Testing and Fractography of One-Sided “Plus Nut” Connections.

Fastener failures have been well studied and documented in the literature due to their importance in mechanical design and forensic analysis. However, the failure mechanisms of “one-sided” fasteners, sometimes called “plus nuts” is less understood. A typical fastener requires access to both sides of the connected materials, such as a bolt head and a corresponding nut. In contrast, a plus nut is installed and tightened from only one side. As part of their installation, the tabs of the plus nuts deploy into the inaccessible space and in the process become work-hardened. Plus nuts typically carry only modest loads, yet plus nuts that are fatigued or overloaded and subject to failure and produce fractures that are less thoroughly characterized than other fastener failures. The unique set of material properties of the nut itself, in combination with eccentric loading can lead to preferential failures and unexpected failure modes. In this work plus nuts were tested to failure in different combinations of loading and installation configuration to establish expected failure modes for different applications. This work aims to predict the failure of one-sided plus nut connections within the context of commonly understood fastener failures. The resulting samples were analyzed fractographically to correlate lab-induced plus nut fractures to the known input testing conditions.