Flexural fatigue characteristics of sandwich structures at different loading frequencies

The effects of frequency on the fatigue behavior of S2 glass fiber-vinylester reinforced sandwich composites with two different PVC cores have been investigated. Flexural fatigue tests were performed on sandwich beams with core densities of 130 and 260 kg/m3 at frequencies of 3 and 15 Hz, at a stress ratio, R=0.1 and at four different load levels viz. 90%, 85%, 80% and 75% of the ultimate load. S-N diagrams were generated and it was observed that the fatigue strength increased with core density, and the number of cycles to failure, Nf, increased with increase in frequency. In all cases failure was dominated by a primary shear crack in the core however, the crack path and crack propagation rates varied with frequency. The fatigue crack growth rate (FCGR) in the core of the H130 sandwich beams was subsequently investigated and the relationship between the crack growth rate, da/dN, and the cyclic stress intensity range, ΔK, was determined. It was found that crack growth rate decreased with increase in loading frequency.