Abstract
The purpose of this study is to investigate how the cyclic frequency of fatigue tests influences the fatigue behavior of fiber reinforced plastics (FRP) under natural convective conditions, especially in a higher cyclic frequency range than those of 1000 or 1800cpm used in conventional fatigue tests with constant stress.
The reduction of rigidity and the raise of surface temperature were observed on the cantilever-type laminated FRP specimens excited sinusoidally by a shaker in a controlled high cyclic frequency of 6000, 8000 or 10000cpm. The time to failure was taken as the time period until the rigidity of a specimen became under 7/8 of its initial value according to the ASTM Standard, D671-63T. Two kinds of FRP were tested: the roving glass cloth reinforced unsaturated polyester resin (R) and the glass-fiber mat reinforced one (M).
The observation of the surface temperature of specimens revealed two patterns in the process of surface temperature increase during fatigue and the cyclic frequency dependency for both FRP laminates. The raise in temperature of specimens M was more remarkable and sensitive for the stress level than that of specimens R.
As for the cantilever bending fatigue strength evaluated on the basis of 1/8 rigidity reduction, a considerably large cyclic frequency effect was recognized. The fatigue strength under higher cyclic frequency loading was remarkably lower in comparison with that under the conventional cyclic frequency loading. It was found that fatigue strength σF is expressed in the form of λf-γ, where λ and γ are constants and f is the cyclic frequency.
