焊接接头超高周疲劳实验研究

本文基于超声疲劳振动技术,设计了三种焊接接头试样(圆形对接焊接试样及其喷丸处理试样和板状十字焊接试样),并利用超声疲劳试验系统测定了其超高周疲劳性能,实验应力比为-1,频率20kHz,实验在室温条件下进行。实验结果表明,圆形对接焊接接头的疲劳性能高于板状十字焊接接头,喷丸处理能提高焊接接头的疲劳强度。将焊接接头的疲劳性能与对应形状的母材进行对比分析,发现焊接接头的疲劳性能远低于母材。在相同疲劳寿命的条件下,圆形焊接接头试件的疲劳强度仅为母材的45%,十字焊接接头试件仅为母材的29%;圆形对接接头在5×106周次以后,试件仍然发生疲劳断裂,而板状十字焊接接头在超高周区域(107～109周次)存在疲劳极限。超声疲劳断口的扫描电子显微镜分析结果显示,圆形焊接接头试件断口位置主要位于熔合区的焊趾处或焊接接头表面几何非连续处,十字接头试件断口位于焊趾处;焊接接头试件裂纹萌生于焊接缺陷、试样表面夹杂或熔合区的不连续处;喷丸处理对焊接接头的裂纹萌生机制没有显著影响。     

A phenomenological model for anisotropic creep in a multipass weld metal

Creep strength of welded joints can be estimated by continuum damage mechanics. In this case constitutive equations are required for different constituents of the welded joint: the weld metal, the heat-affected zone, and the parent material. The objective of this paper is to model the anisotropic creep behavior in a weld metal produced by multipass welding. To explain the origins of anisotropic creep, a mechanical model for a binary structure composed of fine-grained and coarse-grained constituents with different creep properties is introduced. The results illustrate the basic features of the stress redistribution and damage growth in the constituents of the weld metal and agree qualitatively with experimental observations. The structural analysis of a welded joint requires a model of creep for the weld metal under multiaxial stress states. For this purpose the engineering creep theory based on the creep potential hypothesis, the flow rule, and assumption of transverse isotropy is applied. The outcome is a coordinate-free equation for secondary creep formulated in terms of the Norton–Bailey–Odqvist creep potential and three invariants of the stress tensor. The material constants are identified according to the experimental data presented in the literature.     

Numerical study of the plastic behaviour in tension of welds in high strength steels

The influence of the mismatch between material properties and constraint on the plastic deformation behaviour of the heat affected zone of welds in high strength steels is investigated in this study, using finite element simulations. An elastoplastic implicit three-dimensional finite element code (EPIM3D) was used in the analysis. The paper presents the mechanical model of the code and the methodology used for the numerical simulation of the tensile test of welded joints. Numerical results of the tensile test of welded samples with different hypothetical widths for the Heat Affected Zone and various material mismatch levels are shown. The analysis concerns the overall strength and ductility of the joint and in relation to the plastic behaviour of the heat affected zone. The influence of the yield stress, tensile strength and constraint on the stress and plastic strain distribution in the soft heat affected zone is also discussed.     

Upper bound of the limit load for a tensile cylinder with a soft welded (soldered) joint containing a crack

The limit load is one of the main characteristics in estimating the performance of different structures, in particular, structures with soft welded (soldered) joints. In some cases, the difference between the yield strengths of the main material and the joint material is so great that plastic strain is localized in a thin joint. With some features of such strain distribution taken into account, an upper bound of the limit load of a tensile axisymmetric sample with a crack in a welded (soldered) joint is obtained.     

考虑塑性和刚度不匹配的焊接残余应力估算

现有残余应力计算方法未能考虑材料塑性变形和焊接接头刚度不匹配的影响,使得焊接残余应力计算结果和实际残余应力存在较大偏差.在2219-T87铝合金钨极氩弧焊焊接头残余应力测试基础上,提出一种基于非线性有限元和材料弹性模量分区的残余应力—释放应变曲线的残余应力计算方法,研究了材料塑性变形和接头刚度不匹配对焊接残余应力计算的影响.结果表明,焊接接头中非均质材料塑性不匹配可以引起对于残余应力计算的较大误差;材料塑性变形对残余应力的影响大于接头刚度不匹配对残余应力的影响.所提出方法修正了传统方法在焊接接头的残余应力计算中由于未考虑接头非均质材料塑性不匹配而引起的误差.     

Fatigue strength of welded joints based on local, semi-local and nominal approaches

Analysis based on the so-called “local approach” is made to estimate the fatigue strength of welded joints. Numerical analyses or strain gauges are employed for finding the stress and/or strain state in the vicinity of the weld toe. The notch stress intensity factor (NSIF) approach applied to fillet welded joints, as far as the opening angle between the weld and the main plate surface is constant (e.g. 135°, typical for many fillet welds), is able to rationalise the fatigue strength data both for different joint geometries and absolute dimensions. The NSIF approach has been previously developed as an extension of the Linear Elastic Fracture Mechanics (LEFM) to open V-notches and is based on the exponential local stress field around the V-notch tip. Several different “local approaches”, although simpler and more practical than the NSIF, are based on the stress (or strain) values determined beyond the exponential local one. To distinguish such approaches from the NSIF based one, we define the former as semi-local or nominal approaches while the latter is a local approach. The paper underlines that the local approaches, differently from the other ones, are able to unify in a single scatter band the fatigue strength data obtained from welded joints having different geometry and absolute dimensions.     

Dynamic Separation of Resistance Spot Welded Joints: Part II—Analysis of Test Results and a Model

The ultimate strength of resistance spot welded joints fabricated from a wide range of steel grades, weld button size, and sheet thickness are reported for lap-shear and cross-tension specimens subjected to quasi-static and impact loading conditions. Test data are analyzed with respect to energy, impact speed, and loading rate. Loading rate is identified as a critical, test system independent parameter to reflect the strain rate sensitivity of the steels. An equation is fitted to the ultimate strength test data as a function of loading rate which is proposed to predict the separation of spot welded joint under dynamic loading. The model is validated by test data from open literature generated from other type of specimens and/or dynamic test conditions.     

拉压循环载荷下正交异性钢桥面板肋-面板焊缝裂纹扩展分析

采用四步法计算了考虑循环载荷中压应力影响的正交异性钢桥面板的肋-面板焊缝表面裂纹扩展。第一步是基于正交异性钢桥面板的疲劳分析模型,计算肋-面板焊缝处的应力,第二步是通过肋-面板焊缝的三维局部模型,用Schwartz-Neumann交替法计算焊缝表面裂纹的应力强度因子分布,第三步是用二维断裂力学模型和增量塑性损伤模型,计算循环载荷中的压应力对裂纹扩展的影响,第四步是用第二步中的三维裂纹分析结果和第三步中的二维断裂力学模型得到的裂纹扩展公式,计算钢桥面板的肋-面板焊缝表面裂纹扩展。计算结果表明,对应于正交异性钢桥面板肋-面板焊缝处的循环应力,本文所用模型的裂纹尖端反向塑性区导致裂纹扩展率增加50%以上。研究结果为正交异性钢桥面板肋-面板焊缝裂纹的疲劳寿命分析提供了研究基础。     

Very high cycle fatigue behavior of bridge steel welded joint

Very high cycle fatigue (VHCF) behaviors of bridge steel (Q345) welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = ?1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 10⁷ cycles. The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone (HAZ) in the very high cycle regime. The fatigue fracture surface was analyzed with scanning electron microscopy (SEM), showing welding defects such as pore, micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints. The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.     

Effects of laser power density on static and dynamic mechanical properties of dissimilar stainless steel welded joints

The mechanical properties of laser welded joints under impact loadings such as explosion and car crash etc. are critical for the engineering designs. The hardness, static and dynamic mechanical properties of AISI304 and AISI316 L dissimilar stainless steel welded joints by CO₂ laser were experimentally studied. The dynamic strain-stress curves at the strain rate around 10³ s^?1 were obtained by the split Hopkinson tensile bar (SHTB). The static mechanical properties of the welded joints have little changes with the laser power density and all fracture occurs at 316 L side. However, the strain rate sensitivity has a strong dependence on laser power density. The value of strain rate factor decreases with the increase of laser power density. The welded joint which may be applied for the impact loading can be obtained by reducing the laser power density in the case of welding quality assurance.     

A new theoretical approach for structural modelling of riveted and spot welded multi-spot structures

A general theoretical approach based on theory of elasticity is presented in order to define the structural behaviour of riveted and spot welded joints. The new closed form solutions lead to the definition of a joint element useful to FE models of riveted or spot welded multi-spot structures. The objective is an accurate evaluation of the local elastic stiffness of spot joints in FE analysis, which is fundamental to perform a reliable simulation of multi-joint structures and, consequently, a good estimate of loads acting on spots; this makes it possible to introduce structural stress or new general criteria allowing, for example, to predict fatigue behaviour. On the other hand, a low entry of degrees of freedom is needed when several spot joints are present in a complex structure. The goal is to reach a reliable spot region model which can be used as the basis to develop a spot element in FE analysis. In the present paper, based on new closed form solutions, a spot element is introduced, so as to precisely evaluate both local and overall stiffness both of spot welded joints and riveted joints. Based on the stress function approach and the Kirchhoff plate theory in linear elastic hypotheses, closed-form in-plane stress, displacement, moment and transverse shear force solutions are derived for a new bidimensional model, subjected to various types of loads. The capability to simulate spot welds or rivets depends on the definition of two elastic parameters intrinsic in closed form solutions, that tunes the theoretical model according to actual joint behaviour.The proposed joint element combines the precision in the simulation with a very limited number degrees of freedom in the overall finite element model of an actual multi-spot structure.The results obtained using the introduced theoretical framework and spot element approach perfectly match those obtained using very refined FE models and experimental data.     

Fatigue life estimate of C---Mn steel welded cruciform joints

The fatigue life of a manual metal arc welded cruciform joint failing from a root lack of the penetration region is estimated by the application of crack growth relations. A two-parameter relation was used. The initiation life and propagation life of the joint were taken into account to obtain the total fatigue life of the joint from the crack growth parameters. To test the accuracy of the method, the predicted data was compared with the experimental data for a C---Mn type steel welded joints. The results were in good agreement with the experimental data.     

未熔透尺寸对十字焊接接头疲劳性能影响规律试验研究

针对某厂一压力机架的焊接结构，采用极大似然方法，进行了部分熔透焊接十字接头的拉—拉疲劳试验，得到了该类结构的Ｐ－Ｓ－Ｎ对数曲线。研究了未熔透尺寸对接头的应力集中系数、疲劳强度和试件疲劳破坏形式的影响规律，结果表明，在焊缝与母材等强的情况下，当未熔透尺寸２ａ／Ｔ＜０．５时，其疲劳性能无明显减弱；而当未熔透尺寸２ａ／Ｔ＞０．５时，则接头的抗疲劳性能有显著的改变。     

欧洲近海结构用钢研究计划的进展

欧洲近海结构用钢研究计划在第一阶段(1975—80年)关于海洋焊接结构在疲劳载荷下工作性能的研究基础上,于1981—87年继续进行了第二阶段的研究工作。参加国家有欧洲经济共同体6国及挪威、加拿大等。本文综合报道第二阶段研究情况与主要成果,内容包括:板厚对疲劳强度的影响,焊后改进技术与腐蚀疲劳,疲劳载荷及变幅疲劳试验,疲劳分析的断裂力学方法,某些管节点的应力分析等;并对研究计划的背景及今后研究工作的方向作了介绍。      

Dynamic tensile strength of composite laminate joints fastened mechanically

The tensile strength of composite joints is determined under dynamic loading conditions. The composites are laminates made from hybrid fiber reinforced plastic (HFRP) and carbon fiber reinforced plastic (CFRP). Three different mechanically fastened joint configurations are considered: they are the pin-connected, single-lap and double-lap type. The joint strength under dynamic load is found to be lower than that under quasi-static load. The pin-connected joint was the weakest. Investigated also are the influence of geometric parameters for pin-connected HFRP laminate joints. The results shed light on how to improve the bearing strength of mechanical joints when encountering dynamic loads.     

A simpliied fatigue assessment method for transverse illet welded joints

Under the as-welded condition the fatigue crack initiation period was considered nonexistent and Linear Elastic Fracture Mechanics(LEFM) was used to calculate fatigue strength for a range of weld geometries. Fracture mechanics assessment of welded joints requires accurate solutions for stress intensity factor(SIF). However, the solutions for the SIF of complex welded joints are dificult to determine due to the complicated correction factors. Three methods for SIF prediction are discussed on illet welded specimens containing continuous or semi-elliptical surface cracks, including the traditional correction method Mk, the approximate correction method Kt, and the suggested additional crack size method(ac+ae).The new additional crack parameter ae is used to replace the stress concentration effect of weld proile Mk, which simpliies the calculation process. Experimental results are collected to support fatigue strength assessment of the additional crack size method.     

Non-linear and failure behaviour of spotwelds: a “global” finite element and experiments in pure and mixed modes I/II

The paper deals with joint element model used in crashworthiness simulations. The first part of the paper is dedicated to the formulation of a new “global” finite element for spotweld modelling. The mechanical behaviour of the joint is elastic–plastic type and damage is taken into account to model the failure of the welded area. The second part of the paper concerns a new experimental procedure for joint strength analysis in pure and mixed modes I/II and for joint model characterisation. Experiment is based on Arcan principle and results are compared to open literature. In the last part of the paper, the parameters of the new joint model are identified using experiments and used for several shapes of spot-welded specimens. The model predicts reasonably the elastic–plastic part of the response but is unable to predict the post-peak response observed especially in the case of pure shear.     

Evaluation of fatigue strenght of welded structures by local strain measurements

The object of the paper is the experimental evaluation of the local strain conditions near the fatigue-crack starting zones in welded structures, at the toe of weld joints. These zones are characterized by steep gradients for strain and stress. The evaluation is performed using strain gages and finite-element techniques on real cruciform weld joints. The results illustrate the possibilities and the limitations posed by the use of local strain measurements to assess the fatigue strength of welded structures. These factors are usually estimated on the basis of a nominal stress approach.Paper was presented at 1983 SESA Spring Meeting held in Cleveland, OH on May 15–19, 1983.     

Effect of non-persistent joints distribution on shear behavior

The particle flow code 2D (PFC2D) is used to establish a coplanar, non-persistent joint model. Three joint distribution types, namely, both-side (type a), scattered (type b), and central (type c), are set according to their position. Numerical simulations of the direct shear test are conducted to investigate the effect of non-persistent joint distribution and connectivity on shear mechanical behavior. Simulation results are in good agreement with the analytical solutions to Jennings' criterion, and show: (1) type-c and type-b joints have high strength, whereas type-a joints have low strength. Shear strength and modulus increase with a decrease in joint persistency, and the shear displacement that correspond to shear strength increases with a decrease in persistency. (2) The brittle failure characteristics of the sample are evident when the intact rock bridge area is large. Reinforcement at both ends of the joint limits shear deformation, and shear strength can be effectively improved when joint persistency is large. The small-area dispersed reinforcement joint method cannot effectively improve shear strength. (3) The comprehensive shear strength parameters and the shear strength of the non-persistent joints can be predicted well using Jennings' criterion. Cohesion is the dominant factor that controls shear strength.     

A review of the volume-based strain energy density approach applied to V-notches and welded structures

A large bulk of experimental data from static tests of sharp and blunt V-notches and from fatigue tests of welded joints are presented in an unified way by using the mean value of the Strain Energy Density (SED) over a given finite-size volume surrounding the highly stressed regions. When the notch is blunt, the control area assumes a crescent shape and R₀ is its width as measured along the notch bisector line. In plane problems, when cracks or pointed V-notches are considered, the volume becomes a circle or a circular sector, respectively. The radius R₀ depends on material fracture toughness, ultimate tensile strength and Poisson’s ratio in the case of static loads; it depends on the fatigue strength Δσ_A of the butt ground welded joints and the Notch Stress Intensity Factor (NSIF) range ΔK₁ in the case of welded joints under high cycle fatigue loading (with Δσ_A and ΔK₁ valid for 5 × 10⁶ cycles).Dealing with welded joints characterised by a plate thickness greater than 6 mm, the final synthesis based on SED summarises nine hundred data taken from the literature while a new synthesis from spot-welded joints under tension and shear loading, characterised by a limited thickness of the main plate, is presented here for the first time (more than two hundred data).Dealing with static tests, about one thousand experimental data as taken from the recent literature are involved in the synthesis. The strong variability of the non-dimensional radius R/R₀, ranging from about zero to about 1000, makes the check of the approach based on the mean value of the SED severe.