J. Eng. Mater. Technol. 2003;125(3):237-246. doi:10.1115/1.1491574.

Strain recovery after plastic prestrain and associated elastic and inelastic behavior during loading and unloading of DQSK steel sheet are measured. Average tangent modulus and Poisson’s ratio during unloading and reloading are found to differ from their elastic values in the undeformed state, and they also vary as a function of stress. This modulus, often referred to as the “springback modulus,” decreases with plastic prestrain rapidly for prestrain values <2 percent and decays slowly for larger values of prestrain, while the average Poisson’s ratio during unloading increases with plastic prestrain initially rapidly and then remains almost unchanged at larger prestrain. Changes in the springback modulus and Poisson’s ratio are shown to be due to recovery of microplastic strain and not due to viscoelastic effects. Springback modulus and Poisson’s ratio are anisotropic, showing a maximum in modulus and a minimum in Poisson’s ratio at 45 deg to rolling direction. To describe the combination of recoverable inelastic and elastic deformation as a function of plastic prestrain, a set of equations has been developed based upon a previously developed constitutive model. Calculated results are capable of explaining experimental results on modulus and Poisson’s ratio changes. Implication of the results on “springback” is illustrated and empirical relations are obtained.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):247-255. doi:10.1115/1.1543971.

To meet the growing demand for rapid, low-cost die fabrication technology in the sheet metal forming industry, easy-to-machine, polyurethane-based, composite board stock is widely used as a rapid tooling material. However, the failure mechanisms of the rapid prototyped tools are not clearly understood, thus making the prediction of tool life difficult. As a fundamental step for effective tool life estimation, the microstructure and the mechanical properties of the polymer composite tooling material were characterized. A finite element model of 90° V-die bending process was developed, and the effects of process parameters on stress distribution in punch and die were investigated through simulation. The simulation results were verified through experiments using instrumented, laboratory-scale punch and die sets.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):256-259. doi:10.1115/1.1584491.

In this paper, a constitutive equation for superplasticity, which is based on the microstructural mechanism of superplastic deformation taking into account the effects of deformation damage, is incorporated into the finite element method to simulate the superplastic forming process. The effects of strain rate sensitivity, cavity growth and imposed hydrostatic pressure on the strain limit are studied. The predicted results are validated through the comparison with the existing experimental data. It is found that the present model produces accurate results in all cases.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):260-265. doi:10.1115/1.1586938.

This paper presents the development of a generalized method to predict forming limits of sheet metals. The vertex theory, which was developed by Stören and Rice (1975) and recently simplified by Zhu, Weinmann and Chandra (2001), is employed in the analysis to characterize the localized necking (or localized bifurcation) mechanism in elastoplastic materials. The plastic anisotropy of materials is considered. A generalized deformation theory of plasticity is proposed. The theory considers Hosford’s high-order yield criterion (1979), Hill’s quadratic yield criterion and the von Mises yield criterion. For the von Mises yield criterion, the generalized deformation theory reduces to the conventional deformation theory of plasticity, i.e., the J2-theory. Under proportional loading condition, the direction of localized band is known to vary with the loading path at the negative strain ratio region or the left hand side (LHS) of forming limit diagrams (FLDs). On the other hand, the localized band is assumed to be always perpendicular to the major strain at the positive strain ratio region or the right hand side (RHS) of FLDs. Analytical expressions for critical tangential modulus are derived for both LHS and RHS of FLDs. For a given strain hardening rule, the limit strains can be calculated and consequently the FLD is determined. Especially, when assuming power-law strain hardening, the limit strains can be explicitly given on both sides of FLD. Whatever form of a yield criterion is adopted, the LHS of the FLD always coincides with that given by Hill’s zero-extension criterion. However, at the RHS of FLD, the forming limit depends largely on the order of a chosen yield function. Typically, a higher order yield function leads to a lower limit strain. The theoretical result of this study is compared with those reported by earlier researchers for Al 2028 and Al 6111-T4 (Grafand Hosford, 1993; Chow et al., 1997).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):266-276. doi:10.1115/1.1586939.

The goal of this paper is to derive a micromechanics framework to study the kinetics of transformation due to interface migration in elastic-plastic materials. Both coherent and incoherent interfaces as well as interstitial and substitutional atomic diffusion are considered, and diffusional transformations are contrasted with martensitic ones. Assuming the same dissipation for the rearrangement of all substitutional components and no dissipation due to diffusion in an interface in the case of a multicomponent diffusional transformation, we show that the chemical driving force of the interface motion is represented by the jump in the chemical potential of the lattice forming constituent. Next, the mechanical driving force is shown to have the same form for both coherent and frictionless (sliding) interfaces in an elastic-plastic material. Using micromechanics arguments we show that the dissipation and consequently the average mechanical driving force at the interface due to transformation in a microregion can be estimated in terms of the bulk fields. By combining the chemical and mechanical parts, we obtain the kinetic equation for the volume fraction of the transformed phase due to a multicomponent diffusional transformation. Finally, the communication between individual microregions and the macroscale is expressed by proper parameters and initial as well as boundary conditions. This concept can be implemented into standard frameworks of computational mechanics.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):277-282. doi:10.1115/1.1555657.

Al2O3/YAG eutectic composite has been developed for a structural material used in ultra high temperature environments over 1500°C such as a gasturbine. Creep behavior is one of the important material properties in ultra high temperature materials. In the present study, we propose an image-based finite element analysis for estimating the steady state creep behavior of the Al2O3/YAG eutectic composite. In the image-based finite element analysis, microstructure of the material taken by a SEM is modeled into a finite element mesh using a software for image process. Then finite element creep analyses are carried out to obtain the steady state creep behavior of the Al2O3/YAG eutectic composite by using steady state creep constitutive equations for both Al2O3 single crystal and YAG single crystal. The results of steady state creep behavior obtained from the image-based finite element analysis are compared with the experimental results. It is found that the steady state creep behavior of the Al2O3/YAG eutectic composite is accurately estimated by the image-based finite element analysis. Furthermore, we examine the effect of volume fractions of the constituents on the steady state creep behavior of the Al2O3/YAG eutectic composite.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):283-293. doi:10.1115/1.1584481.

This paper presents measurements of the thickness-average residual stress imposed by laser peening near the edge of a thin sheet of a common titanium alloy. The slitting (or crack compliance) method was extended beyond its typical form to make residual stress measurements in this geometry. An analytical method was employed to optimize the experiment design for the near-edge, thin material geometry, where the design included the optimal number and position of strain gages and the most effective set of basis functions for stress computation. Two experiments were performed on a titanium strip that had been laser peened near the edge, using an optimal experiment design. Residual stress was found to be large and compressive near the edge of the sheet, with the compressive stress extending over 38% of the laser peened area.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):294-301. doi:10.1115/1.1584492.

Dynamic compressive stress-strain curves at various strain rates of an Ethylene-Propylene-Diene Monomer Copolymer (EPDM) rubber have been determined with a modified split Hopkinson pressure bar (SHPB). The use of a pulse-shaping technique ensures that the specimen deforms at a nearly constant strain rate under dynamically equilibrated stress. The validity of the experiments was monitored by a high-speed digital camera for specimen edge deformation, and by piezoelectric force transducers for dynamic stress equilibrium. The resulting dynamic stress-strain curves for the EPDM indicate that the material is sensitive to strain rates and that the strain-rate sensitivity depends on the value of strain. Based on a strain energy function theory, a one-dimensional dynamic constitutive equation for this rubber was modified to describe the high strain-rate experimental results within the ranges of strain and strain rates presented in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):302-308. doi:10.1115/1.1584493.

Laser metal forming is an attractive process for rapid prototyping or the rebuilding of worn parts. However, large tensile stress may arise in layers deposited by laser melting of powder. A potential solution is to preheat the substrate before and during deposition of layers to introduce sufficient contraction during cooling in the substrate to modify the residual stress distribution in the deposited layers. To demonstrate the value of this approach, specimens were prepared by depositing stellite F on a stainless steel substrate with and without preheating. Residual stresses were computed by numerical simulation and measured using the crack compliance method. For non-preheated specimens simulation and experiment agreed well and showed that extremely high residual tensile stresses were present in the laser melted material. By contrast, pre-heated specimens show high compressive stresses in the clad material. However, in this case the numerical simulation and experimental measurement showed very different stress distribution. This is attributed to out of plane deformation due to the high compressive stresses which are not permitted in the numerical simulation. A “strength of materials” analysis of the effect of out of plane deformation was used to correct the simulation, Agreement with experimental results was then satisfactory.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):309-314. doi:10.1115/1.1586937.

A FEM analysis was carried out to study the mechanical behavior of a hard coating subjected to a nanoindentation test performed with a Berkovich indenter. The nanoindentation test was simulated by FEM code MSC Marc. The case study is a coating of CVD (Chemical Vapor Deposition) diamond. By the simulation it is possible to obtain the load-displacement curve by which Young modulus and hardness may be evaluated. The paper also analyzes the residual stresses developed at the end of the unloading phase and the influence of the strengthening law to determine the hardness and the elastic modulus of the CVD diamond. The analysis has demonstrated, by the comparison with the experimental results, that the numerical model well describes the behavior of the coating of CVD diamond in the nanoindentation test; in addition it was pointed out that the choice of the hardening law is a crucial aspect in the simulation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):315-323. doi:10.1115/1.1590998.

The effects of variable amplitude loading on fretting fatigue behavior of titanium alloy, Ti-6Al-4V were examined. Fretting fatigue tests were carried out under constant stress amplitude and three different two-level block loading conditions: high-low (Hi-Lo), low-high (Lo-Hi), and repeated block of high and low stress amplitudes. The damage fractions and fretting fatigue lives were estimated by linear and non-linear cumulative damage rules. Damage curve analysis (DCA) and double linear damage rule (DLDR) were capable to account for the loading order effects in Hi-Lo and Lo-Hi loadings. In addition, the predictions by DCA and DLDR were better than that by linear damage rule (LDR). Besides its simplicity of implementation, LDR was also capable of estimating failure lives reasonably well. Repeated two-level block loading resulted in shorter lives and lower fretting fatigue limit compared to those under constant amplitude loading. The degree of reduction in fretting fatigue lives and fatigue strength depended on the ratio of cycles at lower stress amplitude to that at higher stress amplitude. Fracture surface of specimens subjected to Hi-Lo and repeated block loading showed the clear evidence of change in stress amplitude of applied load. Especially, the repeated two-level block loading resulted in characteristic markers which reflected change in crack growth rates corresponding to different stress amplitudes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):324-332. doi:10.1115/1.1590999.

A thermo-elastic-plastic finite element modeling of orthogonal cutting with a large negative rake angle has been developed to understand the mechanism and thermal aspects of grinding. A stagnant chip material ahead of the tool tip, which is always observed with large negative rake angles, is assumed to act like a stable built-up edge. Serrated chips, one of typical shapes of chips observed in single grain grinding experiment, form when analyzing the machining of 0.93%C carbon steel SK-5 with a rake angle of minus forty five or minus sixty degrees. There appear high and low temperature zones alternately according to severe and mild shear in the primary shear zone respectively. The shapes of chips depend strongly on the cutting speed and undeformed chip thickness; as the cutting speed or the undeformed chip thickness decreases, chip shape changes from a serrated type to a bulging one to a wavy or flow type. Therefore, there exists the critical cutting speed over which a chip can form and flow along a rake face for a given large negative rake angle and undeformed chip thickness.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(3):333-340. doi:10.1115/1.1580854.

In structures containing brittle materials, residual and/or heterogenous stresses may prevent cracks to propagate up to failure. Consequently, for such structures, crack arrest has to be accounted for and a weakest link hypothesis may not be applicable. A probabilistic crack propagation model is derived to describe instantaneous or delayed arrest phenomena. A time-dependent regime is induced by slow crack growth experienced by ceramics and glasses. A general expression is obtained in which instantaneous up to infinite propagation times can be modeled in a unified way. The results are illustrated on a case study dealing with propagation of cracks in a thin walled tube submitted to a temperature gradient through its thickness. Different types of propagation/arrest regimes can be identified.

Commentary by Dr. Valentin Fuster


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