Design and Application of Elastic Modulus of Track Pads
What is the elastic modulus range and design basis of under-rail pads for conventional railways?
The elastic modulus of under-rail pads for conventional railways is usually 80-150MPa, which can balance the shock absorption effect and bearing capacity of the pads and adapt to the operating conditions of conventional speed trains. Its design basis is mainly the axle load and operating speed of the line. The axle load of conventional railways is mostly 21t, and the modulus of 80-150MPa can effectively buffer the medium-intensity impact between wheels and rails. The design also needs to refer to the sleeper type. The modulus of pads for wooden sleeper lines can be biased to the lower limit to enhance shock absorption; the modulus of pads for concrete sleeper lines can be biased to the upper limit to improve structural stability. At the same time, the cost of pads in this modulus range is moderate, which meets the economic needs of conventional railways and can control project investment while ensuring performance. In addition, the design needs to consider regional climate differences. The modulus of pads in humid areas needs to be increased by 5%-10% to prevent the modulus from decreasing due to softening of the pads after water absorption.
Why do under-rail pads for high-speed railways adopt a low elastic modulus design?
The elastic modulus of under-rail pads for high-speed railways is usually controlled at 20-50MPa. The low modulus design can greatly improve the elastic deformation capacity of the pads and effectively absorb the high-frequency vibration generated when high-speed trains are running. The speed of high-speed trains can reach 350km/h, and the impact frequency between wheels and rails is high and the duration is short. Low modulus pads can quickly buffer vibration through their own deformation and reduce the transmission of vibration to the track bed and subgrade. The low modulus design can also reduce the contact stress between wheels and rails, avoid fatigue damage of rails caused by long-term high-frequency impact, and extend the service life of rails. In addition, low modulus pads can improve the ride smoothness of trains, reduce noise inside the trains, and improve passengers' riding experience. At the same time, low modulus pads have better compatibility with ballastless tracks, can better fit the track structure, and ensure the long-term stability of track geometric alignment.
What is the necessity of high elastic modulus design for under-rail pads of heavy-haul railways?
The axle load of heavy-haul railway trains can reach more than 30t, and the vertical load between wheels and rails is extremely large. High elastic modulus pads (200-300MPa) can provide sufficient support stiffness to avoid excessive compressive deformation of the pads. High modulus pads have stronger resistance to compressive permanent deformation. Under long-term heavy load, their deformation can be controlled within 3%, which is far lower than the deformation standard of low modulus pads, ensuring the stability of the track structure. The impact force of freight trains in heavy-haul railways is large when starting and braking. High modulus pads can effectively resist instantaneous impact loads and prevent local settlement or deformation of the track. In addition, the pressure distribution on the bearing surface of high modulus pads is more uniform, which can reduce the local stress concentration of sleepers and extend the service life of sleepers. At the same time, high modulus pads have better wear resistance, can adapt to the high-density transportation conditions of heavy-haul lines, and reduce the replacement frequency of pads.
What factors should be considered in the elastic modulus selection of under-rail pads for urban rail transit?
The elastic modulus selection of under-rail pads for urban rail transit should give priority to vibration and noise reduction requirements. The modulus of pads for underground lines should be 30-60MPa, which can effectively reduce the impact of vibration on surrounding residents. Urban rail trains start and stop frequently, and the effect of alternating loads is obvious. When selecting, it is necessary to pay attention to the fatigue resistance of the pads to ensure that the modulus does not decay significantly under long-term alternating loads. The proportion of curve sections in urban rail lines is high, and the lateral load is large. The modulus of pads should be increased by 20%-30% compared with that of straight sections to enhance lateral restraint capacity and prevent rail lateral displacement. In addition, most urban rail lines are underground or elevated structures with limited maintenance space. When selecting, it is necessary to choose pad products with stable modulus and long service life to reduce the workload of later maintenance. At the same time, the insulation performance of the pads should be considered to ensure that the modulus design does not conflict with the insulation requirements and ensure the normal operation of the urban rail signal system.
How to conduct on-site detection of the elastic modulus of under-rail pads?
On-site detection of the elastic modulus of under-rail pads requires the use of a portable compressive modulus tester. During the test, vertical loads should be applied to simulate actual working conditions, and the load size corresponds to the design axle load of the line. Before the test, it is necessary to clean the debris and stains on the surface of the pad to ensure that the contact surface between the pad and the test instrument is flat, so as to avoid affecting the test results due to poor contact. During the test, the deformation of the pad under different loads should be recorded, and the elastic modulus value should be calculated through the stress-strain curve, and compared with the design standard to judge whether it meets the standard. For laid pads, non-destructive testing technology can be used to indirectly evaluate whether the modulus has changed by detecting the internal structure of the pads through ultrasonic waves. In addition, it is necessary to regularly extract pads from different sections for laboratory re-inspection, and comprehensively judge the modulus stability and service status of the pads combined with on-site test data.