Rißfortschritt in ultrafesten Stählen und deren Schweißverbindungen bei dynamischer Beanspruchung

Crack propagation in ultra-high-strength steels and their welded joints under dynamic loading . Reported are results of investigation into the propagation of cracks in the base metal and weld metal of an ultra-high-strength steel. The material used in the investigations was a Ni? Co? Mo? alloy maraging steel with a yield point of 170 kp/mm². The steel was arc welded and TIG welded. The joints exhibited a drop of static strength in the range of 5 to 8 percent related to the base metal. Under zero-to-tension stress cycles the fatigue strength corresponded that of other high-strength steels, under tension-compression stress cycles the steel exhibited a higher fatigue strength. It was possible to show striations with the aid of scanning microscopy. Comparing the track propagation calculated in the microscopic range with the results obtained from the crack growth curves produced approximate agreement.     

Fatigue strength of mash seam welded joints

Reversed bending fatigue tests have been conducted using four series of mash seam welded joints obtained from the coupling of two different steels and plate thicknesses. Fatigue strength was evaluated and the effects of material property changes resulting from welding were studied. Fatigue strength of all series of the welded joints decreased slightly compared with that of the base steel. Type of steel and plate thickness in the welded samples exerted very little influence on fatigue strength. In the welded joints between steels with the same plate thickness, fatigue failure took place at a location away from the weld zone in the plate with the lower strength, while in the welded joints between plates of different thickness, failure occurred at the shoulder between the thin and thick plate, i.e. at the weld zone. Regardless of the type of steel and the plate thicknesses joined, fatigue strengths of the mash seam welded joints were slightly higher than those of the laser welded butt joints.     

Gigacycle fatigue data sheets for advanced engineering materials

Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 10¹⁰ cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 10⁸ cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete.     

Zerstörungsfreie Prüfung von Tiefspaltschweißnähten mittels Durchstrahlung und Ultraschall

Non destructive testing of Narrow-Gap welded joints using radiographic and ultrasonic methods . The intensified demand for economical sources of electrical energy has led to nuclear power plants with increasingly larger pressure vessels, particularly for BWR and PWR installations. The wall thicknesses of these vessels now exceeds 200 mm. The fabrication of these heavy structural components to close construction tolerances has required the development of economic welding processes, which in general are fully mechanised. One such process which has been successfully applied to the joining of heavy sections is Narrow-Gap welding. Investigations on Narrow-Gap welded joints using conventional nondestructive testing methods, such as radiographic and ultrasonic methods, have revealed that weld defects in these weld seams can be readily determined since for this process, the possible zones of weld failures are clearly defined over a very small region. The assessment of the test results according to accepted standards for pressure vessel construction has revealed, that providing the welding conditions are carefully controlled, acceptable weld quality is ensured.     

Fatigue assessment of high‐frequency mechanical impact (HFMI)‐treated welded joints subjected to high mean stresses and spectrum loading

The fatigue strength of welded joints can be improved with various post‐weld treatment methods. High‐frequency mechanical impact treatment is a residual stress modification technique that creates compressive residual stresses at the weld toe. However, these beneficial residual stresses may relax under certain loading conditions. In this paper, previously published fatigue data for butt and fillet welded joints subjected to high stress ratios and variable amplitude cyclic stresses were evaluated in relation to the current International Institute of Welding (IIW) recommendations on fatigue strength improvement and a proposed IIW design guideline for high‐frequency mechanical impact‐treated welded joints. The evaluation showed that the current IIW recommendations resulted in both non‐conservative and overly conservative fatigue strength estimations depending on the applied stress level, whereas the proposed fatigue assessment guideline fitted the current data well.     

Gigacycle fatigue data sheets for advanced engineering materials

Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 10¹⁰ cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 10⁸ cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete. r 2007 Published by Elsevier Ltd.     

The effect of controlled shot peening on fusion welded joints

This work examines the effect of controlled shot peening (CSP) treatment on the fatigue strength of an ASTM A516 grade 70 carbon steel welded joint. Metallurgical modifications, hardness, elemental compositions, and internal discontinuities, such as porosity, inclusions, lack of penetration, and undercut found in treated and untreated fusion welded joints, were characterized. The fatigue results of as-welded and peened skimmed joints were compared. It was observed that the effect of the CSP and skimming processes improved the fatigue life of the fusion weld by 50% on MMA-welded, 63% on MIG-welded, and 60% on TIG-welded samples.     

Overview of the geometrical influence on the fatigue strength of steel butt welds by a nonlocal approach

The paper deals with the high‐cycle fatigue behaviour of steel butt‐welded joints in the as‐welded condition, under constant amplitude loading. It tackles the problem of strength assessment by changing the geometrical shape and features of the weldment. This investigation considers the recommendations available in design standards, particularly referring to the Eurocodes, and it re‐analyses experimental data where a clear profile of the actual weld shape is available. For the available weld profiles, numerical investigations are shown by applying the Implicit Gradient approach for the evaluation of the nonlocal effective stress in the fatigue strength assessment. The results are very promising since the nonlocal approach significantly agrees with design standard recommendations in conventional cases, but it is also able to provide specific indications on the influence of the geometrical features, as well as the tip radius and opening angles, specifying their influence when no explicitly detailed indications are available in the design standards.     

Fatigue behaviour of post weld heat treated electron beam welded AA2219 aluminium alloy joints

This paper reports the effect of post weld heat treatment on fatigue behaviour of electron beam welded AA2219 aluminium alloy. An attempt has been made to enhance the fatigue strength of the electron beam welded joints through post weld heat treatment methods such as solution treatment, artificial aging, solution treatment and artificial aging. Electron beam welding machine with 100 kV capacity has been used to fabricate the square butt joints. Servo hydraulic controlled fatigue testing machine with a capacity of 100 kN has been used to evaluate the fatigue life of the welded joints. Of the three post weld heat treated joints, the solution treated and aged joints are enduring higher number of cycles under the action of cyclic loads.     

An analysis of the reasons for the reduction in reliability of machinery structures under low temperature climate conditions

The influence of low temperature on the reliability of machine parts and on the characteristics of strength of steels and their weld joints is analyzed. It is established that a drop in temperature in impact cyclic loading causes a decrease in fatigue resistance of weld joints of low-alloy steels and a corresponding increase in relative frequency of crack formation in them. A comparison of experimental and service data shows that the increase in service relative frequency of crack formation in welded machine parts at reduced temperature is an indication of their fatigue origin. It was concluded that the increase in the frequency of failures of welded machine parts with a reduction in temperature observed under service conditions is caused by a reduction in their fatigue resistance and an increase in the tendency of their materials toward brittle fracture.     

Fatigue Strength Assessment of Laser Beam Welded Joints Made of AA7075 and Magnetic Pulse Welded Joints Made of AA7075 and 3D-Printed AlSi10Mg

The use of 7000 aluminum alloys has an important role in future lightweight structures in the field of mobility due to the low density and high strength. However, these alloys can only be fusion welded to a limited extent because welding defects can rarely be prevented. For this reason, investigations are carried out to identify the most suitable welding parameters for two processes: laser beam and magnetic pulse welding. Herein, laser beam welding is successfully used to manufacture a roll-formed and longitudinally welded pipe made of AA7075 and joined by magnetic pulse welding with a 3D-printed lug-tube made of AlSi10Mg. The fatigue strength of these pipe joints and of laser beam welded butt joint specimens is determined using load-controlled fatigue tests. For the characterization of the specimens, cross sections are prepared and examined metallographically, which reflect the local weld seam geometry in the joining area. A fatigue assessment is made using linear-elastic approaches. The reference radius concept is applied to map the influence of geometric notches on the fatigue strength, assuming linear-elastic stress–strain behavior. It is shown that the recommended notch stress fatigue class FAT 178 (von Mises stress) can be applied for a safe and reliable fatigue assessment.     

Schwingfestigkeitseigenschaften von Schweißverbindungen hochfester,schweißgeeigneter Baustähle

Fatigue strength properties of welded joints of high-strength weldable structural steel. Particularly with regard to weld joints, high-strength and super high-strength structural steels under unfavourable boundary conditions is hardly superior to normal-strength characteristics of butt joint and cruciform joint samples can be improved by optimizing the weld seam geometry and by subsequent mechanical treatment by shot peening with ceramic balls or with steel scraps, and at determining which difficulties may occur during this process. The practical tests were supported and completed by theoretical calculations according to the FE-method. A final comparison with the standards will illustrate the difficulties in determining admissible fatigue strength values of weld joints.     

Fatigue behavior prediction of welded joints by using an integrated fracture mechanics approach

Current fracture mechanics methods for fatigue assessment of welded joints are based on long crack behavior. The present work introduces a method to predict the fatigue strength of welded joints by means of an integrated fracture mechanics approach (IFMA) that takes into account the fatigue behavior of short cracks. This methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, function of crack length. Firstly, the proposed fracture mechanic method is introduced and compared with the traditional fracture mechanic approach, used mainly for fitness for purpose assessment of welded joints with cracks or other crack-like defects. Then, the method is used for several theoretical and parametric applications to show its ability to predict the influence of different mechanical, geometrical and microstructural parameters in the definition of the fatigue resistance of welded joints. The influence of plate thickness, initial crack length and reinforcement angle on fatigue strength of butt-welded joints has been analysed and results show good agreement with experimental trends. Finally, the method is applied to predict and analyze the fatigue behavior of butt welded and non-load-carrying transverse fillet welded joints, and estimated and experimental results are analysed and compared.     

A simplified fatigue assessment method for high quality welded cruciform joints

The primary goal of this study was to develop an equation relating the geometric parameters to fatigue strength which can be used is routine design assessment. To attain this, the influence of local geometrical weld variations on the fatigue strength of non-load-carrying cruciform fillet welded joints were systematically studied using plane strain linear elastic fracture mechanics (LEFM). The effects of weld toe radius, flank angle and weld size were considered. Both continuous weld toe cracks and semi-elliptical toe cracks with alternate pre-existing defect depths were considered. A previously developed experimental crack aspect ratio development curve was used for assessing the growth of the semi-elliptical cracks using 2D FE models. A total of 152 experimental fatigue data points from six published studies of welded cruciform joints were evaluated. Details of the actual weld toe radius, flank angle and weld size were available for these joints. For the high quality welds evaluated, an assumed initial crack depth of 0.05 mm was found to correlate best with the experimental data. Of all the geometric parameters considered analytically, weld toe radius was found to have the most dramatic influence on fatigue life. A simple equation is proposed which relates welded joint fatigue strength to the ratio weld toe radius/plate thickness for high quality welds.     

Effect of microstructural features on the failure behavior of hybrid laser welded AA7020

Detailed investigations of microstructural feature, mechanical property, fatigue strength, and damage mechanism were conducted on hybrid laser welded 7020‐T651 aluminum alloys used into high‐speed railway vehicles. The results show that the hybrid laser welding process can induce significant changes of microstructures and alloying elements, together with numerous gas pores. Such local modifications degrade the fatigue performance. The tensile strength of welded joints was approximately 74% with respect to the base metal, thus satisfying the design standard. The fatigue property was determined in the low and high cycle regimes. It was found that the fatigue strength of welded joints was fairly inferior to that of the base metal, but far higher than the IIW recommended value. Furthermore, welding defects were well believed to contribute to the shorter fatigue life. The small fatigue crack growth presented highly discontinuous and inhomogeneous due to microstructure and porosity. By contrast, the crack stable growth stage was less sensitive to microstructural features of hybrid welded joints.     

Mikrostrukturelle,mechanische und korrosive Eigenschaften reibrührgeschweißter Stumpfnähte in Aluminiumlegierungen

Microstructural, mechanical and corrosive properties of friction stir welded aluminium joints Friction stir welding (FSW) is a novel solid state welding process. It allows joining of high strength aluminum alloys, generally considered as difficult-to-weld with conventional technologies, without loss in joint strength. Results of investigations on selfmade FSW butt joints of the aluminum alloys 2024-T3 and 6013-T4 are presented. First, the microstructure of the weld seam and heat affected zone is characterised metallographically and by hardness measurements. By tensile, fatigue endurance (SN) and fatigue crack propagation tests it is demonstrated, that especially the FSW-joints of 2024–T3 sustain high mechanical loadings. Investigations on the corrosion properties reveal a certain sensitivity of the 2024-T3 joints to intergranular and exfoliation corrosion.     

Fatigue of high strength steel joints welded with low temperature transformation consumables

In the present paper constant (CA) and variable amplitude (VA) fatigue testing have been carried out on out-of plane gusset fillet welded high strength steel joints. The joints were welded with conventional weld filler material and martensitic low transformation temperature weld filler, LTT, in order to study the influence of the residual stress on the fatigue strength. Residual stress measurements were carried out close to the weld toe using X-ray diffraction technique in order to study the relaxation due to VA fatigue. The residual stress showed different level of relaxation depending on the VA spectrum loading used. The LTT joints show 40% increase in mean fatigue strength compared to the conventional joints in CA. The LTT joints show 12% increase in mean fatigue strength compared to the conventional joints. The LTT joints show 33% increase in mean fatigue strength in CA compared to VA testing. However, the improvement of the fatigue strength is less significant in variable amplitude testing mainly due to the relaxation of the compressive residual stresses.     

船舶工程焊接接头完整性评估与疲劳寿命预测技术综述

介绍了船体结构中含缺陷的焊接接头完整性评估以及疲劳寿命预测方法,对结构完整性评估的K准则、COD准则、J积分准则、SINTAP/FITNET安全性评定方法以及评定流程、国内外标准进行介绍。并对焊接接头的疲劳强度影响因素,如平均应力、残余应力以及接头几何形状应力集中等进行了分析;介绍了焊接接头疲劳寿命预测的常用方法,如基于名义应力的评估方法、基于断裂力学的评估方法、基于损伤力学的评估方法以及其它疲劳评估方法等。此外,还对基于断裂力学法建立的焊接残余应力与疲劳裂纹扩展寿命之间的函数关系进行了介绍,该方法实现了对含残余应力的焊接结构疲劳寿命的精确预测。     

Fatigue strength analysis of laser-hybrid welds in thin plate considering weld geometry in microscale

Utilization of thin plates together with laser-based welding processes allows manufacturing of large weight efficient steel structures. However, the fatigue strength of welds in thin-plate structures with plate thicknesses below 5 mm is observed to have large variation, which brings challenges to fatigue strength assessment. One possible reason for this variation is the increased influence of actual weld geometry that is neglected in common fatigue strength assessment approaches utilizing geometry idealization. To reveal this influence the fatigue strength of 3 mm thick laser-hybrid welded butt joints were studied using the measured microscale weld geometry and the notch stress approach. Notch stresses were defined using Neuber’s stress averaging approach which allows the determination of the fatigue-effective stress without fictitious geometric modifications. For the studied specimens the large scatter of fatigue strength in the high-cycle region could be explained using this approach with high-resolution weld profile measurements combined with thorough finite-element analysis. It was observed that axial misalignment in narrow laser-hybrid welds causes a significant notch stress increase on the root side reducing the fatigue strength dramatically in terms of structural and nominal stress. In order to capture the increased notch stress it is crucial to use a significantly smaller stress averaging length than commonly assumed for welded joints.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.