Modellierung des Ermüdungsrisswachstums in Schweißverbindungen unter Berücksichtigung von Schweißeigenspannungen

The present paper contains research results determined within the framework of a project called IBESS (?Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen“) by the Materials Mechanics Group of the Technische Universität Darmstadt [1]. Aim is to calculate the fatigue life of welded joints by taking into account the effect of residual stresses and the influence of the weld toe geometry. Here, the fatigue life is regarded as period of short fatigue crack growth. Two and three dimensional finite element models, with cracks as initial defects, are constructed for this purpose. Fatigue crack growth analyses are performed by using the node release technique together with the finite element program ABAQUS. The welding residual stresses as well as the plasticity induced crack closure effects are considered. Structural calculations are performed in order to introduce residual stress fields in finite element models. The calculated compressive residual stress field matches the measured one especially in the weld notch area. The effective cyclic J‐integral (ΔJ_eff) is used as crack tip parameter in a relation similar to the Paris equation for the calculation of the fatigue life. For this purpose, a Python code was written for the determination of ΔJ_eff at every crack length phase. The calculated fatigue lives were compared with experimental data and a good accordance between both results was achieved. The impact of welding residual stresses on ΔJ_eff as well as on the fatigue life during short crack growth was investigated. As expected, results revealed that at lower stress amplitude, a compressive residual stress field is favorable to the fatigue life, whilst a tensile residual stress field is unfavorable. The influence of residual stresses can be neglected only for large load amplitudes.     

Ermittlung des Zeitstandverhaltens einer austenitischen Schweißverbindung

Evaluation of the Creep Rupture Behaviour of an austenitic Welded Joint Part II: Numerical Calculations of the Creep behaviour For a welded joint of a 18% Cr-11%Ni-steel the stress and strain distribution is calculated by an inelastic finite-element-analysis using constitutive equations of the different zones. On the base of different criteria, the crack initiation time is calculated. The results are compared with test results of specimens with different geometry taken from different positions of the welded joint.     

Schweißverhalten von Aluminium und seinen Legierungen

Weldability of Aluminium and Al Alloys Aluminium is behind steel the most used metal. Advantages of Al-materials are the low weight, the favourable strength and deformation properties, the good corrosion resistance and the good electrical and thermal conductivity. Welding of aluminium encounters difficulties due to high melting oxide layer at the surface, the strong heat dissipation from the welding region, the tendency to form craters or pores and the high heat extension. This contribution describes the basic facts and special features of arc welding and weld properties of aluminium and Al-alloys during welding.     

Ermittlung des Zeitstandverhaltens einer austenitischen Schweißverbindung

Evaluation of the Creep Rupture Behaviour of an Austenitic Welded Joint. Part I: Experimental Determination of the Creep Behaviour The creep rupture behaviour of a welded joint of 18% Cr-11% Ni-steel was investigated on specimens of different geometry and size. Differences in the creep rupture behaviour between welded joint specimens and specimens taken from the different zones of the welded joint are discussed. The plasticity-and creep behaviour of the different zones is described by constitutive equations.     

Schweißen mit dem Laserstrahl im Vakuum

Welding with the Laser Beam in Vacuum In the vehicle industry, beam welding methods are successfully applied for the joining of high‐quality parts. Here, the focus is mainly put on drive section components. Especially in component manufacturing of couplings, toothed wheel – shaft joints and drive shafts, electron beam welding using small vacuum cycle chambers is applied. The application of powerful solid‐state lasers results, however, often in negative consequences, such as soiling by process emissions or increased tendency to spatter. The application of laser beam welding in vacuum allows for the reduction of these side effects and, at the same time, for the increase of penetration depth and weld quality. This article introduces first results gained from tests made with workpieces from the vehicle industry.     

Erfahrungen beim Schweißen gasempfindlicher hochreiner Werkstoffe

Experiences in Welding Highly Pure, Gas-Sensitive Materials Highly pure steels and non-ferrous react during processing, particularly welding, with atmospheric gases and change their service properties. Hence special measures are often necessary in forming and inert gas welding. Taking the specific example of a high pure chromium steel (superferritic), welding properties, processing data and selection of suitable inert gas torches are described. The possibility of applying this information to other materials such as titanium is discussed.     

Schweißen im Behälter- und Apparatebau

Welding in Tank and Plant Construction Since a relatively limited number of plastics types are used, the welding tasks involved in chemical apparatus engineering can be accomplished with just a few welding techniques are well-known and welding can be carried out efficiently, reliably and economically by this means. In this respect, however, a number of important principles have to be observed. The DVS guidelines provide vital information on questions relating to calculation, design, personnel, equipment, material and testing. Extrusion welding enjoys prime importance as regards quality and economic aspects.     

Modifizierung organischer Konstruktionswerkstoffe für technische Anwendungen

Modification of Organic Engineering Materials for Technological Applications It is reported about experiments for synthesis of novel reactive, thermotropic, liquid‐crystalline polymers (LCPs) as well as about investigations concerning the use of these LCPs as a blend component for the production of modified polyamide and polyester fibres and their properties. These reactive LCPs are synthetically easy accessible p oly e ster i mid a nhydrides ( PEIA ) bearing lateral as well as terminal anhydride groups. The average number of anhydride groups is variable between 4 and 18. Molecular weights of 30 kg/mol up to 80 kg/mol could be obtained. During mixing of the reactive LC‐PEIAs with polyamide 6 [PA 6] or poly(ethylene terephthalate) [PET] in an extruder under melting conditions both components evidently react within some few minutes to form graft‐block‐copolymers containing on their backbone chains lateral and terminal polyamide respectively polyester blocks. These quickly occurring modification reactions are the base for the industrial application in form of a continuously arrangeable “reactive blending‐spinning‐drawing”‐process. Graft‐block‐copolymers synthesised by this way in the sense of “reactive blending” can be processed together with the corresponding adequate matrix material polyamide 6 or polyester into drawable filaments. After spinning and drawing under suitable conditions lc‐PEIA‐fibrils modified by molecules of the basic polymer with diameters of less than 500 nm are detectable in the resulting filaments. The desired “microphase distribution” of the PA‐modified respectively PET‐modified lc‐PEIA‐macromolecules as the reinforcing system components could be achieved. Moreover these graft‐block‐copolymers built by “reactive blending” in‐situ have a high thermodynamic compatibility because of their chemical similarity to the primary structure of the respective matrix materials resulting in a relatively high reinforcing effect. These both aspects as well as the proceeding orientation of the lc‐PEIA‐microphases by the filament drawing cause, though optimising processes are still remaining, a remarkable increase of the tensile strengths as well as clearly improved initial moduli at a simultaneously raised stretchability of the lc‐PEIA‐modified polyamide and polyester filaments. These effects could be achieved with PEIA‐amounts of lower than 5 percentages by weight .     

Meßmethoden und technische Anwendungen der Bruchmechanik

Measuring methods and technical application of fracture mechanics. A method of measuring the plane-strain fracture toughness K_lc using compact-tension-specimens according to American (ASTM) and British (BISRA) standards is described. There exist conditions for the minimum thickness and minimum crack length necessary for the determination of valid K_lc-values. The K_lc-values decrease with decreasing temperature. Own results and pictures of the fracture surfaces of the tempering steel 30 Cr Ni Mo 8 are shown as examples. With low strength steels linear elastic fracture mechanics characterized by K_lc is normally not valid at room temperature. Therefore it is supplemented by the crack-opening-displacement concept (COD-concept) applicable up to macroscopic yielding. — After a survey of three failure categories for cylindrical pressure vessels with axial through the wall cracks and the controlling material properties two examples are given for the application of fracture mechanics to test pressure vessels containing longitudinal slits.     

Größeneinflüsse bei schwingbeanspruchten Schweißverbindungen

Size effects in fatigue of welded joints A survey is presented on size effects that have to be regarded in fatigue of welded joints and how these effects are considered in current design codes. Experimental findings have to be simplified in calculations to account for the existing interactions between structural size, residual welding stresses and mean stresses, the expected effect of throat thickness as well as statistical aspects. A more detailed assessment of these effects may enable better estimates and an optimization of the fatigue strength of welded joints.     

Langzeitfestigkeit von Schweißverbindungen an Deponiedichtungsbahnen aus Polyethylen

Long-term Strength of Welded Joints in Polyethylene Water Proofing Sheets for Refuse Dumps Waterproofing of refuse dumps is nowadays mainly carried out with sheets made from HDPE. Welding is the technique used for joining the sheets. These welded joints are the weakest point in the system. Whereas non-destructive tests give relatively little information on the quality of the weld, peel and tenside creep tests provide a good degree of differentation. The test results available show that it is not so much the welding technique that influences long-term strength, but that it is the creep strength of the base material and the excessive rise in stress due to notch effects at the changes in cross-section which determine the long-term quality of the welded joint.     

Die Anwendung von Schallemission für die Schweißnahtkontrolle

The use of Acoustic Emission as a weld Quality Monitor . The results obtained from our experiments with spot welding, gas tungsten-arc welding, and submerged-arc welding demonstrate the potential of acoustic emission monitoring as a nondestructive test for welds. The real-time nature of the acoustic emission data sets this technique apart from other nondestructive methods used for weld inspection. Since the acoustic emission from weld defects is transient, empirical calibration is required for each application. Defects can be located approximately but the type of defekt cannot be determined.     

Spannungsberechnungen in Schweißnahtmodellen mit Hilfe neuer Werkstoffgesetze

Stress Calculation in a Weld Model with the Help of New Material Laws Based on the incremental formulations of the material theory from Mises for two dimensional problems a subroutine has been developed for using new material laws in finite element programs. Using this subroutine a phenomenological material law has been used to consider the influence of the strain in finite element calculation of inhomogeneous weldments.     

Neue Entwicklungen auf dem Gebiet der Schweißtechnik

Recent Development in the Field of Welding Technique. Because of important advantages as soundness of gap, no overlapping and possibility of mechanization the welding procedure takes a considerable plave in the wide field of joint techniques. Often the mechanical properties of the weld metal exceed the values of the base metal. The following report describes recent welding techniques: welding with cored-wire electrodes, the narrow-gap-welding and electron-beam welding at atmosphere. This procedures are often used for the welding of new grades of materials.     

Messen des Korrosionspotentials an Schweißverbindungen im Kleinbereich

The small scale measurement of corrosion potential in welded joints . The corrodibility of welded joints is reported on. Potential curves give the possibility to backing the incomplete results in this field and permit a well defined statement concerning the corrosion behaviour. Failures of interpretation due to polarisation are indicated. The pulsating method helps to prevent changes of the concentration by reactions and alteration of the pH-value. This method delivers useful informations and detects the corrosion sensibility of welded joints.