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Research Papers

J. Eng. Mater. Technol. 2017;139(3):031001-031001-8. doi:10.1115/1.4035621.

Automotive manufacturers always seek high strength and high formability materials for automotive bodies. Advanced high strength steels (AHSS) are excellent candidates for this purpose. These steels generally show a reasonable degree of formability, in addition to their high strength. One particular type is the twinning-induced plasticity (TWIP) steel, which is a high manganese austenite steel, and represents a second generation in AHSS. In this study, comprehensive deformation analysis of TWIP900CR steel including tensile, bending, Erichsen, and deep drawing of cylindrical cups tests is made. Finite element simulation of U and V shaped bending processes is also performed. Results indicate that the TWIP steel has good mechanical properties and high formability. However, springback is quite significant. The coining force should be considered in order to reduce the amount of springback. For springback prediction, it is found that the Yld2000-2d material model has better prediction capability than the Hill48 model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(3):031002-031002-9. doi:10.1115/1.4035623.

Transformation-induced plasticity (TRIP) effect is the outstanding mechanism of austenitic stainless steel. It plays an important role in increasing formability of the steel due to higher local strain hardening during deformation. In order to better understand forming behavior of this steel grade, the strain-induced martensitic transformation of the 304 stainless steel was investigated. Uniaxial tensile tests were performed at different temperatures for the steel up to varying strain levels. Stress–strain curves and work hardening rates with typical TRIP effect characteristics were obtained. Metallographic observations in combination with X-ray diffraction method were employed for determining microstructure evolution. Higher volume fraction of martensite was found by increasing deformation level and decreasing forming temperature. Subsequently, micromechanics models based on the Mecking–Kocks approach and Gladman-type mixture law were applied to predict amount of transformed martensite and overall flow stress curves. Hereby, individual constituents of the steel and their developments were taken into account. Additionally, finite element (FE) simulations of two representative volume element (RVE) models were conducted, in which effective stress–strain responses and local stress and strain distributions in the microstructures were described under consideration of the TRIP effect. It was found that flow stress curves calculated by the mixture law and RVE simulations fairly agreed with the experimental results. The RVE models with different morphologies of martensite provided similar effective stress–strain behavior, but unlike local stress and strain distributions, which could in turn affect the strain-induced martensitic transformation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(3):031003-031003-9. doi:10.1115/1.4035620.

Ultrasonic consolidation of fiber optics in metals is of major importance allowing surface embedding and protecting the fibers from exposure to open environment. The paper investigates the computational modeling of this process of embedding fibers at the aluminum subsurface. This new method provides an opportunity to develop sensory materials (Mekid et al., 2015, “Towards Sensor Array Materials: Can Failure be Delayed?” Sci. Technol. Adv. Mater., 16(3), p. 034607) and new types of nervous materials (Mekid and Kwon, 2009, “Nervous Materials: A New Approach for Better Control, Reliability and Safety of Structures,” Sci. Adv. Mater., 1(3), pp. 276–285) for structural health monitoring applications. A thermo-mechanical analysis of embedding SiC fiber in aluminum substrate has been conducted. The temperature distribution was obtained using a thermal model with process-dependent heat flux at the sonotrode/foil interface, which is coupled to the structural model in an iterative manner for simulating fiber embedment. The structural model uses a process-dependent plastic flow rule with an isotropic hardening model. A ductile damage model is employed for the first time in simulating such problems in addition to the use of real material properties of the fiber, which has resulted in better numerical results. Both of these factors help in determining the extent of damage particularly to the fiber/sensor being embedded. The experimental test has shown good agreement.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(3):031004-031004-11. doi:10.1115/1.4035764.

In a “very high-temperature reactor” (VHTR), the Nb–V-modified 9Cr–1Mo creep strength enhance the ferritic (CSEF) steel which is the chosen material for fabrication of reactor pressure vessels and piping because of its excellent high temperature thermal and mechanical properties. In such CSEF steel weldments, the hydrogen-induced cracking (HIC) is a critical issue. In the present work, the different levels of hydrogen have been induced in P91 CSEF weld metal to study their effect on HIC. The HIC susceptibility of P91 steel welds has been studied by carrying out the tensile test and flexural test for the different level of diffusible hydrogen. The hydrogen levels in deposited metals have been measured by using the mercury method. The fracture tensile and flexural test samples have been characterized based on the field-emission scanning electron microscope (FE-SEM). It is concluded that higher the level of diffusible hydrogen in deposited metal, more is the susceptibility of P91 steel to HIC. The minimum flexural and tensile strength are 507.45 MPa and 282 MPa, respectively, for 12.54 ml volume of diffusible hydrogen in 100 g of deposited weld metal.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(3):031005-031005-8. doi:10.1115/1.4035894.

The aim of this work is to design a lightweight, creep-resistant blade for an axial single-stage micro-gas turbine. The selected process was casting of an intermetallic titanium/aluminum alloy. All the project phases are described, from the preliminary thermodynamic and geometric stage design, to its three-dimensional (3D) modeling and the subsequent finite element method–computational fluid dynamics (FEM-CFD) analysis, to the manufacturing process of the single rotor blade. The blade making (height 7 cm and chord 5 cm, approximately) consisted in a prototyping phase in which a fully 3D version was realized by means of fused deposition modeling and then in the actual production of a full-scale set of blades by investment casting in an induction furnace. The produced items showed acceptable characteristics in terms of shape and soundness. Metallographic investigations and preliminary mechanical tests were performed on the blade specimens. The geometry was then refined by a CFD study, and a slightly modified shape was obtained whose final testing under operative conditions is though left for a later study. This paper describes the spec-to-final product procedure and discusses some critical aspects of this manufacturing process, such as the considerable reactivity between the molten metal and the mold material, the resistance of the ceramic shell to the molten metal impact at high temperatures, and the maximal acceptable mold porosity for the specified surface finish. The CFD results that led to the modification of the original commercial shape are also discussed, and a preliminary performance assessment of the turbine stage is presented and discussed.

Topics: Design , Blades
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(3):031006-031006-7. doi:10.1115/1.4036068.

A comprehensive study has been conducted to develop proper test methods for accurate determination of failure strengths along different material directions of closed-cell polymer-based structural foams under different loading modes. The test methods developed are used to evaluate strengths and failure modes of commonly used H80 polyvinyl chloride (PVC) foam. The foam's out-of-plane anisotropic and in-plane isotropic cell microstructures are considered in the test methodology development. The effect of test specimen geometry on compressive deformation and failure properties is addressed, especially the aspect ratio of the specimen gauge section. Foam nonlinear constitutive relationships, strength and failure modes along both in-plane and out-of-plane (rise) directions are obtained in different loading modes. Experimental results reveal strong transversely isotropic characteristics of foam microstructure and strength properties. Compressive damage initiation and progression prior to failure are investigated in an incremental loading–unloading experiment. To evaluate foam in-plane and out-of-plane shear strengths, a scaled shear test method is also developed. Shear loading and unloading experiments are carried out to identify the causes of observed large shear damage and failure modes. The complex damage and failure modes in H80 PVC foam under different loading modes are examined, both macroscopically and microscopically.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(3):031007-031007-10. doi:10.1115/1.4035875.

This study concerns the development of peridynamic (PD) strain energy density functions for a Neo-Hookean type membrane under equibiaxial, planar, and uniaxial loading conditions. The material parameters for each loading case are determined by equating the PD strain energy density to that of the classical continuum mechanics. The PD equations of motion are derived based on the Neo-Hookean model under the assumption of incompressibility. Numerical results concern the deformation of a membrane with a defect in the form of a hole, a crack, and a rigid inclusion under equibiaxial, planar, and uniaxial loading conditions. The PD predictions are verified by comparison with those of finite element analysis.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Eng. Mater. Technol. 2017;139(3):034501-034501-2. doi:10.1115/1.4035763.

A new nonholonmic Hamiltonian formulation of ab initio molecular dynamics extends current Ehrenfest, Car–Parrinello, and Born–Oppenheimer formulations, offering potential improvements to modeling methods employed in computational materials design.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;139(3):034502-034502-5. doi:10.1115/1.4035622.

Plasma-enhanced chemical vapor deposition (PECVD) is a well-known method for the synthesis of carbon nanotube (CNT) forests with the electric field in the plasma sheath being responsible for the vertical orientation of CNTs. Here, we investigate the deformation mechanism and mechanical properties of pristine and conformally coated PECVD CNT forests under compressive loading. Our in situ indentation experiments reveal that local buckles form along the height of pristine CNTs progressing downward from the starting point at the tips. For CNT forests coated from their roots to top with alumina using atomic layer deposition (ALD), the deformation mechanism depends strongly on the coating thickness. The buckling behavior does not change significantly when the coating is 5-nm thick. However, with a 10-nm-thick coating, the nanotubes fracture—that is, at both the CNT core and alumina coating. Ex situ indentation experiments with a flat punch reveal 8- and 22-fold increase in stiffness with the 5- and 10-nm coating, respectively. Comparing the behavior of the PECVD forests with CNTs grown with thermal chemical vapor deposition (CVD) shows that the mechanical behavior of PECVD CNTs depends on their characteristic morphology caused by the growth parameters including plasma. Our findings could serve as guidelines for tailoring the properties of CNT structures for various applications in which CNT compliance or deformation plays a critical role.

Commentary by Dr. Valentin Fuster

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