EFFECT OF GMAW TWIN WIRE TANDEM PARAMETERS ON MECHANICAL PROPERTIES OF WELDED JOINTS

Tandem welding has the advantage of higher productivity over a limitation considering narrow groove joints. Tandem has higher deposition and heat input. To study the effect of variation in robotic tandem parameters on fatigue properties of welded joints, the following parameters were investigated, viz. first case root & the second pass using a single wire, subsequent run tandem twin and second case using root pass using a single wire, subsequent run using tandem twin wire welding. Butt joint samples were subjected to ultrasonic testing, mechanical & metallurgical testing, and fatigue testing. Found Group 1 tensile strength, yield strength, % elongation & impact value to be 12 %, 9 %, 23 % & 14 %, respectively, higher than Group 2. Explained variations in microhardness & better fatigue life were observed for Group 1. This investigation will help manufacturers in decision-making while selecting tandem parameters considering productivity or reliability

Optimisation of welding parameters to mitigate the effect of residual stress on the fatigue life of nozzle–shell welded joints in cylindrical pressure vessels.

Doctoral Degree. University of KwaZulu-Natal, Durban.The process of welding steel structures inadvertently causes residual stress as a result of thermal cycles that the material is subjected to. These welding-induced residual stresses have been shown to be responsible for a number of catastrophic failures in critical infrastructure installations such as pressure vessels, ship’s hulls, steel roof structures, and others. The present study examines the relationship between welding input parameters and the resultant residual stress, fatigue properties, weld bead geometry and mechanical properties of welded carbon steel pressure vessels. The study focuses on circumferential nozzle-to-shell welds, which have not been studied to this extent until now. A hybrid methodology including experimentation, numerical analysis, and mathematical modelling is employed to map out the relationship between welding input parameters and the output weld characteristics in order to further optimize the input parameters to produce an optimal welded joint whose stress and fatigue characteristics enhance service life of the welded structure. The results of a series of experiments performed show that the mechanical properties such as hardness are significantly affected by the welding process parameters and thereby affect the service life of a welded pressure vessel. The weld geometry is also affected by the input parameters of the welding process such that bead width and bead depth will vary depending on the parametric combination of input variables. The fatigue properties of a welded pressure vessel structure are affected by the residual stress conditions of the structure. The fractional factorial design technique shows that the welding current (I) and voltage (V) are statistically significant controlling parameters in the welding process. The results of the neutron diffraction (ND) tests reveal that there is a high concentration of residual stresses close to the weld centre-line. These stresses subside with increasing distance from the centre-line. The resultant hoop residual stress distribution shows that the hoop stresses are highly tensile close to the weld centre-line, decrease in magnitude as the distance from the weld centre-line increases, then decrease back to zero before changing direction to compressive further away from the weld centre-line. The hoop stress distribution profile on the flange side is similar to that of the pipe side around the circumferential weld, and the residual stress peak values are equal to or higher than the yield strength of the filler material. The weld specimens failed at the weld toe where the hoop stress was generally highly tensile in most of the welded specimens. The multiobjective genetic algorithm is successfully used to produce a set of optimal solutions that are in agreement with values obtained during experiments. The 3D finite element model produced using MSC Marc software is generally comparable to physical experimentation. The results obtained in the present study are in agreement with similar studies reported in the literature

The application of high power lasers to the welding of tee section joints in ship production

PhD ThesisThe use of computers by naval architects has revolutionised ship design and -construction management. The use of high power laser technology could similarly revolutionise production processes to produce a quantum leap in productivity. Facilitating low heat input materials processing, the laser is suited to various cutting, welding and heat treatment applications in shipbuilding to increase productivity through improved product accuracy. From these processes, the Author has concentrated on the application of high power lasers to the welding of tee section joints - the most common joint configuration in ship structures - by a single sided method (skid welding) to give both the lowest possible heat input and greatest flexibility. -Using a lOkW laser, single pass fully penetrating skid welds may be produced for joints in plate of up to 15mm thick, but using this size of laser, production parameter envelopes to produce visually and structurally sound joints reduce in size as plate thickness increases to greater than 10mm. It is shown that fully penetrating laser skid welds produced in steel conventionally used for surface vessel construction are of superior structural quality to fillet welds as required by classification society rules. The work has shown that achieving process consistency in an automated production based skid welding workstation operating with existing levels of joint tolerance will be dependent not only on well designed laser and beam delivery harware but also on suitable on-line adaptive control systems. It has been demonstrated that by employing laser skid welding for steelwork fabrication, an increase in productivity can be gained, principally through increased processing speed and improved product accuracy.British Shipbuilders: The Science and Engineering Research Council

The safety analysis concept of welded components under cyclic loads using fracture mechanics method

Fracture Mechanics process of Welded Joint is a very vast research area and has many possibilities for solution and prediction. Although the fatigue strength (FAT) and stress intensity factor (SIF) solutions are reported in several handbooks and recommendations, these values are available only for a small number of specimens, components, loading and welding geometries. The available solutions are not always adequate for particular engineering applications. Moreover, the reliable solutions of SIF are still difficult to find in spite of several SIF handbooks have been published regarding the nominal applied SIF. The effect of residual stresses is still the most challenge in fatigue life estimation. The reason is that the stress distributions and SIF modified by the residual stresses have to be estimated. The stress distribution is governed by many parameters such as the materials type, joint geometry and welding processes. In this work, the linear elastic fracture mechanics (LEFM), which used crack tip SIFs for cases involving the effect of weld geometry, is used to calculate the crack growth life for some different notch cases. The variety of crack configurations and the complexity of stress fields occurring in engineering components require more versatile tools for calculating SIFs than available in handbook’s solutions that were obtained for a range of specific geometries and load combinations. Therefore, the finite element method (FEM) has been used to calculate SIFs of cracks subjected to stress fields. LEFM is encoded in the FEM software, FRANC, which stands for fracture analysis code. The SIFs due to residual stress are calculated in this work using the weight function method. The fatigue strength (FAT) of load-carrying and non-load carrying welded joints with lack of penetration (LOP) and toe crack, respectively, are determined using the LEFM. In some studied cases, the geometry, material properties and loading conditions of the joints are identical to those of specimens for which experimental results of fatigue life and SIF were available in literature so that the FEM model could be validated. For a given welded material and set of test conditions, the crack growth behavior is described by the relationship between cyclic crack growth rate, da/dN, and range of the stress intensity factor ( K) , i.e., by Paris’ law. Numerical integration of the Paris’ equation is carried out by a FORTRAN computer routine. The obtained results can be used for calculating FAT values. The computed SIFs along with the Paris’ law are used to predict the crack propagation. The typical crack lengths for each joint geometry are determined using the built language program by backward calculations. To incorporate the effect of residual stresses, the fatigue crack growth equations which are sensitive to stress ratio R are recommended to be used. The Forman, Newman and de Konig (FNK) solution is considered to be the most suitable one for the present purpose. In spite of the recent considerable progress in fracture mechanics theories and applications, there seems to be no, at least to the author’s knowledge, systematic study of the effect of welding geometries and residual stresses upon fatigue crack propagation based completely on an analytical approach where the SIF due to external applied load (Kapp) is calculated using FEM. In contrast, the SIF due to residual stresses (Kres) is calculated using the analytical weight function method and residual stress distribution. To assess the influence of the residual stresses on the failure of a weldment, their distribution must be known. Although residual stresses in welded structures and components have long been known to have an effect on the components fatigue performance, access to reliable, spatially accurate residual stress field data are limited. This work constitutes a systematic research program regarding the concept for the safety analysis of welded components with fracture mechanics methods, to clarify the effect of welding residual stresses upon fatigue crack propagation.Die Bewertung einer Schweißnaht ist ein großes Forschungsgebiet und hat viele Möglichkeiten für Lösungskonzepte und Vorhersagen. Obwohl für die Schwingfestigkeit und die Spannungsintensitätsfaktor (SIF)-Lösungen in verschiedenen Handbüchern Empfehlungen ausgewiesen sind, sind diese Werte nur für eine geringe Anzahl von Proben, Komponenten, Belastungsfälle und Schweißgeometrien verfügbar. Die vorhandenen Lösungsansätze sind nicht immer für spezielle technische Anwendungen geeignet. Darüber hinaus sind zuverlässige bewährte Lösungen von Spannungsintensitätsfaktoren immer noch schwierig zu finden, obwohl verschiedene SIF-Handbücher mit Hinweis auf den anliegenden nominalen SIF veröffentlicht sind. Der Einfluss von Eigenspannungen ist eine der größten Herausforderungen bei der Lebensdauerabschätzung. Aufgrund der Tatsache, dass infolge der Eigenspannungen sowohl die Spannungsverteilung als auch der SIF verändert werden, muss eine Abschätzung erfolgen. Die Spannungsverteilung wird durch viele Parameter beeinflusst, wie zum Beispiel den Werkstoff, die Nahtgeometrie und den Schweißprozess. In der vorliegenden Arbeit wurde für die Berechnung des Ermüdungsrisswachstums unter verschiedenen Kerbfällen das Konzept der linear-elastischen Bruchmechanik (LEBM) verwendet, welches K-Lösungen für die Rissspitze bei unterschiedlichen Fällen der Schweißgeometrie berücksichtigt. Aufgrund der Komplexität der Risskonfigurationen und der Spannungsfelder in praxisrelevanten Komponenten werden weitere Hilfsmittel zur Berechnung von Spannungsintensitätsfaktoren benötigt, welche die herkömmlichen Lösungen in Handbüchern erweitern. Deshalb wurde die Finite Elemente Methode (FEM) zur Berechnung von Spannungsintensitätsfaktoren an Rissen verwendet. Die LEBM wird in der FEMSoftware FRANC berücksichtigt. Die aus Eigenspannungen resultierenden Spannungsintensitätsfaktoren wurden mit Hilfe der Gewichtsfunktionsmethode berechnet. Die Ermüdungslebensdauer (Schwingfestigkeit) von tragenden und nichttragenden Schweißnähten mit ungenügender Durchschweißung beziehungsweise Kerbriss wurden mit Hilfe der LEBM durch Integration der Zyklischen Risswachstumskurve ermittelt. Zur Validierung des FEM-Modells konnte in einigen untersuchten Fällen auf experimentelle Ergebnisse zur Lebensdauer und zum SIF aus der Literatur zurückgegriffen werden, wo identische Geometrien, Materialeigenschaften und Belastungsverhältnisse der Naht vorlagen. Unter Vorgabe des Werkstoffes und der Prüfbedingungen wurde das Risswachstumsverhalten mit dem Zusammenhang von Risswachstumsgeschwindigkeit da/dN und zyklischem Spannungsintensitätsfaktor K mit dem Paris-Gesetz beschrieben. Eine numerische Integration der Paris-Gleichung erfolgte über ein FORTRAN-Programm. Die damit erhaltenen Ergebnisse sind als Ermüdungslebensdauer (Schwingfestigkeit) verwendbar. Die berechneten SIF‘en entlang der Paris-Geraden werden zur Vorhersage des Risswachstums benutzt. Die typischen Risslängen für jede Nahtgeometrie wurden mit Hilfe des eigens integrierten Programmes ermittelt. Zur Berücksichtigung des Einflusses von Eigenspannungen wird empfohlen, Risswachstumsgleichungen zu nutzen, die empfindlich auf das Spannungsverhältnis R reagieren. Für die vorliegende Zielsetzung gilt der Lösungsansatz nach Forman, Newman und de Konig (FNK) als der am besten geeignete. Trotz der jüngsten, beträchtlichen Fortschritte in den bruchmechanischen Theorien und Anwendungen sind systematische Studien zum Einfluss der Schweißgeometrie und der Eigenspannungen auf das Ermüdungsrisswachstum, in welchen der SIF aufgrund extern anliegender Beanspruchungen (Kapp) mit der FEM berechnet wurde, in der Literatur kaum vorhanden. Im Gegensatz dazu wurde der SIF infolge von Eigenspannungen (Kres) mit Hilfe der analytischen Gewichtsfunktionsmethode und der Eigenspannungsverteilung berechnet. Um den Einfluss von Eigenspannungen auf das Versagen einer Schweißverbindung abzuschätzen, muss deren Verteilung bekannt sein. Obwohl die Wirkung von Eigenspannungen auf das Ermüdungsverhalten in geschweißten Strukturen und Komponenten schon lange bekannt ist, ist der Zugriff auf verlässliche und präzise Daten von räumlichen Eigenspannungsfeldern begrenzt. Bezüglich einer konzeptionellen Sicherheitsanalyse von geschweißten Komponenten mit bruchmechanischen Methoden begründet diese Arbeit einen systematischen Ansatz, um den Einfluss von Schweißeigenspannungen auf das Ermüdungsrisswachstum zu verdeutlichen

Development of welding techniques and filler metals for high strength aluminum alloys Annual summary report, 27 Jun. 1964 - 27 Jun. 1965

Welding techniques and filler metals for high strength aluminum alloy

Residual stress effects and damage tolerance behaviour of integral lightweight structures manufactured by FSW and HSM

Estágio realizado na empresa Airbus Operations GmbH, orientado pelo Mr. Marco PacchioneTese de Programa Doutoral. Engenharia Mecânica. Universidade do Porto. Faculdade de Engenharia. 201

Fatigue improvement of steel bridges with high-frequency mechanical impact treatment

This thesis investigates the performance of fatigue-improved welds with high-frequency mechanical impact (HFMI) for application on new bridges. Fatigue strength improvement with HFMI can enable lightweight design of bridges and allow the utilisation of the benefits of high-strength steels. Studies of various bridge types were performed in this thesis showing that 20% material saving is possible in the main load-carrying members through post-weld treatment and the use of increased steel grades (fy > 355 MPa) where necessary. Limitations of the application of HFMI treatment on bridges were also identified, related to the degree of improvement and choice of steel grade.Experimental work of HFMI-treated joints with thick main plates relevant for bridges is scarce in the literature and comprehensive studies on the thickness effect are few. Therefore, the thickness effect was studied based on an established database of 582 fatigue test results of different types of HFMI-treated joints, collected from 28 studies. It was shown that the thickness effect becomes weaker than what is recommended for as-welded joints as a result of HMFI treatment. Fatigue experiments were conducted on a typical fatigue-prone detail in steel bridges with load-carrying plates of 40 and 60 mm which showed a significant fatigue strength improvement after HFMI treatment, exceeding recommended fatigue strengths given by the International Institute of Welding. Based on the fatigue experiments, a weak thickness effect was derived for non-load-carrying transverse attachment joints where the attachment and weld sizes are kept constant.The performance of HFMI-treated welds in composite steel and concrete road bridges was studied through a state-of-the-art review and simulations of variable amplitude in-service stresses in four case-study bridges in Sweden. It was shown that, in such bridges, very high and varying stress ratios are present due to a high portion self-weight stresses, which constitute up to 50% of the highest total stresses. Furthermore, it was revealed that the fatigue-critical locations in HFMI-treated bridges remain unchanged compared with conventional bridges and that compressive overloads pose no detrimental effect that requires additional attention in the fatigue assessment. Variable amplitude experiments with a bridge spectrum load from the case studies were conducted, including both low and high mean stress tests. The low mean stress tests performed equally or better than the constant amplitude fatigue strength, confirming that bridge loads do not pose any additional damaging effect for non-load-carrying transverse attachment specimens. The high mean stress tests clearly reflected the detrimental effect of high tensile self-weight stresses and enabled verification and development of approaches to consider these effects in design

A stress analysis method for fatigue life prediction of welded structures

In the case of structural weldments, the procedure for estimating fatigue life requires information concerning geometry of the object, loads and material. Detailed knowledge of stress fields in the critical regions of weldments is used to determine the fatigue life. The main theme of the research discussed in this thesis is to provide details of the methodology which has been developed to determine peak stress and associated non-linear through thickness stress distribution at the critical weld toe location by using only the geometry dependent stress concentration factors along with appropriate unique reference stress calculated in an efficient manner e.g. without modeling geometrical weld toe details. The peak stress at the weld toe can be subsequently used for estimating the fatigue crack initiation life. The non-linear through thickness stress distribution and the weight function method can be used for the determination of stress intensity factors and for the analysis of subsequent fatigue crack growth. Accurate peak stress estimation requires 3D fine mesh finite element (FE) models, accounting for the micro-geometrical features, such as the weld toe angle and weld toe radius. Such models are computationally expensive and therefore impractical. On the other hand, stresses at sharp weld corners obtained from 3D coarse FE meshes are inaccurate and cannot be used directly for fatigue life estimations. A robust, sufficiently accurate, efficient and practical approach is proposed for fatigue life estimation of welded structures based on 3D coarse mesh FE models. Another objective is to establish a methodology which is capable of accounting for the actual variability of stress concentration factors at welds, welding defects such as misalignment and incomplete penetration resulting from manufacturing processes. The proposed approach is capable of accounting for the effects from use of different material and effect of residual stresses from welding process. Residual stress information is obtained from a welding process simulation model, which has been validated against measured residual stress data. The proposed methodology has been validated using numerical and experimental data by analyzing different weldments of varying geometrical and load configurations. Further, the applicability of the stress field obtained from the proposed methodology is demonstrated by using it in a forward looking “Total Fatigue Life” concept based only on the fracture mechanics approach