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May 02, 2018
Understanding High-Temperature Composites through Testing

Posted by: Casey Whalen

Composite repairs have been used to restore structural integrity to pipes with external corrosion for years in plants, refineries and pipelines. As composites become more common within the oil and gas industry, alternative uses for the material are being sought after, including high-temperature uses.

This significant interest in evaluating the performance of composite repair materials at elevated temperatures led to the development of HT repair systems by Pipe Wrap. These systems use the benefits of either high strength carbon fiber or fiberglass fabric with a proprietary high-temperature epoxy resin. The filler material and adhesive used on this system were also uniquely developed to address the needs of pipe rehabilitation at elevated temperatures.

The resin matrix used in the HT systems, typically Atlas HT or FormaShield HT, is a multi-cured state system allowing for variable glass transition temperatures (Tg) named CS-205. This system requires an elevated step-wise temperature cure that can range up near 260°C to achieve optimum mechanical properties. Because this system is a multi-state curing system, it allows one system to be cured at various temperatures to provide flexibility of matching the required temperature performance as determined by the repair scenario. With a Tg near 260°C, this matrix allows the HT resin to be utilized at temperatures up to 233°C.

As with most resins, strength and modulus decrease at higher temperatures. Figure 1 illustrates a DMA analysis on the Atlas HT system, which shows a gradual reduction in the storage modulus as temperature increases.

Fig_1_HT.PNG

The load transfer filler, EP-HT, was also tested; a general decrease in compressive strength is observed as temperature increases. With a value near 20 ksi at room temperature, the compressive strength drops slowly to 13 ksi near 200°C as shown in Figure 2.

Fig_2_HT.PNG

The adhesive, PPR-HT, was also formulated with additives to optimize properties for pipe repairs at high temperatures. The lap shear strength of the adhesive was determined by performing a double lap shear sample using carbon-steel plates bonded together solely with the adhesive. Figure 3 shows the average load versus temperature. These samples were initially cured at 120°C, and eventually post cured at 230°C. A maximum performance is observed near the cure temperature and slowly begins to taper off as the material approaches its Tg.

Fig_3_HT.PNG

The key elements to consider are the lap shear strength for the adhesive, compressive properties for the filler material and tensile strength and modulus characteristics for the composite wrap. While there are other properties that need to be considered, these properties have the greatest influence on composite repair performance. Other considerations include environmental conditions, cyclic responses and fatigue life.

As the interest of longer repair life increases for elevated temperature conditions, it has become more important to fully understand how the composite repair system behaves at elevated temperatures. Generally speaking, they exhibit a gradual decrease in mechanical performance when exposed to increasing temperature conditions and may cause an unexpected failure if not properly accounted for. Conducting a combination of sub-scale coupon tests, in conjunction with full-scale destructive tests, is essential for engineers to understand the limit state capacity of a given repair at elevated temperatures.

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Category: Feature Articles, Pipe Wrap

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