OpenUCT :: Browsing by Author "Tait, Robert B"

Browsing by Author "Tait, Robert B"

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Open Access
Accurate characterisation of surface fatigue cracks (in steels) using the AC potential drop NDT technique
(2002) Bright, Dominic; Tait, Robert B
In order to maintain the desired level of safety in an industrial environment, it is important that the structural integrity of all the components and equipment being utilised by the facilty is assured. To determine the structural integrity of an engineering system precisely, not only must the depth of the flaw be established, but the crack profile and orientation must also be obtained in order to assess the extent of the crack. The present work described in this thesis discusses the development of an AC potential drop (ACPD) system as a non-destructive testing (NDT) technique for flaw detection and sizing of variously shaped and oriented surface breaking fatigue cracks. The ACPD method was reviewed to establish its viability as an NDT technique, and to understand the response of the output to fatigue cracking. The proposed system using a mains power source with a frequency of 50Hz was investigated as a means of providing a sensitive and reliable measurement device that could be employed to analyse fatigue cracks in industrial components in situ.
Open Access
The detection of defects in tubes and plates using guided waves
(2011) Oaker, Bradley; Tait, Robert B
Eddy current testing is the non-destructive test method of choice for the inspection of condenser tubes. However, unplanned shutdowns of power stations, due to unexpected condenser tube failures, still occur despite rigorous eddy current inspection programs. In addition to the improvement required in the reliability of inspections, there is also a need to shorten the duration of inspections.
Open Access
Determination of the power losses on a tyre-mounted ore-grinding mill
(1998) Tyabashe, Loyiso; Nurick, Gerald N; Tait, Robert B
The objective of this study was to determine the power losses occurring on a tyre-mounted ore-grinding mill designed by Dorbyl Heavy Engineering for Mintek. In order to understand the type of losses occurring on this mill it was decided that a scaled model be built and tested. Such a model mill was designed through the dimensional analysis and similitude technique. This technique enables the reduction of the number of variables to be tested by grouping them into a set of dimensionless parameters, and also allows the results obtained by means of experimentation with the scaled models to be related to the full-scale prototypes. The model mill was constructed in the Mechanical Engineering Workshop at the University of Cape Town. The overall dimensions of the model are 1400 mm long, 1070 mm wide and 892.5 mm high. Its drive-train comprises an electric motor, the wedge and timing belts with the appropriate pulleys, a differential gear set, the tyres, a shell and a disc brake sub-assembly. The tyres used to drive the shell can either be of the pneumatic or solid rubber types. The model mill was designed such that the tyre parameters required in the theoretical solution for power losses, which utilises the Light Vehicle Dynamics Simulation tyre model, could also be measured.
Open Access
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.
Open Access
Eddy current detection of fatigue cracks in drill pipes
(2012) Molteno, Matthew Robert; Tait, Robert B; Wilkinson, Andrew John; Becker, TH
The Vertical Marine Mining technique is a lucrative method used by De Beers to recover bottom lying diamonds from the seabed, mainly off the coast of Namibia. The method employs a 200m length pipe-drilling structure rotated from the surface and acting as a conduit through which the disturbed sediment is transported to the surface for processing. However, cyclic operating stresses combined with corrosion have tended to cause circumferential fatigue cracks to develop on the outside surface of the drill pipe and propagate inwards. As no early warning system exists, these cracks propagate undetected until failure occurs. High operational costs and losses associated with down time, provide a strong motivation for a system to detect fatigue cracking prior to failure, especially given that fatigue crack growth rate and fatigue lifetimes can be estimated using Fracture Mechanics principles. Therefore what is required is an early warning of fatigue crack initiation with non-destructive testing (NOT). The eddy current method is an ideal NDT technique as it does not require contact with the test surface and is highly sensitive to fatigue cracks .. However, this method is generally only sensitive to surface and near surface defects. This proves to be a major limitation - the external drill pipe surface is obstructed by flanges and fittings such that continuous inspection of the external surface would be impossible. Therefore two specialised eddy current methods to allow the detection of external fatigue cracks from inside the drill pipe were reviewed, namely: Pulsed Eddy Current (PEC) and Remote Field Eddy Current (RFEC). It was found that unlike PEC, RFEC is highly sensitive to external circumferential planar defects in ferrous pipes. This, above all, highlighted the suitability of RFEC for drill pipe inspection.
Open Access
The effect of drawing strain on the fatigue behaviour of stainless and carbon steel wires
(2001) Topic, Miroslav; Ball, Anthony; Allen, Colin; Tait, Robert B
A study has been made of the fatigue crack initiation and fatigue crack growth behaviour of three different steels in wire form, namely, an austenitic AISI 304 stainless steel, a corrosion resistant ferritic steel, 3CR12, and pearlitic high carbon steel. The stainless steel wires were produced in the laboratory at a drawing speed of 50 mm min-1, without intermediate annealing, whilst the high carbon pearlitic steel was manufactured commercially. Studies were made on stainless steel wires as a function of drawing strain between 0.09 and 0.585. Fatigue testing was carried out on an ESH servo hydraulic testing machine on both notched and unnotched samples and the S-N curves were used to evaluate the fatigue properties of the steels. Tests were performed with sinusoidal loading and load ratios of R= 0.048 and R=0.22 at a frequency of 2Hz. The microstructural evolution during drawing was characterised by optical and transmission optical microscopy, and x-ray diffraction. Fatigue crack growth and fracture surfaces were studied using scanning electron microscopy. In general, the fatigue limit was enhanced by increased drawing strain, but such strain also increased the subsequent crack propagation rates. The highest value of fatigue limit of 630 MPa was exhibited by the commercial pearlitic steel despite of its high notch sensitivity. Both shot peening of the steel wire surface and reducing the surface roughness by manual polishing increased the fatigue limit between 40 and 25 % respectively. The fatigue limit of AISI 304 stainless steel wire was improved from 215 MPa to 650 MPa after drawing to 0.585 strain. This improvement is attributed to the deformation-induced phase transformation of (ϒ) austenite to α'-martensite. X-ray diffractometer traces show that the amount of strain-induced martensite varied from 8% in the wires drawn at low strain (0.09) to 36% in the wire samples drawn to 0.585 strain. This study has established that approximately 20% of deformation-induced martensite, through drawing strain, is a critical amount which determines the subsequent fatigue response of this steel. If the amount of previously developed martensite is less than the critical amount of 20%, the martensite formed during the fatigue process will act beneﬁcially by retarding fatigue cracking, raising the fatigue limit and resulting in a ductile fatigue fracture surface. However, in the presence of more than 20% of martensite, any martensite induced by cyclic strain will encourage more rapid crack initiation compared to a material containing less than 20% martensite which leads to more brittle fracture surface characteristics. The fatigue limit of 3CR12 steel wire was also improved from 130 MPa to 310 MPa (maximum stress) after drawing to 0.68 strain. The experimental results indicate that the use of drawn 3CR12 ferritic steel for wire application under cyclic conditions is restricted to low stress levels. However, the application of heat treatment and the resultant development of a dual-phase microstructure, improved the fatigue limit to 470 MPa. Based on the findings in this study, recommendations regarding material selection and drawing process optimisation for wire production to improve the fatigue performance of AISI 304 stainless steel is given.
Open Access
The effect of laser shock peening and shot peening on the fatigue performance of aluminium alloy 7075
(2017) Becker, Alexander; Tait, Robert B; George, Sarah
It has been well established that most fatigue cracks initiate from stress concentration sites found on the surfaces of components subject to cyclic fatigue loading. The introduction of residual compressive stresses into the surface layers of components, through various means including shot peening and laser shock peening, can result in local residual compressive stresses which provide a resistance to both crack initiation and propagation, thus leading to an increase in the fatigue life of the components. The effects of both laser shock peening (LSP) and conventional shot peening (SP) on the fatigue properties of both 7075-T6 and 7075-T0 aluminium round bar test specimens were investigated and compared by means of cyclic 3-point bend fatigue testing. This investigation focused on the role that the peening induced microstructure, surface morphology and hardness had on the fatigue life of the test specimens. It was found that both the laser shock peening and shot peening processes substantially increased the fatigue lives compared to unpeened AA7075-T6. The laser shock peening process more than doubled the fatigue life of the specimens and the shot peening process increased the fatigue life by approximately 1.6×. No discernible hardening effects could be determined in the laser shock peened specimens. However, the shot peening process resulted in a distinct hardened region within the surface layers of the AA7075-T6 specimens which was attributed to the longer pressure duration of the shot peening process which results in greater plastic deformation. It was also shown that polishing the shot peened and laser shock peened specimens after their respective peening procedures resulted in a significant increase in fatigue life. Polishing after peening resulted in a 3.4× fatigue life increase in the shot peened test specimens (T6 condition) and a 5.4× fatigue life increase in the laser shock peened test specimens (T6 condition). This result highlights the role that surface roughness plays in component fatigue life. Furthermore, the increase in the average fatigue life of the polished test specimens shows that the depth of the residual compressive stresses induced by the peening processes were deep enough to allow for surfaces layers to be removed from the test specimens without any detrimental effect to the overall average fatigue life of the components. The result also suggests that the magnitudes of the residual stresses induced by the laser shock peening process being greater than those of the shot peening process. The main difference between the peening treatments was demonstrated as originating from the surface roughening effects of the two peening procedures. The laser shock peening process only slightly increased the surface roughness of a polished AA7075-T6 test specimens. The shot peening process severely affected the surface roughness of the test specimens, creating many potential crack initiation sites. The AA7075-O test specimens (annealed) showed no overall improvement in their fatigue life, regardless of the mechanical treatment received. The increased ductility of the specimens during the 3-point bending fatigue process led to stress relieving of the peening induced compressive stresses. The specimens were however still fatigued to failure. This enabled the analysis of the effect of the peening induced surface roughness to be analysed. It was found that the shot peened and laser shock peened surface roughness values were significantly higher than the roughness values of the T6 specimens owing to the increased ductility and thus workability of the test specimens. These increased surface roughnesses resulted in the shot peened test specimens failing before the laser shock peened specimens. Both sets of peened specimens failed before the "as machined" and polished test specimens highlighting the role that their induced surface roughnesses had on their fatigue lives. The cross-sectional microstructures of the peened samples in each material condition showed varied changes in the microstructure of the treated aluminium alloy. There was evidence of a large degree of plastic deformation near the surface of shot peened specimens in both material conditions. However, there was limited evidence of changes to the grains structure of the laser shock peened specimens, in both material conditions. In addition, the ability of the laser shock peening process to recover fatigue life in damaged components was also investigated. This brought into question whether the laser shock peening process can be used on a partially fatigued component at the point of crack initiation, in an attempt to further improve the fatigue life of the component. It was found that the laser shock peening of the cracks initiated in fatigue life recovery process did little to effectively recover fatigue life in the damaged components. A degree of life extension was present as cracks re-initiated after a few thousand cycles and was attributed to crack tip closure. This closure led to a general reduction in the fatigue crack growth rate when compared to laser shock peened/polished test specimens fatigued at the same stress.
Open Access
Evaluation of mechanical properties of textile concrete subjected to different environmental exposures
(2007) Mumenya, Siphila Wanjika; Tait, Robert B; Alexander, Mark Gavin
Within the last decade, a new class of civil engineering materials referred to as High Performance Fibre Reinforced Cementitious Composites (HPFRCC) has been developed. As a subset in this new field, additions of polypropylene (PP) fibres, in the form of woven textile fabrics, have proved most successful as reinforcement to cementitious matrices, leading to the development of a novel composite material: so called "Textile Concrete" (TC). Although the strength and modulus of PP are not very high, careful design through the appropriate placement and high fibre volume fraction of a textile format, has led to sufficient strength and stiffness, while at the same time high toughness has been achieved. In particular the development of a fibrillated core fibre to which is attached an outer layer of ultrasonically welded, or bonded "fluffy" PP fibres, has been most successful, and is a unique feature. These outer fluffy layers provide excellent physical bonding characteristics to the cement matrix, thus overcoming the intrinsic hydrophobic nature and otherwise weak matrix bonding of PP fibres. In South Africa where TC has found ready application, the composite is produced from a fine grained cementitious matrix and a PP textile commercially known as CemForce.
Open Access
An experimental study of the stress intensity factors of semi-elliptical & crescent moon surface fatigue cracks in round bars
(2005) Rambocus, Odesh Sharma; Tait, Robert B
Bibliography: leaves 158-168.
Open Access
Fatigue and fracture behaviour of PVC at elevated temperatures
(2008) Zwiklitz, L von; Tait, Robert B
The research studies revealed that there was a distinct difference in fatigue performance as a result of a temperature increase from 20°C to 45°C, as characterised by both SN and Paris fatigue tests. There was also, but to a lesser degree, an ageing effect. The temperature performance factor for the SN curves was between 1.6 and 4.6, while for the Paris characterisation the temperature performance factor was between 3 and 3.5.
Open Access
Fatigue and fracture behaviour of PVC at elevated temperatures
(2008) Von Zwiklitz, L; Tait, Robert B
A series of failures in underground water-carrying uPVC piping in a luxury resort in Dubai gave rise to a research opportunity to determine the effect of temperature on fatigue life performance of uPVC piping. Two different modes of testing were used to determine this temperature effect, namely SN and Fracture Mechanics Paris testing. The temperatures tested were 20%C and 45%C. In addition to temperature tests, a potential ageing effect was also investigated by comparing pipes which had been in service in the resort, and previously unused piping. The SN tests consisted of externally and symmetrically stressing, across the diameter, sections of pipe from the luxury Madinat Jumeirah resort in Dubai where the failures had occurred. The Paris equation generating FM tests used Compact Tension specimens and produced an equation relating the crack growth rate to the cyclic stress intensity amplitude. In addition, material properties were measured which could then be used for fatigue lifetime predictions. In addition to the lifetime tests, fracture toughness tests were also completed. These were done with a view to determining the fracture toughness of the material, and also to ascertain if there was an orientation effect for crack growth. The potential ageing effect was also investigated. This was achieved by means of using differently orientated specimens. SENB specimens were used to determine circumferential fracture toughness and C-Shaped specimens for longitudinal cracks (the direction of on-site crack growth). Fracture surfaces were inspected and calculations performed to indicate critical flaw sizes were broadly consistent with linear elastic fracture mechanics (LEFM). Using the material properties garnered from the Paris fatigue and fracture toughness tests, lifetime predictions were made and compared to the SN data for cycles to failure The research studies revealed that there was a distinct difference in fatigue performance as a result of a temperature increase from 20%C to 45%C, as characterised by both SN and Paris fatigue tests. There was also, but to a lesser degree, an ageing effect. The temperature performance factor for the SN curves was between 1.6 and 4.6, while for the Paris characterisation the temperature performance factor was between 3 and 3.5.
Open Access
Fracture mechanics based fatigue and fracture toughness evaluation of SLM Ti-6Al-4V
(2015) Dhansay, Nur Mohamed; Tait, Robert B; Becker, Thorsten
The focus of this research project was to determine experimentally the fatigue and fracture toughness characteristic, from a fracture mechanics perspective, of Ti-6Al-4V titanium alloy manufactured by Selective Laser Melting (SLM). Three build orientations are considered where a fatigue crack is grown parallel and two are grown perpendicular to the build orientation. The project then endeavours to generate a fracture mechanics based Paris equation from the fatigue crack growth rate results and together with the fracture toughness, fatigue life predictions may be determined based on crack propagation lifetimes. SLM is an Additive Manufacturing (AM) technique whereby an object is fabricated in a layerwise manner via the use of lasers, directly from a 3D CAD model. This process allows for the manufacture of complex designs in its net or near net shape form, which is not possible with conventional manufacturing techniques. There are minimal amounts of material wastage and it potentially eliminates post manufacture machining and processing costs. Ti- 6Al-4V is used in many applications where high strength at low density is required at moderate temperatures. Corrosion resistance qualities of the alloy are also considered for many applications. Some of the applications where this alloy is used include turbine engine components, aircraft structural components, aerospace fasteners, high-performance automotive parts, marine applications, medical implant devices and sports equipment. Due to the large use of the alloy in industry and with the potential benefits of manufacturing by SLM, there is a great need for investigating SLM Ti-6Al-4V as a viable alternative to conventional casting, forging and machining. There is limited literature covering the fatigue crack growth rate and fracture toughness of SLM Ti-6Al-4V and the effect of build orientation on these characteristics. However, it is clear, from the limited available literature that fatigue crack growth rate behaviour is affected by build orientation, and so this project investigates the effect of these orientations, and aims to contribute to understanding why these orientation effects occur. Since there is even less literature available on the fracture toughness of SLM Ti-6Al-4V with respect to build orientation, this project also endeavours to characterise orientation effects on fracture toughness, if any, and compares these with those of conventionally manufacture Ti-6Al-4V.
Open Access
A fracture mechanics study of the fracture toughness testing techniques applied to brittle materials
(2002) Naidu, Thevashen; Tait, Robert B
This dissertation describes an investigation into the application of fracture mechanics to brittle materials, with particular emphasis on the fracture toughness testing techniques used on these materials.
Open Access
Polymer impregnation of concrete as a means of improving corrosion resistance
(1980) Potgieter, Hentie; Tait, Robert B
The service life of concrete in particularly extreme bacteriological environments has long been a problem that has been facing engineers and concrete materials experts. This is particularly relevant for the case of concrete sewer pipes, the useful life of which is critically limited by corrosion due primarily, and ultimately, to sulphide attack. Sulphides are formed from the sewage sulphates, by bacteria in the slime layers on the walls of the pipe. These diffuse, firstly into the liquid, and then into the sewer atmosphere as hydrogen sulphide, which is then in turn oxidised to sulphuric acid. Hydrogen sulphide gas is well known for its characteristic "rotten eggs" odour, but more important although less known, for its extreme toxicity. The maximum safe concentration in air is only twice that of hydrogen cyanide. It also has the dangerous side effect that the ability to sense it by smell is quickly lost after first encountering the gas, and deaths have occurred in sewers that can be both directly and indirectly attributed to hydrogen sulphide poisoning. The corrosion discussed in this thesis refers primarily to that caused by this bacteriologically created sulphuric acid attack in the space above the liquid, as opposed to sub-liquid level corrosion due to aggressive chemicals, more commonly associated with industrial effluents. This is all discussed more fully in later sections.
Open Access
A probabilistic fracture mechanics model for the tubing degradation of the Koeberg steam generators
(2004) Damon, Randolph; Tait, Robert B
The susceptibility of Steam Generator (SG) Tubes (particularly tubes manufactured from Inconel 600) to primary water stress corrosion cracking (PWSCC) has been a major concern to the nuclear industry since 1971, when this phenomenon was first observed outside the laboratory. Since the susceptibility of Inconel 600 to PWSCC in pure water was first established as a certainty, various electricity utilities have devoted considerable resources to determining the exact nature of this degradation phenomenon and to predicting its consequences. Whereas, the study of the nature of this degradation mechanism has led to many conflicting conclusions, the predictive methods developed have been more successful. Initially, the establishment of the Leak Before Risk of Break principle has allowed various utilities to justify SG operation with cracked tubes. Later, the development of probabilistic simulation methods, most notably by EdF and the Jozef Stefan Institute (JSI), have led to further justification of the existing maintenance regimes and also allowed the ability to do sensitivity studies with regards to various influencing parameters. The current maintenance regime at the Koeberg Nuclear Power Station (near Cape Town, South Africa) utilises tube plugging (when cracks exceed a certain length limit) and online leak detection (based on detection of radioactive Nitrogen 16) as the main means of ensuring safe SG operation. However, the plugging limit used is based on a conservative deterministic analysis, which may be penalising in some situations. Thus, the need for risk-based SG life-time optimisation was evident.
Open Access
Residual stress evaluation of aluminium drill rods
(1995) Segal, Andrew Michael; Tait, Robert B
This thesis describes the design, construction and calibration of an air abrasive centre hole (AACH) residual stress measuring facility as well as its use in an experimental study of residual stresses developed in extruded high strength aluminium drill rods. These drill rods were manufactured by Hulett Aluminium for the mining industry for surface drilling exploration work. Initially a review of available residual stress measurement techniques was undertaken to establish which technique was the most suitable, particularly for residual stress measurement in aluminium rods as a function of processing route, and preferably with the advantages of being reliable, easy to perform, and nominally portable. Furthermore, it was required to establish a residual stress measurement technique which would be well suited for incorporation into the production line of the extruded aluminium drill rods.
Open Access
Residual stress measurement and structural integrity evaluation of SLM Ti-6Al-4V
(2012) Knowles, Craig Russell; Tait, Robert B
The constant drive toward cleaner, more powerful and more efficient jet turbines in the aerospace industry has narrowed the gap between the aircraft performance envelope requirements and the material limits. The most advanced turbine engines are incredibly complex in design and the weight-saving requirements have placed significant pressure on material capabilities and the manufacturing systems. The next generation of manufacturing methodologies are being developed in the Additive Manufacturing (AM) arena from which Selective Laser Melting (SLM) has emerged as a promising candidate for producing highly complex components. Selective Laser Melting is a laser-based AM technique which builds 3-dimensionsal parts from CAD models in a layerwise fashion...
Open Access
Understanding and modelling damage and fracture in nuclear grade graphite
(2011) Becker, Thorsten; Tait, Robert B; Marrow, T J
This thesis studied the crack initiation and propagation characteristics of Nuclear Block Graphite 10 (NBG10) and Gilsocarbon (IM1-24), using the DoubleTorsion (DT) technique.

Browsing by Author "Tait, Robert B"

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