Diego Vergara

Two-Dimensional Numerical Modelling of Hydrogen Diffusion in Metals Assisted by Both Stress and Strain

The present work is based on previous research on the one-dimensional (1D) analysis of the hydrogen diffusion process, and proposes a numerical approach of the same phenomenon in two-dimensional (2D) situations, e.g. notches. The weighted residual method was used to solve numerically the differential equations set out when the geometry was discretized through the application of the finite element method. Three-node triangular elements were used in the discretization, due to its simplicity, and a numerical algorithm was numerically implemented to obtain the hydrogen concentration distribution in the material at different time increments. The model is a powerful tool to analyze hydrogen embrittlement phenomena in structural materials.

Hydrogen Degradation of Cold-Drawn Wires: A Numerical Analysis of Drawing-Induced Residual Stresses and Strains

Abstract Hydrogen degradation represents a problem of major technological concern in the structural integrity of prestressed concrete structures in which cold-drawn wires are the main structural component and can suffer the deleterious effect of hydrogen, with the subsequent risk of catastrophic failure. This paper presents an innovative numerical analysis of hydrogen degradation of cold-drawn prestressing steel wires, focused on the two relevant variables influencing the phenomenon of hydrogen transport by diffusion in the metal: residual stresses generated by manufacture and plastic strains after cold work. To achieve this goal, two real (industrial) drawing processes are analyzed with two main differences, namely, the reduction of cross-sectional area performed at the first drawing stage and the number of drawing steps used in the whole process. Generated results prove the importance of an adequate design of the cold-drawing process with regard to the residual stress-and-strain field and its relevant r...

Interactive Virtual Platform for Simulating a Concrete Compression Test

Laboratory practices in technical degrees are usually crowed, what makes that many students waste the opportunity of putting into practice the knowledge acquired in theoretical classes. Taking this into account, an interactive virtual platform (IVP) is presented in this paper for enhancing students´ self-learning of one of the most commonly material tests used in engineering: the compression test for concrete samples. In order to carry out such innovative teaching approach, a computational modeling of a virtual materials laboratory was developed by authors including an interactive compression test machine. This way, by using this IVP, students can freely interact with the virtual compression machine during out of class study time. According to this approach, the aim of this computational application is essentially didactic, since (i) this tool allows students to get familiar with concrete compression test and, in this way, (ii) the knowledge acquired by means of such tool can be really useful to improve the performance in students´ learning during their later pressential laboratory practices.

New Approach for the Teaching of Concrete Compression Tests in Large Groups of Engineering Students

AbstractThis paper presents a teaching approach aiming to give students the chance of applying theoretical concepts in virtual environments, thereby overcoming limitations in overcrowded classes or in large groups of engineering undergraduates using available testing equipment. The proposed approach deals with enhancing self-learning of one of the most common tests used in materials engineering and/or civil engineering, namely, the compression test of concrete samples. To achieve this goal, two didactic-propose computational tools were developed: a virtual laboratory (VL) and video tutorials (VTs). Furthermore, two different teaching/learning experiences are compared in this paper: (1) using actual laboratory after using both virtual environments (VL and VTs); and (2) using only virtual environments. In both cases the use of these virtual tools improves the student learning outcomes, especially when these resources fulfill a lack of real equipment. Besides, the results of survey questions show the high mo...

Two-dimensional numerical modelling of hydrogen diffusion assisted by stress and strain

This work is based on previous research on the one-dimensional (1D) analysis of the hydrogen diffusion process, and proposes a numerical approach to simulate the phenomenon in two-dimensional (2D) situations, e.g., near notches. The weighted residual method was used to solve numerically the differential equations set out when the geometry was discretized through the application of the finite element technique. This developed procedure can be a suitable practical tool to analyze hydrogen embrittlement phenomena in structural materials.

Hydrogen Diffusion in Metals Assisted by Stress: 2D Numerical Modelling and Analysis of Directionality

Hydrogen diffusion within a metal or alloy is conditioned by the stress-strain state therein. For that reason it is feasible to consider that hydrogen diffuses in the material obeying a Fick type diffusion law including an additional term to account for the effect of the stress state represented by the hydrostatic stress. In this paper the hydrogen transport by diffusion in metals is modelled in notched specimens where loading generates a triaxiality stress state. To this end, two different approaches of stress-assisted hydrogen diffusion, one-dimensional (1D) and two-dimensional (2D), were compared in the vicinity of the notch tip in four notched specimens with diverse triaxiality level at two different loading rates. The obtained results show that the 2D approach predicts lower values of hydrogen concentration than the 1D approach, so that a loss of directionality of hydrogen diffusion, depending on both notch geometry parameters (radius and depth) and loading rate, appears when a 2D approach is considered.

Influence of the Die Bearing Length on the Hydrogen Embrittlement of Cold Drawn Wires

Prestressing steels, obtained by cold drawing, are highly susceptible to hydrogen embrittlement (HE) phenomena. Stress and strain fields produced by cold drawing play an essential role in this process since they affect hydrogen diffusion. Therefore, variations of such fields due to changes in drawing conditions could modify life in-service of these structural components. In this work the effect on HE of a parameter of the wire drawing process, the bearing length, is analyzed by means of diverse numerical simulations by the finite element method (FEM). The results of this work allow the definition of a characteristic value of the die bearing length equal to the wire radius, and demonstrate that the effects of stress-strain fields produced by wire drawing on HE are reduced when the bearing length exceeds such a characteristic value, so that the optimum cold drawing process is that with a bearing length higher than the wire radius.

Effects of Manufacturing-Induced Residual Stresses and Strains on Hydrogen Embrittlement of Cold Drawn Steels

Hydrogen embrittlement (HE) plays a relevant role in the performance of prestressing steel wires. In this framework, the knowledge of residual stresses and plastic strains in wires due to cold-drawing (manufacturing-induced residual stresses), as well as of wires hydrogenation from harsh environments are the keys to successful predictions of wire lives. This paper advances previous analyses of HE in cold-drawn prestressing wires via numerical modelling, first, of the cold-drawing process to gain the distributions of residual stresses and plastic strains, and next, of the stress-strain assisted hydrogen diffusion in wires towards creation the conditions for HE nucleation. Generated results prove the relevant role of residual stress-and-strain field in hydrogen diffusion in the wires, as well as their possible consequences for HE.

Hydrogen Embrittlement of Cold Drawn Prestressing Steels: the Role of the Die Inlet Angle

Cold-drawn prestressing steel wires are susceptible to hydrogen embrittlement (HE) in aggressive environments, and the residual stress-strain states generated in wires by the drawing process play the key role there. Hence, the alterations of the stress-strain fields due to the specific features of the drawing procedure affect the serviceability of wires under HE. On the basis of performed numerical simulations of the process of cold drawing with the use of different drawing dies, the paper addresses the effect of the inlet angle of the die on the residual stress-strain fields in the wires. We also deduce their consequences for the hydrogenation of wires and their susceptibility to HE. The reduction of the die inlet angle is shown to be beneficial for the performance of wires.

Hydrogen-Assisted Rolling-Contact Fatigue of Wind Turbines Bearings

This work analyses the effects of hydrogen-assisted rolling-contact fatigue on bearings of wind turbines. To this end, the well-known ball-on-rod test to evaluate the resistance to rolling-contact fatigue was simulated by the finite element method for obtaining the stress-strain state undergone by the bearings considering their in-service conditions. Results of this paper allow a simple estimation of the hydrogen amount in potential damage zones of the specimen.