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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

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