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Literature review on hybridization To develop this material, hybridization for 18 different fibers was considered. Hybridization of more than one type of fibrous materials was the interest of many materials science researchers. Most of their work was concerned with combining two types of fibers to enhance the mechanical properties of either type acting alone and to reduce the cost. This has been reported in several publications such as Bunsell and Harris (1974), Philips (1976), Manders and Bader (1981), Chow and Kelly (1980), and Fukuda and Chow (1981). Hybridization interested structural engineers as a tool to overcome the problem of a lack of ductility in FRF reinforcing bars. Nanni, Henneke, and Okamoto (1994) studied bars of braided aramid fibers around a steel core. Tamuzs and Tcpfcrs (1995) reported experimental investigations for hybrid fiber bars using different combinations of carbon and aramid fibers. Somboonsong, Frank, and Harris (1998) developed a hybrid FRP reinforcing bar using braided aramid fibers around a carbon fiber core. Harris, Somboonsong, and Frank (1998) used these bars in reinforcing concrete beams to achieve the general load- deflection behavior of concrete beams reinforced with conventional steel.
Design concept and materials To generate ductility, a hybridization technique of different types of fibers has been implemented. Three fibers have been selected with a different magnitude of elongations at failure. Figure 1 shows the stress-strain curves in tension for the selected composite fibers, and Table 1 shows their mechanical properties. The technique is based on combining these fibers together and controlling the mixture ratio so that when they arc loaded together in tension, the fibers with the lowest elongation (LE) fail first, allowing a strain relaxation (an increase in strain without an increase in load for the hybrid). The remaining high-elongation (HE) fibers are proportioned to sustain the total load up to failure. The strain value at failure of the LE fibers presents the value of the yield-equivalent strain of the hybrid, while the HE fiber strain at failure presents the value of ultimate strain. The load corresponding to failure of LE fibers presents the yield-equivalent load value, and the maximum load carried by the HE fibers is the ultimate load value. Ultra-high-modulus carbon fibers (Carbon No. 1) have been used as LE fibers to have as low a strain as possible, but not less than the yield strain of steel (approximately 0.2% for Grade 60 steel). On the other hand. E-glass fibers were used as HE fibers to provide as high a strain as possible to produce a high-ductility index (the ratio between deformation at failure and deformation at yield). High-modulus carbon fibers (Carbon No. 2) were selected as medium-elongation (ME) fibers to minimize the possible load drop during the strain relaxation that occurs after failure of the LE fibers, and also to provide a gradual load transition from the LE fibers to the HE fibers. Based on this concept, a uniaxial fabric was fabricated and tested to compare its behavior in tension with the theoretical predicted loading behavior. The theoretical behavior is based on the rule of mixtures, in which the axial stiffness of the hybrid is calculated by a summation of the relative stiffness of each of its components. The fabric was manufactured by combining different fibers as adjacent yarns and impregnating them inside a mold by an epoxy resin. Figure 2 shows a photo of one of the fabricated samples. Woven glass fiber tabs were provided at both ends of the test coupons to eliminate stress concentrations at end fixtures during testing. The coupons had a thickness of 2 mm (0.08 in.) and a width of 25.4 mm1 in.) and were tested in tension according to ASTM D 3039 specifications. The average load-strain curve for four tested samples is shown in Fig. 3 together with the theoretical prediction. It should be noted that the behavior is linear up to a strain of 0.35%, when the LE fibers started to fail. At this point, the strain increased at a faster rate than the load. When the strain reached 0.90the ME fibers started to fail, resulting in an additional increase in strain without a significant increase in load, up to the total failure of the coupon by failure of the HE fibers. A yield-equivalent load (the first point on the load-strain curve where the behavior becomes nonlinear) of 0.46 kN/mm