Reliable fracture simulations are expected to contribute to the longer life cycle of structures. The phase-field fracture (PFF) model is anticipated to be a robust approach to reproducing crack propagation and branching. Measurement techniques for detecting cracks and fractures have also advanced and are widely used for health monitoring. In the field of structural analysis, a data assimilation method using the full-field measurement data obtained by digital image correlation has been proposed. The data assimilation method is attracting attention as a method of improving the simulation accuracy by utilizing the time-series measurement data obtained from such measurement techniques. In this study, we apply for the first time the nonsequential data assimilation method minimizing the cost function using tree-structured Parzen estimator (DMC-TPE method) to assimilate full-field strain measurement data into PFF simulations. In this paper we validate the proposed data assimilation method through numerical experiments, where data assimilation is performed using the pseudo-measurement data obtained from PFF simulations conducted with parameters assumed as true values. The results demonstrate that the proposed method enables the simultaneous inverse estimation of multiple parameters included in the PFF model. The validation results revealed that, although the DMC-TPE depends on the initially estimated values, the parameters that significantly impact the simulation results were accurately estimated.

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The effect of Al addition on the pressure sintering (so-called spark plasma sintering, SPS) behavior of B₄C powder by transient liquid phase sintering (TLPS) was investigated. First, an accurate sample temperature evaluation in a closed graphite die was performed using elemental standard powders. It was found that as the sintering temperature increased, the measurable die surface temperature exponentially decreased compared to the die internal temperature (i.e., sample temperature) due to thermal radiation. Next, pressure sintering treatment of the mixed powder of B₄C containing 5 vol.% Al was performed in vacuum at a constant compressive stress (50 MPa) and heating rate (2 K/s). Although densification due to Al melting was not observed at 933 K, it was found that densification began at temperatures above 1550 K, where the wettability of Al for B₄C is improved, and reached the final stage of sintering at approximately 2200 K. As a result, the densification temperature of B₄C with Al addition could be shifted to a lower temperature by approximately 250 K compared to B₄C without additives. It was suggested that the formation of Al₃BC₃ between Al and B₄C promoted the rearrangement and shape change of B₄C particles, resulting in densification at low temperatures.

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Due to the complex and nonlinear correlations between microstructures and mechanical properties in dual phase alloys, it is difficult to estimate their strength by conventional methods. This study attempts to model the relationship between microstructure and mechanical properties in sintered and hot-rolled α+β titanium-iron (Ti-Fe) alloys using machine learning. In the preparation of the models, 4-9 microstructural factors were investigated to identify the most important predictors of mechanical properties. A Random Forest (RF) model was found to have best predictive power, producing a good match with experimental data in samples which were outside of the training dataset. Moreover, the average α grain diameters, β phase widths (by intercept method), and β phase area fractions were found to be the strongest predictors of mechanical behavior.

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This paper presents an overview of research studies for achieving high-strength Al alloys. Severe plastic deformation (SPD) is a major process to strengthen the alloys, which is capable of making simultaneous use of all strengthening mechanisms such as grain refinement, particle dispersion (precipitation), solid solution and dislocation accumulation, regardless of alloy type and/or alloy state. High-strengths may also be achieved by syntheses through consolidation of powder mixtures using the SPD process under high pressures such as high-pressure torsion and high-pressure sliding. Future prospect is made to produce Al alloys with the strength more than 1 GPa with a reasonable ductility.

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In this study, a carbon steel tube with a variable wall thickness was fabricated under various conditions by dieless mandrel drawing to evaluate the effect of the Zener-Hollomon parameter on its microstructure. STKM13C low carbon steel tubes with an outer diameter of 6.0 mm and inner diameter of 4.0 mm were drawn under various conditions of heating temperature, feeding speed, and the reduction in area. The feeding speed was set as constant, and the drawing speed was increased or decreased during dieless mandrel drawing to obtain tubes with a variable wall thickness. Thereafter, the wall thickness and microstructure of the drawn tubes were evaluated. Furthermore, the Zener-Hollomon parameter was calculated under various dieless mandrel drawing conditions. Consequently, a tube with variable wall thicknesses, with a wall thickness reduction range of 0 – 21%, was obtained by the dieless mandrel drawing. Furthermore, the Zener-Hollomon parameter increased as the heating temperature decreased, and as the feeding velocity and reduction in area increased. As a result of an increase in the Zener-Hollomon parameter, the average grain size decreased to less than 2 μm in a single pass of the dieless mandrel drawing.

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In terms of safety and environmental impact, trivalent chromium plating is a potential surface treatment which alternates conventional hexavalent chromium plating. In order to improve the tribological properties, including friction characteristics, of the coating from trivalent baths, the authors attempted to incorporate silicon carbide (SiC) fine particles in the coating. Composite coating incorporating SiC particle deposited on a mild steel rod substrate successfully reduced the friction coefficient against alumina rod even while no significant increase in film hardness was achieved by particle incorporation. The effect of the coating hardness on the reduction in friction coefficient was limited. It was supposed that a possible reason for friction reduction was attributed to the reduction in the contact area by the incorporated particles. Coating surface texture modified by incorporating SiC particle should have contributed to the reduced friction because the texture allowed to bury wear debris and SiC particles detached from the film. Such a feature was supposed to promote reduced shear resistance as well as eliminate three-body abrasive wear.

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Sintered Ag nanoparticles display brittle fatigue crack propagation behavior at 298 K because of their submicron-size crystals. On the other hand, at elevated test temperatures above 413 K, ductile fatigue crack propagation characteristics similar to those of soft metals such as solder appear owing to viscous creep behavior at the grain boundaries, and no temperature dependence of the fatigue crack propagation properties was observed. The fatigue crack initiation lives observed experimentally and the fatigue lives derived by a pseudo-fracture mechanics approach using numerical fatigue tests with the fatigue crack propagation law mostly coincided with each other. However, the fatigue crack initiation life was calculated on the excessively safe side because of the long incubation period for crack initiation due to the influence of continued sintering during the tests performed near the sintering temperature. The application of the pseudo-fracture mechanics approach made it possible to derive the fatigue crack initiation law for smooth specimens of sintered Ag nanoparticles with satisfactory accuracy if the test temperature is lower than the sintering temperature.

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Announcement Concerning Article Retraction
The following paper has been withdrawn from the database of Mater. Trans., because a description based on a misinterpretation of the experimental results was found by the authors in advance of publication after acceptance.
Mater.Trans. 52(2011) Advance view.
Improvement in Fatigue Strength of Biomedical β-Type Ti-Nb-Ta-Zr Alloy while Maintaining Low Young’s Modulus through Optimizing ω-Phase Precipitation

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