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For example, the fatigue limit of 45 steel is 34.3mpa after sub fire treatment and 24.5mpa after annealing treatment. After quenching at 840 ¢J and tempering at 420 ¢J, it is 52.9mpa. It can be seen that the mechanical properties of 45 steel are very different after different heat treatment.
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Fatigue characteristics of epoxy phenolic FRP: FRP is a kind of polyphase material, a kind of fiber coincidence material, and a kind of brittle material (elongation less than 5%)
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Fatigue process of FRP: the fatigue failure process of FRP is a comprehensive process of interface degumming, resin cracking and fiber fracture. For FRP laminates, the failure begins in the transverse fiber bundle perpendicular to the direction of alternating stress. The first is the degumming at individual points of the interface, which develops along the fiber direction, causing the transverse fiber bundle to peel off. After a period of time, there are cracks in the resin, and the parts gradually become opaque from translucency. When the cracks further expand, local fibers begin to break, the resin in the damaged area becomes white powder, and finally the parts are damaged.
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Factors affecting the fatigue strength of FRP: FRP is a composite material composed of alkali free fiberglass cloth and synthetic resin. The fatigue strength of FRP varies with different resin types. For the same kind of resin, the fatigue strength of phenolic resin or epoxy resin is higher than that of polyester resin. The fatigue strength of heat resistant type is higher than that of common type. If the strength of resin is the same, the fatigue strength of FRP with high elongation is also high.
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The influence of stress concentration on the fatigue strength of FRP is also serious. Under the static load, FRP has higher notch sensitivity than metal. The stress concentration coefficient of metal is 3, and the stress concentration coefficient of FRP laminate is 3 ~ 9.
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