Browsing by Author "Pretorius, T S"

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    A study of brittle powder compaction using a combined discrete finite element approach
    (1999) Parker, I; Mitchell, G P; Pretorius, T S
    The Discrete Element Method (DEM) is a collection of numerical techniques based on a discontinuum idealisation of a physical system. Discrete Element Methods have been successfully applied to the simulation of the dynamic behaviour of granular media. Using the Finite Element Method to model the behaviour of each discrete element has resulted in the Combined Finite Discrete Element Method. This combination allows researchers to investigate the behaviour of assemblies of particles in which each particle need not behave in an entirely elastic mamrer. More importantly, discrete crack constitutive models may now be applied to each discrete element. The purpose of this thesis is to simulate the macroscopic behaviour of an assembly of brittle particles, using the Combined Finite Discrete Element Method. Brittle behaviour is incorporated via the use of a Mode I and Mode II brittle failure constitutive models. The Mode I and Mode II failure models used are the Rankine and Tresca failure models, respectively. The development of a pre-processor to generate the initial configuration of the particle assembly also formed a major component of the thesis. Algorithms to fill a plane with randomly shaped polygons were developed and implemented for the pre-processor. The cold compaction process is idealised as the compaction of a granular medium, in which each particle is a deformable polygon. Two algorithms are proposed and implemented to generate randomly shaped polygons. A further algorithm was developed to fill a simple bounding polygon with smaller polygons. A_commercial CFDEM code, ELFENExplicit, was used to investigate the effect of particle constitutive model and particle interaction on the macroscopic behaviour of the granular assembly. The built-in linear elastic and Mode I Rankine failure models were used in the initial simulations while a Tresca failure model was implemented to investigate the effect of a Mode II failure model.
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    Vibration problems of skips in mine shafts : the effect of compressive forces in the guides
    (1989) Pretorius, T S
    Investigations into problems involving the vibration of conveyances in deep mining shafts have led to the identification of 'slamming' as a significant event in the initiation of large perturbations in the motion of the skip. Slamming occurs when the flexible rollers on the skip which normally act on the guides are inoperative. The primary concern is that this slamming event can give rise to large lateral loads on the shaft steelwork and is therefore a factor which limits the speed at which the skip can be drawn up the shaft. This study extends previous work to investigate the influence of compressive forces in the guides on the response of the skip and the steelwork. These forces are induced as a result of mining operations and lead to a decrease in the transverse stiffness of the guides. A mathematical model of the slamming event is formulated and a numerical solution for a specific case is performed. An alternative simplified solution is discussed and compared to the initial formulation, with the aim of facilitating the use of previous research results. A model to simulate the response of the skip when the skip rollers are functional is formulated, and numerical solutions of different examples are given. An important conclusion is that the compressive forces can significantly reduce the transverse stiffness of the guides, and should be taken into account in future designs. Bibliography: pages 86-88.