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J. Eng. Mater. Technol. 2016;138(3):031001-031001-11. doi:10.1115/1.4032559.

A comparison is presented between the sensitivity to measurement error of the crack compliance and layer removal methods of residual stress measurement when applied to glass fiber reinforced plastic (GFRP) pipes. This is done by adding random scatter to the exact strain distribution associated with a known stress distribution. This defines strain data that simulate experimental measurements. These data are used to determine the corresponding residual stress distributions. The error in the residual stress distribution when scatter is included can thereby be determined. It is shown that the layer removal and crack compliance methods are equally suitable for the measurement of axial and circumferential stresses in a pipe wound at only ±55 deg. The layer removal method, however, is shown to have significantly lower sensitivity to measurement error when the axial residual stresses in layered GFRP pipes are considered.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031002-031002-11. doi:10.1115/1.4032486.

A crystal-plasticity-based damage model that incorporates material length-scale through use of the slip-plane lattice incompatibility is developed with attention to the physical basis for the evolution of damage in a “bulk” shear deformation and without resort to ad hoc measures of shear deformation. To incorporate the physics of the shear damage process recently found by Kweon et al. (2010, “Experimental Characterization of Damage Processes in Aluminum AA2024-O,” ASME J. Eng. Mater. Technol., 132(3), p. 031008), the development of tensile hydrostatic stress in grains due to grain-to-grain interaction, two existing theories, crystal plasticity, and the void growth equation by Cocks and Ashby (1982, “On Creep Fracture by Void Growth,” Prog. Mater. Sci., 27(3–4), pp. 189–244) is combined to make the model in this study. The effect of the void volume increase onto the constitutive behavior is incorporated by adding the deformation gradient due to the void volume growth into a multiplicatively decomposed kinematics map. Simulations with the proposed model reveal the physics of shear and reproduce the accelerated damage in the shear deformation in lab experiments and industrial processes: the gradient of hydrostatic stress along with the development of macroscopic normal stress (hydrostatic stress) components amplifies the development of the local hydrostatic stress in grains under tensile hydrostatic stress.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031003-031003-5. doi:10.1115/1.4032560.

Tensile flow behavior of 9Cr–2WVTa ferritic/martensitic (RAFM) steel in normalized-tempered condition has been studied based on Voce equation over the temperature range of 25–600 °C. Yield strength (YS) and ultimate tensile strength (UTS) decrease with increase in temperature. However, the elongation decreases with increase in temperature up to 400 °C and then increases beyond 400 °C. True stress–true plastic strain curves at all temperatures are adequately described by the Voce equation. While saturation stress (σs) decreases with temperature, the rate at which the stress approaches the saturation value (nV) increases with temperature. The variation of the stress increment up to saturation stress (σun) with temperature shows a plateau in the temperature range of 200–400 °C. Moreover, the product of σun and nVun·nV) is inversely proportional to the elongation. The relation of elongation to σun·nV can be described by a power law with the exponent of −1.63.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031004-031004-11. doi:10.1115/1.4032661.

This paper discusses the application of a characteristic strain model (CSM) to analyze the creep behavior of rotating components. First, simple cylinders are analyzed at variable loads and different model constants. A closed-form analytical solution for the steady-state stress and the location of the skeletal point in the rotating solid cylinder are obtained. Then, the hollow cylinder behavior is investigated by numerical analysis, and the skeletal point location is shown to be independent of the applied load. Finally, a numerical creep analysis of a steam turbine rotor is carried out with a detailed examination of the stress and creep strain fields in the rotor disk. The existence of multiple skeletal points in the rotor disk, as well as the independence of their locations of the creep data, is shown.

Topics: Creep , Stress , Cylinders , Disks
Commentary by Dr. Valentin Fuster

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