Papers by Keyword: Strain Path

Abstract: Marciniak and Nakajima tests are commonly used in building FLD's, since they allow covering all regions from uniaxial to almost equibiaxial strain paths. In this work, the deviation from equibiaxial strain paths is analyzed as function of the material anisotropic behavior. The numerical results show that material with present equibiaxial stress and strain paths, while for the ones with the paths are neither equibiaxial in stress nor strain. Moreover, it is shown that despite the similarities between the two tests, they present different sensitivity to the control of the blank holder force and to the friction coefficient. Namely, the stress and strain paths in the Marciniak specimen center are more sensitive to the control of the blank holder force. On the other hand, the stress and strain paths in the Nakajima specimen center are more sensitive to the friction coefficient. The deviation from the equibiaxial strain path indicates that the stress ratio is also not necessarily 1.0, meaning that the stress triaxiality and the Lode parameter also present some deviation from the reference values for an equibiaxial stress state. This should be taken into account when analyzing forming limit results.

Abstract: With the goal to define a cost-effective and efficient process to identify adequate materials for sheet metal forming processes, it is crucial to evaluate the formability of materials. Forming limit curves (FLC) are used to analyze the forming and failure limits of sheet metals and dependence of the major (φ₁) and minor strain (φ₂) from the uniaxial stress-strain area through the plane-strain point to the biaxial strain area. According to ISO 12004-2, the FLC is performed by Nakajima or Marciniak tests. Due to the experimental setup and the preconditions, pre-stretching occurs in the specimens and bending and friction effect are the result. The determination of the onset of necking (FLC) results mathematically from a “best-fit inverse parabola” on section lines. In addition, the failure point, i.e. the maximum strain value one frame before failure, is also analyzed. In contrast, tensile, notched tensile and hydraulic bulge tests, which together have a potential to map an alternative FLC, exhibits a linear strain path evolution. The behavior of the various strain paths of Nakajima and the alternative methods are examined for necking and cracking. Furthermore, the fracture surfaces are investigated by confocal laser scanning microscopy to identify influences of the different FLC methods on the fracture mechanics. FLCs were conducted with the Nakajima and the alternative FLC characterization method for a ductile steel (DX54D). To ensure transferability, the tensile tests are also performed with a high-strength steel (DP800). The FLC of the ductile steel, generated through the alternative method, exhibits a similar shape to the Nakajima generated FLC with the advantage of a constant strain rate leading to linear strain paths and a lower number of tests. The same results are achieved for the uniaxial strain tests with DP800.

Abstract: High-strength cable-steel bridge is the “lifeline” of steel structure bridges, which requires high comprehensive mechanical properties, and cold-drawing is the most important process to produce high-strength cable-steel bridge. Therefore, through the ABAQUS platform, a bridge wire drawing model was established, and the simulation analysis on the process of stress strain law and strain path trends for high-strength bridge steel wire from Φ 12.65 mm by seven cold-drawing to Φ 6.90 mm was conducted. The simulation results show that the wire drawing the heart of the main axial deformation, surface and sub-surface of the main axial and radial deformation occurred, with the increase in the number of drawing the road, the overall deformation of the wire was also more obvious non-uniformity. In the single-pass drawing process, the change in the potential relationship of each layer of material was small, and multiple inflection points appeared in the strain path diagram; the change in the seven-pass potential relationship was more drastic, which can basically be regarded as a simple superposition of multiple single-pass pulls.

Abstract: Thermomechanical hot rolling processes are often realized using reverse rolling stands, where the rolled stock is fed forward and backward through the rolling gap. During those processes, material undergoes several strain reversals that significantly alter microstructure evolution of austenite with respect to continuously rolled counterparts. In Nb-microalloyed steels, where precipitation hardening is usually expected, the effects of strain reversal are especially complex. When rolling direction is reversed, both static recrystallization (SRX) kinetics and strain-induced precipitation (SIP) processes are slowed down due to decreasing dislocation density. It affects the competition between driving force for SRX and pinning pressure for SIP and, in turn, changes the non-recrystallization temperature (Tnr), compared to the case where strain path is linear. In the present paper, detailed through-scale analysis of strain path effects in microalloyed austenite will be presented. Physical simulation and detailed microstructural analysis will be employed to study global and local effects in microalloyed austenite after complex deformation histories. Conclusions regarding the influence of strain path changes on the interactions between SRX and SIP will be drawn.

Abstract: The cold rolling texture evolution as a function of strain path in pure titanium with initial typical recrystallized texture has been studied using viscoplastic self-consistent simulations. Three different strain paths, namely unidirectional rolling, two-step cross rolling and multi-step cross rolling have been employed to investigate the effect of strain path change on the evolution of deformation texture. The simulation results indicate that the activation of predominant prismatic slip in unidirectional rolling sample results in the formation of commonly cold rolling fiber texture RD//<10-10> in pure titanium, whereas the increased activity of basal slip over that of prismatic slip is responsible for the strong ND//<hkil> fiber texture in the two cross rolled samples.

Abstract: Lightweight design for vehicle industry is not anymore an optional condition but a mandatory need to reduce the fuel consumption and adhere to environmental regulations. To achieve this goal many single parts have been removed and complex design have been implied. This includes implementation of tailored-welded blanks and multi-layer materials. Due to the increase use of dissimilar materials in a component it is also called as hybrid components. It was observed that due to use of hybrid component the part weight decrease and thus increase fuel efficiency. To continue this aspect, in this bilayer tube flaring is investigated. The metal tubular material from inside and polymer from outside is considered for flaring. The flaring behavior of the tube is analyzed and compared with the single metal layer. The strength difference and effect of that on the formability is discussed and resulted. It was observed that due to contact of lower strength material from outside the formability of the metal tube increased and failure is delayed.

Abstract: In most situations, original Cockcroft criterion underestimates material formability in the first quadrant of FLD. So far, some modified Cockcroft criteria have been reported for different applications. This presentation will focus on the modified Cockcroft criterion which takes strain-path effect into consideration. This paper demonstrates the accuracy of this criterion through limiting dome height test, free bulge test, and the biaxial tensile test using cruciform specimen respectively. The results showed that the modified Cockcroft criterion with strain path effect has good agreement with experimental results.

Abstract: In conventional analysis of instability, a rough prediction of uniform deformation was obtained due to taking material parameters as constants. In this study, the constitutive equation with varying parameters for Zn-5%Al alloy at 340 °C is employed to predict the critical values of uniform strain in tension based on Considere criterion and Hart criterion, respectively. It should address the factor of strain rate in the characterization of the capability of uniform deformation on superplastic alloys, or for that matter, on any rate-dependent material. Comparison and analysis indicated that the results on Hart criterion have the better predictability of uniform deformation than Considere criterion. The Considere criterion is dependent on strain path, while Hart crtierion is merely dependent on the values of strain and strain rate in tension, and is independent on the strain path or the deformation condition or the deformation history. Therefore, the uniform strain vs. strain rate relation can be taken as a quantitative reference for designing a reasonable strain path during superplastic forming with increase of formability and reduction of forming time.

Abstract: The effect of strain path on work hardening and texture for a super austenitic stainless steel was investigated using both experiments and modeling. Compression deformation tests by stepwise changing loading direction in two and three dimensions were performed on cubic specimens at room temperature. The results were compared to uniaxial compression with equal accumulative strain, up to 20%, and uniaxial tension with equal final strain, up to 10% elongation of the longest side. The textures in all samples were analyzed using pole figures from EBSD analysis. Because of the high stacking fault energy of this super austenitic stainless steel, the texture was dominated by <110>-fiber texture in the compressive direction for the uniaxial compression, <111>- and <100>-fiber texture in the tensile direction for the uniaxial tensile test, and a combination of all these for the cube deformation. The density of the texture was much weaker for samples where the loading direction altered, if samples with equal accumulated strain were compared. The cube deformation was also modeled using a crystal plasticity model. The crystal plasticity model consists of a representative volume element (RVE) containing crystal grains with random orientations. The Taylor assumption was used for homogenization between the macro-and subscale. The material parameters in the crystal plasticity model were determined by calibration of its macroscopic response to experimental data. The simulated textures correspond rather well to the experimental results, but the work hardening should be completed to take into account kinematic hardening.

Abstract: In recent decades,the forming area advanced both in terms of material used as well as in flexibilityand process cost reduction. New processes are been studied, including theIncremental Sheet Forming – ISF. The ISF is a process characterized by theproduction of small batches of parts, rapid prototyping, and manufacturingflexibility with reduced operational cost. This study aims to compare thecomputer simulation with real experiments from ISF. The results of strain pathsof the three main strains simulated were consistent with the experimentalmanufacture of a symmetrical sample.