Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.     

Induction of residual compressive stresses in the sub-surface by the adjustment of the micromilling process and the tool´s cutting edge

Additional to the conventional requirements for the component quality, the residual stress state of the sub-surface is becoming increasingly important as it affects the fatigue and wear behaviour of the component. First results indicate that micromilling allows the induction of high residual compressive stresses to increase the component's service life. Therefore, a specific design of the micromilling process with regard to the process parameters (feed per tooth and depth of cut), the use of cooling lubricant as well as the tool's cutting edge aimed for the controlled induction of residual compressive stresses in a hardened high-speed steel is presented.     

An approach to calculate fatigue properties of laser cladded components

Regarding the fatigue strength of laser cladded components only very few investigations exist today. This situation induced an investigation whose most important technical results are presented in this paper. The final goal of this work is to predict the optimal process and material parameters for the laser cladding process regarding the fatigue strength and locations of crack initiation. With the intent of calculating the residual stresses in the coating and the adjacent substrate, a FEM-based weld simulation model was developed. On the basis of the calculated residual stresses, the mechanical load situation during cyclic bending is simulated. In order to verify the FE models, round specimens have been laser cladded and analysed regarding clad quality, dilution and hardness as well as fatigue strength during a four-point bending test. The results from simulations and experiments are used as input for calculating the fatigue strength and locations of crack initiation. The Cobalt-based alloy Stellite 21 has been cladded on two substrate materials: the austenitic steel X5CrNi18-10 and the hardened and tempered steel 42CrMo4. Experimental analyses showed that the fatigue limits of the specimens strongly depend on the core materials. The fatigue strength of the steel 42CrMo4 has been figured out to be lower compared to that of the austenitic steel. It is assumed that these results are associated with different residual stresses. A model to simulate residual stresses induced by the cladding process has successfully been set up. Possibilities of fatigue-limit calculation using two approved models for hardened steels have been investigated.     

渗碳淬硬层残余应力的分布特征

以17Cr2N i2Mo钢齿轮和20CrMnMo钢代表性试样为对象,测试了深层渗碳淬硬层的残余应力。对多种渗碳淬硬层残余应力的分布特征和对疲劳性能影响进行了比较研究。通过硬化层残余应力测试,掌握硬化层残余应力分布特征,可为较合理地确定重载大模数齿轮的有效硬化层深度提供技术支撑。     

Residual Stress Relaxation of Shot-Peened Deformation Surface Layer on Duplex Stainless Steel Under Applied Loading

The relaxation of residual stress in shot-peened surface layer of duplex stainless steel S32205 under static and cyclic loading was investigated. The results reveal that the compressive residual stress is relaxed under applied tensile stress. The relaxation of residual stress in longitudinal direction is more obvious than that in transverse direction in both austenite and ferrite. When the applied stress is beyond the yield strength of the materials, the relaxation of the residual stress becomes drastic. Under cyclic loading, the residual stress relaxation occurs fast in the first few cycles, it then becomes stable gradually. A model was used to quantitatively calculate the residual stress under cyclic loading with different applied tensile stresses. The relaxation behavior is determined by the applied loading, the number of cycles, dislocation density, and the residual stress gradient. The relaxation behavior difference under cyclic loading between ferrite and austenite is discussed.     

A coupled creep plasticity model for residual stress relaxation of a shot-peened nickel-based superalloy

Shot peening is a commonly used surface treatment process that imparts compressive residual stresses into the surface of metal components. Compressive residual stresses retard initiation and growth of fatigue cracks. During component loading history, shot-peened residual stresses may change due to thermal exposure, creep, and cyclic loading. In these instances, taking full credit for compressive residual stresses would result in a nonconservative life prediction. This article describes a methodical approach for characterizing and modeling residual stress relaxation under elevated temperature loading, near and above the monotonic yield strength of INI 00. The model incorporates the dominant creep deformation mechanism, coupling between the creep and plasticity models, and effects of prior plastic strain to simulate surface treatment deformation.     

喷丸强化的基本原理与调控正/切断裂模式的疲劳断裂抗力机制图

20世纪90年代前,汽车上使用的各种圆柱螺旋弹簧(以下称弹簧)承受较低的扭转疲劳载荷,在交变正应力作用下,绝大多数发生的是宏观正断型疲劳断裂,其形貌呈45°斜断口。因为喷丸强化弹簧表层引入的残余应力与外施交变正应力间存在交互作用,所以都利用喷丸强化工艺中的"应力强化机制"提高其疲劳断裂抗力。但21世纪伊始,随着轿车结构的迅速发展,设计者迫切要求弹簧承受的扭转疲劳载荷水平与日俱增,由此导致喷丸强化的弹簧除正断型的疲劳断裂外,时而出现纵向或横向切断型的疲劳断裂,由此引发疲劳断裂抗力发生显著下降,以往很少出现这种难以理解的现象。目前从事喷丸强化工艺技术的弹簧制造行业很少有文献关注这类问题。文中通过逻辑思维对弹簧的受力分析得出的诠释发现:在切断模式下,喷丸引入的残余应力与外施交变切应力之间不存在交互作用,表明"应力强化机制"在改善切断型疲劳断裂抗力中的强化作用已经基本消失。根据作者提出的喷丸强化工艺原理,喷丸同步引入弹簧表层的是由残余应力与循环弹塑性变形改性的组织结构组成的一对"孪生",通过分析作者发现,"孪生"中改性的显微组织结构形成的"组织结构强化机制"取代"应力强化机制"起到了改善切断型疲劳断裂抗力的作用。研究结果还表明,优化的喷丸强化工艺必须同时具备"组织结构强化机制"和"应力强化机制",才能够起到改善正断型和切断型疲劳断裂抗力的作用。     

残余应力对高速列车A7N01铝合金焊接接头疲劳行为的影响

研究了残余应力在疲劳加载过程中的应力松弛行为,采用预拉伸、表面喷丸等表面预制残余应力的方法预制了表面残余应力,并研究了焊接接头的疲劳性能. 结果表明,在经过1×105周次循环载荷后,各个部位残余应力发生较大松弛,在经过2×105周次后,应力松弛较1×105周次时松弛幅度降低. 在2×105周次后,应力松弛不再明显,最终残余应力分布在拉应力20～40 MPa之间. 当引入残余应力后,各种条件下的实际应力循环比发生了明显的变化,当应力比R≥0时,随着R的增大,平均应力增大,试样的疲劳周次显著下降. 残余压应力会使裂纹萌生的周期缩短,同时加快疲劳裂纹扩展速率.     

Fatigue Properties Improvement of Low-Carbon Alloy Axle Steel by Induction Hardening and Shot Peening: A Prospective Comparison

Fatigue fracture is the major threat to the railway axle, which can be avoided or delayed by surface strengthening. In this study, a low-carbon alloy axle steel with two states was treated by surface induction hardening and shot peening, respectively, to reveal the mechanism of fatigue property improvement by microstructure characterization, microhardness measurement, residual stress analysis, roughness measurement, and rotary bending fatigue tests. The results indicate that both quenching and t...     

感应淬火对半轴用非调质钢高周疲劳性能的影响

利用旋转弯曲疲劳试验方法对比研究了一种半轴用铁素体+珠光体型非调质钢感应淬火前后高周疲劳性能的变化。结果表明,试验钢试样感应淬火后表层形成了厚度约1.2 mm、平均显微硬度约650 HV0.3的马氏体组织淬硬层。该淬硬层具有较高的残余压应力和十分细小的原奥氏体晶粒。感应淬火处理后,试验钢的疲劳耐久极限从420 MPa提高到716 MPa。疲劳断口的SEM观察表明,未经感应淬火试样的疲劳裂纹均起源于试样的表面基体,而经表面感应淬火后的疲劳裂纹则起裂于淬硬层边界附近的内部基体。     

Fatigue limit of shot peened LC9 aluminum alloy

The effects of shot peening on the three-point bending fatigue limit are described for LC9 aluminum alloy. Examination of the fatigue fracture surfaces revealed that the fatigue cracks were initiated under the hardened layer, depending upon the compressive residual stress (CRS) distribution. In this case, the local fatigue limit at the fatigue source is about 1.38 times of that of unstrengthened ones. An explanation is given with reference to the concept of internal fatigue limit б_wi, which is regarded as the critical stress to initiate propagative fatigue microcrack in the interior of metal. and an intrinsic property of metal.     

The Effect of Ultrasonic Impact Treatment on the Fatigue Resistance of Friction Stir Welded Panels

In this work, the results of an experimental study for assessing the effects of Ultrasonic Impact Treatment on the fatigue resistance of Friction Stir Welded aluminum alloy panels are presented. Although the significant compressive residual stress introduced on the material by ultrasonic impact treatment (UIT) was expected to cause retardation in the crack growth rate, this was only noted at low initial ΔΚ values. At high ΔΚ values, the effect of UIT practically diminishes. The phenomenon was attributed to the relaxation/redistribution of the residual stresses with fatigue damage. This provides an alarming situation where damage tolerance design relies on models where only the initial residual stress profile is taken into account without knowledge of the potential re-distribution of the residual stresses caused by the fatigue damage accumulation. The findings of this work also indicate that any FCG tests performed can only be considered as case-specific and conclusions can only be drawn for the case studied.     

7075铝合金循环载荷下残余应力松弛的实验研究

采用淬火和喷砂2种强化工艺引入残余应力,分析循环应力作用下7075铝合金应力松弛现象。基于应力松弛机理讨论了2种不同强化工艺下残余应力松弛规律的异同。结果表明,加载应力状态与大小、残余应力的初始分布和冷作硬化是影响残余应力松弛的主要因素。     

微粒子喷丸中碳钢疲劳性能分析

使用旋转弯曲疲劳试验机进行了中碳钢10~9cyc的疲劳实验,分析了直径100μm的钢球和陶瓷球喷丸处理后材料的疲劳性能及实验过程中残余应力的稳定性.与未喷丸试样相比,2种微粒子喷丸试样的疲劳极限分别提高了35%和23%.根据实验过程中对残余应力的跟踪测试,详细分析了残余应力松弛的过程及机理,阐述了疲劳极限提高机理及其控制因素.     

Modelling of residual stress field in spherical mandrelling process

Fatigue life of structural elements with bolt holes depends mainly on residual stress distribution law around these holes. Residual compressive circumferential normal stresses around the hole reduce operating stress magnitudes to minimum values for cyclic tension and this enhances fatigue life and load-carrying capacity of structures. The presence of residual stresses is a result of the manufacturing process. Residual stresses can also be induced deliberately around the holes by means of appropriate working with suitably chosen parameters. Quantitative knowledge of residual stresses is necessary to model the stress field after applying an external load to a structural element in order to assess static or dynamic strength, fatigue strength including. This paper presents a combined approach consisting of experimental and numerical modelling of residual circumferential normal stress distribution when forming holes in workpieces of medium carbon steel by spherical mandrelling (SM). Since the object of study is residual macrostresses (stresses of first type), a mechanical method of their determination has been employed. On the basis of experimental outcomes, a mathematical model has been built and it predicts mean integral value distribution of residual circumferential normal stresses. Since the range of the experimental technique employed is limited by the wall thickness of the bushing being worked, numerical modelling of residual stresses by means of FE simulation has been performed. The numerical results obtained allow this mathematical model to be applied to various wall thicknesses by introducing correction factors for the polynomial coefficients. At the same time, the adequacy of the proposed FE model can be evaluated only by the experimentally obtained mathematical model. The SM efficiency for enhancing the load-carrying capacity of structural elements with cylindrical holes subjected to tension has been proved by means of FE simulation.     

An enhanced analytical model for residual stress prediction in machining

The predictions of residual stresses are most critical on the machined aerospace components for the safety of the aircraft. In this paper, an enhanced analytic elasto-plastic model is presented using the superposition of thermal and mechanical stresses on the workpiece, followed by a relaxation procedure. Theoretical residual stress predictions are verified experimentally with X-ray diffraction measurements on the high strength engineering material of Waspaloy that is used critical parts such as in aircraft jet engines. With the enhanced analytical model, accurate residual stress results are achieved, while the computational time compared to equivalent FEM models is decreased from days to seconds.     

残余应力和组织结构对提高感应脉冲淬火件疲劳性能的作用

结构钢经大功率感应脉冲淬火后,可获得高硬度、高的疲劳极限和高的回火抗力。分析表明,高幅值的残余压应力和“有效晶粒”细化是提高弯曲疲劳性能的主要因素。     

Predictive modeling of machining residual stresses considering tool edge effects

The surface integrity of machined components is defined by several characteristics, of which residual stress is extremely important. Residual stress is known to have an effect on critical mechanical properties such as fatigue life, corrosion cracking resistance, and dimensional tolerance of machined components. Among the factors that affect residual stress in machined parts are cutting parameters and tool geometry. This paper presents a method of modeling residual stress for hone-edge cutting tools in turning. The model utilizes analytical cutting force models in conjunction with an approximate algorithm for elastic–plastic rolling/sliding contact. Oxley’s cutting force model is coupled with a slip line model proposed by Waldorf to estimate the cutting forces, which are in turn used to estimate the stress distribution between the tool and the workpiece. A rolling/sliding contact model, which captures kinematic hardening, is used to predict the machining residual stresses. Additionally, a moving heat source model is applied to determine the temperature rise in the workpiece due to the cutting forces. The model predictions are compared with experimental data for the turning of AISI 52100. Force predictions compare well with experimental results. Similarly, the predicted residual stress distributions correlate well with the measured residual stresses in terms of magnitude of stresses and depth of penetration.     

20CrMnMo钢不同渗碳淬硬层深度的组织与应力分布

以三种渗碳淬硬层深度不同的20CrMnMo齿轮钢试样为研究对象,对其显微组织和硬度梯度进行检测,采用电化学剥层技术,借助X-350A型X射线应力仪,测量各试样深层渗碳淬硬层的残余应力及其分布,将硬度梯度与残余应力的分布进行对照,认为20CrMnMo钢渗碳淬硬层硬度小于400 HV1,均为残余压应力,之后才转换为残余拉应力,这一结果为确保齿面具有可靠的抗接触疲劳性能,合理地确定重载大模数齿轮有效硬化层深度提供技术支撑。     

GH4169疲劳试样加工残余应力的表征及其对低周疲劳寿命的影响

为了探究GH4169合金低周疲劳试样加工过程中表面残余应力的变化及残余应力对低周疲劳性能的影响，采用X射线衍射法对“车 磨 抛”不同加工工艺的试样表面残余应力进行表征及对成品试样进行残余应力层深度测试，通过中子衍射法对试样内部进行残余应力测试。采用电液伺服万能试验机进行了低周疲劳试验。结果表明：GH4169合金低周疲劳试样加工表面残余应力随着切削量的增加由表面残余压应力变为残余拉应力。根据残余应力层深度测试，机加工对试样表面造成残余应力层深度小于0.01 mm，内部残余拉应力减小；根据疲劳试验结果，GH4169合金疲劳寿命与表面轴向残余压应力呈正相关关系。