Surface Tensile Residual Stresses Research Articles

Stress evolution of cold-drawn pearlitic steel wire subjected to uniaxial tension

The aim of this paper is to investigate the effect of uniaxial tension on the residual stress of cold-drawn pearlitic steel wire through X-ray diffraction in combination with layer removal technique. It is found that an extra 0.9% plastic strain is introduced to the steel wire due to the uniaxial tension using a universal electronic tensile machine. The XRD results indicate that uniaxial tension after drawing has dominate influence on the decrease of surface tensile residual stress and the corresponding stress gradient in the drawn wires.

Residual stress influence on fatigue lifetimes of electroplated AISI 4340 high strength steel

ABSTRACTResidual stresses play an important role in the fatigue lives of structural engineering components. In the case of near surface tensile residual stresses, the initiation and propagation phases of fatigue process are accelerated; on the other hand, compressive residual stresses close to the surface may increase fatigue life. In both decorative and functional applications, chromium electroplating results in excellent wear and corrosion resistance. However, it is well known that it reduces the fatigue strength of a component. This is due to high tensile internal stresses and microcrack density. Efforts to improve hard chromium properties have increased in recent years. In this study, the effect of a nickel layer sulphamate process, as simple layer and interlayer, on fatigue strength of hard chromium electroplated AISI 4340 steel hardness – HRc 53, was analysed. The analysis was performed by rotating bending fatigue tests on AISI 4340 steel specimens with the following experimental groups: base material, hard chromium electroplated, sulphamate nickel electroplated, sulphamate nickel interlayer on hard chromium electroplated and electroless nickel interlayer on hard chromium electroplated. Results showed a decrease in fatigue strength in coated specimens and that both nickel plating interlayers were responsible for the increase in fatigue life of AISI 4340 chromium electroplated steel. The shot peening pre‐treatment was efficient in reducing fatigue loss in the alternatives studied.

Effects of workpiece thermal properties on machining-induced residual stresses - thermal softening and conductivity

Workpiece material properties play a key role in controlling the cutting process, and consequently residual stresses. Different materials may behave totally differently under the same cutting conditions; they may produce different types of chip, surface finish, residual stress, etc. The current work examines the effects of two workpiece thermal properties, specifically thermal conductivity ( k) and thermal softening exponent ( m), on machining-induced residual stresses, in order to understand their role in controlling the residual stresses induced in different materials, when cut using the same cutting conditions. Finite element analysis was used to model the process of orthogonal dry cutting, using the arbitrary-Lagrangian-Eulerian technique, and then predict the induced residual stresses. In order to isolate the effects of the examined properties ( k and m), only one material (stainless steel AISI 316L) was used as the base workpiece material, and different values were assigned to its k and m, one at a time. Values up to four times the original magnitudes were used, covering almost all commercial steels and stainless steels. All other material properties and cutting conditions were kept constant. Surface tensile residual stresses were induced in all cases, and a strong effect was found for both properties, k has mainly affected the thickness of the tensile layer, where higher k resulted in thicker layers; it has also induced higher surface tensile residual stresses. On the other hand, higher m (lower softening effects) has significantly increased the magnitude of surface tensile residual stresses, with almost no effect on the thickness of the tensile layer.

Role of Residual Stresses in Stress Relaxation of Prestressed Concrete Wires

In prestressed concrete elements, the stress relaxation losses in steel tendons are of paramount importance to structural design. In this work, experimental and numerical results show a clear relationship between stress relaxation and the presence of residual stresses in steel wires for prestressing concrete. Stress relaxation tests were performed at different initial loads, from 50 to 98% of the rupture load, with three types of wire differing only in their distribution of residual stresses. Cold-drawn wires having the same mechanical properties required by standards (ASTM E421, ISO15630/3) but with different residual stresses will exhibit different stress relaxation losses; such losses increase with increasing surface tensile residual stresses (as a consequence of cold drawing) or increasing inner residual tensile stresses (as a consequence of mechanical postdrawing treatments). Actual expressions for prediction of stress relaxation losses do not take into account residual stresses. Better performance of the wires can be achieved by changing the profile of residual stresses according to the requirements of the structures. This note shows how to deal with this challenge.

Effects of Strain Hardening and Initial Yield Strength on Machining-Induced Residual Stresses

Finite element analysis was used in the current study to examine the effects of strain hardening and initial yield strength of workpiece material on machining-induced residual stresses (RS). An arbitrary–Lagrangian–Eulerian finite element model was built to simulate orthogonal dry cutting with continuous chip formation, then a pure Lagrangian analysis was used to predict the induced RS. The current work was validated by comparing the predicted RS profiles in four workpiece materials to their corresponding experimental profiles obtained under similar cutting conditions. These materials were AISI H13 tool steel, AISI 316L stainless steel, AISI 52100 hardened steel, and AISI 4340 steel. The Johnson–Cook (J–C) constitutive equation was used to model the plastic behavior of the workpiece material. Different values were assigned to the J-C parameters representing the studied properties. Three values were assigned to each of the initial yield strength (A) and strain hardening coefficient (B), and two values were assigned to the strain hardening exponent (n). Therefore, the full test matrix had 18 different materials, covering a wide range of commercial steels. The yield strength and strain hardening properties had opposite effects on RS, where higher A and lower B or n decreased the tendency for surface tensile RS. Because of the opposite effects of A and (B and n), maximum surface tensile RS was induced in the material with minimum A and maximum B and n values. A physical explanation was provided for the effects of A, B, and n on cutting temperatures, strains, and stresses, which was subsequently used to explain their effects on RS. Finally, the current results were used to predict the type of surface RS in different workpiece materials based on their A, B, and n values.

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.

Effect of Abrasive Flow Machining Process on Bending Fracture Strength of WC-Co Alloy.

The bending fracture strength of a commercial WC-Co alloy finished by abrasive flow machining process was examined by three points bending test, using two kinds of specimens pre-processed by wire-EDM and grinding. It was found that this abrasive flow machining process increased the bending fracture strength of wire-EDM pre-processed specimens by 60% and of grinding pre-processed ones by 10% at maximum, respectively. This big strength increase of wire-EDM pre-processed specimens was ascribed to the facts that the abrasive flow machining process changed the surface tensile residual stress to the compressive one and removed micro cracks existed in surface layer. Further the relative small strength increase in grinding pre-processed specimens was discussed on the basis of improvement in surface roughness and decrease in the amount of surface residual stress of compression side, caused by abrasive flow machining process. Finally it was concluded that the abrasive flow machining process is the effective finishing process for a WC-Co alloy with both effects of increasing the strength and giving the enough surface property.

The electro-discharge machining surfaces of high-chromium white irons

Electro-discharge machining (EDM) is widely used with hard and brittle materials, which are difficult to machine using conventional techniques. In the EDM process, spark discharges between the electrically conductive workpiece and a tool electrode increase the temperature so that localized melting occurs. Approximately 15% of this molten metal is removed by flushing, while the remainder resolidifies on the workpiece surface to produce a re-cast layer [1]. This re-cast layer has a rapidly quenched strucmre. A heat-affected layer may also be formed in the workpiece material due to the high temperatures involved in the process. The total depth of the recast layer and the heat affected regions may be between 2 and 130 ~m, and increases with the pulse energy used in the EDM process [2]. The extreme thermal and transformation stresses generated in the re-cast layer may also produce extensive surface cracks, which generally do not propagate deeper than the re-cast layer. The presence of these surface cracks and tensile residual stresses in the re-cast layer can deleteriously affect mechanical properties, such as fatigue resistance [3].

爆発硬化した高マンガンオーステナイト鋳鋼繰返し衝撃材の残留応力

Explosion hardened high manganese austenitic cast steel is being tried for rail crossing recently. From the previous studies, it became clear that high tensile residual stress was generated in the hardened surface layer by explosion and the surface tensile residual stress changed into the compressive stress of about 500MPa after the repetition of impact.In this study, the distribution of biaxial residual stress in the explosion hardened high manganese austenitic cast steel subjected to repeated impact loads was examined and compared with those of original and shot peened steels.The results obtained are summarized as follows.(1) The residual stress distributions in the impact surface layer of the original, explosion hardened and shot peened steels were similar qualitatively as given in the following. The surface compressive stress changes into tension at the depth of 0.01mm and the tensile stress in the surface layer shows a remarkable variation as the depth increases and changes into compressive stress in the inner layer.(2) It seems that the residual stress distribution mentioned above is produced by the combined effects of surface hardened layer, the transverse deformation in impact surface and uneven deformation at hardened layer.(3) The high tensile residual stress in the explosion hardened surface layer did not increase by repeated impact loading, and no macrocrack was osberved in the impact surface layer.

有限要素法による研削加工層の残留応力のシミュレーション解析

The residual stress distribution of the ground surface layer, which may or may not contain secondary phase grains in the matrix, was microscopically analyzed by the finite element method. In this case, the dominating factors of the ground surface layer such as ratio of grinding force, grinding force, etc., were varied. The main results obtained were as follows : (1) As the ratio of grinding force μ (=tangential force Ft/normal force Fn) is increased, the ground surface shows tensile residual stress on the ground surface and compressive residual stress in the next layer down.(2) When the tangential force is small, the ground surface shows compressive residual stress on the ground surface layer and tensile residual stress in the next layer down.(3) The ground surface layer, which contains secondary phase grains in the matrix, shows a remarkable increase of residual stress when directly under a direct grinding point.