Field Trials of Aerospace Fasteners in Mechanical and Structural Applications

Abstract

The present work reports findings for the application of specialized aerospace aluminum rivets, manufactured from Al 7075 (Al-Zn-Mg-Cu) T6 alloy stem/mandrel, with an Al 5056 (Al-Mg) shank or sleeve, which were used for construction rectification of an outdoor louver façade on a high-rise building. These specialized rivets were used to replace failed conventional construction rivets, which consisted of sleeve and mandrel comprised of either all-steel, all-aluminum, or aluminum-steel. The building is in close vicinity to the ocean and exposed to extremely high wind loading, making the rivets susceptible to failure by corrosion and fatigue. The focus of the present work is to report the examination of the specialized replacement rivets following an in-service lifetime of 12 years. The examination revealed that the replacement rivets (mandrel and sleeve) remained intact and uncontaminated, essentially free of corrosion. It is likely that sunlight exposure and the composite nature of the rivets enhanced the performance through age hardening. Analysis of the rivets included visual inspection, optical microscopy, Vickers microhardness testing, and transmission electron microscopy. The aim of the analysis was to correlate microstructures with microhardnesses, using these data to evaluate the ultimate tensile strength (UTS), yield strength (YS), and the potential for further age hardening. The Vickers microhardnesses were observed to have increased by ~8% over the service lifetime of 12 years, which equates to increases in YS (34.8–46.8 MPa) and UTS (23.8–45.6 MPa). Although the results show that there is a large increase in the strength values when comparing the unused rivets to the 12-year-old rivets, this increase in hardness may not necessarily be due purely to natural aging kinetics such as exposure from the sun and outdoor temperature. However, there appears to be some insignificant alteration of the microstructure and mechanical properties as a result of this exposure.