High-speed bending-fatigue testing of composite materials

Abstract

A methodology for high-speed fatigue testing, especially for resin materials, including fibre-reinforced composites, was devised and evaluated. To exert periodic stress on a material at a frequency of more than 200 Hz, a specimen (made of glass fibre-epoxy laminate) was fixed as a cantilever to an electromagnetic vibrator and vibrated at its resonant frequency of the first bending mode by using a resonance tracking control. The shape of the specimen was designed with finite-element vibration analysis to obtain a resonant frequency of more than 200 Hz and a desired strain distribution for inducing fatigue damage under a certain stress level. The rise in temperature during the fatigue testing (due to damping loss) was estimated by applying heat-transfer theory and suppressed by external cooling to keep the specimen at room temperature. To confirm the validity of the devised high-speed-testing method, a completely reversed bending test at 1 Hz was also performed with identical specimens. The results from both testing methods (conducted at 230 Hz and 1 Hz, respectively) were plotted on a single power-law curve in an S-N plot. The good agreement between the two plots suggests that fatigue strength in the high- or giga-cycle region of resin and composites can be evaluated in a very short time if temperature is controlled appropriately.