An integrated theory is developed for predicting the hygrothermomechanical (HGTM) response of fiber composite components. This integrated theory is based on a combined theoretical and experimental investigation. In addition to predicting the HGTM response of components, the theory is structured to assess the combined hygrothermal effects on the mechanical properties of unidirectional composites loaded along the material axis and off-axis, and those of angle-plied laminates. The theory predicts values in good agreement with measured data at the micromechanics, macromechanics, laminate analysis, and structural analysis levels.
An investigation was conducted to characterize the mechanical properties of intraply hybrids made from graphite fiber/epoxy matrix (primary composites) hybridized with varying amounts of secondary composites made from S-glass or Kevlar 49 fibers. The tests were conducted using thin laminates having the same thickness. The specimens for these tests were instrumented with strain gages to determine stress-strain behavior. The results show that the mechanical properties of intraply hybrid composites can be measured using available test methods such as the 10-deg off-axis method for intralaminar shear, and conventional test methods for tensile, flexure, and Izod impact properties. Intraply hybrids have linear stress-strain curves to fracture for longitudinal tension and nonlinear stress-strain curves for intralaminar shear.
The results also showed that combinations of high-modulus graphite/S-glass/epoxy matrix composites exist which yield intraply hybrid laminates with the “best” balanced properties: for example, 100 percent increase in impact resistance and 35 percent increase in tensile and flexural strengths, with no reduction in modulus compared with graphite fiber/epoxy matrix composites. In addition, the results showed that the translation efficiency of mechanical properties from the constituent composites to intraply hybrids may be assessed using a simple equation.
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