The use of strain memory alloys in structural health monitoring systems

There is an increasing drive toward the monitoring of the structural health of costly and safety critical infrastructure. Quite often, however, this does not require the cost and nuisance of active monitoring, and a simple passive system will suffice. One of the most robust, cost effective, and neat methods of passive peak monitoring involves the use of strain memory alloys; which are ferrous alloys that display paramagnetism in the unstrained state, but transform proportionally to display varying degrees of ferromagnetism depending on the level of peak strain induced in the material. This effect is achieved using a transformation in crystal structure from a metastable austenitic structure to a stable strain-induced martensitic structure. Because of the physical properties and the relatively low cost of strain memory alloys it is actually possible to manufacture complete components from this family of materials, and the materials can be tailored in terms of physical as well as transformation properties using alloying chemistry and prior deformation of austenite in the ausforming temperature range. Several dual purpose components have been developed for a variety of different applications, using strain memory alloys: a smart composite laminate, a smart rock anchor, and a smart aircraft bolt among others. This paper presents the general principles involved in using strain memory alloys for structural health monitoring, as well as specific application designs and results.