Publications

2012
J. Lee, Morovat, M. A., Hu, G., Engelhardt, M. D., and Taleff, E. M., “Experimental Investigation of Mechanical Properties of ASTM A992 Steel at Elevated Temperatures,” Engineering Journal, American Institute of Steel Construction, vol. 29, pp. 33–49, 2012. Publisher's VersionAbstract
This paper presents the results of a detailed experimental study into the mechanical properties of ASTM A992 structural steel at elevated temperatures. Critical testing issues, including temperature measurement, temperature control, and extensometer use, along with the testing equipment and procedures are briefly explained. Tensile steady-state temperature tests are conducted on samples of ASTM A992 steel at temperatures up to 1000 °C. Full stress-strain curves, representing steel coupons tested to fracture at elevated temperatures, are generated. Important mechanical properties such as yield stress, tensile strength, proportional limit, elastic modulus and elongation are obtained from the stress-strain curves. Results are compared with elevated-temperature properties specified by Eurocode 3 and by the AISC Specification. When defined as the stress at 2% total strain, the measured yield stress values agree reasonably well with the corresponding values from Eurocode 3 and the AISC Specification. However, for more conventional definitions of yield stress, such as the 0.2% offset yield stress, the agreement is poor. It is observed that the yield stress of steel at elevated temperatures up to about 600 °C is highly dependent on the manner in which yield stress is defined. The effects of displacement loading rates on steel strength and static yielding behavior are also investigated. It is shown that the displacement rate has a large impact on the steel strength at elevated temperatures, especially at temperatures higher than 600 °C. Further work is needed to fully characterize the time-dependent effects on the elevated-temperature stress-strain response of structural steel. Additionally, this paper presents results of Charpy V-Notch (CVN) tests on ASTM A992 steel at elevated temperatures.
A. J. Carpenter, Barnes, A. J., and Taleff, E. M., “High-Temperature Deformation of Magnesium Elektron™ 43,” Materials Science Forum, vol. 735, pp. 93–100, 2012. Publisher's VersionAbstract
Complex sheet metal components can be formed from lightweight aluminum and magnesium sheet alloys using superplastic forming technologies. Superplastic forming typically takes advantage of the high strain-rate sensitivity characteristic of grain-boundary-sliding (GBS) creep to obtain significant ductility at high temperatures. However, GBS creep requires fine-grained materials, which can be expensive and difficult to manufacture. An alternative is provided by materials that exhibit solute-drag (SD) creep, a mechanism that also produces elevated values of strain-rate sensitivity. SD creep typically operates at lower temperatures and faster strain rates than does GBS creep. Unlike GBS creep, solute-drag creep does not require a fine, stable grain size. Previous work by Boissière et al. suggested that the Mg-Y-Nd alloy, essentially WE43, deforms by SD creep at temperatures near 400°C. The present investigation examines both tensile and biaxial deformation behavior of ElektronTM 43 sheet, which has a composition similar to WE43, at temperatures ranging from 400 to 500°C. Data are presented that provide additional evidence for SD creep in Elektron 43 and demonstrate the remarkable degree of biaxial strain possible under this regime (>1000%). These results indicate an excellent potential for producing complex 3-D parts, via superplastic forming, using this particular heat-treatable Mg alloy.
A. J. Carpenter, Taleff, E. M., Louis G. Hector, J., Carter, J. T., and Krajewski, P. E., “A Time-Dependent Material Model for the Simulation of Hot Gas-Pressure Forming of Magnesium Alloy AZ31,” Materials Science Forum, vol. 735, pp. 198–203, 2012. Publisher's VersionAbstract
A time-dependent material constitutive model is developed for the deformation of wrought Mg AZ31 sheet material at 450°C. This material model is used to simulate gas-pressure bulge forming of AZ31 sheet into hemispherical domes. Finite-element-method (FEM) simulations using this material model are compared against experimental data obtained for dome height as a function of forming time under forming conditions identical to those assumed in the simulations. The time-dependent material model predicts experimental dome heights during forming with a quite useful accuracy. The most significant advantage of the time-dependent material model is that it can address the effect of preheating time on forming. Preheating times shorter than ~120 s produce an increase in forming rate. This material model provides a quantitative means of accounting for that effect.
2011
M. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “Creep Buckling of Steel Columns Subjected to Fire,” in In Proceedings of the 35th International Symposium on Bridge and Structural Engineering, London, England, 2011. Publisher's VersionAbstract
One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of applied load. Although material creep and consequently the phenomenon of creep buckling can significantly impact the safety of steel columns subjected to fire, they have received relatively little research attention, and are not currently explicitly considered in code-based design formula for columns at elevated temperatures, such as those in the Eurocode 3 or in the AISC Specification. This paper will propose a preliminary methodology to study the phenomenon of creep buckling in steel columns subjected to fire. Preliminary analytical solutions are presented, and compared with computational predictions for creep buckling. The analytical and computational results clearly indicate that accurate knowledge of material creep is essential in studying creep buckling phenomenon at elevated temperatures. In addition, the results show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.
N. A. Pedrazas, Worthington, D. L., Dalton, A. D., Sherek, P. A., Steuck, S. P., Quevedo, H. J., Bernstein, A. C., Taleff, E. M., and Ditmire, T., “Effects of Microstructure and Composition on Spall Fracture in Aluminum,” Materials Science and Engineering A, vol. 536, pp. 117–123, 2011. Publisher's VersionAbstract
Spall strength was measured as a function of composition and microstructure in three Al materials: a high-purity Al (Al HP), a commercial-purity Al (AA1100) and an alloy of Al containing 3 wt.% Mg (Al–3Mg). The Al HP and AA1100 materials were tested as single-crystal sheets, and the Al–3Mg alloy was tested as polycrystalline sheets having a variety of controlled grain sizes. A high-intensity laser produced shock loadings to create tensile strain rates ranging from 2 × 106 s−1 to 5 × 106 s−1, which caused spall fracture. Crystallographic orientation, relative to the direction of shock propagation, does not discernibly affect spall strength in the Al-HP material. Intermetallic particles, associated with impurity elements, initiate microstructural damage during tensile shock loading and reduce spall strength of the AA1100 material below that of the Al-HP material. The spall strength of the Al–3Mg is lowest among the three materials, and this is a result of the decreased ductility during spall fracture caused by the Mg solid-solution alloying addition. Grain size affects fracture character of the Al–3Mg material, but does not discernibly affect spall strength; the fraction of ductile transgranular fracture, versus brittle intergranular fracture, increases with grain size.
M. A. Morovat, Engelhardt, M. D., Taleff, E. M., and Helwig, T., “Importance of Time-Dependent Material Behavior in Predicting Strength of Steel Columns Exposed to Fire,” Applied Mechanics and Materials, vol. 82, pp. 350–355, 2011. Publisher's VersionAbstract
One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of the applied load. This paper will propose a preliminary methodology to study the phenomenon of creep buckling in steel columns subjected to fire. Analytical solutions using the concept of time-dependent tangent modulus are developed to model time-dependent buckling behavior of steel columns at elevated temperatures. Results from computational creep buckling studies using ABAQUS® are also presented, and compared with analytical predictions. Both analytical and computational methods utilize material creep models for structural steel developed by Harmathy, and by Fields and Fields. The analytical and computational results clearly indicate that accurate knowledge of material creep is essential in studying creep buckling phenomenon at elevated temperatures, and that neglecting creep effects can lead to potentially unsafe predictions of the strength of steel columns subjected to fire.
D. A. Dalton, Worthington, D. L., Sherek, P. A., Pedrazas, N. A., Quevedo, H. J., Bernstein, A. C., Rambo, P., Schwarz, J., Edens, A., Geissel, M., Smith, I. C., Taleff, E. M., and Ditmire, T., “Microstructure Dependence of Dynamic Fracture and Yielding in Aluminum and an Aluminum Alloy at Strain Rates of 2×106 s-1 and Faster,” Journal of Applied Physics, vol. 110, pp. 103509, 2011. Publisher's VersionAbstract
Experiments investigating fracture and resistance to plastic deformation at fast strain rates (>106 s−1) were performed via laser ablation on thin sheets of aluminum and aluminum alloys. Single crystal high purity aluminum (Al-HP) and a single crystal 1100 series aluminum alloy (AA1100) were prepared to investigate the role of impurity particles. Specimens of aluminum alloy +3 wt. % Mg (Al+3Mg) at three different grain sizes were also studied to determine the effect of grain size. In the present experiments, high purity aluminum (Al-HP) exhibited the highest spall strength over 1100 series aluminum alloy (AA1100) and Al+3Mg. Fracture characterization and particle analysis revealed that fracture was initiated in the presence of particles associated with impurity content in the AA1100 and at both grain boundaries and particles in Al+3Mg. The Al+3Mg specimens exhibited the greatest resistance to plastic deformation likely resulting from the presence of magnesium atoms. The Al-HP and AA1100, both lacking a strengthening element such as Mg, were found to have the same Hugoniot elastic limit (HEL) stress. Within the single crystal specimens, orientation effects on spall strength and HEL stress appear to be negligible. Although the fracture character shows a trend with grain size, no clear dependence of spall strength and HEL stress on grain size was measured for the Al+3Mg. Hydrodynamic simulations show how various strength and fracture models are insufficient to predict material behavior at fast strain rates, and a revised set of Tuler-Butcher coefficients for spall are proposed.
M. Morovat, Engelhardt, M., Helwig, T., and Taleff, E., “"High-Temperature Creep Buckling Phenomenon of Steel Columns Subjected to Fire,” pp. 2929–2940, 2011. Publisher's VersionAbstract
One of the critical factors affecting the strength of steel columns at elevated temperatures is the influence of material creep. Under fire conditions, steel columns can exhibit creep buckling, a phenomenon in which the critical buckling load for a column depends not only on slenderness and temperature, but also on the duration of applied load. Although material creep and consequently the phenomenon of creep buckling can significantly impact the safety of steel columns subjected to fire, they have received relatively little research attention, and are not currently explicitly considered in code-based design formula for columns at elevated temperatures, such as those in the Eurocode 3 or in the AISC Specification. This paper will propose a preliminary methodology to study the phenomenon of creep buckling in steel columns subjected to fire. Preliminary analytical solutions are presented, and compared with computational predictions for creep buckling. The analytical and computational results clearly indicate that accurate knowledge of material creep is essential in studying creep buckling phenomenon at elevated temperatures. In addition, the results show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.
2010
E. M. Taleff, Hector, Louis G., J., Bradley, J. R., Verma, R., and Krajewski, P. D., “Local Thinning at a Die Entry Radius during Hot Gas-Pressure Forming of an AA5083 Sheet,” ASME Journal of Manufacturing Science and Engineering, vol. 132, pp. 011016-1–7, 2010. LinkAbstract
Splitting at regions of local thinning below die entry radii is a critically important mechanism of failure in hot gas-pressure forming of sheet materials. Local thinning is controlled by sheet-die friction and die geometry, as well as sheet material properties. In this study, local thinning is investigated at a particularly severe die entry radius during hot forming of a fine-grained AA5083 sheet at 450°C. Particular emphasis is placed on the relationship between local thinning and sheet-die friction conditions. A simple analysis of the mechanics of this thinning phenomenon is presented. Finite element simulation results are presented for different sheet-die friction conditions. Sheet thickness profiles measured from parts produced in forming experiments using three different lubrication conditions are compared with predictions from simulations. Simulation predictions agree well with experimental data for the occurrence and location of thinning below a die entry radius. Additional insights into sheet-die friction for controlling local thinning and preventing premature necking failure are detailed.
E. M. Taleff, Takata, K., and Ichitani, K., “Hot and Warm Deformation of AA5182 Sheet Materials: Ductility and Microstructure Evolution,” in Proceedings of the 12th International Conference on Aluminum Alloys, September 5–9, 2010, 2010, pp. 1231–1236.Abstract
Not available.
J. Qiao and Taleff, E. M., “Superplasticity-Like Creep Behavior of Coarse-Grained Ternary Al Alloy,” Transactions of Nonferrous Metals Society of China, pp. 564–571, 2010. Publisher's VersionAbstract
Enhanced tensile ductilities in coarse grained Al-Mg-Zn and Al-Mg-Fe materials were studied. The materials were Al-2Mg-5Zn, Al-3Mg-5Zn, Al-4Mg-5Zn, Al-3Mg-0.11Fe, Al-3Mg-0.27Fe, and Al-3Mg-0.40Fe. Tensile elongation-to-failure tests were conducted at constant cross-head speed and constant temperatures from 300 to 450 °C. Strain rate change tests were conducted at a constant temperature from 300 to 450 °C and in strain-rate range from 4.31×10−5 to 1.97×10−2 s−1. Experimental results show that over 100% ductilities are consistently achieved in these materials. This superplasticity-like behavior is rate-controlled by solute-drag creep. Although ternary Zn and Fe additions do not have an adverse effect on solute-drag creep and ductility, they increase stress exponent and its sensitivity to Mg content during solute-drag creep. Key words Al-Mg alloys; superplasticity; solute-drag creep; tensile ductility; strain-rate sensitivity
J. - K. Chang, Takata, K., Ichitani, K., and Taleff, E. M., “Abnormal Grain Growth and Recrystallization in Al-Mg Alloy AA5182 following Hot Deformation,” Metallurgical and Materials Transactions A, vol. 41, pp. 1942–1953, 2010. LinkAbstract
Abnormally large grains have been observed in Al-Mg alloy AA5182 sheet material after forming at elevated temperature, and the reduced yield strength that results is a practical problem for commercial hot-forming operations. The process by which abnormal grains are produced is investigated through controlled hot tensile testing to reproduce the microstructures of interest. Abnormal grains are shown to develop strictly during static annealing or cooling following hot deformation; the formation of abnormal grains is suppressed during plastic straining. Abnormal grains grow by static abnormal grain growth (SAGG), which becomes a discontinuous recrystallization process when abnormal grains meet to form a fully recrystallized microstructure. Nuclei, which grow under SAGG, are produced during hot deformation by the geometric dynamic recrystallization (GDRX) process. The mechanism through which a normally continuous recrystallization process, GDRX, may be interrupted by a discontinuous process, SAGG, is discussed.
J. - K. Chang, Takata, K., Ichitani, K., and Taleff, E. M., “Ductility of an Aluminum-4.4 Wt. Pct. Magnesium Alloy at Warm- and Hot-Working Temperature,” Materials Science and Engineering A, vol. 527, pp. 3822-3828, 2010. LinkAbstract
An AA5182 aluminum alloy sheet, containing 4.4wt. pct. magnesium, was subjected to tensile testing at temperatures from 100 to 400° C under strain rates from 10−3 up to 3×10−2s −1 . Flow stress, tensile elongation and reduction-in-area were measured and are correlated with deformation and fracture mechanisms. At slow strain rates and elevated temperatures, solute-drag creep produces large tensile elongations, up to 247%, and large reductions in area, up to 91%. Tensile elongation is greatest when the Zener–Hollomon parameter is in the range of 109 to 1010s −1 . Ductility decreases at the slowest strain rates and highest temperatures because of cavity interlinkage leading to fracture. As strain rate increases and temperature decreases beyond the range of peak ductility, an increased rate of flow localization, i.e. necking, reduces elongation and reduction-in-area. Ductility further decreases with increasing strain rate and decreasing temperature as deformation transitions into logarithmic creep and fracture transitions to a ductile shear mode. At the lowest temperature and fastest strain rate applied, the Portevin–Le Chatelier (PLC) effect is observed and ductility is least.
L. G. Hector, Jr., Krajewski1, P. E., Taleff, E. M., and Carter, J. T., “High-Temperature Forming of a Vehicle Closure Component in Fine-Grained Aluminum Alloy AA5083: Finite Element Simulations and Experiments,” Key Engineering Materials, vol. 433, pp. 197–210, 2010. WebsiteAbstract
Fine-grained AA5083 aluminum-magnesium alloy sheet can be formed into complex closure components with the Quick Plastic Forming process at high temperature (450oC). Material models that account for both the deformation mechanisms active during forming and the effect of stress state on material response are required to accurately predict final sheet thickness profiles, the locations of potential forming defects and forming cycle time. This study compares Finite Element (FE) predictions for forming of an automobile decklid inner panel in fine-grained AA5083 using two different material models. These are: the no-threshold, two-mechanism (NTTM) model and the Zhao. The effect of sheet/die friction is evaluated with five different sheet/die friction coefficients. Comparisons of predicted sheet thickness profiles with those obtained from a formed AA5083 panel shows that the NTTM model provides the most accurate predictions.
E. M. Taleff, Hector, Jr., L. G., Verma, R., Krajewski, P. E., and Chang, J. - K., “Material Models for Simulation of Superplastic Mg Alloy Sheet Forming,” Journal of Materials Engineering and Performance, vol. 19, pp. 488–494, 2010. LinkAbstract
Accurate prediction of strain fields and cycle times for fine-grained Mg alloy sheet forming at high temperatures (400-500 °C) is severely limited by a lack of accurate material constitutive models. This paper details an important first step toward addressing this issue by evaluating material constitutive models, developed from tensile data, for high-temperature plasticity of a fine-grained Mg AZ31 sheet material. The finite element method was used to simulate gas pressure bulge forming experiments at 450 °C using four constant gas pressures. The applicability of the material constitutive models to a balanced-biaxial stress state was evaluated through comparison of simulation results with bulge forming data. Simulations based upon a phenomenological material constitutive model developed using data from both tensile elongation and strain-rate-change experiments were found to be in favorable accord with experiments. These results provide new insights specific to the construction and use of material constitutive models for hot deformation of wrought, fine-grained Mg alloys.
P. A. Sherek, Hector, Jr., L. G., Bradley, J. R., Krajewski, P. E., and Taleff, E. M., “Simulation and Experiments for Hot Forming of Rectangular Pans in Fine-Grained Aluminum Alloy AA5083,” Key Engineering Materials, vol. 433, pp. 185–195, 2010. Link
J. Qiao and Taleff, E. M., “Superplasticity-Like Creep Behavior of Coarse-Grained Ternary Al Alloys,” Transactions of Nonferrous Metals Society of China, vol. 20, pp. 564–571, 2010. LinkAbstract
Enhanced tensile ductilities in coarse grained Al-Mg-Zn and Al-Mg-Fe materials were studied. The materials were Al-2Mg-5Zn, Al-3Mg-5Zn, Al-4Mg-5Zn, Al-3Mg-0.11Fe, Al-3Mg-0.27Fe, and Al-3Mg-0.40Fe. Tensile elongation-to-failure tests were conducted at constant cross-head speed and constant temperatures from 300 to 450 °C. Strain rate change tests were conducted at a constant temperature from 300 to 450 °C and in strain-rate range from 4.31×10−5 to 1.97×10−2 s−1. Experimental results show that over 100% ductilities are consistently achieved in these materials. This superplasticity-like behavior is rate-controlled by solute-drag creep. Although ternary Zn and Fe additions do not have an adverse effect on solute-drag creep and ductility, they increase stress exponent and its sensitivity to Mg content during solute-drag creep.
E. M. Taleff, “An Overview of Hot- and Warm-Forming of Al-Mg Alloys,” Key Engineering Materials, vol. 433, pp. 259–265, 2010. LinkAbstract
Al-Mg alloys exhibit remarkable hot and warm ductilities, which have made the 5000-series alloys a critical part of commercial hot gas-pressure forming operations for the transportation industry. A review of the metallurgical and practical engineering reasons for this success is presented, and new understanding for behaviors in these materials, expected to impact future advances in hot- and warm-forming technology, are described. The excellent formabilities in this material class are fundamentally attributable to two deformation mechanisms, grain-boundary-sliding and solute-drag creep. However, a number of failure mechanisms ultimately limit final ductility and formability. These include cavitation, flow localization and microstructure evolution. The interplay of these mechanisms is discussed in terms of the potential to improve processing windows in forming operations.
2009
J. R. Ciulik and Taleff, E. M., “Dynamic Abnormal Grain Growth: A New Method to Produce Single Crystals,” Sripta Materialia, vol. 61, pp. 895–898, 2009. WebsiteAbstract
Dynamic abnormal grain growth (DAGG) is a newly discovered phenomenon which can be used to produce large single crystals from polycrystalline material in the solid state at temperatures above approximately half the melting temperature. The unique aspect of DAGG, compared to previously understood abnormal grain growth phenomena, is the requirement of plastic straining for initiation and propagation of abnormal grain growth. Our findings demonstrate that DAGG can be used to produce large single crystals of molybdenum in the solid state. Keywords Abnormal grain growth; Molybdenum; X-ray diffraction; Electron backscattered diffraction
R. Verma, Hector, Louis G., J., Krajewski, P. E., and Taleff, E. M., “The Finite Element Simulation of High-temperature Magnesium AZ31 Sheet Forming,” JOM, vol. 61, pp. 29–37, 2009. LinkAbstract
Finite element (FE) simulations will be vitally important to advancing magnesium alloy sheet forming technologies for vehicle component manufacturing. Although magnesium alloy sheet has been successfully formed into complex components at high temperatures, material constitutive model development for FE simulations has not kept pace with the needs of forming process design. This article describes the application of a new material constitutive model in FE simulations for hot forming of magnesium AZ31 alloy sheet. Simulations of forming both simple geometries from laboratory studies and complex parts from production trials are presented and compared with experimental results.

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