A three-dimensional multiscale model of intergranular hydrogen-assisted cracking

We present a three-dimensional model of intergranular hydrogen-embrittlement (HE) that accounts for: (i) the degradation of grain-boundary strength that arises from hydrogen coverage; (ii) grain-boundary diffusion of hydrogen; and (iii) a continuum model of plastic deformation that explicitly resolves the three-dimensional polycrystalline structure of the material. The polycrystalline structure of the specimen along the crack propagation path is resolved explicitly by the computational mesh. The texture of the polycrystal is assumed to be random and the grains are elastically anisotropic and deform plastically by crystallographic slip. We use the impurity-dependent cohesive model in order to account for the embrittling of grain boundaries due to hydrogen coverage. We have carried out three-dimensional finite-element calculations of crack-growth initiation and propagation in AISI 4340 steel double-cantilever specimens in contact with an aggressive environment and compared the predicted initiation times and crack-growth curves with the experimental data. The calculated crack-growth curves exhibit a number of qualitative features that are in keeping with observation, including: an incubation time followed by a well-defined crack-growth initiation transition for sufficiently large loading; the existence of a threshold intensity factor K_(Iscc) below which there is no crack propagation; a subsequent steeply rising part of the curve known as stage I; a plateau, or stage II, characterized by a load-insensitive crack-growth rate; and a limiting stress-intensity factor K_(Ic), or toughness, at which pure mechanical failure occurs. The calculated dependence of the crack-growth initiation time on applied stress-intensity factor exhibits power-law behavior and the corresponding characteristic exponents are in the ball-park of experimental observation. The stage-II calculated crack-growth rates are in good overall agreement with experimental measurements

Effect of flow pattern at pipe bends on corrosion behaviour of low carbon steek and its challenges

Recent design work regarding seawater flow lines has emphasized the need to identify, develop, and verify critical relationships between corrosion prediction and flow regime mechanisms at pipe bend. In practice this often reduces to an pragmatic interpretation of the effects of corrosion mechanisms at pipe bends. Most importantly the identification of positions or sites, within the internal surface contact areas where the maximum corrosion stimulus may be expected to occur, thereby allowing better understanding, mitigation, monitoring and corrosion control over the life cycle. Some case histories have been reviewed in this context, and the interaction between corrosion mechanisms and flow patterns closely determined, and in some cases correlated. Since the actual relationships are complex, it was determined that a risk based decision making process using selected ‘what’ if corrosion analyses linked to ‘what if’ flow assurance analyses was the best way forward. Using this in methodology, and pertinent field data exchange, it is postulated that significant improvements in corrosion prediction can be made. This paper outlines the approach used and shows how related corrosion modelling software data such as that available from corrosion models Norsok M5006, and Cassandra to parallel computational flow modelling in a targeted manner can generate very noteworthy results, and considerably more viable trends for corrosion control guidance. It is postulated that the normally associated lack of agreement between corrosion modelling and field experience, is more likely due to inadequate consideration of corrosion stimulating flow regime data, rather than limitations of the corrosion modelling. Applications of flow visualization studies as well as computations with the k-ε model of turbulence have identified flow features and regions where metal loss is a maximu

Ultimate strength

Concern for the ductile behaviour of ships and offshore structures and their structural components under ultimate conditions. Attention shall be given to the influence of fabrication imperfections and in-service damage and degradation on reserve strength

Computational fluid dynamic and thermal stress analysis of coatings for high-temperature corrosion protection of aerospace gas turbine blades

The current investigation presents detailed finite element simulations of coating stress analysis for a 3-dimensional, 3-layered model of a test sample representing a typical gas turbine component. Structural steel, Titanium alloy and Silicon Carbide are selected for main inner, middle and outermost layers respectively. ANSYS is employed to conduct three types of analysis- static structural, thermal stress analysis and also computational fluid dynamic erosion analysis (via ANSYS FLUENT). The specified geometry which corresponds to corrosion test samples exactly is discretized using a body-sizing meshing approach, comprising mainly of tetrahedron cells. Refinements were concentrated at the connection points between the layers to shift the focus towards the static effects dissipated between them. A detailed grid independence study is conducted to confirm the accuracy of the selected mesh densities. The momentum and energy equations were solved, and the viscous heating option was applied to represent improved thermal physics of heat transfer between the layers of the structures. A discrete phase model (DPM) in ANSYS FLUENT was employed which allows for the injection of continuous uniform air particles onto the model, thereby enabling an option for calculating the corrosion factor caused by hot air injection. Extensive visualization of results is provided. The simulations show that ceramic (silicon carbide) when combined with titanium clearly provide good thermal protection; however, the ceramic coating is susceptible to cracking and the titanium coating layer on its own achieves significant thermal resistance. Higher strains are computed for the two-layer model than the single layer model (thermal case). However even with titanium only present as a coating the maximum equivalent elastic strain is still dangerously close to the lower edge. Only with the three-layer combined ceramic and titanium coating model is the maximum equivalent strain pushed deeper towards the core central area. Here the desired effect of restricting high stresses to the strongest region of the gas turbine blade model is achieved, whereas in the other two models, lower strains are produced in the core central zones. Generally, the CFD analysis reveals that maximum erosion rates are confined to a local zone on the upper face of the three-layer system which is in fact the sacrificial layer (ceramic coating). The titanium is not debonded or damaged which is essential for creating a buffer to the actual blade surface and mitigating penetrative corrosive effects. The present analysis may further be generalized to consider three-dimensional blade geometries and corrosive chemical reaction effects encountered in gas turbine aero-engines. Key words: Thermal coating; Silicon Carbide ceramic; ANSYS; Finite element stress analysis; CFD (computational fluid dynamics); mesh density; total deformation; erosion.</i

Methodology for the simulation of a ship’s damage stability and ultimate strength conditions following a collision

This paper presents a methodology called SHARC developed for the simulation and analysis of a ship’s damage stability and ULS conditions following a collision. SHARC combines three types of methods: advanced nonlinear finite element simulations that simulate the collision scenario, a dynamic damage stability simulation tool called SIMCAP, and a modified Smith method for the ULS analysis of a collision-damaged ship structure. The novelty of the presented methodology is that it can be used for real-time simulations to study the ingress of water through the damage opening of a struck vessel and how it affects the ship’s stability, structural integrity (ULS) and survival capability against, e.g., capsizing. The results for an intact and a damaged oil tanker under noncorroded and corroded structural conditions and various sea states are presented to demonstrate the features of SHARC

OpenSeesPy-based web application for pushover curve computation of RC bridge piers subject to arbitrarily non-uniform corrosion patterns

Existing reinforced concrete (RC) bridge piers are often subject to complex spatially non-uniform steel corrosion patterns typically due to water percolation and exposition to environmental agents. This produces degradation of strength and ductility of the pier, which may significantly influence the seismic performances of bridges. The computation of pushover curves of corroded RC piers can be carried out by fiber-beam-column elements combined with suitable degradation laws for the uniaxial materials. For this purpose, a multi-level fiber-based modeling procedure is proposed based on a partition of the pier into zones characterized by different cross-sections with fiber discretizations reproducing the sectional deterioration pattern. A web application based on OpenSeesPy is defined to implement this procedure. This includes an interface developed by React JS and Boostrap V5 and an APIs layer based on the Flask framework. Through the interface, users can insert the parameters needed for the structural response simulation, which is, then, performed by employing the numerical procedure developed in Python. At the end of the computation, users can visualize and download the results or vary the input parameters to perform new simulations. The web application runs in a Docker container, making it easy to deploy on cloud platforms or on-premises solutions. Numerical simulations of real specimens affected by material deterioration are performed

Analytické a numerické přístupy pro modelování koroze v železobetonových konstrukcích

Corrosion of reinforcement in concrete is one of the most influencing factors causing the degradation of RC structures. This paper attempts at the application of an analytical and numerical approaches to simulation of concrete cracking due to reinforcement corrosion. At first, a combination with detailed analysis of two analytical models proposed by Liu and Weyers (1998) and Li et al. (2006) is suggested and presented. Four distinct phases of the corrosion process are identified and a detailed guide through the mathematical development is described. Next, numerical computations obtained with nonlinear finite element code are presented. The model features the state-of-the-art in nonlinear fracture mechanics modelling and the heterogeneous structure of concrete is modelled via spatially varying parameters of the constitutive law. Finally, the results of the analytical studies are compared to numerical computations and the paper concludes with the sketch of a real-life numerical exampleKoroze ocelové výztuže v betonu je jedním z hlavních příčin degradace železobetonových konstrukcí. Tento příspěvek předkládá možnosti analytických a numerických přístupů modelování rozvoje trhlin v betonu vzniklých působením koroze výztuže. Nejprve je prezentována kombinace a detailní analýza dvou analytických modelů od Liu a Weyerse (1998) a Li a kol. (2006). Jsou identifikovány čtyři fáze vývoje koroze s detailním popisem matematického modelu. Dále jsou prezentovány numerické výpočty získané nelineární konečně prvkostní analýzou. Použitý model využívá nejnovější nástroje nelineárního modelování s uplatněním přístupů lomové mechaniky. Heterogenní struktura betonu je modelována pomocí náhodného pole vstupních parametrů. Na závěr jsou porovnány výsledky analytických a numerických výpočtů a je uveden příklad aplikace na reálné části konstrukc

Buckling strength improvements for Fibre Metal Laminates using thin-ply tailoring

The buckling response and load carrying capacity of thin-walled open cross-section profiles made of Fibre Metal Laminates, subjected to static axial compression loading are considered. These include thin-walled Z-shape and channel cross-section profiles adopting a 3/2 FML lay-up design, made of 3 aluminium layers. The objective of the investigation is the comparison of standard thickness Fibre Reinforced Plastic layers versus thin-ply material technology. Whilst thin ply designs differ only by the layer thickness, they offer an exponential increase in stacking sequence design freedoms, allowing detrimental coupling effects to be eliminated. The benefit of different hybrid materials are also considered. The comparisons involve semi-analytical and finite element methods, which are validated against experimental investigations

Biomimetic flow fields for proton exchange membrane fuel cells: A review of design trends

Bipolar Plate design is one of the most active research fields in Polymer Electrolyte Membrane Fuel Cells (PEMFCs) development. Bipolar Plates are key components for ensuring an appropriate water management within the cell, preventing flooding and enhancing the cell operation at high current densities. This work presents a literature review covering bipolar plate designs based on nature or biological structures such as fractals, leaves or lungs. Biological inspiration comes from the fact that fluid distribution systems found in plants and animals such as leaves, blood vessels, or lungs perform their functions (mostly the same functions that are required for bipolar plates) with a remarkable efficiency, after millions of years of natural evolution. Such biomimetic designs have been explored to date with success, but it is generally acknowledged that biomimetic designs have not yet achieved their full potential. Many biomimetic designs have been derived using computer simulation tools, in particular Computational Fluid Dynamics (CFD) so that the use of CFD is included in the review. A detailed review including performance benchmarking, time line evolution, challenges and proposals, as well as manufacturing issues is discussed.Ministerio de Ciencia, Innovación y Universidades ENE2017-91159-EXPMinisterio de Economía y Competitividad UNSE15-CE296

Long-term management and condition assessment of concrete culvert

Infrastructure is the backbone of national security, economic growth, public safety and other aspects of the society. Nationwide, the condition of America’s infrastructure was graded as “D+” by the American Society of Civil Engineers (ASCE) in 2013. Owners and responsible agencies have employed various cost-effective maintenance and repair methods as well as analytical tools to repair and extend the service life of the infrastructure. However, without a long-term plan, maintenance work can be delayed by lack of funds. Maintenance delay may cause significant reduction in condition state leading to premature failure of the infrastructure. Consequently, a long-term rehabilitation plan is needed to find the best time and the amount of investment needed to avoid catastrophic failures. In this research, three methods are proposed to compute the long-term annual investments for a culvert network. They are modified worst first method, network optimization approach and estimation by the maximum deterioration rate. The performance of a 15-culvert system was evaluated using the above methods. The method based on maximum deterioration rate is very simplistic and can only be used to estimate the lower bound value of the investment. In the modified worst first method, a fixed yearly budget is allocated and the project level corrective actions are suggested for each culvert for each year. Then the budget allocation is changed and the analysis is repeated. One could imagine this procedure as current practice of fixed budget allocation projected into the future. To do the network optimization approach, the computer program LINGO was used for the budget optimization. Developed constraints and one objective function were used based on financial requirements. Based on the simulation results, the modified worst first method is computationally intensive but provides the optimum budget. The method using maximum deterioration rate provides the lower bound value and should be used as the absolute lowest value that should be allocated. The optimization method uses computer programming and provides an upper bound value for small networks. It is anticipated that for large culvert networks the network optimization approach can be used to provide reasonable long-term annual budgets. A long-term maintenance plan for culvert networks can be accomplished only if correct condition states of all culverts are known by inspection. However, the culvert material influences the inspection method. Concrete, metal and plastic are the most common culvert materials. In the USA, 78% of culverts are made of concrete because concrete culverts are strong, durable, and economically preferable. In the research, several common concrete inspection methods were reviewed. By comparing all these methods, ultrasound was found to be the most reliable, fastest, and most widely used NDT method. Ultrasound wave velocity is related to the elastic properties of the material. Thus, a finite element analysis was used to simulate concrete with voids. Concrete blocks with different void sizes and distributions under dry and fully saturated conditions were simulated. Using back-calculated Young’s modulus values, ultrasound wave velocity was computed and compared with experimental results from the literature. A good comparison provided a theoretical basis for the relationship between ultrasound velocity and material porosity. Ultrasound velocity and ultrasound diffusion method to characterize concrete were reviewed in this research. However, these methods are unable to account for the influence of the fluid in voids. Therefore, a new hypothesis of shock wave transmission in voids was studied. When high frequency and high energy ultrasound are applied to concrete, a shock wave will be generated at the edge of the voids and propagate through the voids. An ideal 1-D model was used to simulate shock wave velocity propagation. The high sound pressure of the solid will move like a piston to generate shock wave in voids. First, shock speed was studied by solving the Riemann Problem. After the piston stops, rarefaction will be generated and its speed is much faster than the shock. The interaction of rarefaction waves with shock waves was studied. The results show that when the rarefaction hits the shock, the shock wave velocity is reduced. Furthermore, the energy lost was also studied during the rarefaction interaction with the shock. The total energy is the same but due to the reaction the energy is spread during the propagation. Consequently, bigger voids will allow more of the rarefaction to interact with the shock and the velocity of the shock will decrease. In addition, the energy will spread to a longer volume and the total energy density will decrease, causing a reduction of wave amplitude