Development of probabilistic models for long term reliability of sandwich composites in saline freeze/thaw environment for civil engineering applications

摘要

Structural engineers commonly use balsa wood and PVC foam as core materials for sandwich composite structures. The long-term reliability and damage mechanism of these composite sandwich structures under severe environmental conditions are still unclear. These liability concerns prevent most civil structural engineers from considering this material in infrastructure applications. Also, they must account for the unique nature of the civil construction industry and offer an advantage over traditional building materials such as steel and concrete. Introducing a strength reduction factor (&phis;) will increase the chance for using sandwich composites in civil engineering applications. Using a probabilistic approach is a means to reach to this goal. In this project, the long-term effects of freeze/thaw in a saline environment were simulated by exposing sandwich composites to 100 days of freeze/thaw exposure (-20°C to 20°C) in the presence of a 3% NaCl saline solution. The sandwich panels were comprised of using balsa wood (SB100) and foam core (Airex C70.55) with fiberglass/vinyl ester face sheets, fabricated with vacuum assisted resin transfer molding (VARTM).Samples were tested for core shear, core compression, and peel tests. Results confirmed that exposure reduced the balsa wood core properties significantly, however, PVC foam core shear modulus increased by 25%, and the compression modulus reduced by 25%. Simulated lifetime core shear fatigue curves were also developed and evaluated. A substantial reduction in tensile, shear, and compression properties was observed for the face sheets constructing the sandwich panels. The thermal cycling events degraded the matrix binding, the warp and fill fibers, thus impairing the structural integrity of the cross-ply laminate. After testing, a reliability-based approach was used to examine the "strength reduction" factors (&phis;) of the core materials. Resistance factors of 0.79 and 0.95 were obtained for balsa and foam core composite sandwich materials, respectively. The results of this research will ultimately lead to a probabilistic analysis model that will eventually act as a benchmark to reliably predict the performance of sandwich core materials.