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Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2006. / Welded joints are major causes for concern in the engineering arena for two basic reasons. In the first instance the weld is known to be a region of weakness within a structure and is caused by residual and applied stress concentrations. Secondly, the behaviour of the stress patterns is somewhat difficult to predict accurately due to the difficulty of conforming to the geometry and process parameters. The experimental procedure in this work commenced with specimens being welded with a technique very commonly used in industry. The residual stresses generated by thermal fluctuations from the welding process cannot be predicted easily and is viewed as a problem as increased stress levels promotes failure. Residual stresses were then determined with the use of an ultrasonic stress measuring device. Strain gauges were used to measure strains in the welded specimens and these strains converted to stresses. The results of the two methods were compared and analysed. Compact tensile specimens were used to perform fatigue testing. The results confirmed findings from earlier research such as the proportion of cyclic life spent on initiating the crack. Hardness tests were performed to determine if any relationship existed between fatigue failure, yield strength and hardness. Finally metallurgical analysis revealed the phases and structures of the weld and heat-affected zones. The findings of this research indicate that close relationships exist between the cycles to crack initiation and ultimate fracture, the hardness, yield stress and the fatigue life of the weld as well as between the grain diameter and the yield stress. Furthermore it is shown that there was not enough information gathered in this research to conclude that the life expectancy of 300WA welded steel can be predicted. However recommendations are made for future research in the prediction of failure of the 300WA welded steel.

The aim in this dissertation was to improve our understanding of the effectiveness of shot peening in prolonging fatigue life, of 7075-T6 Aluminium Alloy round bar, taking into consideration surface residual stress, microstructural and micro-hardness parameters. Three point bending, high stress, moderately low cycle, fatigue tests were conducted to study the effects of shot peening and associated surface residual compressive stresses on fatigue life. The influence of shot peening on the microstructure was explored, including the application of mechanical small plastic straining and surface skimming, to vary the surface residual compressive stresses and induce strain hardening. Tests were performed to measure residual stress-depth distribution, plastic straining, micro-hardness, and the microstructure analysed on scanning electron microscopy (SEM) fractographs. The Juvinall and Marshek life prediction model was used in conjunction with the Gerber equation for non-zero mean stress applications to generate a proposed life prediction model for this material which is user-friendly. The proposed life prediction model has a linear equation format with the flexibility to conservatively accommodate most of the various types, and combinations, of treatments applied in this research by the use of customised constants. The results show that there was good correlation between actual and predicted fatigue life as well as useful insights into the role of the microstructure in explaining fatigue life behaviour.

Fatigue fracture usually is defined as the failure of a part at a stress below its breaking stress due to repetitive loading conditions. It is now a well known fact that a part, designed for a maximum stress well below the yield point of the material, may be very apt to fail under cyclic loading conditions over a period of time. The metal appears to "wear out" after a time just like humans do and fatigue failure occurs. Metal fatigue may be contrasted to animal fatigue in that l) its detection before the latter stages is normally difficult if not impossible, 2) the condition is not dissipated by recovery, and 3) damage is cumulative.

Although metal fatigue has been recognized for many years and during that time many persons have tried to find the explanation for it, there is still not a complete theory concerning this phenomenon. Metal fatigue is one of the major problems the modem designer must face. If the design techniques are to progress as other technological advances are made, it is evident that it will involve a better understanding of metal fatigue.

The thermocyclic fatigue behavior of the low-melting solder 43Sn/43Pb/14Bi has been investigated and compared to that of standard 60Sn/4OPb solder via metallographic analysis (using scanning electron microscopy) and evaluation of the degree of fatigue development (using a fatigue scale as a function of thermocycles). Specimens were subjected to shearing strains imposed by several hundred fatigue thermocycles. Both solder types fatigue by the same microstructural failure mechanism as described by other workers. The mechanism is characterized by a preferential coarsening of the solder joint microstructure at the region of maximum stress concentration where cracks originate.

Submitted in partial fulfilment of the requirements for the Degree of Master of Technology: Mechanical Engineering, Durban University of Technology, 2011. / The objective of the present study was to investigate if piezoelectric techniques or sensors can be used in monitoring structural degradation. The study considers experimental results and analytical modelling of a ductile structure under tensile and cyclic loading. Throughout the project the emphasis was placed on the effectiveness of strain measuring sensors. Conventional tensile testing was conducted using a Lloyds testing machine. The testing machine was calibrated to have a lateral movement of 2mm/min (tension force). Rectangular plates were pulled in tension until failure. From that experimental data was produced for a uni-axial loading system. Cyclic testing was carried out using an in-house designed and manufactured fatigue machine. It produced a reciprocating load (force) of 25rad/s on a rectangular plate. Two different sensor measuring instruments (strain gauge and piezoelectric) were used. The strain gauge sensor was attached to a specimen and connected to a Wheatstone bridge. The piezoelectric sensor was attached to the specimen and then linked directly to the capturing system. From these two sensors experimental results were obtained and compared. The mathematical relationships for the rectangular plates were formulated using effective stress-strain behaviour based on the elastic and plastic behaviour of the plates. The analytical and experimental results were compared. Results from this investigation show that piezoelectric sensors can be useful for measuring fatigue failure on a ductile material.

School of Engineering Includes bibliographical references (leaves 248-256)