Yield Point Phenomenon Research Articles

Cyclic and Biaxial Deformation Behavior for Bake-hardenable Steel Sheet

Abstract It is well known that bake hardening treatment on the bake-hardenable steel sheets leads to the increase of yield strength and appearance of yield point phenomena under uniaxial tension deformation. On the other hand, strain paths during forming process and the subsequent dent resistance are usually not monotonic nor uniaxial, so that more investigation under complex strain path and stress state are essential in order to establish its constitutive model and achieve an accurate prediction of dent resistance by numerical simulation. In the present study, the effects of bake hardening treatment on various pre-strain paths and secondary deformations after bake hardening treatment were observed. From the obtained results, the yield drop phenomena did not appear and the Bauschinger effect was observed in cyclic deformation with bake hardening treatment. On the other hand, the yield drop and the subsequent yield plateau followed by workhardening was observed in equi-biaxial tension test as similar to the result of conventional uniaxial tension test result with bake hardening treatment.

Flow softening behavior of Ti–13V–11Cr–3Al beta Ti alloy in double-hit hot compression tests

Abstract

On the yield point phenomenon in low-carbon steels with ferrite-cementite microstructure

Low-carbon steels with a ferrite-cementite microstructure represent a discontinuous yielding resulting in several industrially undesirable problems. Considering the thermal contraction mismatch between the ferrite and cementite phases during the cooling stage to room temperature, which is comparable with the volume expansion due to austenite to martensite transformation, one may expect that these steels, similar to dual-phase steels, exhibit a continuous yielding behavior. In order to examine this phenomenon, different microstructures including ferrite-cementite, ferrite-cementite-martensite and ferrite-martensite were produced using a low-carbon steel and yielding behavior of steel samples with these microstructures were evaluated during uniaxial tensile loading. In addition, the effect of volume expansion due to austenite to martensite transformation and thermal contraction mismatch between ferrite and cementite phases on each microstructure was also evaluated. It was found that for the case of ferrite-cementite steels with very small cementite particles, continuous yielding cannot be observed. Finally, yielding behavior of different steel samples were explained based on the theoretical results obtained.

Distribution of the Mechanical Properties of a Complexly Deformed Component of Low Carbon Steel Influenced by Post-Heat Treatment

The influence of post-heat treatment on the mechanical properties of complexly deformed low carbon steel is studied in this work to improve a forming process to ensure and enhance the overall product quality. Since there is no constant deformation degree, there are big differences in the mechanical properties within the part: material seen a high deformation degree shows increased strength but decreased ductility, which carries the risk of formation of cracks. A post heat treatment reduces that risk by reducing inner stresses and work/strain hardening. This study involves two steel component varying in thickness and forming process: stretching plus bending and stretching only. Post heat treatment after cold forming is necessary to provide the safety margin needed in the application. The forming process which consists of stretching only delivers a more uniform deformation over the length of the nozzle. However, adding bending to stretching keeps the deformation degree lower and leads to a more homogenous property distribution after annealing. Material only exposed to stretching shows strong softening by deforming above critical deformation degree with resulting values below initial properties. Even during annealing of around 40 min, recrystallization took part: more globular grains can be seen. Post heat treatment also homogenizes hardness over the cross-section of the work piece. Increased hardness towards the tension and compression surface side as a result of friction between tool and work piece should be avoided to provide the safety margin also on the more local level. Annealing retrieves the yield point phenomenon and strongly increases ductility up to 35 % and reduces the ratio of tensile yield strength to ultimate tensile strength to more moderate levels of around 0.6.

The yield-point phenomenon and cyclic plasticity of the uniaxially loaded specimens

The article presents the influence of the yield-point phenomenon (YPP) on the cyclic plasticity of the uniaxial cyclically-loaded specimens based on observation of stress–strain responses and imaging of strain fields. The phenomenon and its dependence on cyclic material hardening or softening was studied through experiments conducted on the low-alloy EN 42CrMo4 steel in its normalised state (184HV), which exhibits cyclic hardening, and in its tempered state (296HV), which is subject to cyclic softening. The results of the study express the influence of the YPP on cyclic plasticity through inhomogeneous strain field when specimens are loaded with strain amplitudes within the yield plateau. The YPP is well expressed in uniaxial cyclic experiments of cyclically softening material.

Yield point phenomena in TIMETAL 125 beta Ti alloy

The yield point phenomena in TIMETAL 125 beta-Ti alloy was studied using hot compression tests in the temperature range of 680–880°C and at strain rates of 0.001–1s−1. The yield drop increased with decreasing temperature or increasing strain rate. Indeed, the yield point was sharper at high strain rates and low temperatures. The Mo equivalent was taken as a compositional index to determine the influence of chemical composition on the yield point phenomenon. There was a direct relationship between the yield drop and Mo equivalent at different deformation conditions. The total thermal–mechanical energy (Q) for yielding was determined using the power-law constitutive equation. A second-order polynomial equation was used to establish a relationship between Q and the deformation temperature. The contributions of mechanical and thermal energies (QM and QT) to Q were determined based on a semi-empirical power-law equation. The developed model showed that there is a direct relationship between the amount of mechanical energy and the yield drop. It was also found that the ratio of thermal to the total energy (QT/Q) could increase from 0.2 to more than 0.8 by increasing temperature and decreasing strain rate.

Lüders Band Propagation Monitoring in Railway Steels

The tensile failure behavior of railway wheelset materials was studied with the aid of infrared thermography technology. The tensile specimens were prepared from the actual railway vehicle parts, which were used for over 20 years. Generally, the Lüders bands are appeared and observed in annealed soft steel like low-carbon steel, during yield point phenomenon due to the unbalance of strain. During the tensile testing, a high-speed infrared camera was employed to monitor surface temperature changes in specimen. During the tensile testing, the formation of Lüders bands was clearly observed. Moreover, the propagation of bands along the 45 degree direction of loading axis was also observed. In this investigation, the yield point phenomenon with Lüders band in railway wheelset materials was introduced, and the interpretation of tensile behavior was also discussed.

Microstructure and mechanical behavior of hot compressed Ti–6V–6Mo–6Fe–3Al

Hot compression tests were carried out in temperature range of 680–880°C and at strain rates of 0.001–1s−1 on Ti–6V–6Mo–6Fe–3Al beta titanium alloy. The stress–strain curves showed yield point phenomena, especially at high strain rates and low temperatures. A polynomial equation was developed to relate the yield drop to the Zener–Hollomon parameter. It was found that the yield drop reaches a maximum and remanis nearly constant at high values of the Zener–Hollomon parameter (lnZ>30). The flow curves at high temperatures and low strain rates were typical of dynamic recovery. However, at low temperatures and high strain rates (27<lnZ<31), dynamic softening with the negative work hardening rate was observed at high strains. The microstructural observations showed that the dynamic softening was associated with partial dynamic recrystallization in which numerous grain boundary bulges were formed.

Relaxation mechanism of plastic deformation and its justification using the example of the sharp yield point phenomenon in whiskers

The generality of the dynamic approach to a wide range of problems of the continuum mechanics, including deformation at the rates determining quasi-static deformation conditions has been demonstrated using the example of deformation of cadmium and copper whiskers. The sharp yield point phenomenon has been analyzed in terms of the theory of dislocations and phenomenological integrated criteria of plasticity. It has been shown that the characteristic relaxation times used in these criteria, regardless of the applied model of plasticity, reflect essential properties of the very deformation process itself.

Hot Deformation of Al - 4.5 Mass % Mg Alloy Sheet

Hot deformation of an Al - 4.5 Mg alloy was studied in tension at temperatures of 663 K to 753 K and at strain rates ranging from 10-5to 10-1s-1. Large total elongations of 150 % - 180 % were obtained under deformation conditions for the appearance of yield point phenomenon, where the corresponding stress exponent was ~3. It was discussed that solute-drag dislocation glide mechanism can operate under these conditions and result in large tensile ductility. In addition, dynamic recovery also affects the hot ductility.

Investigation and modeling of the anomalous yield point phenomenon in pure tantalum

The monotonic and cyclic behavior of commercially pure tantalum has been investigated at room temperature, in order to capture and understand the occurrence of the anomalous yield point phenomenon. Interrupted tests have been performed, with strain reversals (tensile or compressive loading) after an aging period. The stress drop is attributed to the interactions between dislocations and solute atoms (oxygen) and its macroscopic occurrence is not systematically observed. InfraRed Thermography (IRT) measurements supported by Scanning Electron Microscopy (SEM) pictures of the polished gauge length of a specimen during an interrupted tensile test reveal the nucleation and propagation of a strain localization band.The KEMC (Kubin–Estrin–McCormick) phenomenological model accounting for strain aging has been identified for several loadings and strain rates at room temperature. Simulations on full specimen using the KEMC model do not show strain localization, because of the competition between viscosity and strain localization. However, a slight misalignment of the sample can promote strain localization.

Hot Deformation Behavior of Beta Titanium Ti-13V-11Cr-3Al Alloy

Hot compression tests were conducted on Ti-13V-11Cr-3Al beta-Ti alloy in the temperature range of 1203 K to 1353 K (930 °C to 1080 °C) and at strain rates between 0.001 and 1 s−1 The stress–strain curves showed pronounced yield point phenomena at high strain rates and low temperatures. The yield point elongation and flow stresses at the upper and lower yield points were related to the Zener–Hollomon parameter. It was found that dynamic recovery at low strain rates and dynamic recrystallization at high strain rates were the controlling mechanisms of microstructural evolution. The results also showed that strain rate had a stronger influence on the hot deformation behavior than temperature. The microstructural observations and constitutive analysis of flow stress data supported the change in the hot deformation behavior of the studied alloy varies with strain rate. For various applied strain rates, the activation energy for hot deformation was calculated in range of 199.5 to 361.7 kJ/mol. At low strain rates (0.001 and 0.01 s−1), the value of activation energy was very close to the activation energy for the diffusion of V, Cr, and Al in beta titanium. The higher value of activation energy for deformation at high strain rates (0.1 and 1 s−1) was attributed to the accumulation of dislocations and the tendency to initiate dynamic recrystallization.

Influence of yttrium content on phase formation and strain hardening behavior of Mg–Zn–Mn magnesium alloy

The aim of this study was to identify the effect of yttrium (Y) addition on the phase development and strain hardening behavior of an extruded Mg–Zn–Mn (ZM31) magnesium alloy. The addition of a small amount (0.3wt.%) of Y in the alloy led to the formation of icosahedral quasicrystalline I (Mg3YZn6) phase. Both I-phase and W-phase (Mg3Y2Zn3) were present in the extruded ZM31+3.2Y alloy, while long period stacking ordered (LPSO) X-phase (Mg12YZn) and Mg24Y5 were observed in the extruded ZM31+6Y alloy. The Y addition significantly refined grains in the extruded state. The presence of I-phase in the extruded ZM31+0.3Y alloy increased hardness, compressive yield strength, and Stage B strain hardening rate. The extruded ZM31+3.2Y alloy exhibited a lower hardness and Stage B hardening rate due to the formation of W-phase. Both extruded ZM31+0.3Y and ZM31+3.2Y alloys showed a yield point phenomenon with an initial negative strain hardening rate. The extruded ZM31+6Y alloy had a high hardness and compressive yield strength without Stage B hardening, suggesting a change of major deformation mode from twinning to slip mainly due to the role of LPSO X-phase. After solution treatment and aging, the hardness and compressive yield strength gradually increased with increasing Y content, while the strain hardening exponent and the extent of Stage B strain hardening decreased due to the dissolution of I- and W-phases and the presence of LPSO X-phase.

Statistical Quantification of the Impact of Surface Preparation on Yield Point Phenomena in Nickel

Nanoindentation was used to evaluate the effect of three surface preparation techniques—mechanical polishing, electropolishing, and ion polishing—on experimental measurements of incipient plasticity in commercially pure Ni 200. Surface preparation techniques are linked to defect densities, estimated with image quality (IQ) and kernel average misorientation (KAM) data obtained from electron backscatter diffraction patterns and the Taylor relation. Minimum yield pressures are insensitive to surface preparation, while mean yield pressure depends on dislocation density, and the maximum yield pressure is likely influenced by defects other than dislocations. KAM coupled with IQ may be a useful non-destructive parameter to relate surface defect density to the resulting changes in the spatial variability of incipient plasticity during a nanoindentation experiment. This analysis makes the assumption that geometrically necessary dislocation density is proportional to total dislocation density; in cases where this condition is not satisfied, the KAM analysis may not be valid.

상온과 343℃에서 Alloy 690TT 증기발생기 전열관의 인장물성치 평가

본 논문에서는 상온과 원전 설계온도에서 증기발생기 전열관의 축방향과 원주방향 응력-변형률 거동과 인장물성치를 파악하기 위해서, 튜브 시편과 링 시편을 이용하여 상온과 343℃에서 Alloy 690TT전열관에 대한 인장시험을 수행하였다. 축방향 인장시험 결과 상온과 343℃에서 모두 항복점 현상이 관찰되었으며, 343℃에서는 Serration이 관찰되었다. 축방향과 원주방향 모두 상온에 비해 343℃에서 강도는 감소하였으나 연신율은 거의 변화가 없었다. 343℃에서 가공경화율은 상온에 비해 약간 감소하였으나, 가공경화 거동의 변화는 없었다. 시험 온도에 관계없이 축방향에 비해 원주방향의 항복강도와 인장강도가 약 5~10% 정도 낮았다. 시편 방향에 관계없이 상온 대비 343℃에서 Alloy 690TT 전열관의 항복강도와 인장강도 감소는 ASME Sec.II의 온도 보정계수에 의해 예측된 것보다 큰 것으로 확인되었다.

Effect of polypropylene fiber processing conditions on fiber mechanical behavior

An investigation into the mechanical behavior of melt‐spun isotactic polypropylene (iPP) fibers is reported. Two different iPP formulations, PH835 and Exxon3854, synthesized using Ziegler–Natta and metallocene catalysts, respectively, and spun at take‐up velocities ranging from 1000 to 3000 m min−1 were subjected to uniaxial tensile loading, cyclic loading and creep tests. The strain rate sensitivity was determined by performing strain rate jumps. Injection molded specimens from the same iPP formulations were tested under the same conditions. The fiber birefringence increases slightly with increasing take‐up velocity, while the crystallinity is approximately insensitive to this process parameter in this range of velocities. Fibers from the two iPP samples behave differently at large plastic strains despite having the same birefringence and crystallinity. Differences are also seen in creep. The behavior of fibers is significantly different from that of the injection molded samples of the same iPP and same crystallinity. These have lower strain hardening rate, smaller failure strains, close to zero strain rate sensitivity and exhibit a yield point phenomenon. The difference is associated with the different nature and spatial organization of the crystals and inter‐crystalline amorphous and mesomorphic phases. © 2014 Society of Chemical Industry

Design and Development of Coupling for Unequal Torque System

The problems occurs at the steel industries, as the overall subject of this paper is the frequent failure of output shaft of gear motor at the pay-off reel and design and development of new type of coupling which will operates according to the situation of the nature of the rotation of pay-off reel rotation and the nature of the rotation of the 4HI cold rolling mill and which will perform efficient working under maximum torque condition without fail. As the mechanism used in the coupling is a ratchet pawl mechanism which work is to provide only one directional rotation and restrict the other. Also the use of spring loaded inserts to, which when reverse motion required, mechanism to engage the coupling hence the newly designed coupling is best suitable as per the nature of the rotation of the pay-off and 4HI cold rolling mill. Keywords: Pay-off reel, 4HI cold rolling mill, Gear motor, Coupling, Ratchet and pawl mechanism. I. Introduction The gear motor is used in cold rolling mill to give motion to the Pay-off reel which is movable with automatic guide table also un-wrapping the coil and the beginning of the process. Pay-off is the standby unit to reduce feeding time of the coil to the cold rolling mill. A cold rolling mill, also known as a reduction mill or mill, has a common construction independent of the specific type of rolling being performed. Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold- rolled products include sheets, strips, bars, and rods, these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the work pieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used. Cold rolling cannot reduce the thickness of a work piece as much as hot rolling in a single pass. Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin-rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Skin-rolling, also known as a skin-pass, involves the least amount of reduction: 0.5-1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield point phenomenon. It is also used to break up the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required. Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small. Cold rolling shapes requires a series of shaping operations, usually along the lines of sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing. If processed by a blacksmith, the smoother, more consistent, and lower levels of carbon encapsulated in the steel makes it easier to process, but at the cost of being more expensive. II. Force analysis of shaft of gear motor The shaft of gear motor goes failure at the weakest section due to overloading, a tangential load act by the metal sheet i.e. torque of cold rolling mill that comes over the Pay-off reel up to 30000 Nm, self-torque 1281 Nm, and bending loading. Also it was found that casing of gear motor goes failure, i.e. it broke. The Gear motor is of 5.5KW and 41 rpm and that of Cold rolling mill driving motor is of 1000KW and initial rpm of motor is 200mpm i.e. meter per minute. The shaft of gear motor goes failure in duration of 1-2 months and each motor cost up to 60,000 Indian rupees. Due to periodic failure of gear motor, company changed the gear motor with separate gear box and motor. One of the reasons of failure is lack of concentration of worker while working. In Continuous process industry many types of roll are used for various applications; however process rolls used are classified in to two as working & conveying. Working rolls are used for changing the product being process. This included cold rolling mill, hot rolling mill rolls used for reduction of sheet thickness & shape. Printing press rolls used for transferring pattern to product. Corrugation rolls used for changing the profile of sheet. Conveying rolls are mainly having application of transferring product from one place to other. This type rolls are smaller diameter and are generally non driven to large diameter used for transferring sheets, paper, and textile products in continuous process industry. The gear motor is used in pay-off reel to wound the metal coil, that motor is connected with Pinch roll of pay-off reel by claw coupling. One lever provided for

降伏点現象の有限ひずみ弾塑性構成モデリング

降伏点現象の有限ひずみ弾塑性構成モデリング

On the nature of the yield point phenomenon

Materials in which the yield point phenomenon occurs exhibit a characteristic drop in the stress–strain curve at the end of the elastic (or mainly elastic) region. In most cases the subsequent plastic deformation takes place locally in deformation bands, often termed Lüders bands. Although first described more than 150years ago, the yield point phenomenon is not yet fully understood. In this paper, a new and simple macromechanical model is presented to explain the yield point phenomenon. It is based on three major points: (i) a realistic or true upper yield point, (ii) typical strain-hardening behaviour common to many materials and (iii) the triaxiality of the stress state that necessarily has to develop at the Lüders front and that determines the stress level at the lower yield strength. The model is evaluated by experimental, analytical and numerical evidence. Combining all evidence, the model basically describes the yield point phenomenon correctly. It may also be applied to related strain-softening phenomena and it explains the comparatively low ductility of some nanocrystalline materials.

Nanoscale analysis of surface oxides on ZnMgAl hot-dip-coated steel sheets

In this work, the first few nanometres of the surface of ZnMgAl hot-dip-galvanised steel sheets were analysed by scanning Auger electron spectroscopy, angle-resolved X-ray photoelectron spectroscopy and atomic force microscopy. Although the ZnMgAl coating itself is exhibiting a complex micro-structure composed of several different phases, it is shown that the topmost surface is covered by a smooth, homogeneous oxide layer consisting of a mixture of magnesium oxide and aluminium oxide, exhibiting a higher amount of magnesium than aluminium and a total film thickness of 4.5 to 5 nm. Especially by the combined analytical approach of surface-sensitive methods, it is directly demonstrated for the first time that within surface imprints--created by industrial skin rolling of the steel sheet which ensures a smooth surface appearance as well as reduced yield-point phenomenon--the original, smooth oxide layer is partly removed and that a layer of native oxides, exactly corresponding to the chemical structure of the underlying metal phases, is formed.