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### Editorial

J. Eng. Mater. Technol. 2008;131(1):010201-010201-1. doi:10.1115/1.3030689.
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This special issue of the Journal of Engineering Materials and Technology (JEMT) contains selected papers from the first symposium organized on shear; SHEAR07 in Nancy, France, 4–7 September, 2007. All papers have successfully gone through the regular peer-review process of JEMT.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Eng. Mater. Technol. 2008;131(1):011001-011001-10. doi:10.1115/1.3026543.

In order to characterize the metal behavior at strain, strain rate, and temperature range encountered in metal forming processes, the rheological compressive test is well adapted and has been often used. Nevertheless, this experimental test is more complicated to realize than the extension one and requires some particular considerations owing to the friction condition occurring between the specimen and the dies. This paper deals with a new specimen shape proposed to realize both static and dynamic compression tests. The independence of the material parameters to die friction is highlighted by means of a pseudo-experimental validation. The proposed specimen shape is validated by compression tests carried out on a 50CD4 steel (norm EN 10 083). The choice of the mathematical form of the constitutive law allowing to characterize its behavior at strains, strain rates, and temperatures corresponding to an extrusion application is then discussed. To replicate more accurately the nonuniformity of the different fields in the specimen, a classical inverse procedure consisting in coupling a finite element model of the compression test with an optimization module is used to determined the rheological parameters.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011002-011002-10. doi:10.1115/1.3026545.

Microelectromechanical systems (MEMS), particularly those with radio frequency (rf) applications, have demonstrated significantly better performance over current electromechanical and solid-state technologies. Surface roughness and asperity microcontacts are critical factors that can affect contact behavior at scales ranging from the nano to the micro in MEMS devices. Recent investigations at the continuum level have underscored the importance of microstructural effects on the inelastic behavior of asperity microcontacts. Hence, a microstructurally based approach that accounts for the inhomogeneous deformation of the asperity microcontacts under cyclic loading and that is directly related to asperity physical scales and anisotropies can provide a detailed understanding of the deformation mechanisms associated with asperity microcontacts so that guidelines can be incorporated in the design and fabrication process to effectively size critical components and forces for significantly improved device durability and performance. A physically based microstructural representation of fcc crystalline materials that couples a multiple-slip crystal plasticity formulation to dislocation densities is used in a specialized finite-element modeling framework. The asperity model and the loading conditions are based on realistic service conditions consistent with rf MEMS with metallic normal contacts. The evolving microstructure, stress fields, contact width, hardness, residual effects, and the localized phenomena that can contribute to failure initiation and evolution in the flattening of single crystal gold asperity microcontacts are characterized for a loading-unloading cycle. It is shown that the nonuniform loading conditions due to asperity geometry and contact loading and the size effects due to asperity dimensions result in significant contribution of the geometrically necessary dislocation densities to stress, deformation, and microstructural evolution of crystalline asperities. This is not captured in modeling efforts based on von Mises continuum plasticity formulations. Residual strains and stresses are shown to develop during the cyclic loading. Localized tensile stress regions are shown to develop due to stress reversal and strain hardening during both loading and unloading regimes. Hardness predictions also indicate that nano-indentation hardness values of the contact material can overestimate the contact force in cases, where a rigid flat surface is pressed on a surface roughness asperity.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011003-011003-9. doi:10.1115/1.3026546.

In the present contribution, the parameter identification of ductile materials is reconceived in the frame of localization phenomena. To describe the damage process on a continuum mechanical basis, the Gurson–Tvergaard–Needleman damage model is implemented in the finite element system Scientific Parallel Computing-Program Module Hyperelastic Plastic (SPC-PMHP), which was developed for parallel computers to solve nonlinear initial boundary value problems within large strain formalism. The softening of the material is responsible for the loss of ellipticity of the differential equations and for the strain localization. A general localization criterion is given to determine when numerical solutions cease to show convergence. This criterion is based on an analysis of the determinant of the acoustic tensor, which includes the material tangent, the stresses, and the deformation gradient. On this account, the onset of localization is significantly affected by material parameters. The parameters are identified on the basis of locally measured displacement fields. As additional information, the strain localization criterion is included in the identification process. A numerical example shows the influence of the localization criterion on the parameter identification.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011004-011004-9. doi:10.1115/1.3026544.

In components under static creep loading condition, the multiaxial stress fields appear due to the plastic constraint and they produce a more brittle type cracking behavior. From a practical standpoint, the characterizations of creep crack growth rates under the multiaxial stress field are important to improve the methods for creep life extension. In this paper, creep crack growth tests were conducted using round bar specimens with sharp circular notches for tungsten-added 12%Cr ferritic heat-resistant steel (W12%Cr steel), and the effect of multiaxiality on creep ductility and creep crack growth rate were investigated. Furthermore, three-dimensional elastic-plastic creep finite element analyses were conducted to clarify the effect of multiaxiality on creep crack growth.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011005-011005-10. doi:10.1115/1.3026547.

A combined experimental and analytical approach is undertaken to identify the relationship between process parameters and fracture behavior in the cutting of a $1mm$ thick alumina samples by a hybrid $CO2$ laser∕waterjet (LWJ) manufacturing process. In LWJ machining, a $200W$ power laser was used for local heating followed by waterjet quenching of the sample surface leading to thermal shock fracture in the heated zone. Experimental results indicate three characteristic fracture responses: scribing, controlled separation, and uncontrolled fracture. A Green’s function based approach is used to develop an analytical solution for temperatures and stress fields generated in the workpiece during laser heating and subsequent waterjet quenching along the machining path. Temperature distribution was experimentally measured using thermocouples and compared with analytical predictions in order to validate the model assumptions. Computed thermal stress fields are utilized to determine the stress intensity factor and energy release rate for different configurations of cracks that caused scribing or separation of the workpiece. Calculated crack driving forces are compared with fracture toughness and critical energy release rates to predict the equilibrium crack length for scribed samples and the process parameters associated with transition from scribing to separation. Both of these predictions are in good agreement with experimental observations. An empirical parameter is developed to identify the transition from controlled separation to uncontrolled cracking because the equilibrium crack length based analysis is unable to predict this transition. Finally, the analytical model and empirical parameter are utilized to create a map that relates the process parameters to the fracture behavior of alumina samples.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011006-011006-8. doi:10.1115/1.3026548.

This study illustrates a method of measuring internal total strain based on the observation that networks of internal boundaries within a polycrystalline material deform locally in a manner congruent with the local metal flow. Appropriate measurements of the development of the spatial anisotropy of such networks with increasing deformation provide a basis for defining several measures of the local total strain. These quantities, called “grain strains” when the boundaries observed are grain boundaries, can serve as an experimental measure of the internal total strain at various locations in a specimen for comparison with computations based on finite element models using various constitutive relations or phase field simulations of grain growth or deformation. Experimental measurements of grain strains at the center of a ferritic steel sheet rolled in nominally 10% increments to 50% total reduction in thickness illustrate the method and correlate well with corresponding strains based on measures of the change in thickness of the sheet and the assumption of plane strain.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011007-011007-6. doi:10.1115/1.3026549.

The development of fatigue in a machine element can be divided into the fatigue crack initiation stage and the crack growth stage. The effects of the surface conditions, such as roughness and surface qualities, on the fatigue strength are considered to be restricted to the fatigue crack initiation stage. Thus, these effects can be evaluated exactly by the investigation on the fatigue crack initiation life. In this research a practical method is presented for detecting fatigue crack initiation during fatigue tests using an ion-sputtered film. Ion-sputtered films were formed on acrylic and steel test pieces having a specially designed round notch. Three-point bending fatigue tests were performed and the instants of fatigue crack initiation on the notched surface of both acrylic and steel test pieces during the tests were identified clearly; thus the fatigue crack initiation life can also be determined experimentally using the presented method. In addition, since the system used to measure the electric resistance of the film, which indicates the crack initiation, is extremely simple, that is, it consists of a variable resistor, a DC power source, and a data recorder, this method is considered very practical for monitoring crack initiation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011008-011008-7. doi:10.1115/1.3026550.

In this paper, we derive some analytical solutions of the Kamal cure rate differential equation. The Kamal model is a first order quasilinear ordinary differential equation, describing the progress of the curing reaction of several thermosetting polymers. All the examined cases refer to isothermal curing processes. The solutions obtained in this paper are all of implicit form. The derived solutions are applied to a repair technique based on the adhesive bonding of polymer matrix composite patches onto damaged or corroded areas. Critical duration times of realistic cure cycles corresponding to composite patch repair are estimated. The practical importance of the proposed analytic solutions is demonstrated through the presented engineering application.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011009-011009-6. doi:10.1115/1.3026556.

Tensile and compressive stress-strain responses were obtained for various densities of polymer foams. These experimental data were used to determine relevant engineering parameters (such as elastic moduli in tension and compression, ultimate tensile strength, etc.) as a function of foam density. A phenomenological model applicable for both compressive and tensile responses of polymeric foams is validated by comparing the model to the experimentally obtained compression and tensile responses. The model parameters were analyzed to determine the effect of each parameter on the mechanical response of the foam. The engineering parameters were later compared to the appropriate model parameters and a good correlation was obtained. It was shown that the model indeed captures the entire compressive and tensile response of polymeric foams effectively.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011010-011010-6. doi:10.1115/1.3026557.

The purpose of this study is to clarify the relationship between ionic conductivity and phase transformation of zirconia system codoped with scandium oxide $Sc2O3$ and ytterbium oxide $Yb2O3$. Aiming to achieve high ionic conductivity as well as high mechanical strength, the authors have also investigated the relationship between phase transformation and mechanical strength. The results have been discussed with respect to both the conductivity and the mechanical strength. The Sc- and Yb-codoped zirconia $(ZrO2)$ used as samples in this study were prepared by a standard solid-state reaction. X-ray powder diffraction (XRD) method was used to determine the crystal structures of the sintered samples. To detect any phase change between room temperature and $1273K$, thermal mechanical analysis (TMA) was conducted. To determine oxygen-ion conductivity in a temperature range from $873to1273K$ in air, impedance measurements were performed with alternating current (ac). Single-cell performance was confirmed under the condition of $26.2Pa$ partial hydrogen pressure. Finally, to measure bending strength, three-point bending tests were performed with a universal testing machine. The results of XRD and TMA showed that codoping of $Sc2O3$ and $Yb2O3$ into $ZrO2$ successfully stabilized the cubic phase when the average radius ratio of these two dopants in total was close to the ideal one for the eight-coordinate. The ac impedance measurement demonstrated that the cubic-phase stabilization achieved a high conductivity. Adequate amounts of dopants produced oxygen vacancies for high conductivity without complex defects: $ZrO2$ system doped with $1mol%$ of $Yb2O3$ and $8mol%$ of $Sc2O3$ showed the highest conductivity at $1273K$ and $0.30S∕cm$. The bending strength decreased with increasing the content of doped $Sc2O3$ from $7mol%to11mol%$, depending on the amount of the tetragonal phase, which contributes to strengthen materials. In the performance test, the $ZrO2$ system stabilized with doping $1mol%$$Yb2O3$ and $8mol%$$Sc2O3$ with thickness of $2.16mm$ showed maximum power density at $1273K$, that is, $210mW∕cm2$. From all the above tests, we recommend that, based on electrical and mechanical considerations, 1Yb8ScSZ is the present best option for an electrolyte material for a solid oxide fuel cell.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011011-011011-11. doi:10.1115/1.3030941.

Spot-weld joints are commonly used to fasten together metal sheets. Because fatigue fracture is the most critical failure mode for these joints under fluctuating loads, understanding their fatigue failure behavior and assessment of their fatigue lives are crucial from the viewpoint of failure prevention in design. In this study, a series of experiments was conducted to study the fatigue failure of spot-welded modified tensile-shear specimens made of a low carbon steel. Two different types of resistance spot welding were investigated (manual and automated). Tests were repeated under different load ranges, and the corresponding fatigue lives were determined. The specimens were also examined under an optical microscope. In the numerical part of this study, a finite element analysis was carried out using commercial software, ANSYS , to determine the stress and strain states within the specimens. The material nonlinearity, local plastic deformations around the welds during loading, and the residual stresses and strains developed after unloading as a result of plastic deformations were taken into account. Based on the predicted stress and strain states, fatigue analyses were performed using several models for life assessment. Then, the measured and predicted fatigue lives were compared, and the suitability of the models was discussed. Among the strain-based models, Coffin–Manson and Morrow’s means stress models yielded the best predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011012-011012-6. doi:10.1115/1.3030943.

Nucleation of fatigue cracks at nonmetallic primary inclusions in high cycle fatigue of martensitic steel is computationally investigated. We explore the capabilities of an elastic interphase material adhered to the inclusion surface to alter the driving force for fatigue crack nucleation in the matrix. By varying the elastic stiffness of the encapsulating interphase, the stresses and cyclic plastic strains are examined in the matrix in the proximity of a partially debonded inclusion, a worst case scenario for nucleation. The matrix is modeled as elastic-plastic with pure kinematic hardening expressed in a hardening minus dynamic recovery format. The inclusion and interphase are modeled as isotropic linear elastic. An idealized spherical, homogeneous inclusion is considered to facilitate parametric study. A nonlocal average value of the maximum plastic shear strain amplitude was used in a modified form of the Fatemi–Socie parameter in the proximity of inclusions as a fatigue indicator parameter to facilitate comparative parametric study of potency for crack nucleation.

Commentary by Dr. Valentin Fuster

### SHEAR BEHAVIOR AND RELATED MECHANISMS IN MATERIALS PLASTICITY

J. Eng. Mater. Technol. 2008;131(1):011101-011101-9. doi:10.1115/1.3030882.

The microstructure development was investigated in torsion deformed NiAl. High strain torsion of solid bars was done with a Paterson rock deformation machine at temperatures between 700 K and 1300 K under a confining pressure of 400 MPa. The maximum shear strains and shear strain rates applied were 19 and $2.2×10−4 s−1$, respectively. The shear stress–shear strain curves are characterized by a peak at low shear strains, which is followed by softening and a steady state at high shear strains. Increasing shear strain leads to grain refinement, with the average grain size decreasing with temperature. Moreover, a steady state grain aspect ratio and inclination of the grain long axis with respect to the shear plane is observed. With increasing shear strain, the fraction of low angle grain boundaries goes over a maximum and approaches a steady state of about 20–40%. The development of the microstructure is characterized by two different temperature regimes. Up to 1000 K, continuous dynamic recrystallization characterized by limited grain growth takes place, leading to a transformation of low into high angle grain boundaries. At temperatures above 1000 K, discontinuous dynamic recrystallization occurs by massive grain growth. The results are qualitatively discussed on the basis of models dealing with dynamic recrystallization.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011102-011102-10. doi:10.1115/1.3030896.

Texture formation and Swift effect were investigated in torsion deformed NiAl. High-strain torsion of solid bars was done with a Paterson rock deformation machine at temperatures between 700 K and 1300 K under a confining pressure of 400 MPa. The maximum shear strains and shear strain rates applied were $19×10−4 s−1$ and $2.2×10−4 s−1$, respectively. Textures were measured by diffraction of neutrons, electrons, and synchrotron radiation. The textures consist of an oblique cube and Goss component, the intensity of which depends on the initial texture and deformation temperature. The axial lengthening and shortening observed are related to the Goss and the oblique cube components, respectively. There is qualitative agreement between experiment and simulation at low temperature and low shear strains. With increasing temperature, continuous and discontinuous dynamic recrystallization take place, strongly influencing the development of texture and Swift effect.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011103-011103-8. doi:10.1115/1.3030939.

The development of ideal orientations within the steady-state region of hot torsion flow curves of fcc and bcc metals undergoing “continuous” dynamic recrystallization is analyzed. It is well known that in fcc metals, e.g., Al deformed at $400°C$ and above, the experimentally observed end texture consists of the twin-symmetric $B(112¯)[11¯0]/B¯(1¯1¯2)[1¯10]$ component, whereby the $(hkl)[uvw]$ indices correspond to the shear plane $z$ and the shear direction $θ$, respectively. In bcc iron however, only one of the self-symmetric $D1(112¯)[111]$ and $D2(1¯1¯2)[111]$ components dominates (the former in the case of positive shear or clockwise rotation about the $r$-axis, and the latter during negative shear). The tendency toward a single end orientation imposes certain limitations on grain refinement, as this would ultimately imply the coalescence of subgrains of or close to this orientation, and therefore the disappearance of existing high angle boundaries $(≥15 deg)$. It is believed that the preference of D1 over D2, or vice versa, could be related to phenomena other than glide-induced rotations, e.g., grain boundary migration resulting from differences in work hardening rates. In this paper, the standard Taylor model is first used to predict the texture evolution in simple shear under the full-constraint rate-sensitive scheme. This is then coupled with an approach that takes into account grain boundary migration resulting from differences in dislocation densities within grains of varying orientations. The preliminary results are in agreement with experimental findings, i.e., grains with initial orientations close to D2 grow at the expense of neighboring grains during negative shear and vice versa.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011104-011104-11. doi:10.1115/1.2969258.

The present article addresses the following question: How is it that shears are so common in the plastic deformation of metallic alloys? An answer is sought in a geometric description of the shear flow when the deformation is produced by slip systems gliding according to the Schmid law. Such flows are represented schematically by what is called “simple shear” and a kinematic study is done of the way these shears can be produced by the joint activity of various slip systems. This implies specific conditions on the glide rates, which can be known analytically thanks to adequate parametrizations. All the possible shears have been calculated in the case of cubic metals deforming with identical critical resolved shear stresses (Bishop and Hill polyhedron). Three dimensional representations are given in the space of the Bunge angles associated with the principal directions of the shears. A special attention has been given to the number of slip systems involved. Most of the shears are not far from some combination of two or three systems. This is quantified by defining the misorientation $ω$ between a shear taken at random and the set of shears produced by the glide on two or three octahedral slip systems. It is found that in most cases, $ω<15 deg$. The maximum value of $ω$ (30.5 deg) is found for the orientations called Cube and U in rolled metals.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011105-011105-14. doi:10.1115/1.3030880.

This work brings forward a twofold contribution relevant to the adiabatic shear banding (ASB) process as a part of dynamic plasticity of high-strength metallic materials. The first contribution is a reassessment of a three-dimensional finite deformation model starting from a specific scale postulate and devoted to cover a wide range of dissipative phenomena, including ASB-related material instabilities (strong softening prefailure stage). The model, particularly destined to deal with impacted structures was first detailed by (Longère2003, “Modelling Adiabatic Shear Banding Via Damage Mechanics Approach  ,” Arch. Mech., 55, pp. 3–38; 2005, “Adiabatic Shear Banding Induced Degradation in a Thermo-Elastic/Viscoplastic Material Under Dynamic Loading  ,” Int. J. Impact Eng., 32, pp. 285–320). The second novel contribution concerns numerical solution of a genuine ballistic penetration problem employing the above model for a target plate material. The ASB trajectories are shown to follow a multistage history and complex distribution pattern leading finally to plugging failure mechanism. The corresponding analysis and related parametric study are intended to put to the test the pertinency of the model as an advanced predictive tool for complex shock related problems.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011106-011106-6. doi:10.1115/1.3030940.

A theoretical analysis of plane strain incompressible velocity fields is first carried out. Two types of local fields are distinguished according to the sign of $det(L)$, where $L$ is the velocity gradient tensor, whereas $det(L)=0$ is associated with simple shear. A geometrical interpretation is used to illustrate the various decompositions of $L$. Finally, it is suggested that the condition $det(L)=0$ can be used to predict the occurrence of shear bands during metal forming processes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011107-011107-8. doi:10.1115/1.3030942.

Two experimental devices that promote simple shear are used to investigate the plastic behavior of metals under very large strains. First, researches on the anisotropic behaviors of sheets of metals performed with the help of the planar simple shear test are reviewed. In particular, it is shown that, with this device, stage IV may be reached and analyzed on polycrystals as well as on single crystals. The second part is devoted to equal channel angular extrusion, which is known to promote grain refinement after several passes. A direct comparison of the crystallographic textures measured on sheared and on extruded samples confirms that the extrusion promotes massively simple shear. Besides, the grain refinement is measured with a dedicated transmission electron microscopy (TEM) attachment. It is shown that the grain size decreases regularly for a low carbon steel as well as for copper, down to around $1 μm$. It is argued that the sustained hardening in stage IV is a mechanical signature of the grain size decrease. The trend is interpreted and reproduced quantitatively with the help of a simple modeling approach.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011108-011108-15. doi:10.1115/1.3030973.

Torsion experiments were carried out on pure magnesium (99.9%) and the magnesium alloy AZ71 under free-end conditions of testing. The alloy had an axisymmetric initial texture, while the pure Mg samples were prepared from a rolled plate with a nonaxisymmetric initial texture. The torque as a function of the twist angle was measured at different temperatures (room temperature, $150°C$, and $250°C$). During twisting, systematic shortening of the samples was observed (Swift effect). The evolution of the crystallographic texture was analyzed by electron backscattering diffraction measurements. The occurrence of dynamic recrystallization (DRX) was detected in pure Mg at $250°C$. The Swift effect in the axisymmetric samples was simulated with the “equilibrium equation” approach using polycrystal modeling. In the nonaxisymmetric samples, the texture was simulated at different angular positions with the help of the viscoplastic self-consistent model. The changes in the textures due to DRX were explained in terms of the Taylor factor. Finally, the texture evolution was interpreted with the help of the behavior of ideal orientations and persistence characteristics of hexagonal crystals in simple shear.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;131(1):011109-011109-5. doi:10.1115/1.3030938.

By analyzing the deformation of $α$—isotactic polypropylene through cyclic uniaxial compression at different temperatures—conclusions are drawn on the contribution of the crystalline phase and the amorphous phase to the hardening curve. The deformation of the crystalline phase, which deforms mainly by simple shear of the crystallites, strongly depends on the properties of the amorphous phase. A separation of strain in a relaxing and a quasipermanent part, as introduced by the work of Hiss (1999, “Network Stretching, Slip Processes and Fragmentation of Crystallites During Uniaxial Drawing of Polyethylene and Related Copolymers  ,” Macromolecules, 32, pp. 4390–4403), is undertaken. By this experimental procedure it is possible to characterize the deformation dependence of several physical quantities such as Young’s modulus or the stored energy associated to each loading-unloading cycle. Furthermore specific transition strains, A, B, C, and D, can be determined where the recovery properties change. It is demonstrated that beyond point C the strain hardening can be described by the simple rubber hardening model of Haward (1987, “The Application of a Simplified Model for the Stress-Strain Curve of Polymers  ,” Polymer, 28, pp. 1485–1488).

Commentary by Dr. Valentin Fuster

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