Research Papers

J. Eng. Mater. Technol. 2014;136(4):041001-041001-10. doi:10.1115/1.4027857.

The prediction of temperature-dependent fatigue deformation and damage in directionally solidified and single-crystal nickel-base superalloy components used in the hot section of gas turbine engines requires a constitutive model that accounts for the crystal orientation in addition to the changing deformation mechanisms and rate dependencies from room temperature to extremes of the use temperature (e.g., 1050 °C). Crystal viscoplasticity (CVP) models are ideal for accounting for all of these dependencies. However, as the models become more physically realistic in capturing the true cyclic deformation mechanisms, increases the requirements to achieve an accurate model calibration. As a result, CVP models have yet to become viable for life analysis in industry. To make CVP models an industry relevant tool, the calibration times must be reduced. This paper explores methods to reduce the calibration time. First, a series of special calibration experiments are conceived and conducted on each relevant orientation and microstructure. Second, a set of parameterization protocols are used to minimize parameter interdependencies that reduce the amount of iteration required during the calibration. These experimental and calibration protocols are exercised using the CVP model of Shenoy et al. (2005, “Thermomechanical Fatigue Behavior of a Directionally Solidified Ni-Base Superalloy,” ASME J. Eng. Mater. Technol., 127(3), pp. 325–336) by calibrating a directionally solidified Ni-base superalloy across an industry relevant temperature range of 20 °C to 1050 °C.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(4):041002-041002-7. doi:10.1115/1.4028006.

It is the objective of this study to conduct realistic simulations of the arc-height development in shot-peened Almen strips using the finite element (FE) method. Unlike our earlier work which is devoted to relaxation of shot peening induced residual stress, in this paper, the focus is on peen forming as a result of repeated spherical impingement. Specifically, a 3D FE model with 1500 randomly distributed shots bombarding an Almen strip was developed. Strain rate dependent plasticity was considered and an artificial material damping was applied to control the undesired high-frequency oscillations. The solution further adopts both explicit dynamic and implicit quasi-static analyses to simulate the entire arc-height development in the Almen strips. Quantitative relationships between the resulting equivalent plastic strain and the associated residual stress distribution for a given shot velocity and shot numbers are established and discussed. The work also considers the effect of repeated impacts upon the induced residual stress field using a large number of random shots. Attention was further devoted to the effect of the strip constraint upon the outcome of the impingement. Our results indicate that the proposed FE model is a powerful tool in investigating the underlying mechanisms of the peening treatment.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(4):041003-041003-36. doi:10.1115/1.4028121.

The recently growing demand for production and applications of microscale devices and systems has motivated research on the behavior of small volume materials. The computational models have become one of great interests in order to advance the manufacturing of microdevices and to reduce the time to insert new product in applications. Among the various numerical and computational techniques, still the approaches in the context of continuum theories are more preferable due to their minimum computational cost to simulation on realistic time and material structures. This paper reviews the methods to address the thermal and mechanical responses of microsystems. The focus is on the recent developments on the enhanced continuum theories to address the phenomena such as size and boundary effects as well as microscale heat transfer. The thermodynamic consistency of the theories is discussed and microstructural mechanisms are taken into account as physical justification of the framework. The presented constitutive model is calibrated using an extensive set of microscale experimental measurements of thin metal films over a wide range of size and temperature of the samples. An energy based approach is presented to extract the first estimate of the interface model parameters from results of nanoindentation test.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(4):041004-041004-8. doi:10.1115/1.4028336.

The weight of a bipolar plate is one of the most crucial properties from the viewpoint of improving the power density of a proton-exchange membrane fuel cell (PEMFC) stack. Aluminum alloys have good material characteristics such as low electrical resistivity, high thermal conductivity, and low density. Furthermore, they are less expensive and easily machinable compared to graphite when used for fabricating bipolar plates. In this study, the use of AA5052 for fabricating a bipolar plate was investigated. The results of the feasibility experiments conducted to develop fuel cells with AA5052 bipolar plates having multiple microchannels were presented. The formability of microchannels under various types of pulsating loads was estimated for different punch loads and die radii using 0.3 mm thick AA5052 sheets. For a 0.1 mm die radius, the optimum formability was obtained for five cycles of sine wave dynamic loading with a maximum load of 90 kN. The experimental results demonstrated the feasibility of the proposed technique for fabricating bipolar plates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(4):041005-041005-7. doi:10.1115/1.4028256.

Anisotropic strain-hardening behavior of the TX51 (Mg–5Sn–1Ca) magnesium alloy sheets was investigated in the temperature range of 25–300 °C and at an initial strain rate of 5 × 10−4 s−1. Tensile tests were carried out with the loading axis oriented at 0 deg, 45 deg, and 90 deg to the rolling direction (RD) to explore the effects of temperature on the anisotropic strain-hardening behavior of the sheets after hot rolling and annealing. The anisotropic strain-hardening behavior of the TX51 sheet was due to the crystallographic texture as well as mechanical fibering of the microstructure. The former was manifested by the development of a relatively sharp basal {0001} texture, and the latter was caused by alignment in the RD of CaMgSn coarse particles. Kocks–Mecking type plots showed stage III and stage IV strain-hardening behavior at all test temperatures. The directionality of flow stress and initial strain-hardening rates in stage III were discussed based on the Schmid factors of material.

Commentary by Dr. Valentin Fuster

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