Search Results (25)

The constitutive model is widely employed to characterize the rheological properties of metallic materials under high-temperature conditions. It is typically derived from a series of high-temperature tests conducted at varying deformation temperatures, strain rates, and strains, including hot stretching, hot compression, separated Hopkinson pressure bar testing, and hot torsion. The original experimental data used for establishing the constitutive model serves as the foundation for developing phenomenological models such as Arrhenius and Johnson–Cook models, as well as physical-based models like Zerilli–Armstrong or machine learning-based constitutive models. The resulting constitutive equations are integrated into finite element analysis software such as Abaqus, Ansys, and Deform to create custom programs that predict the distributions of stress, strain rate, and temperature in materials during processes such as cutting, stamping, forging, and others. By adhering to these methodologies, we can optimize parameters related to metal processing technology; this helps to prevent forming defects while minimizing the waste of consumables and reducing costs. This study provides a comprehensive overview of commonly utilized experimental equipment and methods for developing constitutive models. It discusses various types of constitutive models along with their modifications and applications. Additionally, it reviews recent research advancements in this field while anticipating future trends concerning the development of constitutive models for high-temperature deformation processes involving metallic materials. Full article

The concept of “material effort” appears in continuum mechanics wherever the response of a material to the currently existing state of loads and boundary conditions loses its previous, predictable character. However, within the material, which still descriptively remains a continuous medium, new physical structures appear and new previously unused physical features of the continuum are activated. The literature is dominated by a simplified way of thinking, which assumes that all these states can be characterized and described by one and the same measure of effort—for metals it is the Huber–Mises–Hencky equivalent stress. Quantitatively, perhaps 90% of the literature is dedicated to this equivalent stress. The remaining authors, as well as the author of this paper, assume that there is no single universal measure of effort that would “fit” all operating conditions of materials. Each state of the structure’s operation may have its own autonomous measure of effort, which expresses the degree of threat from a specific destruction mechanism. In the current energy sector, we are increasingly dealing with “low-cycle thermal fatigue states”. This is related to the fact that large, difficult-to-predict renewable energy sources have been added. Professional energy based on coal and gas units must perform many (even about 100 per year) starts and stops, and this applies not only to the hot state, but often also to the cold state. The question arises as to the allowable shortening of start and stop times that would not to lead to dangerous material effort, and whether there are necessary data and strength characteristics for heat-resistant steels that allow their effort to be determined not only in simple states, but also in complex stress states. Do these data allow for the description of the material’s yield surface? In a previous publication, the author presented the results of tension and compression tests at elevated temperatures for two heat-resistant steels: St12T and 26H2MF. The aim of the current work is to determine the properties and strength characteristics of these steels in a pure torsion test at elevated temperatures. This allows for the analysis of the strength of power turbine components operating primarily on torsion and for determining which of the two tested steels is more resistant to high temperatures. In addition, the properties determined in all three tests (tension, compression, torsion) will allow the determination of the yield surface of these steels at elevated temperatures. They are necessary for the strength analysis of turbine elements in start-up and shutdown cycles, in states changing from cold to hot and vice versa. A modified testing machine was used for pure torsion tests. It allowed for the determination of the sample’s torsion moment as a function of its torsion angle. The experiments were carried out at temperatures of 20 °C, 200 °C, 400 °C, 600 °C, and 800 °C for St12T steel and at temperatures of 20 °C, 200 °C, 400 °C, 550 °C, and 800 °C for 26H2MF steel. Characteristics were drawn up for each sample and compared on a common graph corresponding to the given steel. Based on the methods and relationships from the theory of strength, the yield stress and torsional strength were determined. The yield stress of St12T steel at 600 °C was 319.3 MPa and the torsional strength was 394.4 MPa. For 26H2MH steel at 550 °C, the yield stress was 311.4 and the torsional strength was 382.8 MPa. St12T steel was therefore more resistant to high temperatures than 26H2MF. The combined data from the tension, compression, and torsion tests allowed us to determine the asymmetry and plasticity coefficients, which allowed us to model the yield surface according to the Burzyński criterion as a function of temperature. The obtained results also allowed us to determine the parameters of the Drucker-Prager model and two of the three parameters of the Willam-Warnke and Menetrey-Willam models. The research results are a valuable contribution to the design and diagnostics of power turbine components. Full article

Modeling of friction stir welding (FSW) is challenging, as there are large gradients in both strain rate and temperature (typically between 450 and 500 °C in aluminum alloys) that must be accounted for in the constitutive law of the material being joined. Constitutive laws are most often calibrated using flow stresses from hot compression or hot torsion testing, where strain rates are much lower than those seen in the stir zone of the FSW process. As such, the current work employed a recently developed method to measure flow stresses at high strain rates and temperatures in AA 2219-T67, and these data were used in the development of a finite element (FE) simulation of FSW. Because heat generation during FSW is primarily a function of friction between the rapidly spinning tool and the plate, the choice of friction law and associated parameters were also studied with respect to FE model predictions. It was found that the Norton viscoplastic friction law provided the most accurate modeling results, for both the transient and steady-state phases of an FSW plunge experiment. It is likely that the superior performance of the Norton law was its ability to account for temperature and rate sensitivity of the plate material sheared by the tool, while the Tresca-limited Coulomb law favored contact pressure, with essentially no temperature or rate dependence of the local material properties. With optimized friction parameters and more accurate flow stresses for the weld zone, as measured by a high-pressure shear test, a 65% overall reduction in model error was achieved, compared to a model that employed a material law calibrated with hot compression or hot torsion test results. Model error was calculated as an equally weighted comparison of temperatures, torques, and forces with experimentally measured values. Full article

Cold-formed steel (CFS) sections, increasingly favored in the construction industry due to their numerous advantages over hot-rolled steel, have received limited attention in research concerning the flexural behavior of galvanized iron (GI)-based CFS at elevated temperatures. Understanding how these materials and structures behave under elevated temperatures is crucial for fire safety. The authors have performed experimental studies previously on GI-based CFS under elevated temperatures. In that study, CFS sections made of GI of grade E350 of 1.5 m long and 2 mm thickness were used. Built-up beam sections were tested under two-point loading after heating to 60 and 90 min durations and subsequently cooling them down using air and water. This study aims to uncover the influence of different stiffener configurations on the load carrying capacity of sections under elevated temperature parametrically. With the experimental study results from previous studies as a reference, authors used FEM analysis to comprehensively study the behavior of GI-based CFS sections under fire. Vertical, horizontal, and not providing a stiffener were the configurations selected to study the beams parametrically. Parametric analysis confirmed that different stiffener configurations did not alter the predominant failure mode, which remained distortional buckling across all specimens. Beams with vertical stiffeners demonstrated superior performance compared to those with horizontal stiffeners in parametric analysis. Lateral–torsional buckling was observed in the reference specimen, lacking stiffeners due to inadequate restraint at the supports. Full article

Hot deformation constitutive analysis and processing maps of ultrasonic melt treated (UST) A5052 alloy were carried out based on a hot torsion test in this study. The addition of the Al–Ti master alloy as a grain refiner with no UST produced a finer grain size than the UST and pure Ti sonotrode. The Al3Ti phase particles in the case of the Al–10Ti master alloy acted as a nucleus for grain refinement, while the Ti atoms dissolved in the melt from the sonotrode were considered to have less of a grain refinement effect, even under UST conditions, than the Al3Ti phase particles in the Al–Ti master alloy. The constitutive equations for each experimental condition by torsion test were derived. In the processing maps examined in this study, the flow instability region was not present under UST in the as-cast condition, but it existed under the no UST condition. The effects of UST examined in this study are considered as (i) the uniform distribution of Ti solutes from the sonotrode and (ii) the reduction of pores by the degassing effect. After the homogenization heat treatment, most instability regions disappeared because the microstructures became uniform following the decomposition of intermetallic compounds and distribution of solute elements. Full article

Recrystallization kinetics and two critical temperatures—the non-recrystallization temperature T_NR and the static recrystallization critical temperature T_SRCT—of five Nb, Ti, and V microalloyed steel grades are evaluated. The experimental examination is realized by employing isothermal double-hit compression tests and continuous hot torsion tests, both performed on a Gleeble^® 3800 thermo-mechanical simulator. The experimental results are used for the critical assessment of predicted T_NR using four empirical equations from the literature, and for the validation of simulated T_NR and T_SRCT. The thermokinetic computer simulations are realized using the mean-field microstructure modeling software MatCalc. Analysis shows that higher microalloying contents increase both critical temperatures, T_NR and T_SRCT, whereby the effect of recrystallization retardation of Nb is more pronounced than that of Ti or V. The most accurate reproduction of the experimental recrystallization behavior of the five examined steel grades is realized by the employed physics-based simulation approach. Full article

In this study, a lightweight fiber-reinforced plastic (FRP) helical gear was fabricated to investigate the potential application of FRP in automobile parts that require high loads and reduced noise. High-performance aramid FRP processed using the wet-laid method was used in the tooth region, and SCR420 steel was used in the inner hub region. A hot-forming system that combines rapid induction heating and water channel cooling methods was developed to reduce the cycle time. The cooling water flow conditions were analyzed to precisely control the mold temperature. Additionally, a rotating extraction system was developed to mitigate the extraction difficulty owing to the helix angle to the extraction direction. Using the innovative hot-forming system developed in this study, a helical gear without any process-induced defects was fabricated with a significantly reduced cycle time. The performance of the gear was successfully estimated using gear durability, torsional strength, and motion noise tests. The use of FRP materials offers significant potential to realize lightweight components; however, certain challenges related to their properties that may limit their application must be addressed on a case-by-case basis. Full article

This paper presents the results of physical modelling of the process of multi-pass rolling of a wire rod with controlled, multi-stage cooling. The main goal of this study was to verify the possibility of using a torsion plastometer, which allows conducting tests on multi-sequence torsion, tensile, compression and in the so-called complex strain state to physically replicate the actual technological process. The advantage of the research methodology proposed in this paper in relation to work published so far, is its ability to replicate the entire deformation cycle while precisely preserving the temperature of the deformed material during individual stages of the reproduced technological process and its ability to quickly and accurately determine selected mechanical properties during a static tensile test. Changes in the most important parameters of the process (strain, strain rate, temperature, and yield stress) were analyzed for each variant. After physical modelling, the material was subjected to metallographic and hardness tests. Then, on the basis of mathematical models and using measurements of the average grain size, chemical composition, and hardness, the yield strength, ultimate tensile strength, and plasticity reserve were determined. The scope of the tests also included determining selected mechanical properties during a static tensile test. The obtained results were verified by comparing to results obtained under industrial conditions. The best variant was a variant consisting of physically replicating the rolling process in a bar rolling mill as multi-sequence non-free torsion; the rolling process in an NTM block (no twist mill) as non-free continuous torsion, with the total strain equal to the actual strain occurring at this stage of the technological process; and the rolling process in an RSM block (reducing and sizing mill) as tension, while maintaining the total strain value in this block. The differences between the most important mechanical parameters determined during a static tensile test of a wire rod under industrial conditions and the material after physical modelling were 1.5% for yield strength, approximately 6.1% for ultimate tensile strength, and approximately 4.1% for the relative reduction of the area in the fracture and plasticity reserve. Full article

Curved composite twin-girder bridges are suitable for mountainous areas, due to their advantages of light self-weight, excellent mechanical performance, and fewer construction requirements. It has been found that many composite twin-girder bridges collapsed due to fatigue failure. However, the literature review showed no relevant studies on the fatigue performance of curved composite twin-girder bridges. Because of this, the specimen of 1:2 scale curved composite twin-girder bridge in accordance with the design scheme of Xizhen Bridge in China was designed and tested. Three possible fatigue details were selected: cruciform connections, transverse attachments, and transverse splices named Class I, Class II, and Class III. For the test data of nominal stress (NS), equations were proposed to convert the strain value into the internal force of the fatigue detail position. The stress caused by torsion accounts for 2.8% of the total stress, which is almost negligible. The fatigue evaluation process based on the hot spot stress (HSS) S-N curve method is presented. The HSS method is more conservative than the NS S-N curve method in predicting the fatigue life of complex structures with high-stress concentrations. Full article

An overview is presented on the structural and phase transformations and physical and mechanical properties of those multicomponent copper-based shape memory alloys which demonstrate attractive commercial potential due to their low cost, good shape memory characteristics, ease of fabrication, and excellent heat and electrical conductivity. However, their applications are very limited due to brittleness, reduced thermal stability, and mechanical strength—properties which are closely related to the microstructural features of these alloys. The efforts of the authors of this article were aimed at obtaining a favorable microstructure of alloys using new alternative methods of thermal and thermomechanical treatments. For the first time, the cyclic martensitic transformations during repeated quenching, methods of uniaxial megaplastic compression, or torsion under high pressure were successfully applied for radical size refinement of the grain structure of polycrystalline Cu-Al-Ni-based alloys with shape memory. The design of the ultra- and fine-grained structure by different methods determined (i) an unusual combination of strength and plasticity of these initially brittle alloys, both under controlled heat or hot compression or stretching, and during subsequent tensile tests at room temperature, and, as a consequence, (ii) highly reversible shape memory effects. Full article

Ti-6Al-4V profiles prepared by hot extrusion are usually accompanied by bending and twisting. The hot twist-stretch straightening is an effective strategy such that the bending deflection and twisting angle can be simultaneously decreased by a single straightening process. In addition, utilizing stress relaxation effect, the residual stress and springback can be greatly reduced by holding the straightening temperature and strain constant for a period after twist-stretch straightening. In this study, the hot deformation behaviors of the Ti-6Al-4V profile were revealed by experiments. The tensile model was obtained by uniaxial tensile tests within ranges of temperatures (500–700 °C) and strain rates (5 × 10⁻⁵–1 × 10⁻³ s⁻¹). The creep constitutive model was acquired with stress relaxation experiments in ranges of temperatures (500–700 °C) and pre-strain of 1.5%. Then, the coupled thermo-mechanical model of hot twist-stretch straightening was established. Based on orthogonal experiment strategy, the effects of straightening temperature, stretch strain, and holding time on the bending deflection and torsion angle of profile were investigated systematically and the process was optimized. The straightening accuracy is significantly affected by straightening temperature and holding time. By using optimized process parameters in practical straightening experiments, the deflection/length and angle/length after straightening does not exceed 2‰ and 2.5‰°/mm, respectively, which is basically consistent with the numerical simulation result. Full article

Reports in the literature show that severe plastic deformation can improve mechanical strength, ductility, and corrosion resistance of pure magnesium, which suggests good performance for biodegradable applications. However, the reported results were based on testing of small samples on limited directions. The present study reports compression testing of larger samples, at different directions, in pure magnesium processed by hot rolling, equal channel angular pressing (ECAP), and high pressure torsion (HPT). The results show that severe plastic deformation through ECAP and HPT reduces anisotropy and increases strength and strain rate sensitivity. Also, scaffolds were fabricated from the material with different processing histories and immersed in Hank’s solution for up to 14 days. The as-cast material displays higher corrosion rate and localized corrosion and it is reported that severe plastic deformation induces uniform corrosion and reduces the corrosion rate. Full article

This work presents the results of the numerical and physical modelling of the hot torsion of a hardly deformable 5XXX series aluminium alloy. Studies were conducted on constrained torsion with the use of the STD 812 torsion plastometer. The main purpose of the numerical tests was to determine the influence of the accuracy of the mathematical model describing the changes in the yield stress of the tested material on the distribution of strain parameters and on the stress intensity. According to the preliminary studies, in the case of numerical modelling of the torsion test, the accuracy of the applied mathematical model describing the changes in the rheological properties of the tested material and the correct definition of the initial and boundary conditions had a particularly significant impact on the correctness of the determination of the strain parameters and the intensity of stresses. As part of the experimental tests, physical modelling of the hot torsion test was conducted. The aim of this part of the work was to determine the influence of the applied strain parameters on the distribution and size of grain as well as the microhardness of the tested aluminium alloy. Metallographic analyses were performed using light microscopy and the electron backscatter diffraction method. Due to the large inhomogeneity of the deformation parameters and the stress intensity in the torsion test, such tests were necessary for the correct determination of the so-called representative area for metallographic analyses. These types of studies are particularly important in the case of the so-called complex deformation patterns. The paper also briefly presents the results of preliminary research and future directions in which it is planned to use complex deformation patterns for physical modelling of selected processes combining various materials. Full article

The size evolution of niobium carbonitrides Nb(C,N) and the evolution of the composition of an austenitic matrix in 316Nb stainless steel were simulated using DICTRA software. For the first time, the complete nine-element composition of steel was taken into account during isothermal and even anisothermal heat treatments. A reduced model was then proposed to optimize the calculation time for complex heat treatments. The change in the mean Nb content in austenite due to Nb(C,N) evolution during different heat treatments was studied. It qualitatively agrees with experimental data as obtained by electron probe microanalysis. Furthermore, the model was successfully applied to explain the effect of heat treatments on the recrystallization behavior of 316Nb steel during hot torsion tests. Moreover, the effect of the thermodynamic database and the number of alloying elements chosen was discussed. We showed that taking into account seven or even nine elements greatly improves the accuracy compared to usual simplified compositions. The proposed method can be useful in designing heat treatments promoting or conversely hindering recrystallization for a wide variety of Nb-bearing steels. Full article

Hot deformation is one of the main technological stages of products made from metallic materials. It is strictly required to decrease the costs of developing optimized technologies at this stage without a significant decrease in the products’ quality. The present investigation offers an algorithm to unite three different models to predict the hot deformation behavior, fracture, and microstructure evolution. The hot compression and tension tests of the AISI 316Ti steel were conducted using the thermomechanical simulator Gleeble 3800 for the models’ construction. The strain-compensated constitutive model and the Johnson–Mehl–Avrami–Kolmogorov (JMAK)-type model of the grain structure evolution show a satisfactory accuracy of 4.38% and 6.9%, respectively. The critical values of the modified Rice and Tracy fracture criteria were determined using the experimental values of the relative cross-section reduction and finite element calculation of the stress triaxiality. The developed models were approved for the stainless AISI 316Ti steel by the hot torsion with tension test. Full article