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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):031004-031004-6. doi:10.1115/1.4030197.

Friction stir spot welding (FSSW) was performed on 2024-T3 Al alloy to study the effect of pin and shoulder geometry (cylindrical and triangular shapes) on stir zone (SZ) characteristics and mechanical properties of welded samples. The process was conducted at a constant dwell time of 5 s and shoulder plunge depth of 0.3 mm. The rotational speed varied in the range of 630–1600 rpm. Based on the observations, the change in the pin shape led to changes in the SZ area, hook geometry, and mechanical properties of the joints. At low rotational speeds, the strengths of the joints made by the triangular pin were higher than those formed by the cylindrical pin. On the other hand, the change in the shoulder geometry from cylindrical to triangular reduced the joint strengths. However, the effect of pin geometry on the strengths was more than the shoulder geometry. Moreover, the strengths were directly related to the SZ area. Scanning electron microscopy of the welded sections revealed formation of cracks in the SZ as a result of S phase local melting at the tool rotational speed of 1600 rpm.

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

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