焊接接头低周被劳裂纹扩展的统计分析与可靠性最弱环模型

在相同的试验条件下,对１６ＭｎＲ压力容器用钢焊接接头焊缝区、热区和母材区,分别做了１５根试样的低周疲劳试验,获得了三个我的低周波劳寿命分布与裂纹扩展规律。试验表明,Ｐａｒｉｓ公式中的两个参数ｍ和ｃ分别服从正态分布和对数正的两个裂纹和蔗间的扩展寿命进行了预测,预测结果与试验符合良好。文中还对焊接接头三个区的低周疲国筌 统计分析,获得了相应的概率密度函数,初步提出了整个焊接接头的可靠性最弱环模型。     

焊接接头热影响区的海水腐蚀疲劳裂纹扩展速率

本文采用T型板状焊接接头试件承受弯曲循环载荷进行空气中和海水自由腐蚀条件下的疲劳裂纹扩展试验(海水中试验条件为R=0-0.1,加载频率0.2Hz,海水温度20℃±1℃),得到了海上平台钢E36-Z35焊接接头热影响区在空气中和海水中裂纹扩展速率da/dN～AK曲线和Paris公式中相应的C和m值,并进行了讨论. 应用本文得到的试验数据,用断裂力学方法对板状焊接接头在空气中和海水中自由腐蚀条件下的疲劳裂纹扩展寿命进行了估算,结果表明,在适当考虑裂纹起始寿命的情况下,用断裂力学方法估算的疲劳裂纹扩展寿命与试验结果相当符合.文中还讨论了初始裂纹尺寸的选取对估算裂纹扩展寿命的影响.     

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。     

点焊接头疲劳寿命预测研究进展

概述点焊接头疲劳损伤理论与寿命预测方法的研究进展,分析点焊接头的疲劳强度影响因素和裂纹萌生扩展机理,对点焊接头疲劳寿命估算方法进行了对比,对寿命估算模型进行了评价,最后对点焊接头疲劳寿命预测研究的未来发展趋势提出了看法．     

大型汽轮机部件低周疲劳安全寿命的设计和评定

文中提出了汽轮机部件低周疲劳安全寿命的设计和评定方法。介绍了汽轮机部件低周疲劳安全寿命的含义和计算方法。该方法以概率论和统计学为基础，把汽轮机部件的低周疲劳寿命处理为随机变量，对低周疲劳试验数据进行统计分析来确定材料低周疲劳寿命的分布参数，使用可靠性理论来确定汽轮机部件的低周疲劳安全寿命。文中给出了低周疲劳寿命服从正态分布和对数正态分布时汽轮机部件低周疲劳安全寿命的计算方法和应用实例。该方法考虑了汽轮机运行参数随机性和材料低周疲劳特性离散性的影响，为汽轮机部件低周疲劳寿命的设计、评定和诊断提供了科学的依据。     

钢构件应力集中区低周疲劳分析及实验研究

针对两个具有应力集中区的特定钢构件的低周疲劳问题进行理论分析和实验研究。文中提出了一套低周疲劳分析的相关近似方法 :由材料静力参数估算其疲劳参数 ,并利用实验数据加以修正 ,获得了循环应力 -应变曲线和应变 -疲劳寿命曲线 ;进而把非线性有限元法和局部应力应变分析结合在一起计算疲劳寿命。理论结果和实验结果作了比较 ,从而证明本文提出的分析方法是行之有效的。     

沥青混合料疲劳过程的损伤力学分析

采用损伤力学方法研究沥青混合料的疲劳失效问题。针对悬壁梁弯曲疲劳试件,推导出疲劳过程中应力场、损伤场和疲劳裂纹形成寿命的工程封闭公式。根据沥青混合料特点,提出一种模拟疲劳裂纹扩展的特征单元失效模式,从而将疲劳裂纹形成与扩展两个阶段统一用损伤力学理论进行描述和分析。本文对沥青混合料试件的疲劳裂纹形成寿命与扩展寿命分段进行了预测,还对疲劳过程中刚度衰减及位移幅值的演化过程进行了数值模拟计算。理论预期与实验结果吻合良好。     

TC4钛合金超高周次轴向振动疲劳试验

应用基于压电超声疲劳试验技术开发的20kHz轴向振动疲劳试验系统,完成了室温下TC4钛合金超高周疲劳试验,获得了TC4合金在107～109周次范围内的轴向振动疲劳寿命曲线(S-N曲线);运用C.Paris推导公式预测了TC4合金材料的寿命,得到各应力水平下破坏率为50％、95％、99％的安全寿命.结果表明:在疲劳循环大于107周次时,试件仍会发生疲劳断裂,疲劳强度随循环次数的增加而下降,并不存在明显的疲劳极限.TC4合金的S-N曲线在107～109周次的范围内呈连续下降型.在轴向振动超高周疲劳试验中,试件的裂纹扩展寿命只占其在50％破坏率下疲劳安全寿命的一小部分,其疲劳寿命主要由试件的裂纹萌生寿命决定.     

考虑T-stress的非正态随机参数疲劳裂纹扩展寿命的可靠性分析

基于断裂力学的疲劳裂纹扩展寿命问题的研究常常将裂纹尖端应力展开项的高次项忽略,引起了裂纹扩展模拟的误差,本文考虑高次项T-stress对裂纹扩展角的影响,对裂纹扩展过程做了数值模拟,结果显示相同裂纹扩展长度下,考虑T-stress会延长裂纹扩展寿命。文章首先采用修正的Paris-Erdogan 公式计算了两端承受均布拉伸载荷的边缘斜裂纹板的疲劳裂纹扩展寿命,裂纹扩展方向采用两参数修正的最大拉应力准则。由于结构尺寸,材料特性和载荷等因素具有不确定性,导致疲劳裂纹扩展过程带有一定的随机性,本文以材料属性和载荷为随机变量,在随机有限元法的基础上,结合计算可靠度的四阶矩法,Edgeworth级数展开方法,提出随机参数服从任意分布时的结构疲劳裂纹扩展寿命可靠度的计算方法。分析了参数为非正态分布时的平板裂纹扩展寿命可靠度随裂纹扩展的变化过程。本文方法可预测工程中板裂纹的扩展行为,以及预测裂纹板的可靠度。     

焊接接头材质和力学非均匀体低周疲劳特性研究

焊接接头是由母材区、热影响区和焊缝组成的材质和力学性质非均匀体,与均匀材料相比,有关非均匀体低周疲劳特性的研究还相当少,许多问题尚未完全搞清.本文通过16MnR压力容器用钢焊接接头的应变控制低周疲劳试验,采用宏观试验与微观分析相结合的方法,考虑了焊接接头材质和力学非均匀性的影响,对其疲劳特性进行了较为深入的研究,得出一些新的十分有意义的结论.     

Plastic strip model of cracked weld joint with residual stresses

The effect of residual stresses on the fracture behavior of a cracked weld joint is studied by making use of the continuous dislocation formulation. Considered are the plastic zone length of the strip model zone and the opening displacement of a crack that is normal to both weld line and base metal boundary; they depend on the character of the yield stresses for the base metal (BM), weld material (WM), and heat affected zone (HAZ). The crack driving force is found to increase with the tensile residual stress while crack initiation and growth are suppressed if the residual stress is compressive. Moreover, the plastic zone and crack opening displacement are found to decrease linearly with the HAZ yield strength as the HAZ width is increased for HAZ yield strength greater than that of BM.     

Mechanical modeling and fracture analysis for a non-homogeneous weldment with a crack perpendicular to the interface

A new mechanical model is established for a non-homogeneous weldment with HAZs and fusion zones treated as functionally graded materials. The interface-perpendicular anti-plane fracture problems are analyzed for the HAZ and the weld metal, respectively, by the methods of Fourier integral transform and Cauchy singular integral equation. Lobatto–Chebyshev collocation method put forth by Erdogan and Gupta is employed to obtain numerical results of stress intensity factor (SIF). Parametric studies yield three conclusions: (1) overmatching is more beneficial than undermatching to the reduction of the SIF of a HAZ crack, however, the latter is more effective than the former in reducing the SIF of a weld-metal crack; (2) the optimum value of mismatch factor is 1.0, and values too greater or too smaller than this should be avoided in engineering design; (3) when the mismatch factor is unequal to 1.0, the SIF could be reduced by increasing the absolute value of the non-homogeneity parameters of HAZs.     

Crack propagation resistance of Zeron 100 weld metal fabricated using the GTA and SMA welding processes

It is envisaged that super duplex stainless steels, as currently used in the offshore oil and gas industries, will find application in the emergent renewable energy sector in areas such as offshore wind, wave and tidal electricity/hydrogen generation. Such applications typically involve engineering components experiencing fluctuating loads. Sub-critical flaws inherent in welded joints are ideal sites for crack initiation and subsequent propagation leading to fast fracture. The current paper investigates the fatigue performance of two Zeron 100 weld metals in a benign environment (laboratory air). The effects of residual stresses and misalignment inherent from the welding process are also considered. The crack propagation threshold and the intrinsic crack propagation resistance of both weld metals was found to be similar to that of the base metal. However, the fracture toughness of the base metal was superior to the GTA weld metal, which was in turn better than the SMA weld metal.     

A study on the damage evaluation of weld heat affected zone

From the micro- and macroscopic points of view, the damage evolution of weld-simulated heat affected zone (HAZ) is studied. In the framework of continuum damage mechanics (CDM), the ductile and low cycle fatigue (LCF) damage evolution laws of HAZ have been examined. Two alternative laws of damage are proposed in this paper, which may meet the need for describing damage evolution of ductile rupture and LCF fracture, respectively.     

Effect of welding process on fatigue crack growth behaviour of austenitic stainless steel welds in a low alloy (Q & T) steel

Fatigue crack growth behaviour from a lack of penetration (LOP) defect in austenitic stainless steel weld metals of cruciform joints made of a low alloy high strength (Q & T) steel has been studied to understand the effect of two welding processes, namely, shielded metal arc welding (SMAW) and flux cored arc welding (FCAW). Fatigue crack growth studies were carried out at a stress ratio of R = 0 and a frequency of 90 to 110 Hz in a resonant testing equipment (Rumul, Model:8601). Crack growth rates were relatively lower in the weld metal obtained by flux cored arc welding process. Microstructural features observed revealed marked difference in the morphology of delta ferrite for the welded joints obtained from the above two welding processes. Long streaks of delta ferrite in austenite matrix were found in case of SMAW-weld metal which seem to have lowered the resistance to the fatigue crack propagation. A discontinuous network of delta ferrite found in austenite matrix in the case of FCAW-weld metal seems to have contributed to slower propagation of fatigue crack. Fractographic features also substantiate the observed trends in the fatigue crack growth behaviour.     

High temperature fracture parameter for a weld interface crack

A fracture parameter is introduced to characterize crack growth at elevated temperature for a weld interface crack. Finite element analyses are made for a compact tension (CT) specimen under constant load where an interface is modeled along the crack plane to simulate the dividing line of the weld metal (WM) and base metal (BM). A constitutive relation for an elastic-creeping material is used. The proposed time dependent fracture parameter is obtained for different creep constants of WM and BM. A transition time and fracture parameter for the homogeneous and welded specimens are defined such that the results can be normalized and presented in a general form for assessing the crack growth life.     

A NEW CYCLIC J-INTEGRAL FOR LOW-CYCLE FATIGUE CRACK GROWTH

The constitutive equation under the low-cycle fatigue (LCF) was discussed, and a two-dimensional (2-D) model for simulating fatigue crack extension was put forward in order to propose a new cyclic J-integral. The definition, primary characteristics, physical interpretations and numerical evaluation of the new parameter were investigated in detail. Moreover, the new cyclic J-integral for LCF behaviors was validated by the compact tension (CT) specimens. Results show that the calculated values of the new parameter can correlate well with LCF crack growth rate, during constant-amplitude loading. In addition, the phenomenon of fatigue retardation was explained through the viewpoint of energy based on the concept of the new parameter.     

An extended finite element method for modeling near-interfacial crack propagation in a layered structure

The extended finite element method (XFEM) is applied for the simulation of near-interfacial crack propagation in a metal–ceramic layered structure. An experimental evidence indicates that, in a ceramic–metal–ceramic sandwich structure, a near-interfacial crack in the ceramic layer can be drawn to or deflect away from the metal layer depending on the difference in elastic properties across the interface. To model near-interfacial fracture, only the Heaviside functions are used for the XFEM, and the vector level set method is employed for efficient evaluation of the enrichment functions. The crack propagation paths predicted by the XFEM simulation are found to be consistent with the experimental observation.     

基于多因素的焊接结构疲劳裂纹扩展速率分析

以Donahue等提出的疲劳裂纹扩展速率计算模型为基础,通过引入形状系数、张开比和残余应力等参数,建立了适用于焊接结构的疲劳裂纹扩展速率计算模型,分析了多种因素对焊接结构疲劳裂纹扩展速率的影响规律。结果表明,焊板厚度和焊缝余高的变化均会对焊接结构疲劳裂纹的扩展速率产生影响,在对焊接结构表面形状进行设计时应保有一定的焊缝余高;有效应力比的增大会降低焊接结构疲劳裂纹的扩展速率,且裂纹深度的变化不会改变有效应力比对焊接结构疲劳裂纹扩展速率的影响;残余应力的增大会提高焊接结构疲劳裂纹的扩展速率,且残余应力对疲劳裂纹扩展速率的促进作用随着裂纹深度的增加而增大,在对焊接结构的疲劳性能进行设计时须考虑残余应力对结构性能的影响。     

Characterization of mechanical properties by indentation tests and FE analysis – validation by application to a weld zone of DP590 steel

In this work, the mechanical properties of the metal active gas (MAG) weld zone and heat affected zone (HAZ) were characterized utilizing the continuous indentation method together with its finite element (FEM) analysis. To verify the measured properties, uni-axial tension and three point bending tests were performed for DP590 welded specimens. For numerical simulations, the isotropic hardening law was assumed along with the non-quadratic anisotropic yield function, Yld2000-2d. As for the failure criterion of the base material and weld zones particularly for the failure evaluation in the uni-axial tension test, Hill’s bifurcation theory and the M–K theory were applied to calculate the forming limit diagrams considering all measured properties including strain-rate sensitivity.