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### Research Papers

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
J. Eng. Mater. Technol. 2016;138(3):031005-031005-13. doi:10.1115/1.4033070.

In this paper, a multiscale approach has been developed for investigating the rate-dependent viscoplastic behavior of polymer matrix composites (PMCs) with thermal residual stress effect. The finite-volume direct averaging micromechanics (FVDAM), which effectively predicts nonlinear response of unidirectional fiber reinforced composites, is incorporated with improved Bodner–Partom model to describe the viscoplastic behavior of PMCs. The new micromechanical model is then implemented into the classical laminate theory, enabling efficient and accurate analysis of multidirectional PMCs. The proposed multiscale theory not only predicts effective thermomechanical viscoplastic response of PMCs but also provides local fluctuations of fields within composite microstructures. The deformation behaviors of several unidirectional and multidirectional PMCs with various fiber configurations are extensively simulated at different strain rates, which show a good agreement with the experimental data found from the literature. Influence of thermal residual stress on the viscoplastic behavior of PMCs is closely related to fiber orientation. In addition, the thermal residual stress effect cannot be neglected in order to accurately describe the rate-dependent viscoplastic behavior of PMCs.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031006-031006-6. doi:10.1115/1.4032849.

This study investigates the influence of titanium (Ti) and magnesium (Mg) additions on aluminum (Al) alloys in order to evaluate the relationship between the structure and properties of the new alloys. The alloys obtained at elevated temperatures mainly consist of Al–2Mg–1Ti, Al–2Mg–3Ti, Al–4Mg–2Ti, and Al–6Mg–2Ti alloys, as well as α and τ solid solution phases of intermetallic structures. Microstructural analyses were performed using X-ray diffraction (XRD), optical microscope, and energy dispersive spectrometry (EDS) techniques. Test results show that the average grain size of the alloys decreased with the addition of Ti inclusions during the casting and solidification processes, and the smallest grain size was found to be 90 μm for the Al–6Mg–3Ti alloy. In addition, tensile properties of the Al–Mg–Ti alloys were initially improved and then worsened after the addition of higher concentrations of Ti. The highest tensile and hardness values of the alloys were Al–4Mg–2Ti (205 MPa) and Al–6Mg–3Ti (80 BHN). The primary reasons for having higher mechanical properties may be attributed to strengthening of the solid solution and refinement of the grain size and shape during the solidification process. For this study, the optimum concentrations of Ti and Mg added to the Al alloys were 4 and 2 wt.%, respectively. This study may be useful for field researchers to develop new classes of Al alloys for various industrial applications.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031007-031007-11. doi:10.1115/1.4032882.

An optically transparent woven glass fiber-reinforced polyester composite has been fabricated. This composite has been used as an interlayer in the fabrication of a laminated glass-composite window panel for application in blast-resistant windows. The transparency of the glass fiber-reinforced composite was achieved by matching the refractive index of the polyester matrix with that of glass fibers. Various chemical additives have been investigated for their effectiveness in modifying the refractive index of the polyester matrix. The composite interlayer's mechanical properties under both quasi-static and dynamic loading conditions have been characterized in this study. The window panels were tested under various blast loading conditions. The panels perform well under U.S. General Services Administration (GSA) specified C, D, and E blast loading levels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031008-031008-12. doi:10.1115/1.4032749.

This work presents a hyperviscoelastic model, based on the Hencky-logarithmic strain tensor, to model the response of a tire derived material (TDM) undergoing moderately large deformations. The TDM is a composite made by cold forging a mix of rubber fibers and grains, obtained by grinding scrap tires, and polyurethane binder. The mechanical properties are highly influenced by the presence of voids associated with the granular composition and low tensile strength due to the weak connection at the grain–matrix interface. For these reasons, TDM use is restricted to applications involving a limited range of deformations. Experimental tests show that a central feature of the response is connected to highly nonlinear behavior of the material under volumetric deformation which conventional hyperelastic models fail in predicting. The strain energy function presented here is a variant of the exponentiated Hencky strain energy, which for moderate strains is as good as the quadratic Hencky model and in the large strain region improves several important features from a mathematical point of view. The proposed form of the exponentiated Hencky energy possesses a set of parameters uniquely determined in the infinitesimal strain regime and an orthogonal set of parameters to determine the nonlinear response. The hyperelastic model is additionally incorporated in a finite deformation viscoelasticity framework that accounts for the two main dissipation mechanisms in TDMs, one at the microscale level and one at the macroscale level. The new model is capable of predicting different deformation modes in a certain range of frequency and amplitude with a unique set of parameters with most of them having a clear physical meaning. This translates into an important advantage with respect to overcoming the difficulties related to finding a unique set of optimal material parameters as are usually encountered fitting the polynomial forms of strain energies. Moreover, by comparing the predictions from the proposed constitutive model with experimental data we conclude that the new constitutive model gives accurate prediction.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031009-031009-13. doi:10.1115/1.4033036.

A dislocation density-based crystal plasticity framework, a nonlinear computational finite-element methodology adapted for nucleation of crack on cleavage planes, and rational crystallographic orientation relations were used to predict the failure modes associated with the high strain rate behavior of aluminum-bonded composites. A bonded aluminum composite, suitable for high strain-rate damage resistance application, was modeled with different microstructures representing precipitates, dispersed particles, and grain boundary (GB) distributions. The dynamic fracture approach is used to investigate crack nucleation and growth as a function of the different microstructural characteristics of each alloy in bonded composites with and without pre-existing cracks. The nonplanar and irregular nature of the crack paths were mainly due to the microstructural features, such as precipitates and dispersed particles distributions and orientations, ahead of the crack front. The evolution of dislocation density and the subsequent formation of localized plastic slip contributed to the blunting of the propagating crack(s). Extensive geometrical and thermal softening resulted in localized plastic slip and had a significant effect on crack path orientations and directions along cleavage planes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031010-031010-4. doi:10.1115/1.4033071.

Modern techniques of severe plastic deformation (SPD) used as a means for grain refinement in metallic materials rely on simple shear as the main deformation mode. Prediction of the mechanical properties of the processed materials under tensile loading is a formidable task as commonly no universal, strain path independent constitutive laws are available. In this paper, we derive an analytical relation that makes it possible to predict the mechanical response to uniaxial tensile loading for a material that has been preprocessed by simple shear and, as a result, has developed a linear strain gradient. A facile recipe for mechanical tests on solid bars required for this prediction to be made is proposed. As a trial, it has been exercised for the case of commercial purity copper rods. The method proposed is recommended for design with metallic materials that underwent preprocessing by simple shear.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031011-031011-10. doi:10.1115/1.4033033.

In this study, a new magnesium (Mg) alloy containing 0.4% Ce was developed using the technique of disintegrated melt deposition followed by hot extrusion. The tensile and compressive properties of the developed Mg–0.4Ce alloy were investigated before and after heat treatment with an intention of understanding the influence of cerium on the deformation and corrosion of magnesium. Interestingly, cerium addition has enhanced the strength (by 182% and 118%) as well as the elongation to failure of Mg (by 93% and 8%) under both tensile and compressive loadings, respectively. After heat treatment, under compression, the Mg–0.4Ce(S) alloy exhibited extensive plastic deformation which was 80% higher than that of the as-extruded condition. Considering the tensile and compressive flow curves, the as-extruded Mg–0.4Ce and the heat treated Mg–0.4Ce(S) alloys exhibited variation in the nature and shape of the curves which indicates a disparity in the tensile and compressive deformation behavior. Hence, these tensile and compressive deformation mechanisms were studied in detail for both as-extruded as well as heat treated alloys with the aid of microstructural characterization techniques (scanning electron microscope (SEM), transmission electron microscope (TEM), selective area diffraction (SAD), and X-ray diffraction (XRD) analysis. Furthermore, results of immersion tests of both as-extruded and heat treated alloys revealed an improved corrosion resistance (by ∼3 times in terms of % weight loss) in heat treated state vis-a-vis the as-extruded state.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031012-031012-8. doi:10.1115/1.4033072.

A new multiscale modeling approach is proposed to predict the contributions of dynamic strain aging (DSA) and the resulting negative strain rate sensitivity (NSRS) on the unusual strain-hardening response of Hadfield steel (HS). Mechanical response of HS was obtained from monotonic and strain rate jump experiments under uniaxial tensile loading within the 10−4 to 10−1 s−1 strain rate range. Specifically, a unique strain-hardening model was proposed that incorporates the atomic-level local instabilities imposed upon by the pinning of dislocations by diffusing carbon atoms to the classical Voce hardening. The novelty of the current approach is the computation of the shear stress contribution imposed on arrested dislocations leading to DSA at the atomic level, which is then implemented to the overall strain-hardening rule at the microscopic level. The new model not only successfully predicts the role of DSA and the resulting NSRS on the macroscopic deformation response of HS but also opens the venue for accurately predicting the deformation response of rate-sensitive metallic materials under any given loading condition.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031013-031013-8. doi:10.1115/1.4033275.

This paper reports the results obtained in a study on the effect of the addition of TiO2 nanoparticles on the mechanical properties and microstructural characteristics of photocatalytic concretes. In the hardened state, tests to determine the compressive strength and modulus of elasticity were carried out. Also, microstructural aspects of the samples were investigated. In the fresh state, the influence of the addition of TiO2 on the concrete compaction and conduction calorimetry curves was evaluated. The results obtained indicated a better mechanical and microstructural behavior of concrete with addition of TiO2.

Topics: Concretes
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031014-031014-18. doi:10.1115/1.4033283.

Many mathematical models based on the advanced theory of bending to predict bending characteristics for monolithic sheet materials are available in the literature. In this work, a similar approach is utilized to develop bending models for a bilayer laminated sheet material. The principal stresses and strains through the thickness and change in relative thickness, at specified bend curvatures, are obtained as a function of increasing curvature during bending. Additionally, three-dimensional (3D) finite element (FE) based models for bilayer laminate bending are developed to overcome simplifications of the analytical models. In order to experimentally validate the two models, a new experimental bend test-jig is developed and experiments are performed on bilayer steel–aluminum laminate for different clad to matrix thickness ratios. These experiments have enabled continuous measurements of strain along the width at the bend line and through the laminate thickness at one of the specimen edges using an online strain mapping system based on digital image correlation (DIC) method. Analytical model results indicate how the through-thickness strain distribution and relative thickness of the specimen in bending are influenced by the location and thickness of the soft clad material. The FE model and experimental results exhibit similar trends in the relative thickness change for different geometric arrangements of steel–aluminum layers. The tangential and radial stresses decrease in magnitude with increasing aluminum clad thickness ratios. The 3D FE model of laminate bending provided strain predictions across the specimen width at the bend line on the tension and compression sides that increased with increasing clad thickness ratios. Also, relative thickness data from the 3D FE model showed uniaxial and plane strain stress states at the edge and midwidth sections of the test specimen. The results from analytical and FE models and from DIC and microscopic thickness measurements indicate that thickness at the bend line increases with increasing clad thickness for the case of clad layer on the compressive side of the laminate (i.e., C-C case) and vice versa for clad layer on the tensile side (C-T).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;138(3):031015-031015-8. doi:10.1115/1.4033322.

In this study, we employed pressurized creep tubes to investigate the biaxial thermal creep behavior of Inconel 617 (alloy 617) and Haynes 230 (alloy 230). Both alloys are considered to be the primary candidate structural materials for very high-temperature reactors (VHTRs) due to their exceptional high-temperature mechanical properties. The current creep experiments were conducted at 900 °C for the effective stress range of 15–35 MPa. For both alloys, complete creep strain development with primary, secondary, and tertiary regimes was observed in all the studied conditions. Tertiary creep was found to be dominant over the entire creep lives of both alloys. With increasing applied creep stress, the fraction of the secondary creep regime decreases. The nucleation, diffusion, and coarsening of creep voids and carbides on grain boundaries were found to be the main reasons for the limited secondary regime and were also found to be the major causes of creep fracture. The creep curves computed using the adjusted creep equation of the form $ε=Aσcosh−1(1+rt)+Pσntm$ agree well with the experimental results for both alloys at the temperatures of 850–950 °C.

Topics: Creep , Alloys , Stress
Commentary by Dr. Valentin Fuster