AbstractA method is described for evaluating flexural fatigue properties of unsaturated polyesters. With the use of the Sonntag fatigue machine and a modified version of the Prot principle, dynamic parameters and fatigue properties can be readily determined. Testing of castings with no reinforcement reveals two different modes of resin fracture, brittle and plastic. A plastic fracture implies that the resin yields before breaking. Reproducibility of the results is directly related to the type of fracture, ranging from very good for a plastic resin to very poor for a brittle. Close proximity of yield points and endurance limits for plastic resins confirms validity of Prot's method of extrapolating the latter parameter. Incorporation of two layers of glass cloth in positions corresponding to the outermost plies in a laminate shows that failure at the glass–resin interface may be the overriding cause of laminate breakdown. Both plastic and brittle resins become stiffer in the presence of glass, but only the plastic resin is realizing its inherent strength to give a stronger reinforced casting. The brittle resin is, on the other hand, weaker with glass present because of inability to dissipate local stress concentrations at the warpfilling crossovers. The break amplitude is a particularly significant parameter because it is expected to be the same in a normal laminate as in these castings with only the outermost plies present. Additional plies would merely increase stiffness and, correspondingly, the stress to reach amplitude coincident with failure at the outermost plies. Use of relatively thick test bars (0.5 in.) permits insertion of a thermocouple in the neutral plane without affecting dynamic properties. Measurement of the temperature rise during flexing allows assessment of the damping capacity of the glass–resin system, which is of particular importance for resonance fatigue. By this means, three different types of glass–resin systems could be distinguished: (1) weak and high damping (plastic), (2) strong and low damping (brittle), (3) strong and high damping (tough). For a particular resin, the behavior is determined by the basic composition as well as by degree of cure. For well‐cured polyester samples, several links were found between dynamic parameters, such as stiffness, fatigue strength and damping, and basic composition. Distance between crosslinks in the polyester backbone thus appears to have a direct bearing on strength and damping. Such relationships are valuable in that they make possible predictions of the fatigue behavior of a given polyester form
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