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Local cyclic deformation behavior and microstructure of railway wheel materials
Materials Science and Engineering A 387–389 (2004) 481–485
Local cyclic deformation behavior and microstructure of railway wheel materials F. Walther∗ , D. Eifler Institute of Materials Science and Engineering, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany Received 25 August 2003; received in revised form 13 January 2004
Abstract The current investigations concentrate on the relation between the loading and environmental conditions, the local microstructure and the fatigue behavior of highly stressed railway wheel and tire steels. Experiments under stress control and total strain control were performed at ambient temperature with servohydraulic testing systems. Superimposed mean loadings allow an evaluation of cyclic creep and mean stress relaxation effects. Strain, temperature and electrical measuring techniques were used to characterize the cyclic deformation behavior of specimens from different depth positions of the cross-sections of UIC-specified wheel components (UIC: International Railway Union). The measured values show a strong interrelation. The microstructural characterization of the different material conditions was done by light and scanning electron microscopy together with digital image processing. © 2004 Elsevier B.V. All rights reserved. Keywords: Cyclic deformation behavior; Railway wheel and tire steels; Microstructure; σ, ε-hysteresis measurements; Temperature and electrical measurements
1. Introduction The wheel/rail-contact is characterized by high contact forces and small contact areas. Extreme and complex mechanical service loadings with static and dynamic stresses are superimposed by elevated temperatures in the contact area as a result of slipping and braking. The optimization of the wheel/rail-system requires a detailed knowledge of the cyclic deformation behavior of the wheel materials. But only a few systematic investigations are concerned with this topic [1,2]. As a consequence of the industrial heat treatment of complete wheels microstructural gradients occur and the cyclic deformation behavior shows a strong dependence on the local microstructure and can not be quantitatively compared to specimens of homogeneous medium carbon steels with similar chemical compositions, e.g. SAE 1045 [3,4]. The response of the investigated wheel materials to the applied cyclic loading can be described with mechanical hysteresis measurements. Additional temperature and electrical measurements are used to complement the plastic strain ∗ Corresponding author. Tel.: +49-631-2052413; fax: +49-631-2052137. E-mail address: [email protected] (F. Walther).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.05.034
amplitude εa,p . Thereby the change of temperature T and voltage U yield integral signals from the tested material volume. Apart from the geometry, the electrical resistance and consequently the voltage depend on the specific electrical resistance of the material. During cyclic loading, softening and hardening processes influence the defect density and therefore the specific electrical resistance in a characteristic manner. The evaluated values of the electrical resistance and the voltage, respectively, are independent from a gauge length and could be helpful tools to assess the fatigue behavior of complex components. The change of the voltage is also measurable in the unloaded state and can be used as a reference value to describe the proceeding fatigue damage. The presentation of εa,p , T and U versus the number of cycles yield characteristic cyclic deformation, temperature and voltage curves corresponding to the actual fatigue state [5,6].
2. Materials The chemical compositions and the monotonic properties are UIC-conformable. The investigated railway steels contain about 0.52 (R7) and 0.63 (B6) wt.% C, 0.4 and
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Fig. 1. SEM-micrographs of the tread (a) and flange (b) of a wheel R7; the cross-section center of a tire B6 (c).
0.5 wt.% Si, 0.8 and 0.9 wt.% Mn, 0.3 wt.% Cr and small quotas of Cu, Mo and Ni [7]. The industrial heat treatments cause different ferritic-pearlitic microstructures in the rim of the wheels and in the tires, with different ferrite quotas as a function of the distance to the surface. Within monoblock wheels R7 (UIC 812-3 V) the ferrite quota increases with increasing surface distance from about 6 area% near the tread to a maximum of about 20 area%. In tires B6 (UIC 810-1 V) maximum ferrite quotas around 2–3 area% are present in the cross-section center (cf. Fig. 1). In SEM-micrographs area-specific cementite lamellae spacings of 0.14–0.19 m (R7) and 0.11–0.15 m (B6) were identified. A correlation between the ferrite quota and the Vickers hardness in the range 230–300 HV10 (R7) and 300–330 HV10 (B6) could be found for each component along the cross-section [7].
3. Experiments and methods The axial stress-controlled and total-strain-controlled experiments with different load ratios were performed at ambient temperature, using triangular waveforms with a frequency of 5 Hz in servohydraulic testing systems until 2 × 106 cycles. For the cycle-dependent recording of mechanical hysteresis-loops an extensometer was fixed to the polished
specimens surface. Highly accurate temperature measurements were performed with thermocouples in the middle of the gauge length and at the shafts. Temperature changes T are exclusively caused by plastic deformations in the gauge length. A DC power supply with a direct current of 8 A was fixed to both shafts. To measure the difference of the voltage U two wires were connected at the transitions from the gauge length to the shafts. No temperature rise is caused by the applied current. All measured signals are amplified and digitized by DAQ-devices [7].
4. Results The results were achieved with specimens taken from different depth positions with tangential orientation to the cross-sections of the components. To identify critical stress amplitudes and for the estimation of the endurance limit, stepwise load increase tests were performed. The circles in the cross-section drawings of the Figs. 2a and 4a represent the 19 (B6) and 14 (R7) different specimen positions. Dependent on the microstructure and the appropriate cyclic deformation curves determined at a stress amplitude of σ a = 500 MPa (B6) and 400 MPa (R7) the cross-sections of the components can be divided into 4 (B6) and 3 (R7) areas of comparable behavior.
Fig. 2. Stress control, cyclic deformation curves (a); εa,p , T, U–Nf -relation at Nf /2 (b).
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Fig. 3. SEM-surface investigations at 57.5% Nf (a) and Nf (b) (vertical loading direction).
Fig. 2a shows characteristic plastic strain amplitudes of the different specimen positions versus the number of cycles for single step tests with the tire steel B6 at σ a = 500 MPa. In a typical curve, subsequent to position-dependent incubation intervals, cyclic softening with increasing and cyclic hardening with decreasing plastic strain amplitudes are obtained. The final fatigue state is predominated by crack growth. Increasing plastic strain amplitudes after smaller incubation intervals are responsible for a decrease in the number of cycles to failure Nf . As a function of the individual specimen position and microstructure, respectively, the three measured physical values at Nf /2 result in similar curves (Fig. 2b). Decreasing plastic strain amplitudes, changes of temperature and voltage lead to substantially longer lifetimes. For the evaluation of cyclic deformation characteristics and failure mechanisms in defined fatigue states surface polished flat specimens from the tread and the flange of wheels R7 were investigated in the SEM. Fig. 3a shows in- and extrusions and first microcracks after 7 × 103 cycles (57.5% Nf ) with σ a = 400 MPa. The crack path could be correlated with the microstructure of the specimens. The crack runs in the ferritic seam at the grain boundaries (marked in Fig. 3b with white arrows). A variety of stress amplitudes and total strain amplitudes at a load ratio of R = −1 were investigated for a comparison of the cyclic deformation behavior in the selected component areas, e.g. tread and abrasion reserve, respectively, wear limit. Total strain control leads to less pronounced cyclic
softening and hardening processes compared to stress control, as shown in the cyclic deformation curves of Fig. 4a for the three areas of R7 and total strain amplitudes of 2.5 and 4.0 × 10−3 . For both loading parameters the plastic strain amplitude increases with the ferrite quota and reaches the maximum in the flange (area 3). Regarding the lifetime only small differences occur. With T, N- and U, N-curves additional information about the effective cyclic deformation processes can be derived (cf. [8]). For identical loading parameters under stress control and total strain control, the lower ferrite quota in the tread and near-surface areas, respectively cause in general lower εa,p -, T- and U-values and higher lifetimes in comparison with the others. Cyclic stress–strain-curves (CSS-curves) are used to describe the cyclic deformation behavior. The substitution of εa,p by T or U taken at Nf /2 leads to cyclic σ a , T(CST-) or σ a , U- (CSU-) curves as shown in Fig. 4b for total-strain-controlled experiments in the flange (area 3) of wheels R7. The mathematical descriptions are given with a modified relation according to Morrow in the diagram. The measured values are equivalently qualified for a description of the cyclic deformation behavior. The mean stress sensitivity is of great technical relevance for the cyclic deformation and service behavior of railway materials. In stress-controlled experiments with superimposed mean stresses cyclic creep was detected and described in form of εm,p , N-curves. Positive (negative) mean
Fig. 4. Total strain control, cyclic deformation curves (a); CSS-, CST- and CSU-curves (b).
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Fig. 5. Stress control, influence of mean stresses on εa,p , N-curves (a); εa,p (Nf /2) as a function of σ m (b).
stresses lead to higher (lower) plastic strain amplitudes, positive (negative) plastic mean strains εm,p and decreasing (increasing) lifetimes. As a function of the mean stress different shapes of the cyclic deformation curves can be observed. In Fig. 5a the cyclic deformation curves for the tire steel B6 at σ a = 500 MPa and mean stresses in the range −300 MPa ≤ σ m ≤ +200 MPa are exemplarily shown for the cross-section center (area 4). During the first cycles at σ m = 0 MPa and σ m = ± 100 MPa, no or only very small plastic deformation is measured. Because of the distinct exceeding of the yield strength of about 600 MPa, the loading parameters σ a = 500 MPa and σ m = +200 MPa lead to a decrease of εa,p within the first 100 cycles to a minimum of about 1.2 × 10−3 . Until specimen failure cyclic softening combined with cyclic creep strains up to 10−1 is present. With σ m = −300 MPa the compression strength is exceeded. εa,p start is 0.7 × 10−3 and the further deformation is characterized by cyclic hardening until specimen failure at 1.3 × 106 cycles. The minimum of εm,p is −0.3 × 10−1 . The influence of the mean stress on the cyclic deformation behavior is summarized in Fig. 5b for the wheel and tire areas with the greatest differences in the cyclic deformation behavior and microstructure, respectively. The tran-
sition from negative to positive σ m -values results generally in a material- and position-dependent increase of εa,p (Nf /2). Similar trends are observed for the changes in temperature and voltage. For the total-strain-controlled experiments in Fig. 6a (R7, area 2) εa,t = 3 × 10−3 and superimposed total mean strains between −3 × 10−3 ≤ εm,t ≤ +3 × 10−3 were used. From the very first cycle, mean stresses are effective. The influence of the mean strains depends mainly on the cyclic stability of the mean stresses. Mean stress relaxation is generally of great interest regarding the cyclic deformation and service behavior of wheels and in particular of tires. At the wear limit (area 2) of wheels R7, initially high positive and negative mean stresses between −80 MPa ≤ σ m ≤ +90 MPa relax relatively fast to values between −15 MPa ≤ σ m ≤ 2 MPa. Independent from the selected mean strains, the plastic strain amplitudes are arranged in a very close scatterband. Beyond that for R7 and B6 no significant differences in the area-specifical T, N- and U, N-curves as well as in the lifetimes can be determined. The plastic strain amplitudes plotted in Fig. 6b for the areas 1 and 3 of R7 versus the total mean strain result in a nearly horizontal fitting line. The difference in εa,p between area 3 and area 1 is about 0.2
Fig. 6. Total strain control, influence of total mean strains on σ m , N-curves (a); σ m (N = 3) and εa,p (Nf /2) as functions of εm,t (b).
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× 10−3 . Furthermore, the mean stress values at the beginning of the tests at N = 3 cycles are given as a function of εm,t . It is clearly shown, that a smaller ferrite quota (area 1) tends for positive (negative) mean strains to higher (smaller) mean stresses. Compared with area 3 there is at |εm,t | = 3 × 10−3 a coefficient of about 1.4–1.8. For mean-strain-free experiments σ m is area-independent nearby zero.
strain amplitudes, positive (negative) plastic mean strains and decreasing (increasing) lifetimes. Total-strain-controlled experiments with superimposed mean strains showed pronounced mean stress relaxation which occurs the more completely the larger the plastic strain amplitude is. The cyclic deformation behavior and the lifetime are nearly not affected.
5. Conclusions
Acknowledgements
Because of the the industrial heat treatments, the railway materials R7 and B6 are characterized by microstructural gradients over the wheel and tire cross-sections, respectively. Thus, the local cyclic deformation behavior strongly differs. More or less pronounced cyclic softening and hardening processes occur during cyclic loading in stress control or total strain control. Beside the plastic strain amplitude, temperature and voltage measurements are suitable techniques to characterize the cyclic deformation behavior of railway wheels and tires at loading conditions with and without mean loadings as well. εa,p , T and U show a strong interrelation. In stress-controlled experiments with superimposed mean stresses cyclic creep was determined. In comparison to the reference curve at σ m = 0 MPa, mean stresses lead to a variation of the cyclic deformation curves. Positive (negative) mean stresses lead to higher (lower) plastic
The support of this work by Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.
References [1] A. Blarasin, T. Giunti, M. La Sana, V. Stabielli, P. Vanolo, An integrated approach to manage fatigue performance of railway wheels, in: X.-R. Wu, Z.-G. Wang (Eds.), Fatigue’99, Proc. of the 7th Int. Fat. Congr., vol 4/4, Higher Education Press, Beijing, 1999, pp. 2603–2608. [2] A. Ekberg, P. Sotkovszki, Int. J. Fat. 23 (2001) 29. [3] H.-J. Christ, Encyclopedia of Mater. Sci. Technol. (2001) 2955. [4] Y. Pan, E. Macherauch, D. Eifler, Z. Mat.-wiss u. Werkstofftech. 23 (1992) 343. [5] O. Buck, Phys. Stat. Sol. 2 (1962) 535. [6] M. Schelp, D. Eifler, Mater. Sci. Eng. A319–321 (2001) 652. [7] F. Walther, D. Eifler, Z. Metallkd. 92 (2001) 882. [8] H. Harig, D. Dengel, Fat. Eng. Mater. Struc. 3 (1980) 113.
Local cyclic deformation behavior and microstructure of railway wheel materials F. Walther∗ , D. Eifler Institute of Materials Science and Engineering, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany Received 25 August 2003; received in revised form 13 January 2004
Abstract The current investigations concentrate on the relation between the loading and environmental conditions, the local microstructure and the fatigue behavior of highly stressed railway wheel and tire steels. Experiments under stress control and total strain control were performed at ambient temperature with servohydraulic testing systems. Superimposed mean loadings allow an evaluation of cyclic creep and mean stress relaxation effects. Strain, temperature and electrical measuring techniques were used to characterize the cyclic deformation behavior of specimens from different depth positions of the cross-sections of UIC-specified wheel components (UIC: International Railway Union). The measured values show a strong interrelation. The microstructural characterization of the different material conditions was done by light and scanning electron microscopy together with digital image processing. © 2004 Elsevier B.V. All rights reserved. Keywords: Cyclic deformation behavior; Railway wheel and tire steels; Microstructure; σ, ε-hysteresis measurements; Temperature and electrical measurements
1. Introduction The wheel/rail-contact is characterized by high contact forces and small contact areas. Extreme and complex mechanical service loadings with static and dynamic stresses are superimposed by elevated temperatures in the contact area as a result of slipping and braking. The optimization of the wheel/rail-system requires a detailed knowledge of the cyclic deformation behavior of the wheel materials. But only a few systematic investigations are concerned with this topic [1,2]. As a consequence of the industrial heat treatment of complete wheels microstructural gradients occur and the cyclic deformation behavior shows a strong dependence on the local microstructure and can not be quantitatively compared to specimens of homogeneous medium carbon steels with similar chemical compositions, e.g. SAE 1045 [3,4]. The response of the investigated wheel materials to the applied cyclic loading can be described with mechanical hysteresis measurements. Additional temperature and electrical measurements are used to complement the plastic strain ∗ Corresponding author. Tel.: +49-631-2052413; fax: +49-631-2052137. E-mail address: [email protected] (F. Walther).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.05.034
amplitude εa,p . Thereby the change of temperature T and voltage U yield integral signals from the tested material volume. Apart from the geometry, the electrical resistance and consequently the voltage depend on the specific electrical resistance of the material. During cyclic loading, softening and hardening processes influence the defect density and therefore the specific electrical resistance in a characteristic manner. The evaluated values of the electrical resistance and the voltage, respectively, are independent from a gauge length and could be helpful tools to assess the fatigue behavior of complex components. The change of the voltage is also measurable in the unloaded state and can be used as a reference value to describe the proceeding fatigue damage. The presentation of εa,p , T and U versus the number of cycles yield characteristic cyclic deformation, temperature and voltage curves corresponding to the actual fatigue state [5,6].
2. Materials The chemical compositions and the monotonic properties are UIC-conformable. The investigated railway steels contain about 0.52 (R7) and 0.63 (B6) wt.% C, 0.4 and
482
F. Walther, D. Eifler / Materials Science and Engineering A 387–389 (2004) 481–485
Fig. 1. SEM-micrographs of the tread (a) and flange (b) of a wheel R7; the cross-section center of a tire B6 (c).
0.5 wt.% Si, 0.8 and 0.9 wt.% Mn, 0.3 wt.% Cr and small quotas of Cu, Mo and Ni [7]. The industrial heat treatments cause different ferritic-pearlitic microstructures in the rim of the wheels and in the tires, with different ferrite quotas as a function of the distance to the surface. Within monoblock wheels R7 (UIC 812-3 V) the ferrite quota increases with increasing surface distance from about 6 area% near the tread to a maximum of about 20 area%. In tires B6 (UIC 810-1 V) maximum ferrite quotas around 2–3 area% are present in the cross-section center (cf. Fig. 1). In SEM-micrographs area-specific cementite lamellae spacings of 0.14–0.19 m (R7) and 0.11–0.15 m (B6) were identified. A correlation between the ferrite quota and the Vickers hardness in the range 230–300 HV10 (R7) and 300–330 HV10 (B6) could be found for each component along the cross-section [7].
3. Experiments and methods The axial stress-controlled and total-strain-controlled experiments with different load ratios were performed at ambient temperature, using triangular waveforms with a frequency of 5 Hz in servohydraulic testing systems until 2 × 106 cycles. For the cycle-dependent recording of mechanical hysteresis-loops an extensometer was fixed to the polished
specimens surface. Highly accurate temperature measurements were performed with thermocouples in the middle of the gauge length and at the shafts. Temperature changes T are exclusively caused by plastic deformations in the gauge length. A DC power supply with a direct current of 8 A was fixed to both shafts. To measure the difference of the voltage U two wires were connected at the transitions from the gauge length to the shafts. No temperature rise is caused by the applied current. All measured signals are amplified and digitized by DAQ-devices [7].
4. Results The results were achieved with specimens taken from different depth positions with tangential orientation to the cross-sections of the components. To identify critical stress amplitudes and for the estimation of the endurance limit, stepwise load increase tests were performed. The circles in the cross-section drawings of the Figs. 2a and 4a represent the 19 (B6) and 14 (R7) different specimen positions. Dependent on the microstructure and the appropriate cyclic deformation curves determined at a stress amplitude of σ a = 500 MPa (B6) and 400 MPa (R7) the cross-sections of the components can be divided into 4 (B6) and 3 (R7) areas of comparable behavior.
Fig. 2. Stress control, cyclic deformation curves (a); εa,p , T, U–Nf -relation at Nf /2 (b).
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Fig. 3. SEM-surface investigations at 57.5% Nf (a) and Nf (b) (vertical loading direction).
Fig. 2a shows characteristic plastic strain amplitudes of the different specimen positions versus the number of cycles for single step tests with the tire steel B6 at σ a = 500 MPa. In a typical curve, subsequent to position-dependent incubation intervals, cyclic softening with increasing and cyclic hardening with decreasing plastic strain amplitudes are obtained. The final fatigue state is predominated by crack growth. Increasing plastic strain amplitudes after smaller incubation intervals are responsible for a decrease in the number of cycles to failure Nf . As a function of the individual specimen position and microstructure, respectively, the three measured physical values at Nf /2 result in similar curves (Fig. 2b). Decreasing plastic strain amplitudes, changes of temperature and voltage lead to substantially longer lifetimes. For the evaluation of cyclic deformation characteristics and failure mechanisms in defined fatigue states surface polished flat specimens from the tread and the flange of wheels R7 were investigated in the SEM. Fig. 3a shows in- and extrusions and first microcracks after 7 × 103 cycles (57.5% Nf ) with σ a = 400 MPa. The crack path could be correlated with the microstructure of the specimens. The crack runs in the ferritic seam at the grain boundaries (marked in Fig. 3b with white arrows). A variety of stress amplitudes and total strain amplitudes at a load ratio of R = −1 were investigated for a comparison of the cyclic deformation behavior in the selected component areas, e.g. tread and abrasion reserve, respectively, wear limit. Total strain control leads to less pronounced cyclic
softening and hardening processes compared to stress control, as shown in the cyclic deformation curves of Fig. 4a for the three areas of R7 and total strain amplitudes of 2.5 and 4.0 × 10−3 . For both loading parameters the plastic strain amplitude increases with the ferrite quota and reaches the maximum in the flange (area 3). Regarding the lifetime only small differences occur. With T, N- and U, N-curves additional information about the effective cyclic deformation processes can be derived (cf. [8]). For identical loading parameters under stress control and total strain control, the lower ferrite quota in the tread and near-surface areas, respectively cause in general lower εa,p -, T- and U-values and higher lifetimes in comparison with the others. Cyclic stress–strain-curves (CSS-curves) are used to describe the cyclic deformation behavior. The substitution of εa,p by T or U taken at Nf /2 leads to cyclic σ a , T(CST-) or σ a , U- (CSU-) curves as shown in Fig. 4b for total-strain-controlled experiments in the flange (area 3) of wheels R7. The mathematical descriptions are given with a modified relation according to Morrow in the diagram. The measured values are equivalently qualified for a description of the cyclic deformation behavior. The mean stress sensitivity is of great technical relevance for the cyclic deformation and service behavior of railway materials. In stress-controlled experiments with superimposed mean stresses cyclic creep was detected and described in form of εm,p , N-curves. Positive (negative) mean
Fig. 4. Total strain control, cyclic deformation curves (a); CSS-, CST- and CSU-curves (b).
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Fig. 5. Stress control, influence of mean stresses on εa,p , N-curves (a); εa,p (Nf /2) as a function of σ m (b).
stresses lead to higher (lower) plastic strain amplitudes, positive (negative) plastic mean strains εm,p and decreasing (increasing) lifetimes. As a function of the mean stress different shapes of the cyclic deformation curves can be observed. In Fig. 5a the cyclic deformation curves for the tire steel B6 at σ a = 500 MPa and mean stresses in the range −300 MPa ≤ σ m ≤ +200 MPa are exemplarily shown for the cross-section center (area 4). During the first cycles at σ m = 0 MPa and σ m = ± 100 MPa, no or only very small plastic deformation is measured. Because of the distinct exceeding of the yield strength of about 600 MPa, the loading parameters σ a = 500 MPa and σ m = +200 MPa lead to a decrease of εa,p within the first 100 cycles to a minimum of about 1.2 × 10−3 . Until specimen failure cyclic softening combined with cyclic creep strains up to 10−1 is present. With σ m = −300 MPa the compression strength is exceeded. εa,p start is 0.7 × 10−3 and the further deformation is characterized by cyclic hardening until specimen failure at 1.3 × 106 cycles. The minimum of εm,p is −0.3 × 10−1 . The influence of the mean stress on the cyclic deformation behavior is summarized in Fig. 5b for the wheel and tire areas with the greatest differences in the cyclic deformation behavior and microstructure, respectively. The tran-
sition from negative to positive σ m -values results generally in a material- and position-dependent increase of εa,p (Nf /2). Similar trends are observed for the changes in temperature and voltage. For the total-strain-controlled experiments in Fig. 6a (R7, area 2) εa,t = 3 × 10−3 and superimposed total mean strains between −3 × 10−3 ≤ εm,t ≤ +3 × 10−3 were used. From the very first cycle, mean stresses are effective. The influence of the mean strains depends mainly on the cyclic stability of the mean stresses. Mean stress relaxation is generally of great interest regarding the cyclic deformation and service behavior of wheels and in particular of tires. At the wear limit (area 2) of wheels R7, initially high positive and negative mean stresses between −80 MPa ≤ σ m ≤ +90 MPa relax relatively fast to values between −15 MPa ≤ σ m ≤ 2 MPa. Independent from the selected mean strains, the plastic strain amplitudes are arranged in a very close scatterband. Beyond that for R7 and B6 no significant differences in the area-specifical T, N- and U, N-curves as well as in the lifetimes can be determined. The plastic strain amplitudes plotted in Fig. 6b for the areas 1 and 3 of R7 versus the total mean strain result in a nearly horizontal fitting line. The difference in εa,p between area 3 and area 1 is about 0.2
Fig. 6. Total strain control, influence of total mean strains on σ m , N-curves (a); σ m (N = 3) and εa,p (Nf /2) as functions of εm,t (b).
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× 10−3 . Furthermore, the mean stress values at the beginning of the tests at N = 3 cycles are given as a function of εm,t . It is clearly shown, that a smaller ferrite quota (area 1) tends for positive (negative) mean strains to higher (smaller) mean stresses. Compared with area 3 there is at |εm,t | = 3 × 10−3 a coefficient of about 1.4–1.8. For mean-strain-free experiments σ m is area-independent nearby zero.
strain amplitudes, positive (negative) plastic mean strains and decreasing (increasing) lifetimes. Total-strain-controlled experiments with superimposed mean strains showed pronounced mean stress relaxation which occurs the more completely the larger the plastic strain amplitude is. The cyclic deformation behavior and the lifetime are nearly not affected.
5. Conclusions
Acknowledgements
Because of the the industrial heat treatments, the railway materials R7 and B6 are characterized by microstructural gradients over the wheel and tire cross-sections, respectively. Thus, the local cyclic deformation behavior strongly differs. More or less pronounced cyclic softening and hardening processes occur during cyclic loading in stress control or total strain control. Beside the plastic strain amplitude, temperature and voltage measurements are suitable techniques to characterize the cyclic deformation behavior of railway wheels and tires at loading conditions with and without mean loadings as well. εa,p , T and U show a strong interrelation. In stress-controlled experiments with superimposed mean stresses cyclic creep was determined. In comparison to the reference curve at σ m = 0 MPa, mean stresses lead to a variation of the cyclic deformation curves. Positive (negative) mean stresses lead to higher (lower) plastic
The support of this work by Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.
References [1] A. Blarasin, T. Giunti, M. La Sana, V. Stabielli, P. Vanolo, An integrated approach to manage fatigue performance of railway wheels, in: X.-R. Wu, Z.-G. Wang (Eds.), Fatigue’99, Proc. of the 7th Int. Fat. Congr., vol 4/4, Higher Education Press, Beijing, 1999, pp. 2603–2608. [2] A. Ekberg, P. Sotkovszki, Int. J. Fat. 23 (2001) 29. [3] H.-J. Christ, Encyclopedia of Mater. Sci. Technol. (2001) 2955. [4] Y. Pan, E. Macherauch, D. Eifler, Z. Mat.-wiss u. Werkstofftech. 23 (1992) 343. [5] O. Buck, Phys. Stat. Sol. 2 (1962) 535. [6] M. Schelp, D. Eifler, Mater. Sci. Eng. A319–321 (2001) 652. [7] F. Walther, D. Eifler, Z. Metallkd. 92 (2001) 882. [8] H. Harig, D. Dengel, Fat. Eng. Mater. Struc. 3 (1980) 113.