J. Eng. Mater. Technol. 2013;136(1):010201-010201-1. doi:10.1115/1.4026012.
Commentary by Dr. Valentin Fuster

Research Papers

J. Eng. Mater. Technol. 2013;136(1):011001-011001-14. doi:10.1115/1.4025425.

Accurate modeling of workpiece material behavior in machining is critical to analyze and design a process. The workpiece material behavior in the machining process involves dynamic flow stress and damage/fracture behavior, which are very difficult to be determined. In this study, the extended split Hopkinson pressure bar (SHPB) test is conducted to determine the dynamic flow stress curves of 7075-T651 aluminum alloy, which enables the strain, strain rate and the temperature obtained in the test to approach that in the cutting condition. A damage criterion under the typical stress state of orthogonal cutting is established to reflect the material damage initiation in primary shear zone. The damage criterion parameters of 7075-T651 alloy are determined by comparing the numerical and experimental results of the proposed inner high-pressure piercing fracture test. The orthogonal cutting test and simulation of 7075-T651 alloy are conducted. It is demonstrated that the determined flow stress and the established damage criterion produces realistic process outputs in agreement with experimental results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;136(1):011002-011002-14. doi:10.1115/1.4025508.

The mechanical responses of small volume metallic compounds are addressed in this work through developing a nonlocal continuum theory. In this regard, a thermodynamic-based higher-order strain-gradient plasticity framework for coupled thermoviscoplasticity modeling is presented. The concept of thermal activation energy and the dislocations interaction mechanisms are taken into consideration to describe the choice of thermodynamic potentials such as Helmholtz free energy and rate of dissipation. The theory is developed based on the decomposition of the thermodynamic conjugate forces into energetic and dissipative counterparts, which provides the constitutive equations to have both energetic and dissipative gradient length scales. The derived constitutive model is calibrated against the experimental data of bulge test conducted on thin films.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;136(1):011003-011003-8. doi:10.1115/1.4025758.

This work extends the generalized plasticity model for structural metals under cyclic loading proposed by Lubliner et al. (1993, “A New Model of Generalized Plasticity and its Numerical Implementation,” Int. J. Solids Struct., 22, pp. 3171–3184) to incorporate temperature-dependence into the elastic-plastic response. Proposed flow equations satisfy the Clausius–Duhem inequality through a thermodynamically consistent energy functional and retain key aspects of conventional plasticity models: Mises yield surface, normal plastic flow, and additive decomposition of strain. Uniaxial specialization of the 3D rate equations leads to a simple graphical method to estimate model properties. The 3D integration scheme based on backward Euler discretization leads to a scalar quadratic expression to determine the plastic strain rate multiplier and has a symmetric algorithmic tangent matrix. Both properties of the integration lead to a computationally efficient implementation especially suited to large-scale, finite element analyses. In comparison studies using experimental data from a Cottrell–Stokes test, the modified rate equations for the generalized plasticity model capture a thermally activated increase in the flow stress.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;136(1):011004-011004-6. doi:10.1115/1.4025695.

The experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (350–500 °C) and strain rates (0.005–0.5 s−1), were employed to develop constitutive equations in a commercially pure aluminum (AA1070). The effects of temperature and strain rate on the hot deformation behavior were represented by Zener–Hollomon parameter including Arrhenius term. The results show that the hardening rate and flow stress are evidently affected by both deformation temperature and strain rate. The power law, exponential, and hyperbolic sinusoidal types of Zener–Hollomon equations were used to determine the hot deformation behavior of AA1070. The results suggested that the highest correlation coefficient was achieved for the hyperbolic sine law for the studied material. So the proposed deformation constitutive equations can give an accurate and precise estimate of the flow stress for AA1070, which means it can be used for numerical simulation of hot forming processes and for choosing proper forming parameters in engineering practice accurately.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;136(1):011005-011005-5. doi:10.1115/1.4025976.

The influence of initial texture on rollability is investigated using cuneal AZ31 Mg alloy sheets. Upon large thickness reduction, the sheet with initial basal texture has many edge cracks, whereas the sheet is crack-free if its normal direction is orthogonal to c-axis of hexagonal close packed (HCP) lattice. Microstructural analysis shows that the former one has heterogeneous grain structure owing to grain-boundary-related recrystallization, and by contrast the later one has a more uniform microstructure for the twin-related recrystallization. The initial nonbasal texture can lead to excellent rollability and anisotropic deformation, based on which a new iterative approach of rolling is proposed, which may achieve large reduction in few passes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;136(1):011006-011006-7. doi:10.1115/1.4026035.

This paper studies the accuracy of a technique which is capable of predicting and modeling a wide range of creep life in Ni-based superalloys. The θ-projection method was applied to characterize the creep behavior of the Ni-based superalloy IN-792 at 800 °C. Constant load creep tests have been carried out over a wide range of loads at the constant operating temperature. Creep curves were fitted using either 4-θ or 6-θ equation by the use of a nonlinear least-square technique. The results showed that both 4- and 6-θ projection parameters revealed a good linearity as a function of stress. Comparison of experimental creep curves with those predicted using both of the utilized θ-projection techniques showed that the techniques fit the experimental data at high strain values very well while the 6-θ approach describes much better the creep curves at low strain region.

Topics: Creep , Superalloys , Stress
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;136(1):011007-011007-16. doi:10.1115/1.4025916.

The current work focuses on evaluation of the effective elastic properties of cementitious materials through a voxel based finite element analysis (FEA) approach. Voxels are generated for a heterogeneous cementitious material (type-I cement) consisting of typical volume fractions of various constituent phases from digital microstructures. The microstructure is modeled as a microscale representative volume element (RVE) in ABAQUS® to generate cubes several tens of microns in dimension and subjected to various prescribed deformation modes to generate the effective elastic tensor of the material. The RVE-calculated elastic properties such as moduli and Poisson's ratio are validated through an asymptotic expansion homogenization (AEH) and compared with rule of mixtures. Both periodic (PBC) and kinematic boundary conditions (KBC) are investigated to determine if the elastic properties are invariant due to boundary conditions. In addition, the method of “Windowing” was used to assess the randomness of the constituents and to validate how the isotropic elastic properties were determined. The average elastic properties obtained from the displacement based FEA of various locally anisotropic microsize cubes extracted from an RVE of size 100 × 100 × 100 μm showed that the overall RVE response was fully isotropic. The effects of domain size, degree of hydration (DOH), kinematic and periodic boundary conditions, domain sampling techniques, local anisotropy, particle size distribution (PSD), and random microstructure on elastic properties are studied.

Commentary by Dr. Valentin Fuster

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