Fiber-reinforced polymer (FRP) materials are incredibly strong and durable. But even they lose strength over time under service conditions, reaching a point where they are no longer Fit For Service.
Below we explain how FRP materials change over time due to their unique damage mechanisms.
FRP strength comes from its unique material properties
The strength of FRP composites comes from the combination of fibers (typically glass or carbon) embedded within a polymer matrix that provides corrosion protection and binds the fibers together. This unique composition provides high tensile strength, making FRP materials ideal for applications where resistance to stretching forces is crucial.
The challenge is understanding how the material weakens over time so we can predict when FRP equipment will no longer be able to do what it was designed to do — before it fails.
There are several key factors to keep in mind about FRP materials:
- FRP is a non-linear viscoelastic material.
- FRP strength declines from stress, strain and chemical attack over time.
- Damage mechanisms affecting FRP are completely different from metal alloys.
- Polymer damage occurs faster than fiber damage.
- Construction and design codes tend to focus on tensile strength; however, the polymer component, which is key to Fitness For Service of FRP, contributes only a small fraction of tensile strength.
- Bending behaviour indicates polymer condition rather than tensile strength.
- Cracks form when the polymer is stretched beyond its limit; as damage increases, cracks form more easily and the polymer becomes brittle.
- All polymer damage relates to changes in elasticity or mechanical behaviour.
- Overt flaws such as cracks and voids are not good indicators of damage or current strength of the material.
Assessing the strength of FRP
Assessing the strength of FRP is complicated because both the polymer and the fiber demonstrate non-linear viscoelastic behaviour. This gets even more complicated as the strength reduction of glass fibers and many other reinforcements is due to changes in the strain at failure. Most polymers (especially thermoset polymers) experience more rapid change of BOTH Young’s modulus and strain at failure than glass or carbon fiber.
Furthermore, the polymer almost always fails mechanically before the reinforcement does – which stops the polymer’s corrosion protection and load transfer to the reinforcement.
FRP Fitness For Service evaluation
Preventing failures and optimizing the service life of any FRP structure is the result of a balance between the service loads applied and the actual capacity of the structure. This is the basis of Fitness For Service or Suitability For Service evaluation. The goal is to identify and monitor polymer condition up to the point where serious damage is likely to occur and equipment can no longer remain in service.
First, let’s clarify the distinction between FRP failure modes and underlying damage mechanisms:
What are FRP failure modes?
Failure modes relate only to the way that failure appears to the naked eye. They are observed through visual inspection.
What are FRP damage mechanisms?
Damage mechanisms are the underlying processes and changes that lead to the appearance of a failure mode. They cannot usually be detected with the naked eye and often require techniques such as non-destructive evaluation and analytical techniques involving destructive testing, including microscopy.
For fiberglass reinforced plastic (FRP), there are 3 damage mechanisms that can occur:
- Damage to the matrix, or resin
- Damage to the reinforcement fibers
- Damage to the interface of the matrix
These can be expanded to identify that both mechanical and chemical forces play a role in the damage.
Creep Rupture
A common result of these damage mechanisms that leads to failure of FRP is known as “Creep Rupture”.
The damage mechanisms identified all relate to constituents of FRP that combine to give it strength and stiffness. For this reason, changes in stiffness, also known as creep, will generally result from damage.
The relationship of damage mechanisms, common failure modes and laminate flexural modulus are shown in Figure 1. Creep is also observed in some qualification tests, such as for developing the pressure rating of pipe. At this writing, it is not possible to a priori quantify the relationship of each damage mechanism to the changes in stiffness, and it is used as a proxy to indicate progression of bulk damage from accumulated damage mechanisms. In some cases where the resin has been damaged, such as in reduction of the glass transition temperature, the damage does not immediately show as creep.
Laminate stiffness
Changes to the matrix or resin that appear to conventional, visual inspection as corrosion barrier damage may also affect the laminate stiffness.
Damage to the corrosion barrier can affect stiffness, usually in proportion to the thickness of the corrosion barrier. UltraAnalytix® non-destructive evaluation has found that damage to corrosion barriers is often detected and quantified before visual inspection. Learn more about FRP corrosion barrier assessment.
Sometimes, these detections serve to reduce the stiffness values calculated, thus providing conservative values to owners.
Adhesive bonds are commonly used to join FRP structures (secondary bonds). They are used for joining shell and pipe sections as well as most repairs. The post-processing for UltraAnalytix will reliably provide information on the condition of these bonds. Several third parties have verified UltraAnalytix analysis of several types of joints: FRP-to-FRP joints such as for pipe, reinforcements to FRP tanks and structures, FRP and carbon fiber applied to steel and repairs to FRP.
Changes in elasticity key to remaining strength
Over the past 15 years and thousands of inspections, UTComp has demonstrated how changes in stiffness can be used as a proxy for overall polymer condition. We originally expressed this as a Percentage of Design Stiffness: the term has since changed to Polymer Damage Status (PDS) as a result of our work with the committee developing a new part for the API 579 Fitness for Service standard.
The proposed new part includes a focus on remaining elasticity, not just stiffness, of FRP. This includes a new method for calculating the Remaining Strength Factor (RSF) of the material using simple measurements of PDS and FRP thickness.
With this calculation, RSF provides a simple explanation of the FRP’s retained elastic properties right up to the point that a fracture or failure is likely to occur.
The good news is that even when polymer damage has reached the proposed minimum allowable RSF, failure is not imminent. Asset owners will still have many months or even a couple of years to repair or replace the equipment in most cases.
Learn more about UltraAnalytix non-destructive evaluation (NDE) for FRP assets.
Updated Sept. 2, 2025