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Research Papers

J. Eng. Mater. Technol. 2017;139(4):041001-041001-9. doi:10.1115/1.4036443.

High stress regions around corrosion pits can lead to crack nucleation and propagation. In fact, in many engineering applications, corrosion pits act as precursor to cracking, but prediction of structural damage has been hindered by lack of understanding of the process by which a crack develops from a pit and limitations in visualization and measurement techniques. An experimental approach able to accurately quantify the stress and strain field around corrosion pits is still lacking. In this regard, numerical modeling can be helpful. Several numerical models, usually based on finite element method (FEM), are available for predicting the evolution of long cracks. However, the methodology for dealing with the nucleation of damage is less well developed, and, often, numerical instabilities arise during the simulation of crack propagation. Moreover, the popular assumption that the crack has the same depth as the pit at the point of transition and by implication initiates at the pit base has no intrinsic foundation. A numerical approach is required to model nucleation and propagation of cracks without being affected by any numerical instability and without assuming crack initiation from the base of the pit. This is achieved in the present study, where peridynamics (PD) theory is used in order to overcome the major shortcomings of the currently available numerical approaches. Pit-to-crack transition phenomenon is modeled, and nonconventional and more effective numerical frameworks that can be helpful in failure analysis and in the design of new fracture-resistant and corrosion-resistant materials are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041002-041002-9. doi:10.1115/1.4036586.

Precracked 304 stainless steel (304SS) compact tension (CT) specimens repaired by laser with addition of different weight fractions of nano-tungsten carbide (nano-WC) were studied to investigate the effects of nano-WC on the fracture behavior and microstructure. Crack open displacements (CODs) measured by a digital image correlation (DIC) system were compared among specimens with different treatments. Microstructures were examined by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS). The results indicate an overall improvement of microstructure and fracture behavior. The specimen repaired by the addition of 5% nano-WC shows the most significant improvement from the current study. Both metallurgical bonding at the interface and fine equiaxial grains in the repaired layer are observed. The densification process of the repaired layer is also improved. In addition, an approximately 10–30% reduction of COD values was observed as the applied load varied from 1 to 20 kN. However, excessive addition of nano-WC led to the agglomeration and inhomogeneous distribution of WC nanoparticles in the repaired layers, resulting in the formation of microcracks. The fracture parameter COD shows a close relationship with the microstructure in laser repaired specimens with different powder ratio addition.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041003-041003-7. doi:10.1115/1.4036584.

The multicomponent NiCoCrAlTaY coating as bond layer as well as the zirconia stabilized by yttrium oxide (YSZ) coating as top ceramic layer was deposited on duplex vane surface by plasma spray-physical vapor deposition (PS-PVD) system. The thickness and microstructure of thermal barrier coatings (TBCs) under the influence of duplex vane geometry were presented in this article. It has been proven that the entire surface of duplex vane was covered by NiCoCrAlTaY and YSZ coatings. The position with thickest coating was found close to the leading edge and trailing edge of the vane. In those places, the coating was approximately 80–100% thicker than in the other areas on duplex vane. The obtained results indicate that it is possible to manufacture the TBCs including metallic bond layer and top ceramic layer by PS-PVD process on multiple vanes for gas turbines.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041004-041004-7. doi:10.1115/1.4036585.

The paper presents experimental results of microcutting brittle materials (granite). The analysis was conceived on the observed interaction between the workpiece and two tools of different shapes. Experiment was based on scratching the workpiece surface with diamond tools. Applied tools had tip radius R0.2 and R0.15 mm. The experiment determined the changes in the value of perpendicular and tangential components of the cutting force based on the geometric properties of tools, as well as the changes of the specific energy of microcutting granite (Jošanica and Bukovik types). The experiment has shown that reduction of tool radius causes reduction of the cutting force intensity and specific cutting energy. Because of its physical/mechanical properties, more energy is required for micromachining granite “Jošanica” than “Bukovik.” Based on the topography of the surface, the value of critical tool penetration depth was established, after which the brittle fracture is no longer present. For granite “Jošanica” values of critical penetration depth are 6 and 5 μm when micromachining with tools R0.2 and R0.15 mm, while for Bukovik those values are 6.5 and 5.5 μm. The paper should form the basis for understanding the phenomena which occur during microcutting brittle materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041005-041005-3. doi:10.1115/1.4036587.

This overview/survey assesses the state of the discipline for the failure of homogeneous and isotropic materials. It starts with a quick review of the many historical but unsuccessful failure investigations. Then, it outlines the dysfunctional current state of the field for failure criteria. Finally, it converges toward the technical prospects that can and very likely will bring much needed change and progress in the future.

Topics: Failure
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041006-041006-8. doi:10.1115/1.4036687.

This paper focuses on micromechanical finite element (FE) modeling of the effects of size and morphology (particularly elongation or aspect ratio (AR) along the loading direction) of martensite particles and the ferrite grains on the overall mechanical behavior of dual-phase (DP) steels. To capture the size-effect of the martensite particles and ferrite grains, the core and mantle approach is adapted in which a thin interphase of geometrically necessary dislocations (GNDs) is embedded at the martensite–ferrite boundaries. It is shown that as the martensite particles size decreases or their aspect ratio increases, both the strength and ductility of DP steel increase simultaneously. On the other hand, as the ferrite grain size decreases or its aspect ratio increases, the overall strength increases on the expense of the ductility. The conclusions from this study can be used in guiding the microstructural design of DP steels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041007-041007-7. doi:10.1115/1.4036588.

Cement paste is a material with heterogeneous composite structure consisting of hydrated and unhydrated phases at all length scales that varies depending upon the degree of hydration. In this paper, a method to model cement paste as a multiphase system at molecular level for predicting constitutive properties and for understanding the constitutive mechanical behavior characteristics using molecular dynamics is presented. The proposed method creates a framework for molecular level models suitable for predicting constitutive properties of heterogeneous cement paste that could provide potential for comparisons with low length scale experimental characterization techniques. The molecular modeling method followed two approaches: one involving admixed molecular phases and the second involving clusters of the individual phases. In particular, in the present study, cement paste is represented as two-phase composite systems consisting of the calcium silicate hydrate (CSH) phase combined with unhydrated phases tricalcium silicate (C3S) or dicalcium silicate (C2S). Predicted elastic stiffness constants based on molecular model representations employed for the two phases showed that, although the individual phases have anisotropic characteristics, the composite system behaves as an isotropic material. The isotropic characteristics seen from two-phase molecular models mimic the isotropic material nature of heterogeneous cement paste at engineering scale. Further, predicted bulk modulus of the composite system based on molecular modeling is found to be high compared to the elastic modulus, which concurs with the high compression strength of cement paste seen at engineering length scales.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041008-041008-11. doi:10.1115/1.4036709.

In this study, a representative volume element (RVE) homogenization approach is proposed to predict the mechanical properties of a lithium-ion battery (LIB) cell, module, and pack in an electric vehicle (EV). Different RVE models for the LIB jellyroll and module are suggested. Various elastic properties obtained from RVE analyses were compared to the analytic solution. To validate the approach suggested, the elastic responses of two types of homogenized LIB module for various loading cases were compared to the model where all the jellyroll and module components were described fully. Additionally, parametric studies were conducted to determine the relationship between design parameters of the jellyroll components and the elastic behavior of LIB jellyroll and module. The results obtained in this study provide useful information for both LIB cell developers, at the concept design stage, and engineers of electric vehicles who want to predict the mechanical safety of a battery pack.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041010-041010-8. doi:10.1115/1.4037022.

This study explores the application of a proper orthogonal decomposition (POD) and radial basis function (RBF)-based surrogate model to identify the parameters of a nonlinear viscoelastic material model using nanoindentation data. The inverse problem is solved by reducing the difference between finite element simulation-trained surrogate model approximation and experimental data through genetic algorithm (GA)-based optimization. The surrogate model, created using POD–RBF, is trained using finite element (FE) data obtained by varying model parameters within a parametric space. Sensitivity of the model parameters toward the load–displacement output is utilized to reduce the number of training points required for surrogate model training. The effect of friction on simulated load–displacement data is also analyzed. For the obtained model parameter set, the simulated output matches well with experimental data for various experimental conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041011-041011-9. doi:10.1115/1.4037020.

Traditionally, the mechanical properties of materials have been characterized using the uniaxial tension test. This test is considered adequate for simple forming operations where single axis loading is dominant. Previous studies, however, have noted that the data acquired from this type of testing are not enough and additional details in other axes under simultaneous deformation conditions are important. To analyze the biaxial strain, some studies have suggested using the limiting dome height test and bulge test. However, these tests limit the extent of using multi-axial loading and the resulting stress pattern due to contact surfaces. Therefore, researchers devised the biaxial machine which is designed specifically to provide biaxial stress components using multiple and varying loading conditions. The idea of this work is to evaluate the relationship between the dome test data and the biaxial test data. For this comparison, cruciform specimens with a diamond shaped thinner gage in the center were deformed with biaxial stretching on the biaxial testing machine. In addition, the cruciform specimens were biaxially stretched with a hemispherical punch in a conventional die-punch setting. Furthermore, in each case, the process was simulated using a three-dimensional (3D) model generated on abaqus. These models were then compared with the experimental results. The forces on each arm, strain path, forming, and formability were analyzed. The differences between the processes were detailed. It was found that biaxial tests eliminated the pressurization effect which could be found in hemispherical dome tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(4):041012-041012-13. doi:10.1115/1.4037021.

Cold expansion (CX) is a material processing technique that has been widely used in the aircraft industry to enhance fatigue life of structural components containing holes. CX introduces compressive hoop residual stresses that slow crack growth near the hole edge. The objective of this paper is to predict residual stresses arising from cold expansion using two different finite element (FE) approaches, and compare the results to measurement data obtained by the contour method. The paper considers single-hole, double-hole, and triple-hole configurations with three different edge margins. The first FE approach considers process modeling, and includes elastic–plastic behavior, while the second approach is based on the eigenstrain method, and includes only elastic behavior. The results obtained from the FE models are in good agreement with one another, and with measurement data, especially close to the holes, and with respect to the effect of edge margin on the residual stress distributions. The distribution of the residual stress and equivalent plastic strain around the holes is also explored, and the results are discussed in detail. The eigenstrain method was found to be very useful, providing generally accurate predictions of residual stress.

Commentary by Dr. Valentin Fuster

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