The Relationship Between Compression Set Resistance and Long-Term Elasticity Retention of Rail Pads
What long-term impact does excessive compression set of under-rail pads have on track stiffness?
Excessive compression set causes the pad to gradually lose elasticity and undergo irreversible plastic deformation under long-term cyclic train loads, with continuous thickness reduction. Thickness reduction decreases the initial elastic settlement of the track, gradually increasing the overall track stiffness and forming a "stiffness hardening" phenomenon. This uneven increase in stiffness enlarges the stiffness difference between track sections, generating periodic impact loads as trains pass, and exacerbating wheel-rail wear and component fatigue. Over time, the dynamic geometric dimensions of the track become difficult to maintain, and the frequency of maintenance and repair increases by more than 40%.
What are the differences in compression set resistance between rubber pads and polyurethane pads, and why are their applicable scenarios different?
Rubber pads have relatively weak compression set resistance, with a permanent deformation rate usually between 20%-25%, but they offer good elasticity and low cost, making them suitable for ordinary-speed light-load lines. Polyurethane pads have a more stable molecular structure, with a compression set rate ≤10%-half that of rubber pads-and better aging and wear resistance, suitable for high-speed and heavy-haul lines. High-speed lines have high requirements for track stiffness stability, and heavy-haul lines have large loads with high plastic deformation risks for pads; therefore, polyurethane pads must be used. Ordinary-speed lines have small loads and lower requirements for stiffness stability, so rubber pads can meet service needs.
Why does the compression set rate of under-rail pads rise sharply in high-temperature environments?
High temperatures accelerate the thermal motion of the pad material's molecular chains, causing relaxation of the cross-linked structure of rubber or polyurethane and a significant decrease in the material's elastic modulus. In high-temperature environments above 70℃, the molecular chains of rubber pads are prone to fracture or rearrangement, leading to a sharp reduction in their ability to resist plastic deformation, and the compression set rate can rise to more than 35%. In addition, high temperatures reduce the bonding force between the pad and the sleeper/rail, increasing the pad's slip under load and exacerbating plastic deformation, forming a vicious cycle of "high temperature-deformation-stiffness change".
What is the relationship between the compression set performance of under-rail pads and installation preload?
Excessively high installation preload causes the pad's initial compression rate to exceed the design value (e.g., over 25%), placing the pad material in a high-stress state for a long time, accelerating fatigue damage to the molecular chains, and significantly increasing the compression set rate. Insufficient preload allows excessive reciprocating compression deformation of the pad under train loads, with increased deformation per compression cycle, also accelerating the accumulation of permanent deformation. Only within the designed preload range, with the pad's compression rate maintained at 15%-20%, can the molecular chain stress be in an optimal state, maximizing the delay of permanent deformation.
How to quickly evaluate whether the compression set of under-rail pads exceeds the limit on site?
The "thickness measurement method" can be used: select new pads of the same batch as a benchmark, measure the actual thickness of in-service pads on site, and calculate the thickness reduction rate. If the thickness reduction rate of rubber pads exceeds 25% and that of polyurethane pads exceeds 10%, it indicates excessive compression set. At the same time, observe the pad's appearance: if the pad shows obvious depression, flattening, loss of its original elastic contour, or cannot quickly recover after train passage, its permanent deformation is severe. In addition, combined with track smoothness test data, if the rail surface longitudinal level deviation of a section frequently exceeds the limit, it is highly probable that the compression set of the pads in that section is excessive.