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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

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

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

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

Influence of High Strain Rate Compression on Microstructure and Phase Transformation of NiTi Shape Memory Alloys

Description: Since NiTi shape memory alloy (SMA) was discovered in the early 1960s, great progress has been made in understanding the properties and mechanisms of NiTi SMA and in developing associated products. For several decades, most of the scientific research and industrial interests on NiTi SMA has focused on its superelastic applications in the biomedical field and shape memory based “smart” devices, which involves the low strain rate (around 0.001 s^-1) response of NiTi SMA. Due to either stress-induced martensite phase transformation or stress induced martensite variant reorientation under the applied load, NiTi SMA has exhibited a high damping capacity in both austenitic and martensitic phase. Recently, there has been an increasing interest in exploitation of the high damping capacity of NiTi SMA to develop high strain rate related applications such as seismic damping elements and energy absorbing devices. However, a systematic study on the influence of strain, strain rate and temperature on the mechanical properties, phase transformation, microstructure and crystal structure is still limited, which leads to the difficulties in the design of products being subjected to high strain rate loading conditions. The four main objectives of the current research are: (1) achieve the single loading and the control of strain, constant strain rate and temperature in high strain rate compression tests of NiTi SMA specimens using Kolsky (split Hopkinson) compression bar; (2) explore the high strain rate compressive responses of NiTi SMA specimens as a function of strain (1.4%, 1.8%, 3.0%, 4.8%, and 9.6%), strain rate (400, 800 and 1200 s^-1), and temperature (room temperature (294 K) and 373 K); (3) characterize and compare the microstructure, phase transformation and crystal structure of NiTi SMAs before and after high strain rate compression; and (4) correlate high strain rate deformation with the changes of microstructure, phase transformation characteristics and crystal structure. ...
Date: May 2016
Creator: Qiu, Ying
Partner: UNT Libraries

Presto 4.20 user's guide : addendum for shock capabilities.

Description: This is an addendum to the Presto 4.20 User's Guide to document additional capabilities that are available for use in the Presto{_}ITAR code that are not available for use in the standard version of Presto. Presto{_}ITAR is an enhanced version of Presto that provides capabilities that make it regulated under the U.S. Department of State's International Traffic in Arms Regulations (ITAR) export-control rules. This code is part of the Vivace product, and is only distributed to entities that comply with ITAR regulations. The enhancements primarily focus on material models that include an energy-dependent pressure response, appropriate for very large deformations and strain rates. Since this is an addendum to the standard Presto User's Guide, please refer to that document first for general descriptions of code capability and use. This addendum documents material models and element features that support energy-dependent material models.
Date: June 1, 2011
Creator: Spencer, Benjamin Whiting
Partner: UNT Libraries Government Documents Department

Fragmentation in Biaxial Tension

Description: We have carried out an experiment that places a ductile stainless steel in a state of biaxial tension at a high rate of strain. The loading of the ductile metal spherical cap is performed by the detonation of a high explosive layer with a conforming geometry to expand the metal radially outwards. Simulations of the loading and expansion of the metal predict strain rates that compare well with experimental observations. A high percentage of the HE loaded material was recovered through a soft capture process and characterization of the recovered fragments provided high quality data, including uniform strain prior to failure and fragment size. These data were used with a modified fragmentation model to determine a fragmentation energy.
Date: June 13, 2006
Creator: Campbell, G H; Archbold, G C; Hurricane, O A & Miller, P L
Partner: UNT Libraries Government Documents Department

FLASHFOAM : a triboluminescent polymer foam for mechanical sensing.

Description: The formulation and processing of a brittle polyurethane foam containing triboluminescent powder additives is described. Two powder additives, known to exhibit triboluminescence, were individually examined: triethylammonium tetrakis (dibenzoylmethanato) europate [NEt3H][Eu(DBM)4] and ordinary table sugar (sucrose, C12H22O11). In each instance, the powders were mixed into the polyol component of the foam. When combined with the isocyanate component, the resulting foams had these powders incorporated into their cellular structure so as to induce a triboluminescent response upon crushing during impact testing. The triboluminescent response of foam specimens containing each of these powder additives was characterized by measuring: the time rate of change in the optical output (measured as Watts), the peak optical output, the total integrated output (Watt-seconds), during the impact event. Foams containing the europate compound were found to yield several orders of magnitude higher output when compared to the sugar-containing foam. Strain rate and concentration of the powder (in the foam) were important variables with respect to optical output. Both the peak and total triboluminescent output increased with increasing powder concentration. Peak output was also found to increase with increasing strain rate. However, the total output was found to be roughly constant for a given concentration regardless of strain rate (over the strain rate range: 20 sec-1< e& < 150 sec-1). At very low strain rates, no triboluminescent response was measured.
Date: July 1, 2003
Creator: Dentinger, Paul M.; Whinnery, LeRoy L., Jr. & Goods, Steven Howard
Partner: UNT Libraries Government Documents Department

A viscoplastic micromechanical model for the yield strength of nanocrystalline materials

Description: In this paper we present a micromechanical approach based on Fast Fourier Transforms to study the role played by dislocation glide and grain boundary (GB) accommodation in the determination of the plastic behavior of nanostructured materials. For this, we construct unit cells representing self-similar polycrystals with different grain sizes in the nanometer range and use local constitutive equations for slip and GB accommodation. We study the effect of grain size, strain rate and pressure on the local and effective behavior of nanostructured fcc materials with parameters obtained from experiments and atomistic simulations. Predictions of a previous qualitative pressure-sensitive model for the effective yield strength behind a shock front are substantially improved by considering strain partition between slip and GB activity. Under quasiestatic conditions, assuming diffusion-controlled mechanisms at GB, the model predicts a strain-rate sensitivity increase in nanocrystalline samples with respect to the same coarse-grained material of the same order as in recently published experiments.
Date: March 14, 2006
Creator: Lebensohn, R; Bringa, E & Caro, A
Partner: UNT Libraries Government Documents Department

On the response of rubbers at high strain rates.

Description: In this report, we examine the propagation of tensile waves of finite deformation in rubbers through experiments and analysis. Attention is focused on the propagation of one-dimensional dispersive and shock waves in strips of latex and nitrile rubber. Tensile wave propagation experiments were conducted at high strain-rates by holding one end fixed and displacing the other end at a constant velocity. A high-speed video camera was used to monitor the motion and to determine the evolution of strain and particle velocity in the rubber strips. Analysis of the response through the theory of finite waves and quantitative matching between the experimental observations and analytical predictions was used to determine an appropriate instantaneous elastic response for the rubbers. This analysis also yields the tensile shock adiabat for rubber. Dispersive waves as well as shock waves are also observed in free-retraction experiments; these are used to quantify hysteretic effects in rubber.
Date: February 1, 2010
Creator: Niemczura, Johnathan Greenberg (University of Texas-Austin)
Partner: UNT Libraries Government Documents Department

Final Report LDRD 04-ERD-021

Description: In this project, we performed experiments and simulations to establish constitutive models for plastic behavior and to determine the deformation mechanism of nanocrystalline materials at different grain sizes (<100 nm) and high strain rates (>10{sup 6}/s). The experiments used both laser-induced shocks and isentropic compression to investigate, for the first time, the high-strain-rate deformation of nanocrystalline Ni. Samples were characterized using transmission electron microscopy, nanoindentation, profilometry, and x-ray diffraction before and after loading. We validated constitutive models using both atomistic molecular dynamics and continuum simulations performed at the boundary of their current computational possibilities to match experimental scales.
Date: February 23, 2007
Creator: Bringa, E
Partner: UNT Libraries Government Documents Department

DYNA3D Material Model 71 - Solid Element Test Problem

Description: A general phenomenological-based elasto-plastic nonlinear isotropic strain hardening material model was implemented in DYNA3D for use in solid, beam, truss, and shell elements. The constitutive model, Model 71, is based upon conventional J2 plasticity and affords optional temperature and rate dependence (visco-plasticity). The expressions for strain hardening, temperature dependence, and rate dependence allow it to represent a wide variety of material responses. Options to capture temperature changes due to adiabatic heating and thermal straining are incorporated into the constitutive framework as well. The verification problem developed for this constitutive model consists of four uni-axial right cylinders subject to constant true strain-rate boundary conditions. Three of the specimens have different constant strain rates imposed, while the fourth specimen is subjected to several strain rate jumps. The material parameters developed by Fehlmann (2005) for 21-6-9 Nitronic steel are utilized. As demonstrated below, the finite element (FE) simulations are in excellent agreement with the theoretical responses and indicated the model is functioning as desired. Consequently, this problem serves as both a verification problem and regression test problem for DYNA3D.
Date: January 24, 2008
Creator: Zywicz, E
Partner: UNT Libraries Government Documents Department

Compressive Properties of a Closed-Cell Aluminum Foam as a Function of Strain-Rate and Temperature.

Description: The compressive deformation behavior of a closed-cell Aluminum foam (ALPORAS) manufactured by Shinko Wire. Co. in Japan was evaluated under static and dynamic loading conditions as a function of temperature. High strain rate tests (1000 - 2000/s) were conducted using a split-Hopkinson pressure bar(SHPB). Quasi-static and intermediate strain rate tests were conducted on a hydraulic load frame. Little change in the flow stress behavior as a function of strain rate was measured. The deformation behavior of the Al-foam was however found to be strongly temperature dependent under both quasistatic and dynamic loading. Localized deformation and stress state instability during testing of metal foams will be discussed in detail since the behavior over the entire range of strain rates indicates nonuniform deformation.
Date: January 1, 2001
Creator: Gray, G. T. (George T.), III; Liu, C. (Cheng); Trujillo, C. P. (Carl P.); Jacquez, B. (Benito); Mukai, T. & Cady, C. M. (Carl McElhinney)
Partner: UNT Libraries Government Documents Department

Influence of temperature and strain rate on the compressive behavior of PMMA and polycarbonate polymers

Description: Compression stress-strain measurements have been made on commercial polymethylmethacrylate (PMMA) and polycarbonate (PC) polymers as a function of tcmperature (-197 C to 220 C) and strain rate. A split-Hopkinson-pressure bar (SJIPU) was used to achieve strain rates of about 2500 s-' and a servohydraulic tester was used for lower strain rate testing (0.001 to 5 s-'). The mechanical response of these transparent polymers is quite different. The strength of PC is weakly dependent on strain rate, only moderately dependent on temperature, and remains ductile to -197OC. In contrast, the strength of PMMA is linearly dependent on temperature and strongly dependent on strain rate. Significantly, PMMA develops cracking and fails in compression with little ductility ( 7 4 % total strain) at either low strain rates and very low temperatures (-197OC) or at high strain rates and temperatures very near ambient.
Date: January 1, 2001
Creator: Cady, C. M. (Carl McElhinney); Lopez, M. F. (Mike F.); Gray, G. T. (George T.), III; Idar, D. J. (Deanne J.) & Blumenthal, W. R. (William R.)
Partner: UNT Libraries Government Documents Department

Sierra/SolidMechanics 4.22 user's guide : addendum for shock capabilities.

Description: This is an addendum to the Sierra/SolidMechanics 4.22 User's Guide to document additional capabilities that are available for use in the Presto{_}ITAR code that are not available for use in the standard version of Sierra/SolidMechanics (Sierra/SM). Presto{_}ITAR is an enhanced version of Sierra/SM that provides capabilities that make it regulated under the U.S. Department of State's International Traffic in Arms Regulations (ITAR) export-control rules. This code is part of the Vivace product, and is only distributed to entities that comply with ITAR regulations. The enhancements primarily focus on material models that include an energy-dependent pressure response, appropriate for very large deformations and strain rates. Since this is an addendum to the standard Sierra/SolidMechanics User's Guide, please refer to that document first for general descriptions of code capability and use. This addendum documents material models and element features that support energy-dependent material models.
Date: October 1, 2011
Partner: UNT Libraries Government Documents Department

Material science experiments on the Atlas Facility

Description: Three material properties experiments that are to be performed on the Atlas pulsed power facility are described; friction at sliding metal interfaces, spallation and damage in convergent geomety, and plastic flow at high strain and high strain rate. Construction of this facility has been completed and experiments in high energy density hydrodynamics and material dynamics will begin in 2001.
Date: January 1, 2001
Creator: Keinigs, R. K. (Rhonald K.); Atchison, W. L. (Walter L.); Faehl, R. J. (Rickey J.); Lindemuth, I. R. (Irvin R.); Anderson, W. E. (Wallace E.); Bartsch, R. R. (Robert Richard) et al.
Partner: UNT Libraries Government Documents Department