Environmental effects on the hybrid glass fiber/carbon fiber composites

Fiber reinforced polymer composites (FRPCs) have been widely used to replace conventional metals due to the high specific strength, fatigue resistance, and light weight. In the power distribution industry, an advanced composites rod has been developed to replace conventional steel cable as the load-bearing core of overhead conductors. Such conductors, called aluminum conductor composite core (ACCC) significantly increases the transmitting efficiency of existing power grid system without extensive rebuilding expenses, while meeting future demand for electricity. In general, the service life of such overhead conductors is required to be at least 30 years. Therefore, the long-term endurance of the composite core in various environments must be well-understood.;Accelerated aging by hygrothermal exposure was conducted to determine the effect of moisture on the glass fiber (GF)/carbon fiber (CF) hybrid composites. The influence of water immersion and humid air exposure on mechanical properties is investigated. Results indicated that immersion in water is the most severe environment for such hybrid GF/CF composites, and results in greater saturation and degradation of properties. When immersed directly in water, the hybrid GF/CF composites exhibit a moisture uptake behavior that is more complex than composite materials reinforced with only one type of fiber. The unusual diffusion behavior is attributed to a higher packing density of fibers at the annular GF/CF interface, which acts as a temporary moisture barrier. Moisture uptake leads to the mechanical and thermal degradation of such hybrid GF/CF composites. Findings presented here indicate that the degradation is a function of exposure temperature, time, and moisture uptake level. Results also indicate that such hybrid GF/CF composites recover short beam shear (SBS) strength and glass transition temperature (Tg) values comparable to pre-aged samples after removal of the absorbed moisture.;In the hygrothermal environment (60°C/85% R.H. air), the hybrid GF/CF composites exhibit theoretical Fickian behavior before reaching the pseudo-saturation level. However, with continued exposure, the composites then exhibit weight loss, a phenomenon attributed to a combination of a dehydration reaction and a hydrolysis reaction. These reactions resulted in loss of low molecular weight molecules present in the epoxy matrix. Results indicate that a small amount of moisture-induced damage occurred in the hybrid composites, although the mechanical and thermal properties were only slightly diminished. In fact, the retained tensile strength was equivalent to the rated tensile strength.;The influence of thermal exposure on the strength of hybrid GF/CF composites was thoroughly investigated. Results indicate that tensile strength and modulus degrade as a function of temperature and time. When the aging temperature is close to Tg, degradation was increased along with increasing temperature. Results also indicate that storage modulus can be used as an index to predict the temperature dependence of tensile strength and the long-term isothermal aging behavior, at least within specific domains.;The effect of thermal cycling on the hybrid composites was also investigated. Thermal degradation during thermal cycling is attributed to two distinctive mechanisms. The primary degradation results from isothermal aging, while additional thermal fatigue damage occurs at sub-zero temperatures due to increasing residual stress. An empirical normalized tensile strength-storage modulus correlation provides an index of thermal fatigue damage.;Various forms of environmental aging and their effects on mechanical and thermal properties of hybrid GF/CF composites were assessed. The results and analysis provides an insight into fundamental mechanisms of degradation associated with overhead conductors with the hybrid GF/CF composite cores.