Ductile Fracture of TRIP780 Sheets under Multi-axial Loading

Matthieu Dunand,Dirk Mohr 한국소성가공학회 2010 기타자료 Vol.2010 No.6

The loading path to fracture in multi-axial experiments is determined through hybrid experimental-numerical analysis. A series of multi-axial fracture experiments are carried out on specimens extracted from TRIP780 steel sheets. It involves three different types of full-thickness specimens: notched tensile specimens, tensile specimens with central hole and disc specimens for punch testing. The experimental program characterizes the onset of fracture over a wide range of stress states between uniaxial tension and equi-biaxial tension. Using an anisotropic Hill’48 plasticity model, the evolution of local stress and strain fields is evaluated through detailed finite element analysis of each experiment. The accuracy of this hybrid experimental-numerical loading history identification is evaluated by comparing the local digital image correlation measurements with the FE predictions. In particular, uncertainties affecting the identification of the state of loading at the onset of fracture are quantified.

Finite element analysis of a tin oxide-coated copper foam anode in lithium-ion batteries based on a fully coupled diffusionalmechanical modeling

K. J. Jeong(정경재),H. -H. Cho(조훈휘),H. N. Han(한흥남),D. C. Dunand(David C. Dunand) 한국소성가공학회 2019 한국소성가공학회 학술대회 논문집 Vol.2019 No.5

Hydrogen-induced transformation superplasticity in zirconium

Hong, E.,Dunand, D.C.,Choe, H. Pergamon Press ; Elsevier Science Ltd 2010 International journal of hydrogen energy Vol.35 No.11

Commercially-pure zirconium is alloyed and dealloyed repeatedly with hydrogen at 810 <SUP>o</SUP>, thereby cyclically triggering phase transformations between hydrogen-free α-Zr and hydrogen-alloyed β-Zr. Under an externally applied tensile stress, the internal mismatch stresses produced by the α-β transformations are biased, resulting in the accumulation of strain increments after each chemical cycle in the direction of the applied stress. Two key parameters, i.e., half-cycle time and applied stress, are examined to determine their effects on the strain increments. A tensile strain of 133% is achieved without fracture after multiple chemical cycles, demonstrating for the first time transformation superplasticity in zirconium induced by isothermal hydrogen cycling.

Microstructure and compressive behavior of ice-templated copper foams with directional, lamellar pores

Park, Hyeji,Choi, Myounggeun,Choe, Heeman,Dunand, David C. Elsevier Sequoia 2017 Materials science & engineering Structural materia Vol.679 No.-

<P><B>Abstract</B></P> <P>Copper foams are fabricated by directional freezing of aqueous suspensions of nanometric CuO powders followed by ice sublimation, reduction to Cu in Ar–5% H<SUB>2</SUB> gas and sintering. During slurry solidification, parallel, lamellar, centimeter-long ice dendrites grow, pushing the CuO powders into lamellar interdendritic spaces. Upon subsequent ice sublimation, the ice dendrites create lamellar pores surrounded by CuO walls that are subsequently reduced to copper and sintered; these ice-templated walls display surface micropores and, depending on the reduction/sintering parameters, internal micropores. Varying the main processing parameters – powder fraction in the slurry (from 13 to 19vol%) and casting temperature (from −10 to −30°C) – has a strong effect on the foam microstructure: (i) porosity (varying from 45% to 73%) is inversely related to slurry powder fraction, (ii) oriented lamellar macropores width increases from 15 to 64µm with decreasing slurry fraction and increasing freezing temperature and (iii) oriented lamellar Cu wall width increases from 19 to 63µm with increasing slurry fraction and freezing temperature. The resulting Cu foams show oriented, lamellar macropores (beneficial to permeability) and walls micropores (which increase the surface area) and are promising for use in electrochemical cells given the simplicity, scalability, low cost, and microstructure tunability associated with the ice-templating process. The ice-templated Cu foams, with pore directions parallel and perpendicular to the direction of compressive loading, show ductile compressive behavior with high yield stress, ductility and energy absorption; they are compared to model predictions and literature data of Cu lotus foams with elongated, cylindrical pores.</P>

Printed Origami Structures (Adv. Mater. 20/2010)

Ahn, Bok Yeop,Shoji, Daisuke,Hansen, Christopher J.,Hong, Eunji,Dunand, David C.,Lewis, Jennifer A. WILEY-VCH Verlag 2010 Advanced Materials Vol.22 No.20

<B>Graphic Abstract</B> <P>Bok Y. Ahn, Jennifer Lewis, and co-workers report on p. 2251 a new method for creating complex 3D structures that combines direct-write assembly with a wet-folding origami technique. Planar lattices composed of a titanium hydride ink are printed, and then folded, rolled, or molded into the desired shape. These 3D objects are then transformed into metallic or ceramic structures by thermal annealing. <img src='wiley_img_2010/09359648-2010-22-20-ADMA201090069-content.gif' alt='wiley_img_2010/09359648-2010-22-20-ADMA201090069-content'> </P>

Numerical and experimental investigation of (de)lithiation-induced strains in bicontinuous silicon-coated nickel inverse opal anodes

Cho, H.H.,Glazer, M.P.B.,Xu, Q.,Han, H.N.,Dunand, D.C. Elsevier Science 2016 Acta materialia Vol.107 No.-

<P>A volume expansion of up to similar to 310% occurs upon the lithiation of silicon in Si-coated nickel inverse opal anodes, which causes (de)lithiation-induced mismatch stresses and strains between the Si and Ni during battery cyclical (dis)charging. These (de)lithiation-induced mismatch strains and stresses are modeled via sequentially coupled diffusion- and stress-based finite element (FE) analysis, which takes the mechanical contact between the Si and Ni phases into account, as well as the complex geometry and material properties of the Si-coated Ni inverse opal anode system. During lithiation, compressive strains up to 0.2% are developed in the Ni scaffold since the Si active layer expands. A rapid recovery of these lithiation-induced mismatch strains occurs during subsequent delithiation, though full recovery is not achieved. Strain histories upon multiple (de)lithiation cycles vary with the choice of various mechanical contact conditions employed between the two phases, since the mechanical contact properties determine how the contacted phases interact mechanically. The numerically predicted strains are compared with experimental strain data collected in operando using X-ray diffraction. The simulated strain histories agree with the measured data, enabling the possibility of predicting mechanical performance and eventual degradation using only numerical modeling. In particular, the FE model indicates that plastic deformation occurs first in the lithiated Si active layer, then in the Ni scaffold. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>

Printed Origami Structures

Ahn, Bok Yeop,Shoji, Daisuke,Hansen, Christopher J.,Hong, Eunji,Dunand, David C.,Lewis, Jennifer A. WILEY-VCH Verlag 2010 Advanced Materials Vol.22 No.20

<B>Graphic Abstract</B> <P>Printed origami structures are fabricated by combining direct-write assembly of planar lattices with a wet-folding origami technique. This novel approach provides a low-cost, versatile route to create three-dimensional structures from diverse materials, whose shapes range from simple polyhedra to intricate origami forms. <img src='wiley_img_2010/09359648-2010-22-20-ADMA200904232-content.gif' alt='wiley_img_2010/09359648-2010-22-20-ADMA200904232-content'> </P>

Microstructure and Mechanical Properties of Reticulated Titanium Scrolls

Hong, Eunji,Ahn, Bok Y.,Shoji, Daisuke,Lewis, Jennifer A.,Dunand, David C. WILEY‐VCH Verlag 2011 Advanced Engineering Materials Vol.13 No.12

<P><B>Abstract</B></P><P>Reticulated titanium scrolls are produced by printing titanium hydride lattices composed of two orthogonal layers of ink filaments, which are then rolled into cylinders and reduced to titanium upon partial vacuum sintering. The resulting three‐dimensional titanium scrolls contain a hierarchical pore size distribution composed of macroporosity between patterned filaments and micropores within each filament. These reticulated architectures exhibit an attractive combination of stiffness, strength, and ductility when tested in uniaxial compression.</P>

Experimental and modeling study of compressive creep in 3D-woven Ni-based superalloys

Cho, Hoon-Hwe,Erdeniz, Dinc,Sharp, Keith W.,Dunand, David C. Elsevier 2018 ACTA MATERIALIA Vol.155 No.-

<P><B>Abstract</B></P> <P>Micro-architectured Ni-based superalloy structures, with Ni-20Cr-3Ti-2Al (wt.%) composition and γ/γ′-microstructure, are created by a multi-step process: (i) non-crimp orthogonal 3D-weaving of ductile, 202 μm diameter Ni-20%Cr wires, (ii) gas-phase alloying with Al and Ti, (iii) simultaneous transient-liquid phase (TLP) bonding between wires and homogenization within wires <I>via</I> interdiffusion, (iv) solutionizing to create a single-phase solid solution, and (v) aging to precipitate the γ′ phase. The creep behavior of these 3D-woven γ/γ′ nickel-based superalloys is studied under uniaxial compression <I>via</I> experiments at 825 °C and <I>via</I> finite element (FE) analysis, using a 3D model of the woven structures obtained through X-ray micro-tomography. The creep strain rate for the woven Ni-based superalloy is higher than that for the bulk superalloy due to the lower solid volume fraction of the woven structure, while the creep exponents are identical. The compressive creep behavior is sensitive to the geometry of the woven structures: fewer wires perpendicular to the load and fewer bonds between wires cause lower creep resistance of the woven structure, due to a reduction in load transfer from the longitudinal wires (which are primarily load-bearing) and the perpendicular wires. Creep buckling of longitudinal wires drastically reduces creep resistance of the woven structure, confirming the importance of maintaining longitudinal wires vertical and parallel to the uniaxial compression direction. Finally, reducing wire cross-section, <I>e.g., via</I> oxidation, reduces creep resistance. The oxidation kinetics of the wire structures at 750, 825, and 900 °C displayed parabolic rate constants comparable to commercial Ni-based superalloys, but indicates that up to 35% of the wire cross-section is oxidized after 7 days at 825 °C, such that oxidation-resistant coatings are needed for long-term use in oxidative environment.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Compressive creep behavior of hot-pressed Mg_1.96Al_0.04Si_0.97Bi_0.03

Michi, Richard A.,Kim, Gwansik,Kim, Byung-Wook,Lee, Wooyoung,Dunand, David C. Elsevier 2018 Scripta materialia Vol.148 No.-

<P><B>Abstract</B></P> <P>The compressive creep behavior of hot-pressed Mg<SUB>1.96</SUB>Al<SUB>0.04</SUB>Si<SUB>0.97</SUB>Bi<SUB>0.03</SUB>, a promising thermoelectric material, is investigated at 500 °C. At stress levels between 81 and 212 MPa, dislocation creep with stress exponent <I>n</I> = 7.6 ± 0.3 is observed. No diffusional creep is observed, likely attributable to a dispersion of ~1 μm Bi-, Al-, and O- rich particles which pin grain boundaries. Mg<SUB>1.96</SUB>Al<SUB>0.04</SUB>Si<SUB>0.97</SUB>Bi<SUB>0.03</SUB> exhibits similar creep behavior to previously studied silicides, but is significantly more creep resistant than other thermoelectric materials, PbTe and Bi<SUB>2</SUB>Te<SUB>3</SUB>. This makes Mg<SUB>1.96</SUB>Al<SUB>0.04</SUB>Si<SUB>0.97</SUB>Bi<SUB>0.03</SUB> an excellent material for thermoelectric power generation systems subjected to high stresses and temperatures.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>