Effects of Loading Frequency on the Fatigue Behavior of Sandwich Composites

摘要

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/m~3 at frequencies of 3 and 15Hz, at a stress ratio of 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 the failure mechanisms were characterized. Fatigue strength decreased with increased stress and increased with core density. It was observed that frequency of testing and stress levels influenced the fatigue life. The fatigue limit appeared to be unaffected by loading frequency but the number of fatigue cycles to failure, N_f, increased with the increase in frequency and time to failure, t_f decreased with the increase in frequency. For the H130 sandwich beams at a frequency of 3Hz and at 90% of the ultimate load (0.9P_(ult)), the fatigue crack initiated on the compression side and propagated horizontally as a Mode Ⅰ crack for approximately 2-5mm parallel to the interface and just below the bond line in the core. Thereafter, the crack kinked to Mode Ⅱ at approximately 45° to the vertical traversing toward the tension side. The crack then propagated in the core as a 2nd stage mode Ⅰ crack to the edge of the specimen. In all cases the Mode Ⅰ crack propagated above the bond line in the core. The crack angle (Mode Ⅱ) decreased with the decrease in loads. Increased loading frequency of 15Hz at 0.9P_(ult) resulted in a substantially longer Mode Ⅰ crack length ( +- 45mm ). For the R260 sandwich beams, the fatigue crack propagation path included Mode Ⅰand Mode Ⅱ fatigue cracks at both frequencies (3Hz and 15 Hz) as in the H130 beams. Additionally, the mode Ⅰ fracture comprised of core fracture above the bond line followed by clean debonding between the core and the face sheet.