Browsing by Author "Kapci, Mehmet Fazil"

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Citation - WoS: 24
Citation - Scopus: 23
An Atomistic Study on the Help Mechanism of Hydrogen Embrittlement in Pure Metal Fe
(Pergamon-Elsevier Science Ltd, 2024) Hasan, Md Shahrier; Kapci, Mehmet Fazil; Bal, Burak; Koyama, Motomichi; Bayat, Hadia; Xu, Wenwu
The Hydrogen Enhanced Localized Plasticity (HELP) mechanism is one of the most important theories explaining Hydrogen Embrittlement in metallic materials. While much research has focused on hydrogen's impact on dislocation core structure and dislocation mobility, its effect on local dislocation density and plasticity remains less explored. This study examines both aspects using two distinct atomistic simulations: one for a single edge dislocation under shear and another for a bulk model under cyclic loading, both across varying hydrogen concentrations. We find that hydrogen stabilizes the edge dislocation and exhibits a dual impact on dislocation mobility. Specifically, mobility increases below a shear load of 900 MPa but progressively decreases above this threshold. Furthermore, dislocation accumulation is notably suppressed at around 1 % hydrogen concentration. These findings offer key insights for future research on Hydrogen Embrittlement, particularly in fatigue scenarios.
Citation - Scopus: 2
Design of Bio-Joint Shaped Knee Exoskeleton Assisting for Walking and Sit-to
(Springer International Publishing, 2019) Kapci, Mehmet Fazil; Unal, Ramazan
In this study, a bio-joint shaped knee joint exoskeleton is presented. This design is meant for avoiding misalignment of the exoskeleton joint with the biological knee joint. For this purpose a cam mechanism has been designed to prevent the misalignment from translation of the femur on tibia. Additionally, walking and sit-to-stance is passively assisted with a spring element that is activated with the heel contact. A single spring is used for both walking and sit-to-stance, due to the similar characteristics of the gait cycle and initial phases of the sit-to-stance in joint stiffness. © 2018 Elsevier B.V., All rights reserved.
Citation - WoS: 9
Citation - Scopus: 9
Edge Dislocation Depinning From Hydrogen Atmosphere in Α-Iron
(Pergamon-Elsevier Science Ltd, 2024) Kapci, Mehmet Fazil; Yu, Ping; Marian, Jaime; Liu, Guisen; Shen, Yao; Li, Yang; Bal, Burak
Understanding the dislocation motion in hydrogen atmosphere is essential for revealing the hydrogen-related degradation in metallic materials. Atomic simulations were adopted to investigate the interaction between dislocations and hydrogen atoms, where the realistic hydrogen distribution in the vicinity of the dislocation core was emulated from the Grand Canonical Monte Carlo computations. The depinning of edge dislocations in alpha-Fe at different temperatures and hydrogen concentrations was then studied using Molecular Dynamics simulations. The results revealed that an increase in bulk hydrogen concentration increases the flow stress due to the pinning effect of solute hydrogen. The depinning stress was found to decrease due to the thermal activation of the edge dislocation at higher temperatures. In addition, prediction of the obtained results was performed by an elastic model that can correlate the bulk hydrogen concentration to depinning stress.
Citation - WoS: 5
Citation - Scopus: 5
Experimental and Molecular Dynamics Simulation-Based Investigations on Hydrogen Embrittlement Behavior of Chromium Electroplated 4340 Steel
(ASME, 2021) Dogan, Ozge; Kapci, Mehmet Fazil; Esat, Volkan; Bal, Burak
In this study, chromium electroplating process, corresponding hydrogen embrittlement, and the effects of baking on hydrogen diffusion are investigated. Three types of materials in the form of Raw 4340 steel, Chromium electroplated 4340 steel, and Chromium electroplated and baked 4340 steel are used in order to shed light on the aforementioned processes. Mechanical and microstructural analyses are carried out to observe the effects of hydrogen diffusion. Mechanical analyses show that the tensile strength and hardness of the specimens deteriorate after the chrome-electroplating process due to the presence of atomic hydrogen. X-ray diffraction (XRD) analyses are carried out for material characterization. Microstructural analyses reveal that hydrogen enters into the material with chromium electroplating process, and baking after chromium electroplating process is an effective way to prevent hydrogen embrittlement. Additionally, the effects of hydrogen on the tensile response of alpha-Fe-based microstructure with a similar chemical composition of alloying elements are simulated through molecular dynamics (MD) method.
Citation - WoS: 1
Citation - Scopus: 2
Investigation of Hydrogen Diffusion Profile of Different Metallic Materials for a Better Understanding of Hydrogen Embrittlement
(Gazi Univ, 2023) Kapci, Mehmet Fazil; Bal, Burak
In this study, hydrogen diffusion profiles of different metallic materials were investigated. To model hydrogen diffusion, 1D and 2D mass diffusion models were prepared in MATLAB. Iron, nickel and titanium were selected as a material of choice to represent body-centered cubic, facecentered cubic, and hexagonal closed paced crystal structures, respectively. In addition, hydrogen back diffusion profiles were also modeled after certain baking times. Current results reveal that hydrogen diffusion depth depends on the microstructure, energy barrier model, temperature, and charging time. In addition, baking can help for back diffusion of hydrogen and can be utilized as hydrogen embrittlement prevention method. Since hydrogen diffusion is very crucial step to understand and evaluate hydrogen embrittlement, current set of results constitutes an important guideline for hydrogen diffusion calculations and ideal baking time for hydrogen back diffusion for different materials. Furthermore, these results can be used to evaluate hydrogen content inside the material over expensive and hard to find experimental facilities such as, thermal desorption spectroscopy.
Investigation of Temperature and Pressure Effect on the Hydrogen Sorption Kinetics in the Interface of Mg/MgH2 by Molecular Dynamics
(International Association for Hydrogen Energy, IAHE, 2022) Kapci, Mehmet Fazil; Wu, Zhen; Bal, Burak
Molecular dynamics simulations were performed in order to analyze the hydrogen sorption kinetics between αMgH2 and hcp Mg structures under different temperatures and pressures. Results showed that hydrogen desorption from magnesium hydride and absorption by hcp magnesium increase at the higher temperatures. During the hydrogen desorption from magnesium hydride and absorption into hcp magnesium, crystallographic orientation change in the magnesium atoms was observed. At 400 °C, the pressure was found to have a negative impact during the hydrogen desorption from magnesium hydride due to the prevention of recrystallization. © 2023 Elsevier B.V., All rights reserved.
Numerical Study of Magnesium-Based Metal Hydride Reactor Incorporating Multi-Phase Heat Exchanger for Thermal Energy Storage System
(Scanditale AB, 2020) Yao, Jing; Zhu, Pengfei; Ren, Jianwei; Kapci, Mehmet Fazil; Bal, Burak; Kurko, Sandra V.; Zhang, Z. X.
Metal hydride based thermal energy storage system is regarded as a promising method due to its good reversibility, low cost, and no by-product. Multi-phase heat exchange has much higher heat transfer coefficient than single-phase fluid heat exchange, thus facilitating the steam generation. In this study, a two-dimensional model of the metal hydride reactor using multi-phase heat exchange is proposed to estimate the performance and its feasibility of application in the concentrated solar power system. The results show that the velocity of the heat transfer fluid should match well with the thermal conductivity of the metal hydride bed to maintain the heat flux at a relatively constant value. The match of thermal conductivity of 3 or 5 W/(m·K) and fluid velocity of 0.0050 m/s results in the heat flux up to about 19 kW/m2, which is increased by 3 orders of magnitude than single-phase heat exchange. This study helps to facilitate the widespread application of metal hydride based thermal energy storage system in the concentrated solar power system. © 2024 Elsevier B.V., All rights reserved.
Parameter Analysis of a Biomass Based SOFC-Engine Polygeneration System for Cooling, Heating and Power Production
(Scanditale AB, 2020) Zhu, Pengfei; Guo, Leilei; Yao, Jing; Ren, Jianwei; Kapci, Mehmet Fazil; Bal, Burak; Zhang, Z. X.
In order to meet the demand of clean and efficient energy conversion technology, a novel combined cooling, heating and power (CCHP) system fueled by biomass is proposed. This system is consists of biomass gasification unit, solid oxide fuel cell, IC engine unit and absorption refrigeration chiller. Thermodynamic model of the CCHP system are developed and then parameter analysis is adopted to optimize the performance of this system. The effect of air equivalent ratio (ER), steam biomass ratio (S/B) and the fuel utilization factor of SOFC (μ) on the performance of the entire system are studied. The results show that increase of S/B and μ will prompt the electrical efficiency, while the increase of ER has a negative effect on electrical efficiency. The exergy analysis shows that the exergy destruction of biomass gasification process and engine is larger, which is 454.5 kW and 207.2 kW respectively. On the contrary, exergy destruction of SOFC and absorption refrigeration chiller are 15.9 kW and 52.8 kW, respectively. © 2024 Elsevier B.V., All rights reserved.
Citation - WoS: 4
Citation - Scopus: 4
A Phenomenological Hydrogen Induced Edge Dislocation Mobility Law for Bcc Fe Obtained by Molecular Dynamics
(Pergamon-Elsevier Science Ltd, 2024) Baltacioglu, Mehmet Furkan; Kapci, Mehmet Fazil; Schoen, J. Christian; Marian, Jaime; Bal, Burak
Investigating the interaction between hydrogen and dislocations is essential for understanding the origin of hydrogen-related fractures, specifically hydrogen embrittlement (HE). This study investigates the effect of hydrogen on the mobility of 1/2<111>{110} and 1/2<111>{112} edge dislocations in body-centered cubic (BCC) iron (Fe). Specifically, molecular dynamics (MD) simulations are conducted at various stress levels and temperatures for hydrogen-free and hydrogen-containing lattices. The results show that hydrogen significantly reduces dislocation velocities due to the pinning effect. Based on the results of MD simulations, phenomenological mobility laws for both types of dislocations as a function of stress, temperature and hydrogen concentration are proposed. Current findings provide a comprehensive model for predicting dislocation behavior in hydrogencontaining BCC lattices, thus enhancing the understanding of HE. Additionally, the mobility laws can be utilized in dislocation dynamics simulations to investigate hydrogen-dislocation interactions on a larger scale, aiding in the design of HE-resilient materials for industrial applications.
Citation - WoS: 38
Citation - Scopus: 40
The Role of Hydrogen in the Edge Dislocation Mobility and Grain Boundary-Dislocation Interaction in Α-Fe
(Pergamon-Elsevier Science Ltd, 2021) Kapci, Mehmet Fazil; Schoen, J. Christian; Bal, Burak
The atomistic mechanisms of dislocation mobility depending on the presence of hydrogen were investigated for two edge dislocation systems that are active in the plasticity of alpha-Fe, specifically 1/2<111>{110} and 1/2<111>{112}. In particular, the glide of the dislocation pile-ups through a single crystal, as well as transmission of the pile-ups across the grain boundary were evaluated in bcc iron crystals that contain hydrogen concentrations in different amounts. Additionally, the uniaxial tensile response under a constant strain rate was analyzed for the aforementioned structures. The results reveal that the presence of hydrogen decreases the velocity of the dislocations -in contrast to the commonly invoked HELP (Hydrogen-enhanced localized plasticity) mechanism-, although some localization was observed near the grain boundary where dislocations were pinned by elastic stress fields. In the presence of pre-exisiting dislocations, hydrogen-induced hardening was observed as a consequence of the restriction of the dislocation mobility under uniaxial tension. Furthermore, it was observed that hydrogen accumulation in the grain boundary suppresses the formation of new grains that leads to a hardening response in the stress-strain behaviour which can initiate brittle fracture points. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.