Finite element analysis on relationships between the J-integral and CTOD for stationary cracks in welded tensile specimens

In this study the effects of weld strength mismatching and geometry parameters on the relationship between the J-integral and the crack tip opening displacement (CTOD) are investigated. Numerical analysis was carried out by an ABAQUS two-dimensional elastic–plastic analysis mode. The work was performed for center-cracked welded specimens with uniform tensile load. In the specimens the weld strength mismatch, M, was from 0.8 to 1.2, the crack length, a/W, from 0.1 to 0.5, the weld width, h/c, from 0.1 to 0.5. The main results indicate that weld strength mismatching has only a weak influence on the relationship between the J-integral and CTOD at low load levels, but there is a strong effect at high load levels. The yield strength of weld metal may be used for low load levels and the yield strength of base metal may be used for high load levels, when the basic relationship of J-integral versus CTOD is utilized to treat the problem of welded joints. The results also show that the crack size and weld width have an influence on the relationship between the J-integral and CTOD at high load levels. Because the equivalence between the J-integral and CTOD breaks down at high load levels, the relationship between J-integral and CTOD becomes more complex, and the weld strength and geometry mismatching factors must be included.     

金属膜片联轴器接触面的动力学建模与振动特性分析

为探究金属膜片联轴器等多螺栓连接多层结合面的动力学等效建模方法,提出了两种基于虚拟材料层模拟螺栓连接的建模方法。通过赫兹接触理论和分形几何理论求得虚拟材料参数,探究膜片组的结构参数对膜片联轴器整机轴向振动传递特性的影响规律。同时,将金属膜片组件和联轴器整机作为实验对象,进行实验与仿真数据的对比研究。研究表明：金属膜片组件前5阶固有频率平均偏差在5%以内,联轴器整机阻抗曲线吻合较好,频段内的平均偏差在5%以内,曲线峰值对应的频率偏差在3%以内;虚拟材料法适用于多层螺栓连接结合面的等效建模,将每层结合面均等效为一层虚拟材料具有更高的计算精度;膜片数量主要影响第1阶固有频率,随着膜片数量增多隔振效果降低;膜片厚度会对多个固有频率位置产生影响,随着膜片厚度增大固有频率向高频移动,隔振效果逐渐降低。     

Performance evaluation of solar air heater having expanded metal mesh as artificial roughness on absorber plate

A parametric study of artificial roughness geometry of expanded metal mesh type in the absorber plate of solar air heater duct has been carried out and compared with smooth duct. The performance evaluation in terms of energy augmentation ratio (EAR), effective energy augmentation ratio (EEAR) and exergy augmentation ratio (EXAR) has been carried out for various values of Reynolds number (Re) and roughness parameters of expanded metal mesh roughness geometry in the absorber plate of solar air heater duct. It is found that the augmentation ratios decrease at faster rate with Re in the order of EAR, EEAR and EXAR. It is also found that augmentation ratios increase with increase in duct depth and intensity of solar radiation. The artificially roughened solar air heater duct performs better as per EAR or heat energy gain criteria for any values of Re and roughness parameters of expanded metal mesh. The EAR is high for the parameters of expanded metal mesh type roughness geometry which create more turbulence, however the pump work required for flow of air will also increase. The EXAR is a more suitable criterion to incorporate the quality of heat collected and pump work required. The EXAR is more for higher duct depth and low Re range. Based on EXAR the suitable design parameters of expanded metal mesh roughness geometry are determined.     

The effect of mechanical heterogeneity and limit load of a weld joint with longitudinal weld crack on the J-integral and failure assessment curve

According to the CEGB R6 (Rev.3) approach, the influence of strength mis-matching and the limit load of a weld joint with a longitudinal weld crack on the J-integral and failure assessment curve can be studied by using an elastic–plastic finite element method for Center-Cracked Panel (CCP) specimens. The results indicate that the values of the J-integral and the shapes and positions of the failure assessment curves are greatly affected by the strength mis-matching factor M, a ratio of weld metal yield strength to that of base metal. If the limit load of the base metal is adopted to normalize the applied load, then the greater the value of M, the larger the safe area is in the failure assessment curve (FAC). However, if the limit load of the weld metal is adopted to normalize the applied load, then the greater the value of M, the smaller the safe area is. Therefore, for the undermatched and evenmatched joints, it is safer to choose the limit load of the base metal as the normalized load, and for the overmatched joints, it is safer to choose the limit load of the weld metal as the normalized load. Moreover, when M is less than 0.8 for the former situation, the option 1 curve of the R6 is not a conservative assessment curve. Considering that there is no simple theoretical formula which is suitable for calculating the limit load of a mechanical heterogeneous weld joint under plane stress and a variety of crack geometries, it is recommended that no matter what the strength of the overmatched or undermatched weld joint is, it is safer to use the limit load of that metal which has the higher strength grade of base metal and weld metal as the normalized load.     

Evaluation of reaction mechanism of coal-metal oxide interactions in chemical-looping combustion

The knowledge of reaction mechanism is very important in designing reactors for chemical-looping combustion (CLC) of coal. Recent CLC studies have considered the more technically difficult problem of reactions between abundant solid fuels (i.e. coal and waste streams) and solid metal oxides. A definitive reaction mechanism has not been reported for CLC reaction of solid fuels. It has often been assumed that the solid/solid reaction is slow and therefore requires that reactions be conducted at temperatures high enough to gasify the solid fuel, or decompose the metal oxide. In contrast, data presented in this paper demonstrates that solid/solid reactions can be completed at much lower temperatures, with rates that are technically useful as long as adequate fuel/metal oxide contact is achieved. Density functional theory (DFT) simulations as well as experimental techniques such as thermo-gravimetric analysis (TGA), flow reactor studies, in situ X-ray photo electron spectroscopy (XPS), in situ X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to evaluate how the proximal interaction between solid phases proceeds. The data indicate that carbon induces the Cu-O bond breaking process to initiate the combustion of carbon at temperatures significantly lower than the spontaneous decomposition temperature of CuO, and the type of reducing medium in the vicinity of the metal oxide influences the temperature at which the oxygen release from the metal oxide takes place. Surface melting of Cu and wetting of carbon may contribute to the solid-solid contacts necessary for the reaction.     

Constructal architecture for heating a stream by convection

In this paper we consider the fundamental problem of how to heat a stream to a specified exit temperature such that the overall fuel consumption is minimal. As illustration, we consider metal slabs that move at constant speed through a very slender enclosure with fixed total volume and arbitrary (nonuniform) distribution of cross-sectional area (heat transfer contact area). The heating is provided by a large number of heaters, which are distributed arbitrarily along the enclosure. The combustion gases flow in the x direction, which is oriented against the direction of the metal stream. The heat transfer is by convection. We show that minimal heat consumption is achieved when the heaters and the heat transfer contact area are distributed nonuniformly. The density of heaters per unit length must decrease as x^?0.8 toward the entrance of the metal stream, and the heat transfer contact area must increase in proportion with x. These features suggest that the metal must move not as a single stream but as a tree-shaped flow. The metal enters in several parallel streams, which serve as tributaries to larger streams, leading to a single stream that exits at the specified temperature.     

Effects of atmospheres on bonding characteristics of silver and alumina

Joints prepared using the silver–copper oxide based reactive air brazing (RAB) technique are known to experience a significant decrease in joint strength when exposed to a high-temperature reducing environment. To investigate the effects of ambient atmosphere on the bonding characteristics of ceramic joints brazed with Ag–CuO filler metals, alumina joints prepared using a series of Ag–CuO compositions were exposed to a reducing hydrogen atmosphere and re-oxidized in air at 800 °C. As previously reported, joints exposed only to hydrogen revealed significant reduction in flexural strength and exhibited interfacial de-bonding between the filler metal and the alumina substrate. In the case of the joints brazed with a filler metal containing a high copper content, 8 mol% CuO, the formation of interfacial porosity caused by the reduction of interfacial oxide phases led to an extremely weak interface, which was not recovered after subsequent reoxidation in air at 800 °C. However, no such microstructural change or formation of interfacial porosity was observed in joints brazed with filler metals containing no or low copper content and the substrate/filler metal interface remained intact even though interfacial strength was found to be relatively weak. Subsequent reoxidation of these joints resulted in the recovery of interfacial strength and flexural strength. The results clearly indicate that interfacial adhesion in this type of braze filler metal is significantly influenced by the oxidizing/reducing characteristics of the surrounding atmosphere during high-temperature exposure. XPS analysis conducted on the in situ fractured surfaces of as-brazed and hydrogen-treated samples prepared using a filler metal with 2 mol% CuO indicates that it is the concentration of oxygen in the silver matrix which is critical to the level of bond strength between silver and alumina. This was confirmed by exposing an equivalent set of joining specimens to an inert atmosphere at high temperature, which subsequently also displayed low flexural strengths and de-bonding along the silver/alumina interface.     

Stress intensity factors for a pipe-plate welded joint

A major component of any linear elastic fracture mechanics model for fatigue crack growth is the calculation of the crack tip stress intensity factor. This is particularly difficult for welded joints due to the complex geometry. While some data are available for cracks in welded T-plate joints, there is relatively little data available for larger cracks in more complex tubular joints. Such cracks are of significant interest since the most practical application of fracture mechanics models is the prediction of remaining life for cracks discovered in service.

A pipe-plate joint has been developed as a simplified model of tubular joint geometries for fatigue studies. Two such specimens have been tested in air, with detailed monitoring of crack growth behaviour using potential drop techniques. These data were used to obtain crack growth rate data from which estimates of stress intensity factors were made. Separately, finite element analyses for various discrete crack configurations were performed. The results of these analyses are presented and discussed, with particular emphasis on the accuracy of the results and the implications for fracture mechanics modelling.     

A new steel expansion joint for industrial plants: Bubble joint

In power, steel, chemical and other industrial plants, ducts are essential to carry air and effluent gases. Ducts are designed such that under the temperature they can expand and contract freely in order to eliminate internal thermal stresses. To allow for free movements, expansion joints are introduced at the points of change in direction and also in long duct runs. Whenever large lateral movements are encountered, either a very deep single expansion joint or a toggle section with two joints is currently used in practice. In this article, a new type of steel expansion joint is proposed by introducing a bubble in the center of the single expansion joint with the goal of increasing its lateral movement capacity. The lateral movement capacity of the proposed joint is investigated by finite element simulation taking into account both material and geometrical nonlinearities. The results show that the new steel bubble joint can accommodate substantially more lateral movements than the conventional steel expansion joint. The new bubble joint provides an economical solution for retrofitting an existing plant where the available space is limited.     

Effect of weld metal properties on fatigue crack growth behaviour of gas tungsten arc welded AISI 409M grade ferritic stainless steel joints

The effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on fatigue crack growth behaviour of the gas tungsten arc welded ferritic stainless steel joints was investigated. Rolled plates of 4 mm thickness were used as the base material for preparing single ‘V’ butt welded joints. Centre cracked tensile (CCT) specimens were prepared to evaluate fatigue crack growth behaviour. Servo hydraulic controlled fatigue testing machine was used to evaluate the fatigue crack growth behaviour of the welded joints. From this investigation, it was found that the joints fabricated by duplex stainless steel filler metal showed superior fatigue crack growth resistance compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Higher yield strength, hardness and relatively higher toughness may be the reasons for superior fatigue performance of the joints fabricated by duplex stainless steel filler metal.     

Contact mechanics and thermal conductance of carbon nanotube array interfaces

A model is developed in this work to predict the thermal contact resistance of carbon nanotube (CNT) array interfaces with CNT arrays synthesized directly on substrate surfaces. An analytical model for contact mechanics is first developed in conjunction with prior data from load–displacement experiments to predict the real contact area established in CNT array interfaces as a function of applied pressure. The contact mechanics model is utilized to develop a detailed thermal model that treats the multitude of individual CNT–substrate contacts as parallel resistors and considers the effects on phonon transport of the confined geometry that exist at such contacts. The influence of CNT array properties, e.g. diameter and density, are explicitly incorporated into the thermal model, which agrees well with experimental measurements of thermal resistances as a function of pressure for different types of interfaces. The model reveals that: (1) ballistic thermal resistance dominates at the CNT array interface; (2) the overall performance of CNT array interfaces is most strongly influenced by the thermal resistance at the contacts between free CNT ends and the opposing substrate surface (one-sided interface) or the opposing CNT array (two-sided interface); and (3) dense arrays with high mechanical compliance reduce the thermal contact resistance of CNT array interfaces by increasing the real contact area in the interface.     

Finite element analysis of the effect of weld geometry and load condition on fatigue strength of lap joint

Many welded lap joints are subject to fluctuating loads, and fatigue failure plays a dominant role in the failure of such structures. Based on the concepts of linear elastic fracture mechanics, the effects of weld geometry, load conditions and the boundary constraints on fatigue strength of a ferrite–pearlite steel lap joint were investigated in this paper using the finite element method. Paris's power law was used to predict the fatigue life of the joints. Various weld geometry including the leg length, flank angle and the size of lack-of-penetration were considered during the calculation of fatigue strengths. The loads include tension, bending and their combinations. It was found that the existence of a root crack has no influence on the fatigue strength of the joint, under the relevant load conditions. The existence of a toe crack is also of no influence on the fatigue strength of the joint if the applied loads, e.g. DOB>0 in this paper, produce a compressive stress field at the top region of the main plate. For a lap joint with a free transverse (Y direction in this paper) boundary constraint at the main plate, a joint with a smaller size of lack-of-penetration, a reasonably large weld leg and smaller flank angle is recommended to be used in engineering practice, in order to obtain a higher fatigue strength. For a lap joint, with transverse fixed boundary constraint at the main plate, the fatigue strength increases with a decrease of weld size but the influence of flank angle depends on the type of load carried. It was also found that the size reduction in FE model is a significant influence on the calculated fatigue strength; the use of reduced size FE model gives a much higher overestimate of fatigue strength of the joint.     

Low-temperature contacts through SixNy-antireflection coatings for inverted a-Si:H/c-Si hetero-contact solar cells

High-temperature contact firing of screen printable metal pastes is getting more problematic as silicon wafers used in solar cell production are becoming thinner. Besides, an electronic degradation of the Si_xN_y/c-Si interface occurs at these temperatures especially if the Si_xN_y layer is directly deposited onto high-quality absorbers as on the front side of a-Si:H/c-Si hetero-contact solar cells of inverted geometry. The latter structure has been proposed as an easy producible high-efficiency solar cell. Low-temperature alternatives such as local ablation of Si_xN_y with 355 nm laser radiation are examined with regard to the stability of the electronic quality of passivated areas between the openings in the Si_xN_y layer. Contactless time-resolved microwave conductivity measurements (TRMC) are applied to measure changes in electronic passivation after these treatments. Subsequent galvanic metallization of the openings is optimized for its use as ohmic contacts.     

Experimental determination of the interfacial heat transfer during cooling and solidification of molten metal droplets impacting on a metallic substrate: effect of roughness and superheat

The interfacial heat transfer between a solidifying molten metal and a metallic substrate is critical in many processes such as strip casting and spray deposition. As the molten metal cools down and solidifies, the interface undergoes a change from the initial liquid/solid contact to a solid/solid contact, leading to very dynamic variations in the rate of interfacial heat transfer. This article presents the results of an experimental study of the contact heat transfer when molten nickel or copper droplets are dropped on an inclined metallic substrate. The interfacial heat transfer coefficient, h, between the melt and the substrate is evaluated by matching model calculations with the top splat surface temperature history measured by a fast-response pyrometer. The results suggest that a high value of the interfacial heat transfer coefficient h (10⁴ to 3×10⁵ W/m² K) is achieved when the molten splat is in contact with the substrate, followed by a smaller value (<10⁴ W/m² K) during the later stages of solidification and the solid cooling phase. A parametric study was performed to investigate the effect on h of the metal/substrate materials combination, the melt superheat, and the substrate surface roughness, and some of the results are also presented.     

Free convective heat transfer in an enclosure with an internal louvered blind

A numerical study has been conducted of free convection in a tall vertical enclosure with an internal louvered metal blind. The study considers the effects of Rayleigh number, enclosure aspect ratio, and blind geometry on the convective heat transfer. The numerical model has been validated against experimental measurements and the results have been presented in terms of an empirical correlation for the average Nusselt number. The correlation is applicable to an enclosure with an internal metal blind. It has been shown that the Nusselt number correlation can be combined with a simple one-dimensional model to closely predict the enclosure U-value.     

Methane decomposition to produce COx-free hydrogen and nano-carbon over metal catalysts: A review

Catalytic decomposition of methane (CDM) is a promising technology for producing CO_x-free hydrogen and nano-carbon, meanwhile it is a prospective substitute to steam reforming of methane for producing hydrogen. The produced hydrogen is refined and can be applied to the field of electronic, metallurgical, synthesis of fine organic chemicals and aerospace industries. However, the CDM for CO_x-free hydrogen production is still in its infancy. The urgent for industrial scale of CDM is more important than ever in the current situation of huge CO_x emission. This review studies CDM development on Ni-based, noble metal, carbon and Fe-based catalysts, especially over cheap Fe-based catalyst to indicate that CDM would be a promising feasible method for large hydrogen production at a moderate cheap price. Besides, the recent advances in the reaction mechanism and kinetic study over metal catalysts are outlined to indicate that the catalyst deactivation rate would become more quickly with increasing temperature than the CDM rate does. This review also evaluates the roles played by various parameters on CDM catalysts performance, such as metal loading effect, influences of supports, hydrogen reduction, methane reduction and methane/hydrogen carburization. Catalysts deactivation by carbon deposition is the prime challenge found in CDM process, as an interesting approach, a molten-metal reactor to continually remove the floated surface solid carbons is put forwarded in accordance to overcome the deactivation drawback. Moreover, particular CDM reactors using substituted heating sources such as plasma and solar are detailed illustrated in this review in addition to the common electrical heating reactors of fixed bed, fluidized bed reactors. The development of high efficiency catalysts and the optimization of reactors are necessary premises for the industrial-scale production of CDM.     

Comparative finite element analysis of the stress–strain states in three different bonded solid oxide fuel cell seal designs

One of the critical issues in designing and fabricating a high performance planar solid oxide fuel cell (pSOFC) stack is the development of the appropriate materials and techniques for hermetically sealing the metal and ceramic components. A second critical issue is ensuring that the brittle ceramic cell constituents, i.e. the electrodes and electrolyte, exhibit high mechanical reliability by mitigating potential sources of thermal-mechanically induced stresses that can lead to fracture during operation and/or shutdown. A foil-based sealing approach is currently being developed that appears to offer good hermeticity and mechanical integrity, while minimizing the generation of high stresses in either of the joint's substrate materials. Based on the concept's viability, demonstrated in prior experimental work, numerical analyses were conducted to evaluate the behavior and benefits of the seal in a configuration prototypic of current pSOFC stack designs. This paper presents recent results from finite element (FE) simulations of a planar cell using the foil-based seal, along with companion analyses of the more conventionally employed glass-ceramic and brazed joints. The stresses and deformations of the components were evaluated at isothermal operating and shutdown temperatures. The results indicate that the foil seal is able to accommodate a significant degree of thermal mismatch strain between the metallic support structure and the ceramic cell via elastic deformations of the foil and plasticity in the foil-to-cell braze layer. Consequently the cell stresses in this type of seal are predicted to be much lower than those in the glass-ceramic and brazed designs, which is expected to lead to improved stack reliability. This ability to accommodate large thermal strain mismatches allows the design requirement of thermal expansion matching between ceramic and metal stack components to be relaxed and expands the list of candidate materials that can be considered for the metal frames and interconnects.     

Use of the cylindrical instability stress for blunt metal loss defects in linepipe

Assessment of metal loss defects in gas pipelines can be analysed by a number of methods. In analyses with finite element methods a failure criterion is required. A material property is introduced, the cylindrical instability stress, which determines the plastic collapse of cylindrical pressure containing vessels. The use of this is extended to cover blunt metal loss defects. Some published finite element studies of defected vessels are re-analysed using this failure criterion.

The cylindrical instability stress is a more accurate failure criterion for plastic collapse in pipelines and pressure vessels than commonly used measures such as flow stress, Specified Minimum Yield Stress plus 10 ksi or multiplied by 1.1. It can be used in determining burst pressure of defected and un-defected pressure vessels and piping.     

SIF evaluation and stress analysis of drillstring threaded joints

In a study by Tafreshi and Dover [1], [2], [3] stress analysis of drillstring threaded joints under axial, bending and torsion loadings was carried out using the finite element method. Stress concentrations were determined on a variety of threaded joints in order to predict the fatigue life of these components. Here some of those results are presented in more detail. In this study it is shown that in the case of bending, axisymmetric solid elements with non-linear, asymmetric deformation with fourier interpolation can be employed which reduces the computational time and modelling in comparison with the full three-dimensional analysis. Quasi semi-elliptic circumferential internal and external cracks in tubes with the same dimensions of the pins or boxes of a series of joints are modelled and analysed using the J-integral method. The result of this fully three-dimensional crack analysis shows the same trend as experimental [3] and published results [4].     

Electrodeposition of PEM fuel cell catalysts by the use of a hydrogen depolarized anode

Up to 30% of the expensive catalyst metal in conventional fuel cell catalysts is not utilized in fuel cells caused by an absence of contact to either the ion conducting, electron conducting or educt phase. This contact can be improved by in situ electrodeposition with a precursor layer which is mostly done in a galvanostatic mode in the literature. In this paper electrochemical deposition with a hydrogen depolarized anode is described and so a potentiostatic electrodeposition under the control of the working-electrode potential and dry working-electrode conditions is enabled. This potentiostatic electrodeposition with a hydrogen depolarized anode significantly increases the performance of the fuel cell.