Static and Fatigue Behavior of Sandwich Panels with GFRP Skins and Governed by Soft-Core Shear Failure

Sandwich panels composed of glass fiber-reinforced polymer (GFRP) skins and low-density polyurethane core are investigated for potential structural applications such as lightweight insulated roofing or cladding systems. Ten 1,220x318x78-mm panels, including three control specimens, were tested under high-cycle fatigue at maximum load levels of 45-70% of the ultimate static load. Fatigue tests were carried out to failure, thereby enabling the development of the complete fatigue life (S-N) curve and stiffness degradation. Digital imaging correlation was used to map shear deformations and showed that shear contributes about 90% of the deflection. Both static and fatigue failure modes were due to core shear failure. The study showed that fatigue threshold was 37% of the ultimate static capacity, and that to sustain at least 2 million cycles, service load should be limited to 45% of ultimate. The maximum design service live and snow loads, as well as wind load based on the most critical conditions in Canada, were all below the fatigue threshold of the panel. The panels had a strength safety factor of 2.1 relative to the factored maximum design live and snow load, and 6.5 relative to the factored maximum wind load. Deflection limits may govern for some service load conditions, but is not critical when only wind load considered. Stiffness degradation reached a maximum of 15-20%, but residual deflections upon unloading were insignificant.