Cyclic loading of magnetite bearing rocks: modifications of structure, magnetic and elastic properties

Repeated seismic activity can cause progressive failure of rock masses due to material fatigue [1]. To simulate induced seismic loading, an iron ore with alternating quartz- and magnetite-rich layers from the Sydvaranger mine (Finnmark/Norway) was subjected to laboratory uniaxial compressional cyclic loading at low stresses in the range of elastic deformation (about 6 MPa static ±3 MPa dynamic pressure) and frequencies related to induced seismicity (10 – 100 Hz) [2]. Some of the experiments were performed until material failure occurred at up to 2 million cycles. Changes in magnetic behaviour were identified by measurements of magnetic susceptibility from Verwey transition in magnetite (about 120 K) to room temperature after intervals of 150 000 loading cycles. Magnetic domains were analysed with magnetic force microscopy. Deformation-related surface structures on magnetite and quartz were identified by reflected light microscopy and high-resolution scanning electron microscopy and compared to results from Raman spectroscopy. Dynamic mechanical analysis (DMA) was used for cyclic loading, which is a common method for determination of viscoelastic behaviour, especially of soft materials with pronounced viscoelasticity like polymers. Suitability of DMA for determination of relative changes of viscoelastic parameters during cyclic loading will be discussed under consideration of the relatively high Young’s modulus and almost ideal elastic behaviour of the iron ore under the chosen experimental conditions. [1] Gischig et al. (2016) Rock Mech Rock Eng, 49, 2457-2478. [2] https://www.geosig.com (2009), Seismic Signals and Sensors.