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Master's degrees completed 2008


Mr Buys 2008: "Rock Bolt Condition Monitoring Using Ultra Sonic Guided Waves"

The resin anchored rock bolt is used extensively in the mining industry to stabilize the roof and prevent it from collapsing. However, there are different defects associated with a resin anchored rock bolt. Examples are partially encapsulated bolts, over-spinned bolts and corroded bolts. These defects reduce the integrity of the roof, and thereby have an effect on the safety and productivity of the mines.
The integrity of the rock bolts is a critical issue for the mining industry because of its influence on the safety of mining operations. Different research groups around the world have addressed the problem of determining rock bolt integrity. The most promising technique found in the literature study was based on guided ultrasonic waves (Beard and Lowe, 2003).
This study extended the previous work by Beard and Lowe (2003) using guided ultrasonic waves, to investigate damage in more realistic embedded bolts which deviate from pure cylinders. The fundamental L(0,1) mode in its lower frequency range, as suggested by Beard and Lowe was utilized. This was done through the use of finite element model simulations of various defect scenarios, which were compared to experimental measurements on bolts. Defects like loss of resin encapsulation, voids and local corrosion cracks were addressed. The time traces of the different finite element defect scenarios could be directly compared to experimental time traces which distinguish this study from the analytical approach.
Some finite element modelling issues were investigated and it was found that the time step is critical if an implicit solver is used, whereas for an explicit solver the element size is critical if accurate answers are needed. Furthermore it was also apparent that the boundary of the mortar has an influence on the results. The method used in the study was to move the boundaries far enough to prevent interference. This however increases the model size and thereby the computer resources required.
Axisymmetric defects were modelled using axisymmetric finite elements to reduce the problem size. These models gave results comparable to the measured bolts. Three-dimensional finite element models seemed to be promising for simulating non-axisymmetric defects. It was found that it is not possible to solve large three-dimensional models without energy absorbing boundaries.
Axisymmetrical and three dimensional finite element models of a partially encapsulated bolt and a bolt with a local corrosion crack were built. It was possible to detect simulated local corrosion cracks with the finite element models. Clear reflections for the crack in the bolt could be seen. If the bolt, resin and rock are cracked, different reflections will be detected. These different reflections complicated the interpretation of the results.
Once the integrity of models such as these has been established, the models could in principle be used to train neural networks for use in commercial equipment. The present study was limited to lower frequencies because of computer resource limitations.  Basic principles and modelling issues could however be addressed and it may be expected that these principles could soon be extended to higher frequencies with a new generation of computers.

Supervison: Prof PS Heyns

 
Mr DS Agenbag 2008: "Longitudinal Handling Characteristics of a Tailless Gull-Wing Aircraft"

A handling quality investigation was performed on the swept gull-wing configuration. The swept gull-wing configuration is tailless and has a wing with a transition in the sweep and dihedral angle. An example of this type of aircraft is the Exulans. This aircraft is currently under development at the University of Pretoria. The handling quality study was focused on pitch axis dynamics. The Exulans is a research testbed that will be used to investigate the swept gull-wing configuration and its special controls by means of full-scale flight testing. Variable wing sweep, twisting elevons and winglets will be investigated as means of control. These control devices are configured in such a way as to have minimum impact on the performance of the aircraft. The handling qualities of the swept gull-wing configuration have to be acceptable while using these different control strategies.

The study was launched to investigate whether a gull-wing configuration aircraft will have satisfactory handling qualities at CG-positions associated with the most favourable aerodynamic performance. There is an aerodynamic performance gain in designing an aircraft so that the CG falls on the so-called 'E-point'. The E-point is the centre of pressure for an elliptical circulation distribution. An elliptical circulation distribution is associated with the highest Oswald efficiency for an aircraft.

Time domain simulation techniques and frequency domain analysis techniques were used to analyze the handling qualities of the gull-wing configuration. The C-star criterion was used to analyze handling qualities with time domain simulation data as input. Comparative time domain simulations were performed between the Exulans and other aircraft to compare handling qualities. Eigenvalue analysis was used together with the thumbprint criteria to investigate inherent gull-wing airframe dynamics. The Shomber-Gertsen and Military Specification 8785 criteria were also used for the same purpose. The Neal-Smith method was used to investigate the effect of control authority on handling qualities and the effect of a pilot. The Monnich and Dalldorff criterion was used to evaluate gust-handling qualities. An analysis chart by Fremaux and Vairo was used to evaluate the tumbling susceptibility of the gull-wing configuration.

The pitch handling quality investigation shows sufficient promise that the swept gull-wing configuration will have acceptable handling qualities with the CG placed at positions associated with optimized aerodynamic performance. Analysis showed that the swept gull-wing configuration is potentially prone to tumbling. With low static margins, the configuration should exhibit improved handling qualities in gusty conditions when compared to existing tailless aircraft.

It is recommended that a lateral handling quality study be performed before full-scale flight testing commences on the Exulans. In addition, the possibility of wingtip stall must be investigated for the case of the swept gull-wing configuration.

Supervisors: Prof NJ Theron and Mr RJ Huyssen.




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