Analyzing a material’s performance requires an understanding not only of its molecular behavior, but also how that behavior translates into macroscopic performance. Usually these are separate fields of research, but when they are explored together, and their connection is understood, technical teams can draw on a larger body of knowledge to design products. This is especially important when developing products that are more technical and complex in nature, such as composite materials, which are anisotropic.
Though this approach can be costly, investing in in-house and third-party R&D and hiring knowledgeable engineers and scientists to interpret the results establishes a solid foundation for product development. Composite designs that are based on more comprehensive understanding of the physical characteristics and performance of the individual components are superior to those that are developed employing less exact testing and analysis.
Building Better Solutions
Composite materials used in pipeline strengthening are multi-component systems that include a reinforcing fiber, such as glass or carbon, a saturating resin, a primer polymer and a filler compound. Understanding the role of each component in the system on both microscopic and macroscopic scales are critical to ensuring a design that is based on limit state analysis. In simpler terms, being able to identify which component fails first is critical to a successful composite repair design.
Recent research carried out by ClockSpring|NRI revealed that, when used in a composite repair system, a very stiff filler material can be the first component to fail. This failure can lead to premature disbondment of the defect area and the eventual premature failure of the composite system. Without this knowledge, it would be easy to misinterpret a failure by assuming that the composite laminate is “weak” and that improved filler properties might be the way to increase the ultimate strength of the reinforced member.
A thorough understanding of the components and their properties is critical both for improving existing products and developing new ones. Without it, companies could end up investing in improving the wrong component of an engineered composite repair (ECR) system and end up no closer to producing a better and more effective product.
Another example that illustrates why it is so important to understand the micro and macro scale performance of composite product is issues that arise during the post-curing phase. All epoxy systems designed for high-temperature applications require post-curing to reach their full design capacity. On a micro-level scale, when an epoxy reacts with an amine, the molecules form chains. If the curing schedule defined by the manufacturer is not followed, this reaction does not reach completion, which means not all the chains are formed. The result is epoxy vitrification — the situation in which the epoxy appears to be cured but has failed to achieve its ultimate material properties.
If a proper cure schedule is not executed in the field, a composite system can experience premature failure. This is a phenomenon that is still not well understood by the industry at large and is a focus of current educational efforts at ClockSpring|NRI.
It also is worth mentioning that codes and standards have been slow to incorporate the proper language about this topic for structural strengthening, leading to improper and misunderstood marketing statements such as, “The system is designed for high-temperature operations and cures at room temperature.” The reality is that greater precision is required. The ultimate design properties of high-temperature composites can only be achieved when the proper cure protocol is followed.
Improving Product Knowledge
Research programs undertaken to achieve a better understanding of the components of ECRs have revealed some interesting results.
In one of these initiatives, researchers evaluated how composite fabric fiber and saturating resin contribute to the performance of composite materials on a pipeline. The test program was undertaken using multiple ECR systems with various resin and fiber properties. Using products developed in house, the research team undertook a large testing protocol to understand the differences in performance of various products and to identify the causes for those differences. The team found out through this program that sometimes similar products can perform differently.
There were two key takeaways from this test project. One is that repairs with higher through-wall modulus systems (example epoxy-based systems) outperformed lower through-wall modulus systems (example polyurethane systems). Another is that for both carbon- and glass-based systems using the same fiber type and repair thickness, decreasing the through-thickness modulus of an ECR results in diminished strain reduction efficiency.
In another program, researchers looked into the limitations of the maximum repair thickness. Within the ASME PCC-2 Article 4.1 standard, there are multiple design equations for determining the required thickness of a composite material to repair a damaged pipe. None of these equations, however, addresses a maximum repair thickness (aka a cap) for a design. While this may seem unnecessary, based on findings from numerous in-house test programs, not having a cap is problematic and is not conducive to the practical application of composite materials.
Following the assumption that continuing to add composite layers increases the strength of an ECR could lead to a less effective repair because at a certain thickness, adding layers can result in a less robust repair, compared to what the design equations show. In other words, every composite repair system reaches a critical maximum repair thickness level beyond which the law of diminishing returns begins to take effect.
In this research program, several products were tested to determine the ultimate repair thickness. It was discovered that as the wrap thickness is increased, the amount of strain reduction begins to plateau at some point. This is plotted as wrap thickness divided by pipe wall thickness on the X-axis and percent strain reduction in the Y-axis — meaning that a 0.18-in. wall pipe and a 0.18-in. thick composite would be represented at Point 1 on the X-axis. Test results indicate that at some stage, adding repair thickness to a defect area does not provide additional strength to the repair.
Live Pressure Testing
When pipes are under internal pressure, it is common for pressure to be reduced before a composite repair is applied. This is a safety protocol for some industries. It also is done under the assumption that reducing the pressure better engages the composite. In many cases, composite repair installations take place without shutting off flow through the pipe. Instead, the repair is carried out with the pipe at full operating pressure or some form of reduced pressure.
ASME PCC-2 4.1 and ISO 24817 provide design equations in which increasing installation pressure reduces the required composite repair thickness. This is counter-intuitive from a safety design perspective.
An internal test program studied the effect internal pipe pressure has on composite reinforcement systems during installation. Full-scale testing analyzed the effects on the burst pressure and the cyclic pressure fatigue life of a pipe with a simulated 50 percent wall loss corrosion defect.
The results showed that installation pressure has little impact on the long-term performance of composite repairs, provided the defect region is not yielded. It is important to note that at the installation pressures tested, the defect remained in the elastic region. In general, it appears that installation pressure should not be considered a determining factor in the design of the composite repair. It seems best though to conservatively neglect the installation pressure used in the design of the composite repair. A thicker repair helps to lower the strains in the defect region, with a higher stiffness composite system having the lowest strain provided that other factors remain constant.
Investing in Performance
Product testing is important for quality assurance, meeting regulatory requirements, and verifying manufacturing process, but it is arguably most critical when developing products for specific applications.
Well defined test methods remove the grey areas and provide owners and operators with a clearly defined set of product strengths and boundary limitations. Investing in extensive testing in the course of product developments delivers products the industry can trust rather than products that end up teaching costly lessons in the field.
Tags: ClockSpring|NRI, composite repair, June 2019 Print Issue
Eri Vokshi is a professional engineer with ClockSpring|NRI. The author would like to thank Davie Peguero, Matt Green and the ClockSpring|NRI technical department for their input.