Evolution of mechanical properties for a dual-phase steel subjected to different loading paths

The evolution of the mechanical properties of a dual-phase (DP590) steel sheet after being prestrained by uniaxial tension, plane strain and equal biaxial stretching was investigated. Specimens were first loaded using the three prestraining modes. Then, from the prestrained specimens, a few sub-sized samples were machined along the rolling direction and the transverse direction for further uniaxial tension testing. Six loading paths were provided. Equal biaxial stretching was performed using a cruciform specimen. The evolution of work hardening performance, elastic modulus, yield stress and tensile stress under the six loading paths were discussed in detail. The results indicate that loading paths can affect the latent work hardening performances, strain hardenability, yield stress and tensile stress evolution as well as the elastic modulus decrease during plastic deformation. The uniaxial tension–uniaxial tension path results in a cross-softening phenomenon, the largest yield stress enhancement and a mild maximum tensile stress increase. The equal biaxial stretching-uniaxial tension path leads to a cross-hardening phenomenon, the least yield stress enhancement and the largest tensile strength increase maximum tensile strength. The elastic modulus of DP590 steel not only changes with the accumulated plastic strain but also varies with the loading paths. The largest decrease of the elastic modulus equal biaxial stretching–uniaxial tension can reach 12.7% beyond 8% equivalent strain, which is 5.2% greater than that in the monotonic uniaxial tension path.     

Some experimental results on yield condition in plane stress state

Summary In the present paper, the determination of yield locus curves for deep drawing sheet metal is described. The experiments are based on the combination of biaxial tension tests on plane cross-shaped specimens with uniaxial tension and compression tests performed on conventional specimens. The influence of preloading (prestrain) on the hardening behaviour is discussed. The material coefficients are computed through the equalization of the test results by means of a quadratic formulation of the yield condition. Finally, the material coefficients are given as functions of the prestrain of specimens in the rolling direction.With 9 Figures     

Effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet

The effect of prestrain with a path change on the strain rate sensitivity of AA5754 sheet was investigated. Prestrain magnitudes between 0% and 12% were applied in plane strain in either the transverse or longitudinal (rolling) material direction. Samples were then loaded in uniaxial tension in the longitudinal direction at strain rates of 0.001/s and 0.1/s. Results show that when a path change is involved between prestrain and subsequent uniaxial loading, the strain rate sensitivity of the hardening rate at 0.1/s compared to 0.001/s is reduced. The rate sensitivity of the yield stress remains constant with increasing magnitudes of prestrain, while the rate sensitivity of the elongation to failure decreases with increasing prestrain. A permanent softening of the flow stress is also observed, which is greater when the path change is combined with a change in orientation.     

Large-strain Bauschinger effect in austenitic stainless steel sheet

Large-strain Bauschinger effect in cold-rolled austenitic stainless steel sheet is investigated after large amounts of prestrain. The material is prestrained in uniaxial tension, and the tensile properties of the prestrained material are measured in different angles with respect to the prestraining direction. By comparing the differences in the yield stresses in different orientations, the effect of prestraining on material anisotropy is studied. The method is applied to AISI 304-type stainless steel sheet. The test results are analyzed using a combined isotropic–kinematic hardening model. The results indicate that this kind of material shows a considerable Bauschinger effect. Transient and permanent softening is observed in the experiments. The experimental Young's modulus also seems to decrease with prestrain.     

Calculation of yield stresses and plastic strain ratios

Yield stresses and plastic strain ratios of aluminium, copper, brass and steel sheets having various textures, which are characterized by the orientation distribution functions, have been calculated as a function of angle to the rolling direction using the Bunge method based on Taylor's minimum energy theory and another method suggested by the present authors. The calculated results are compared with the measured ones. For steels, the two methods yield almost identical yield stress results. The Bunge method yields higher average plastic strain ratios than the measured data, while their variation with the angle to the rolling direction agrees very well with the measured values. The plastic strain ratios calculated by the second method are in very good agreement with the measured data in their average values but show smaller variations with the angle to the rolling direction than the measureD. Therefore, combination of the two methods can yield very good agreement between calculated and measured plastic strain ratios. For the f c c metals, the calculated yield stresses and plastic strain ratios are in good agreement with measured data, regardless of the calculation method.     

Prediction of the yielding behaviour of rolled polyoxymethylene

The yield stress anisotropy of rolled polyoxymethylene (a semicrystalline polymer) is predicted by treating the polymer as a polycrystalline aggregate having two types of crystal slip system. The model first predicts the texture development on cold rolling, then the angular variation in yield stress for various uniaxial stress and plane strain compression tests. At present the model has four disposable parameters (the critical resolved shear stresses for chain and prism slip, a strain hardening coefficient and a minimum yield stress on reverse yielding), which are amenable to experimental confirmation.     

Forming and fracture limits of aluminium alloy

Abstract

Forming and fracture limits of an AA 3104-H19 aluminium alloy sheet were studied by hydraulic bulging and Marciniak type deep drawing and tensile tests. The alloy appeared to be highly anisotropic, exhibiting distinctly different fracture patterns in the rolling and transverse directions. The preferred fracture direction was transverse to the rolling direction. In the tensile test, samples loaded in the rolling direction failed transverse to the rolling direction, but in the transverse direction, the fracture was inclined at ～55° to the tensile axis. In some cases, two such competing fractures in the characteristic directions could be observed. Scanning electron microscopy studies revealed a typical ductile fracture pattern. The fracture occurred by shearing in the through thickness direction, and typical alternating shear lips in a direction inclined at ～45° to the through thickness direction could be observed. Forming limit diagrams for both rolling and transverse directions were determined from the experiments. The measured limit strains in uniaxial tension were predicted well by the modified Rice–Tracey theory, but in equibiaxial tension, the theory overestimated the fracture limit strains.     

Critical plane approach with two families of microcracks for modelling of unilateral fatigue damage

New multiaxial fatigue damage model based on the critical plane approach is proposed. Two different physical mechanisms of the fatigue damage development on each potential failure plane (critical plane) are considered. In general, each critical plane contains two families of a parallel microcracks. The proposed model reproduces simultaneously fatigue damage induced anisotropy, the influence of positive and negative mean stresses, unilateral fatigue damage, microcrack closure effect and fatigue behaviour under variable amplitude loading. The expression for the equivalent stress in the damage evolution equation includes the stress intensity for the amplitudes as well as joint invariants for the mean values of the stress tensor and for the vectors associated with the directions of microcracks. The theoretical predictions are compared with experimental data under uniaxial cyclic loading of brass specimens. The influence of positive and negative mean stresses on the fatigue life of brass is investigated.     

Effect of prestrain paths on mechanical behavior of dual phase sheet steel

The tensile and fatigue performance of dual phase (DP600) sheet steel was investigated with specimens, as-received, and with two different prestrain path conditions, uniaxial and plane strain. First, tensile tests of the as-received condition of DP600 were performed to obtain mechanical properties, specifically the uniform elongation, for determining the prestrain levels of the specimens. Then three prestrain levels from each strain path were applied onto the sheet steel. Tensile and fatigue specimens were prepared from the prestrained coupons. Mechanical properties were obtained from the uniaxial tests of the as-received and prestrained specimens for comparison. Fatigue testing was also conducted with strain controlled to acquire fatigue properties. The fatigue life curves were plotted as a function of strain range and Neuber factor. The uniaxially prestrained specimens exhibited higher fatigue strength than that of the as-received ones for the long life region, but the opposite effect was observed for the short life region of less than 10⁴ reversals.     

Comparison of thermomechanical stresses produced in work rolls during hot and cold rolling of Cartridge Brass 1101

Thermomechanical stresses play an important role in defining the life of the work roll used in hot rolling process. In this research temperature dependent mechanical properties of cartridge brass are determined experimentally using high temperature compression tests at different temperatures and strain rates. Real life measurements are made from a brass rolling mill as input data for the simulation boundary conditions. Hot rolls are made of AISI H11 hot work tool steel. Temperature dependent mechanical properties of AISI H11 steel are used. Thermal and mechanical stresses produced in the work rolls during hot rolling process are predicted using a thermoplastic finite element approach in the ABAQUS Standard software. Hot rolling is compared with cold rolling to determine the effects produced on the work rolls. A criterion is introduced to compare the severity of stresses produced on the rolling surfaces in case of hot rolling and cold rolling based on the yield stress of the roller material for different temperatures. A method for separating thermal and mechanical stresses in the simulation is also described.     

Textural Characteristic of 70-30 Brass Rolled by Cross Shear Rolling in Single Direction

70-30 brass is rolled with 90% reduction bycross shear rolling in single direction with speed ra-tio 1.39.The sheet is divided into five layers alongrolling plane normal to measure macroscopic statis-tical unsymmetric textures in every layer are des-cribed and analysed by means of three dimensionalorientation distribution function.The results indi-cate that the main textures in every layer of brassrolled by cross shear rolling in single direction arethe same as the main textures of brass rolled byconventional rolling.But the intensities,peak posi-tions and scatters of every texture component in{110}<112>are different,namely,there is amacroscopic statistical unsymmetry.It is found thatthe textures in every layer of brass rolled by crossshear rolling in single direction can be considered asthe textures of brass rolled by common rolling insingle direction at identical shear forces,themacroscopic statistical unsymmetry depends on theshear forces which are exerted on the layer.     

AZ31B镁合金挤压板材力学性能的各向异性

在AZ31B镁合金板材的板面内沿不同方向进行单向拉伸和压缩试验,研究挤压板材的力学性能。结果表明,变形AZ31B镁合金板材具有显著的各向异性和拉压非对称性。在板面内,沿挤压方向拉伸时的屈服应力明显地比沿同方向压缩和沿其他方向拉伸或压缩时的高(约2倍);沿45°斜向拉伸的屈服应力和抗拉强度较低,而延伸率最高;这种非对称性主要表现为屈服非对称和塑性流动非对称,即拉压的屈服应力不相等和拉压应力-应变曲线形状不同,压缩曲线表现出特殊的"S"型。基于晶体塑性理论,讨论了引起变形镁合金的各向异性和拉压非对称性力学性能的变形机理。     

Induced anisotropy by the Mullins effect in filled silicone rubber

This study is concerned with the experimental characterization of anisotropy induced by the Mullins effect in a particle-reinforced silicone rubber. Experimental data concerning the influence of type and direction of initial loading on the subsequent stress softening are quite scarce. In this scope, a set of experimental tests were carried out on a filled silicone rubber. Uniaxial tensile tests and bulge tests were used to precondition the samples, i.e., to induce some primary stress softening. In both cases, subsequent uniaxial tensile tests were conducted on preconditioned specimens. The first set of experiments consists of a uniaxial tension path followed by uniaxial tension along different directions. It appears that the stress softening varies from a maximum in the same direction load to a minimum in the orthogonal direction, with respect to the first tensile load direction. Next, the bulge test is proposed as an original way to yield very different biaxial tensile strain-histories for first loading path. The fact that the biaxiality ratio varies from the pole (uniaxial tension) until the bulge border (planar tension), permits to analyze second tensile load curves in a material that experienced a more complex first load path. These experimental data allow to discuss the most appropriate criteria to describe the strain-induced anisotropy phenomenon.     

Experimental identification of yield surface of Al–Cu bimetallic sheet

Design of structures made form metal layered composite with a gradient variation of physical properties requires knowledge of their behaviour in the small of elasto-plastic strain. The aim of the research was the experimental investigation of these behaviours. Preliminary tests were carried out on standard flat specimens made from aluminium-copper layered composite, which was obtained by rolling process. Each component (layer) in the state before connecting into a composite was tested independently. The test specimens were cut from metal sheets in different directions (in the range 0–90°). The primary strength tests showed a large anisotropy of mechanical properties. Further studies, in the main part, were associated with the investigations of evolution of yield surfaces for Al–Cu bimetal and components in the range of strains from proportional limit to 0.3%. The investigations were realised by monotonic tensile tests of mini specimens, which were cut out in different directions from the large-size specimens and put to the initial deformation 0.75% in the direction of rolling. The method gave possibility to realising tests in the plane stress state and build for aluminium, copper and Al–Cu bimetal experimental yield surfaces. Evolution of yield surface with increasing levels of plastic deformation was studied. Analysis of their shape showed that aluminium, copper and Al–Cu bimetal had isotropic hardening. It was shown that the law of mixtures applied to the determination of the yield surface of Al–Cu bimetal was applicable only in a short range of elasto-plastic deformation (0.05–0.2%) and for specimens cut at an angle 0–45° from the large-size specimen.     

The characterization of shape memory effect for low elastic modulus biomedical β-type titanium alloy

This work investigates the textures of biomedical TiNbTaZr alloy rolled by 99% cold reduction ratios in thickness. The relationship between textures and superelasticity of the specimens treated at 873 K and 1223 K for 1.2 ks is studied. The microstructure of tensile specimen is investigated by transmission electron microscopy. Textures of cold-rolled and heat-treated specimens are studied. During unloading, the anisotropy of superelastic strain and pure elastic strain in the heat-treated specimens is observed. Superelastic strain along rolling direction and transverse direction is larger than those along 45° from rolling direction while pure elastic strain shows the highest value along 45° from rolling direction in the specimen treated at 873 K. For the specimen treated at 1223 K, higher pure elastic strain is obtained along rolling direction. The maximum recovered strain around 2.11% is obtained along rolling direction.     

Micromechanical modeling of the work-hardening behavior of single- and dual-phase steels under two-stage loading paths

Work-hardening behavior of single-phase steel and dual-phase steel which is made of hard martensite surrounded by soft ferrite is analyzed by using an elastoplastic crystal plasticity model in conjunction with the incremental self-consistent model. Two-stage loading paths consisting of uniaxial tension, unloading and subsequent uniaxial tension/compression for various directions are applied. Bauschinger effect and transitional re-yielding behavior, which depends on the direction of the second loading path, are predicted and analyzed with respect to the distribution of the residual resolved shear stresses within the material. These features, which are caused by the inhomogeneity of the residual stress field, are especially pronounced in the case of the dual-phase steel because of the strong mechanical contrast between ferrite and martensite phases.     

Investigation of Non‐Linear Springback for High Strength Steel Sheets by ESPI

Abstract: The springback of the sheet metals after large deformations during deep drawing is not a strongly linear process with a constant Young’s modulus, but the stress–strain behaviour during the unloading phases shows considerably non‐linear and inelastic effects. Unloading of two types of steel sheets for cold forming, a cold‐rolled high‐strength microalloyed steel and a low‐carbon steel sheet, has been analysed using the method of electronic speckle pattern interferometry (ESPI). The specimens were investigated by uniaxial tension tests, and the influences of different testing parameters upon springback were analysed. The experimental measurements showed that the stress–strain curve during unloading is non‐linear, the influence of the prestrain path upon unloading is minor, and the secant moduli of unloading curves decrease with increasing prestrain. When the prestrain value becomes high enough, a saturated value for the secant modulus is approached. An empirical relation was found to describe the changes in the unloading modulus in accordance with the prestrain value.