Research Papers

J. Eng. Mater. Technol. 2014;136(2):021001-021001-9. doi:10.1115/1.4026232.

In this paper, time-independent plasticity is addressed within the thermodynamic framework with internal variables by Rice (1971, “Inelastic Constitutive Relations for Solids: An Internal Variable Theory and Its Application to Metal Plasticity,” J. Mech. Phys. Solids, 19, pp. 433–455). It is shown in this paper that the existence of a free energy function along with thermodynamic equilibrium conditions directly leads to associated flow rules. The time-independent inelastic behaviors can be fully determined by the Hessian matrix at the nondegenerate critical point of the free energy function. The normality rule of Hill and Rice (1973, “Elastic Potentials and the Structure of Inelastic Constitutive Laws,” SIAM J. Appl. Math., 25, pp. 448–461) or the Il'yushin (1961, “On a Postulate of Plasticity,” J. Appl. Math. Mech. 25, pp. 746–750) postulate is just a stability requirement of the thermodynamic equilibrium. The existence of a free energy functional which is not a direct function of the internal variables, along with thermodynamic equilibrium conditions also leads to associated flow rules. The time-independent inelastic behaviors with the free energy functional can be fully determined by the quasi Hessian matrix at the quasi critical point of the free energy functional. With the free energy functional, the thermodynamic forces conjugate to the internal variables are nonconservative and are constructed based on Darboux theorem. Based on the constructed nonconservative forces, it is shown that there may exist several possible thermodynamic equilibrium mechanisms for the thermodynamic system of the material sample. Therefore, the associated flow rules based on free energy functionals may degenerate into nonassociated flow rules. The symmetry of the conjugate forces plays a central role for the characteristics of time-independent plasticity.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(2):021002-021002-8. doi:10.1115/1.4026193.

Semisolid extrusion of metals involves extrusion of metallic alloys with a microstructure consisting of spherical solids in a liquid matrix. In this research, the effect of cooling rate during forward semisolid extrusion on microstructure and mechanical properties of 7075 aluminum was investigated. Semisolid microstructure was prepared according to the recrystallization and partial melting (RAP) method. Optimum semisolid temperature and holding time which were resulted in a suitable microstructure for specimens were determined at 580 °C for 10 min. Different cooling rates were applied during semisolid extrusion and the resulted mechanical properties were studied. Tensile properties of semisolid extruded rods in T6 condition were also compared with those of conventionally extruded specimen. The results indicate that utilizing optimum values of semisolid extrusion parameters, namely, temperature and time of heating as well as cooling rate severity, brings both the possibility to obtain mechanical properties of conventionally extruded specimens and to get advantages of semisolid forming technique. Experimental results also show that increment of cooling rate and extrusion pressure improves the mechanical properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(2):021003-021003-11. doi:10.1115/1.4026271.

Temperature-dependent crystal viscoplasticity models are ideal for modeling large-grained, directionally solidified Ni-base superalloys but are computationally expensive. This work explores the use of reduced-order models that are potentially more efficient with similar predictive capability of capturing temperature and orientation dependence. First, a transversely isotropic viscoplasticity model is calibrated to a directionally solidified Ni-base superalloy using the response predicted by a crystal viscoplasticity model. The unified macroscale model is capable of capturing isothermal and thermomechanical responses in addition to secondary creep behavior over the temperature range of 20–1050 °C. A second approach is an extreme reduced-order microstructure-sensitive constitutive model that uses an artificial neural network to provide a set of parameters that depend on orientation, temperature, and strain rate to give a first-order approximation of the material response using a simple constitutive model. This simple relationship is then used in a Neuber-type fatigue notch analysis to predict the local response.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(2):021004-021004-9. doi:10.1115/1.4025424.

A microstructure-based fatigue model is employed to predict fatigue damage in 4140 steel. Fully reversed, strain control fatigue tests were conducted at various strain amplitudes and scanning electron microscopy was employed to establish structure-property relations between the microstructure and cyclic damage. Fatigue cracks were found to initiate from particles near the free surface of the specimens. In addition, fatigue striations were found to originate from these particles and grew radially outward. The fatigue model used in this study captured the microstructural effects and mechanics of nucleation and growth observed in this ferrous metal. Good correlation of the number of cycles to failure between the experimental results and the model were achieved. Based on analysis of the mechanical testing, fractography and modeling, the fatigue life of the 4140 steel is estimated to comprise mainly of small crack growth in the low cycle regime and crack incubation in the high cycle fatigue regime.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(2):021005-021005-8. doi:10.1115/1.4026474.

In the present study, viscoelastic response of an active fiber composite (AFC) is investigated by conducting stress relaxation and creep deformation tests, and the quasi-linear viscoelastic (QLV) constitutive model is used to describe the viscoelastic response of the AFC. The AFC under study consists of unidirectional long piezoelectric ceramic fibers embedded in an epoxy polymer, encapsulated between two Kapton layers with interdigitated surface electrodes. The relaxation and creep experiments are performed by loading the AFC samples along the longitudinal axis of the fibers, under several strain and stress levels at three temperatures, namely 25 °C, 50 °C, and 75 °C. The experimental results reveal the nonlinear viscoelastic behavior of the composite. Next, simulation and prediction of the viscoelastic response, including stress relaxation and creep deformation of the material, are done by using semi-analytical QLV model in which a relaxation time-dependent function is used, which also depends on strain and temperature. The results from the model are compared with those from the experiments. In general, the experimental and simulation results are in good agreement, except in the case of some of the creep responses, where considerable discrepancies are seen between the experimental and analytical approaches. Possible reasons for these differences are discussed in details.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(2):021006-021006-10. doi:10.1115/1.4026596.

Full size creep test specimens, i.e., conventional uniaxial creep test specimen and Bridgman notch specimens are usually used to determine the full set of material constants for any creep model. However, in many situations, sufficient material is not available for theses specimens to be manufactured from it. Therefore, small creep test specimens have been introduced and used to determine (i) creep constants and (ii) the remaining life time for engineering components. Two commonly used small creep specimen types, i.e., the impression and the small ring creep tests, are used in this paper to determine the steady state creep constants. However, these specimen types are limited for use in determining the secondary creep properties, i.e., they are unable to determine the full set of material creep constants for creep damage models. In this paper the recently developed small two-bar creep test specimen and the newly developed small notched specimen test are described and used to determine a full set of material constants for Kachanov and Liu-Murakami creep damage models. The small notched specimen manufacturing, loading and testing procedures are described in this paper. P91 steel at 600 °C and (Bar-257) P91 steel at 650 °C have been used to compare the material constants obtained from the small two-bar and the small notched creep test specimens with those obtained from the conventional uniaxial creep test specimens and Bridgman notch specimens. The results show remarkably good agreement between the two sets of results.

Topics: Creep , Steel , Testing
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2014;136(2):021007-021007-6. doi:10.1115/1.4026617.

This paper evaluates the elastic stability and vibration characteristics of circular plates made from auxetic materials. By solving the general solutions for buckling and vibration of circular plates under various boundary conditions, the critical buckling load factors and fundamental frequencies of circular plates, within the scope of the first axisymmetric modes, were obtained for the entire range of Poisson's ratio for isotropic solids, i.e., from −1 to 0.5. Results for elastic stability reveal that as the Poisson's ratio of the plate becomes more negative, the critical bucking load gradually reduces. In the case of vibration, the decrease in Poisson's ratio not only decreases the fundamental frequency, but the decrease becomes very rapid as the Poisson's ratio approaches its lower limit. For both buckling and vibration, the plate's Poisson's ratio has no effect if the edge is fully clamped. The results obtained herein suggest that auxetic materials can be employed for attaining static and dynamic properties which are not common in plates made from conventional materials. Based on the exact results, empirical models were generated for design purposes so that both the critical buckling load factors and the frequency parameters can be conveniently obtained without calculating the Bessel functions.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Eng. Mater. Technol. 2014;136(2):024501-024501-5. doi:10.1115/1.4026838.

Carbon nanotube (CNT)-aluminum (Al) nanocomposites were prepared using friction stir welding (FSW) processing, and then the mechanical properties of these nanostructured materials were determined using the universal MTS machine. The fabrication of the CNT-metal composite consisted of the following steps: (a) homogenizing the CNTs and Al powder at three different ratios: 0/100, 25/75, and 50/50, (b) compacting the mixtures into grooves that were initially machined into the substrate (2024-T3) for the three cases, (c) incorporating CNTs in a substrate by the FSW process, and (d) validating the dispersion of the CNTs into the Al substrates after the characterization steps. Scanning electron microscopy (SEM) analysis and other physical characterization tests (e.g., mechanical, metallography, and fracture surfaces) were conducted on the prepared substrates. Test results showed that CNTs were dispersed and aligned uniquely in the different locations of the metal structures depending on the FSW zones: advancing, retreating, transverse, and stir zone regions. The mechanical properties of each zone were also compared to the distribution of CNTs. The advancing side had the highest amount of CNTs mixed into the aluminum substrate while retaining the yield strength (YS); however, the elongation was reduced. The retreating side had little to no CNTs distributed into the substrate and the mechanical properties were not significantly affected. The stir zone YS had little influence of the CNTs at the lower CNT/Al powder ratio (25/50), but a significant effect was noticed at the higher ratio of 50/50. The elongation to failure was significantly affected for both cases. The transverse zone YS and elongation to failure was significantly reduced by the powder mixtures. These results may open up new possibilities in the aircraft and other manufacturing industries for future development in the field.

Commentary by Dr. Valentin Fuster

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