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Oxide scale must be removed before cold drawing wire, otherwise it will cause bad surface quality, inferior die life and many wire ruptures. The nature of oxide scales and the methods of scale removal are reviewed, with particular emphasis on mechanical descaling. This is the major concern of the research, therefore a cantilever bending test has been developed to assess the ease of removal of the scale on commercial steel rod surface in the laboratory. A scanner method and a Finite Element model have been developed to evaluate the critical strain for scale cracking and removal after cantilever bending. Scanning electron microscopy (SEM), together with electron backscattered diffraction (EBSD), energy diffraction spectrum (EDS) and X-ray element mapping analysis, was used to characterize the scale before and/or after bending test. The effects of laying temperature, cooling conditions, ageing time, relative humidity and temperature, and coil positions on scale cracking and removal behaviour were studied. It was found that laying temperature has a larger effect on descalability than cooling conditions. The effect of relative humidity and temperature on descalability depended on a critical holding time. Beyond it, relative humidity and temperature had no further effect on descalability. The higher the environmental temperature, the less the critical holding time. Ageing time had an effect on descalability, but the effect was relatively small. Failure in tension started with first cracks formed at the places with high stress concentration. As tensile strain increased, new cracks formed midway between the existing cracks. Crack spacing stayed uniform but decreased until the scale segments spalled off the rod surface. The crack spacing increased with scale thickness and decreased with strain applied. Scale cracking and spallation mechanisms in compression depended on the relative shear strength of the oxide, the buckling stability of the layer and the relative shear strength of the interface. Spallation always required the propagation of a crack at the interface. The residual sub-layer left on the rod surface of EAF steel after the bending test was identified as magnetite. On the same sample, copper enrichment was found at the scale/metal interface, but within the metal side, and silicon enrichment was found at the scale/metal interface, but within the scale side.

A dissertation submitted to the Faculty of Engineering, University of the Wttwatersrand, Johannesburg, in fulfilment of th e degree of Master of science in Engineering Johannesburg 1988 / This dissertation addresses the question of the fracture behaviour of notched concrete and mortar beams. The major purpose of the work was to study the development of the micro-cracked zone and identify the point at which main crack growth began, and thus to characterise concrete and mortar at the start of main crack growth. Notched concrete and mortur beams of width 100 mm, depth either 200 mm or 300 nun, and with a span/depth ratio of three, were tested. Measurements of midspan deflection, midspan load, surface displacements across the fracturing section and ultrasonic pulse transit time were made. Ordinary Portland cement and mineral aggregates were used for the concrete and mortar beams. The J'■integral, surface displacements across the fracturing section and ultrasonic pulse transit time measurements were used to detect the onset of main crack growth. It was found that a reduction in the load carrying capacity of concrete and mortar is possible due to micro-cracking only. The value of the J-integral at the start of main crack growth was found to be essentially the same for concrete and mortar. The value of the J-integral at the start of micro-cracking was ■ < found to be essentially the same for concrete and mortar, and about 40% of the value of the J-integral at the start of main crack growth. The value of the J-integral at the start of micro-cracking and at the start of main crack growth was found, on average, to increase for an increase in beam depth. Surface displacements across the fracturing section showed the tension zone at the start of main crack growth to be approximately twice the size of the compression zone for both concrete and mortar. The'size of the micro-cracked zone, as determined from surface displacements across the fracturing section, was found to be 42% of the residual ligament depth for concrete, and 41% of the residual ligament depth for mortar. Scatter in the results was found to be considerable, thus meaning that only general trends could be identified

Ce travail de thèse s’intéresse aux alliages à base nickel exposés au milieu primaire des réacteurs à eau pressurisée : ceux-ci, et en particulier, l’alliage 600, contenant environ 16% de chrome, montrent, en service, une sensibilité à un phénomène de corrosion localisée appelé corrosion sous contrainte (CSC). La corrosion sous contrainte aboutit, à terme, au développement de fissures intergranulaires nécessitant le remplacement des matériaux de structure. La compréhension de ces phénomènes constitue donc un enjeu majeur dans le cadre de la sûreté et du prolongement de la durée de vie des réacteurs, avec, également, des aspects économiques évidents. Le rôle de cette étude est d’apporter des éléments de compréhension quant au rôle de l’hydrogène dans ces phénomènes de corrosion sous contrainte. L’objectif de ce travail était double : d’une part, déterminer la source principale de l’hydrogène absorbé par l’alliage lors de exposition au milieu primaire, et d’apporter des éléments permettant de caractériser le mécanisme responsable de son absorption. D’autre part, un second objectif consistait à évaluer dans quelle mesure l’hydrogène absorbé par l’alliage pouvait jouer un rôle dans ces phénomènes de CSC, notamment, en regard de ses interactions possibles avec les défauts de structure du matériau. À cette fin, des techniques de traçage isotopique mises en œuvre lors de la corrosion de ces alliages en milieu primaire ont été utilisées, la pénétration des traceurs étant ensuite analysées par spectrométrie de masse d’ions secondaires. Ces analyses ont permis de montrer que l’hydrogène absorbé provenait principalement de la dissociation de la molécule d’eau lors de l’édification du film d’oxyde passif. Par ailleurs, la création de défauts de structure dans le matériau, et leur étude par annihilation de positons et microscopie électronique en transmission, après création ou après interaction avec l’hydrogène introduit par chargement cathodique, ont permis de caractériser les interactions de cet élément avec les défauts. Ces interactions sont importantes, et mènent à une réorganisation des défauts (coalescence, migration), mais sont transitoires, leur intensité dépendant de l’activité locale de l’hydrogène en solution. Ces résultats ont permis la proposition d’un nouveau modèle d’amorçage et de propagation des fissures de CSC. / Since the late 1960s, a special form of stress corrosion cracking (SCC) has been identified for Alloy 600 exposed to pressurized water reactors (PWR) primary water: intergranular cracks develop during the alloy exposure, leading, progressively, to the complete ruin of the structure, and to its replacement. The main goal of this study is therefore to evaluate in which proportions the hydrogen absorbed by the alloy during its exposure to the primary medium can be responsible for SCC crack initiation and propagation. This study is aimed at better understanding of the hydrogen absorption mechanism when a metallic surface is exposed to a passivating PWR primary medium. A second objective is to characterize the interactions of the absorbed hydrogen with the structural defects of the alloy (dislocations, vacancies…) and evaluate to what extent these interactions can have an embrittling effect in relation with SCC phenomenon. Alloy 600-like single-crystals were exposed to a simulated PWR medium where the hydrogen atoms of water or of the pressuring hydrogen gas were isotopically substituted with deuterium, used as a tracer. Secondary ion mass spectrometry depth-profiling of deuterium was performed to characterize the deuterium absorption and localization in the passivated alloy. The results show that the hydrogen absorption during the exposure of the alloy to primary water is associated with the water molecules dissociation during the oxide film build-up. In an other series of experiments, structural defects were created in recrystallized samples, and finely characterized by positron annihilation spectroscopy and transmission electron microscopy, before or after the introduction of cathodic hydrogen. These analyses exhibited a strong hydrogen/defects interaction, evidenced by their structural reorganization under hydrogenation (coalescence, migrations). However, thermal desorption spectroscopy analyses indicated that these interactions are transitory, and dependent on the local hydrogen activity in the bulk material. Finally, these results allowed a new model describing SCC crack initiation and propagation to be formulated.

Cotyledonal- or transverse-cracking (TVC) in certain cultivars of snapbeans, Phaseolus vulgaris ~. seeds, clearly evident during germination, seriously places affected seedlings at a competitive disadvantage. TVC is an inherited trait and occurs across cell walls of cotyledons rather than along cell walls. Thus, it might be hypothesized that internal pressure resulting from swelling of storage proteins during imbibition might account for cellular rupture. To further elucidate this possibility, experiments were designed to compare electrophoretic patterns of storage proteins from seeds of snapbeans resistant and susceptible to TVC, and to correlate the different patterns of polyacrylamide gel el ectrophoretograrns of these proteins to TVC. One hundred seeds were selected randomly from a bulk sample of 225 g from each of 17 seed lots representing 15 cultivars, seed coats removed and cotyledons finely ground (60 mesh). Seed flours were defatted twi ce with hexane (50 ml / g) at 4°C and defatted flours reground with a mortar and pestle. Seed proteins were extracted in 0.5 M NaCl (solvent to four ratio of 10:1) at pH 7.5 for 1 h at 4oc with centrifugation at 10,000 g for 30 min. Separated proteins were subjected to electrophoresis under denaturing and non-denaturing conditions and molecular weight of different protein brands determined. Different protein banding patterns were identified and correlated to the TVC phenomenon. The data showed visual differences between banding patterns of resistant and susceptible cultivars. While the electrophoretic technique shows observable differences in cultivars expressing differential TVC, it is not clear which protein bands are associated with the TVC phenomenon. For plant breeders to employ this tool in screening for TVC resistant snapbean cultivars, further requirements are needed.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

7xxx series Al-Zn-Mg-Cu-base wrought Al-alloy products are widely used for aerospace structural applications where monotonic and cyclic mechanical properties are of prime concern. Microstructure of these commercial alloys usually contains brittle coarse constituent particles or inclusions of Fe-rich intermetallic compounds and Mg2Si, typically in the size range of 1 to 50 micron. Plastic deformation and fracture of 7xxx series alloys (as well as of numerous other wrought Al-alloys) is associated with gradual microstructural damage accumulation that involves cracking of the coarse constituent particles, growth of voids around the cracked particles, and the void coalescence. To understand and model the microstructural damage evolution processes such particle cracking, quantitative microstructural data associated with the damage nucleation are required under monotonic as well as cyclic loading conditions. In the past quantitative characterization of particle cracking damage in these alloys has been problematic. However, with recent advances in digital image analysis and stereology based techniques, it is now possible to quantitatively characterize the damage nucleation in hot-rolled 7075(T6) Al-alloy (a typical alloy of 7xxx series) due to cracking of the Fe-rich coarse constituent particles. The objectives of this work are: * Quantitative characterization of the cracking of Fe-rich constituent particles as a function of strain under quasi-static loading. This involves measurements of number density of cracked particles, volume fraction of the cracked particles, their size, shape, and orientation distribution, as well as nearest neighbor distribution and two-point correlation functions to quantify spatial dispersion of the cracked particles in a series of interrupted uniaxial tensile test specimens at different strain levels. * Quantitative characterization of the cracking of Fe-rich constituent particles under cyclic loading to study the differences between the particle cracking damage due to monotonic and cyclic loading.