High-temperature Stress-strain Curves Research Articles

A Forward Identification Method for High-Temperature Stress–Strain Curves of 7075 Aluminum Alloy Sheet Considering the Necking Stage

The necking phenomenon of metal sheet under high temperatures is serious and continues over a longer duration. It is difficult to describe the high-temperature mechanical properties of materials only on the basis of hardening behavior before necking. To obtain the high-temperature stress–strain curve considering diffuse necking stage, a forward identification method based on strain measurement is proposed in this study. Here, the strain field of the minimum cross-section in the necking region of the specimen is obtained using a DIC (digital image correlation) measurement technique, and the average axial true stress–strain curve is calculated. Then, the average axial true stress–strain curve is modified using the modified Bridgeman formula. Taking 7075 aluminum alloy as an example, the high temperature equivalent stress–strain curve considering the diffuse necking stage is obtained. Compared with the traditional method, the maximum effective strain range is expanded from 0.05 to 0.8 due to the consideration of the necking stage. The obtained curve is characterized by a coupled viscoplastic–damage constitutive model and embedded in ABAQUS through the user subroutine VUMAT to simulate the hot tensile process. The relative error of force–displacement between the simulation and the experiment was 2.4%, validating the ability of the presented method. This study provides theoretical guidance and a scientific basis for the application and forming control of hot stamping processes.

Towards Interrelating the Nature of High Temperature Stress–Strain Curves and Creep Curves with Concomitant Microstructure Evolution

Towards Interrelating the Nature of High Temperature Stress–Strain Curves and Creep Curves with Concomitant Microstructure Evolution

Analysis of Stress-Strain Curves to Predict Dynamic Recrystallization During Hot Deformation of M300 Grade Maraging Steel

18Ni-2000 MPa maraging steel (M300 grade) is extensively used for performance critical aerospace applications where strength-toughness balance is the basis for material selection. The key to achieve the desired mechanical properties in the final product is to process the material under conditions that will impart an equiaxed, fine grained microstructure. This in turn is controlled by the selected deformation processing conditions such as strain, strain rate and temperature. Therefore, hot deformation behavior of the material is usually studied under wide temperature, and strain rate ranges to map the microstructural evolution with specimens deformed to a given strain. However, the effect of strain on the microstructural evolution, its effect on the initiation and termination of dynamic recrystallization are essential to obtain balanced mechanical properties. In this study, analysis of stress-strain curves was conducted with an intention to obtain fine prior austenite grain (PAG) size through dynamic recrystallization (DRX), and the same was verified experimentally from the microstructures evolved through hot isothermal compression tests on cylindrical specimens subjected to different strains. A single peak type DRX curve was identified for analysis from the high temperature stress-strain curves. The theoretically determined critical strain was used to experimentally verify the initiation of DRX (DRXI) and transition from DRX dominant region to grain growth dominant region (DRXT). Hot isothermal compression tests have been conducted at T=1100°C and e = 0.1s−1 and obtained PAG size of 7.68 µm in the specimen deformed to theoretically determined optimum strain of 0.6, thereby validating the used models.

High temperature stress–strain curves of MnS and their applications in finite element simulation

Abstract A novel method for testing stress–strain curves of non-metallic materials was presented. The high temperature stress–strain curves of MnS were preliminarily obtained and corrected to account for the influence of friction. Using the finite element method, the influence of deformation parameters on the deformation evolution of MnS inclusions was investigated based on the experimental reference data. The corresponding physical experiment was designed for comparative analysis. The results indicate that the experimental high-temperature deformation data of MnS are highly reliable. In the process of matrix deformation, the shapes of MnS inclusions change from spherical to ellipsoidal and even to lamellar. There are some differences in the morphological deformation of MnS inclusions located at different positions. With the increase in the initial size of MnS inclusions, the risk of failing the inclusion-flaw inspection increases and the forging quality further deteriorates.

A new critical point on the stress-strain curve: Delineation of dynamic recrystallization from grain growth

A new critical point termed ‘stabilisation stress’ (σstab) is identified in high temperature stress-strain curves. σstab represents the beginning of a stage in the hot working process where grain growth becomes the dominant microstructural phenomenon. This point is detected using an irreversible thermodynamics approach. Predicted correlations between σstab and grain growth are experimentally validated for structural steels, enabling the delineation of dynamic recrystallization from grain growth based on stress-strain curves.

Formation Mechanism of Inclusion Defects in Large Forged Pieces

Nonmetallic inclusions mixed into large forged metal objects destroy the continuity in the metal and affect the quality of the forged product. Research on how inclusions affect the plastic deformation of a matrix shows the significance of the formation mechanism of inclusion defects. For upset forging, the nonlinear finite element model was shown to be appropriate for the ingot hot-forging process by comparing the results with experiments involving plastic and hard inclusions inserted into the forged piece. The high-temperature stress-strain curves of MnS plastic inclusions were obtained experimentally. The results show how, during upsetting, the morphology of MnS plastic inclusions varies from spherical to ellipsoidal, until finally becoming flat in shape. The larger the inclusion is, the larger the degree of deformation of the inclusion is, and large inclusions enhance the risk of the final product failing to pass inspection for inclusion flaws. Strain significantly concentrates in the matrix near a hard inclusion. When the hard inclusion reaches a certain size, conical fractures form on both sides of the inclusion. To pass inclusion-flaw inspection and close hole defects to the extent possible, the flat-anvil upsetting is recommended. Finally, the inclusion-deformation state obtained by finite element simulation is verified experimentally.

Material properties for process simulation

This paper reviews the recent developments in material property modelling and its applications in processing simulation. Many material properties needed by process simulation can now be readily provided, such as the solidification properties and high temperature stress–strain curves. The solidification properties are affected by changes in composition within the specification range of an alloy; such changes in properties then affect casting simulation results. The mechanical properties are calculated by considering two competing deformation mechanisms (dominated by either dislocation glide or dislocation climb), with automatic selection of the dominant mechanism. Sample calculations are given for a variety of engineering alloys, including steels, aluminum, titanium, and nickel-based superalloys. The material properties data calculated can now be passed directly into commercial computer-aided engineering packages for casting and deformation simulation.

Microsample Testing of Single Crystalline Ti-52 at%Al and Ti-55.5 at%Al

ABSTRACTDislocation activity in two-phase commercial TiAl alloys occurs most readily in the γ-TiAl phase, and measurements of the CRSS of single crystalline γ-TiAl provides a solid foundation for understanding the mechanical performance of these alloys. Single crystals of γ-TiAl with greater than 54.5 at%Al have been grown using the optical float zone crystal growing technique, but single crystals with lower Al content, closer to that of commercial alloys, have not. In the present study, polycrystalline ingots of Ti-52 at%Al have been heat treated to form very large grains, and microsample tensile specimens, which have a nominal gage cross-section of 250μm × 300μm, a gage length of 250μm and an overall length of 3 mm, have been machined from within single grains. Microsample high-temperature stress-strain curves for Ti-55.5 at%Al single crystals oriented along the [001] and [010] are presented. Tensile tests results for Ti-52 at%Al tested at 1073K along the [237], [173] and [344] orientations will also be discussed.

The transition from multiple- to single-peak recrystallization during the hot working of austenite

For the three free-machining steels and a low alloy steel tested, the high temperature stress-strain curves obtained in torsion testing in the approximate range 850–1150°C and at a constant strain rate in the approximate range 10 −3−5 s −1 are characteristic of the occurrence of dynamic recrystallization. In both types of steel, there is a change in the shape of the stress-strain curves from a form with a distinct peak followed by regular oscillations to a form with a single peak when a critical value of the Zener-Hollomon parameter (characteristic of the material) is exceeded. The criteria proposed to predict the transition from regular oscillations in flow stress to a single peak have been shown to be equivalent for a constant ratio (adjusted to transition experimental data) of the critical strain for the onset of recrystallization to the strain to the first peak in flow stress. It is now shown that a common strain ratio for the transition of both types of material (independent of the initial grain size and hot-working conditions) is derived from an available dynamic recrystallization model and that this strain ratio again yields the equivalence of the “critical strain condition” and the “relative grain size model”.

An interpretation of high temperature stress-strain curves of polycrystalline metals in low strain range

Etude de la partie lineaire des courbes contrainte deformation a haute temperature des alliages d'aluminium Al-Cu 4%, Al-Mg 5%, Al-Si 11,5% et de l'aluminium pur. Difference entre cette partie de la courbe et le module d'elasticite determine par des essais dynamiques

Viscoelastic properties of thermoplastics at elevated temperatures

A method of measurement and analysis, based on the parallel plate plastometer, has been developed for the determination of the viscoelastic properties of plastic materials at elevated temperatures. It is shown theoretically and verified experimentally that, for linear viscoelastic materials, measurements from constant load and constant plate separation experiments may be expressed, using the appropriate normalization factors, as deformation-time and stress-time curves respectively. The necessary criteria for interpreting such measurements in terms of a mechanical model are discussed. The method has been applied to vinyl chloride-acetate resin, polyethylene resin compounds, and a cellulose ester compound. The data show that these compounds are linear viscoelastic materials, that is, the deformation (or any of its components) is proportional to the applied stress at a fixed value of the time. Data are presented which show that at elevated temperatures the viscoelastic properties of these materials are representable by a simple mechanical model consisting of a pure elastic spring, a retarded Voigt element, and a pure viscous dashpot coupled in series. The elastic, delayed elastic, and viscous components of the deformation are, therefore, quantitatively separable and are completely characterized by four constants, two elastic moduli and two viscosity coefficients. High temperature stress-strain curves at various rates of loading, and recovery behavior after periods of continuous deformation, are predicted theoretically and verified experimentally. The variation of the viscoelastic constants with temperature and composition has been evaluated. The principal viscosity (viscosity of pure dashpot) is the only viscoelastic constant which obeys a linear log η 3 vs. 1 T relation over wide ranges of temperature, thus permitting the calculation of activation energies for viscous flow. It is shown that the disappearance of elastic effects is associated with a sharp decrease of the retardation time (time constant of the delayed elastic deformation). A study of polyethylenes of various molecular weights shows that the effect of increasing molecular weight is to increase the resistance to both the elastic and the viscous types of deformation and to increase the temperature at which elastic effects become negligible. The principal viscosity varies with the molecular weight according to Flory's relation (linear log η 3 vs. M 1 2 ). The variation of the other viscoelastic constants with the molecular weight is more complex.