Reliability Of Welded Joints Research Articles

Softening mechanism and failure behavior of 9Cr oxide dispersion strengthened steel resistance spot welded joint

Oxide dispersion strengthened (ODS) steel is one of the most promising materials for nuclear applications. However, there are fewer studies on the spot welding of ODS steel, and the softening mechanism of ODS steel resistance spot welded joint is unclear. The microstructural features of the ODS resistance spot welded joint was characterized to analyze the softening mechanism. The failure behavior and tensile shear performance were studied combined with finite element simulation. The results showed that the fusion boundary has the lowest hardness and the softening zone is divided into fusion zone(FZ) softening and heat affected zone(HAZ) softening according to different location in the joint. In FZ softening, the softening is mainly caused by the presence of δ-ferrite and fewer oxides as well as lower dislocation density. In HAZ softening, the softening is mainly caused by larger grains, less oxides. There are two types of failure, the fracture initiated at HAZ softening shows a brittle feature, while initiated at FZ softening shows a ductile feature. The softening leads to a reduction in tensile-shear strength. Thus, the control of softening zone is critical to improve the reliability of welded joints.

Fatigue Reliability Based on Predicted Posterior Stress Ranges Determined from Strain Measurements of Wind Turbine Support Structures

In the present paper, an approach for updating the continuous stress range distribution of a welded connection of a wind turbine support structure with predicted information from strain measurements is presented. Environmental conditions, such as wind or, in offshore fields, waves and currents, in combination with rotor excitations generate cyclic stresses affecting the reliability of welded joints of the support structure over the service life. Using strain measurements, these conditions can be monitored, and the resulting stress ranges, under consideration of measurement, mechanical and material uncertainties, can be reconstructed. These stress ranges can be used as an input for updating the prior probability density function (PDF) of the stress ranges predicted by the overall dynamics and a detailed design analysis. Applying Bayesian probability theory and decision theoretical implications, the predicted posterior probability density of the stress ranges is calculated based on the design information and uncertainties. This approach is exemplified, and it is shown how the predicted stress ranges and the design stress ranges are distributed. The prior and the predicted posterior stress ranges are used for a reliability calculation for potentially entering a pre-posterior decision analysis.

Fatigue reliability of welded joints accounting for uncertainties in weld geometry

Fatigue reliability assessment of welded joints can consider the uncertainties from various sources and can establish the design criteria and inspection plans. The present paper aims to address the uncertainties explicitly. The notch strain approach that accounts for the effects of the misalignment, local notch, and crack-like imperfection, which is referred to as secondary notch, is employed. Sensitivity analyses indicate that uncertainties in misalignment, flank angle, weld toe radius together with secondary notch are all important sources of the total uncertainty. The practical difficulty in the determination of the uncertainty models of the geometric parameters is discussed, and the acceptance limits for fabrication are used to derive the uncertainty models. And finally, the impacts of acceptance limits on the fatigue reliability of welded joints are assessed. The results indicate that the weld toe radius and secondary notch depth significantly affect the reliability index and the fatigue reliability increases with the weld quality of the welded joint becomes higher.

Analysis of Reliability of Welded Connections Liquefied Gas and Condensate Pipelines

To assess the strength and reliability of welded joints, the metal of welded joints of the unstable condensate pipeline was tested for static tension and low-cycle durability. The tests were carried out on defect-free specimens, specimens with defects such as lack of fusion, undercut, single pore, single slag inclusion and specimens with defects combining lack of fusion and undercut. A comparative analysis of the effect of defects combining lack of penetration and undercut and separately located defects such as lack of penetration, undercut, single pore, single slag inclusion on tensile strength and low-cycle durability of welded joints is carried out.

Investigations of fracture toughness and reliability of welded joints from low-carbon and low-alloy steels

ABSTRACT The paper presents results of studies on technological defectiveness, structural and mechanical heterogeneity and crack resistance characteristics of welded joints from lowcarbon and low alloy steels, including tests at low temperatures. The computational and experimental complex for evaluating the strength, reliability and safety of welded structures is described, within the framework of which a number of applied problems are solved based on the fracture mechanics criteria.

СРЕДНЕКВАДРАТИЧНЫЕ ОТКЛОНЕНИЯ АТОМОВ – НАНОМАСШТАБНЫЙ ИНДИКАТОР СОСТОЯНИЯ ПОВЕРХНОСТИ

Актуальность исследования связана с повышением надежности сварных соединений паросиловых установок и тем самым с рациональным использованием материалов изделий за счет совершенствования и развития методов изучения их ресурсных физических характеристик. Цель: тестирование новых наномасштабных структурных характеристик III рода для оценок состояния поверхности. Объект: образцы жаропрочной стали трубных поверхностей нагрева паровых котлов. Методы: физическое моделирование условий эксплуатации путем термоциклирования в электропечи МИМП-10УЭ, холодного циклического деформирования с применением гидравлического пресса, рентгенометрия образцов с оценкой среднеквадратичных отклонений атомов и внутренних структурных напряжений на рентгеновских дифрактометрах типа ДРОН, анализ морфологии поверхности с помощью портативного электронного микроскопа типа PENSCKOPE с увеличением ´20. Результаты. Проведены экспериментальные исследования, связанные с количественной оценкой колебательной амплитуды атомов и ее эволюцией при циклических термических и механических нагрузках. В результате исследований предложен новый диагностический параметр в виде колебательной амплитуды атомов , отражающий фундаментальную природу сил межатомных связей. Продемонстрирована корреляция между среднеквадратичными отклонениями атомов (микромасштабный уровень свойств), представляющими микроструктурные характеристики III рода, с макромеханическими характеристиками и (макромасштабный уровень свойств). Так как условный предел текучести и временное сопротивление разрушению связаны с прочностью и разрушением, то и среднеквадратичные отклонения атомов могут быть использованы в качестве индикатора поврежденности и признака достижения предельных состояний. Параметр может служить также характеристикой текущего индивидуального физического состояния металла. Предложена новая методология оценки текущего и наступления предельных состояний. Показано, что восстановление структуры контролируется на атомном микромасштабном уровне по колебательной амплитуде и напряжениям I рода.

Application of adaptive surrogate models in time-variant fatigue reliability assessment of welded joints with surface cracks

The time-variant fatigue reliability of welded joints subjected to stochastic loading is assessed based on the PHI2 method, which takes the time-variant reliability problem as a two-component parallel system reliability problem. The complex fatigue process that takes place at the weld toe is modeled as a semi-elliptical surface crack growth process. Surrogate models, representing surface crack sizes at a time instant, are applied to solve the reliability problem involving time-consuming fatigue crack growth analyses. Polynomial regression models and Kriging interpolation models are both employed. Their corresponding adaptive procedures are also adopted to improve the reliability results. Some adjustments of the active refinement algorithm based on the Kriging model are proposed to make it more suitable for the present study. The time-variant fatigue reliability assessment of a T-plate welded joint is carried out. Without using the adaptive procedures, the Kriging models can better represent the crack sizes compared with the polynomial models. The outcrossing rate formulated by the PHI2 method is sensitive to the reliability index of the first component of the two-component parallel system. The effectiveness of the adaptive procedures, which improve the reliability results, is demonstrated.

Dynamic Bayesian network-based system-level evaluation on fatigue reliability of orthotropic steel decks

Fatigue fractures can be frequently observed in welded joints in orthotropic steel decks (OSDs) after just a few decades of operation, which become the major deterioration mechanism deterring the serviceability of OSDs. In this paper, a novel dynamic Bayesian network (DBN) model has been established for the fatigue reliability analysis of OSDs at system-level. The exact inference algorithm is applied in the DBN model with discrete variables. Special modifications have been made on the existing algorithm to improve the computational efficiency in dealing with the deck system consisting of a considerable number of joints. Using the DBN model, the fatigue reliability of welded joints can be predicted and updated with the inspection and monitoring results at system-level. At the same time, a framework is established for the system-level reliability considering the fatigue fracture of rib-to-deck (RD) joints, the dominant cracking pattern affecting the serviceability of OSDs. For illustration, a typical OSD bridge in China has been selected to carry out a case study. To derive the stress spectrum required by the DBN model, the stochastic traffic model is employed, and the influence-based Monte Carlo simulations have been carried out. As a result, the fatigue reliability can be predicted at both component- and system-levels. Meanwhile, the observation of the traffic and the inspection result has been fused into the DBN model to update the deteriorating state of the deck system. Besides, the effect of enhancement and maintenance has been highlighted, including the enhancement in fatigue strength at the construction stage, and the repair and traffic control during the operation stage.

Prediction of fatigue limit improvement in needle peened welded joints containing crack-like defects

PurposeThe purpose of this paper is to estimate the acceptable defect size amax after needle peening (NP) and predict the fatigue limit improvement through the use of NP for an austenitic stainless steel welded joint containing an artificial semi-circular slit on a weld toe.Design/methodology/approachResidual stress and hardness distribution were measured. Microstructures around the weld toe were observed to clarify the cause for the change in hardness after NP. Finite element method analysis was used to analyze the change in the stress concentration following NP. Fracture mechanics was used to evaluate amax after NP. The fatigue limits before and after NP were predicted by determining amax for several levels of stress amplitude.FindingsThe tensile residual stress induced at the surface of the weld toe prior to NP changed to a compressive residual stress after NP. The residual stress near the surface layer after NP exceeded the yield stress prior to NP due to the increase in yield stress as a result of work hardening as well as the generation of a deformation-induced martensitic structure. The stress concentration was reduced due to the shape improvement caused by NP. The estimation value of amax after NP and the prediction results of fatigue limits were in good agreement with the fatigue test results.Practical implicationsThe proposed method is useful in improving the reliability of welded joints used in large steel structures, transportation equipments and industrial machines.Originality/valueFrom an engineering perspective, it is essential to estimate amax and the fatigue limit of welded joints with crack-like defects. However, it is unclear as to whether it is possible to predict amax and the effects of NP on the fatigue limit for stainless steel welded joints.

Technology of thermal welding with ultrasonic weld joint treatment as applied to NPP formworks

Increasing the strength, reliability and durability of welded structures is an important problem in welding engineering. A significant influence is exerted by residual stresses. Uneven heating of the product during welding causes its uneven temperature deformation. The product material solidity prevents the free temperature deformation of its individual parts, resulting in the formation of stresses and plastic deformation of some parts of the joint metal during welding, and after cooling, welding stresses and deformations remain in the product.The paper describes an innovative technology of thermal welding with ultrasonic weld joint treatment as applied to aluminum sliding formworks used in NPP construction. Ultrasonic treatment in the process of welding greatly increases the strength of formwork weld joints by reducing their residual stresses, grain size and degassing. The structure and properties of aluminum welds become identical to the base metal. Thermal welding with ultrasonic weld joint treatment will improve the reliability of welded joints and increase their time in service. The paper presents the results of theoretical and experimental studies of ultrasonic effects on the welded joints and heat-affected zone.

Manufacture of high-quality ribbed titanium panels

Titanium alloys are widely used in the aerospace industry on account of their high strength, fatigue strength, impact strength, and corrosion resistance. Airplane components such as ribbed panels must satisfy strict requirements on the quality of weld joints and the dimensional precision. The welding of titanium alloys leads to the formation of flaws at the joint and impairs the performance of the part; their geometric dimensions are distorted on account of the welding stress; pores and microstress appear in the seam; and the mechanical and other characteristics are impaired. That calls for comprehensive analysis of the manufacture of ribbed titanium panels, all the way to the heat treatment of the final product, so as to discover the processes that are most likely to generate defects. The manufacture of the workpieces—including their preparation for welding, the welding conditions, and subsequent heat treatment—affects the static and cyclic strength of welded ribbed panels, with variation in the number of weld-seam defects and their total area. The results obtained in the present work permit prediction of the properties and reliability of welded structures and the maintenance of identical properties of the basic metal and the seam metal, with the guaranteed absence of external and internal effects, on the basis of the selected production processes. It is found that the properties and reliability of weld joints will match those of the basic VT20 titanium alloy in ribbed panels in the following conditions: welding with specific cooling rate in the phase-transition temperature range; final milling, turning, degreasing, and drying of the surface of the joined edges; light annealing after welding in air; subsequent sandblasting; and gas-laser layout cutting in a nitrogen or argon atmosphere, with or without milling.

К вопросу о надежности сварных соединений сталей при эксплуатации в условиях низких климатических температур

The article discusses the issues of ensuring of operational reliability of welded joints in steel structures for critical applications operating at low climatic temperatures. It is established that the zone of permanent joints have a pronounced structural heterogeneity, which is in the process of operation shows a tendency to the formation of microdefects leading to the rapid destruction of the structure as a whole. It is shown that structural heterogeneity in the area of the permanent connection can be reduced by applying methods of adaptive pulse-arc welding

Simulation of Structure Formation Processes of Dissimilar Steels Welded Joints Using an Intermediate Layer

This paper shows the results of solving a 3D problem to define types of structures and tensions which can appear during the butt contact welding process of dissimilar steels through low carbon steel inserts. The finite element method to calculate welded structures was used. The thickness of inserts was the main variable parameter. According to the results of numerical simulation using inserts can increase the reliability of welded joints between pearlitic high-carbon steel and austenitic chromium-nickel steel. The best result was obtained by using an insert with a thickness less than 20 mm. Structural studies of the welded joints between high-carbon steel and chromium-nickel steel through low-carbon inserts confirm the results of mathematical modeling.

A method for evaluating the reliability of welded joints on the basis of statistical processing of radiographic inspection results

The mechanical properties, radiographic and metallographic analyses of the structure of welded joints produced by argon-shielded arc welding after repair by resistance spot welding are compared. It is shown that the suitability for repair improves if the method of argon-shielded arc welding of defects is replaced by resistance spot welding.

Characteristic nonmetallic inclusions in welded joints oil and gas of pipelines

Trunk pipelines work under a cyclic mechanical load because when oil or gas pumped pressure is changing constantly - pulsates. Therefore, fatigue phenomena are a common cause of accidents. Often fatigue phenomena are formed near nonmetallic inclusions. To know how the characteristics of non-metallic inclusions influenced on the probability of an accident, need to use the most modern research methods. Modern methods of study found that on the accident rate of welded joints of pipelines most influence have morphological type, composition and size nonmetallic inclusion, this effect more important than common level of pollution by nonmetallic inclusions. The article presents the results of investigations of welded joints produced with the use of different popular welding materials. We used the methods, described in the state standards, scanning electron microscopy, spectral microprobe analysis and nanoindentation. We found that non-metallic inclusions are stress concentrators because they have shrunk. Between metal and non-metallic inclusions formed blank space due to shrinkage, it strengthens the differential properties of this boundary. Nonmetallic inclusion is not fixed, it can move. This means that in a pulsation, nonmetallic inclusion strikes on the metal inside. We found that the most dangerous emergency inclusion size group 5-6 acute form. The data that we have received means that the control of quality and reliability of welded joints need define contamination by characteristics nonmetallic inclusions, it more important than total pollution.

Increasing the reliability of welded joints in high-strength V-1461 aluminium–lithium alloy

Filler materials for welding V-1461 alloy, ensuring high cracking resistance, corrosion resistance and mechanical properties, are selected. The results of tests of the welded joints in static tensile loading and bending and the impact toughness of the weld metal and low-cycle fatigue strength are presented. The experimental results show that the application of impact ultrasound treatment increases by an order of magnitude the values of low-cycle fatigue resistance of the welded joints as a result of the formation of the nanostructured surface layers. The level of residual stresses in welded joints is determined.

Increasing the reliability of welded joints in V-1461 high-strength aluminium–lithium alloy

Filler materials for welding V-1461 alloy, increasing the cracking resistance, corrosion resistance, and mechanical properties, are selected. The results of the tests of the welded joints by static tensile loading and bending and the impact toughness of the weld metal and low-cycle fatigue are presented. The results show that the application of impact ultrasound treatment increases by an order of magnitude the values of low-cycle fatigue of the welded joints as a result of the formation of a nanostructured surface layer. The level of residual stresses in the welded joints is determined.

Specialized equipment for automatic welding of aircraft structures

The main technical characteristics of new equipment for automatic welding of aircraft structures characterized by high quality and reliability of welded joints produced from aluminium and titanium alloys are described. The efficiency and advantages of equipment in comparison with similar foreign equipment are outlined.

Improving the reliability of welded joints in bulk-quenched electrosteel rail

Metallurgical methods of improving the reliability of the weld seam in bulk-quenched electrosteel rail are considered: optimization of the chemical composition in terms of the basic elements and total impurity content; improvement in plastic properties of the rail by reducing the hardness; and removal of nonmetallic inclusions from the steel. Instability is seen in the weldability of the rails and also in the composition, with the same set of mechanical properties. The need for improvement in welding technology is shown; without the elimination of existing problems, the weldability of electrosteel rail cannot be guaranteed.

ОЦЕНКА НАДЕЖНОСТИ СВАРНЫХ СОЕДИНЕНИЙ ТОНКОСТЕННЫХ КОНСТРУКЦИЙ ПО РЕЗУЛЬТАТАМ ИХ НЕРАЗРУШАЮЩЕГО КОНТРОЛЯ

The problem of estimating the reliability of welded thin-walled structures under static loading is solved. The stochastic nature of the stability of structures is determined by the presence of defects in welded joints. The earlier introduced by the present authors condition when the maximum value of the membrane component of the plastic crack opening reaches its critical value is used as the failure criterion for surface defects. The probability distribution density of the depth of defects is determined based on the corresponding density for detected defects and the probability of their detection. For the Poisson-type distribution of a random value of the number of crack-like defects, the problem is reduced to determining the probability of the absence in the welded joint of cracks with the opening in excess of the allowable value. A case of correcting unallowable defects is studied, where the reliability of the welded joints is determined by the defects overlooked duringthe inspection. Numerical analyses of the reliability of welded joints based on the results of the ultrasonic detection are presented. Key words: welded joint, defect, failure criterion, defect detection probability, reliability.