True Strain Values Research Articles

Processing Maps of the Ti-6Al-4V Alloy in a Forging Process Design

The paper presents the results of a complex study of the Ti-6Al-4V alloy, conducted with the application of the dynamic material modelling (DMM) method, in a wide range of temperatures, strain rates and strains. A compression test was carried out in a wide range of temperatures (800 – 1100°C) and strain rates (0.01 – 100 s-1), up to the constant final true strain value of 0.9. The obtained stress-strain curves were a basis for determining deformation activation energy with the use of an Arrhenius plot and a correlation between the Zener-Hollomon parameter and flow stress, for which the constitutive equation proposed by Sellars was used. The power dissipation efficiency parameter was calculated. The maps of power dissipation as the function of temperature and strain rate were plotted in the form of the isoclines of the power dissipation efficiency parameter expressed in %. The processing maps exhibited the range of occurrence and recrystallization of the primary α phase, the degree of the β phase recrystallization progress against the background of the process deformation windows and instability flow domain. An analysis of the influence of process parameters up on the microstructure and hardness changes was conducted.

Kinematic study of necking in a semi-crystalline polymer through 3D Digital Image Correlation

This study is devoted to the necking phenomenon and to the kinematic aspects associated with this plastic instability. The material studied is a High Density PolyEthylene deformed in tension. Full-field 3D-DIC (Digital Image Correlation) strain measurements were carried out on one face of a tensile specimen. On the opposite face, the longitudinal strain was measured in the central section using video-traction®, an extensometer based on marker tracking. This last measurement was used with feedback to control the strain rate in the specimen center. Specific data processing was run on the 3D-DIC measurements to obtain strain and strain rate maps up to true strain levels greater than 1.8. Even with measurements made on a relatively small strain rate range (smaller than 1 decade), we highlighted how much the necking phenomenon depends on the strain rate. In particular, we found that, around the yield point, the strain localization is more pronounced with higher strain rates. By analyzing the evolution of the strain rates in the necking shoulder regions, we carried out direct and original measurements of the strain level corresponding to the onset of necking stabilization (Natural Draw Ratio). This was possible because our original experimental protocol enabled us to obtain such strain maps even when the true strain values were significantly greater than 1. From an experimental point of view, we found that a video-extensometer based on marker tracking cannot efficiently measure the strain rate in the specimen center at large strain levels when the markers are greatly very deformed. In addition, a wide range of new experimental results (2D and 1D strain-rate and velocity data) is presented in this paper and we consider that this data should be particularly useful for the validation of numerical simulations of necking in polymers.

Effect of Technical Quality of Thermomechanical Die Forging of AA2099 Alloy

Abstract The paper presents the results of investigations of a multicomponent third-generation aluminium alloy, classified as AA2099. The actual forging conditions were determined basing on the assessment of the quality of side surface of specimens subjected to compression in Gleeble 3800 simulator and on flow curves of the alloy, as well as numerical modelling of forging process performed with application of QForm 3D v.7 software. Compression tests were realized at temperatures 400-500 °C, with a strain rate of 0.001-100 s-1, up to a specified constant true strain value of 0.9. Microstructure examination in as-delivered state was performed with application of Leica DM 4000M optical microscope. The obtained results of isothermal deformation of specimens were correlated with the analysis of a characteristic layered pancake-type microstructure. The simulation of die forging of a complex-shape forging (high-current contact tip used in power engineering) at the temperature 500 °C, was performed. The shape of a forging makes it possible to fully analyse the influence of thermomechanical process conditions on technical quality of a product. The simulation of forging process showed full correctness of material flow, with no signs of instability. At the same time, the analysis of investigations allowed to prepare and realize the industrial forging trials for a forging of a very complex shape, in a single step, at the temperature 500 °C, with application of thermomechanical treatment. The forging attained high quality of shape and surface. Directional specimens were taken, in order to be subjected to microstructure examination and hardness testing. The data obtained from industrial tests, combined with the results of testing using Gleeble simulator as well as from numerical modelling, make up the guidelines for mechanical processing of AA2099 alloy at the temperatures 470-500 °C.

Technological Aspect of Processing Maps for the AA2099 Alloy

Results of an experimental and modelling study of forming processes in the AA2099 Al–Cu–Li alloy, for a wide range of temperatures, strains and strain rates, are presented. The analyses are based on tensile testing at 20 °C at a strain rate of 0.02 s−1 and uniaxial compression testing in the temperature range 400–550 °C at strain rates ranging from 0.001 to 100 s−1, for constant values of true strain of 0.5 and 0.9. The stability of plastic deformation and its relationship with a sensitivity of stress to strain rate are considered. The power dissipation efficiency coefficient, η(%), and the flow instability parameter, ξ ≤ 0, were determined. The complex processing maps for hot working were determined and quantified, including process frames for basic forging processes: conventional forging and for near-superplastic and isothermal conditions. A significant aspect is the convergence of power dissipation when passing through the 500 °C peak. Deformation, temperature and strain-rate-dependent microstructures at 500 °C for strain rates of 0.1, 1, 10 and 100 s−1 are described and analysed for the conventional die forging process frame, corresponding to 465–523 °C and strain rates of 50–100 s−1.

Multi-passes warm rolling of AZ31 magnesium alloy, effect on evaluation of texture, microstructure, grain size and hardness

In this study the effect of multi-passes warm rolling of AZ31 magnesium alloy on texture, microstructure, grain size variation and hardness of as cast sample (A) and two rolled samples (B & C) taken from different locations of the as-cast ingot was investigated. The purpose was to enhance the formability of AZ31 alloy in order to help manufacturability. It was observed that multi-passes warm rolling (250°C to 350°C) of samples B & C with initial thickness 7.76mm and 7.73 mm was successfully achieved up to 85% reduction without any edge or surface cracks in ten steps with a total of 26 passes. The step numbers 1 to 4 consist of 5, 2, 11 and 3 passes respectively, the remaining steps 5 to 10 were single pass rolls. In each discrete step a fixed roll gap is used in a way that true strain per step increases very slowly from 0.0067 in the first step to 0.7118 in the 26th step. Both samples B & C showed very similar behavior after 26th pass and were successfully rolled up to 85% thickness reduction. However, during 10th step (27th pass) with a true strain value of 0.772 the sample B experienced very severe surface as well as edge cracks. Sample C was therefore not rolled for the 10th step and retained after 26 passes. Both samples were studied in terms of their basal texture, microstructure, grain size and hardness. Sample C showed an equiaxed grain structure after 85% total reduction. The equiaxed grain structure of sample C may be due to the effective involvement of dynamic recrystallization (DRX) which led to formation of these grains with relatively low misorientations with respect to the parent as cast grains. The sample B on the other hand showed a microstructure in which all the grains were elongated along the rolling direction (RD) after 90 % total reduction and DRX could not effectively play its role due to heavy strain and lack of plastic deformation systems. The microstructure of as cast sample showed a near-random texture (mrd 4.3), with average grain size of 44 & micro-hardness of 52 Hv. The grain size of sample B and C was 14μm and 27μm respectively and mrd intensity of basal texture was 5.34 and 5.46 respectively. The hardness of sample B and C came out to be 91 and 66 Hv respectively due to reduction in grain size and followed the well known Hall-Petch relationship.

Deformation Microstructure and Mechanisms in a Metastable β Titanium Alloy Exhibiting TWIP and TRIP Effects

Titanium alloys typically exhibit a limited ductility (typically 20%) and little strain-hardening. An alloy design with new concept was conducted aiming at improving both ductility and strain hardening while keeping the mechanical resistance at an excellent level. An experimental validation was illustrated with the Ti-12(wt.%)Mo alloy, exhibiting true stress - true strain values at necking, of about 1000MPa and 0.38, respectively, with a large strain hardening rate close to the theoretical limit. In order to clarify the origin of this outstanding combination of mechanical properties, detailed microstructural investigation and phase evolution analysis were conducted by means of in-situ synchrotron XRD, in-situ light microscopy, EBSD mapping and TEM microstructural analysis. In the deformed material, combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects are observed. Primary strain/stress induced phase transformations (β->ω and β->α’’) and primary mechanical twinning ({332}<113> and {112}<111>) are simultaneously activated in the β matrix. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {332}<113> twinning and the subsequent secondary mechanisms are shown to be dominant at the early stage deformation process. The evolution of the deformation microstructure results in a high strain hardening rate (~2GPa) bringing both a high tensile strength and a large uniform elongation.

A modified Zerilli–Armstrong constitutive model to predict hot deformation behavior of 20CrMo alloy steel

True stress and true strain values were obtained from isothermal hot compression tests conducted on a Gleeble thermal simulation machine, in a wide range of temperatures (1173–1373K) and strain rates (1.5×10−3–1.5×10−2s−1). The experimental data were used to develop a modified Zerilli–Armstrong constitutive model. The predicted flow stresses using the developed model were compared with experimental values. A correlation coefficient (R) of 0.989 and an average absolute relative error (AARE) of 7.71% between the measured and calculated flow stresses have been obtained. Comparing with a modified Johnson–Cook model developed in the authors’ previous study, the accuracy, the number of material constants involved and the computational time required of the model were evaluated.

STUDY ON HIGH STRAIN RATE COMPRESSION SUPERPLASTICITY OF AS-EXTRUDED AZ31 MAGNESIUM ALLOY

The grain size of as-extruded AZ31 magnesium alloy was refined by isothermal annealing pretreatment through orthogonal experiment. By using the Gleeble-3800 thermal simulator, the compression superplasticity of as-extruded AZ31 magnesium alloy was studied. The high strain rate superplastic compression was realized. The process parameters of the superplastic compression were established and the mechanism of the superplastic deformation was analyzed. The effects of deformation temperature and strain rate on the superplastic flow were investigated. The results indicated that at 250°C–300°C and strain rate at 1×10-2 s -1, the true strain values were all more than 2.03. As the temperature was 300°C and the strain rate was 1×10-2 s -1–1×10 s -1, the true strain values were all more than 2.18. The results showed that the as-extruded AZ31 magnesium alloy being refined presented good compression superplasticity. The main mechanism for the superplastic compressive deformation of the as-extruded AZ31 magnesium alloy was grain-boundary sliding, meanwhile, dynamic recrystallization also played a harmonious role during the superplastic deformation.

Study on cryorolled Al–Cu alloy using X-ray diffraction line profile analysis and evaluation of strengthening mechanisms

Al–Cu alloy sheets were rolled at room temperature and at liquid nitrogen temperature to true strain values of 0.69 and 1.38. The microstructural changes and variation in mechanical properties due to the effect of rolling temperature was evaluated. X-ray diffraction line profile analysis was performed to calculate crystallite size, microstrain and dislocation density. Irrespective of the amount of reduction, cryorolled samples exhibited reduced crystallite size along with enhanced lattice strain and dislocation density than room temperature rolled samples. At both the reductions, cryorolled samples exhibited better hardness and strength compared to the room temperature rolled samples and the enhanced properties are attributed to the higher dislocation density of the cryorolled samples. The electron backscatter diffraction results indicated that the grains are submicron in size in rolled conditions. It also revealed that majority of the fraction of special grain boundaries are Σ1 boundaries with decreasing tendency of Σ3 boundaries. Quantitative contributions of different strengthening mechanisms were performed on the rolled samples and it was observed that significant contribution comes from grain boundary strengthening.

Influence of strain and strain rate on microstructural evolution during superplasticity of Mg–Al–Zn sheet

Strain-induced abnormal grain growth was observed along the gage length during high-temperature uniaxial tensile testing of rolled Mg–Al–Zn (AZ31) sheet. Effective strain and strain rates in biaxial forming of AZ31 sheets also affected the nature of grain growth in the formed sheet. For the uniaxial testing done at 400 °C and a strain rate of 10−1 s−1, abnormal grain growth was prevalent in the gage sections that experienced true strain values between 0.2 and 1.0. Biaxial forming of AZ31 at 5 × 10−2 s−1 and 400 °C also exhibited abnormal grain growth at the cross sections which experienced a true strain of 1.7. Uniaxially tested sample at 400 °C and a strain rate of 10−3 s−1, however, showed no abnormal grain growth in the gage sections which experienced true local strain values ranging from 1.0 to 2.3. The normalized flow stress versus temperature and grain size compensated strain rate plot showed that the deformation kinetics of the current AZ31 alloy was similar to that reported in the literature for AZ31 alloys. Orientation image microscopy (OIM) was used to study the texture evolution, grain size, and grain boundary misorientation during uniaxial and biaxial forming. Influence of deformation parameters, namely strain rate, strain, and temperature on grain growth and refinement were discussed with the help of OIM results.

Rolling and Annealing Effects on Microstructure and Hardness of Commercial 405 Stainless Steel

The cold-rolling experiments for several values of true strain namely 5 percent, 10 percent, and 15 percent respectively have been carried out on commercial SS-405 steel samples at 350 o C; the as-rolled samples were cut into several pieces in size of 10 x 10 x 5 mm 3 , and some pieces were annealed to 550 o C for 24 hours. All samples were then mounted and polished before etching in order to observe the grain boundaries. The microstructure observation on all samples was carried out by using optical microscope (MO), meanwhile X-ray diffraction technique was employed in order to support the identification of the existing phases and to verify changes with respect to crystal orientation; the hardness tests were carried out by using Vickers micro hardness tester. The microstructure observation supported by X-ray diffraction results shows that the phase grains of rolled sample tends to take the oblong-shape, accompanied by a preferred orientation predominantly inclined toward the (110) plane. The microhardness testing results show that there has been an increase in the hardness of the as-rolled samples; Mainly because of the nearly negligible thickness of the original sample (being only 5 mm in size), the 15 percent as-rolled samples exhibits only a slight reduction in hardness compared to the 10 percent as-rolled samples; The main cause of this effect is the movement of some dislocations infiltrating the surface resulting in the reduction of the inner-stress in the bulk of the samples. In the rolled-annealed samples there is a very significant reduction in hardness compared to the as-rolled samples. Here the main cause is the recrystallization process taking place during annealing, which tends to significantly reduce the dislocations.

Mechanical properties, microstructure and micro-texture evolution for 1050AA deformed by equal channel angular pressing (ECAP) and post ECAP plane strain compression using two loading schemes

Electron back scattered diffraction (EBSD) was used to document the microstructure and microtexture developed due to cross deformation of commercial purity 1050 aluminum alloy. The material was first deformed in an equal channel angular pressing die (ECAP) to one up to four passes, via route B c and then deformed in plane strain compression (PSC), in two different directions, to two axial true plastic strain values of 0.5 and 1.0. The study provides a documentation of the evolution of microstructure parameters namely; cell size, average misorientation angle, fraction of submicron cell size and fraction of high angle grain boundaries. These microstructure parameters were investigated on the plane normal to the loading direction in PSC (plane constituted by the rolling and transverse directions). These microstructure parameters were compared to those achieved due to the ECAP process only. The ideal rolling texture orientations are depicted and crystal orientation maps were generated. The spatial distribution of grains having these orientations are revealed through these maps. The fraction of the main texture components for a 15° spread around the specified orientations was experimentally calculated and a quantitative idea on the evolution of microtexture was also presented. Deformation path change was shown to be an effective tool for manipulating texture as well as microstructure.

EBSD investigation of the microstructure and microtexture evolution of 1050 aluminum cross deformed from ECAP to plane strain compression

Electron backscattered diffraction (EBSD) was used to document the microstructure and texture developed due to cross deformation of commercial purity 1050 aluminum alloy. The materials are first deformed in equal channel angular pressing die (ECAP) to different number of passes; 1,4, 8, 12, and 16 passes, via route BC and then deformed in plane strain compression (PSC) to two axial true plastic strain values of 0.5 and 1.0. Deformation path change was proven to be a very effective tool for manipulating the evolution of microstructure and microtexture. The study provides a documentation of the evolution of microstructure parameters namely cell size, misorientation angle, fraction of submicron grain size, and fraction of high angle grain boundaries. These microstructure parameters were investigated on two planes; the plane normal to the loading direction in PSC (RD–TD) and that plane normal to the transverse direction (RD–ND). These microstructure parameters are compared to those achieved due to the ECAP process only. The ideal rolling texture orientations are depicted and crystal orientation maps were generated. The spatial distribution of grains having these orientations is revealed through these maps. The fraction of the main texture components for a 10° spread around the specified orientations is experimentally calculated and a quantitative idea on the evolution of microtexture is also presented.

Using Contact-less Optical Systems to Characterize and Measure Deformation of TWIP Materials

There is evident huge progress in the contact-less systems for measuring deformation by means of the photogrammetry especially during last ten years. From the forming point of view is very important that in many cases photogrammetry can improve knowledge about material deformation behavior under plastic deformation. This paper deals with characterization and description quite special deformation behavior of so-called TWIP (TWinning Induced Plasticity) materials by these contact-less optical systems (in this paper is used system ARAMIS). Beside the revealing deformation behavior there is also effort to find out development of anisotropy during whole static tension test and comparison of results from those measured at deformation values specified by the commonly used standard ČSN ISO 12 275. Other important results can be found during measurement of FLD for TWIP materials according standard ISO 12004-2. There is also possibility to compare values of true strain measured by optical system and results from numeric simulation program – in this case it was widely used simulation program for sheet forming from ESI company (France) PAM-STAMP 2G - comparing of real values of true strain and results from simulation, their mapping and final graphical display of differences between these values.

Texture Manipulation in Commercial Purity Aluminum by Deformation Path Change from ECAP to Plane Strain Compression

Electron back scattered diffraction (EBSD) was used to document the microstructure and texture developed due to cross deformation of commercial purity 1050 aluminum alloy. The materials was first deformed in equal channel angular pressing die (ECAP) to one and two passes, via route BC and then deformed in plane strain compression (PSC) to two axial true plastic strain values of 0.5 and 1.0. The study provides a documentation of the evolution of microstructure parameters namely; cell size, misorientation angle, fraction of submicron grain size and fraction of high angle grain boundaries. These microstructure parameters were investigated on the plane normal to the loading direction in PSC (RD-TD). These microstructure parameters are compared to those achieved due to the ECAP process only. The ideal rolling texture orientations are depicted and crystal orientation maps were generated. The spatial distribution of grains having these orientations are revealed through these maps. The fraction of the main texture components for a 10o spread around the specified orientations is experimentally calculated and a quantitative idea on the evolution of microtexture is presented.

In-situ synchrotron diffraction and digital image correlation technique for characterizations of retained austenite stability in low-alloyed transformation induced plasticity steel

Direct measurement and quantification of phase transformation in a low-alloyed transformation induced plasticity steels depending on the tensile load as well as determination of the real true stress and true strain values were carried out in-situ using high energy synchrotron radiation . Digital image correlation technique was used to quantify more precisely the true strain values. The aim of the work was to obtain a better understanding of the phase transformation of commercial low-alloyed transformation induced plasticity steel depending on the true strain and true stress values.

Effect of strengthening mechanisms on cold workability and instantaneous strain hardening behavior during grain refinement of AA 6061-10 wt.% TiO 2 composite prepared by mechanical alloying

The mechanical alloying (MA) of AA 6061 alloy reinforced with 10 wt.% fine anatase-titania composites powder milled with different timings (1, 5, 10, 20, 30, and 40 h) was cold consolidated and sintered. The main purpose of this study is to investigate the effect of microstructure and the various strengthening mechanisms such as solid solution, grain size, precipitate, dislocation and dispersion strengthening during grain refinement of AA 6061-10 wt.% TiO 2 composite via MA on cold working and strain hardening behavior. The sintered composite preforms were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. The strengthening mechanisms were estimated by using simplified models available in the literatures. The evaluation of cold deformation behavior under triaxial stress condition through room temperature cold-upsetting tests (incremental loads) was studied by correlating the strengthening mechanisms. Among the developed strengthening mechanisms the grain size and dislocation strengthening mechanisms diminished the deformation capacity of the composites. The strain hardening behavior was also examined by proposing instantaneous strain hardening index ( n i ). The value of maximum instantaneous strain hardening index for ultrafine-grained composite was to be about twice that of coarse-grained composite at lower true axial strain value. Also, the coarse grained 5 h composite was observed to be the best one as it exhibited a better strain hardening or deformation behavior.

Manufacture and characterisation of agarose microparticles

The mechanical properties of agarose microparticles formed by shearing bulk gels dispersed in cold water using a high speed rotor/stator device were investigated. The influence of agarose concentration on bulk gel texture (Young’s modulus, stress and strain at failure) and microparticle size (Sauter mean diameter and size distribution) was assessed. Results obtained showed that bulk gels from 1 to 8% (w/w) showed a linear increase in Young’s modulus with concentration, while the true strain values at failure levelled at high concentrations (>3%). High speed shearing of the bulk gels yielded final particle sizes for the l% and 2% w/w microgel suspensions significantly lower than for the more concentrated gels (3–8% w/w) which are statistically identical ( p < 0.005) at 103 ± 2 μm, irrespective of concentration. Rheological investigation of the microparticle suspensions, over a range of bulk gel concentrations, showed that microparticles form a solid like suspension at high volume fractions, becoming fluid like above a well defined yield stress. Overall, a simple and low-cost procedure has been developed to produce an array of agarose microparticles that can confer a range of textural functionalities to beverages from liquids to fluid gels.

New Fe–Cr–Mo–Ga–C composites with high compressive strength and large plasticity

A novel nanostructured Fe–Cr–Mo–Ga–C composite derived from a bulk metallic glass-forming alloy composition has been prepared with different carbon content to improve the ductility of this high-strength material. Excellent compressive mechanical properties were found for both samples, which essentially consist of soft gallium-enriched dendrites embedded in a hard chromium- and molybdenum-enriched matrix. The coexistence of the soft and hard phases leads to a composite material with a high fracture strength combined with large ductility. The obtained values of true strength and true strain are 2.9 GPa and 36% for the alloy with lower carbon content and 3.0 GPa and 16% for the alloy with higher carbon content, respectively. It is worth noting that such excellent compressive behavior has never been observed before for any crystalline or glassy structured material.

Failure Properties of Cervical Spinal Ligaments Under Fast Strain Rate Deformations

The failure responses of the anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum were examined in vitro under large strain-rate mechanical loading. To quantify the failure properties for 3 cervical spinal ligaments at strain rates associated with traumatic events. There exists little experimentation literature for fast-rate loading of the cervical spine ligaments. The small amount of available information is framed only in extensive experimental coordinates, and not in the context of strains. Bone-ligament-bone complexes were strained at fast rates, in an incrementally increasing loading protocol using a servohydraulic mechanical test frame. Failure loads and displacements were converted to engineering and true stress and strain values, and compared for the different ligaments (anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum), spinal levels (C3-C4, C5-C6, and C7-T1), and for male versus female specimens. There were no significant differences in force or true stress for gender or spinal level. There was a significant difference in force and true stress for ligament type. A difference was found between the posterior longitudinal ligament and ligamentum flavum for failure force, and between the ligamentum flavum and both the anterior and posterior longitudinal ligaments for failure true stress. No significant differences were found in true strain for ligament, gender, or spinal level. The mean ligament failure true strain was 0.81. Failure true strains were approximately 57% of the failure engineering strains. Once the injury mechanisms of the cervical spine are fully understood, computational models can be employed to understand the potentially traumatic effects of clinical procedures, and mitigate injury in impact, falls, and other high-rate scenarios. The soft tissue failure properties in this study can be used to develop failure tolerances in fast-rate loading scenarios. Failure properties of the anterior and posterior longitudinal ligaments were similar, and the same properties can be used to model both ligaments.