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Research Papers

J. Eng. Mater. Technol. 2015;137(3):031001-031001-14. doi:10.1115/1.4029908.

The predictive capability of the Sehitoglu–Boismier unified constitutive and life model for Mar-M247 Ni-base superalloy is extended from a maximum temperature of 871 °C to 1038 °C. The unified constitutive model suitable for thermomechanical loading is adapted and calibrated using the response from isothermal cyclic experiments conducted at temperatures from 500 °C to 1038 °C at different strain rates with and without dwells. The flow rule function and parameters as well as the temperature dependence of the evolution equation for kinematic hardening are established. Creep and stress relaxation are critical to capture in this elevated temperature regime. The coarse-grained polycrystalline microstructure exhibits a high variability in the predicted cyclic response due to the variation in the elastic response associated with the orientation of the crystallographic grains. The life model accounts for fatigue, creep, and environmental damage under both isothermal and thermomechanical fatigue (TMF).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031002-031002-8. doi:10.1115/1.4030004.

The temperature and concentration play an important role on rheological parameters of the gel. In this work, an experimental investigation of thermorheological properties of aqueous gel Carbopol Ultrez 20 for various concentrations and temperatures has been presented. Both controlled stress ramps and controlled stress oscillatory sweeps were performed for obtaining the rheological data to find out the effect of temperature and concentration. The hysteresis or thixotropic seemed to have negligible effect. Yield stress, consistency factor, and power law index were found to vary with temperature as well as concentration. With gel concentration, the elastic effect was found to increase whereas viscous dissipation effect was found to decrease. Further, the change in elastic properties was insignificant with temperature in higher frequency range of oscillatory stress sweeps.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031003-031003-8. doi:10.1115/1.4030066.

Surface enhancement techniques such as shot peening are extensively used to increase the fatigue life of components in gas turbine engines. Due to the combined thermomechanical nature of the loading encountered within an engine, aeroengine designers have avoided incorporating the beneficial effects in their analysis. This can lead to overdesign and early retirement of critical engine components. A finite element modeling procedure is introduced that incorporates the shot peening residual stresses on a fir-tree turbine disk assembly. Unlike traditional equivalent loading approaches, the method models the actual impact of shots on the assembly and is the first time this approach is used to introduce peening residual stresses in turbine disks. In addition, the stability of these residual stresses in response to cyclic thermomechanical loadings at the contact interface is also studied. The results reveal that thermomechanical overload can nearly fully relax the shot peening residual stresses within the first cycle due to the combined effects of decreased material yield strength and plastic deformation. This work will enable aeroengine designers to assess critical surface treated components for structural integrity, optimal design, and residual life.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031005-031005-9. doi:10.1115/1.4030196.

In this study, sonication dispersion technique was employed to infuse 0.1–0.4 wt.% carbon nanofibers (CNFs) into polyester matrix to enhance thermomechanical properties of resulting nanocomposites. The effect of dispersion conditions has been investigated with regard to the CNF content and the sonication time. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) micrographs revealed excellent dispersion of 0.2 wt.% CNF infused in polyester, resulting in enhanced mechanical responses. Polyester with 0.2 wt.% CNF samples resulted in 88% and 16% increase in flexural strength and modulus, respectively, over the neat one. Quasi-static compression tests showed similar increasing trend with addition of CNF. Fracture morphology study of tested samples revealed relatively rougher surface in CNF-loaded polyester compared to the neat due to better interaction between the fiber and the matrix. Dynamic mechanical analysis (DMA) study exhibited about 35% increase in the storage modulus and about 5 °C increase in the glass transition temperature (Tg). A better thermal stability in the CNF-loaded polyester was observed from the thermogravimetric analysis (TGA) studies. Best results were obtained for the 0.2 wt.% CNF loading with 90 mins of sonication and 50% sonication amplitude. It is recommended that this level of sonication facilitates suitable dispersion of the CNF into polyester matrices without destroying the CNF's structure.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031006-031006-8. doi:10.1115/1.4030339.

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(6), pp. 433–455), Yang et al. (2014, “Time-Independent Plasticity Related to Critical Point of Free Energy Function and Functional,” ASME J. Eng. Mater. Technol., 136(2), p. 021001) established a model of time-independent plasticity of three states. In this model, equilibrium states are the states with vanishing thermodynamic forces conjugate to the internal variables, and correspond to critical points of the free energy or its complementary energy functions. Then, the conjugate forces play a role of yield functions and further lead to the consistency conditions. The model is further elaborated in this paper and extended to nonisothermal processes. It is shown that the incremental stress–strain relations are fully determined by the local curvature of the free energy or its complementary energy functions at the critical points, described by the Hessian matrices. It is further shown that the extended model can be well reformulated based on the intrinsic time in the sense of Valanis (1971, “A Theory of Viscoplasticity Without a Yield Surface, Part I. General Theory,” Arch. Mech., 23(4), pp. 517–533; 1975, “On the Foundations of the Endochronic Theory of Viscoplasticity,” Arch. Mech., 27(5–6), pp. 857–868), by taking the intrinsic time as the accumulated length of the variation of the internal variables during inelastic processes. It is revealed within this framework that the stability condition of equilibrium directly leads to Drucker (1951, “A More Fundamental Approach to Stress–Strain Relations,” First U.S. National Congress of Applied Mechanics, pp. 487–497) and Il'yushin (1961, “On a Postulate of Plasticity,” J. Appl. Math. Mech., 25(2), pp. 746–750) inequalities, by introducing the consistency condition into the work of Hill and Rice (1973, “Elastic Potentials and the Structure of Inelastic Constitutive Laws,” SIAM J. Appl. Math., 25(3), pp. 448–461). Generalized inequalities of Drucker (1951, “A More Fundamental Approach to Stress–Strain Relations,” First U.S. National Congress of Applied Mechanics, pp. 487–497) and Il'yushin (1961, “On a Postulate of Plasticity,” J. Appl. Math. Mech., 25(2), pp. 746–750) for nonisothermal processes are established straightforwardly based on the connection.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031007-031007-5. doi:10.1115/1.4030340.

In the present investigation, the applicability of a previously developed closed form energy based framework to predict low cycle fatigue (LCF) life of aluminum 6061-T6 was extended from room temperature to elevated temperature. The three different elevated temperatures considered in the present investigation were 75 °C, 100 °C, and 125 °C which were below the creep activation temperature for aluminum 6061-T6. Like the room temperature life assessment framework, the elevated temperature life assessment framework involved computation of the Ramberg–Osgood cyclic parameters from the average plastic strain range and the average plastic energy obtained from an axial isothermal-mechanical fatigue (IMF) test. The temperature dependent cyclic parameters were computed for 25 °C (room temperature), 75 °C, and 100 °C and then extrapolated to 125 °C utilizing functions describing the dependence of the cyclic parameters on temperature. For aluminum 6061-T6, the cyclic parameters were found to decrease with increase of temperature in a quadratic fashion. Furthermore, the present energy based axial IMF framework was found to be able to predict the LCF life of aluminum 6061-T6 at both room and elevated temperatures with excellent accuracy.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031008-031008-7. doi:10.1115/1.4030356.

Improving yield strength and asymmetry is critical to expand applications of magnesium alloys in industry for higher fuel efficiency and lower CO2 production. Grain refinement is an efficient method for strengthening low symmetry magnesium alloys, achievable by precipitate refinement. This study provides guidance on how precipitate engineering will improve mechanical properties through grain refinement. Precipitate refinement for improving yield strengths and asymmetry is simulated quantitatively by coupling a stochastic second phase grain refinement model and a modified polycrystalline crystal viscoplasticity φ-model. Using the stochastic second phase grain refinement model, grain size is quantitatively determined from the precipitate size and volume fraction. Yield strengths, yield asymmetry, and deformation behavior are calculated from the modified φ-model. If the precipitate shape and size remain constant, grain size decreases with increasing precipitate volume fraction. If the precipitate volume fraction is kept constant, grain size decreases with decreasing precipitate size during precipitate refinement. Yield strengths increase and asymmetry approves to one with decreasing grain size, contributed by increasing precipitate volume fraction or decreasing precipitate size.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031009-031009-7. doi:10.1115/1.4030430.

The corrosion behavior of 9Cr ferritic–martensitic heat-resistant steel was investigated in water and chloride environment at room temperature (RT). The results of linear polarization, electrochemical impedance spectroscopy (EIS), and potentiodynamics (PD) polarization tests on long-term exposure show that 9Cr ferritic–martensitic steel has weaker corrosion resistance and greater pitting corrosion tendency in higher chloride concentrations. Corresponding scanning electron microscopy (SEM) observation displays that higher concentration chloride promotes the pitting initiation. During long-term exposure, pitting susceptibility decreases, the average pit size increases, and the density declines in higher chloride concentrations. Pits in the grains and along the grain boundaries are observed by optical microscope (OM), and it indicates that inclusions in grains and carbide particles at grain boundaries are the sites susceptible to pitting initiation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031010-031010-6. doi:10.1115/1.4030412.

Vanadium pentoxide (V2O5) was chosen as a sintering aid to lower the sintering temperature of the ZnO–TiO2 system. The effect of V2O5 on the sintering behavior and material properties of ZnO–TiO2 ceramics and cermets made of ZnO–TiO2 ceramics and copper (Cu) was investigated as a function of V2O5 percentage and sintering temperature. Densities and hardness of the specimens were improved with an increase of V2O5 up to 2 wt. %. The sintering temperature of the specimens can be reduced to below 1000 °C. The properties of ZnO–TiO2 ceramics and cermets made from ZnO–TiO2 ceramics and Cu with V2O5 are strongly dependent on the sintering temperature. The density of ZnO–TiO2 ceramics and cermets was increased up to 95%, 90% of theoretical density at 900–920 °C, 960–1000 °C, respectively, for 4 hrs.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):031011-031011-7. doi:10.1115/1.4030413.

Reactively bonded solder joints with Al/Ni exothermic films attract much attention in semiconductor and microelectromechanical systems (MEMS) industries. Higher bond strength of the joints is required for long-term mechanical reliability. We have investigated the strength of rectangular-solid single crystal silicon (SCS) specimens with reactively bonded Sn-3.5Ag solder joint by using specially developed four-point bending test equipment. In this paper, the influences of Al/Ni exothermic film thickness and metallic interlayer on the strength are discussed. The strength increases with increasing Al/Ni film thickness and pressure load during bonding. Metallic interlayer between the solder and SCS also affects the strength because fracture origin is dependent on the types of metals. The obtained results suggest that reacted NiAl is durable against external forces compared with the solder and interlayer.

Commentary by Dr. Valentin Fuster

Discussion

J. Eng. Mater. Technol. 2015;137(3):035501-035501-2. doi:10.1115/1.4030341.

The paper by Ladani et al. [1] presents an investigation of the mechanical properties, including anisotropy and strain rate sensitivity, of Ti6Al4V test specimens fabricated using the electron beam melting (EBM) additive manufacturing method. The authors use tensile testing and nanoindentation tests on fabricated Ti6Al4V specimens of different build orientations. Analyses of the results include calculating strain rate sensitivity, examining fracture surfaces of the sample pieces, and interpreting nanoindentation data. This discussion of the paper [1] highlights and critiques some key elements of the work, namely, the build orientations used to assess mechanical anisotropy, the tensile testing procedures and reporting, and the strain rate sensitivity observations, as mentioned in Ref. [2].

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(3):035502-035502-2. doi:10.1115/1.4030357.

According to classical mechanics definition, if the material properties are the same in all direction, material is called isotropic. The word isotropy means uniformity in all orientations and it is derived from the Greek isos “equal” and tropos “way.” Consequently, if material properties are not isotropic, they are called anisotropic. This is a general definition of isotropy. What Pettit et al. [1] in their discussion mean most likely is what is so called “plastic anisotropy,” using Lankford coefficient, which is used to evaluate the distortion in yield surface in metal forming operations and is used for materials that have experienced large plastic deformation during a process. This is a measure of plastic anisotropy, rather than a general measurement of variation of properties in different orientations.

Commentary by Dr. Valentin Fuster

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