PRODUCT DESCRIPTION
Related solutions: Explore our pultruded FRP profiles, FRP rebar, pultrusion machines, pultrusion dies.
What Is an FRP Bolt?
An FRP bolt (Fiber Reinforced Plastic bolt) is a structural fastener manufactured from continuous glass fiber or carbon fiber reinforcement embedded in a thermosetting resin matrix — typically vinyl ester, epoxy, or isophthalic polyester. The composite is formed via pultrusion or filament winding, producing a fastener with a highly directional fiber architecture optimized for tensile performance along the bolt axis. FRP bolts are used in two distinct engineering contexts:
- Geotechnical / Tunnel Rockbolts — ground support elements in mechanized shield tunneling and mining, where the bolt must be cleanly sheared by a TBM cutter head after serving its support function
- Industrial & Structural Fasteners — bolting solutions for chemical, marine, electrical, and infrastructure applications where corrosion resistance, electrical isolation, or weight reduction are primary design drivers
Material Composition & Manufacturing
| Property | GFRP (Glass Fiber) | CFRP (Carbon Fiber) |
|---|---|---|
| Fiber Type | E-glass or ECR-glass | PAN-based carbon fiber |
| Resin System | Vinyl ester / Epoxy | Epoxy |
| Fiber Volume Fraction | 60–70% | 55–65% |
| Manufacturing Method | Pultrusion / Filament winding | Pultrusion |
| Surface Finish | Sand-coated / ribbed / threaded | Smooth or ribbed |
| Color | Yellow, white, natural | Black |
Resin selection matters: Vinyl ester resins provide strong chemical resistance to acids and solvents. Epoxy resins offer higher mechanical performance and better fiber-matrix adhesion. Isophthalic polyester is a cost-effective option for moderate chemical environments.
Mechanical & Physical Properties
| Parameter | Typical Value | Test Standard |
|---|---|---|
| Tensile Strength (longitudinal) | 700 – 1,000 MPa | ASTM D7205 |
| Tensile Capacity (tunnel-grade bolt) | ~203 kN | Project spec / EN standard |
| Elastic Modulus (longitudinal) | 40 – 50 GPa | ASTM D3039 |
| Shear Strength | 90 – 150 MPa | ASTM D2344 |
| Compressive Strength | 300 – 500 MPa | ASTM D695 |
| Flexural Strength | 600 – 900 MPa | ASTM D790 |
| Density | 1.8 – 2.1 g/cm³ | — |
| Weight vs. Steel | ~25% of equivalent steel bolt | — |
| Thermal Expansion Coefficient | 6 – 10 × 10⁻⁶ /°C | ASTM E831 |
| Operating Temperature Range | −40°C to +120°C | Resin-dependent |
| Electrical Resistivity | >10¹² Ω·cm | ASTM D257 |
| Thermal Conductivity | 0.3 – 0.5 W/m·K | — |
Engineering Note: Unlike isotropic steel, FRP is an orthotropic material — mechanical properties are higher in the longitudinal (fiber) direction than transverse. Always apply directional load factors when designing connections with FRP fasteners.
Dimensional Specifications
Industrial / Structural FRP Bolts
| Parameter | Specification |
|---|---|
| Metric Thread Sizes | M8, M10, M12, M16, M20, M24 |
| Imperial Thread Sizes | 5/16″-18, 3/8″-16, 1/2″-13 UNC |
| Standard Lengths | 50 mm – 6,000 mm (custom available) |
| Head Styles | Hex, flat countersunk, low-profile, encapsulated |
| Nut & Washer | Molded GRP nut and washer sets available |
| Thread Tolerance | 6H/6g (metric) per ISO 965 |
| Grade Equivalent | Grade 5.1 (cold-formed, heat-treated) |
Tunnel / Geotechnical FRP Rockbolts
| Parameter | Specification |
|---|---|
| Bolt Diameter | Ø18 mm, Ø20 mm, Ø22 mm, Ø25 mm |
| Length Range | 1,500 mm – 12,000 mm |
| Anchorage Type | Resin capsule anchor / mechanical end plate |
| Surface Profile | Helical ribs or sand-coated for resin bond |
| Bond Strength | ≥ 15 MPa (resin encapsulation) |
| Plate Size | 150×150×10 mm (standard) |
| Shear Performance | Cleanly sheared by TBM at designed bolt spacing |
Performance Advantages Over Alternative Materials
| Property | FRP Bolt | Carbon Steel (Grade 8.8) | 316L Stainless Steel | Titanium Grade 5 |
|---|---|---|---|---|
| Corrosion Resistance | good | Poor | Good | good |
| Electrical Conductivity | Non-conductive | Conductive | Conductive | Conductive |
| Magnetic | Non-magnetic | Magnetic | Slightly magnetic | Non-magnetic |
| Density (g/cm³) | 1.8 – 2.1 | 7.85 | 8.0 | 4.43 |
| Tensile Strength (MPa) | 700 – 1,000 | 830 | 515 | 900 |
| TBM Shearability | good | Not suitable | Not suitable | Not suitable |
| Relative Cost | Medium | Low | High | Very High |
Applications
Mechanized Tunnel Boring & Mining FRP rockbolts reinforce the ground ahead of the TBM shield face. Their intentionally low shear strength allows the rotating cutter head to sever them cleanly during advance, reducing costly machine stoppages and cutter head damage that steel bolts would cause. Used extensively in metro rail, road tunnel, and hydropower headrace tunnel construction. Chemical Processing & Industrial Plants Reactors, scrubbers, pipe racks, and platforms handling HCl, H₂SO₄, NaOH, or chlorinated compounds demand fasteners that resist chemical attack. FRP bolts maintain structural integrity without the pitting, crevice corrosion, or stress corrosion cracking that degrades stainless steel in aggressive media. Marine & Offshore Infrastructure Jetty decking, offshore platform gratings, desalination plant structures, and coastal installations are permanently exposed to chloride-laden saltwater. FRP reduces galvanic corrosion between dissimilar metals and reduces coating maintenance cycles. Electrical Substations & Telecommunications FRP’s dielectric properties make it the standard choice for bus bar supports, transformer mounting, antenna structures, and switchgear enclosures where bolted connections must not form conductive paths or create grounding faults. Water & Wastewater Treatment Clarifier mechanisms, aeration equipment, filter press frames, and dosing system supports in continuously wet, chemically active environments. FRP fasteners extend maintenance intervals by reducing corrosion-driven replacement cycles. Truck, Trailer & Refrigerated Panel Bodies Low-profile encapsulated FRP panel bolts with integrated sealing collars allow single-operator installation of FRP wall panels to steel framing. The non-conductive, non-corrosive head prevents condensation and galvanic damage at the panel penetration point — a persistent failure mode with steel fasteners in insulated bodies.
Installation Guidelines
- Apply torque values as specified by the manufacturer — FRP does not yield visibly before failure the way steel does, making audible or tactile over-torque detection unreliable
- Use calibrated torque wrenches; avoid impact drivers unless specifically approved for the product
- For galvanic isolation applications, use non-metallic (GRP or nylon) washers and nuts throughout the connection
- Cut to custom length using a fine-tooth carbide or diamond-tipped blade; secure the cut end immediately with resin sealer to prevent fiber wicking
- Avoid point loading perpendicular to the bolt axis — design connections to transfer load axially through the fastener
- Store away from prolonged direct UV exposure; confirm UV stabilizer content with manufacturer for long-term outdoor installations
Frequently Asked Questions
Q: What is the tensile capacity of a standard tunnel-grade FRP rockbolt? A: Tunnel-grade FRP rockbolts typically achieve a tensile capacity of approximately 203 kN at an elastic modulus of 45 GPa for a 22 mm diameter bolt. Actual values depend on fiber volume fraction, resin system, and manufacturing quality. Always request certified load test data and material traceability documents for geotechnical applications. Q: Why is low shear strength a design advantage for tunnel FRP bolts? A: In mechanized shield tunneling, the TBM cutter head must advance through previously installed rockbolts. Steel bolts cannot be sheared without causing severe damage to cutting tools and machine downtime. FRP rockbolts are engineered to shear cleanly at predictable loads — this is not a weakness but a functional design requirement unique to the application. Q: Can FRP bolts be used in both acidic and alkaline environments? A: Vinyl ester-based FRP offers broad chemical resistance, performing well in most acids (HCl, H₂SO₄ up to moderate concentrations) and alkalis (NaOH). However, strong oxidizing acids (concentrated HNO₃, chromic acid) and certain organic solvents can attack the resin matrix. Always specify the exact chemical environment so the correct resin system can be confirmed. Request a chemical resistance chart from the manufacturer. Q: What is the maximum continuous operating temperature for FRP bolts? A: Standard vinyl ester and epoxy FRP fasteners are rated for continuous service to +120°C. High-temperature epoxy systems extend this to approximately +150°C – +180°C. Cryogenic performance to −40°C is generally acceptable. FRP is not suitable for applications with flame exposure or continuous temperatures exceeding the resin’s heat deflection temperature (HDT). Q: How does the elastic modulus of FRP compare to steel, and does it matter? A: FRP has an elastic modulus of 40–50 GPa versus 200 GPa for steel. This means FRP bolts deflect more under equivalent load — a critical consideration in precision-tolerance assemblies or preloaded joints. However, in many corrosion-resistant or electrically isolated applications, absolute stiffness is secondary to durability. Engineers should calculate joint stiffness and bolt elongation using FRP-specific modulus values, not steel assumptions. Q: Are FRP bolts compliant with international standards? A: FRP fasteners for industrial applications are typically tested per ASTM D7205 (tensile), ASTM D2344 (shear), and ISO 10406 (FRP reinforcing elements). Tunnel rockbolts are tested to project-specific geotechnical standards or national equivalents (e.g., EN 14490 for soil nailing, or project-level pull-out test protocols). Request full material certification, third-party test reports, and batch traceability documentation for specification-critical procurement. Q: Can FRP bolts be reused after removal? A: FRP fasteners subjected to significant tensile or shear loading should not be reused without inspection. Unlike steel, FRP may sustain internal fiber damage that is not visible externally. For safety-critical or structural connections, treat FRP bolts as single-use fasteners unless the manufacturer explicitly states otherwise and provides an inspection protocol. Q: What torque values should be applied to FRP bolts? A: Torque values vary by bolt diameter, thread pitch, resin system, and joint material. As a general guide, FRP bolts are tightened to approximately 30–60% of the equivalent steel torque value for the same thread size to avoid crushing the fiber matrix. The manufacturer’s specified torque table must always take precedence. Never estimate torque for FRP based on steel bolt tables.
Require certified material data sheets, chemical resistance charts, or project-specific engineering consultation? Contact our technical team with your application parameters.
Complete the assembly with our matching FRP nuts, pultruded in the same resin systems for consistent chemical and thermal performance.
Request a quote: Need this product for your project? Contact us for a quote — our engineering team can help with specifications, custom sizes, and bulk orders.
