arXiv subjects

Tomohisa Kojima

Publications and source records attributed to Tomohisa Kojima.

5 recordsLinked to original sources

arXiv2025

Investigation of PINN Stability and Robustness for the Euler-Bernoulli Beam Problem

Physics-Informed Neural Networks (PINNs) encounter significant training difficulties when applied to doubly-clamped beam problems, and the underlying causes are not fully understood. This study investigates the PINN loss landscape to identify the failure mechanisms of two primary formulations: the high-order strong formulation and the energy-based formulation. The results demonstrate that the Strong Formulation suffers from landscape ill-conditioning driven by the boundary conditions (BCs), leading to convergence issues in the doubly-clamped case. Conversely, while the energy-based formulation requires only lower-order derivatives, its loss functional can become indefinite, causing optimization difficulties near saddle points. Based on strain field benchmarks against Finite Element Method (FEM), it is found that the strong formulation, combined with a BC handling method and the L-BFGS optimizer, yields the best performance across three classical boundary condition cases. These findings clarify distinct, formulation-dependent failure modes, offering a diagnostic foundation for developing robust physics-based surrogate models for complex beam systems.

cs.CE↗

arXiv2023

Effect of shatterproof polymer film application on the fracture types and strength of glass subject to bending load

Shatterproof polymer films are widely for windows used because they can be easily installed on existing glass windows to improve safety. Applying them to glass plates has been reported to not only prevent fragments from scattering but also increase load-bearing capacity and penetration resistance. However, the clarification of their mechanism and quantitative evaluation are still insufficient because the effect of film application on the strength and failure mode of glass under quasi-static loading has not been investigated. In this study, three-point bending tests and fracture surface observations were conducted on a float glass with a shatterproof polymer film. The stress field formed inside the glass was visualised during the tests using the photoelastic method. By varying the support span of the specimen, the deformation mode was varied to generate three types of failures: bending, shear caused by Hertzian contact stress, and mixed-mode failures. Under the conditions in the present study, the breaking loads of the specimens with and without film were almost the same; however, the fracture surface observation indicated that the area subjected to shear failure caused by Hertzian contact stress was larger with film application. Finally, the effect of the film thickness on the breaking load due to bending deformation was theoretically predicted.

cond-mat.mtrl-sci↗

arXiv2023

Dynamic Cavitation Inception by Wave Propagation Across Solid-Fluid Interface with Varying Solid Surface Wettability

Fluid-structure interaction (FSI) problems are important because they may induce serious damage to structures. In some FSI problems, the interaction mechanism is strongly dependent on the wave propagation across the solid-fluid interface. In this study, we attempted a quantitative evaluation of the effect of the solid surface wettability on the wave propagation across the solid-fluid interface with FSI in the case of longitudinal wave propagation vertically towards the interface. During the experiments, while the water was continuously compressed by the solid buffer motion, cavitation bubbles appeared being originated from the buffer-water interface as a result of the transmitted tensile wave propagating across the interface in a cycle. It was confirmed that interfacial boundary condition as wettability could change the wave transmission behavior owing to changes in the cavitation occurrence. It was also confirmed that the worse the wettability, the severer the cavitation intensity, and the greater the difference between the energy lost by the buffer and the energy stored in the water. Consequently, the effect of the cavitation inception on the wave propagation at the solid-fluid interface with FSI could be quantitatively evaluated by considering the energy transferred from the solid to the water.

physics.flu-dyn↗

arXiv2021

Evaluation of dynamic fracture toughness of a bonded bi-material interface subject to high-strain-rate shearing using digital image correlation

High-strain-rate shear tests were conducted on a three-layered bonded test piece comprising a central aluminum layer with PMMA resin layers bonded on both sides. Upon calculating the displacement field and the strain field using digital image correlation (DIC), the crack tip was located, and the fracture toughness was evaluated at the Aluminum/PMMA bonding interface. As a result of the DIC, it was possible to determine the process by which 1) the elastic stress wave propagated to the aluminum section, 2) the wave was transmitted to the PMMA section, and 3) the crack developed at the interface. The tip of the crack was identified using displacement distributions obtained using DIC. The fracture toughness of the interface was evaluated using the stress intensity factor. The true interfacial stress was calculated by correcting the strain value at the interface obtained using DIC. The distribution of the stress suggested that mode II fracture appears in the present test method when the crack is sufficiently shorter than the length of the bonding interface, and mode I and mode II fractures appear when the crack is longer in comparison. Although the value of the stress intensity factor was disturbed by the error of the DIC analysis, it was confirmed that the obtained values were similar regardless of the difference in the crack length, upon averaging the stress intensity factor values from the crack tips to the long-range with a ratio of 1 to the subset in DIC. As the obtained stress intensity factor value was similar to the values calculated in the related literature, it can be concluded that the method proposed in this study yields a reasonable stress intensity factor.

cond-mat.mtrl-sci↗

arXiv2020

Numerical analysis of wave propagation across Solid-Fluid interface with Fluid-Structure interaction in circular tube

Fluid-structure interaction (FSI) and wave propagation in engineering structures can cause severe damage to piping systems or fluid machines, inducing serious accidents. In these phenomena, the mechanism of structural damage depends on the wave propagation across the fluid-solid interface. Previous studies reported that disagreements between the induced pressure value on the solid-fluid movable interface and the value predicted by the classical one-dimensional theory arose from the effects of two-dimensional wave propagation. To address this problem, in this study, a two-dimensional axisymmetric simulation of wave propagation across the solid-fluid interface with FSI was conducted. The simulation was performed using ANSYS Autodyn with a Lagrangian solver for solids and Eulerian solver for water. The results showed that radial wave propagation caused by the dynamic effect of the tube and water's inertia affected the peak pressure on the solid-fluid interface. The peak pressure was attenuated near the tube wall because of the inertial effect of the tube and fluid expansion. By calculating the mean pressure and axial stress to compare the simulated peak pressure with that from one-dimensional acoustic theory, it was indicated that the transition region for transmitted pressure was located immediately after the solid-fluid interface. In this region, the transmitted peak pressure may exceed the value predicted by one-dimensional acoustic theory. The transition region was oriented in the axial direction from the interface. In addition, prediction of the transmitted peak pressure with one-dimensional acoustic theory was suggested via normal wave speed in the unconfined fluid from a safety engineering perspective, although the circumferential stress generated in the tube enclosing fluid can be sufficiently accurately predicted using the same theory with the Korteweg speed.

physics.flu-dyn↗