Influence of hygrothermal environment on thermal and mechanical properties of carbon fiber/fiberglass reinforced matrix composite materials
The absorption and diffusion of water in a carbon fiber/glass fiber hybrid composite was investigated. Water-sorption experiments, mechanical property tests and dynamic mechanical analysis (DMA) were performed after immersion in water at different temperatures for up to 32 weeks. The moisture uptake mechanism exhibited by the hybrid fiber system was determined to be more complex than the single fiber type. Weight-change profiles for the composites fitted the theoretical Fickian diffusion curve during the initial immersion time, but diverged substantially as time progressed. The shear properties and the glass transition temperature (Tg) were sensitive to the effects of hygrothermal environment, and values for both decreased with increasing water uptake. Microscopic inspection of water-soaked samples showed no cracking when the absorption was less than saturation. The thermal and mechanical properties were mostly retained (after drying), provided
High-temperature strength and storage modulus in unidirectional hybrid composites
Abstract The mechanical properties of pultruded rods of unidirectional hybrid glass/carbon-epoxy composites were evaluated with respect to retention of high-temperature properties up to 250 °C. Two prototype matrices were produced using developmental epoxy formulations with glass transition temperatures (Tg) of 176 and 202 °C, respectively. The two prototype composites showed diminished tensile strength at high temperatures, but with different temperature thresholds. The high-Tg composite retained full strength to 170 °C, while the lower-Tg composite retained full strength to 80 °C. Matrix softening and loss of fiber–matrix adhesion was a major factor affecting the strength reduction observed at high temperatures. The storage modulus, measured by dynamic mechanical analysis (DMA), showed temperature dependence nearly identical to the tensile strength for both composites. The correlation between storage modulus and tensile strength was analyzed in terms of the effect of temperature on the shear modulus of the matrices. The storage modulus behavior is presented as a basis for projecting the strength-temperature dependence of unidirectional composites.
ROOM- AND HIGH-TEMPERATURE MAGNETIC PROPERTIES OF Sm(CobalFexCu0.088Zr0.025)7.5 (x=0.30) SINTERED MAGNETS
The effect of Fe content on room- and high-temperature magnetic properties of Sm(CobalFexCu0.088Zr0.025)7.5 (x=0-0.30) sintered magnets, prepared by the conventional powder metallurgy method, has been systematically studied. With increasing Fe content, the remanences BT and maximum energy products (BH)max at room temperature gradually increase and reach optimal values of 1.055 T with x=0.27 for BT, and 205 kJ/m3 with x=0.21 for (BH)max, respectively, and then drop sharply with further Fe increase. An excessive addition of Fe (x≥0.24) results in rapid deterioration of permanent magnetic properties due to the formation of a considerable amount of FeCo soft magnetic phase. It was found that the effect of Fe content on high temperature stability is huge. The lower the Fe content, the smaller the temperature coefficient of coercivity. When x=0.07, a temperature coefficient of coercivity of -0.14%/K (293-723 K) can be achieved, as compared with -0.23%/K for x=0.21. BT of 0.725 T, bHc of 517 kA/m, iHc of 764 kA/m, and (BH)max of 95 kJ/m3 at 723 K were obtained, and the B-H demagnetization curve remains a straight line.