Material Strength and Bolt Preload
Joint stiffness starts with the fishplate material. Fishplates rolled from high-strength steel such as Q355B to GB/T 1591 develop a connection stiffness 10-15 percent higher than fishplates of ordinary carbon steel such as Q235 to GB/T 700, at the same section, because the higher yield strength delays plastic deformation of the contact zone under the rail end.
Bolt preload is the second controlling variable. Stiffness reaches its maximum when the preload is held between 60 percent and 70 percent of the bolt yield strength; below that band the joint works loose under each axle passage, and above it the bolt is at risk of yielding during installation. Preload is therefore controlled with a calibrated torque wrench rather than by feel, and bolts are re-tightened every six months to compensate for short-term relaxation.
Contact Area Between Fishplate and Rail
The contact area determines how much of the bolt load is transferred into the rail web in shear and friction rather than through local crushing. Gaps, foreign material and wear all reduce it, and the effect is disproportionate: a 10 percent reduction in contact area typically reduces joint stiffness by 8-12 percent.
Control measures are straightforward but must be systematic: clean the contact faces before assembly, remove burrs and debris, repair worn seating faces and replace fishplates whose contact profile no longer matches the rail web. In practice, contact area loss rather than material strength is the most common reason a joint measures soft during inspection.
Static and Dynamic Stiffness Testing
Stiffness is measured, not inferred. The static method applies symmetric loads at both ends of the rail joint, records the relative displacement at the joint, and derives stiffness from the load-displacement curve as stiffness equal to load divided by displacement.
Because the actual load path is cyclic, dynamic testing is added: a vibration exciter applies sinusoidal loading and the stiffness values at different frequencies are recorded, so the performance under dynamic forces can be compared with the static result.
Load direction must be consistent with the rail axis, otherwise additional bending moments distort the result
Joint displacement is measured on both sides of the gap to separate joint deformation from rail deflection
The static and dynamic values should be reported together, since a large difference indicates a loose preload or a contact problem
Consequences of Insufficient Joint Stiffness
Insufficient stiffness produces a large relative displacement at the rail joint, which converts every wheel passage into an impact rather than a smoothly transferred load. The consequences accumulate along the track:
Increased wheel-rail noise and reduced passenger comfort
Accelerated wear of rails, fishplates and bolts
Maintenance cycles shortened by 30-50 percent and higher operating cost
Progressive deterioration of the joint geometry, which further reduces stiffness in a self-reinforcing cycle
Fishplate Types and How to Improve Joint Stiffness
| Fishplate type | Stiffness relative to rail base metal | Typical application |
|---|---|---|
| Double-ended fishplate | 70-75 percent | Ordinary railways |
| Oblique joint fishplate | 80-85 percent | High-speed and heavy-haul railways |
| Insulated fishplate | 5-10 percent below an equivalent steel fishplate | Electrified railways with track circuits |
Oblique joint fishplates achieve their advantage because stress distribution across the section is more uniform. Insulated fishplates lose stiffness through the insulating layer, but the design still meets the needs of electrified track.
Where measured stiffness is below requirement, the sequence is: select a higher-strength material, then increase section thickness, since raising thickness from 16 mm to 20 mm increases stiffness by 15-20 percent. Bolt preload is then re-established to the 60-70 percent band, contact faces are cleaned and dressed, and fishplates that no longer seat correctly are replaced.
Frequently Asked Questions
Q: What factors affect the connection stiffness of fishplates?
Material strength, bolt preload and the contact area between fishplate and rail. High-strength Q355B gives 10-15 percent more stiffness than Q235, preload is optimal at 60-70 percent of yield strength, and a 10 percent contact area loss cuts stiffness by 8-12 percent.
Q: How is the connection stiffness of fishplates tested?
Static testing applies symmetric loads at both ends of the joint and derives stiffness from the load-displacement curve. Dynamic testing applies sinusoidal loads with a vibration exciter and records stiffness at different frequencies, with the load direction aligned to the rail axis.
Q: What impact does insufficient connection stiffness have on train operation?
It increases relative displacement at the joint, causing strong impact as trains pass, more wheel-rail noise, lower passenger comfort, accelerated wear of rails, fishplates and bolts, and maintenance cycles shortened by 30-50 percent.
Q: How do different fishplate types differ in connection stiffness?
Double-ended fishplates reach 70-75 percent of rail base metal stiffness for ordinary lines, oblique joint fishplates 80-85 percent for high-speed and heavy-haul lines, and insulated fishplates run 5-10 percent below an equivalent steel fishplate.
Q: How can the connection stiffness of fishplates be improved?
Use higher-strength material, increase section thickness, for example from 16 mm to 20 mm for a 15-20 percent gain, hold bolt preload at 60-70 percent of yield strength with a torque wrench, clean the contact faces and replace worn fishplates.