Atomistic Simulation of Hydrogen Embrittlement of Grain Boundaries in Metals

It has been known for about a century that hydrogen contamination causes severe degradation in the mechanical properties of metals. This phenomenon is generally termed as ‘hydrogen embrittlement’ (HE). In this thesis, the underlying mechanisms behind HE phenomenon were elucidated on an atomic scale. The H segregation at various grain boundaries (GBs) and its influence on the structure, mechanical properties, deformation mechanisms and failure response of GBs were examined by atomistic simulations. First, H segregation at various GBs was studied in this thesis. The results indicated that H segregation properties were very sensitive to GB structures. The effects of H atoms on the mechanical behaviour and plastic deformation of GBs were then examined. It was shown that H atoms modified the behaviour of dislocation nucleation and caused the yield stress of dislocation nucleation to increase or decrease. Different deformation mechanisms were directly responsible for this modification. In addition, H segregation increased the critical shear stress and impeded the coupled GB motion, irrespective of the GB structures. During GB migration, H vacancy clusters cannot grow, which suggests that the coupled GB motion may help to resist H-induced intergranular embrittlement. The role of H atoms in changing the interaction of dislocations with GBs was also investigated. Several interaction mechanisms such as dislocation transmission, nucleation and reflection were reported for different glide planes and GB structures. Segregated H atoms transformed these interaction mechanisms into ones involving dislocation absorption for most of GBs. This disordered the atomic structure of GBs and established a local stress state, which promoted the ultimate failure of GBs due to the formation of vacancies

Reverse time migration for inverse acoustic scattering by locally rough surfaces

Consider the inverse scattering of time-harmonic point sources by an infinite rough surface which is supposed to be a local perturbation of a plane. A novel version of reverse time migration is proposed to reconstruct the shape and location of the rough surface. The method is based on a modified Helmholtz-Kirchhoff identity associated with a special rough surface, leading to a modified imaging functional which always reaches a peak on the boundary of the rough surface for sound-soft case and penetrable case, and hits a nadir on the boundary of the rough surface for sound-hard case. Numerical experiments are presented to show the powerful imaging quality, especially for multi-frequency data.Comment: 29 pages,26 figure

Simultaneous recovery of a locally rough interface and the embedded obstacle with the reverse time migration

Consider the inverse acoustic scattering of time-harmonic point sources by an unbounded locally rough interface with bounded obstacles embedded in the lower half-space. A novel version of reverse time migration is proposed to reconstruct both the locally rough interface and the embedded obstacle. By a modified Helmholtz-Kirchhoff identity associated with a planar interface, we obtain a modified imaging functional which has been shown that it always peaks on the local perturbation of the interface and on the embedded obstacle. Numerical examples are presented to demonstrate the effectiveness of the method.Comment: 21 pages, 19 figure

MDFL: Multi-domain Diffusion-driven Feature Learning

High-dimensional images, known for their rich semantic information, are widely applied in remote sensing and other fields. The spatial information in these images reflects the object's texture features, while the spectral information reveals the potential spectral representations across different bands. Currently, the understanding of high-dimensional images remains limited to a single-domain perspective with performance degradation. Motivated by the masking texture effect observed in the human visual system, we present a multi-domain diffusion-driven feature learning network (MDFL) , a scheme to redefine the effective information domain that the model really focuses on. This method employs diffusion-based posterior sampling to explicitly consider joint information interactions between the high-dimensional manifold structures in the spectral, spatial, and frequency domains, thereby eliminating the influence of masking texture effects in visual models. Additionally, we introduce a feature reuse mechanism to gather deep and raw features of high-dimensional data. We demonstrate that MDFL significantly improves the feature extraction performance of high-dimensional data, thereby providing a powerful aid for revealing the intrinsic patterns and structures of such data. The experimental results on three multi-modal remote sensing datasets show that MDFL reaches an average overall accuracy of 98.25%, outperforming various state-of-the-art baseline schemes. The code will be released, contributing to the computer vision community

Unityによるマイクロマグネティクスシミュレーションのリアルタイム可視化

マイクロマグネティクスとは，磁石内部に現れる原子磁気モーメントによって作られる磁化構造やその動的な変化を扱う分野であり，ハードディスクのヘッドやMRAMのシミュレーションなどに用いられる．マイクロマグネティクスシミュレーションでは，磁気モーメントの相互作用があるため計算量が多くその動きは予測しずらい．シミュレーションを理解するには可視化する必要がある．また，解析サイクルを早くできるように，GPUの高い演算能力を用いてリアルタイムにシミュレーションすることは有用である．本研究では，ゲームエンジンのUnityを用いてマルチプラットフォーム対応のマイクロマグネティクスシミュレーションのリアルタイム可視化システムを開発した．本システムでは，物理シミュレーションの部分はグラフィクスAPI(OpenGL，Direct3Dなど)に内蔵されるGPGPU機能(Compute shader)で高速に計算される．計算されたシミュレーションの結果はUnityのビルトインレンダリングパイプラインに送り，リアルタイムに描画される．プラットフォームにとらわれず，あらかじめドライバーなどをインストールする必要もなく，ほとんどのデバイスで実行可能のため，本システムは簡易的なシミュレーションを行う場面で有効である．本文ではC++(CUDA)とUnity(Compute shader)を用いた二種類のシミュレーションシステムを実装し，比較，評価した．同じ手法と条件でマイクロマグネティクスシミュレーションを実行すると，Compute shaderを用いた場合はCUDAより2倍，CPUより約40倍速かった．1152粒子の条件で，最新のスマートフォンだと20FPS以上のフレームレートが得られる．小規模のシミュレーションであれば，スマートフォンなどのデバイスでもリアルタイムにシミュレーションと可視化が可能となった．学生などのユーザーはスマートフォンなどのデバイスで手軽にシミュレーションができ，教育用途に役立つ可能性がある．電気通信大学202

Magnetic Crosstalk Suppression and Probe Miniaturization of Coupled Core Fluxgate Sensors

This paper demonstrates the probe structure optimization of coupled core fluxgate magnetic sensors through finite element analysis. The obtained modelling results have been used to optimize the probe structures from horizontal- to vertical- arrangements for magnetic crosstalk suppression and probe miniaturization. The finite element analysis show that with the same distance between each adjacent fluxgate elements, the magnetic crosstalk is suppressed by 6 times and the volume is reduced by 2 times after the optimization. Furthermore, the miniaturized probes with low magnetic crosstalk have been designed and implemented. The experimental results which showed more than 5 times suppression of magnetic crosstalk verified the simulation results. Therefore, the results provide detailed reference to cope with the contradiction between volume miniaturization and magnetic crosstalk suppression in magnetic sensor-array design