Multi-Axial Fatigue Properties under Tension-Torsion Loading for Short-Glass-Fiber-Reinforced Phenolic-Resin Matrix Composite

Abstract

Static and fatigue tests under varying load ratio of tension and torsion at room temperature were carried out with short-glass-fiber-reinforced phenolic-resin matrix composites (GF/Phenol) made by injection-molded (S-series) and transfer-molded (P-series) processes. We investigated the short fiber content (Vƒ = 0%, 20%, and 50%) and stress ratio α = τ/σ effect on static and fatigue properties. Static strength and elastic modulus in uniaxial loading conditions were higher with increasing short fiber content. Furthermore, sensitivity for short fiber content of the injection-molded process was higher than that of the transfer-molded one. Static strength showed good agreement with the Tsai-Hill criterion. Relationships between the maximum principal stress σp1, max and number of cycles to failure Nf were approximately linear in the whole range of fatigue life. Normalizing σp1, max with the principal stress of static strength σp1, 0 gave S-N curves that depended on Vƒ, α and the molding processes. For unified evaluation of multi-axial fatigue life for GF/Phenol, non-dimensional effective stress σ* by the Tsai-Hill criterion was applied. Relationships between the σ* and N onto a double logarithmic chart was presented in the form of Basquin's exponential law without dependence on molding-process, Vƒ and α. The material constant n in Basquin's law showed a slope of σ*-N curves of S-series (n = 26.3) was equivalent to P-series (n = 27.0). It has been confirmed the multi-axial fatigue life of GF/Phenol could be predicted by using σ* with unique S-N curve.