Browsing by Author "Cloete, T J"

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    Development and proving of a split Hopkinson pressure bar used for high strain rate materials testing
    (2001) Marais, Stephen Thomas; Tait, Robert B; Nurick, Gerald N; Cloete, T J
    As a result of increasing demand to improve analysis of manufacturing techniques and safety in structures, it is necessary to determine material properties at high strain rates. Conventional screw-driven or servo-hydraulic methods of testing materials at high strain rates are not adequate as oscillations and stress waves are set-up within the testing apparatus. These oscillations and stress waves foul the transducer reading, thus making the data obtained unusable. To overcome these limitations the split Hopkinson pressure bar (SHPB) was developed. A SHPB facility was developed at UCT to measure material properties at high strain rates. The development, discussed in this thesis, included the design and manufacturing of a SHPB, strain gauge amplifiers and a data acquisition system. In addition to this a data processing package, including a dispersion correction routine was also developed.
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    Development of an intermediate strain rate compression testing machine
    (2014) Stander, Melchior; Cloete, T J
    The further development of the wedge bar Intermediate Strain rate Tester (IST) presented by Cloete and Oxtoby [1] is presented in this dissertation. The concept uses a wedge mechanism to deform 5 mm diameter, 5 mm long specimens at strain rates in the region of 10 s, up to strains of 30. As impact principles are used to start and stop the experiment, it takes less than 1 of the testing duration to reach testing speed and to stop the experiment. The kinetic energy stored in the wedge bar helps ensure a near constant loading rate over the duration of the experiment. The yield stress is captured at above 80 of the average strain rate for the experiment for most of the materials investigated. The design work focuses primarily around the design of the new load-frame, load-cell, loading platforms and wedge bar displacement sensor. Finite Element Analysis (FEA) was used to investigate the dynamic response of the IST and modify the design of components where necessary. The modifications made the load-frame and load-cell result in a significant improvement in the quality of the measured signals. The wedge bar displacement sensor performed well. The back pressure driven loading concept shows potential but needs more development before it is suitable for routine testing. A range of common engineering materials was tested at quasi static, intermediate and high strain rates and compared to the results found in the literature. The results match well, however, friction effects dominate the large strain response of the metal specimens. In its current configuration the IST is used effective for testing polymers up to large strains as well as yield stress and small strain measurement of metals. For materials for which specimen friction effects are a problem, interrupted tests can be used to extend the maximum strain achievable.
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    Implementation of material models for high strain rate applications as user-subroutines in abaqus/explicit
    (2003) Bonorchis, Dean; Cloete, T J; Nurick, Gerald N
    The general purpose finite element program, Abaqus, has the facility to allow users to supplement its existing material model library with user-defined material models (VUMATs). This thesis involves the implementation and verification of the Johnson-Cook and Zerilli-Armstrong material models as VUMATs. The same version of the Johnson-Cook material model is contained in Abaqus and was therefore used as a benchmark. These material models are suitable for high strain, high strain rate and high temperature applications. The implementation of the material models was verified by comparing simulation results obtained using the Abaqus version of the Johnson-Cook material model with the simulation results obtained using the VUMATs of the Johnson-Cook and Zerilli-Armstrong material models. Firstly, this verification process was followed using single and multiple element tests with varying prescribed loading conditions. The verification process was then extended by performing a more "realistic" set of Taylor test simulations. The Taylor test simulation results were also compared with published experimental results for validation purposes.
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    An investigation of impact breakage of rocks using the split Hopkinson pressure bar
    (Southern African Institute of Mining and Metallurgy, 2006) Bbosa, L; Powell, M S; Cloete, T J
    Discrete element methods (DEM) are being used to provide detailed impact histories of the particles in comminution devices, such as mills. To match this immense detail of information, far more informative breakage tests than those that are generally conducted are now required. The split Hopkinson pressure bar apparatus is used in this study, as it allows the calculation of breakage forces and absorbed energies. The geometry of rock particles has been identified as significant, so this project undertook to identify the influence of shape on the breakage pattern of blue stone. Comparisons are then made between the breakage pattern of angular rocks and rounded, milled rocks for single impact fracture and consecutive impact loading at low energy. Results of this experiment indicate that although breakage for both geometries occurs over a similar energy range, rounded particles have the greater probability of fracture because they absorb more of the impact energy for a given loading. Size distributions of progeny show that five pebbles or more are sufficient to predict the distribution of most particles in small energy regimes. Cumulative impact testing shows that considerably more energy is required to break a rock through cumulative damage than through a single impact—this is of considerable importance in the light of the indications from DEM simulations that most breakage in a mill will be from cumulative damage rather than single impact breakage.
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    Numerical investigation of dispersion in Hopkinson Pressure Bar
    (2005) Govender, Reuben Ashley; Cloete, T J; Nurick, Gerald N
    The Hopkinson Pressure Bar (HPB) is used as a load-time or displacement-time transducer in impact or blast experiments. The Split Hopkinson Pressure Bar (SHPB) is the accepted form of material testing for strain rates between 102 S-I and 104 S-I. Explicit Finite Element Analysis (FEA) codes are increasingly used to model HPB experiments numerically, due to the complicated boundary conditions imposed by tensile and shear SHPB experiments. However, most publications on numerical modelling of HPB experiments have focussed on the response of the specimen and have paid very little attention to the modelling of the stress wave propagation in the cylindrical bars. This dissertation focuses on the numerical modelling of stress wave propagation in HPBs.