Stiffness Evolution of Polyurethane Rail Pads under Long-Term Creep
Q1: What is the essential difference between creep of polyurethane pads and rubber pads?
A1: Creep of rubber pads mainly comes from irreversible slippage of polymer chains under stress, which is extremely sensitive to temperature, with large creep deformation and poor recovery at high temperatures. Creep of polyurethane pads is controlled by micro-phase separation structure and cross-linking network. Within normal temperature range, its creep is dominated by recoverable viscoelastic deformation with low irreversible deformation. However, under long-term high pressure and high-frequency vibration, cross-linking network damage leads to unrecoverable plastic deformation, showing continuous stiffness increase and elasticity decline.
Q2: How does pad creep affect the fastener system during long-term service?
A2: Creep reduces pad thickness and clip elastic deformation, decreasing effective clamping force. Meanwhile, creep changes the dynamic-static stiffness ratio, increases dynamic stiffness, reduces vibration reduction effect and amplifies wheel-rail impact. Higher stiffness transfers more vibration to bolts and clips, aggravating fatigue damage. In ballastless tracks, pad creep may induce local fastener gaps, worsening track smoothness.
Q3: How do train load, temperature and service time jointly affect creep rate?
A3: Higher train load increases internal stress and accelerates molecular chain slippage, raising creep rate. Higher temperature accelerates polymer movement, concentrating creep deformation in a short time, with faster development in summer. Longer service time leads to accumulated internal damage and slow continuous creep. Under the combined effect, creep is most prominent in heavy-haul, high-temperature and long-service sections.
Q4: When must the pad be mandatorily replaced due to stiffness evolution?
A4: When the permanent compression deformation exceeds the standard limit (usually 5%~10%) or the dynamic stiffness increases by more than 30% from the initial value, the vibration reduction performance is greatly attenuated. If accompanied by surface cracking, chalking or delamination, replacement is required immediately even if the deformation is within the limit to avoid fragment contamination and uneven stress.
Q5: How to improve creep resistance of polyurethane pads through formula and structure optimization?
A5: In formula, increase cross-linking density, select high-modulus polyol and isocyanate systems, and add anti-aging and anti-creep additives to reduce molecular slippage. In structure, adopt lattice, corrugated or gradient design to optimize stress distribution and reduce local creep; control hardness within a reasonable range to avoid brittleness or excessive creep. In addition, surface wear-resistant treatment reduces additional deformation and enhances overall creep resistance.