Effect of a pulsed electromagnetic field on stress relaxation in concentrator-containing flat specimens

Experimental results of evaluating the pulsed electromagnetic field effect on tensile stress relaxation in specimens with hole a concentrator are presented. The pulsed electromagnetic field is shown to induce local inelastic strain of a metal in the vicinity of the concentrator, which is greatly dependent on pulsed field parameters and concentrator sizes, determining the ratio of thermal-nonthermal current effects.     

FE analysis of residual stress relaxation in a girth-welded duplex stainless steel pipe under cyclic loading

This study intends to characterize the residual stress relaxation in a girth-welded duplex stainless steel pipe exposed to cyclic loading. FE thermal simulation of the girth welding process is first performed to identify the weld-induced residual stresses. 3-D elastic–plastic FE analyses incorporated with the cyclic plasticity constitutive model which can describe the cyclic stress relaxation are next carried out to evaluate reconstruction of the residual stresses under cyclic mechanical loading. The results unveils that considerable reduction of the residual stresses in and around the girth weld occur even after the initial few loading cycles and degree of the stress relaxation is dependent on the magnitude of applied cyclic loading.     

焊接工艺参数对Q345钢平板焊接残余应力的影响

为研究焊接工艺参数对Q345钢平板焊接残余应力的影响,对采用药芯焊丝半自动焊接后的8 mm厚平板焊缝结构进行仿真模拟,在经验数值范围内设置不同的焊接工艺参数值,分析平板在横向和厚度方向的焊接残余应力分布情况。研究结果表明:横向的最大焊接残余应力分布在热影响区,且随着焊接速度的增大和焊接层间温度的降低而降低;沿厚度方向的最大焊接残余应力为115.92 MPa,位于平板中间层,随着焊接速度的增大而先减小后增大;平板焊接在横向的残余应力远大于厚度方向的应力。根据焊接残余应力的变化情况,运用二元回归分析法对横向和厚度方向的最大焊接残余应力进行函数拟合与检验,并开展多因素拟合模型的分析,得到焊接速度和焊接层间温度对焊接残余应力的综合影响规律。通过研究残余应力的变化趋势可选定焊接残余应力最小时的工艺参数范围,实现焊接工艺参数优化。     

Investigation of residual stress relaxation under cyclic load

Compressive residual stresses induced by mechanical surface treatment such as shot peening, autofretage, hole expansion, laser shock peening, and low-plasticity burnishing can be highly beneficial to fatigue resistance. Cyclic relaxation of compressive residual stress, however, reduces the benefit. An analytical model is proposed for estimation of residual stress relaxation. Parameters considered by the model include the magnitude and distribution of the residual stress, the degree of cold working required, the applied alternating and mean stresses, and the number of applied loading cycles. An elasto–plastic finite element model was used to demonstrate the model.     

Measuring residual stress and the resulting stress intensity factor in compact tension specimens

The measurement of residual stress through the remaining ligament of a compact tension specimen was studied. In the crack compliance method, a slot or notch is successively extended through the part, and the resulting strain is measured at an appropriate location. By using a finite element simulation of a specimen preloaded beyond yield, three techniques for determining the original residual stress from the measured strains were compared for accuracy and sensitivity to measurement errors. A common beam-bending approximation was substantially inaccurate. The series expansion method proved to be very versatile and accurate. The fracture mechanics approach could determine the stress intensity factor caused by the residual stresses with a very simple calculation. This approach offers the exciting possibility of determining the stress intensity factor prior to a fatigue or fracture test by measuring strains during the specimen preparation.     

Effect of residual surface stress on the fatigue behavior of a low-alloy powder metallurgy steel

In this study we have characterized the fatigue behavior of a low-alloy powder metallurgy (P/M) sintered steel, composed mainly of martensite and bainite with 10.4% porosity. After each processing step, the residual axial surface stress was measured by X-ray diffraction. Significant compressive surface stresses were generated during the machining of the fatigue specimens. A heat-treatment at 175 °C after machining had no effect on these residual stresses, but polishing the surface resulted in a 20% reduction in compressive stresses. The residual surface stresses had no apparent effect on the fatigue behavior of the sintered steel. Rather, the fatigue behavior was controlled by sub-surface defects consisting of weakly bonded particles located in regions of high porosity.     

Effect of tempering under stress on the relaxation resistance and endurance of cold-deformed steel wire

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 118–120, May–June, 1990.     

振动时效工艺在消除膨胀波纹管残余应力中的应用

膨胀波纹管体冷压成型后存在较大的残余应力常导致管端面严重变形和尺寸不稳定,已成为制约管体焊接连接质量和效率提高技术瓶颈。探索振动时效法用于消除或减弱波纹管体残余应力的可行性。结合室内试验和现场试验数据,建立振动时效法适用的关键技术、工艺参数、评价方法以及后处理分析手段。采用所建立的评价体系,通过振动时效参数曲线观测和盲孔法残余应力测量,评定了该试验振动过程及振动时效效果有效性,这证明振动时效工艺可显著降低膨胀波纹管残余应力,能够满足膨胀波纹管焊接等工程需要,提高其尺寸稳定性。     

Thermomechanical relaxation of residual stress in shot peened nickel base superalloy

Abstract

The thermal and thermomechanical behaviour of the relaxation of the residual stresses of a shot peened Astroloy superalloy under tensile cyclic loads has been evaluated by X-ray diffraction and investigated. The stress relaxation under purely thermal conditions (550 and 650°C) and thermomechanical conditions (pulsating tensile loading at 650°C) as afunction of the exposure time is presented. The purely thermal relaxation is interpreted by annihilation and reorganisation of the crystalline defects induced by shot peening, whereas the mechanical relaxation is linked to cyclic plasticity of materials. In consequence, the thermomechanical relaxation is essentially due to the complex mechanism of the concurrent thermal and mechanical effects. A model is used to predict the residual stresses induced by the specified shot peening conditions and their relaxation under the specified thermal/thermomechanical conditions.

MST/1963     

Effect of stress-raiser sharpness on corrosion-fatigue of steel specimens

The effect of notch sharpness and corrosive media on the fatigue of 5-mm-diameter steel specimens was studied. It was established that the influence of stress raisers in a corrosive medium is weaker than in atmospheric air, though this difference may disappear in the case of specimens with very sharp notches.     

Effect of residual stresses on crack behaviour in single edge bending specimens

Residual stresses due to manufacturing processes, such as welding, change the load bearing capacity of cracked components. The effects of residual stresses on crack behaviour in single edge bending specimens were investigated using Finite element analyses. Three parameters (J, Q and R) were used to study the crack behaviour. The J‐integral predicts the size scale over which large stresses and strains exist, the constraint parameter Q describes the crack‐tip constraint as a result of geometry, loading mode and crack depth and the constraint parameter R is used to describe the constraint resulting from residual stresses. To carry out a systematic investigation on the effect of residual stresses on the J‐integral and crack‐tip constraints, models under different combinations of residual stresses and external loads with different crack depths were analysed. It has been shown that the crack‐tip constraint R increased by tensile residual stresses around the crack‐tip. On the other hand, the constraint parameter R decreased and tended to zero at high external load levels.     

Effect of electric current on stress relaxation in metal

We present an analysis of the process of shear stress relaxation upon action of a high-density current pulse. We show the effect of the current density on the stress relaxation rate and the growth of the plastic component of the strain. We propose a phenomenological model describing the behavior of the material upon relaxation, taking into account the effect of the current pulse.Translated from Problemy Prochnosti, No. 2, pp. 68–72, February, 1996.     

Influence of residual stresses in the stress relaxation of cold drawn wires

This study demonstrates the influence of residual stresses in steel wires on the stress relaxation losses. Standard stress relaxation tests were performed on four types of wire, all with the same mechanical properties but with different residual stresses. Surface residual stresses were measured by X-ray diffraction. The experimental results show that stress relaxation losses decrease as the value of surface tensile residual stresses decrease. The role, sometimes controversial, of initial pre-stretching and heat treatments on stress relaxation losses can also be understood in the light of the residual stresses induced during cold-drawing.     

An experimental study on residual stress relaxation in low-cycle fatigue of Inconel 718Plus

Residual stress relaxation induced by the application of mechanical loads is determined by the nature of residual stress, the elasto-plastic material properties, and the type of applied load. Despite the importance of the first load cycle, analytical models available in the literature generally assumed residual stress relaxation as a continuous process. Residual stress induced by machining on Inconel 718Plus superalloy cylindrical specimens was measured before and after the application of load cycles under strain control. Low-cycle fatigue tests were carried out at room temperature for different strain amplitudes, and X-ray diffraction measurements were performed before and after 10 and 100 cycles. A comprehensive analytical model was derived to describe the relaxation process associated with the initial cycles and that associated with the continuous application of load cycle, which is based on the plastic strain energy per cycle W and requires the evaluation of parameters that are only dependent on the material and not on the strain amplitude.     

Calculation of fatigue limits of case-hardened specimens with consideration of mean stresses and residual stresses

 The strength of case-hardened parts under cyclic loading can be determined in an experimental manner, but this can produce a large amount of costs due to the expenditure of material and testing time. Therefore, a calculation method is developed, that strongly reduces the number of experiments. The presented model is based on Weibull's weakest-link concept and allows to compute the survival probability of a case-hardened part which is loaded close to the fatigue limit. The essential quantities in this model are the distribution of Vicker's hardness of the material, the exponents of the Weibull distribution of the volume and the surface, the residual stress state, the surface roughness and the surface oxidation depth. By integrating the survival probabilities of the surface and the volume, the survival probability of the entire specimen or part can be calculated, which allows to compute the fatigue limit. The necessary parameters have to be determined from reference specimens. The model is successfully examined by comparing experimental and calculated results of smooth and notched case-hardened specimens under alternating torsion, rotating bending, repeated tension and tension-compression. Received 20 August 2001 / Accepted 11 November 2001