Crack propagation analysis of welded thin-walled joints using boundary element method

 Tube-to-plate nodal joints under cyclic bending are widely used in the road transport and agricultural industry. The square hollow sections (SHS) used in these constructions are thin-walled and cold formed, and they have thicknesses of less than 4 mm. Some fatigue failures have been observed. The weld undercut may affect the fatigue life of welded tubular joints especially for thin-walled sections. The undercut dimensions were measured using the silicon imprint technique. Modelling of thin-walled cruciform joints, as a simplification of welded tubular joints, is described in this paper to determine the effect of weld undercut on fatigue propagation life. The Boundary Element Analysis System Software (BEASY) is used. The results of the effect of weld toe undercut from this analysis are compared with results from previous research to determine the comparative reduction in fatigue life between thin-walled joints (T=3 mm) and those made of thicker sections (T=20 mm). The loss in fatigue strength of the thin-walled joints is found to be relatively more than that for thicker walled joints. A 3D model of a tube to plate T-joint is also modelled using the boundary element software, BEASY. The nodal joint consists of a square hollow section, 50×50×3 SHS, fillet welded to a 10-mm thick plate, and subjected to cyclic bending stress. Fatigue analyses are carried out and the results are compared with the only available S–N design curve.     

铝合金T型接头激光+GMAW复合焊残余应力数值分析

目的 研究激光+GMAW复合焊中不同激光功率参数对铝合金T型接头残余应力的影响,从而提高焊接性能。方法 分别考虑了热弹塑性理论、传热学以及T型接头几何特性,建立了铝合金T型接头激光+电弧复合焊残余应力的数值分析模型。采用双椭球体热源模型表征电弧热输入与熔滴晗,采用锥体热源模型对激光深熔焊进行描述。基于所建立的T型接头模型,使用ANSYS有限元软件对12 mm厚铝合金激光+ GMAW焊T型接头残余应力进行模拟计算,并研究其分布特征;使用X射线衍射法对T型接头处的残余应力进行测量从而对所建模型的准确性进行验证。同时,对比了不同激光功率下铝合金T型接头对残余应力的影响规律。结果 当激光功率分别为2、3、4、5 kW时,铝合金T型接头路径L3上的纵向残余应力最大值分别为270、263、258、251 MPa,米塞斯-等效应力最大值分别为265、261、257、250 MPa。结论 后焊的焊缝A对焊缝B有明显的热处理作用,使应力明显降低;在T型接头焊缝及近缝区,横向残余应力和厚度方向残余应力峰值均比纵向残余应力峰值小,且随着激光功率的增大,焊缝及近缝区拉应力峰值不断减小。     

Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress - Part II: Application of toughness correction methodology

This part II paper presents the verified results of the toughness correction methodology for welded joints of wide plates. The equivalent CTOD ratio, β_r, is applied to the fracture data of the welded joints from lower to upper ductile-brittle transition temperature region. In the part I paper, β_r is defined as the ratio of CTOD in the standard fracture toughness specimen to CTOD in the wide plate with welding residual stress at the same level of the Weibull stress. In this part II paper, the equivalent CTOD ratio, β_r, under the welding residual stress field has been verified for assessment of constraint loss effects on CTOD fracture toughness of wide plate. Fracture assessments have been conducted by applying the methodology for “After Weld Notch” and “Before Weld Notch” type welded joints. It has been found that an excessive conservatism observed in the conventional procedure is reasonably reduced by applying the proposed methodology.     

Brittle failure susceptibility of welded joints in 09G2MFB steel subjected to the long-term effect of elevated temperatures

09G2MFB thick plate steel (TU 14-1-4473-88) with increased brittle failure resistance, including after the long-term effect of elevated surface temperatures, was developed for welded lined structures of the type of the jacket of the blast furnaces and air heaters. Long-term holding at 450°C has only a slight effect on critical brittleness temperature T₅₀ which does not exceed 20°C and is not associated with weakening of the grain boundaries. The brittle failure susceptibility of the welded joints in 09G2MFB steel depends on the structure in the weld zone. The formation of this structure depends on the cooling rate. The highest susceptibility to brittle failure after the thermal effect is shown by the steel with martensitic and bainitic structures.Translated from Problemy Prochnosti, No. 9, pp. 35–40, September, 1990.     

Microstructure characteristics of thick aluminum alloy plate joints welded by fiber laser

It's difficult to weld high strength thick plate since the groove is huge when using traditional arc welding, and the weld tends to be softened and large deformation could occur after multi-layer welding. All of these can affect the industrial application of high strength thick plate wielding. In this case, developing advanced welding technology and welding material is necessary to optimize the microstructure and performance of the welds. Fiber laser has many advantages such as good monochrome and high quality laser beam. In order to decrease the heat damage to the base metal from the welding heat source, low heat input is employed for welding thick plate. Fiber laser is applied in the welding of 20 mm thick Al–Zn–Mg–Cu alloy with super narrow gap filler wire. The microstructure comparison of Al–Mg–Mn alloy and Al–Mg–Mn–Zr–Er alloy welded joints reveals that a huge amount of fine equiaxed grains is formed in the weld zone of Zr and Er micro-alloying Al–Mg–Mn alloy welding wire and a great number of precipitation strengthening phases are precipitated in the weld zone after the heat treatment of welded joints in the entirety.     

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.     

热轧带钢尾部横裂缺陷的成因及解决

热轧过程中对于厚规格产品，易发生尾部横裂缺陷，经分析其产生原因是由于粗轧过程中板带尾部局部温降大，从而在变形过程中由于延伸不均而造成横向裂纹，通过改善板型及冷却条件，可以很好的解决这一问题。     

双面双弧焊对高强钢焊接效率和质量影响分析

对高强钢10CrNi3MoV钢厚板进行了双面双弧焊接试验研究。结果表明,焊接长2 m、厚50 mm的钢板,双面双弧焊与传统单弧MAG焊相比,生产效率提高1.4倍,焊接接头力学性能满足指标要求;双面双TIG打底的热影响区金相组织为板条状马氏体,焊缝区为板条马氏体+针状铁素体。     

Influence of the filler metal on the mechanical properties of Ti-6Al-4V electron beam weldments

The influence of various filler metals on the mechanical properties of 17 mm thick Ti-6Al-4V electron beam welded joints has been analysed. Autogeneous welded joints exhibit higher toughness when compared to the parent plate but this improvement was less marked than that observed in plasma arc welded joints. To achieve better toughness, without suffering unacceptable losses of strength, different morphologies of commercially pure titanium filler metals have been employed. Using 0.50 mm thick sheet as filler metal leaded to maximum toughness but as counterpart a significant decrease in strength was observed. To obtain high toughness while maintaining a high strength level 0.25 mm sheet and 1 mm diameter filler metals are recommended. Fractographic examination of the failed specimens helped to explain the fracture behaviour of the different welded joints.     

A transient finite element simulation of the temperature and bead profiles of T-joint laser welds

Laser welding is a high power density welding technology, which has the capability of focusing the beam power to a very small spot diameter. Its characteristics such as high precision and low and concentrated heat input, helps in minimizing the micro-structural modifications, residual stresses and distortions on the welded specimens. In this study, finite element method (FEM) is adopted for predicting the bead geometry in laser welding of 1.6 mm thick AISI304 stainless steel sheets. A three-dimensional finite element model is used to analyze the temperature distribution in a T-joint weld produced by the laser welding process. Temperature-dependent thermal properties of AISI304 stainless steel, effect of latent heat of fusion, and the convective and radiative boundary conditions are included in the model. The heat input to the model is assumed to be a 3D conical Gaussian heat source. The finite element code SYSWELD, along with a few FORTRAN subroutines, is employed to obtain the numerical results. The T-joint welds are made using a Nd:YAG laser having a maximum power of 2 kW in the continuous wave mode. The effect of laser beam power, welding speed and beam incident angle on the weld bead geometry (i.e. depth of penetration and bead width) are investigated. Finally, the shapes of the molten pool predicted by the numerical analysis are compared with the results obtained through the experimentation. The comparison shows that they are in good agreement.     

Numerical calculation of residual stress development of multi-pass gas metal arc welding under high restraint conditions

During welding, residual stresses build-up created by the steep thermal gradient that occurs in the weld zone from localized heating and cooling, and phase transformations appearing in low-alloyed structural steel is inevitable. Welding of rather simple test plates do not cover the actual structural effects, which have to be considered during real component welding. However, the resulting welding-induced residual stress state is highly influenced by the structural characteristics, i.e. restraint conditions, of the welded construction. Therefore, a unique large-scale testing facility providing a specific shrinkage restraint while welding and subsequent cooling was used for the present investigations. Hereby, a six bead multi-pass gas metal arc weld of 20 mm thick structural steel S355J2 + N was welded under shrinkage restraint. The residual stresses were experimentally and numerically investigated, and compared to an analysis of plates welded under force-free support and free shrinkage conditions.The experimentally determined and calculated residual stresses using both 2D and 3D numerical models are in a good agreement. Furthermore, the influence of a shrinkage restraint on the residual stress distribution is both experimentally and numerically shown for the present test set-up.     

铝合金厚板搅拌摩擦焊焊缝金属流动行为研究进展

搅拌摩擦焊接过程中,焊缝成形与金属流动行为密切相关,而金属流动又取决于焊接工艺参数、搅拌头形貌、材料本身性能及温度场分布等影响因素。与铝合金薄板搅拌摩擦焊不同,厚板焊接时焊缝上部、下部温差太大,导致焊缝材料流动形态发生较大变化。基于焊缝成形理论,从焊缝金属流动分析方法、影响因素出发,分析厚板搅拌摩擦焊焊缝金属流动形态及特征,探讨焊缝中疏松、未焊透、包铝伸入、弱连接等缺陷的形成原因,揭示厚板搅拌摩擦焊焊缝成形机理。研究结果为铝合金厚板搅拌摩擦焊焊接技术在航空、航天等高科技领域的广泛应用提供科学依据和理论基础。     

Failure Analysis of Laser Weld Joint of X2NiCoMo18-8-5 Steel

Laser welded T-shaped steel joints were received in debris form. The debris were recovered from the crashed component. Different metallurgical analysis was made on these debris to reach the root cause of the failure. In this regard, optical microscope and scanning electron microscope were utilized for the confirmation of different observations. Huge deposits of oxidation products were observed on the thick part (i.e., plate) of the T-shaped joint. For the confirmation of these products a set of simulated experiments were also performed.     

Measurement of residual stresses in a multi-layer explosive welded joint with successive milling technique

In the multi-layer welded joint of titanium-tantalum (Ti-5Ta/Ti-5Ta/Ta/substrate of stainless steel (SUS304) the second layer of plate Ti-5Ta is 4mm thick, and the third plate Ta is only 1 mm thick. It is almost impossible to measure the stresses near the weld with cutting strip technique. Using a successive milling technique the inplane elastic strain releases normal to the thickness direction are measured. With the finite element method (FEM) inherent strain distribution along thickness z-direction is evaluated according to the elastic strain releases. Subsequently, assuming that the inherent strains (plastic strains resulting from the welding process) are the initial strains of the FEM analysis for the welded residual stresses, these are used further to evaluate the residual stress distributions along the thickness z-direction in the multi-layer explosive welding joint.     

316L/S32101异种金属焊接接头的显微组织与力学性能研究

目的 研究乏燃料水池用钢板316L与覆板S32101双相不锈钢的焊接性、接头不同区域显微组织特征及接头与母材之间的性能差异.方法 利用氩弧焊接技术对5 mm厚的316L底板与3 mm厚的S32101覆板以搭接的形式进行焊接,利用金相显微镜、扫描电镜、维氏显微硬度仪和电子万能材料试验机对焊接接头的宏观形貌、显微组织以及力学性能进行研究.结果 316L/S32101焊缝组织主要由铁素体基体、晶界树枝状奥氏体以及晶内细小片状奥氏体所组成;316L侧靠近焊缝处存在一个较窄的熔合区,其组织由奥氏体基体和少许细小分散的铁素体组成,而S32101侧靠近焊缝处组织则由粗大铁素体晶粒和沿晶粒边界分布的若干小块状奥氏体组成.从316L母材区到焊缝区,硬度显著增大,而从焊缝区到S32101母材区,硬度变化很小;焊接接头的抗拉强度高达510 MPa,为两侧316L和S32101母材强度的87.9％和88.6％.结论 在焊接电流为240 A和焊接速度为300 mm/min的条件下,可以通过氩弧焊获得成形良好的搭接接头,且接头的力学性能优异.     

The fracture behaviour of a welded tubular joint: an ESIS round robin on failure assessment methods: Part V: screening method by required toughness and plastic stability considerations

A simplified failure assessment method, which is meant to be appropriate for a first screening of the considered structure with respect to its defect sensitivity, is applied to the T-joint of the ESIS round robin [Fracture behaviour of a welded tubular joint--round robin on failure assessment methods, First Information Package GKSS, 1994; Proceedings of 10th European Conference on Fracture, Berlin, vol. 1, 1994, p. 787]. The method enables one to identify fracture critical parts and to predict roughly the behaviour of a crack-like defect. It consists of an overall required toughness criterion to predict whether or not cleavage fracture can be initiated by a surface crack, and of a simplified stability analysis of the subsequent tearing process. This information can provide the basis to decide whether or not a more detailed failure assessment is required. Combined with analytical calculations of the overall plastic limit loads, the loading and fracture behaviour of the T-joint could be predicted by this simple and cost-effective method with reasonably good accuracy.     

钛合金电子束焊接在船舶领域应用的前景

船舶用钛合金焊接接头要求具有较好的抗冲击性能和抗疲劳性能,针对较大壁厚结构件,一般的焊接技术效率低且焊接质量较难保证,真空电子束焊接技术在这方面具有优势。本文综述了钛合金电子束焊接的特点,对其在船舶应用的前景,尤其是在大厚板焊接、冲击性能和疲劳性能等方面的优异表现进行了介绍,船舶用钛合金厚板焊接的长服役寿命可靠性是采用电子束焊接结构制造的根本目的。     

Microstructure and mechanical properties of friction stir welded 18Cr–2Mo ferritic stainless steel thick plate

In this study, microstructure and mechanical properties of a friction stir welded 18Cr–2Mo ferritic stainless steel thick plate were investigated. The 5.4 mm thick plates with excellent properties were welded at a constant rotational speed and a changeable welding speed using a composite tool featuring a chosen volume fraction of cubic boron nitride (cBN) in a W–Re matrix. The high-quality welds were successfully produced with optimised welding parameters, and studied by means of optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and standard hardness and impact toughness testing. The results show that microstructure and mechanical properties of the joints are affected greatly, which is mainly related to the remarkably fine-grained microstructure of equiaxed ferrite that is observed in the friction stir welded joint. Meanwhile, the ratios of low-angle grain boundary in the stir zone regions significantly increase, and the texture turns strong. Compared with the base material, mechanical properties of the joint are maintained in a comparatively high level.     

Parametric representation of weld fillets using shell finite elements—a proposal based on minimum stiffness and inertia errors

The objective of the present work is to introduce a methodology capable of modelling welded components for structural stress analysis. The modelling technique was based on the recommendations of the International Institute of Welding; however, some geometrical features of the weld fillet were used as design parameters in an optimization problem. Namely, the weld leg length and thickness of the shell elements representing the weld fillet were optimized in such a way that the first natural frequencies were not changed significantly when compared to a reference result. Sequential linear programming was performed for T-joint structures corresponding to two different structural details: with and without full penetration weld fillets. Both structural details were tested in scenarios of various plate thicknesses and depths. Once the optimal parameters were found, a modelling procedure was proposed for T-shaped components. Furthermore, the proposed modelling technique was extended for overlapped welded joints. The results obtained were compared to well-established methodologies presented in standards and in the literature. The comparisons included results for natural frequencies, total mass and structural stress. By these comparisons, it was observed that some established practices produce significant errors in the overall stiffness and inertia. The methodology proposed herein does not share this issue and can be easily extended to other types of structure.     

Effects of several TIG weld repairs on the axial fatigue strength of AISI 4130 aeronautical steel‐welded joints

Welded joints of airframes critical to the flight‐safety are commonly repair welded during its operational live. In this study, the effect of up to three weld repairs by gas tungsten arc welding (GTAW) on the axial fatigue strength of AISI4130 steel used in an airframe critical to the flight‐safety was investigated. The tests were performed on hot‐rolled steel plate specimens, 0.89 mm thick, with load ratio R= 0.1, constant amplitude, at 20 Hz frequency and room temperature. The results obtained indicated that the axial fatigue strength decreased with the GTAW process itself, and with the subsequent repair cycles, as a consequence of microstructural and microhardness changes and of weld profile geometry factors, which induced high stress concentration at the weld toe.