Mechanical and damage analysis along a flat-rolled wire cold forming schedule

Numerical simulation is used to study patented high-C steel flat-rolled wire cold forming processes. An elasto-plastic power law, identified from mechanical tests, is used into Forge2005? finite element (FEM) package in order to describe the material behaviour during wire drawing followed by cold rolling. A through-process approach has been favoured, transferring residual wire-drawing stresses and strain into the flat-rolling preform. This mechanical analysis, associated with a triaxiality study, points to dangerous areas where fracture may initiate due to high tensile stresses. Lema?tre’s isotropic damage criterion, including crack closure effect, a -1/3 cut-off value of stress triaxiality, and tension/compression damage asymmetry, has been used and has confirmed the previous analysis. A number of non-coalesced voids nucleated on inclusions have been observed in the Scanning Electron Microscopy (SEM), especially in high-deformation zones (“blacksmith’s cross”). Their evolution has been simulated in the FEM model using spherical numerical markers, which deform into oblate or prolate ellipsoids. The deformation-induced morphological evolution of voids observed in the SEM compares well with the geometrical evolution of the markers, which suggests that the morphologies observed do not result from micro-crack propagation, but from material transport of the nucleated voids.     

Cold drawn steel wires—processing, residual stresses and ductility Part II: Synchrotron and neutron diffraction

Cold drawing of steel wires leads to an increase of their mechanical strength and to a drop in their ductility. The increase of their mechanical strength has long been related to the reduction of the various material scales by an intense plastic deformation. Besides, it was discussed in the companion paper that large plastic deformation leads to the loss of the material hardening capabilities and that, in such a case, residual stresses preserve the elongation to failure of wires. Experimental measurements of residual stresses inside the wire have therefore been undertaken. In this paper, lattice parameters as measured using synchrotron diffraction are compared with those calculated using the residual stress fields as determined by the finite‐element method. There is a major disagreement between experimental and numerical results that is too large to be attributed to the errors of the finite‐element analyses. Therefore, neutron diffraction experiments have also been performed. These measurements show that there is a significant variation of the lattice parameter with the drawing level, which is not inherited from residual stresses, and that variation is very sensitive to the cooling rate after processing. It is therefore proposed that cold drawing would induce a phase transformation of the steel, possibly a martensitic transformation.     

Corrosion-assisted cracking in progressively drawn pearlitic steels: A materials science approach

This is a summary of the achievements of a major research line at the Materials Science Department of the University of La Coruña (Spain) in the field of environmentally assisted cracking in general and hydrogen degradation in particular. It deals with a materials science approach to the study of corrosion-assisted cracking of progressively drawn pearlitic steels for use in civil engineering. The approach is based on the fundamental idea of materials science of linking the microstructure of different steels (progressively oriented as a consequence of the process of manufacturing by cold drawing) with their macroscopic stress-corrosion behavior (increasingly anisotropic as the degree of cold drawing increases). In the first part of the research, we performed metallographic analysis and gave special attention to the evolution with cold drawing of the two basic microstructural levels: pearlite colonies (first level) and pearlitic lamellae (second level). For the first microstructural level, we observed progressive eleongation and orientation of the pearlite colonies in the direction of cold drawing (axis of the wire). For the second microstructural level, the analysis revealed an increase in the closeness of packing (with a decrease in the interlamellar spacing) and progressive orientation of the pearlitic lamellar microstructure in the direction of cold drawing. Therefore, in the process of cold drawing, both the pearlite colonies and pearlitic lamellar microstructure tend to align in a direction quasiparallel to the axis of the wire. In the second part of the research, we carried out a program of stress-corrosion-cracking tests under both cathodic and anodic conditions to promote two very different mechanisms of cracking: hydrogen-assisted cracking (HAC) and localized anodic dissolution (LAD). Both types of stress corrosion tests confirm that steels subjected to cold drawing exhibit anisotropic behavior connected with clear changes in the direction of crack propagation, which approaches the axis of the wire or the direction of cold drawing. For both mechanisms (HAC and LAD), there is a strong correlation between the angles of microstructural orientation (on the levels of pearlitic colonies and lamellae) and the angles of propagation of macroscopic cracks, which clearly demonstrates the influence of the oriented microstructure (and, thus, of the process of manufacturing by increasing cold drawing) on the macroscopic corrosion-assisted behavior of steel wires.     

Fracture Mechanics of cracks originating from holes treated for notch stress relieving

A tensile plate with a circular hole is investigated. The notch stresses are calculated, for an optimum notch arrangement as well as cold expansion of the notch. The stress-intensity factor is calculated for a crack originating at a single circular hole surrounded by two axially arranged circular stress-relieving notches. Furthermore, the influence of cold expansion of the main notch, which is to be relieved, and of the combination of cold expansion and stress-relieving notches on the stress intensity factor is investigated. The weight-function is given for all the cases above. It has been computed with good approximation by a Finite-Element-Method.     

Determination of critical stress triaxiality along yield locus of isotropic ductile materials under plane strain condition

It is widely accepted that failure due to plastic deformation in metals greatly depends on the stress triaxiality factor (TF). This article investigates the variation of stress triaxiality along the yield locus of ductile materials. Von Mises yield criteria and triaxiality factor have been used to determine the critical limits of stress triaxiality for the materials under plane strain condition. A generalized mathematical model for triaxiality factor has been formulated and a constrained optimization has been carried out using genetic algorithm. Finite element analysis of a two dimensional square plate has been carried out to verify the results obtained by the mathematical model. It is found that the set of values of the first and the second principal stresses on the yield locus, which results in maximum stress triaxiality, can be used to determine the location at which crack initiation may occur. Thus, the results indicate that while designing a certain component, such combination of stresses which leads the stress triaxiality to its critical value, should be avoided.     

A procedure for determining the true stress-strain curve over a large range of strains using digital image correlation and finite element analysis

A procedure for determining the stress-strain curve including post-necking strain is proposed. Hourglass type specimens were used for tensile tests, and the stress-strain curves were identified through an iteration process using finite element analysis. The strain at the position of minimum diameter was measured by digital image correlation. This procedure was applied to carbon steel of various degrees of cold work. The radius of the minimum section of the hourglass type specimen was changed in order to investigate the effect of stress triaxiality on the failure strain. The procedure could derive the stress-strain curve including the post-necking strain. From the obtained curve, it was shown that the stress-strain curves for different degrees of cold work were almost identical when the plastic strain by the cold working was added to the strain. Furthermore, it was revealed that the true stress-strain curve could be approximated well by the power law equation and the curve could be estimated by using the stress-strain relation for before-necking strain.     

Residual stress in pearlitic steel rods during progressively cold drawing measured by X-ray diffraction

This research aims to investigate the residual stress evolution in pearlitic steels during progressively cold drawing. To this end, pearlitic steel rods of 13 mm diameter are continuously cold-drawn to wires of 5.3 mm, and then the residual stresses of multi-pass drawing rods are evaluated by X-ray diffraction technique. The XRD results indicate that the residual stresses of pearlitic steel rods increase gradually with the growing of drawing strain, from stress-free of original rod to high tensile stress of 650 MPa in the last pass drawing wire.     

Notch root radii effects in the fatigue of polymers

Wöhler-type rotating bending fatigue tests have been performed on PVC cantilever specimens containing various sharp and blunt notches of known geometries. An attempt has been made to analyse the data obtained using linear elastic fracture mechanics concepts. Results from the sharp-notched specimens show a good correlation on a stress intensity factor basis and a fatigue limit is revealed. For blunt notches a stress intensity factor had to be calculated allowing for small flaws at the notch root, and some measure of correlation of the stress intensity factor at the fatigue limit for the various specimens is obtained.     

Analyse der Wirkung von Kerben,Mittel‐ und Eigenspannungen auf die Schwingfestigkeit des hochumgeformten Werkstoffbereichs von Spaltprofilen

Analysis of the effect of notches, mean and residual stresses on the fatigue strength of severely deformed material areas of linear flow split profiles The results of fatigue experiments on specimens extracted from the highly deformed area of linear flow split profiles are presented. The Focus is on the effect of mean stresses and notches, which is influenced by the previous forming process. The residual stress state inside a component as well as inside specimens is described. By numerical analyses considering residual stresses their effect on fatigue life can be analysed. Using Weibulls Weakest‐Link Appoach parameters, which were derived by testing smooth specimen are transferred to notched ones.     

Cold drawn steel wires—processing, residual stresses and ductility—part I: metallography and finite element analyses

Cold drawing steel wires lead to an increase of their mechanical strength and to a drop of their ductility. The increase of their mechanical strength has long been related to the reduction of the various material scales by plastic deformation, but the mechanisms controlling their elongation to failure have received relatively little attention. It is usually found that heavily deformed materials show a tendency to plastic strain localization and necking. However, in this paper it is shown that, though the steel wires are plastically deformed up to strain levels as high as 3.5, a significant capability of plastic deformation is preserved in as‐drawn wires. This apparent contradiction is resolved by the existence of residual stresses inside the wire. Finite element analyses have been conducted in order to show that residual stresses, inherited from the drawing process, are sufficient to produce a significant hardening effect during a post‐drawing tensile test, without introducing any hardening in the local material behaviour. The main conclusion of this paper is that once the material has lost its hardening capabilities, residual stresses, inherited from the process, control the elongation of cold drawn wires. The finite element method allowed also the determination of the residual stress field that would lead to the best agreement between the simulated and the experimental stress strain curve of as‐drawn wires.     

The Effect of Notch Geometry on Tensile Strength at Low and Intermediate Strain Rates

Abstract: Mechanical behaviour of notched components can significantly be influenced by notch geometry. In this work, triangular, circular and square notches are examined by experiment. Two materials including a st52 steel and brass are employed for the investigation. The experiments are conducted at low and intermediate strain rates. It is observed that: (i) the yield stress of the brass and the specimens with circular and square notches decreases as the notch length increases such that for 8 mm notches, the value of yield stress converges to the yield stress obtained for plain specimens at the same strain rate. (ii) For triangular notches the yield stress rises with the increase of notch length up to a minimum and then begins to decline. (iii) The effect of notch geometry on yield stress is significant such that for triangular notches, in particular for higher notch lengths, the increase of yield stress is much more profound than the other two notch geometries.     

逐层剥离法测量高碳钢盘条残余应力

基于ASTXSTRESS3000型应力仪,结合化学腐蚀逐层剥离技术的基础上,研究了热轧高碳钢盘条及其拉拔钢丝的表面轴向应力,并测量了盘条内部铁素体的轴向应力分布.研究表明盘条及钢丝的表面状态对残余应力的测量值有重要的影响,盘条轴向应力沿横截面直径方向的分布比较均匀,接近于零应力的水平.经过应变量为1.8的连续拉拔形变后,钢丝外表面的轴向应力值明显提高,由原始盘条表面的6.4MPa,增加到500MPa左右.     

Role of Microstructural Anisotropy in the Hydrogen-Assisted Fracture of Pearlitic Steel Notched Bars

This paper analyzes the role of microstructural anisotropy generated by cold drawing in the hydrogen-assisted fracture of progressively drawn pearlitic steel notched bars. Very different notched geometries were used to generate quite distinct stress triaxiality distributions (and thus very different constraint levels) in the vicinity of the notch tip, and fracture surfaces were classified in accordance with three micromechanical models of hydrogen-assisted micro-damage on the basis of the micro-fracture maps assembled by scanning electron microscopy over the fractured area of the notched specimens.     

Fracture and yield behavior of adhesively bonded joints under triaxial stress conditions

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter.     

EFFECTS OF DRAWING STRAIN AND BLUING ON THE FATIGUE STRENGTH OF EUTECTOID STEEL WIRES

Abstract— The effects of bluing, associated with drawing strain, on the fatigue strength of eutectoid steel wires have been investigated. The fatigue limit increases by bluing and the increase is more significant with higher drawing strain. The peak in the fatigue limit with regard to the drawing strain in the wires, at a strain of 2.5, disappears after bluing. On the other hand, in the ferritic steel wires investigated for comparison, the fatigue limit gradually increases with the drawing strain up to 7.7. Furthermore, no appreciable change in the fatigue limit due to bluing is found. Based on the results of hardness tests on fatigue specimens with- and without-bluing, it is deduced that the decrease of the fatigue limit beyond the peak drawing strain in the eutectoid steel wire can partly be attributed to insufficient locking of the high-density dislocations by solute atoms. The effect of relaxation of residual stress during bluing is also briefly discussed.     

Numerical study of compact shear (Mode II) type test specimen geometry

The Iosipescu shear test specimen geometry has been investigated by a number of research workers in recent years with conflicting results. The paper describes a numerical study of a compact shear test specimen, based on the Iosipescu geometry, which is proposed to investigate size effects in shear failure. A range of geometries has been studied and the extreme cases are reported. Results are presented for the largest absolute principal stresses together with a detailed study of the stresses between and around the roots of the two notches introduced in the test specimens. The results for the largest absolute principal stresses show that tensile stresses are created at the roots of the two notches. These tensile stresses may result in Mode I failure and probably account for the Mode I or mixed mode fracture observed in tests using the Iosipescu geometry. The results for the distribution of stresses between the roots of the two notches show that deep notches increase the likelihood of shear fracture prior to tensile failure. Shallow notches give a stress distribution similar to that developed in the indirect tensile test and hence tensile failure is likely to precede shear failure in such cases. Further numerical and experimental work is proposed.      

Unified triaxiality at the crack tip subjected to plane strain condition under mixed‐mode (I + II) fracture

The new model of stress triaxiality, subjected to plane strain condition under mixed‐mode (I + II) loading, at the yield loci of the crack tip, has been formulated using unified strength theory. It evaluates critical values of triaxiality for various convex and non‐convex failure criteria, unlike the existing model. It shows the effects of Poisson's ratio and intermediate principal stress for materials whose strength in tension and compression is either equal or unequal. Further, on this basis, the crack initiation angles are predicted for various crack inclinations and compared with those obtained from other fracture criteria. The plastic zone shapes supplement the results. Critical yield stress factor, a significant parameter at the crack tip got lowered as the difference among the three principal stresses reduced to a minimum. The crack initiation angles obtained from the model showed good agreement with those obtained from G‐, S‐, and T‐criterion.     

INFLUENCE OF AXISYMMETRIC DEFORMATION PROCESSING METHODS ON THE MECHANICAL PROPERTIES OF Cu-Nb COMPOSITES

It has been shown that deformation processed Cu-19% Nb alloys with good strength and electrical conductivity can be developed in sizes that are useful for engineering applications. Mot extrusion of bundled sub-elemental Cu-19% Nb wires followed by cold drawing to make a composite wire of diameter equal to that of the initial sub-elemental wires resulted in a 67% increase in the ultimate tensile stress. However, on subsequent cold drawing of this composite wire the strength increased at a slower rate than that obtained on continuing cold drawing of the sub-elemental wire and the strength differential decreased. In addition, after cold drawing to equivalent diameters the electrical conductivity of the composite wire was less than that of the sub-elemental wire. These results indicate that while high strengths and good electrical conductivities can be produced in larger size deformation processed Cu-Nb composites by a process of bundling, extrusion and cold drawing of sub-elemental wires, there appears to be a limit to the amount of subsequent cold drawing feasible before the benefit in properties ceases.     

Micromechanics of fracture in notched samples of heavily drawn steel

This paper analyzes the consequences on fracture of the combined effects of triaxial stress states generated by notches of very different geometries and microstructural evolution produced by a heavy cold drawing when eutectoid high-strength prestressing steels are manufactured. The anisotropic fracture behaviour of these materials with high level of strain hardening is rationalized on the basis of the markedly oriented pearlitic microstructure of the drawn steels which influences the operative micromechanism of fracture in this case.