UTComp founder and CTO Geoff Clarkson is leading the project team developing a new part for the API 579-1 / ASME-FFS-1 Fitness For Service standards to include in-service fiber-reinforced polymer (FRP) composite assets for the first time.
This blog post is part of a series on the proposed update to API 579-1 / ASME-FFS-1 and how the changes will impact Fitness For Service inspection and assessment of FRP assets.
Bridging the FRP-steel assessment divide
Developing the new part for the API-579 Fitness For Service standard has challenged the expert project team to reframe how they think about and explain key concepts involved in FRP assessment in ways that make sense to colleagues who more familiar with steel.
A good example is how the group reconciled Remaining Strength Factor (RSF) — which applies to steel — and Polymer Damage Status (PDS) — which UTComp has used for many years as a proxy for overall polymer condition and strength in FRP.
Remaining Strength Factor (RSF)
RSF for steel assessment
RSF is used as the acceptance criteria for Fitness For Service assessment of steel pipes, pressure vessels and other equipment. Basically, it’s a ratio that expresses the strength of damaged in-service steel equipment compared to its undamaged, as-new state.
The minimum allowable RSF for steel is 0.9 or 90%, although this can be adjusted depending on construction codes and other criteria. When RSF is below that minimum, the damaged equipment must be repaired or replaced.
RSF for steel looks at all the changes in the material that can be seen or measured directly, such as thickness loss due to corrosion and the loads that produce stress. These changes can be related back to the construction codes because the mechanical properties of steel remain constant.
RSF is straightforward for steel. It’s more complicated for FRP.
RSF for fiber-reinforced polymer (FRP) materials
Unlike steel, FRP is a mixture of materials in which each component has different mechanical properties. FRP is also affected by different damage mechanisms, and in most cases damage starts in the resin before any cracks, leaks or other overt flaws appear in the glass/fiber components.
Applying RSF to FRP must include measurements such as thickness AND account for viscoelastic changes in the mechanical properties that occur in different ways in the polymer and fiber reinforcement. These changes cannot be related back to the construction code. See all standards and codes that apply to FRP.
Put it this way: compared to the minimum allowable RSF of 90% for steel that’s in-service, brand-new FRP might only have an RSF of 0.7 or 70%. This is due to the nature of the material itself as well as other factors such as manufacturing inconsistencies or uneven curing of the resin.
Yet, based on decades of experience with FRP in a wide range of operating environments, we know that FRP equipment is still fit for service at 70% or even 60% PDS.
So how do we talk about remaining stiffness or strength of FRP equipment in a way that makes sense to people who know steel?
PDS = Polymer Damage Status
The purpose of API 579 is to determine whether a piece of equipment can remain in service and continue to operate — and PDS is critical to determining the RSF of FRP.
With respect to FRP, we do this by focusing on changes that happen in the polymer BEFORE a failure occurs (i.e., before a crack becomes visible).
For many years, UTComp has used patented algorithms to analyze ultrasonic data to calculate changes in stiffness that result from damage to the resin. We originally expressed this calculation as Percentage of Design Stiffness (PDS). In many cases, PDS values below 40% indicate end of service life or a very high risk of failure.
(It’s interesting to note that, even with PDS values in the 40% range, FRP may retain 60% to 90% of its original tensile strength.)
For the new part of API 579, the working group determined how to calculate RSF from PDS and remaining thickness, and changed what the PDS abbreviation stands for.
More correctly, PDS now stands for Polymer Damage Status — how much of the polymer’s original elasticity has been retained. The proposed change doesn’t alter what PDS represents, but it was felt that the name change makes the underlying concept easier to grasp. The simple measurement of PDS and FRP thickness can then be used to calculate RSF. This makes inspection requirements clear.
The new minimum allowable RSF for FRP equipment will be 0.125, or one-eighth, in most cases.
WRC-601 guidance for assessing FRP equipment
With the publication of the Welding Research Council (WRC) Bulletin WRC-601, and a new part now in development for the API-579 Fitness For Service standard, there’s a fundamental change under way in how reliability engineers assess FRP composites.
WRC-601 follows in the footsteps of 600 other bulletins that inform the code, providing technical background and validation for a methodology developed over 20 years of assessing FRP and other polymeric materials used in pressure vessels, tankage, piping, vehicles, building structures, and other assets in a wide range of industries. Learn more about WRC-601.
Also see our four-part educational series for understanding and evaluating FRP assets throughout their lifecycle.
Article updated Sept. 2, 2025