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Quantitative data on the stress-strain behavior of walls and roofs of underground mines are unavailable. It is established practice to use roof bolts to support roofs in mines. Smart materials have been developed for roof bolts that act as sensors in systems capable of monitoring the stress/strain history of mine roofs with the goal of detecting structural defects and damage. The systems can be designed for a passive mode to monitor peak strain or an active mode for real-time alarm response. The systems are low-cost and capable of long-term operation which, in the case of waste isolation facilities, can involve very long time periods. The smart materials to be used in this application are TRIP (Transformation Induced Plasticity) steels. TRIP steels are materials that change state from austenitic, nonmagnetic to a martensitic, ferromagnetic phase as the material undergoes straining. There is a direct correspondence of the peak strain level experienced in the material with the percentage of ferromagnetism, hence monitoring the relative amount of the ferromagnetic content will indicate the level of strain (and therefore, stress). The phase transition that accompanies the straining is irreversible so the monitor material indicates the peak strain until that value is subsequently exceeded. Materials research discussed will cover the selection of compositions with a suitable ferromagnetic response and the development of thermomechanical treatments to achieve high tensile strength required for this application.
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