Smart materials for structural health monitoring.

Abstract

A new philosophy in structural health monitoring was explored, with the view to the creation of a smart mining bolt: one which would bear the normal load of any bolt used in South African gold mining tunnels, but at the same time be capable of monitoring its own level of damage. To this end, a survey of various smart materials currently used in structural health monitoring applications, was conducted, and a group known as strain memory alloys isolated as holding the most promise in this regard. Strain memory alloys give an indication of peak strain based on an irreversible transformation from paramagnetic austenite to ferromagnetic martensite, which occurs in direct proportion to the amount of strain experienced by the material. A measurement of magnetic permeability can therefore be correlated to peak strain. An extensive study of the alloying chemistry, material processing and transformation characteristics was therefore carried out, including an analytical model for the quantification of the energy associated with martensitic nucleation, at a dislocation-disclination level. The conditions within typical South African gold mining tunnels were evaluated, and a smart mining bolt design produced, based on the loading and environmental conditions present. Several material formulations were then proposed, melted, tested and evaluated against the relevant strength, corrosion and transformation criteria. A suitable material was selected and further tested. A working prototype bolt has been produced, and in situ tests of complete bolts, are scheduled to take place shortly.