Browsing by Author "Pearce, Roger"
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Item Open Access An assessment of a copper-containing drawing-quality steel sheet(College of Aeronautics, 1967-10) Pearce, RogerItem Open Access An assessment of the critical strains which can be sustained by a mar-ageing steel under bi-axial tension(College of Aeronautics, 1966) Pearce, RogerMar-ageing steels are being specified more and more in advanced engineering applications, and, frequently, the components are manufactured from sheet material. Consequently, the evaluation of the sheet forming properties of this material is of paramount importance; this investigation forms part of this general program. The aim here is two-fold, firstly, to determine the maximum strain which can be sustained under bi-axial stress, and secondly, to suggest any way in which this value can be increased.Item Open Access Cavitation of superplastic alloys: a study using a hot microhardness technique(Cranfield University, 1980-01) Sohal, Gurnam Singh; Pearce, RogerThis thesis is concerned with the study of cavitation behaviour of two-phase superplastic alloys, using mainly a hot microhardness technique. Cavitation is a serious problem in copper and iron base alloys, it drastically limits the elongation to failure and may affect the service properties of superplastically formed components. There is some empirical evidence to suggest that, for cavitation to occur, the two phases involved must be 'incompatible' that is, they show marked differences in certain properties. Ideally, this phenomenon would be investigated directly by an in-situ study of the deformation behaviour of each phase separately in the microduplex alloy at the superplastic temperature and hence pinpointing the phase incompatibilities, but existing techniques do not allow for this. Here, a hot microhardness tester was commissioned and used to determine the hardness and activation energy of each phase in-situ in a duplex alloy in thc superplastic temperature range. Zn/Al eutectoid and 60/40 brass were here used as models to represent a non-cavitating and cavitating class of superplastic alloys. In addition, a series of alloys were produced of compositions corresponding to those of the individual phases in these systems over a supcrplastic temperature range. These alloys were hot tensile. tested to find the ductility and ten'fle strength of each phase of superplastic alloy. The hot hardness data was correlated with the hot tensile data for the alloys over the whole temperature range from 0.4 to 0.84 Tm. Variation of Young's modulus with temperature was determined by a vibration technique and activation energy calculated form LnH versus Tm plots for all the alloys. In 60; 40 brass, the amount E of -T cavitation decreased with an increase in temperatur% and this alloy gave a maximum elongation of ti 110 pct at 250°C 1,0.45 Trrj, but it still exhibited extensive cavitation. In the Zn/Al eutectoid, no cavitation was observed apart from that associated with a few large inclusions. The hardness values of the two phases in a/ß brass were very different at the superplastic temperature, as were the load extension curves from the tensile tests and the calculated activation energies. In. the Zn/Al eutectoid however, the mechanical properties of the two phases were similar, though the activation energies were different. Finally, an attempt was made to explain the occurrence of cavitation in Supral 150 and certain steels, in some cases experimentally and in others from data available in the literature.Item Open Access Ductility, its measurement and relevance to sheet metal forming operations(College of Aeronautics, 1966) Pearce, RogerItem Open Access The effect of varying strain-ratio on the hydraulic bulging behaviour of aluminium sheet(College of Aeronautics, 1966-11) Bott, Charles H.; Pearce, RogerAnnealed commercial-purity aluminium sheet was cold-rolled up to 32%, and the effect of this treatment on the strain-ratio (r) in various directions in the sheet plane was evaluated. Up to approximately 16% cold reduction ro, r454 and r901 remained approximately constant, while the average strain-ratio, r, showed no change. At cold reductions in excess of 16% ro, and rgo, fall steadily, while the fall in r4s, is less pronounced. Specimens were then 'electromarked' with an array of 0.1 in. dia. circles and bulged, using a pvc 'punch' technique. Plots of natural thickness strain (e) vs. bulge height (h max) show that, for a given height, the strain distribution is more even for an annealed material than for a cold-worked one, due to the effect of work-hardening. The relationship between polar thickness strain and uniaxial uniform elongation (eu) shows a discontinuity at about 10% eu and a further plot of h and r against eu suggests that this is associated with the change in strain-ratio. Thus, bulge height increases linearly with increasing uniform elongation at a constant strain-ratio, but in a more complex fashion with varying strain-ratio. Increased r gives decreased € at the pole, producing a more even strain distribution over the bulge.Item Open Access Extended plasticity in commercial-purity zinc sheet(College of Aeronautics, 1968-01) Naziri, Harry; Pearce, RogerSome microstructures are presented, following on from CoA Memo. No. 137. Annealed c.p. zinc deforms predictably, while the as rolled metal retains the equiaxed grain-size produced by room-temperature rolling. Some evidence of grain growth is apparent in the material strained at 0.2 in/min.Item Open Access Investigation into the effect of material properties and processing upon the springback characteristics of a 1.0% chromium 0.3% molybdenum steel(College of Aeronautics, 1966-09) Pearce, RogerThe principal properties which affect springback are known; these are modulus, yield-strength and rate of work hardening. A sheet of 1.0% Cr/O, 3% Mo in an steel (0.064" x 36" X 126") has been tested in an attempt to map out the variation in these and other relevant properties and so to see whether this property variation could be the cause of the unsatisfactory roll-forming behaviour experienced, (Fig. 1). Finally, various heat-treatments have been tried, to determine their affect on relevant mechanical properties and their variation. Testing has been carried out on the Instron tensile testing machine and heat-treatment in a conventional laboratory furnace.Item Open Access Removal of internal porosity in Supral 150 by hot isostatic pressing(Cranfield University, 1985) Ahmed, Hamayun Kabeer; Pearce, RogerIn recent years, considerable concern has been shown about the effects SPF cavitation has on the mechanical properties of superplastic alloys. This investigation was undertaken to ascertain whether Hot Isostatic Pressure (HIP) eliminated this cavitation in Supral 150 and correspondingly brought about an improvement in the mechanical properties. It was found that the density increased with various isothermal anneals; the activation energy for this process was close to that for grain boundary diffusion in aluminium (61.93 KJ mol-1). The rate of cavity sintering was seen experimentally to be enhanced by the application of pressures greater than 7 HPa, and had an activation energy of 62.42KJ mol-1. Complete cavity closure occurred when the external pressure was greater than the flow stress of the material at thaý temperature and strain-rate; the ratio of external pressure (Pe) to flow stress (of) increased with falling HIP temperature. The activation energy at constant strain-rate (Q-) associated with plastic flow under conditions of hole closure was found to be 53.54KJ mol-1; giving a corresponding activation energy at constant stress (Qa), which is close to that for lattice diffusion in aluminium. The alloy used contained a high level of hydrogen which caused blistering on heat treatment, and was also responsible for the reappearance of porosity in subsequently heat-treated material previously returned to theoretical density; the extent of which was decreased by the use of higher temperatures and pressures or by vacuum degassing the material prior to HIP. Post SPF room temperature ductility was enhanced by HIP. The scatter in the 0.2% PS and UTS values found in as-received SPF specimens was not altered by the use of low pressure HIP (up to 35 MPa), although higher pressures (100 NPa) did slightly enhance these values and drastically reduced the scatter. Room temperature fracture of as-received Supral was by a 450 ductile shear mechanism. In the SPF cavitated material, the external characteristics Of fracture had a more jagged appearance, as the cavitation alters the route of the propagating crack. SPF material which has been HIPped to remove cavitation, fails in a manner similar to the as-received material.Item Open Access Some aspects of anisotropic plasticity in sheet metals(College of Aeronautics, 1968-03) Pearce, RogerThe prediction of the yielding and flow behaviour of materials under complex stress systems from tensile test or other easily determined data has been the aim of engineers for many years. The yield criteria of Tresca and then Mises for isotropic metals are useful, but the realisation that anisotropy is the rule rather than the exception, especially in sheet metals led to the examination of Hill's anisotropic theory by various workers. In the present paper the stress-strain curves of various sheet metals are determined in uniaxial and balanced biaxial tension. As far as yielding behaviour is concerned it is concluded that the theory is reasonably satisfactory for materials where anisotropy is described with r = 1, with certain anomalies for materials with r< 1. As far as flow behaviour is concerned, the theory only applies for materials for r> 1. Crossing of the uniaxial and biaxial curves is observed for certain metals at low strains and this is not predicted by the theory. More work is necessary on low-r materials to resolve these matters.Item Unknown Some aspects of electro-chemical grid-marking(College of Aeronautics, 1969-04) Pearce, Roger; Drinkwater, Ian C.Introduction The value of fine grids, electromarked (1) on to metal sheets before pressing, so that the type and magnitude of the strain developed in the critical region of the pressing can be assessed, is undisputed. Keeler (2), Meyer and Newby (3), and Goodwin (4), have all discussed practical applications of this technique and Palmer (5) has reviewed the field and introduced the concept of a relative safety factor (MSF). However it may be useful to see what this 'mark' consists of, how the marking affects the subsequent metal performance, what the limitations of the process are and other peripherical issues. It is these aspects of the process which is the concern of this paper.Item Open Access Superplasticity in Zn-based alloys(Cranfield University, 1972-02-17) Naziri, Hurmuze; Pearce, RogerThis thesis is concerned with two-basic zinc-based superplastic . alloys, the virtually. single-phase Zn --0.4 wt % Al alloy and the two-phase Zn - Al eutectoid alloy. -The first investigation is concerned with superplasticityin the Zn - 0.4 % wt Al alloy, while the second investigation is devoted to the effect-of copper additions on-the behaviour of the Zn. - Al eutectoid superplastic alloy and the possible mechanisms operating during deformation. A-zinc - 0.4 Al alloy was developed, which showed a. remarkable degree of superplasticity. at room. temperature, and. elongations of greater than 500% could be obtained at the relatively fast crosshead velocity of 0.1 inch/min. The strain7rate sensitivity (m) was found to increase with strain up to 300% on elongation (. 35 -*-. 5)-parallel to the rolling . direction. and it was also found to be anisotropic in-the plane of the sheet. The strain ratio r was also strain dependent and varied in the plane of the sheet. At 900 to the rolling direction, the strain ratio. increased from . 35 to . 75 after an elongation of 350%, that is, tending to unity (isotropy). Texture determination after straining showed a marked change and agreed qualitatively with the change in strain ratio. Grain growth occurred at room temperature, but was observed to be inhibited on superplastic deformation. Surface observations after deformation revealed that grainboundary sliding was taking place; fracture behaviour, though characteristically ductile in nature, varied with strain-rate. This alloy obeyed the Hall-Petch relationship above. a certain critical grain size, below which it was inapplicable, due to the occurrence of superplasticity. Thin-foil transmission-electron microscopy showed the importance of a dislocation recovery Mechanism in the interpretation. of this deformation behaviour, and calculations based on the current theories of volume and grainboundary diffusion, grainboundary sliding and dislocation climb/recovery, showed-that a grainboundary sliding/dislocation-climb recovery model could reasonably predict the observed. strain rate sensitivity and strain-rate. Biaxial tests confirmed the anisotropic properties observed in uniaxial. tests and also in punch-stretching the effect of-friction on cup height was contrary to that observed with workhardening metals. In the second investigation, measurements of. the-flow stress (af) and. strain-rate. sensitivity (m). over a wide range of strain- . rates (6)-and temperatures between -75-to 3000C have been made on a range of superplastic alloys, based on. the Zn - Al eutectoid, but with additions of up to 1% copper. These additions do not significantly affect af or m above about-1500C. The peak in the m. -ý curve is not displaced. by these-copper additions at temperatures above 1500C. Increasing the grain-size (L) increases af for. temperatures above about 600C but decreases. of-below this temperature.. With increasing-grain-size, the peak in m-value moves towards lower L, The dependence of af vs.. La showed that the exponent a. was strain-rate dependent and varied, from 0.6 to I at 2500C. The exponent b in the relation ý vs. 1 was independent F of stress and varied from 1.8 at 2500C to 3 at L 200C. The activation energy was grain-size and stress-dependent, and for the smallest grain size (. 55 jim), a value. of about 14 k. cal/mole was obtained in the superplastic region, while a value of near 20 k-cal/mole was obtained at the low 6 region. Creep-rate - measurements-at room-temperature showed that the secondary creep- rate decreased with increasing copper. content,, by a factor-of about 120 between 0 and 1.0% copper. . Increasing the grain size from 0.55 Um to 1.75 Um decreased the creep-rate further by a-factor varying from 10-50 times and thus an overall gain in creep resistance of 1200 times can be obtained. Uniaxial tension, torsion and_camplastometer tests were. used to cover strain-rates between. 10 3. to 104'. -mirCl in order experimentally to determine the multistage. af -e curve. Hot-totsion stress-strain curves were typical of those shown in previous published work on hot working, and showed that a steady-state stress was obtained during superplasticity. . In uniaxial tension, elongations of greater than 1000% were obtained at the very high initial strain-rate of 1.6 x 101 min-1. Grain coarsening occurred during deformation and was found to be strain-rate and strain dependent. Thin-foil transmission electron microscopy showed the extensive activity. of dislocations and diffusional processes. Dislocation-free structures and rounded interphase boundaries-were observed under . superplastic. conditions. Also, though many of the grains appeared equiaxed after deformation, a variety of odd-shaped grains were observed, particularly in hot torsion, where there was evidence of accelerated spherodization. Fracture. behaviour was dependent on grain size, temperature and strain-rate sensitivity. Calculated of curves,. from current theories, suggested that a dislocation-climb-recovery/grainboundary sliding model, based on grainboundary diffusivity was the operating mechanism during superplasticity.Item Open Access The testing of steel for deep drawing(College of Aeronautics, 1966) Pearce, RogerItem Open Access The warm forming of aluminium/magnesium alloy sheet(Cranfield University, 1979-08) Shehata, Farouk; Pearce, RogerThe desire to reduce the overall motor vehicle weight, and so use less fuel, means that aluminium alloys are now being considered for car bodies. Unfortunately, increase in strength generally leads to a decrease in ductility, because of the alloying additions. Warm forming however , of the Al/Mg series of alloys restores and enhances ductility to a remarkable degree. Cont/d