API/ASME educational series (Part 1)
Fiber-reinforced polymer (FRP) materials deliver versatile, durable performance in many industrial applications, from chemical processing vessels and ductwork to water purification tanks and structural components.
With 70+ years of industrial usage, FRP is relatively new compared to steel or concrete, but it offers exceptional advantages in corrosion resistance, weight reduction and design flexibility.
This blog covers the fundamentals of FRP design, including how it’s fabricated, its unique mechanical and viscoelastic properties, and how engineers use standards like ASME RTP-1 and API 579 to design reliable equipment.
This guide (part 1 of the API/ASME educational series for development of a new part for API 579-1/ASME FFS-1) will help you understand how FRP performs in industrial applications.
Also see this video by UTComp founder and CTO Geoff Clarkson:
FRP applications and fabrication methods
Where is FRP used?
FRP materials are used across many industries due to their chemical resistance and lightweight mechanical strength. Common applications include:
- Piping systems
- Chemical storage tanks
- Scrubbers and ducting
- Expansion joints
- Pressure vessels (e.g., for reverse osmosis or hydrogen storage)
FRP’s adaptability means it can be custom-fabricated into complex geometries, a valuable trait for equipment operating in chemically aggressive environments.
How is FRP made?
FRP consists of two main constituents:
- Resin (Polymer Matrix): Usually a thermosetting polymer like epoxy, polyester, or vinyl ester.
- Reinforcement (Fibers): Most commonly glass fibers, although carbon and other fibers are also used.
The fabrication process involves saturating the fiber reinforcement with resin and then curing the assembly into a solid composite. Key fabrication methods include:
1. Contact Molding
- Hand Lay-Up: Reinforcement is manually placed and saturated with resin using rollers.
- Resin Transfer Molding (RTM): Dry reinforcement is placed in a mold and infused with resin under vacuum.
2. Filament Winding
- Fibers are wound onto a rotating mandrel and saturated with resin, allowing for optimized strength in specific directions (e.g., hoop or axial).
3. Continuous Winding
- A high-speed method used for long pipes, where the mandrel moves continuously during resin application and curing.
4. Joining
- Since thermoset FRPs can’t be melted, components are joined with adhesives or mechanically fastened (e.g., flanges, bolted joints).
Structural composition
Industrial-grade FRP equipment typically has two major layers:
- Corrosion Barrier (high resin content, ~85–100%): Protects the structural layers from chemical attack.
- Structural Layers (43–80% resin by volume): Engineered for strength, with unidirectional or woven reinforcements.
The corrosion barrier is often omitted from mechanical strength calculations, ensuring conservative and safe design.
FRP mechanical properties: tension vs. bending
Understanding how FRP behaves under load is critical for sound design. Unlike metals, FRPs are composite and often orthotropic, meaning their mechanical properties differ depending on the direction of load and fiber orientation.
Tensile properties
FRP derives the majority of its tensile strength from its fibers—typically 87–97%, while the resin matrix contributes a minor portion (3–13%).
- Tensile Modulus (Young’s Modulus) measures stiffness under axial tension.
- Standards like ASTM D638 are commonly used to test tensile strength and modulus.
- Directionality is key: Tensile strength is highest along the direction of fiber alignment.
Flexural (bending) properties
Flexural modulus is tested using methods like ASTM D790 and tends to be significantly lower than tensile modulus—typically around 70–75% of tensile stiffness.
This difference becomes especially relevant in buckling scenarios or when external pressure is applied to cylindrical vessels.
Example:
- Tensile Modulus: 10 GPa
- Flexural Modulus: 7 GPa
These differences underscore why design methods must treat tensile and bending behaviors separately.
FRP’s viscoelastic behavior
One of the unique aspects of FRP is its viscoelasticity—a time- and stress-dependent behavior where properties evolve under sustained loads or exposure to heat and chemicals.
What is viscoelasticity?
Unlike metals, which typically exhibit elastic (recoverable) or plastic (permanent) deformation, FRPs display time-dependent strain. This is influenced by:
- Stress duration
- Operating temperature
- Chemical environment
Over time, mechanical properties such as stiffness can degrade, especially under high stress or temperature. This is why most design codes only allow new material properties for design calculations—there’s no built-in allowance for aging or degradation.
Long-term hydrostatic design basis
For piping systems and some vessels, long-term performance is evaluated using the Long-Term Hydrostatic Design Basis (LTHDB). This involves:
- Pressurizing samples to failure over time
- Plotting pressure vs. time on a log-log scale
- Deriving a conservative maximum pressure for a given design life
Standards such as ASTM D2992 and ASME RTP-1 use this approach for pressure rating FRP components like reverse osmosis vessels and piping.
FRP design approaches
Designing with FRP requires specialized knowledge and methods that differ from metals due to its layered, anisotropic nature. There are several accepted design approaches in industry codes:
1. Allowable Stress Design (ASD)
ASD is the most common method. It treats FRP as a linear elastic material and applies a factor of safety (FS) to the tensile strength.
- ASME RTP-1 and Section X of the ASME Boiler and Pressure Vessel Code often use FS = 10 for continuous loads and FS = 5 for occasional loads.
- Example: If tensile strength is 20,000 psi, allowable stress = 2,000 psi (using FS = 10).
2. Allowable Strain Design
This approach uses strain rather than stress as the controlling parameter.
- Uses the equation:
Allowable Stress = Young’s Modulus × Allowable Strain - Strain limits are typically based on the polymer’s strain-to-failure, which is usually lower than that of the fibers.
This method is conservative and accounts for failure mechanisms like matrix cracking that might not trigger immediate structural failure.
3. Classical Lamination Theory (CLT)
CLT models the behavior of layered composites. It uses matrix math to calculate the laminate’s modulus and strength based on:
- Fiber orientation
- Ply stacking sequence
- Moduli of the resin and fiber
CLT can estimate tensile and bending stiffness in any direction and is useful for finite element modeling or layer-specific analysis.
It’s especially valuable when:
- Material data is unavailable
- Reverse engineering from cutouts is required (e.g., burning out resin and measuring residual fibers)
4. Finite Element Analysis (FEA)
For complex geometries, FEA can model FRP behavior under load. The model must treat FRP as:
- Orthotropic, with directional stiffness
- Layered, if possible, using data from CLT or actual testing
FEA allows prediction of stresses and strains in individual layers, improving confidence in structural performance.
5. Load and Resistance Factor Design (LRFD)
Less common in pressure equipment but used in civil engineering (e.g., FRP bridges or cooling towers), LRFD applies resistance factors to material strengths.
- Typically uses the 5th percentile strength from testing
- Resistance factors vary from 0.25 to 0.65
- Often ignores chemical environment unless modified
While conservative, LRFD is often too simplified for chemically exposed equipment.
FRP standards and codes
Several standards govern FRP design and construction:
- ASME RTP-1: Design and fabrication of vessels
- ASME Boiler and Pressure Vessel Code Section 10: Boiler & pressure vessel code for FRP
- ASME NM.2: Non-metallic piping
- EN 13121 / ISO 14692: European and international pressure vessel codes
- ASTM D3299 / D4097: Fabrication standards
Most use ASD or allowable strain methods and are based on properties of new, undamaged FRP.
See more about API and ASME standards and codes for in-service FRP composite assets.
Design best practices are key to long FRP lifespan
FRP is a versatile, resilient material for industrial design; however, it is also complex.
Directional properties, viscoelastic behavior and sensitivity to fabrication quality mean it must be designed thoughtfully, using validated methods and conservative assumptions.
Key takeaways:
- Use only new material properties in design
- Recognize the difference between tensile and flexural behavior
- Design for directionality—fiber orientation matters
- Validate designs using ASD, strain limits, or lamination theory
- Apply appropriate standards like ASME RTP-1 or ISO 14692
With correct design and fabrication, FRP equipment can last decades (even more than 60 years!) while withstanding the most aggressive industrial environments.