Why Grade Matching Matters and the Mechanical Differences Between Grades
The fishplate, also called the joint bar, is bolted to both rail ends at a joint and transfers bending moment and shear force from one rail to the next. Because the joint is the weakest point of the track, the fishplate must carry a defined share of the rail load, and that share depends directly on the strength grade of the fishplate material. If the grade is lower than the load demands, the fishplate deforms plastically, cracks, and eventually loses its restraint on the rail ends, which is why the material strength grade of the fishplate must match the load-bearing capacity of the rail joint.
The two grades most commonly used for fishplates differ in yield strength, tensile strength, hardness and alloy content. These differences determine the load that the joint can carry and the fatigue life of the fishplate under alternating wheel loads.
| Property | 8.8 Grade | 10.9 Grade |
|---|---|---|
| Yield strength | Greater than or equal to 640MPa | Greater than or equal to 900MPa |
| Tensile strength | Greater than or equal to 800MPa | Greater than or equal to 1000MPa |
| Rockwell hardness | Greater than or equal to HRC28 | Greater than or equal to HRC35 |
| Carbon content | 0.25%-0.35% | 0.35%-0.45% |
| Chromium content | None specified | 1.0%-1.5% |
| Typical application | GB 60kg/m rails on ordinary lines | Heavy-haul and outer standard rails |
| Joint bearing capacity | About 80% of the rail | About 90% of the rail |
Matching the Grade to the Line and Axle Load
The 8.8 grade is suited to ordinary-speed lines laid with GB 60kg/m rails, where the joint bearing capacity reaches about 80% of the rail and the traffic load stays within the elastic range of the material. Outer standard heavy-haul rails require the 10.9 grade because the rail yield strength is at least 900MPa, the axle load is 30 tons or more, and the shear force at the joint is about 2.5 times that of ordinary-speed lines. The 8.8 grade with a 640MPa yield strength quickly develops plastic deformation under that shear force, the adhesion between fishplate and rail is lost, and stress concentration becomes severe. The 10.9 grade, with yield strength about 1.4 times that of the 8.8 grade, keeps the joint stress amplitude within 70% of the yield strength and delivers about twice the fatigue life, which ensures long-term stability on heavy-haul lines.
What Happens When the Fishplate Is Weaker Than the Rail
When the fishplate strength is lower than the rail, the fishplate becomes the weak link of the joint. Stress concentrates on the fishplate while the rail stress is reduced, reversing the intended load sharing. For example, if the fishplate strength is 20% lower than the rail, the stress amplitude at the fishplate can reach 1.5 times that of the rail, and fatigue cracks appear early. Crack propagation leads to fracture of the fishplate, the joint loses its restraint, and misalignment and displacement follow. Stress concentration also accelerates shear failure of the joint bolts, further damaging the integrity of the joint. The fishplate strength must therefore be equal to or higher than the rail strength so that the joint stress distribution stays uniform.
On-Site Detection of the Fishplate Strength Grade
The grade can be verified on site in three ways. First, a portable hardness tester measures Rockwell hardness: the 8.8 grade reads at least HRC28 and the 10.9 grade at least HRC35. Second, samples are taken for a tensile test to measure yield strength and tensile strength, and the values are compared with the grade limits. Third, a spectrometer checks the material composition: the 10.9 grade has a carbon content of 0.35%-0.45% and a chromium content of 1.0%-1.5%, while the 8.8 grade has a carbon content of 0.25%-0.35% and no specified chromium. The surface of a properly supplied fishplate carries a strength grade mark; products without a mark must undergo full testing before use to avoid misapplication.
Balancing Cost and Safety in Grade Selection
Ordinary-speed inland lines with GB rails can use the 8.8 grade, which costs about 30% less than the 10.9 grade and meets the bearing demand with a better cost-performance ratio. High-speed and heavy-haul lines with outer standard rails must use the 10.9 grade; the material cost rises by about 20%, but joint failure is avoided and long-term maintenance cost is reduced. Temporary lines may use a lower grade according to the actual load class, while lines intended for long-term operation must be selected at the highest grade they will see. The core of the balance is accurate matching to the line load and rail grade, avoiding both over-specification that raises cost and under-specification that creates a safety risk.
Frequently Asked Questions
Can an 8.8-grade fishplate be used on a heavy-haul line?
No. The shear force at the joint is about 2.5 times that of ordinary-speed lines, and the 8.8 grade yields at 640MPa, well below the 900MPa yield of the heavy-haul rail. Plastic deformation and fatigue fracture would follow quickly.
How is the hardness check performed in the field?
A portable Rockwell hardness tester is pressed against a cleaned, flat area of the fishplate. The reading is compared with the grade limit: at least HRC28 for the 8.8 grade and at least HRC35 for the 10.9 grade.
Why does the 10.9 grade contain chromium?
Chromium improves hardenability and corrosion resistance and supports the higher strength of the grade. A spectrometer reading of 1.0%-1.5% chromium is one of the composition checks that confirms the 10.9 grade.
What is the consequence of joint stress concentration in a weak fishplate?
Stress concentrates on the fishplate, fatigue cracks initiate and propagate, the fishplate fractures, the joint loses restraint, and misalignment and bolt shear failure follow. The joint can fail suddenly under traffic.
Is a fishplate with a higher grade always better?
Not automatically. A higher grade costs more and can be less ductile if not properly heat treated. The grade should match the rail and the line load so that cost, strength and toughness are balanced.
What should be done with fishplates that carry no grade mark?
Unmarked fishplates must not be installed on load-bearing joints. They should be quarantined and subjected to hardness, tensile and composition testing, and only reinstated if the results match the required grade.