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.     

Eigenspannungen und Mikrostruktur von thermisch getrenntem Stahlblech

Residual stresses and microstructure of thermal cutted steel sheet A research project of Doppelmayr Seilbahnen GmbH and the Swiss Federal Laboratories of Materials Testing and Research (EMPA) was launched to investigate the residual stress depth profiles induced by different thermal cutting methods. The measurements have been performed by X‐ray stress analysis. Pronounced differences in the stress levels at the surface and for the depth profiles have been obtained for the different cutting methods (plasma, microplasma, autogenous (gas), and laser cutting). Near the surface a small region of compression stresses due martensitic or bainitic transformation was found. With increasing depth a transition to tensile stresses occurs, which are caused by the contraction in the heat affected zone (HAZ) during cooling. The highest tensile (237MPa) and compression stresses (‐550MPa) have been obtained for the laser cut samples, while the microplasma cut samples showed the lowest residual stresses (max. 180MPa/‐56MPa).     

The measurement of the distribution of residual stresses through the thickness of a welded joint

A new method for the measurement of residual stresses in welded joints of complex cross section is described. The method involves the measurement of strain changes in the body as a narrow slot is cut in the plane of interest, in small increments of depth. A finite element model of the cross section is used to relate the strain changes at the measurement points to the residual stresses across the slot plane.
The accuracy of the method is demonstrated by using it to measure a known residual stress field in a cold bent bar. The method has been used in combination with a block removal technique to measure the through wall distribution of axial residual stresses at a circumferential butt weld in a cylinder with a protruding root bead. It could also be applied to the measurement of residual stresses in other joint configurations, such as fillet welds or T-butt welds.     

A new method for measurement of three dimensional residual stresses in a multi-pass butt welded joint

In this paper, a new method for the measurement of three dimensional residual stresses in a multi-pass butt welded joint is presented. The method involves the measurement of strain changes at a through hole surface as a coupon of material with the hole is separated from the body of interest and split. The finite element calculations are used to relate the strain changes at the measurenent points to the initial residual stresses at the hole location. The accuracy of the method is demonstrated by using it to measure a known stress field in a bent beam. The method has been used to measure the through thickness distribution of three dimensional residual stresses in a multi-pass butt welded joint. It could also be applied to the measurement of residual stresses in other welded joints.     

Residual stress field determination in welds by means of X‐ray,synchrotron and neutron diffraction

To what extent the welding residual stresses influence fatigue is still unclear and matter of debate. An important reason for this lack of clarity is that the exact determination of residual stress fields in welds is complicated which leads to conservative assumptions about these stresses in the fatigue design codes. The advances in the diffraction analysis of materials offer the opportunity for the full‐field residual strain mapping in welds albeit at the cost of time and technical complexity. In this work residual stress field determination in welded S1100QL specimens by means of the x‐ray, synchrotron and neutron diffraction techniques was undertaken. The results revealed that the maximum values of surface residual stresses are not as frequently assumed, as high as the yield strength in small scale specimens. At the weld toe which could serve as a fatigue crack initiation site, even lower residual stresses than the weld centreline could be present. The in‐depth measurements revealed that the effective part of the residual stress field which could be decisive for the fatigue failure initiation is concentrated at the surface of the weld.     

A residual stress dependent multiaxial fatigue life model of welded structures

In this paper, the influence of the residual stress on the fatigue performance of a welded structure under multiaxial loading modes is studied. First, the local stress state at weld toe is modified via introduction of the residual stress, and a new fatigue life estimation model considering the effect of the residual stress is established by modifying our recently proposed critical plane method. Second, the basic theory and procedure of the finite element simulation on the calculation of the welding residual stress are presented. Finally, a numerical simulation of an aluminum alloy flange‐to‐tube welding process is conducted, and the calculated residual stress is verified with X‐ray diffraction measurement. Furthermore, the performance of the proposed fatigue life estimation model is verified by the experimental data obtained in the fatigue test under different loading modes. It confirms that the consideration of the residual stress is important, especially under the out‐of‐phase loading mode.     

Effect of prestress on low-cycle fatigue

An investigation was conducted on HY-80 steel to show how low-cycle fatigue life is influenced by imposition of various stresses prior to fatigue testing. One cycle of static prestress was imposed on each specimen at loads either above or below the yield strength of the material and fatigue testing was done at various levels of stresses. Tests were conducted on notched and tee-fillet welded beam specimens. The magnitude and type of residual stresses at the toe of tee-fillet welds were determined by a hole drilling technique. Experimental data show that residual stresses induced by prestressing can—according to their sign—influence fatigue life. It was observed in the notched beam tests that when stress cycling was performed at stress levels below the yield strength of the material, tensile prestress increased fatigue life and that the higher the prestress, the greater the increase in life. At stress levels above the yield strength, tensile prestress has little or no effect on fatigue life. Compressive prestresses were found to be detrimental to fatigue life regardless of the stress levels at which the fatigue tests were run. The effects of prestress on the tee-weldments were not delinated as clearly. Fatigue life was increased by tensile prestress but no adverse effects were observed for compressive prestress.     

Neutron Stress Measurement of W‐Fiber Reinforced Cu Composite

Stress measurement methods using neutron and X‐ray diffraction were examined by comparing the surface stresses with internal stresses in the continuous tungsten‐fiber reinforced copper‐matrix composite. Surface stresses were measured by X‐ray stress measurement with the sin²ψ method. Furthermore, the sin²ψ method and the most common triaxal measurement method using Hooke's equation were employed for internal stress measurement by neutron diffraction. On the other hand, microstress distributions developed by the difference in the thermal expansion coefficients between these two phases were calculated by FEM. The weighted average strains and stresses were compared with the experimental results. The FEM results agreed with the experimental results qualitatively and confirmed the importance of the triaxial stress analysis in the neutron stress measurement.     

Nutzung von Low‐Transformation‐Temperature‐Werkstoffen (LTT) zur Eigenspannungsreduzierung im Elektronenstrahlschweißprozess

Low‐Transformation‐Temperature materials (LTT) were designed to reduce delay as well as residual tensile stress in welds on carbon‐manganese steels. Using the volume expansion effect during a martensitic transformation these materials counteract the volume shrinkage during cooling. While this positive effects on residual stress relief by Low‐Transformation‐Temperature‐alloys has been proven in various studies, these alloys have always been used in large volumes as additional filler material in electric arc welding processes. Modular heat fields initiated by an electron‐beam‐welding‐process offers the potential of a time‐activated initiation of compressive stresses triggered by phase transformation of Low‐Transformation‐Temperature‐alloys. Developing a technology able to reduce residual stress and thus the deformation of complex welded components is the aim. The first approach of Low‐Transformation‐Temperature‐material used in the electron beam process and its behaviour is presented here.     

变截面梁的应力计算及其分布规律研究

为了合理计算预应力混凝土变截面箱形梁的剪应力,客观反映其应力分布状况,首先推导了任意变截面梁剪应力计算的一般公式,在此基础上,考虑箱形梁的梁高、底板厚度、腹板厚度沿跨度的变化,导出了变截面箱形梁的剪应力实用计算公式。应用导出的公式,结合矩形及箱形变截面悬臂梁算例,分析了变截面梁的应力分布状况。研究结果表明,变截面梁横截面上的最大剪应力并不发生在截面重心轴处,而是在重心轴以下区域或梁底缘处;变截面箱形梁的底板受有很大剪应力作用,为了合理设计变截面箱形梁,不应采用薄底板,而且应加强其配筋及构造处理。     

Modelling and Statistical Treatment of Residual Stress Distributions in an Edge Welded Stainless Steel Beam

Abstract: Experimental, finite element analysis and statistical studies of residual stresses in edge welded type 316H stainless steel beams are presented. The experimental stress measurements were previously performed by different laboratories using neutron and synchrotron X‐ray diffraction. An analytical model to describe the magnitude and distribution of the residual stresses is presented. Results from finite element analyses are also provided. It is found that there is general agreement between the trends in the residual stresses derived from analysis and measurement. However, the scatter in results is substantial and a statistical framework for treating residual stresses using Bayesian statistics based on experimental and simulation results is described. The Bayesian analytical approach that uses the analytical model as a reference permits the model to be applied to circumstances outside the experimental conditions.     

Verbesserung des Eigenspannungszustands beim Kaltfließpressen durch den aktiven Einsatz eines gesteuerten Gegenstempels

By using an active counter punch during cold extrusion, the residual stress of the component can significantly be improved, which can increase its fatigue strength. In general, the workpieces produced through full‐forward extrusion are characterized by a strongly unfavourable stress state: The resulting high axial tensile residual stresses on the surface as well as the steep stress gradient accelerate the formation and the growth of cracks during cyclic loading. A system developed by the authors, consisting of a counter punch actively employed during the forming process allows improving the residual stress state of the final product. This result is obtained through a homogenization of the deformations in the samples during the cold extrusion. Therefore, a finite element simulation of the process was developed in Simufact Forming®. The simulation was experimentally validated by using the two austenitic steels 1.4307 and 1.4404. Another focus is the development of suitable measuring strategies for the X‐ray residual stress analysis with the sin²Ψ‐method to ensure the quality of the measurement results despite coarse‐grained and anisotropic microstructures. The results of the simulations and the experiments show that his technique has a considerable potential to increase the product quality without modifying the process time.     

ZrO2-Ni等离子喷涂涂层的残余应力

对不同镍含量的ＺｒＯ２－Ｎｉ等离子喷涂层应用Ｘ射线应力分析技术测试涂层表面应力,用曲率测试应力分析方法测定涂层平均应力,两者有较好的相关性,均表明涂层的残余应力为较小张应力。据测试的曲率半径估算了二层、三层、和六层阶梯涂层各层的平均残余应力和涂层喷涂的平均温度,结果表明多层过渡结构以残余应力作用甚微,所估算的喷涂温度与实际操作条件基本相符。     

Effects of Post-Weld Heat Treatments on the Residual Stress and Mechanical Properties of Laser Beam Welded SAE 4130 Steel Plates

The distribution of the residual stresses in laser beam welded SAE 4130 and the effect of stress relief after various post-weld heat treatments (PWHT) were measured by using the x-ray diffraction method. The mechanical properties and microstructure were also examined. Experimental results show that the tensile residua] stresses increased with the heat input of the laser beam. Most of the residual stresses were relieved by PWHT at 530°C for 2 hours followed by furnace cooling to 50°C. The strength of the as-welded samples decreased in comparison with the base metal. The yield strength of the weldment increased after PWHT at 320°C for 2 hours in comparison with the as-welded sample. The elogation of the laser beam welded samples increased after PWHT in comparison with the as-welded samples.     

Analyse der Mikrostruktur und des Eigenspannungszustandes seilverstärkter Aluminiumstrangpressprofile

Analysis of the microstructure and the residual stress state of aluminium extrusions reinforced with ropes Closed profiles of the aluminium wrought alloy EN AW‐6060 have been in‐situ‐reinforced during the extrusion process with ropes built by wires drawn of the nickel‐based alloy Inconel 601 (2.4851). These profiles have been investigated with non‐destructive and semi‐destructive testing methods regarding the residual stresses evolving during cooling from extrusion temperature to room temperature. The results obtained via X‐ray diffraction and hole drilling method have been compared with simulations using a simplified model of the compound investigated. Thereby, compressive residual stresses appearing during the cooling process have been detected parallel to the rope axis nearby the rope‐matrix‐interface because of plastic flow of the matrix material. Additionally, the microstructure of the extruded composite has been analysed focussing on the contingent formation of intermetallic phases at the rope‐matrix‐interface and on the influence of the reinforcements on the longitudinal weld.     

Procedure to Perform a Validated Incremental Hole Drilling Measurement: Application to Shot Peening Residual Stresses

Abstract: The paper describes a test rig designed to check and assess the accuracy of the incremental hole drilling (IHD) method. An external load produces a controlled linearly through thickness variable uniaxial stress field (reference bending stress), known with good accuracy, that can be applied and removed at each hole increment. After the separation between the bending relaxed strain from the residual stress relaxed strain, it is possible to reproduce the bending stress distribution in order to have complete confidence of the residual stress measurement. The bending verification of the IHD method was already proposed by other investigators before, but residual stress measurements were then performed on independent configurations. The proposed testing procedure gives a ‘real time’ verification of the residual stress measurement. Any experimental malfunctioning due to the operator inexperience, or any error during the stress calculation from the relaxed strain would produce an evident difference between the expected reference bending stress and the IHD bending stress output. Moreover, the reference bending stress helped for understanding that the not perfect hole cylindrical shape causes an underestimate of the predicted stress near the surface. A zero depth offset correction was proposed. This correction was tested on the reference bending stress, and then applied to the residual stress prediction. Three shot peening residual stresses IHD measurements were successfully validated by means of the bending stress; moreover, they were in good agreement with independent X‐ray diffraction measures also proposed in the paper.     

偏心轴向荷载作用下薄壁箱梁的约束扭转翘曲应力研究

以薄壁箱梁的约束扭转分析理论为基础,将薄壁箱梁所受偏心轴向荷载作为一种外加双力矩荷载考虑,建立偏心轴向荷载作用下薄壁箱梁约束扭转的双力矩这一广义内力的计算公式。为了便于计算翘曲应力,进一步推导了扭转中心位置及主扇性坐标的实用计算公式。通过对一模型箱梁进行计算,并与按通用有限元软件ANSYS壳单元计算结果进行比较,验证了该文方法和所推导公式的正确性。详细分析箱室高宽比以及悬臂板宽度变化对偏心轴向荷载作用下薄壁箱梁约束扭转翘曲应力的影响。研究结果表明：箱室高宽比及悬臂板宽度对悬臂板端部翘曲应力的影响最大,对腹板与上翼缘、下翼缘交接处翘曲应力的影响相对较小;在偏心轴向分布荷载作用下,悬臂箱梁固定端横截面上控制点处的翘曲应力可达到初等梁应力的12%,不容忽视。     

Residual stress measurement in AlSi alloys

In this work, the relaxation of residual stresses inside a sample made of the aluminum alloy AlSi7Mg0.3 after tempering is described. The comparison of stress evaluation by X‐ray diffraction and incremental hole drilling method combined with electronic speckle pattern interferometry strain determination gives the opportunity to evaluate micro stresses together with first order macro stresses. Compressive stresses within the surface of a cold worked sample are relaxed by tempering. The X‐ray diffraction evaluation is supported by the analysis of a stress‐free sample through incremental hole drilling method.     

Investigation of residual stresses in a repair‐welded rail head considering solid‐state phase transformation

Repair welding for recovery from local damage of a rail head surface is known to cause high residual stress and can accelerate fatigue in the rail. This study examines repair‐welded rails by applying experimental and numerical approaches. In the former approach, two newly manufactured rail specimens and four repair‐welded rail specimens with two different weld depths were prepared, and their residual stresses were measured with a sectioning method. In the latter approach, a finite element repair welding simulation model was developed that adopted a prescribed temperature method with a moving block as an input heat source, and the thermal strain caused by the volume change due to solid‐state phase transformation was considered. Overall, the residual stresses correlated well between the experimental and numerical approaches. The measured high compressive residual stress of ?290 MPa seems to be beneficial to prevent a crack initiation in the rail surface.     

Susceptibility of 7050-T7451 electron beam welded specimens to stress corrosion

Susceptibility to stress corrosion tests were carried out on electron beam welded specimens made from 7050-T7451 aluminium alloy. As a comparison, specimens made from base material were tested too. The resistance of the welded material was high: the tensile properties were only slightly lower than those of the base material. After 30 day exposure to a corrosive environment (alternate immersion in a 3.5% NaCl solution), the tensile properties of the welded material were considerably reduced, while the same properties were only slightly affected in the base material. The combined effect of stress and corrosion was only slightly detrimental for the base material and very detrimental for the welded material. At the lowest stress level tested, about 25% of the ultimate stress, the welded specimens failed after a mean life of 90 days. Considerable residual stresses associated with the welding process were measured in a plate. A test was performed to verify the possibility of stress corrosion cracking promoted by the welding residual stresses. In actual fact, no cracks were observed, but the corrosion rate increased, particularly in the areas affected by the higher residual stresses.