372 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju loss normalized by the fiber surface area is negligible compared to that of the resin.Weight loss studies on sized fibers [105]indicate that the sizing/coupling agent is released within a short time period (~24 h)of aging beyond which the weight loss trend is similar to the corresponding unsized fibers. Mechanical properties Single fiber specimens were tested at room temperature in tension using the single-filament test.Figures 9.4 and 9.5 show the normalized failure strength and failure strain,respectively,of unsized T650-35 carbon fibers aged at 343C.The strengths and failure strains are normalized with respect to their corresponding unaged values,such that the observed decreases are a reflection of the reduction resulting from isothermal aging.A minimum of ten fibers were tested for each condition considered,and the standard deviation is shown as the error bar across the measured mean values.There is some scatter in the strength and failure strain data which is typically encountered in single fiber testing as failure is sensitive to the presence of flaws over the fiber gage length.Fibers aged at the elevated temperature of 343C show a large decrease in the strength after 1,000 h of aging.The significant decrease in mechanical strength signifies that the carbon fibers should not be treated as static entities when composites containing these fibers are aged at this temperature for extended periods.However,the reported data are a worst case scenario in which all surfaces of the fibers are exposed during aging.In situ fibers of the composite have only a small fraction of their total surface area exposed to the oxidizing environment and should,therefore,suffer less degradation.Further,test data from single-filament testing [105]indicate that application of fiber sizing may 1.4 1.4 1.2 T650-35 aged at 343C 1.2 T650-35aged@343℃ 0.3 8 0.6 04 urens amje pazyeuoN 0.4 0.2 0 235547 764 235 547 764 100 Aging Time,hrs Aging Time,hrs Fig.9.4.Normalized strength of unsized Fig.9.5.Normalized failure strain of T650-35 carbon fibers aged at 343C unsized T650-35 fibers aged 343Closs normalized by the fiber surface area is negligible compared to that of the resin. Weight loss studies on sized fibers [105] indicate that the sizing/coupling agent is released within a short time period (∼24 h) of aging beyond which the weight loss trend is similar to the corresponding unsized fibers. Mechanical properties Single fiber specimens were tested at room temperature in tension using the single-filament test. Figures 9.4 and 9.5 show the normalized failure strength and failure strain, respectively, of unsized T650-35 carbon fibers aged at 343°C. The strengths and failure strains are normalized with respect to their corresponding unaged values, such that the observed decreases are a reflection of the reduction resulting from isothermal aging. A minimum of ten fibers were tested for each condition considered, and the standard deviation is shown as the error bar across the measured mean values. There is some scatter in the strength and failure strain data which is typically encountered in single fiber testing as failure is sensitive to the presence of flaws over the fiber gage length. Fibers aged at the elevated temperature of 343°C show a large decrease in the strength after 1,000 h of aging. The significant decrease in mechanical strength signifies that the carbon fibers should not be treated as static entities when composites containing these fibers are aged at this temperature for extended periods. However, the reported data are a worst case scenario in which all surfaces of the fibers are exposed during aging. In situ fibers of the composite have only a small fraction of their total surface area exposed to the oxidizing environment and should, therefore, suffer less degradation. Further, test data from single-filament testing [105] indicate that application of fiber sizing may Fig. 9.4. Normalized strength of unsized T650-35 carbon fibers aged at 343°C unsized T650-35 fibers aged 343°C G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju Fig. 9.5. Normalized failure strain of 372