Probabilistic properties associated with residual life against fatigue failure under random overloads with retardation phenomenon are investigated through computer simulations, where ductile fracture mode is simultaneously taken into analysis. First, based upon a fatigue crack growth model under plural overloads proposed by one of the authors, a system of random differential equations is formulated under random overloads. Next, the so-called reset function, which describes a size of a newly created yielding zone near crack tip by an overload, is newly proposed for steel materials. Based upon the bi-modal approach for failure occurrence against random overloads, in which both a fatigue failure mode and a ductile failure mode are taken into account, the residual life distribution for each failure mode is numerically estimated through computer simulations. Finally, residual life probability distribution function of a cracked component is numerically shown under two failure modes.

This study investigates optimal maintenance analysis against fatigue failure based upon computer simulation approach with a new probabilistic model using a random field. Probabilistic models reported so far often face difficulties in accurately representing the variability in fatigue crack growth resistance and encounter computational problems in analytical approaches. Our model, having been introduced in earlier works, addresses these issues by generating fatigue crack growth samples through computer simulations, thereby facilitating practical applications to practical maintenance problems.In this paper, this model is applied to derive and analyze history-independent optimal maintenance strategies, focusing on three key factors: material inhomogeneity, crack detection variability in inspections, and uncertainty on initial crack lengths in virgin components. Numerical examples demonstrate that (i) the inspection time interval gradually decreases as the number of inspections increases as well as having been qualitatively pointed out and (ii) the proposed method using the new probabilistic model can derive optimal maintenance strategies even under increased model complexity and uncertain factors.

In the very high cycle fatigue for high strength steels, it is often reported that the fatigue crack tends to occur around the interior inclusion and a characteristic fine granular area (FGA) is formed around the inclusion. Outside the FGA, the fracture surface becomes rather flat within the fish-eye region. Remaining region outside the fish-eye is the fracture surface in the rapid crack propagation. Another typical aspect on such an interior induced fracture is the fact that the FGA formation period after starting cyclic loadings occupies a large portion of the total fatigue life. Thus, the FGA formation mechanism is one of the most important subjects in the very high cycle fatigue. From this point of view, the authors have attempted to construct the FGA formation model to interpret the fatigue fracture process inside the FGA. In this concept, polygonization is first caused around the inclusion before any micro-cracking during the long sequence of cyclic loadings. Then, micro-debondings take place at the boundaries between the polygonised layer and the matrix of the steel. The micro-debondings can also occur inside the polygonised layer. These micro-debondings can occur isolatedly and intermittently, and the number of micro-debondings tends to increase and some of them tend to coalesce to one another along the cyclic loadings. If the entire region of the FGA is fully debonded in such a way, a penny-shape crack is formed. Once the penny-shape crack is formed, it can grow continuously following the well-known Paris' law.

When alcohol solution adheres to the amorphous polymer under loading, the crazing occurs even at loads below the allowable strength of the material and eventually leads to the rupture. Since unexpected deterioration and rupture due to alcohol adhesion significantly affect the reliability of plastic products, there is a need to ensure safety by predicting the product life made of plastic materials. This research aims to elucidate the progress of crazing due to the adhesion of alcohol to plastic materials by using the image analysis with the fractal dimension. A quarter-elliptical fixture method was applied to PMMA specimens under 0.5% and 0.8% strain with In-situ observation of the microscope. The experimental results revealed that the observation of the number and area of the crazing was insufficient because of the combination of the multiple crazing as time passed. Therefore, we proposed the image analysis with fractal dimension to quantitatively evaluate the complexity of the crazing with time dependence. The number of crazing units just before fracture was 207 at 0.5% strain and 173 at 0.8% strain, and the percentage of the area occupied by crazing was 19% at 0.5% strain and 22% at 0.8% strain. On the other hand, the fractal dimension converged to 1.55 for both strains, and the fractal dimension can be judged to be at risk of fracture when it reached 1.5. We believed that the fractal dimension was useful to predict the fracture of plastic materials under loading with alcohol adhesion.

Two types of low-carbon steels with different Al contents were used, and low-oxygen weld metals with an oxygen content of 10 ppm were fabricated. A bead-on-plate welding experiments were performed with a tungsten-inert-gas welding system and subsequent water cooling using a welding nozzle that prevents oxygen from entering the molten pool with a double shielding gas. The effect of Al content on the composition of inclusions that act as the starting point for acicular ferrite formation in the early and late stages of the austenite-to-ferrite transformation during cooling after welding was clarified. Optical micrographs and images obtained by scanning electron microscope equipped with energy dispersive X-ray spectroscopy detector showed that acicular ferrite formed within austenite grains on inclusions with a chemical composition and size that were considered to be ineffective for acicular ferrite formation in low-Al steel. This suggests that in low-oxygen region, Al uniformly dissolved in the austenite grains has a large effect on the formation of acicular ferrite.

This study investigates the low-temperature deposition of SiO₂ films using plasma-enhanced chemical vapor deposition (PECVD) for flexible electronics applications, focusing on heat-sensitive substrates, such as polyethylene terephthalate (PET). We compared SiO₂ films deposited at 400 kHz and 13.56 MHz plasma excitation frequencies using tetraethyl orthosilicate (TEOS) as the precursor. Films were characterized for deposition rate, refractive index, stress, density, and surface morphology. Si substrate was used for evaluation. Results exhibit that 400 kHz deposition yields enhanced film properties compared to 13.56 MHz, exhibiting higher deposition rates, increased refractive index (up to 1.475), compressive stress, and slightly higher density, while maintaining smooth surfaces. These improvements are attributed to intensified ion bombardment effects in the low-frequency plasma. The 400 kHz PECVD process demonstrates potential for fabricating high-quality dielectric and passivation layers in flexible electronic devices at temperatures below 60°C, addressing the critical need for low-temperature processes in next-generation flexible electronics development.

Polylactic acid (PLA) resin has been attracting more and more attention because it is not only a biomass-based carbon-neutral material, which does not increase CO₂ emission, but also an eco-friendly biodegradable plastic, although the mechanical strength and biodegradability of PLA is not very high. So, by using Chemicals Informatics (CI) based on public literature data, additives effective for improving the mechanical strength and biodegradability of the PLA resin were explored in order to make the PLA resin available for wider use. As a result, adipic acid and 3,3'-dithiodipropionic acid were discovered as effective additives. To clarify the mechanism that improves the mechanical strength and biodegradability with these additives, molecular simulations were conducted. As a result of the simulation, it was found that the lattice matching between PLA and the additives (adipic acid and 3,3'-dithiodipropionic acid) is very good and that this lattice matching increases adhesion between PLA and the additives and improves the mechanical strength. It was also found that the lattice matching constructs the diffusion path of water molecules and enhances the hydrolysis, which accelerates the biodegradability. The effects of the additives were confirmed by tensile testing and hydrolysis tests.