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Research Papers

J. Eng. Mater. Technol. 2013;135(4):041001-041001-5. doi:10.1115/1.4024395.

Instrumented indentation is commonly used for determining mechanical properties of a range of materials, including viscoelastic materials. However, most—if not all—studies are limited to a flat substrate being indented by various shaped indenters (e.g., conical or spherical). This work investigates the possibility of extending instrumented indentation to nonflat viscoelastic substrates. In particular, conical indentation of a sphere is investigated where a semi-analytical approach based on “the method of functional equations” has been developed to obtain the force–displacement relationship. To verify the accuracy of the proposed methodology selected numerical experiments have been performed and good agreement was obtained. Since it takes significantly less time to obtain force–displacement relationships using the proposed method compared to conducting full finite element simulations, the proposed method is an efficient substitute of the finite element method in determining material properties of viscoelatic spherical particles using indentation testing.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(4):041002-041002-8. doi:10.1115/1.4024545.

In this paper, the effect of surface damage induced by focused ion beam (FIB) fabrication on the mechanical properties of silicon (Si) nanowires (NWs) was investigated. Uniaxial tensile testing of the NWs was performed using a reusable on-chip tensile test device with 1000 pairs of comb structures working as an electrostatic force actuator, a capacitive displacement sensor, and a force sensor. Si NWs were made from silicon-on-nothing (SON) membranes that were produced by deep reactive ion etching hole fabrication and ultrahigh vacuum annealing. Micro probe manipulation and film deposition functions in a FIB system were used to bond SON membranes to the device's sample stage and then to directly fabricate Si NWs on the device. All the NWs showed brittle fracture in ambient air. The Young's modulus of 57 nm-wide NW was 107.4 GPa, which was increased to 144.2 GPa with increasing the width to 221 nm. The fracture strength ranged from 3.9 GPa to 7.3 GPa. By assuming the thickness of FIB-induced damage layer, the Young's modulus of the layer was estimated to be 96.2 GPa, which was in good agreement with the literature value for amorphous Si.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(4):041003-041003-10. doi:10.1115/1.4024394.

Uniaxial tension tests were conducted on thin commercially pure (CP) titanium sheets subjected to electrically assisted deformation using a new experimental setup to decouple thermal–mechanical and possible electroplastic behavior. The observed absence of stress reductions for specimens air-cooled to near room temperature motivated the need to reevaluate the role of temperature on modeling the plastic behavior of metals subjected to electrically assisted deformation, an item that is often overlooked when invoking electroplasticity theory. As a result, two empirical constitutive models, a modified-Hollomon and the Johnson–Cook models of plastic flow stress, were used to predict the magnitude of stress reductions caused by the application of constant dc current and the associated Joule heating temperature increase during electrically assisted tension experiments. Results show that the thermal–mechanical coupled models can effectively predict the mechanical behavior of commercially pure titanium in electrically assisted tension and compression experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(4):041004-041004-9. doi:10.1115/1.4024791.

Materials with customized spatial gradients in mechanical properties are increasingly used in high performance applications requiring enhanced resistance to contact loads, wear, and fatigue. In many engineering materials, multiple property and microstructural gradients may occur simultaneously with depth. In this manuscript, two case carburized steels are analyzed for their gradient in hardness with depth, emphasizing the resulting variation in surface hardness under increasing indentation loads. A parametric study using finite element analysis is then conducted in order to characterize the influence of individual property gradients on the surface indentation response of graded materials. It is shown that the measured surface hardness value decreases rapidly under increasing surface indentation loads in materials with sharp negative hardness gradients. It is also shown that this trend is independent of the magnitude of the strain hardening exponent of the material, as well as the gradient in strain hardening exponent. Gradients in elastic properties were also shown to have negligible influence on surface hardness trends for a fixed gradient in hardness. Finally, it is revealed that the depth of subsurface plastic deformation increases with sharper gradients in hardness, while being insensitive to changes in strain hardening exponent. For elastically graded materials, a decreasing gradient in elastic modulus limits the depth of plastic deformation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(4):041005-041005-9. doi:10.1115/1.4025171.

The Q-state Monte Carlo, Potts model is used to investigate 2D, anisotropic, grain growth of single-phase materials subject to temperature gradients. Anisotropy is simulated via the use of nonuniform grain boundary surface energies, and thermal gradients are simulated through the use of variable grain boundary mobilities. Hexagonal grain elements are employed, and elliptical Wulff plots are used to assign surface energies to grain lattices. The mobility is set to vary in accordance with solutions to a generalized heat equation and is solved for two separate values of the mobility coefficient. Among other findings, the results reveal that like isotropic grain growth, under the influence of a thermal gradient, anisotropic grain growth also demonstrates locally normal growth kinetics.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(4):041006-041006-9. doi:10.1115/1.4025192.

In this paper, a new small-sized (two-bar) specimen type, which is suitable for use in obtaining both uniaxial creep strain and creep rupture life data, is described. The specimen has a simple geometry and can be conveniently machined and loaded (through pin-connections) for testing. Conversion relationships between the applied load and the corresponding uniaxial stress, and between the measured load-line deformations and the corresponding uniaxial minimum creep strain rate, have been obtained, based on the reference stress method (RSM), in conjunction with finite element analyses. Using finite element analyses the effects of the specimen dimensions on reference stress parameters have been investigated. On this basis, specimen dimension ratio ranges are recommended. The effects of friction, between the loading pins and the specimen surfaces, on the specimen failure time, are also investigated. Test results obtained from two-bar specimen tests and from corresponding uniaxial specimen tests, for a P91 steel at 600 °C, are used to validate the test method. These results demonstrated that the specimen type is capable of producing full uniaxial creep strain curves. The advantages of this new, small, creep test specimen, for determining uniaxial creep data, are discussed and recommendations for future research are given.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(4):041007-041007-9. doi:10.1115/1.4025319.

Tension–tension fatigue tests were conducted on an electrodeposited copper film with a thickness of 12 μm under four levels of maximum stress and two levels of mean stress. Statistical characteristics of the measured fatigue lives were analyzed using three estimation methods for cumulative distribution function and five probability distributions in order to identify the dominant probability distribution for the fatigue life of copper film. It was found that while the 3-parameter Weibull distribution provided the best fit for the measured data in most cases, the other distributions also provide a similar coefficient of correlation for the fit. The absence of the dominant probability distribution was discussed with considerations of the deformation mode and the scanning electron microscope (SEM) measurements of fatigue-fractured surfaces. Based on the statistical analysis, the probabilistic stress-life (PSN) curves were obtained for statistical prediction of fatigue life of the copper film in the intermediate life regime.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(4):041008-041008-13. doi:10.1115/1.4025292.

A microstructure-based internal state variable (ISV) plasticity-damage model was used to model the mechanical behavior of a porous FC-0205 steel alloy that was procured via a powder metal (PM) process. Because the porosity was very high and the nearest neighbor distance (NND) for the pores was close, a new pore coalescence ISV equation was introduced that allows for enhanced pore growth from the concentrated pores. This coalescence equation effectively includes the local stress interaction within the interpore ligament distance between pores and is physically motivated with these highly porous powder metals. Monotonic tension, compression, and torsion tests were performed at various porosity levels and temperatures to obtain the set of plasticity and damage constants required for model calibration. Once the model calibration was achieved, then tension tests on two different notch radii Bridgman specimens were undertaken to study the damage-triaxiality dependence for model validation. Fracture surface analysis was performed using scanning electron microscopy (SEM) to quantify the pore sizes of the different specimens. The validated model was then used to predict the component performance of an automotive PM bearing cap. Although the microstructure-sensitive ISV model has been employed for this particular FC-0205 steel, the model is general enough to be applied to other metal alloys as well.

Commentary by Dr. Valentin Fuster

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