliest tribology studies he intuitively realised the potential influence of roughness in shelling phenomena observed at distances greater than those predicted by Hertz s theory (an event not explicable at the time). A few years later, Andrews [2] highlighted the essential role of surface roughness in the transmission of frictional forces between wheel and rail surfaces and, consequently, in adhesion and wear processes. In 1982, at the first Contact Mechanics and Wear of Wheel-Rail System Greenwood [3] spotlighted the results of more than 20 years research on the contact of rough surfaces. He indicated those areas where Hertz s theory could still be considered to hold and those where it was not possible to overlook the microscopic structure of the contacting surfaces.

3 Since then, an ever-increasing number of studies has focussed the attention on the role of wheel and rail surface roughness in some of the most important contact phenomena. 1.1 The real contact area and its role on wheel-rail contact performance The main implication of taking into account the materials surface features, is the concept of real contact area (RCA). As everybody knows, the contact of rough surfaces gives rise to an incomplete interface which can be figured as a sequence of voids and parts actually in contact. The sum of these parts represents the real contact area which is, in practice, a more or less large fraction of the nominal contact area, depending on the external load applied to the contacting bodies. As regards the wheel-rail contact interface, at least two major areas can be identified in which the real contact area is the primary factor affecting contact performance: adhesion and stressrelated contact phenomena. Evaluation of adhesion properties is essential for achieving high speed operation involving high driving/braking force and, as known, dry or lubricated conditions (water, oil or some other media) can greatly affect adhesion. Moreover, lubrication has proved very useful for reducing friction at the gauge corner of a high rail profile and lateral force on low rail of vehicle running surface. Thus, the amount of lubricant at the wheel-rail interface needs to be tailored to achieve acceptable adhesion [4]. In light of the above considerations, research strives ultimately to elucidate wheel/rail creepage under wet or water lubricated contact interface. So, the central issue is: what role does surface microstructure play in these phenomena? Several workers have addressed this issue, some focusing on the influence of surface roughness features and real contact area on adhesion. For example, the experimental findings of Ohyama and Ohya [5] and Chen et al. [6-7] concerning the tribological aspects of adhesion under water lubricated conditions, showed that higher adhesion coefficients are obtained for rougher surfaces than for smoother ones. In the attempt to translate this empirical observation into a theoretical model, the EHL (elastohydrodynamic lubrication) theory, or other simplified numerical methods have been adopted. According to these studies, surface roughness, as well as water viscosity and rolling speed, are key factors in the development of wheel-rail creepage. What is more the decrease of adhesion coefficients with increasing speed, observable when water exists on contacting surfaces, has been explained by the reduction of the real area of contact.

4 Minimising surface damage of rail and wheel is another major aspect of material integrity that can reduce maintenance costs and enhance operational safety of running trains. Recent studies on the contact of surface microasperities [8] have shown that, although rail materials are designed to operate in the elastic regime, a thin surface layer of a few ten of microns exhibits evident plastic deformations. This finding suggests the need for detailed analysis of the contact mechanics at the microscopic scale, so as to gain a deeper insight into the actual stress distribution, the localisation of plastic deformations and the development of rolling contact fatigue. 1.2 Use of ultrasound for evaluating contact parameters Though mainly used for non-destructive evaluation and testing, ultrasonic waves have now been employed for over 30 years also for investigating contact parameters. The simple observation that the amplitude of the ultrasonic wave reflected by a stressed contact region decreases as applied load increases, prompted scientists to exploit the potential of ultrasound for studying contact-related problems. Since then, a number of theoretical models have been proposed and experimental techniques devised aimed at gaining a understanding of the mechanism underlying ultrasonic wave/contact interface interaction. In general, when two surfaces of different materials (ideally smooth and perfectly adherent) come into contact and an ultrasonic wave impinges thereon, the reflection and transmission are governed by the ratio between the difference and sum of the acoustic impedances of the two media. In the specific case, when the materials are homogeneous the incident wave is transmitted entirely through the interface. But, as previously mentioned, in reality adhesion is usually far from perfect and the contact between the two engineering surfaces (which are not smooth but more or less rough) gives rise to a so called incomplete interface in which a sequence of cavities and contact islands are present at the same time. A train of ultrasonic waves incident on this kind of interface is partially transmitted and partially reflected depending on the stress state of the contact region, which influences the amount and size of the air cavities. Since air has very low acoustic impedance compared to solids, an air gap forms an impenetrable obstacle for ultrasounds, which are thus entirely reflected, while the part actually in contact satisfies the theoretical assumption of perfect adherence, thus transmitting all the incident wave impinging thereon. As a result, ultrasonic transmission is incomplete, ranging from 0 (when no load is applied and a negligible part of the materials is in actual contact) to 1 (when the load is so high that all the gaps are filled due to deformation of the materials).

5 These properties can be usefully exploited also for studying the wheel-rail contact. The authors recently proposed applying the ultrasound technique for evaluating the size and shape of contact area [9], real contact area [10] and contact pressure [11]. As explained in detail later, visualisation of the contact ellipse using this experimental technique requires analysing and processing the reflection data, assuming all those points in which there is some transmission through the interface, to belong to the contact region, excluding all those in which the incident wave is reflected in its entirety. Concerning the RCA, although the decrease in ultrasonic reflection from the interface is clearly related to an increase in RCA, the quantitative relationship between the two was not easy to define. To date, the only theoretical model which allows to calculate the value of RCA directly from reflection measurements is the one developed by Krolikowski et al. in the early 1990 s [12]. In this model, reflection is expressed as a function of the angular frequency as follows: 1 RCA 1 1 RCA NCA NCA R (1) 2 2 K Z 2 where RCA/NCA is the ratio real/nominal contact area, is the angular frequency of the incident wave, K is the contact stiffness and Z the acoustic impedance (which is the same for the two contacting bodies). Since frequency is the only variable, acoustic impedance and RCA and K parameters being constant, the best fitting of the experimental R( ) data with the least squares method allows to obtain the value of RCA so as to achieve the best possible the agreement between theory and experiments. 2 Materials and methods 2.1 Wheels and rails Experimental tests were carried out on rail and wheel specimens supplied by the Railway Technical Research Institute (Tokyo, Japan) as follows: 860 mm diameter flanged wheel 500 mm diameter cylindrical wheel (non-flanged) JIS 50 rail (50 Kg/m) The original components were cut as described in [9] and the surfaces were treated with different grades of sandpaper (and diamond compound in one case) so as to obtain graded surface roughness. In most cases treatment allowed to obtain isotropic roughness, but few

6 analyses were carried out reproducing on the samples a unidirectional roughness similar to that observed under real conditions (see Fig. 1). This difference was introduced to verify whether a preferential direction in the surface texture could alter the RCA in any way. Light microscope images of the rail surface. From left to right: isotropic and unidirectional roughness. After sanding and before coupling and loading, the surfaces was carefully polished and degreased with acetone to avoid any possible solid or liquid contamination. Lastly, roughness of all the specimens was measured after each surface treatment as follows: 1) Isotropic roughness: 4 profiles obtained by rotating the measuring direction of 45 along the diameters of a 4 mm diameter circle of, so as to obtain a statistically significant value of RMS roughness 2) Unidirectional roughness: 8 profiles (4 length-wise and 4 cross-wise) 4 mm long Figure 2 shows some of the wheel and rail profiles measured. As can be seen the type and magnitude of asperities distribution differs between longitudinal and transverse profiles for unidirectional roughness. For isotropic roughness, in order to assign a unique roughness value to the whole wheel-rail system, the so called combined roughness c was calculated according to Tsukizoe and Hisakado [13] as follows: 2 c where 1 and 2 are the RMS roughness of the two contacting surfaces calculated as the mean of 4 measurements. After surface finishing, an almost continuous scale of RMS roughness was obtained, over the range m. This can be considered representative of conditions most commonly found on track 1 even though, of course, roughness conditions may (2) 1 Japanese Railways report roughness values in the range m for used rails

7 vary strongly depending on whether the rail is new ( m) or ground (in which case, roughness increases sharply up to 5-7 m). The values obtained after different treatment, as well as calculated combined roughness are reported in Tables 1 and 2. Wheel and rail samples were then coupled and loaded up to N (at intervals of 2000 N) by means of a hydraulic jack; the applied load was checked in real time by placing an HBM load cell connected to a PC between the jack and the wheel sample. The maximum contact pressure achievable with this load system was in the range MPa.

8 JIS 50 Rail JIS 50 Cylindrical Rail, Abrasive Wheel Paper 500 mm #40 (Unidirectional (Isotropic (Isotropic Roughness) Roughness) 40 4 Lapped, R =0.08 µm q µm Paper #240, R q=0.29 µm Paper #240, R q =0.33 µm Lapped, R q =0.09 µm 0-2 Paper #120, R q =0.59 µm Paper #120, Transversal R q=0.5 direction µm Paper #80, R =0.85 µm Paper #80, R =0.91 µm q q Paper Paper #40, #40, R R q =1.42 µm q Longitudinal =1.32 µm direction mm mm Figure 2 Roughness profiles of the specimens used during the experimental tests

9 Table 1 Treatments applied to the wheel and rail tested (isotropic roughness) Average RMS roughness (microns) Treatment JIS 50 Rail Cylindrical 500 mm wheel Combined roughness Abrasive paper # plus diamond compound 6, 3 and 1 micron Abrasive paper # Abrasive paper # Abrasive paper # 120 (Rail) and #40 (Wheel) Abrasive paper # 240 (Rail) and #60 (Wheel) Abrasive paper # Abrasive paper # Table 2 Treatments applied to the wheel and rail tested (unidirectional roughness) Average RMS roughness (microns) JIS 50 Rail Flanged 860 mm wheel Treatment Longitudinal Transversal Longitudinal Transversal Abrasive paper # plus diamond compound 6, 3 and 1 micron NM 0.06 Abrasive paper # NM 0.40 Abrasive paper # NM 1.18 NM = not measured

10 2.2 The ultrasonic set-up As previously mentioned, the experimental tests consist basically of analysing the ultrasonic reflection from the wheel-rail contact interface. This can be done using a set-up very similar (if not identical) to that commonly used in the railway industry, for ultrasonic non-destructive testing of axles, wheel rims, etc. In particular, size and shape of nominal contact area was evaluated using a 10 MHz frequency immersion focussed probe, while for the real contact area calculations 3 immersion unfocussed probes of frequency 5, 10 and 15 MHz were employed. In the latter case, since the ultrasonic beam diameter is of roughly the same size as the transducer in the region of interest (0.25, 6.35 mm), the evaluation of RCA refers to the same zone in the contact area, centred on the maximum stress point. The choice of immersion technique as opposed to the contact one, was dictated by the fully automatic inspection facility which is easily achieved by moving the probe with a three-dimensional movement system remotely controlled via PC, so as to be able to scan the contact region with a defined step. An ultrasonic detector (Krautkramer HIS2) and a digital oscilloscope (HP 54520A) complete the measurement system. All the equipment is controlled by means of a customised software developed in Lab View environment, which is also able to display the contact area in real time. A detailed description of the connections can be found in [9]. 2.3 Acquisition of ultrasonic data Once the wheel-rail system was ready, the potential contact region was scanned with the focussed probe in order to map the contact ellipse. This process is useful for ensuring the correct positioning of the samples, for measuring size and shape of the contact area (and then calculate the average contact pressure) and for evaluating, at a glance, the contact stress distribution across the entire interface. Usually a 10x10 or a 8x8 mm squared region was scanned, with a 0.1 mm step. In this way a number of or 6400 terms matrices were constructed, in which every term represents the value of the ultrasonic reflection. Then by false-colour post processing the data, the contact conditions can be instantaneously verified also detecting any anomalies in the geometry of the contacting bodies as illustrated in Fig. 3, which shows the contact area obtained with a wheel having marked surface waviness.

11 Example of contact area obtained after an ultrasonic test. The irregular shape is due to the presence of waviness on the wheel surface*** Analysis of the ultrasonic reflection from the wheel-rail interface, also shows the position of peak contact pressure. In the majority of cases, this was found to be located approximately in the centre of the contact ellipse, but sometimes, because of local irregularities in the wheel or rail or misalignments it was observed to shift towards the periphery of the contact region. At this point, the unfocussed probe was positioned with its axis aligned with the maximum stress point, and the amplitude of the signal was recorded for each load step and frequency. This allowed to obtain an experimental relationship between reflection and frequency for comparison with Krolikowski s theoretical model (1). As previously mentioned the RCA 1.0 RCA = 1.57% Coefficient of Ultrasonic Reflection RCA = 6.49% RCA = 9.12% RCA = 11.61% RCA = 4.27% 2000 N Experimental 4000 N 6000 N 8000 N N Krolikowski Model Frequency (MHz)

12 appears as a parameter in the equation, thus an iterative fitting procedure allows to adjust the value of RCA until the best fit between theory and experimental data is attained. The diagram in Fig. 4 shows an example of the procedure with the final RCA values Figure 4 Experimental (markers) and theoretical (solid line) relationship between the coefficient of ultrasonic reflection and the frequency of the incident wave (combined RMS roughness = 1.94 mm). The values of the RCA for each load are obtained after the least squares fitting process. The above procedure was repeated varying the load and surface conditions, obtaining a number of experimental relationships between nominal and real contact area and the applied load, and between the real contact area and the RMS roughness value. 3 Results 3.1 Nominal contact area The first result of ultrasound analysis is the visualisation of the size and shape of the contact area for the two cases tested. Figure 5 shows some pictures processed from the ultrasonic reflection data for increasing loads and different surface treatments. Contact ellipses obtained from the experimental tests for different surface treatments and a load of N. Top: 500 mm diameter cylindrical wheel, bottom: 860 mm diameter flanged wheel.***

13 As can be clearly seen, the shape of the contact area is in general elliptical and no significant alterations are produced by variations in roughness. In fact, the values of nominal contact area are agree fairly well with Hertz s theory. These findings are consistent with earlier studies [14] which indicate that surface roughness only affects the nominal contact area when very low pressures are applied. As expected, the ellipse obtained by the cylindrical wheel has a semiaxis ratio quite close to 1, being a classical Hertzian cylinder vs. cylinder contact with crossed axis and very similar radius (250 and 300 mm respectively for wheel and rail). For the 860 mm flanged wheel on the other hand, the area appears to be longer in the direction of motion. 3.2 Real contact area By processing the experimental data obtained varying the reflection with ultrasonic wave frequency for each applied load and surface treatment, it was possible to determine the behaviour of the real contact area versus RMS combined roughness (for isotropic roughness). For unidirectional roughness the variable representing surface conditions was taken as sandpaper grade. In Fig. 6, the real-to-nominal contact area ratio is plotted versus applied load. For isotropic roughness RCA increases almost linearly with increasing loads. Real contact area (ratio real/nominal contact area) = 0.12 m c = 0.44 m c = 0.77 m c c = 0.95 m = 1.16 m c = 1.25 m c = 1.94 m c Applied load (N) Figure 6 Variation of the Ratio Real/Nominal Contact Area vs. applied load for different combined roughness levels (isotropic roughness, 500 mm diameter wheel and. JIS 50 rail) Note the significant difference between the RCA values measured at maximum load (10000 N corresponding to a peak contact pressure of about 700 MPa). In fact, when in contact, the lapped surfaces give a RCA about 7 times higher than the roughest case (R q =1.94 m)

14 These results are consistent with earlier experimental findings using the same technique [15] and with numerical analysis [16]. The explanation lies in the fact that the roughest surfaces are characterized by considerable asperity heights with small radius of curvature, while the smoothest ones have small asperities with greater radius of curvature. Thus to obtain the same percentage of real contact, it is necessary to apply a higher contact pressure over the roughest contact interface. Analysis of the unidirectional roughness contacts revealed the same trend, as shown in Fig Ratio Real/Nominal Contact Area Abrasive Paper #40 Abrasive Paper #220 Lapped Applied Load (N) Figure 7 Variation of the Ratio Real/Nominal Contact Area vs. applied load for different surface treatments (unidirectional roughness, 860 mm diameter wheel and. JIS 50 rail) In this case it emerges that, as lapping treatment produces almost the same roughness for both isotropic and unidirectional working (transverse and longitudinal direction), the real contact area is very similar regardless of whether the sanding grooves take a preferential direction or not. More in general, we can state that enhancing accuracy of the surface treatment, tends to decrease the difference between transverse and longitudinal roughness, converging towards similar values when roughness is very low. Though a direct comparison between isotropic and unidirectional roughness is not theoretically possible, due to the large differences between longitudinal and transverse roughness found for unidirectional sanding, by comparing the contact areas obtained for similar treatment (sandpaper #220 and #240) we can see that the unidirectional contact exhibits a significantly smaller RCA for a given load (Fig. 8). However this issue will be subject of further investigations in future, considering also that the contact of strongly anisotropic surfaces has been treated theoretically only by few studies [18] and that, at the present, there are no experimental evidences regarding differences in real contact area compared with the isotropic case.

15 0.8 Ratio Real/Nominal Contact Area Abrasive Paper #220 Unidirectional Abrasive Paper #240 Isotropic Applied Load (N) Figure 8 Variation of the Ratio Real/Nominal Contact Area vs. applied load: comparison between isotropic and unidirectional roughness for the same surface treatment Figure 9 shows the variation of the real-to-nominal contact area ratio for increasing RMS roughness for three loads (4000, 6000 and N, isotropic roughness). The experimental results indicate that the RCA tends to decrease exponentially with increasing combined roughness: this behaviour was frequently observed for all loads. Real Contact Area (ratio real/nominal contact area) N N 6000 N Combined roughness ( m) Figure 8 Variation of the Ratio Real/Nominal Contact Area vs. combined roughness for three levels of load (case of the isotropic roughness)

16 4 Conclusions An understanding of the fundamental role that surface roughness plays in wheel-rail contactrelated phenomena, is basic to an understanding of the mechanisms underlying surface microasperities interaction and warrants further investigation. In this context, the present investigation is intended as a contribution to elucidating how surface conditions may influence the formation of the real contact area The results of a series of experimental tests using the ultrasonic method on two wheel-rail systems having increasing grades of surface roughness, showed that increases of one order of magnitude in combined roughness led to a roughly sevenfold reduction in RCA under a given load. Comparison between samples with isotropic and unidirectional roughness showed that dominant surface roughness directionality seems to produce a moderate decrease in the size of RCA compared to isotropic roughness. Nevertheless when the surface is very smooth, hence with very similar longitudinal and transverse roughness, no significant differences were observed in the RCA for a given load. The potential practical implications of these results are numerous: high surface roughness (due to corrugation for example) tends to decrease the RCA size, at the same time minimising the heat exchange between the two contacting surfaces and causing an increase in contact stresses locally. This can result in plastic deformations and, in the long run, in fatigue failures especially at the top of the railhead. Further, roughness adversely affects acoustic emission due to rolling noise and vibrations transmitted to the vehicle during motion. On the other hand, high roughness produces a beneficial effect on the adhesion coefficient under wet conditions as several workers have demonstrated in the past. In conclusion, considering that optimisation of the wheel-rail system performance is influenced by the variability of many parameters (often with contrasting effects) detailed studies are needed for evaluating the effective role of all the physical entities involved in contact phenomena. This should allow to define strategies for materials and maintenance techniques to be used, with the ultimate aim of enhancing comfort, safety and prolonging the life of high speed trains. References [1] B. Paul, A review of rail-wheel contact stress problems, in Arnold D. Kerr (ed.) Railroad Track Mechanics and Technology, Pergamon Press London (1975),

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