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This paper discusses efforts related to the development of smart bolts to be used as structural attachment fixtures. The work has been directed at meeting the high-strength application requirements for the aircraft industry and selected applications within the construction industry. The bolts are fabricated from metastable austenitic steel materials which progressively and irreversibly transform from a nonferromagnetic, austenitic parent phase to a thermodynamically stable, ferromagnetic martensitic phase as a function of applied strain. Thus, the ferromagnetic response of a bolt in service can be used to indicate the degree of inelastic deformation, i.e., the peak strain, that the bolt has experienced up to that particular point. A combination of bolt alloy chemistry and thermomechanical treatment has been utilized to produce a sharp ferromagnetic response with respect to small plastic deformation strains. The extreme strength of the materials allows for a centrally drilled access hole for the placement of sensitive Hall sensor probes to detect any inelastic material behavior along the length of the bolt without removal from the structure. A discussion of the smart materials behavior of the bolts will be followed by a presentation of recent test results which illustrate the structural bolt monitoring technique with some possible applications.
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