Magnetic fatigue of magnetite: effect of cyclic loading on the magnetic and microstructural behavior at elevated temperatures
Geophysics
Final Report Abstract
The present study demonstrates the influence of temperature and time-dependent cyclic loading conditions on the mineralogical and magnetic properties of magnetite. We were able to show that cyclic uniaxial compression at 70 ± 30 MPa (data not shown) and 150 ± 30 MPa at 10000 cycles, can significantly change the induced magnetization of a magnetite-quartz ore due to mineral transformation and that the first stage of mechanical fatigue, which is a precursor of the failure of a rock, is closely associated with these transformations. The first thousand cycles are the most sensitive for changes in magnetic susceptibility and its anisotropy. Further loadings do not significantly affect the magnetic susceptibility which then remains more or less constant. When oxygen is available during cyclic loading, mechanical fatigue in magnetite is accompanied by its partial transformation into hematite with a quite significant drop in magnetic susceptibility up to 23%. Vacuum experiments indicated only little changes in magnetic susceptibility up to 4% as a result of irreversible deformation due to the formation of slip steps whereas magnetic fabric parameters like the degree of anisotropy P and the shape parameter T do not show any trend or significant difference. This observation suggests that changes in the degree of anisotropy can be rather correlated with the formation of hematite from magnetite than to plastic deformation in magnetite. The increase of P and T values in air-treated samples can be either related to the change in the shape of the magnetite grains due to microcracks decorated by hematite or the effect of the strong single-crystal anisotropy of hematite. The intensity of oxidation is strongly dependent on temperature and time. We also tested how mechanical fatigue modifies structural and magnetic properties of magnetite under enhanced temperatures and at different frequencies. Laboratory mechanical fatigue compression was carried out on magnetite-rich powders between 30° and 500°C at 1, 10 and 100 Hz under very low uniaxial static stress of 2.55 MPa modulated with 1.27 MPa using a DMA system. Again, magnetic susceptibility decreases due to magnetite to hematite transformation, but we also observed fracturing of the magnetite grains and defect structures in the magnetite lattice indicating that magnetite reacts very sensitive to cyclic deformation. This was confirmed by DMA experiments performed on solid 5x5x3 mm pieces between 30 and 350° and at room temperature under uniaxial static stress of 6.6 MPa modulated with 3 MPa. All these experiments showed that slip markings on the magnetite grain surface occurred. Calorimetric and low-temperature magnetometry across the Verwey transition showed that this first order transition in magnetite is very sensitive to deformation and we interpret the broadening of the transition to be related to the defects in the magnetite lattice. It is questionable if time-dependent magnetic susceptibility measurements can be used in practice as an effective proxy parameter for the detection of mechanical fatigue because oxidation of magnetite to hematite can have manifold reasons in nature. Therefore, its application for the detection of seismomagnetic effects during geological engineering activities, especially in drilling operations or earthquake monitoring needs further studies. However, lattice defects can be seen in high-resolution transmission electron microscopy and X-ray diffraction, as well as in heat capacity and magnetometry across the Verwey transition. Therefore this project clearly show evidence that magnetite is very sensitive to mechanical fatigue.
Publications
- (2021). Effect of cyclic loading at elevated temperatures on the magnetic susceptibility of a magnetite-bearing ore. Geophys. J. Int. 228, 1346-1360
Dudzisz, K., Walter, M., Krumholz, R., Reznik, B., Kontny, A.
(See online at https://doi.org/10.1093/gji/ggab400)
