Browsing by Author "Cloete, Trevor J"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Open Access
    Influence of Ball Bearing Size on the Flight and Damage Characteristics of Blast-Driven Ball Bearings
    (Multidisciplinary Digital Publishing Institute, 2022-01-21) Langdon, Genevieve S; Qi, Ruixuan; Cloete, Trevor J; Chung Kim Yuen, Steeve
    This paper presents insights into the influence of ball size on the flight characteristics and damage of a ball bearing embedded in a rear detonated cylindrical charge. It includes results from a post-test damage analysis of ball bearings from previously reported experiments. Computational simulations using Ansys Autodyn were used to provide extra information about the velocity variation during flight and the damage sustained by the ball bearings during the blast event. The influence of bearing size (diameter and mass) was investigated using the validated simulation models to extend the dataset beyond the initial experimental work. The peak bearing velocity is influenced by the charge mass to ball bearing mass ratio and the aspect ratio of the charge. Larger ball bearings require extra momentum to accelerate them to higher velocities, but their higher surface area means a greater portion of the explosive charge is involved in transferring kinetic energy to the projectile. Tensile spalling was to be the major damage mechanism within the ball bearings. The charge aspect ratio also influenced the hydrostatic pressure propagation within the ball bearing itself, affecting the location and degree of internal cracking within the bearings. These findings will prove valuable to blast protection engineers considering the effects of embedded projectiles in improvised explosive devices.
  • Thumbnail Image
    Item
    Open Access
    Observations and Conclusions of Dynamics Students’ Mathematical Fluency
    (South African Society for Engineering Education, 2015-07-29) Craig, Tracy S; Cloete, Trevor J
    The course Dynamics I in mechanical engineering is a challenging course for many reasons, one of them being its mathematical demands. A collaboration between the first author (a mathematics lecturer and mathematics education researcher) and the second author (a mechanical engineer and the Dynamics I lecturer) sought to answer the question “What specific and identifiable mathematical difficulties are experienced by the Dynamics I students?” The observational results of this, in essence, ethnographic case study suggest that there are two levels of mathematical challenge, namely specific symbolic and computational difficulties as well as the need for well-developed problem-solving processes. We discuss our observations and provide pedagogic advice for lecturers of mathematics to help ease the transition to Dynamics I.
  • Thumbnail Image
    Item
    Open Access
    Performance of microstructural finite element models in predicting the effective modulus of trabecular bone
    (2020) Lawrence, Claire; Cloete, Trevor J; Nurick, Gerald N
    Trabecular bone is made up of an irregular, interconnecting framework of rod- and plate-like struts [1], therefore the mechanical properties of the bone may only be determined through experimental testing or detailed Finite Element modelling. Experimental testing requires a sample to be removed from the body, which is not possible in living patients. As such, there is a drive to move away from experimental testing and focus instead on creating accurate patient-specific Finite Element models from CT scans of the bone. The computational “gold-standard” Finite Element model used for trabecular bone, namely the voxel-based method, uses solid tetrahedral elements, which are extremely resource intensive. Vanderoost, et al [2] developed an alternative Finite Element code which discretises the structure into a series of beams and shells. This beam-shell approach vastly reduces the size of the mesh and, consequently, the processing time required for the simulation. In this work, an analysis cycle was developed to determine the apparent modulus of a structure using the beam-shell Finite Element model [2]. The cycle imports micro-CT scans of a structure, discretises the structure into a beam-shell mesh, performs a Finite Element simulation and outputs the apparent modulus of the structure along with a reconstructed image. The analysis cycle was validated by analysing over 3000 artificially generated images, comprising various configurations of cubic lattices, Kelvin cell lattices and octet truss lattices, and comparing the modulus output by the analysis cycle to baseline results obtained through the simulation of known node and element data. The analysis cycle provided predictions within 10% of the baseline value for most lattices, however there were issues associated with the rasterisation of the input images and postprocessing which caused variation in the results. Overall, it was determined that the analysis cycle is capable of capturing the apparent modulus of a variety of different structures. Micro-CT scans of 127 bone specimens were run through the analysis cycle. The results from the beam-shell analysis were compared to results from experimental testing [3] and an equivalent voxel-based analysis. There was a clear trend in both the beam-shell and voxel-based data, however the voxel-based method produced stiffer results than the beam-shell method overall. The beam-shell method showed more scatter than the voxel-based method, but contained less significant outliers. The effective modulus, i.e. the modulus of an inner core region, was determined for 17 of the bone specimens and compared to equivalent experimental results. The beam-shell method captured the increase in stiffness between the apparent modulus and the effective modulus as regularly as the voxel-based method, given appropriate boundary conditions were applied. The results produced by both methods can be improved by the removal of machining artifacts and improved segmentation of the micro-CT scans. This work confirms that the beam-shell method is capable of capturing the apparent modulus of a trabecular bone sample, however the scatter in the data must be reduced for it to be considered a viable alternative to the voxel-based method. It was found that the beamshell method is equally capable of predicting the relationship between apparent modulus and effective modulus as the voxel-based method. In both the beam-shell results and voxel-based results, the accuracy of a particular data point could only be determined by considering the results in reference to additional simulation and experimental data points. In light of these results, researchers should be cautious in reporting simulation results for trabecular bone without additional verification.
  • Thumbnail Image
    Item
    Open Access
    Simple rule, hidden meaning: the scalar product in engineering mathematics
    (Elephant Delta, 2017-09-13) Craig, Tracy S; Cloete, Trevor J
    Engineering is a highly mathematical field of study with different university courses requiring proficiency at different types of mathematics. Engineering dynamics requires the skilful use of vectors in various ways and proficiency at vector arithmetic, algebra and geometry is of vital importance to incoming students. This paper reports on findings from the administering of a vector proficiency assessment instrument across two semesters of a dynamics course. Findings suggest that problems requiring use of the scalar product embedded within a context are of the highest difficulty level. We argue that the geometric role of the scalar product is weakly understood by the majority of students, leading to poor performance at any problem requiring more than a basic calculation. We suggest that lecturers of engineering mathematics foreground the geometric role and that lecturers of engineering courses be aware of the level of challenge manifest in these problems.