Temperature dependence of the tensile strength of glass fiber–epoxy and glass fiber–unsaturated polyester composites

Epoxy and unsaturated polyester resins reinforced with random‐planar orientation of short glass fibers were prepared and the temperature dependence of their tensile strength was studied. The tensile strength decreases as the temperature increases, and this tendency can be expressed in terms of critical fiber length l c and apparent interfacial shear strength τ:$$ \begin{array}{*{20}c} {\sigma _{cs} \efDot \frac{{2\tau }}{\pi }\left[ {2 + \ln \frac{{\left( {{{1 - l_c } \mathord{\left/ {\vphantom {{1 - l_c } {2L}}} \right. \kern-\nulldelimiterspace} {2L}}} \right)\sigma _f \sigma _m v_f + \sigma _m \sigma '_m v_m }}{{\tau ^2 }}} \right],} \hfill & {L \gE l_c } \hfill \\ {\sigma _{cs} \efDot \frac{{2\tau }}{\pi }\left[ {2 + \ln \frac{{\tau \left( {{L \mathord{\left/ {\vphantom {L d}} \right. \kern-\nulldelimiterspace} d}} \right)\sigma _m v_f + \sigma _m ^2 v_m }}{{\tau ^2 }}} \right],} \hfill & {L < l_c } \hfill \\ \end{array} $$ where σ cs is the tensile strength of composite reinforced with random‐planar orientation of short fibers, L is the fiber length, d is the fiber diameter, σ f is the tensile strength of fiber, σ m is the tensile strength of matrix, u f is the volume fraction of fiber, v m is the volume fraction of matrix, and σ′ m is the stress of the matrix at fracture strain of the composite. The experimental strength values at room temperature are considerably smaller than the theoretical values, and this difference can be explained by the thermal stress produced during molding due to the large difference in the thermal expansion coefficient between glass fiber and matrix resin.