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.     

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.     

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     

Surface residual stress evaluation in double-electrode butt welded steel plates

Surface residual stress evaluation for double-electrode welding was studied. The stresses were monitored after each operational step: positioning, implementing of constraints, welding and constraints removal. The measurements were performed at the deposited metal, heat affected zone, base metal close to the weld joint and along the plate using the X-ray diffraction method. It was observed differences in the stress evaluations for double-electrode welding which resulted in lower bending distortions and higher values of surface residual stresses, compared with single-electrode welding. This behavior is associated with the stress distribution just after the welding processes in both heat affected zone and base metal close to the fillet for double-electrode welding. The main results from the laboratorial tests indicated lower values of the bending distortions for double-electrode welding compared with the single-electrode. In relation to the residual stress, the double-electrode welding generated, in general, higher stress values in both longitudinal and transversal directions.     

The effect of a residual stress field on fracture initiation in RPV steel

Whether flaws in structures containing residual (secondary) stresses will extend under particular operational (primary) loads depends on the extent to which the residual stress field affects: (a) the nature and distribution of initiators; (b) the combined (primary + secondary) stresses and strains experienced by potential initiators. This paper compares fractographic data from specimens loaded by only a primary stress with data from specimens also containing a tensile residual stress field. Three‐dimensional elastic–plastic finite element calculations are used to characterize the stress–strain conditions at the initiation sites at the onset of brittle fracture. The introduction of a residual stress changes the dominant stage in fracture nucleation from microcrack extension to particle cracking. This offsets some of the decrease in fracture toughness expected when the residual stress field increases specimen constraint.