Browsing by Author "George, Sarah L"

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    Open Access
    Changes in material characteristics of AISI 430 stainless steel plates subjected to repeated blast loading
    (2017) Shangase, Thobani Paul; Yuen, Steeve Chung Kim; George, Sarah L
    Structures deform at high strain rates and temperatures when exposed to impulsive loads. To accommodate the macro change there are microstructural changes that occur, i.e., grain morphology and shear banding. Most studies report on macroscopic response, i.e., large inelastic deformation and tearing of the structure, while limited studies have reported on microscopic changes that develop in the structure. The microstructure is directly related to the mechanical properties and performance of the material. Therefore, understanding the effect of high strain rate loadings on the microstructural evolution and subsequent mechanical properties of metals and alloys is necessary for mechanical design. The main objective of this research was to investigate microstructural changes to characterise the strain distribution and plastic deformation, owing to impulsive loading. Features within the microstructure that could be used to characterise deformation included grain size morphology changes, the presence of shear bands and sub-grain networks. The electron backscatter diffraction (EBSD) technique, which used Kikuchi patterns to characterise the strain distribution in the crystal of the deformed material, was also used as a characterisation tool. The first step in the experimental procedure was to select the appropriate material to investigate these microstructural changes. There was also the systematic investigation into the use of single and double heat treatments. These were used to achieve a large equiaxed grain structure, which was desirable from a microstructural point of view but was not desirable for blast-resistant material selection. The two-step heat treatment was concluded to be the most suitable heat treatment for the annealing and homogenisation of the AISI 430 stainless steel plates. The AISI 430 stainless steel plates used were 244 mm by 244 mm in size and had a circular exposed area of 106 mm. These plates were subjected to repeated explosive blasts, using a plastic explosive (PE4). The charge mass was varied for each test and the stand-off distance was kept constant at 150 mm for uniform loads and 13 mm for localised loads. Two plates were selected to investigate the uniform loading scenario. The first plate, a torn plate, used a charge mass of 30 g and one blast and the second plate, an inelastically-deformed plate, used a charge mass of 10 g and was exposed to three blasts. These two plates offered the same overall charge load with a different strain path. A further two plates were chosen for the investigation into the localised loading scenario. One plate, a petalled plate, used a 6 g charge mass and was exposed to two blasts and the second plate, an inelastically-deformed plate, used a 5 g charge mass and was also exposed to two blasts. The latter two plates offered an investigation into the effect of an increased charge load, where charge load affected the strain rate of the deformation resulting from the blast load. All four plates were sectioned across the midline of the dome and then ground and polished to a mirror finish, using OP-S. The polished samples were analysed, using optical microscopy and EBSD. In addition, Vickers hardness tests were carried out along the midline of the sectional plate profiles, in order to evaluate the extent of strain hardening. All the plates showed either a response of inelastically deforming or of complete or partial tearing failures when subjected to blast loads. For inelastic deformation failures, a global dome was characteristic of the uniform loading condition and an inner dome superimposed by the global dome was characteristic of the localised loading condition. Variation of charge mass and the number of blasts showed an increasing linear relationship between the impulse and midpoint deflection. The macrostructure showed a large variation of failures in the localised condition. The microstructural characterisation results produced micrographs showing regions of long, flat grains with multiple sub-grain networks, indicating deformed microstructures of the blast loaded plates. Parts of the microstructures displayed equiaxed/recrystallised grains characteristic of restoration processes, owing to high temperatures. Vickers hardness tests indicated an increase in material hardness as the number of blasts was increased, with a maximum hardness in the central region of the plates. In the first investigation, into uniform loading, the material characterisation results, combined with the fractography results, indicated brittle failure modes typical of high strain rate failures in strain rate sensitive materials, such as the chosen AISI 430 stainless steel plates. In the second investigation, into localised loading, the material characterisation results, combined with the fractography results, indicated a more ductile failure, owing to a 1 g incremental increase of charge mass, which confirmed the strain rate sensitivity of this material.
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    Open Access
    Critical analysis of simulated thermomechanical processing of aluminium can body stock
    (2015) Hyde, Chase Kennedy; Knutsen, Robert D; George, Sarah L
    Hot Plane Strain Compression (PSC) testing is a thermomechanical testing method used to simulate the deformation condition of industrial rolling. Thermomechanical processing (TMP) factors such as the amount of strain, strain rate and temperature all influence the microstructural evolution. The geometry of the PSC test sample and anvil are important factors in order to achieve the plane strain condition and acceptable strain distribution within deformed sample. Geometrical factors such as the breadth ratio (BR) relates the the samples breadth (b) to anvils face width (w) and this ratio has a significant effect on the breadth spread of the sample. The height ratio (HR) relates w to the samples height (h) and this ratio has a significant effect on the strain distribution. Two different geometric PSC testing configurations were investigated for this study, the one configuration had less favourable geometric ratios with a BR of 3 and a HR of 1 and the other configuration had more favourable ratios, with the BR of 4.62 and the HR of 1.3. This investigation is to evaluate the feasibility of a newly installed TMP machinery, the Gleeble 3800, to simulate the hot finishing rolling conditions by the use of hot PSC tests for the production of the can body stock (CBS) aluminium alloy AA3104. Single hot PSC tests were carried out at temperatures of 300, 350 and 400 ⁰C at strain rates of 10, 30 and 100 sec-1 and multi-pass hot PSC tests were carried out to simulate the different rolling passes experienced on the hot finishing rolling mill of the production of the aluminium alloy AA3104. The strain rate, temperature control, flow stress and microstructural flow were investigate to establish whether PSC testing is feasible on the Gleeble 3800.
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    Microstructure and texture development in AISI430 ferritic stainless steel
    (2017) Masindi, Rabelani Rofhiwa; Knutsen, Robert D; George, Sarah L
    AISI 430 ferritic stainless steel (FSS) is the most widely used FSS alloy due to good resistance to stress corrosion cracking. Owing to the chemical composition range, AISI 430 alloys undergo a partial phase transformation of ferrite to austenite when subjected to hot rolling temperatures. Consequently, the alloys consist of ferrite and austenite during processing. The presence of austenite and ferrite influences the microstructure evolution and texture development during hot rolling and subsequent annealing heat treatments. Two AISI 430 FSS heats of varying austenite volume fraction were used in this study. The two AISI 430 FSS heats were deformed using the first three passes of the Steckel mill hot rolling process. Post deformation heat treatments namely: continuous phase transformation and martensite tempering heat treatments were performed after three successive simulated Steckel mill passes. Microstructure analyses were performed using light microscopy and Electron Backscattered Diffraction (EBSD). The microstructure analyses were performed in order to determine microstructure evolution and texture development during hot deformation and post deformation heat treatments. The difference in austenite volume fraction in the respective heats A and B has profound influence on the possibilities for microstructure and texture evolution. For the higher austenite volume fraction heat A, the post-deformation path for austenite decomposition can lead to two very different textures in the prior austenite regions. During continuous diffusional transformation from austenite to ferrite the final texture is influenced by expected variant selection as well as growth selection during the prolonged isothermal heat treatment. The result is relatively strong {001}<110> texture and comparably very weak γ-fiber texture. In the case of the martensite tempering process the γ-fiber texture that is inherited from the austenite to martensite diffusionless transformation is maintained in the prior austenite regions. The mode of post-deformation heat treatment does not significantly impact on texture development in heat B where the texture is dominated by recovery and growth in the primary ferrite phase.