An experimental and numerical study of the isothermal and non-isothermal warm formability of an AA3003 aluminum alloy brazing sheet is presented. Forming limit diagrams were determined using warm limiting dome height (LDH) experiments with in situ strain measurement based on digital image correlation (DIC) techniques. Forming limit curves (FLCs) were developed at several temperature levels (room temperature, 100ºC, 200ºC, 250ºC, and 300ºC) and strain-rates (0.003, 0.018, and 0.1s-1). The formability experiments demonstrated that temperature has a significant effect on formability, whereas forming speed has a mild effect within the studied range. Elevating the temperature to 250C improved the formability more than 200% compared to room temperature forming, while forming at lower speeds increased the limiting strains by 10% and 17% at room temperature and 250ºC, respectively. Non-isothermal deep draw experiments were developed considering an automotive heat exchanger plate. A parametric study of the effects of die temperature, punch speed, and blank holder force on the formability of the part was conducted. The introduction of non-isothermal conditions in which the punch is cooled and the flange region is heated to 250C resulted in a 61% increase in draw depth relative to room temperature forming. In order to develop effective numerical models of warm forming processes, a constitutive model is proposed for aluminum alloy sheet to account for temperature and strain rate dependency, as well as plastic anisotropy. The model combines the Barlat YLD2000 yield criterion (Barlat et al., 2003) to capture sheet anisotropy and the Bergstrom (1982) hardening rule to account for temperature and strain rate dependency. Stress-strain curves for AA3003 aluminum alloy brazing sheet tested at elevated temperatures and a range of strain rates were used to fit the Bergstrom parameters, while measured R-values were used to fit the yield function parameters. The combined constitutive model was implemented within a user defined material subroutine that was linked to the LS-DYNA finite element code. Finite element models were developed based on the proposed material model and the results were compared with experimental data. Isothermal uniaxial tensile tests were simulated and the predicted responses were compared with measured data. The tensile test simulations accurately predicted material behaviour. The user material subroutine and forming limit criteria were then applied to simulate the isothermal warm LDH tests, as well as isothermal and non-isothermal warm deep drawing experiments. Two deep draw geometries were considered, the heat exchanger plate experiments developed as part of this research and the 100 mm cylindrical cup draw experiments performed by McKinley et al. (2010). The strain distributions, punch forces and failure location predicted for all three forming operations were in good agreement with the experimental results. Using the warm forming limit curves, the models were able to accurately predict the punch depths to failure as well as the location of failure initiation for both the isothermal and non-isothermal deep draw operations.

The deformation mechanism of an ultrafine grained (UFG) Al-Zn alloy has been studied. In this work, Al-15wt%Zn alloy was processed by equal channel angular extrusion (ECAE) route A at 100oC to achieve UFG structure. The deformation mechanism was studied by performing tensile test with various strain rates. Scanning electron microscopy and transmission electron microscopy were used to investigate the microstructure evolution in Al-15wt%Zn alloy with increasing ECAE passes. The observation indicated that the super saturated Al-Zn alloy would decompose and precipitate Zn particles during ECAE process. Increasing ECAE passes, the aluminum grain size was reduced, but the size of Zn particles was increased. However, the net effect of increasing ECAE passes is softening of this Al-Zn alloy. The tensile properties of the UFG Al-Zn alloy can be summarized as follows. (1)The UFG Al-Zn alloy possesses higher tensile strength and elongation as compared to commercial purity Al (AA1050). (2)The strain rate sensitivity of the UFG Al-Zn alloy increases significantly with increasing number of ECAE pass, which might be related to the refined aluminum grain size. After processed by 4-16 ECAE passes, the activation volume of the UFG Al-Zn alloy falls in the range of 25 b3~40 b3, which remains nearly constant value with increasing tensile strain. It is suggested that the controlling mechanism responsible for the tensile deformation of the UFG Al-Zn alloy might be related to a grain boundary mediated mechanism. (3)With increasing ECAE passes, the total tensile elongation of the UFG Al-Zn alloy increases but the uniform elongation show little change. This indicates that the increase in total elongation is mainly due to the contribution from an enhanced post-uniform elongation (PUE). It is suggested that the enhanced PUE might be related to the increase in strain rate sensitivity, which is resulted from the refinement of grain size. More detailed studies are needed to understand the deformation mechanism.

In this study, mechanical properties of Al-Zn alloys were conducted, with various parameters including Zn contents, grain size, and tensile strain rate. Experimental samples were all manufactured with friction stir processing method. Samples of Al-Zn alloys with the grain size of 1.5£gm, 1£gm, or 0.5£gm and five Zn concentration were pulled in tension at strain rate of 10-3s-1,10-4s-1 and 10-5s-1 . The data set were then used to draw engineering and true tensile stress vs. strain curves , flowing stress vs. Zn contents curves, Hall-Petch equation curves, m vs. Zn contents curves and m vs. grain size curves. Quantitative analysis were conducted to discover that solid solute softening and inverse Hall-Petch relation were present in Al-Zn alloys, which were more prominent at slower tensile strain rate when grain size was less than 1£gm and the Zn contents was higher than 10wt%. Quantitative analysis of strain rate sensitivity (m) showed the trends of increasing value of m with higher Zn contents and smaller grain sizes when solid solute softening and inverse Hall-Petch relation were present. The high grain-boundary diffusion coefficient of Zn which accelerates the efficiency of dynamic recovery are considered the main reason. The effect gets more prominent with increasing Zn contents , smaller grain size , and slower tensile strain rate. For Zn concentration higher than 10wt%, dynamic recovery may drive inverse Hall-Petch relation to appear when grain size is about 1£gm large.

Metal matrix composites (MMCs) of Al - 4 wt.% Cu reinforced with different volumetric fractions of B4C particles were produced by hot pressing. The effect of aging temperature on the age hardening response of the composites was studied and compared with the characteristics exhibited by the matrix alloy. Reinforcement addition was found to considerably affect the age hardening behavior. Detailed transmission electron microscopy and differential scanning calorimetry observations were made to understand the aging response of the composites. The low strain rate and high strain rate deformation behavior of the MMCs were determined utilizing low velocity transverse rupture tests and true armor-piercing steel projectiles, respectively. Increasing the volume fraction of B4C led to a decrease in flexural strength. The flexural strength vs. strain rate plots showed a slight increase in strength followed by a decrease for all samples. The mechanical performance of the composites and the unreinforced alloy were greatly improved by heat treatment. The MMCs were found to be inferior to monolithic ceramics when used as facing plates in armors.

The effects of reactive metal particles on the microstructure and mechanical properties of epoxy-based composites are investigated in this work. To examine these effects castings of epoxy reinforced with 20-40 vol.% Al and 0-10 vol.% Ni were prepared, while varying the aluminum particle size from 5 to 50 microns and holding the nickel particle size constant at 50 microns. In total eight composite materials were produced, possessing unique microstructures. The microstructure is quantitatively characterized and correlated with the composite constitutive response determined from quasi-static and dynamic compressive loading conditions at strain-rates from 1e-4 to 5e3 /s. Microstructures from each composite and at each strain rate were analyzed to determine the amount of particle strain as a function of bulk strain and strain rate. Using computational simulations of representative microstructures of select composites, the epoxy matrix-metallic particle and particle-particle interactions at the mesoscale under dynamic compressive loading conditions were further examined. From computational simulation data, the stress and strain localization effects were characterized at the mesoscale and the bulk mechanical behavior was decomposed into the individual contributions of the constituent phases. The particle strain and computational analysis provided a greater understanding of the mechanisms associated with particle deformation and stress transfer between phases, and their influence on the overall mechanical response of polymer matrix composites reinforced with metallic particles. The highly heterogeneous composite microstructure and the high contrasting properties of the individual constituents were found to drive localized deformations that are often more pronounced than those in the bulk material. The strain rate behavior of epoxy is shown to cause a strain rate dependent deformation response of reinforcement particle phases that are typically strain rate independent. Additionally, the epoxy matrix strength behavior was found to have a higher dependence on strain rate due to the presence of metal particle fillers. Discrepancies between experimental and simulation mechanical behavior results and these findings indicate a need for epoxy constitutive models to incorporate effects of particle reinforcement on the mechanical behavior.

Static abnormal grain growth (SAGG) was studied in Al-Mg alloy AA5182 sheet by varying four processing parameters: deformation temperature, strain rate, annealing temperature, and annealing time. SAGG is a secondary recrystallization process related to geometric dynamic recrystallization (GDRX) and requires both deformation at elevated temperature and subsequent static annealing. A minimum temperature is required for both SAGG and GDRX. Recrystallized grains only develop at strains larger than the critical strain for SAGG, [epsilon]SAGG. The size of the recrystallized grains is inversely related to and controlled by the density of SAGG nuclei, which increases as local strain increases. The results of this study suggest that SAGG is controlled by two thermally-activated mechanisms, dynamic recovery and recrystallization. During deformation, dynamic recovery increases as deformation temperature increases or strain rate decreases, increasing the critical strain for SAGG. SAGG is subject to an incubation time that decreases as annealing temperature increases. SAGG can produce grains large enough to reduce yield strength by 20 to 50 percent. The results of this study suggest strategies for avoiding SAGG during hot-metal forming operations by varying processing conditions to increase [epsilon]SAGG. / text

Η εργασία αυτή ασχολείται με τον υπολογισμό της δυναμικής απόκρισης επίπεδων μεταλλικών πλαισίων που υποβάλλονται σε σεισμική διέγερση, λαμβάνοντας υπόψη την επίδραση του ρυθμού παραμόρφωσης στις ιδιότητες του υλικού κατασκευής.Από την έρευνα προέκυψαν πολύ χρήσιμα συμπεράσματα που αιτιολογούν, σε ικανοποιητικό βαθμό, την απρόβλεπτη συμπεριφορά μεταλλικών κατασκευών κατά τη διάρκεια ισχυρών σεισμικών γεγονότων στο παρελθόν. / This work elaborates with the calculation of the dynamic response of plane steel frames subjected to earthquake motions taking care of strain rate effect on material properties. The obtained results justify, in a satisfactory way, the unpredictable behavior of steel structures observed in the past due to severe earthquake ground motions.

This research takes a multidisciplinary approach to comprehensively investigate the material and mechanical properties as well as pore water chemistry of the Bearpaw shale. This made it possible to characterize how these properties relate to the mechanical strength of this material. The results of this research challenge our ideas of the hydrogeology and of the geological history of the region. Core samples of the Bearpaw Formation and the overlying glacial till were collected from a field site in southern Saskatchewan, Canada. A combination of laboratory tests including multi-staged oedometer tests, constant rate of strain oedometer tests, specialized triaxial swell tests, along with pore water chemistry and finite element modelling were used to meet the following objectives: (1) To investigate the material properties and compression behaviour of the Bearpaw in addition to assessing disturbance due to specimen size; (2) Examine the time dependent behaviour of the Bearpaw and the transferability of time rate models developed for soft soils to stiff soils; (3) Examine the swelling potential and behaviour of the Bearpaw Formation and the influence of boundary conditions on this behaviour, while assessing the applicability of the swell concepts developed for compacted materials to a naturally swelling clay material; and (4) Constrain the depositional age of the till overlying the Bearpaw Shale. Contrary to what is seen in soft soils, smaller sized specimens were found to reduce disturbance, and produce more accurate and consistent results. Creep was found to follow the same laws as it does in soft soils, calling into question whether the use of preconsolidation pressure to predict geological history in stiff clays is appropriate. There was significant variation in the observed swell pressures of samples of the same size and depth. Finally, the glacial till at site was found to belong uniquely to the Battleford Formation and ranges in age from 22,500 to 27,500 years which is much younger (over 100,000 years younger) than previously believed. / Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2010-01-29 01:34:14.071

No description available.

This work investigates the extinction limits of laminar diffusion counterflow flames for various gaseous (methane, syngas, biogas) fuels using a high flow rate counterflow burner designed and built for this work. Equal momenta of the fuel and oxidizer streams were not maintained to provide data to check the fidelity of the numerical schemes and their chemical mechanisms at "non-standard" conditions. Strain rate values at extinction were obtained as a function of fuel mole fraction. Preliminary work with the new burner found that the methane extinction limit results were consistent with results from literature. The results provide insight into the extinction limit conditions of the aforementioned fuels. The strain rate was found to increase with increasing fuel mole fraction. Extinction limit results indicated that fuels with the highest concentration of hydrogen have the greatest extinction limit, which is believed to be attributed to the high diffusivity and reactivity of hydrogen.