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Dynamic behaviour of railway fastening setting pads
Engineering Failure Analysis 14 (2007) 364–373 www.elsevier.com/locate/engfailanal
Dynamic behaviour of railway fastening setting pads I.A. Carrascal *, J.A. Casado, J.A. Polanco, F. Gutie´rrez-Solana Departamento de Ciencia e Ingenierı´a del Terreno y de los Materiales E.T.S. de Ingenieros de Caminos, Canales y Puertos, Universidad de Cantabria, Avenida de las Castros s/n, 39005 Santander, Spain Available online 19 May 2006
Abstract Thermoplastic elastomer railway pads, placed between the base of the steel rails and the prestressed concrete sleepers, play a major role in the general maintenance of the state of the overall structure of a railway line. These rail setting pads provide the elasticity of the line and damp the vibrations which the rail transmits to the sleeper, thus avoiding the cracking of the concrete sleepers and preventing the wear and tear of the ballast. They also provide electrical insulation between the rails. This paper describes a test methodology for determining the dynamic behaviour of railway fastening setting pads in working conditions, which may involve a wide range of temperatures associated to the wear and the hardening they undergo during the cyclical load process applied on them caused by the passing of the trains. To this end, measurements of the evolution of the stiffness of the pads have been made, both static and dynamic. Also associated energy parameters have been measured, establishing an index of the degree of deterioration undergone by these pads. 2006 Elsevier Ltd. All rights reserved. Keywords: Railway fastening; Setting pads; Dynamic behaviour; Stiffness; Dissipated energy
1. Introduction The widespread use of the concrete sleeper has led to substantial changes in railway fastening systems, since these require an elastic element to be placed between the rail and the sleeper itself in order to avoid, as far as possible, impact loads between the two components and to counteract the excessive stiffness of the concrete (3– 5 times stiffer than wooden sleepers) [1]. This element is called the ‘elastic setting pad’. As a new railway is subjected to the repetitive loads caused by the passing of train wheels, it starts to undergo smaller and smaller plastic deformations until, with time, the system adjusts and reaches an elastic regimen similar to that of homogenous solids. The elasticity of the rail is quantified through the use of the vertical stiffness parameter, k, which is defined as the relation between a load applied on a rail and the vertical deformation produced by this load. This value is valid only in the viscoelastic domain, that is, for small deformations.
*
Corresponding author. Tel.: +34 942 200915; fax: +34 20 18 18. E-mail address: [email protected] (I.A. Carrascal).
1350-6307/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2006.02.003
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365
The vertical stiffness of the rail support could be quantified using the stiffness of its components, assuming that the rail support is a set of the system of ballast, platform and setting pad, so that an increase in the stiffness of the pad will lead to an increase in the stiffness of the rail support. It has been verified that a high value of stiffness in the setting pad increases the dynamic overloads due to the non-suspended masses, thus accelerating the deterioration of the railway, while a low value leads to an excessive subsidence of the rail, with a substantial increase in stress in the rails [2]. Thus, once the optimum stiffness value is obtained, it is important to delimit this nominal value, both the higher and the lower limits. In the Technical Specification for the supply of fastenings [3], both the static and the dynamic stiffness values are delimited. The static vertical stiffness value, ks, must be within the range of 80 6 ks 6 125 kN/mm, while the dynamic stiffness value, kd, must be inside ks 6 kd 6 2 Æ ks. Due to the nature of the material of the pads, these stiffness values can be altered by different environmental agents such as temperature, which can fluctuate between 20 and 80 C on the railway track, humidity or the ageing undergone by the pads due to the continuous mechanical forces of fatigue under compression. The technical specification considers the study of ageing through fatigue by means of a test at 3 · 106 cycles with an inclined load, allowing an increasing pad stiffness of 25% with respect to ks after the test. The work described in this paper has verified how all of these variables, present in normal working conditions, affect the stiffness of the pad, establishing the circumstances under which it can reach levels outside the specification and thus, out of service. 2. Component under study Two different kinds of setting pads have been used in this work depending on the type of rail (UIC 54 and UIC 60). Both pads have been injected by MONDRAGON company. The geometry of each of these is shown in Table 1. In the width dimension, only the working portion of the pad is considered when referring to properties such as the dampening and absorption of vibrations. The 2 mm thick tips of the elastic setting pad are not included. The surfaces of both sides of the elastic setting pad are made up of a series of oblongs of nominal measurements of 8.45 mm width and 14 mm length. The oblongs have a thickness of 2 mm and a conicity of 10. The mission of the oblongs is to absorb the efforts produced by the circulation of the vehicles, providing the railway track with elasticity. It works rather like small springs which, when compression loads appear, are deformed and shortened, thus absorbing some of the stress. It should be noted that the oblongs of the two sides of the pad do not coincide in position, so that the action of the oblongs of one side does not interfere with those of the other [4]. Fig. 1 shows an aerial view of the setting pad and the other components of the fastening system set up without the rail. Table 1 Dimensions of setting pads in mm Nomenclature
Width
Length
Thickness
PAE-1 (UIC-54) PAE-2 (UIC-60)
140 ± 1 148 ± 1
180 ± 1 180 ± 1
þ0:15 70:05 þ0:15 70:05
Fig. 1. Fastening system without rail.
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The material with which the setting pads have been injected is a thermoplastic elastomer. This type of material combines the processing characteristics of thermoplastics with their physical properties, typical of vulcanised rubbers, such as a high elastic deformation. They are of interest mainly because they do not require any process of vulcanisation, slow irreversible processes performed by means of heating. With this material the passage from the melted to the solid state takes place by cooling. This is a fast and reversible process [5]. The injected material is called HYTREL, manufactured by Du Pont, and is a thermoplastic polyester elastomer (TPEE). 3. Experimental methodology For both the stiffness and the fatigue tests, a vertical load is applied on the setting pad by means of a device shown in Fig. 2. The device simulates real working conditions, replacing the sleeper with a steel support with dimensions of 162 · 165 mm. The load is applied by means of a rail piece equipped with a kneecap to ensure the verticality of the efforts. The mean vertical descent recorded by four displacement transducer of ±5 mm, one on each corner of the support, is considered as a measure of the deformation of the setting pad. The load is applied by means of an actuator of ±250 kN capacity. The influence of temperature on the mechanical behaviour of the setting pad is evaluated by placing the test device inside a climatic chamber adapted to the test machine as shown in Fig. 3. Shore D hardness measurements were also made in order to find a correlation between this parameter and the evolution of the mechanical behaviour of the pad [6]. The static vertical stiffness tests were performed following the guidelines given in technical specification [3], three load cycles being performed, the last of these being measured. The load sequence with its corresponding speeds and waiting times is shown in Fig. 4 (maximum force, Fmax = 95 kN and minimum force, Fmin = 20 kN). The static behaviour of the PAE-1 pads was studied at different temperatures, 10, 20, 50 and 80 C. For the dynamic stiffness tests [7], 1000 sinusoidal load cycles of between 20 and 95 kN were applied at a frequency of 5 Hz, the stiffness measurement being taken in the last 10 cycles. The dynamic stiffness was analysed for different temperatures: 20, 40, 60 and 80 C. The evaluation of the effect of a high number of cycles was carried out by means of a test of 2 · 106 cycles with the same load levels at room temperature and the effects of different load levels is analysed by means of a Locati test [8] with four load levels (20/95, 20/110, 20/125 and 20/140 kN) of 50,000 cycles each.
Fig. 2. Test device.
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Fig. 3. Test device in climatic chamber.
Fig. 4. Sequence of static stiffness test.
The geometry of the setting pad made it difficult to control the evolution of temperature throughout the fatigue test with contact methods, so that infrared thermography techniques are used. In order to evaluate the influence of the initial stiffness of the pad on the fatigue behaviour, as well as the environmental conditions, several PAE-2 pads were aged using several methods. These treatments are listed below: Normal (N): state of reception (humidity H = 0.5%) Modified (M): physical change, elimination of 20 oblongs (4 g of mass) Climatic (C): treatment in climatic chamber for 25 cycles of 36 h. with variations between 20 and 70 C and 0 and 90% HR Dry (D): treatment in stove for 2 months at 100 C (H = 0%) Humid (H): immersion in water at 40 C for 2 months (H = 2.3%) In each condition, the pads were subjected to a fatigue of 200,000 cycles at 5 Hz between values of 18 and 93 kN, their static stiffness being measured before and after this test.
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4. Results and analysis 4.1. Static and dynamic tests The graph in Fig. 5 shows the evolution of the thickness reduction of the PAE-1 type pad in the third load– unload cycle applied in the static case at temperatures of 10, 20, 50 and 80 C. The thickness reduction of the pad varies between maximum values of 0.93 mm at 80 C and a minimum of 0.59 mm at 10 C. Fig. 6 shows the good linear correlation between the static stiffness and the Shore D hardness with the temperature of the pad. For extreme temperature values, the stiffness practically reaches the established limits (80– 125 kN/mm), 128.0 kN/mm for 10 C and 80.6 kN/mm for 80 C. The graph in Fig. 7 represents the evolution of the thickness reduction of the PAE-1 type pad in the last of the 1000 cycles applied, at temperatures of 20, 40, 60 and 80 C. A difference can be observed in relation to the above static behaviour. Thus, for 80 C in the dynamic case, the thickness of the pad is reduced by only 0.6 mm, having reached a deformation 35.5% higher in the static regime. This difference translates not only into a reduction in the deformations but also into a drop in the dissipated energy at the beginning (Eddo) and at the end (Eddf) of the dynamic tests in relation to the dissipated energy in the static state (Eds). In other words, the material in the dynamic state resembles more an elastic body that does not waste energy and, as in the static state, this is even more so the lower the temperature (Fig. 8). Fig. 9 shows the evolution of the dynamic stiffness throughout the 1000 cycles applied for the different temperatures, showing that there is a quantitative change in stiffness values between 40 and 60 C. This indicates that the glass transition temperature of this material could be within this temperature range.
Fig. 5. Static behaviour at different temperatures.
Fig. 6. Evolution of stiffness and hardness with temperature.
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Fig. 7. Dynamic behaviour at different temperature.
Fig. 8. Dissipated energy in static and dynamic states.
Fig. 9. Evolution of dynamic stiffness.
Fig. 10 shows the dynamic stiffness values measured in the first of the cycles, kdo, and that measured in cycle 1000, kd, as well as the variation between these two parameters, Dkd. It can be observed that as the temperature of the pad increases, both parameters become more and more different from one another. This fact corroborates the change in properties observed above between 40 and 60 C. At 40 C the Dkd does not exceed 1.5%, reaching 10% for 60 C.
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Fig. 10. Initial and final dynamic stiffness and their variation.
Representing the energy dissipated, Ed, per cycle along the dynamic test (Fig. 11), it is verified that Ed increases as the temperature of the pad increases, remaining practically constant for temperatures of 20 and 40 C. The dissipated energies decrease with the number of cycles applied, due to the change in geometry of the pad caused by its progressive thickness reduction during the test. This drop in the Ed is sharper for the highest temperatures. 4.2. Conventional fatigue test In order to verify the ageing undergone by the pad due to a high cycle fatigue process, a PAE-1 pad was subjected to 2 · 106 load cycles at room temperature and with the same loading variation level as in the tests at different temperatures. The graph in Fig. 12 shows the evolution of the dynamic stiffness and the energy dissipated per cycle. It can be observed that the greatest variation for both parameters takes place in the first 200,000 cycles, approximately, and is less pronounced after this. Before and after the fatigue test, static tests were carried out in order to evaluate the loss of properties from the study of the static stiffness and the dissipated energy. The evolution of the last cycle of each test and the values obtained are shown in Fig. 13. The value of the static stiffness increases after fatigue by 18.5%, while the value for dissipated energy in the static test drops by 41.6%. If the same parameters are analysed in the dynamic test and the difference between the beginning and the end of the test are analysed, the variations are quite similar, 17.3% for dynamic stiffness and 40.0% for dissipated energy. At the same time, the temperature of the pad increases only 7 C after 2 · 106 cycles.
Fig. 11. Evolution of Ed throughout the 1000 cycles.
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Fig. 12. kd and Ed of 2 · 106 cycles.
Fig. 13. Static test before and after fatigue.
4.3. Accelerated fatigue test The graph in Fig. 14 shows the evolution of stiffness and temperature of the pad throughout the Locati test. It can be observed that with the first loading variation level, the first 50,000 cycles, almost the same stiffness is reached as that attained at the end of the 2 · 106 cycles of the conventional fatigue test. On the other hand, the final pad temperature obtained in the long fatigue test is reached at the end of the second load variation level
Fig. 14. Stiffness and Temperature in the Locati test.
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Fig. 15. Variation in dissipated energy at each loading level.
in the accelerated test, that is 50,000 cycles more with a maximum force which is 15 kN greater than the first level. The graph of Fig. 15 shows the exponential growth of the dissipated energy of the pad as the load variation level increases. It represents the dissipated energy at the beginning (Edo) and at the end (Edf) of each loading level. The same graph shows an exponential drop of the variation in the dissipated energy, DEd, between the beginning and the end of each fatigue block. 4.4. Ageing tests Table 2 shows the initial static vertical stiffness values, ks0, of the PAE-2 pads for each ageing condition. In order to verify the possible loss of operability after the 200,000 cycles, the stiffness and the dissipated energy were evaluated for the whole cycle by means of a static stiffness test. (Figs. 16 and 17, respectively). It can be observed that a low initial stiffness value of the pad, whether this is through physical modification or environTable 2 Initial stiffness, ageing conditions PAE-2 type
ks0 (kN/mm)
Modified (M) Humid (H) Dry (D) Normal (N) Climatic (C)
84.2 99.1 107.3 110.7 111.9
Fig. 16. Increase in stiffness starting from initial value.
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Fig. 17. Ed and its variation as a function of the intial k.
mental ageing, leads to its premature deterioration. This fact is reflected in a greater loss in stiffness and in dissipated energy per cycle in the pad, after the fatigue process. 5. Conclusions The deterioration in the railway setting pad produced by its use in normal working conditions can be evaluated using the value of dissipated energy, Ed, per cycle in both dynamic and static conditions. This energetic parameter varies linearly with temperature, allowing a direct correlation to be established with the stiffness and hardness of the pad. Hardness correlates linearly with stiffness and so an index of the degree of degradation in the pad can be established. This parameter is very useful as it can be easily measured by a non destructive test on the own track. Extreme temperature values on the railway track (20 and 80 C) establish threshold working values for setting pads. Under normal working conditions, the environmental ageing of the setting pads does not generate a great decrease in the mechanical performance characteristics. In the worst case, for pads with an excessive degree of humidity, condition H, a stiffness increase of 12% after the fatigue process, can be obtained. References [1] Alias J, Valdes A. La vı´a de ferrocarril; 1990. [2] Lopez-Pita A. La rigidez vertical de la vı´a y el deterioro de las lineas de alta velocidad. Revista de obras pu´blicas. No. 3415; 2001. p. 7– 26. [3] GIF. Pliego de bases para el suministro de sujeciones; 1999. [4] RENFE, Planos de la placa acodad de asiento PAE-1 (P16.5075.00) y PAE-2 (P16.5076.00). [5] Lo´pez Manchado MA, Arroyo Ramos M. Rev Pla´sticos Modernos 1999;78(522):677–83. [6] UNE-EN ISO 869. Pla´sticos y ebonitas. Determinacio´n de la dureza de indentacio´n por medio de un duro´metro (dureza Shore). [7] UNE-EN 13481-2. Aplicaciones ferroviarias. Vı´a. Requisitos de funcionamiento para los sistemas de sujecio´n. Parte 2: Sistemas de sujecio´n para las traviesas de hormigo´n; 2003. [8] Locati L. Programmed fatigue test, variable amplitude rotat. Metall Ital 1952;44(4):135–44.
Dynamic behaviour of railway fastening setting pads I.A. Carrascal *, J.A. Casado, J.A. Polanco, F. Gutie´rrez-Solana Departamento de Ciencia e Ingenierı´a del Terreno y de los Materiales E.T.S. de Ingenieros de Caminos, Canales y Puertos, Universidad de Cantabria, Avenida de las Castros s/n, 39005 Santander, Spain Available online 19 May 2006
Abstract Thermoplastic elastomer railway pads, placed between the base of the steel rails and the prestressed concrete sleepers, play a major role in the general maintenance of the state of the overall structure of a railway line. These rail setting pads provide the elasticity of the line and damp the vibrations which the rail transmits to the sleeper, thus avoiding the cracking of the concrete sleepers and preventing the wear and tear of the ballast. They also provide electrical insulation between the rails. This paper describes a test methodology for determining the dynamic behaviour of railway fastening setting pads in working conditions, which may involve a wide range of temperatures associated to the wear and the hardening they undergo during the cyclical load process applied on them caused by the passing of the trains. To this end, measurements of the evolution of the stiffness of the pads have been made, both static and dynamic. Also associated energy parameters have been measured, establishing an index of the degree of deterioration undergone by these pads. 2006 Elsevier Ltd. All rights reserved. Keywords: Railway fastening; Setting pads; Dynamic behaviour; Stiffness; Dissipated energy
1. Introduction The widespread use of the concrete sleeper has led to substantial changes in railway fastening systems, since these require an elastic element to be placed between the rail and the sleeper itself in order to avoid, as far as possible, impact loads between the two components and to counteract the excessive stiffness of the concrete (3– 5 times stiffer than wooden sleepers) [1]. This element is called the ‘elastic setting pad’. As a new railway is subjected to the repetitive loads caused by the passing of train wheels, it starts to undergo smaller and smaller plastic deformations until, with time, the system adjusts and reaches an elastic regimen similar to that of homogenous solids. The elasticity of the rail is quantified through the use of the vertical stiffness parameter, k, which is defined as the relation between a load applied on a rail and the vertical deformation produced by this load. This value is valid only in the viscoelastic domain, that is, for small deformations.
*
Corresponding author. Tel.: +34 942 200915; fax: +34 20 18 18. E-mail address: [email protected] (I.A. Carrascal).
1350-6307/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2006.02.003
I.A. Carrascal et al. / Engineering Failure Analysis 14 (2007) 364–373
365
The vertical stiffness of the rail support could be quantified using the stiffness of its components, assuming that the rail support is a set of the system of ballast, platform and setting pad, so that an increase in the stiffness of the pad will lead to an increase in the stiffness of the rail support. It has been verified that a high value of stiffness in the setting pad increases the dynamic overloads due to the non-suspended masses, thus accelerating the deterioration of the railway, while a low value leads to an excessive subsidence of the rail, with a substantial increase in stress in the rails [2]. Thus, once the optimum stiffness value is obtained, it is important to delimit this nominal value, both the higher and the lower limits. In the Technical Specification for the supply of fastenings [3], both the static and the dynamic stiffness values are delimited. The static vertical stiffness value, ks, must be within the range of 80 6 ks 6 125 kN/mm, while the dynamic stiffness value, kd, must be inside ks 6 kd 6 2 Æ ks. Due to the nature of the material of the pads, these stiffness values can be altered by different environmental agents such as temperature, which can fluctuate between 20 and 80 C on the railway track, humidity or the ageing undergone by the pads due to the continuous mechanical forces of fatigue under compression. The technical specification considers the study of ageing through fatigue by means of a test at 3 · 106 cycles with an inclined load, allowing an increasing pad stiffness of 25% with respect to ks after the test. The work described in this paper has verified how all of these variables, present in normal working conditions, affect the stiffness of the pad, establishing the circumstances under which it can reach levels outside the specification and thus, out of service. 2. Component under study Two different kinds of setting pads have been used in this work depending on the type of rail (UIC 54 and UIC 60). Both pads have been injected by MONDRAGON company. The geometry of each of these is shown in Table 1. In the width dimension, only the working portion of the pad is considered when referring to properties such as the dampening and absorption of vibrations. The 2 mm thick tips of the elastic setting pad are not included. The surfaces of both sides of the elastic setting pad are made up of a series of oblongs of nominal measurements of 8.45 mm width and 14 mm length. The oblongs have a thickness of 2 mm and a conicity of 10. The mission of the oblongs is to absorb the efforts produced by the circulation of the vehicles, providing the railway track with elasticity. It works rather like small springs which, when compression loads appear, are deformed and shortened, thus absorbing some of the stress. It should be noted that the oblongs of the two sides of the pad do not coincide in position, so that the action of the oblongs of one side does not interfere with those of the other [4]. Fig. 1 shows an aerial view of the setting pad and the other components of the fastening system set up without the rail. Table 1 Dimensions of setting pads in mm Nomenclature
Width
Length
Thickness
PAE-1 (UIC-54) PAE-2 (UIC-60)
140 ± 1 148 ± 1
180 ± 1 180 ± 1
þ0:15 70:05 þ0:15 70:05
Fig. 1. Fastening system without rail.
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The material with which the setting pads have been injected is a thermoplastic elastomer. This type of material combines the processing characteristics of thermoplastics with their physical properties, typical of vulcanised rubbers, such as a high elastic deformation. They are of interest mainly because they do not require any process of vulcanisation, slow irreversible processes performed by means of heating. With this material the passage from the melted to the solid state takes place by cooling. This is a fast and reversible process [5]. The injected material is called HYTREL, manufactured by Du Pont, and is a thermoplastic polyester elastomer (TPEE). 3. Experimental methodology For both the stiffness and the fatigue tests, a vertical load is applied on the setting pad by means of a device shown in Fig. 2. The device simulates real working conditions, replacing the sleeper with a steel support with dimensions of 162 · 165 mm. The load is applied by means of a rail piece equipped with a kneecap to ensure the verticality of the efforts. The mean vertical descent recorded by four displacement transducer of ±5 mm, one on each corner of the support, is considered as a measure of the deformation of the setting pad. The load is applied by means of an actuator of ±250 kN capacity. The influence of temperature on the mechanical behaviour of the setting pad is evaluated by placing the test device inside a climatic chamber adapted to the test machine as shown in Fig. 3. Shore D hardness measurements were also made in order to find a correlation between this parameter and the evolution of the mechanical behaviour of the pad [6]. The static vertical stiffness tests were performed following the guidelines given in technical specification [3], three load cycles being performed, the last of these being measured. The load sequence with its corresponding speeds and waiting times is shown in Fig. 4 (maximum force, Fmax = 95 kN and minimum force, Fmin = 20 kN). The static behaviour of the PAE-1 pads was studied at different temperatures, 10, 20, 50 and 80 C. For the dynamic stiffness tests [7], 1000 sinusoidal load cycles of between 20 and 95 kN were applied at a frequency of 5 Hz, the stiffness measurement being taken in the last 10 cycles. The dynamic stiffness was analysed for different temperatures: 20, 40, 60 and 80 C. The evaluation of the effect of a high number of cycles was carried out by means of a test of 2 · 106 cycles with the same load levels at room temperature and the effects of different load levels is analysed by means of a Locati test [8] with four load levels (20/95, 20/110, 20/125 and 20/140 kN) of 50,000 cycles each.
Fig. 2. Test device.
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Fig. 3. Test device in climatic chamber.
Fig. 4. Sequence of static stiffness test.
The geometry of the setting pad made it difficult to control the evolution of temperature throughout the fatigue test with contact methods, so that infrared thermography techniques are used. In order to evaluate the influence of the initial stiffness of the pad on the fatigue behaviour, as well as the environmental conditions, several PAE-2 pads were aged using several methods. These treatments are listed below: Normal (N): state of reception (humidity H = 0.5%) Modified (M): physical change, elimination of 20 oblongs (4 g of mass) Climatic (C): treatment in climatic chamber for 25 cycles of 36 h. with variations between 20 and 70 C and 0 and 90% HR Dry (D): treatment in stove for 2 months at 100 C (H = 0%) Humid (H): immersion in water at 40 C for 2 months (H = 2.3%) In each condition, the pads were subjected to a fatigue of 200,000 cycles at 5 Hz between values of 18 and 93 kN, their static stiffness being measured before and after this test.
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4. Results and analysis 4.1. Static and dynamic tests The graph in Fig. 5 shows the evolution of the thickness reduction of the PAE-1 type pad in the third load– unload cycle applied in the static case at temperatures of 10, 20, 50 and 80 C. The thickness reduction of the pad varies between maximum values of 0.93 mm at 80 C and a minimum of 0.59 mm at 10 C. Fig. 6 shows the good linear correlation between the static stiffness and the Shore D hardness with the temperature of the pad. For extreme temperature values, the stiffness practically reaches the established limits (80– 125 kN/mm), 128.0 kN/mm for 10 C and 80.6 kN/mm for 80 C. The graph in Fig. 7 represents the evolution of the thickness reduction of the PAE-1 type pad in the last of the 1000 cycles applied, at temperatures of 20, 40, 60 and 80 C. A difference can be observed in relation to the above static behaviour. Thus, for 80 C in the dynamic case, the thickness of the pad is reduced by only 0.6 mm, having reached a deformation 35.5% higher in the static regime. This difference translates not only into a reduction in the deformations but also into a drop in the dissipated energy at the beginning (Eddo) and at the end (Eddf) of the dynamic tests in relation to the dissipated energy in the static state (Eds). In other words, the material in the dynamic state resembles more an elastic body that does not waste energy and, as in the static state, this is even more so the lower the temperature (Fig. 8). Fig. 9 shows the evolution of the dynamic stiffness throughout the 1000 cycles applied for the different temperatures, showing that there is a quantitative change in stiffness values between 40 and 60 C. This indicates that the glass transition temperature of this material could be within this temperature range.
Fig. 5. Static behaviour at different temperatures.
Fig. 6. Evolution of stiffness and hardness with temperature.
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Fig. 7. Dynamic behaviour at different temperature.
Fig. 8. Dissipated energy in static and dynamic states.
Fig. 9. Evolution of dynamic stiffness.
Fig. 10 shows the dynamic stiffness values measured in the first of the cycles, kdo, and that measured in cycle 1000, kd, as well as the variation between these two parameters, Dkd. It can be observed that as the temperature of the pad increases, both parameters become more and more different from one another. This fact corroborates the change in properties observed above between 40 and 60 C. At 40 C the Dkd does not exceed 1.5%, reaching 10% for 60 C.
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Fig. 10. Initial and final dynamic stiffness and their variation.
Representing the energy dissipated, Ed, per cycle along the dynamic test (Fig. 11), it is verified that Ed increases as the temperature of the pad increases, remaining practically constant for temperatures of 20 and 40 C. The dissipated energies decrease with the number of cycles applied, due to the change in geometry of the pad caused by its progressive thickness reduction during the test. This drop in the Ed is sharper for the highest temperatures. 4.2. Conventional fatigue test In order to verify the ageing undergone by the pad due to a high cycle fatigue process, a PAE-1 pad was subjected to 2 · 106 load cycles at room temperature and with the same loading variation level as in the tests at different temperatures. The graph in Fig. 12 shows the evolution of the dynamic stiffness and the energy dissipated per cycle. It can be observed that the greatest variation for both parameters takes place in the first 200,000 cycles, approximately, and is less pronounced after this. Before and after the fatigue test, static tests were carried out in order to evaluate the loss of properties from the study of the static stiffness and the dissipated energy. The evolution of the last cycle of each test and the values obtained are shown in Fig. 13. The value of the static stiffness increases after fatigue by 18.5%, while the value for dissipated energy in the static test drops by 41.6%. If the same parameters are analysed in the dynamic test and the difference between the beginning and the end of the test are analysed, the variations are quite similar, 17.3% for dynamic stiffness and 40.0% for dissipated energy. At the same time, the temperature of the pad increases only 7 C after 2 · 106 cycles.
Fig. 11. Evolution of Ed throughout the 1000 cycles.
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Fig. 12. kd and Ed of 2 · 106 cycles.
Fig. 13. Static test before and after fatigue.
4.3. Accelerated fatigue test The graph in Fig. 14 shows the evolution of stiffness and temperature of the pad throughout the Locati test. It can be observed that with the first loading variation level, the first 50,000 cycles, almost the same stiffness is reached as that attained at the end of the 2 · 106 cycles of the conventional fatigue test. On the other hand, the final pad temperature obtained in the long fatigue test is reached at the end of the second load variation level
Fig. 14. Stiffness and Temperature in the Locati test.
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Fig. 15. Variation in dissipated energy at each loading level.
in the accelerated test, that is 50,000 cycles more with a maximum force which is 15 kN greater than the first level. The graph of Fig. 15 shows the exponential growth of the dissipated energy of the pad as the load variation level increases. It represents the dissipated energy at the beginning (Edo) and at the end (Edf) of each loading level. The same graph shows an exponential drop of the variation in the dissipated energy, DEd, between the beginning and the end of each fatigue block. 4.4. Ageing tests Table 2 shows the initial static vertical stiffness values, ks0, of the PAE-2 pads for each ageing condition. In order to verify the possible loss of operability after the 200,000 cycles, the stiffness and the dissipated energy were evaluated for the whole cycle by means of a static stiffness test. (Figs. 16 and 17, respectively). It can be observed that a low initial stiffness value of the pad, whether this is through physical modification or environTable 2 Initial stiffness, ageing conditions PAE-2 type
ks0 (kN/mm)
Modified (M) Humid (H) Dry (D) Normal (N) Climatic (C)
84.2 99.1 107.3 110.7 111.9
Fig. 16. Increase in stiffness starting from initial value.
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373
Fig. 17. Ed and its variation as a function of the intial k.
mental ageing, leads to its premature deterioration. This fact is reflected in a greater loss in stiffness and in dissipated energy per cycle in the pad, after the fatigue process. 5. Conclusions The deterioration in the railway setting pad produced by its use in normal working conditions can be evaluated using the value of dissipated energy, Ed, per cycle in both dynamic and static conditions. This energetic parameter varies linearly with temperature, allowing a direct correlation to be established with the stiffness and hardness of the pad. Hardness correlates linearly with stiffness and so an index of the degree of degradation in the pad can be established. This parameter is very useful as it can be easily measured by a non destructive test on the own track. Extreme temperature values on the railway track (20 and 80 C) establish threshold working values for setting pads. Under normal working conditions, the environmental ageing of the setting pads does not generate a great decrease in the mechanical performance characteristics. In the worst case, for pads with an excessive degree of humidity, condition H, a stiffness increase of 12% after the fatigue process, can be obtained. References [1] Alias J, Valdes A. La vı´a de ferrocarril; 1990. [2] Lopez-Pita A. La rigidez vertical de la vı´a y el deterioro de las lineas de alta velocidad. Revista de obras pu´blicas. No. 3415; 2001. p. 7– 26. [3] GIF. Pliego de bases para el suministro de sujeciones; 1999. [4] RENFE, Planos de la placa acodad de asiento PAE-1 (P16.5075.00) y PAE-2 (P16.5076.00). [5] Lo´pez Manchado MA, Arroyo Ramos M. Rev Pla´sticos Modernos 1999;78(522):677–83. [6] UNE-EN ISO 869. Pla´sticos y ebonitas. Determinacio´n de la dureza de indentacio´n por medio de un duro´metro (dureza Shore). [7] UNE-EN 13481-2. Aplicaciones ferroviarias. Vı´a. Requisitos de funcionamiento para los sistemas de sujecio´n. Parte 2: Sistemas de sujecio´n para las traviesas de hormigo´n; 2003. [8] Locati L. Programmed fatigue test, variable amplitude rotat. Metall Ital 1952;44(4):135–44.