Effect of Thermal Exposure on Residual Properties of Wet Layup Carbon Fiber Reinforced Epoxy Composites

Abstract

Ambient cured wet layup carbon fiber reinforced epoxy composites used extensively in the rehabilitation of infrastructure and in structural components can be exposed to elevated temperature regimes for extended periods of time of hours to a few days due to thermal excursions. These may be severe enough to cause a significant temperature rise without deep charring as through fires at a small distance and even high-temperature industrial processes. In such cases, it is critical to have information related to the post-event residual mechanical properties and damage states. In this paper, composites are subjected to a range of elevated temperatures up to 260 °C over periods of time up to 72 h. Exposure to elevated temperature regimes is noted to result in a competition between the mechanisms of post-cure that can increase the levels of mechanical characteristics, and the deterioration of the resin and the bond between the fibers and resin that can reduce them. Mechanical tests indicate that tensile and short beam shear properties are not affected negatively until the highest temperatures of exposure considered in this investigation. In contrast, all elevated temperature conditions cause deterioration in resin-dominated characteristics such as shear and flexure, emphasizing the weakness of this mode in layered composites formed from unidirectional fabric architectures due to resin deterioration. Transitions in failure modes are correlated through microscopy to damage progression both at the level of fiber-matrix interface integrity and through the bulk resin, especially at the inter-layer level. The changes in glass transition temperature determined through differential scanning calorimetry can be related to thresholds that indicate changes in the mechanisms of damage.

Seawater Effects on Thermally Aged Ambient Cured Carbon/Epoxy Composites: Moisture Kinetics and Uptake Characteristics

“…In addition, hydrolysis reactions in the bulk resin can also result in this increase following a drop. Thermal aging at 232 °C was noted earlier [62] to result in a complex state of deterioration and a fairly rapid drop in glass transition temperature, as determined through dynamic scanning calorimetry (DSC) after 4 h of aging. Resin-dominated mechanical characteristics demonstrated a range of deteriorative mechanisms, confirming a complex thermally induced state, which results in the variation in uptake, as seen based on fairly localized damage.…”

“…The two highest rates are at 66 • C and 232 • C, with the rest varying, without a clear trend in the rate with the change in temperature. This is due to the multiple mechanisms at play even in the first stage of moisture uptake based on prior thermal history, which leads to competition between mechanisms of post-curing and deterioration at the bulk polymer and fiber-matrix levels [62,66,80]. The current condition of immersion in seawater for an extended period of time (72 weeks) following a range of pre-immersion thermal aging regimes thus adds further coupling and interaction of phenomena other than that investigated in the comprehensive study of the coupling between plasticization and physical aging in an epoxy in a wet environment [32] and that of water content on physical aging reported earlier by Zheng et al [81].…”

“…In addition, there are temperature requirements for rods [59], risers [11], and tools used in downhole operations at temperatures between 150 and 232 • C [60]. In these and other similar applications, there is a need for assurance of the post-incident operational structural integrity [61,62], and thus the need for a comprehensive understanding of long-term durability under harsh conditions [63] and longer-term exposure to seawater combined with other loading conditions and exposures [64]. With the increasing use of non-autoclave process-based composites in the marine and offshore sectors, there is a need to further study the effects of thermal history and seawater exposure on the long-term integrity and residual characteristics of these materials.…”

“…The current investigation focuses on the effects of seawater immersion on the moisture uptake characteristics of carbon/epoxy composites aged for periods up to 72 h at temperatures up to 232 • C. It builds on the research reported in [62,66], with the aim of better understanding moisture kinetics and the resulting post-exposure moisture uptake characteristics as part of an overall effort to better characterize and predict the long-term durability of these non-autoclave cured, lower cost, carbon/epoxy composites.…”