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Indentation induced deformation in metallic materials.

Description: Nanoindentation has brought in many features of research over the past decade. This novel technique is capable of producing insights into the small ranges of deformation. This special point has brought a lot of focus in understanding the deformation behavior under the indenter. Nickel, iron, tungsten and copper-niobium alloy system were considered for a surface deformation study. All the samples exhibited a spectrum of residual deformation. The change in behavior with indentation and the materials responses to deformation at low and high loads is addressed in this study. A study on indenter geometry, which has a huge influence on the contact area and subsequently the hardness and modulus value, has been attempted. Deformation mechanisms that govern the plastic flow in materials at low loads of indentation and their sensitivity to the rate of strain imparted has been studied. A transition to elastic, plastic kind of a tendency to an elasto-plastic tendency was seen with an increase in the strain rate. All samples exhibited the same kind of behavior and a special focus is drawn in comparing the FCC nickel with BCC tungsten and iron where the persistence of the elastic, plastic response was addressed. However there is no absolute reason for the inconsistencies in the mechanical properties observed in preliminary testing, more insights can be provided with advanced microscopy techniques where the study can be focused more to understand the deformation behavior under the indenter. These experiments demonstrate that there is a wealth of information in the initial stages of indentation and has led to much more insights into the incipient stages of plasticity.
Date: December 2005
Creator: Vadlakonda, Suman
Partner: UNT Libraries

Analyses of coupled hydrological-mechanical effects during drilling of the FEBEX tunnel at Grimsel

Description: This paper presents analyses of coupled hydrological-mechanical (HM) processes during drilling of the FEBEX tunnel, located in fractured granite at Grimsel, Switzerland. Two and three-dimensional transient finite-element simulations were performed to investigate HM-induced fluid-pressure pulses, observed in the vicinity of the FEBEX tunnel during its excavation in 1995. The results show that fluid-pressure responses observed in the rock mass during TBM drilling of the FEBEX tunnel could not be captured using current estimates of regional stress. It was also shown that the measured pressure responses can be captured in both two and three-dimensional simulations if the stress field is rotated such that contraction (compressive strain rate) and corresponding increases in mean stress occur on the side of the drift, where increased fluid pressure spikes were observed.
Date: September 2, 2003
Creator: Rutqvist, J.; Rejeb, A.; Tijani, M. & Tsang, C.-F.
Partner: UNT Libraries Government Documents Department

Importance of Deviatoric Stress in Modeling Void Growth in Ductile Fracture

Description: In this paper, the authors extend an upper bound result of Cocks [1989] to obtain a formula for porosity growth in ductile metals. Pressure and deviatoric stress are included. The matrix plasticity model involves the plastic strain rate as a power of the deviatoric stress, with a yield stress. The results show that the deviatoric stress cannot be neglected in calculating the porosity growth, even for high triaxialities.
Date: January 1, 1999
Creator: Tonks, D.L.; Zurek, A.K.; Thissell, W.R.; Rivera, J.M. & Hixson, R.S.
Partner: UNT Libraries Government Documents Department

Ductility Characterization of U-6Nb and Ta-W Alloys

Description: We have previously evaluated the ductility behaviors of U-6Nb and pure Ta. One important observation was that both alloys have very stable necking ductility independent of test conditions. In contrast, uniform ductility varied significantly depending upon strain rates and temperatures. In general, higher strain rate and lower temperature reduce the uniform ductility. Using literature data, we have developed two dynamic ductility models to predict the ductility behaviors of pure-Ta and water-quenched U-6Nb respectively under extreme conditions. In this study we further evaluate the aging effect on U-6Nb and the W-addition effect on Ta. For U-6Nb, the objective is to determine whether or not the ductility degradation by low-temperature aging mostly measured in quasi-static condition can still be observed under dynamic loading (high strain rate) condition. For Ta-W alloys, the focus is to identify the key control parameter so that the optimal condition of high-strength/high-ductility of Ta-10W can be achieved for certain defense-related applications.
Date: September 15, 2006
Creator: Sun, T & Cervantes, O
Partner: UNT Libraries Government Documents Department

Materials Response under extreme conditions

Description: Solid state experiments at extreme pressures, 10-100 GPa (0.1-1 Mbar) and strain rates (10{sup 6}-10{sup 8} s{sup -1}) are being developed on high-energy laser facilities. The goal is an experimental capability to test constitutive models for high-pressure, solid-state strength for a variety of materials. Relevant constitutive models are discussed, and our progress in developing a quasi-isentropic, ramped-pressure, shockless drive is given. Designs to test the constitutive models with experiments measuring perturbation growth due to the Rayleigh-Taylor instability in solid-state samples are presented.
Date: October 6, 2005
Creator: Remington, B A; Lorenz, K T; Pollaine, S & McNaney, J M
Partner: UNT Libraries Government Documents Department

Effect of Grain Size on the Acoustic Emission Generated During Plastic Deformation of Copper

Description: Acoustic emission signals from polycrystalline Al 1100 samples during plastic deformation were analyzed with respect to the strain rate and grain size. A kinematic model is proposed to account for the observed behavior. An experimental acoustic emission parameter, equivalent to the average energy of the acoustic events, correlates satisfactorily with the computed energy of moving dislocations during the deformation process. Both energies attain a maximum value for a certain grain size and are directly dependent on the strain rate.
Date: May 1, 1980
Creator: Baram, J. & Rosen, M.
Partner: UNT Libraries Government Documents Department

Effect of Forging Strain Rate and Deformation Temperature on the Mechanical Properties of Warm-Worked 304L Stainless Steel

Description: Stainless steel 304L forgings were produced with four different types of production forging equipment – hydraulic press, mechanical press, screw press, and high-energy rate forging (HERF). Each machine imparted a different nominal strain rate during the deformation. The final forgings were done at the warm working (low hot working) temperatures of 816 ◦C, 843 ◦C, and 871 ◦C. The objectives of the study were to characterize and understand the effect of industrial strain rates (i.e. processing equipment), and deformation temperature on the mechanical properties for the final component. Some of the components were produced with an anneal prior to the final forging while others were deformed without the anneal. The results indicate that lower strain rates produced lower strength and higher ductility components, but the lower strain rate processes were more sensitive to deformation temperature variation and resulted in more within-part property variation. The highest strain rate process, HERF, resulted in slightly lower yield strength due to internal heating. Lower processing temperatures increased strength, decreased ductility but decreased within-part property variation. The anneal prior to the final forging produced a decrease in strength, a small increase in ductility, and a small decrease of within-part property variation.
Date: February 1, 2010
Creator: Switzner, Nathan T
Partner: UNT Libraries Government Documents Department

Servohydraulic methods for mechanical testing in the Sub-Hopkinson rate regime up to strain rates of 500 1/s.

Description: Tensile and compressive stress-strain experiments on metals at strain rates in the range of 1-1000 1/s are relevant to many applications such as gravity-dropped munitions and airplane accidents. While conventional test methods cover strain rates up to {approx}10 s{sup -1} and split-Hopkinson and other techniques cover strain rates in excess of {approx}1000 s{sup -1}, there are no well defined techniques for the intermediate or ''Sub-Hopkinson'' strain-rate regime. The current work outlines many of the challenges in testing in the Sub-Hopkinson regime, and establishes methods for addressing these challenges. The resulting technique for obtaining intermediate rate stress-strain data is demonstrated in tension on a high-strength, high-toughness steel alloy (Hytuf) that could be a candidate alloy for earth penetrating munitions and in compression on a Au-Cu braze alloy.
Date: October 1, 2005
Creator: Crenshaw, Thomas B. & Boyce, Brad Lee
Partner: UNT Libraries Government Documents Department

Constitutive modeling using the Taylor impact test

Description: The Taylor test can be conveniently divided into three fairly distinct stages: (1) initial transient behavior after impact characterized by nonlinear plastic wave propagation. (2) quasi-steady propagation of the plastic wave front. The duration of this stage is a function of specimen caliber and material. (3) terminal transient behavior, during which most of the deceleration of the undeformed section takes place. After the initial transient is complete, which varies with the strain at which the plastic wave front propagates, the motion is well behaved, in the sense that a one-dimensional analysis can be effectively applied. This paper contains such an analysis. Results are supported by an example from which the state of stress for an OFHC copper specimen is deduced.
Date: September 1, 1995
Creator: Jones, S.E.; Maudlin, P.J. & Foster, J.C. Jr.
Partner: UNT Libraries Government Documents Department

Enhanced tensile ductility in Al-Mg alloys by solid-solution interactions

Description: The development of methods for obtaining high tensile elongation in aluminum alloys is of great importance for the practical forming of near-net-shape parts. Current superplastic alloys are limited in use by high material costs. The utilization of solute-drag creep processes, the approach used in this study, to obtain enhanced tensile ductility in aluminum alloys has lead to tensile elongations of up to 325% in simple, binary Al-Mg alloys with coarse grain sizes. This method has the advantage of lowering processing costs in comparison with superplastic alloys because a fine grain size is not necessary. Whereas superplastic alloys typically have a strain-rate sensitivity of m = 0.5, the enhanced ductility Al-Mg alloys typically exhibit m = 0.3 where maximum ductility is observed. Although a strain-rate sensitivity of rn = 0.5 can lead to elongations of over 1000% (superplastic materials) a value of m = 0.3 is shown experimentally to be sufficient for obtaining elongations of 150% to a maximum observed of 325%. Enhanced ductility is also affected strongly by ternary alloying additions, such as Mn, for which a preliminary understanding is pursued.
Date: November 29, 1995
Creator: Taleff, E.M.; Henshall, G.A.; Lesuer, D.R.; Nieh, T.G. & Wadsworth, J.
Partner: UNT Libraries Government Documents Department

Multiple mechanisms of thermally activated plastic flow in shocked and unshocked tantalum. Revision 1

Description: We argue that the principal features of the plastic flow behavior of Ta can be described a model that incorporates a two-component Peierls-type mechanism and an ``obstacle`` mechanism in series. We compare results of calculations based on such a model with flow data for unalloyed Ta before and after shock loading to 45 GPa for 1.8 {mu}s. Our data suggest that the shock loading changes only structural parameters.
Date: April 22, 1996
Creator: Gourdin, W.H. & Lassila, D.H.
Partner: UNT Libraries Government Documents Department

Superplastic forming using NIKE3D

Description: The superplastic forming process requires careful control of strain rates in order to avoid strain localizations. A load scheduler was developed and implemented into the nonlinear finite element code NIKE3D to provide strain rate control during forming simulation and process schedule output. Often the sheets being formed in SPF are very thin such that less expensive membrane elements can be used as opposed to shell elements. A large strain membrane element was implemented into NIKE3D to assist in SPF process modeling.
Date: December 4, 1996
Creator: Puso, M.
Partner: UNT Libraries Government Documents Department

Measurement of the stress/strain response of energetic materials as a function of strain rate and temperature: PBX 9501 and Mock 9501

Description: We have measured the stress/strain behavior of PBX 9501, Mock 900-21 and two new mocks consisting of monoclinic granular sugar embedded in (1) a BDNPA-F/estane binder (a 9501 material mock; a hard organic crystal embedded in a plastic) and (2) neat estane (an LX-14 mock) at strain rates from 10{sup -3} to 10{sup -1}, at two L/D`s and at two temperatures (25 and 60 C). We find that the compressive strength falls with increasing temperature and rises with increasing strain rate. We also find that the new 9501 sugar mock most closely resembles the behavior of the 9501 explosive and differences may be attributable to the different ages of the estane binder used.
Date: September 1, 1995
Creator: Funk, D.J.; Laabs, G.W.; Peterson, P.D. & Asay, B.W.
Partner: UNT Libraries Government Documents Department

A viscoplastic model of expanding cylindrical shells subject to internal explosive detonations

Description: Thin cylindrical shells subjected to internal explosive detonations expand outwardly at strain-rates on the order 10{sup 4} s{sup {minus}1}. At approximately 150% strain, multiple plastic instabilities appear on the surface of these shells in a quasi-periodic pattern. These instabilities continue to develop into bands of localized shear and eventually form cracks that progress in a way that causes the shell to break into fragments. The entire process takes less than 100 microseconds from detonation to complete fragmentation. Modeling this high strain-rate expansion and generation of instabilities prior to fragmentation is the primary focus of this paper. Applications for this research include hypervelocity accelerators, flux compression generators, and explosive containment vessels for terrorist threats and power plants.
Date: December 31, 1998
Creator: Martineau, R.L.; Anderson, C.A. & Smith, F.W.
Partner: UNT Libraries Government Documents Department

Nanorheology of Liquid Alkanes

Description: We report molecular dynamics simulations of liquid alkanes, squalane and tetracosane, confined between moving walls to which butane chains are tethered, effectively screening the details of the wall. As in an experiment, heat is removed by thermostatting the tethered molecules. Results obtained at high strain rates, typical of practical applications, suggest little or no difference between the bulk rheology and confined flow, and the occurrence of a high degree of slip at the wall-fluid interface at the conditions studied. At relatively low velocities and high densities, tetracosane shows the formation of fully-extended chains at certain wall spacings.
Date: September 1, 1997
Creator: Gupta, S.A., Cochran, H.D., Cummings, P.T.
Partner: UNT Libraries Government Documents Department

Influence of twinning on the constitutive response of Zr: Experiments and modeling

Description: The stress-strain response of Zr due to twinning is distinctly different from that due to slip as a function of temperature and strain rate. When the applied stress is lower than the transition stress, dislocation slip is the dominant deformation mechanism. The traditional MTS model is shown to adequately represent the constitutive behavior of Zr. Above the transition stress twinning becomes the dominant deformation mechanism where the flow stress increases linearly with strain. In this regime the rate-dependent strain hardening can be described by equations based on thermal activation theory that are very similar to the formula used in the MTS model.
Date: May 1, 1997
Creator: Chen, Shuh Rong & Gray, G.T. III
Partner: UNT Libraries Government Documents Department

Influence of strain rate and temperature on the mechanical behavior of iron aluminide-based alloys

Description: Iron aluminides are receiving increasing attention as potential high temperature structural materials due to their excellent oxidation and sulfidation resistance. Although the influence of strain rate on the microstructure/property relationships of pure iron and a variety of iron alloys and steels has been extensively studied, the effect of strain rate on the stress-strain and deformation response of iron aluminides remains poorly understood. In this paper the influence of strain rate, varied between 0.001 and 10{sup 4} s{sup {minus}1}, and temperature, between 77 & 1073{degree}K, on the mechanical behavior of Fe-40Al-0.1B and Fe-16.12Al-5.44Cr-0.11Zr-0.13C-1.07Mo-006Y, called FAP-Y, (both in at.%) is presented. The rate sensitivity and work hardening of Fe-40Al and the disordered alloy based on Fe-16% Al are discussed as a function of strain rate and temperature.
Date: April 1, 1995
Creator: Gray, G.T.
Partner: UNT Libraries Government Documents Department

Modeling of Final Structure Obtained Under High Strain-Rate Deformation

Description: This paper discusses results of experimental research and a model implementation of the hot deformation process in the two-phase region under high strain rate. Hot compression tests were employed to determine the behavior of deformed microalloyed steel over a range of strain rates (1 x 10{sup -3} s{sup -1} - 2.5 x 10{sup 3} s{sup -1}) and temperatures (650 C - 850 C). The thermomechanical history of the material is consequently integrated in the simulation and compared with the experimental results.
Date: January 1, 1999
Creator: Majta, J. & Zurek, A.K.
Partner: UNT Libraries Government Documents Department

On the embrittlement of Zircaloy-4 under RIA-relevant conditions.

Description: The extended use of Zircaloy cladding in light water reactors degrades its mechanical properties by a combination of irradiation embrittlement, coolant-side oxidation, hydrogen pickup, and hydride formation. The hydrides are usually concentrated in the form of a dense layer or rim near the cooler outer surface of the cladding. Utilizing plane-strain ring-stretch tests to approximate the loading path in a reactivity-initiated accident (RIA) transient, we examined the influence of a hydride rim on the fracture behavior of unirradiated Zircaloy-4 cladding at room temperature and 300 C. Failure is sensitive to hydride-rim thickness such that cladding tubes with a hydride-rim thickness >100 {micro}m ({approx}700 wppm total hydrogen) exhibit brittle behavior, while those with a thickness <90 {micro}m ({approx}600 wppm) remain ductile. The mechanism of failure is identified as strain-induced crack initiation within the hydride rim and failure within the uncracked ligament due to either a shear instability or damage-induced fracture. We also report some preliminary results of the uniaxial tensile behavior of low-Sn Zircaloy-4 cladding tubes in a cold-worked, stress-relieved condition in the transverse (hoop) direction at strain rates of 0.001/s and 0.2/s and temperatures of 26-400 C.
Date: December 19, 2001
Creator: Daum, R.S.; Majumdar, S.; Billone, M.C.; Bates, D.W.; Koss, D.A. & Motta, A.T.
Partner: UNT Libraries Government Documents Department