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Title: High strain rate properties of geological materials
Author: Braithwaite, C. H.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2009
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The dynamic response of various geological materials has been investigated through a series of plate impact experiments. The materials involved were supplied from various mines by De Beers and Rio Tinto and were generically termed: sandstone, scilified siltstone, kimberlite, quartz/feldspathic gneiss, biotite schist, amphibolite, amphibolitic gneiss, basalt and iron ore. Investigations into compressional, shear and tensional behaviour were carried out. This project was part of a larger international study to develop models for the explosive loading of rock in a mining environment. This model is known as the Hybrid Stress Blasting Model, or HSBM. For this model to be accurate and relevant to the mining process it is essential to have dynamic data on the various rock types concerned. The materials were found to have a wide range of properties, with a density range of 1.93 – 4.46 g cm-3 and a range of longitudinal sound speeds of 1.97 – 6.86 km s-1. Two of the materials were found to be porous, the iron ore and sandstone. A large number of mineral phases was identified within the specimens. The compressional response of all of the materials was measured. The porous materials demonstrated behaviour dominated by their compaction, with a curved Hugoniot relation for sandstone, and an obvious Hugoniot elastic limit (HEL) at around 3 GPa in the iron ore longitudinal data. Additionally, while not exhibiting porosity, the biotite schist also had a curved Hugoniot. It is postulated that this is due to the presence of clay in the material, something that was shown to be the case in the mineral analysis. The other materials all showed a linear Hugoniot represented the data in σ – μp space. Additional experiments confirmed that at the linear relationship implied that the shock velocity was not significantly changing with pressure over the pressure regime studied. As well as the compressional response, the unloading of a number of the materials was also investigated. It was found that in all loading/unloading cycles there was some irretrievable energy loss. In a number of the materials it appears that the energy lost on loading increases with pressure, as more damage is done to the material. The opposite trend was observed in the biotite schist. Basalt consistently showed a higher percentage of energy lost than the other materials, with the exception of sandstone, where substantial energy loss associated with the collapse of pore occurred. Shear strength and lateral stress were measured using stress gauges for amphibolite, iron ore, sandstone, quartz/feldspathic gneiss, kimberlite, siltstone and basalt. With the exception of sandstone and biotite schist, all of the materials were found to have an obvious HEL. These ranged in value from 1.3 GPa (gneiss) to 5 GPa (siltstone). This HEL was not obvious in many cases from the longitudinal data as it would be in many other materials. It is speculated that the nature of non-elastic deformation in rocks, namely brittle cracking would possibly account for this observation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available