Influences of freeze–thaw cycles (FTC) on tensile properties and mixed-mode I/II fracture response of epoxy resin materials

Abstract

Epoxy resins (ERs) are widely used in manufacturing polymeric components. However, their mechanical properties and fracture resistance often deteriorate under fluctuating environmental conditions, particularly freeze–thaw cycles (FTC) common in rainy and snowy environments. The general understanding of the degradation process induced by FTC on the mechanical and fracture properties of ERs helps engineers develop predictive models. While a few studies have addressed the effect of FTC on the tensile properties of bulk ERs, very little research has been done to study mixed-mode I/II fracture behavior under such cycling. This work deals with the gap by studying the aging effects of 21, 42, and 63 FTC on the tensile properties and mixed-mode I/II fracture toughness of one type of ER being used frequently as a matrix phase for the manufacturing of fibrous composites. In this regard, dumbbell-shaped and cracked short-bend beam (SBB) specimens were used to obtain the tensile and mixed-mode I/II fracture responses, respectively, after exposure to FTC. All the specimens experienced FTC by thawing it in water at 25 °C for 12 h followed by storing the specimens in a freezer at −25 °C for 12 h each. The findings revealed that all aged specimens exhibited significantly lower tensile strain and higher tensile strength compared to the non-aged specimen. Opposite to the increase in tensile strength, a steady decrease of mixed-mode I/II fracture toughness with increasing number of FTC was observed, evidencing the destructive effect of FTC on residual fracture toughness for pre-cracked ER specimens. The reduction in experimental effective fracture toughness slows after the 42nd freeze–thaw cycle, forming a plateau. Therefore, the endurance limit for the tested ER can be set at 0.35 to 0.5 times the fracture toughness of a non-aged specimen. Finally, the mixed-mode I-II fracture envelope was established with regard to the GMTS criterion for both non-aged and aged specimens, by considering the deleterious effect of FTC on either the critical distance or FPZ existing near the crack tip. The GMTS theoretical predictions aligned well with the experimental results.