Axisymmetric Compression Tests Research Articles

Enhancing Axisymmetric Compression Testing: Modeling and Addressing Flow Curve Errors

Axisymmetric compression testing (ACT) stands as a pivotal experimental technique, widely embraced for its capacity to identify the plastic flow attributes inherent to diverse materials. However, the accuracy of the flow curve obtained from ACT can be significantly affected by errors introduced by the inevitable barrelling and foldover, which can lead to erroneous interpretation of deformation phenomena. The existing analytical and numerical solutions for estimating effective stress in ACT are first reviewed. A new closed‐form solution that is insensitive to friction and foldover is then presented. The solution is based on a multilayer model of the sample geometry, which considers the instantaneous barrelling of the sample. The multilayer solution is validated against a reference numerical solution and shows good agreement, even after a significant foldover. The results indicate that the proposed framework can significantly improve the accuracy of material flow characterization. This can be of significant value in the fields of material processing, manufacturing, and materials science.

Performance evaluation of the Generalized Bounding Surface Model in the simulation of Cajicá clay subjected to monotonic loading

Different constitutive models, based on principles of mechanics and experimental evidence, have been developed over several decades to represent and predict the stress-strain behavior of soils subjected to various loading conditions. The Generalized Bounding Surface Model (GBSM) is one of them and is defined as a fully three-dimensional elastoplastic constitutive model for saturated cohesive soils that employ the bounding surface plasticity in conjunction with a nonassociative flow rule and a soil microfabric-inspired rotational hardening rule. The GBSM has been successfully validated on numerous laboratory reconstituted soils, but not on natural or undisturbed soil samples. Hence, in this paper the predictive capabilities of the GBSM are evaluated in the simulation of the monotonic behavior of an undisturbed cohesive soil called “Cajicá clay” from the high plain of Bogotá in Colombia. In the first instance, an isotropic consolidation test and a set of axisymmetric triaxial compression and extension tests are conducted using an automated triaxial equipment, to experimentally describe the response of the undisturbed soil. From experimental data, the parameters associated with the GBSM model are calibrated to finally evaluate its capabilities in the simulation of a cohesive natural soil. A comparison between experimental data and numerical simulations is presented to show both performance and advantages of the GBSM model.

Influence of X-Ray beam exposure on the development of gas bubbles during triaxial testing of sand using 3D synchrotron micro-computed tomography

The constitutive behavior of granular materials is highly influenced by the degree of saturation and whether a saturated or unsaturated framework is adopted to model the behavior of granular materials. Conventional axisymmetric triaxial compression (CTC) testing of saturated specimens tends to assume that the specimen remains saturated during shearing. X-ray computed tomography (CT) has allowed for 3D in-situ measurements beyond the global scale to investigate microscale and localized events such as fabric evolution. The literature reported several CT-coupled geotechnical experiments of saturated specimens; however, no study examined how X-ray exposure might affect the specimen, specifically the radiolysis of pore water. In this study, we present the synchrotron micro-computed tomography (SMT) results of experiments performed on sand specimens without shearing (acrylic tubes) and with shearing (CTC experiments) to assess the influence of X-ray exposure on the development of the gas phase of saturated sand. The observed phase changes were dependent on the initial pore water pressure and duration of exposure to the X-ray; the behavior of individual gas bubbles was dependent on the bubble’s surrounding sand grains and pore throat sizes leading to changes in the degree of saturation.

Prediction of characteristic points of single-peak flow curve through statistical technique and artificial neural network

ABSTRACT Artificial neural network (ANN) is an input–output modeling technique that has been successfully implemented in the field of material science to predict material behavior in terms of constitutive models. The material behavior considered here is the stress–strain behavior of the material at elevated temperatures that is characterized by a single-peak flow curve consisting of characteristic points viz. critical stress, peak stress and steady-state stress. The present work focuses on predicting these stress values along with the corresponding strain values through statistical regression analysis (SRA), ANN, and multi-layer complex neural network (MLCNN) with reasonable accuracy. The flow curves obtained from the axisymmetric compression test of 304LN austenitic stainless steel, performed at constant temperatures and constant strain rates, are used to train the models. The MLCNN has performed best while SRA has performed relatively worst with the current dataset due to rigorous and thorough computation. Both MLCNN and ANN are observed to perform better than SRA because of their non-parametric nature of handling data.

ACTAS: A New Framework for Mechanical and Frictional Characterization in Axisymmetric Compression Test

There are two common methods to interpret the results of an Axisymmetric Compression Test (ACT): the Cylindrical Profile Model (CPM) and the Avitzur model; however, both of the two and all other models available in the literature ignore the unavoidable foldover phenomenon, which breaks the models to provide reliable friction-free flow stress curves. Here, a novel numerical framework (called ACTAS) is presented that incorporates the foldover. ACTAS can be used to both simulate and analyze ACT. Ten finite element models are used to benchmark ACTAS. The results show the reliability of the proposed method in estimating the average and pointwise stress-strain curves and friction factors. Moreover, a new solution is provided by coupling the CPM and the Avitzur model (called A-CPM), to obtain reliable average flow curves even after the onset of foldover.

A modified Johnson-Cook constitutive model for improved thermal softening prediction of machining simulations in C45 steel

A new modified Johnson-Cook model is developed to capture the non-linear thermal softening behaviour below the austenite transformation temperature (723°C) in C45 ferritic-pearlitic steel, which is not captured by the original Johnson-Cook (JC) model. The physical mechanisms responsible for this behaviour are also studied. The model is developed by experimental quasi-static and high strain rate axisymmetric compression testing from 20°C to 720°C and implemented into 3D finite element drilling simulations. The improvement over the original JC model is demonstrated by comparison to experimentally measured torque and thrust force.

Hot compression behaviour and microstructural evolution in aluminium based composites: an assessment of the role of reinforcements and deformation parameters

The response of two different types of aluminium matrix composites (AMCs) reinforced with silicon carbide ceramic particulates or nickel metallic particulates to hot compression testing parameters was evaluated. The composites were produced via two-step stir-casting technique. Axisymmetric compression testing was performed on the samples at different deformation temperatures of 220 and 370 °Ϲ, 0.5 and 5 s−1 strain rates and total strains of 0.6 and 1.2. The initial and post-deformed microstructures were studied using optical and scanning electron microscopy. The results show that flow stress was significantly influenced by imposed deformation parameters and the type of reinforcements used in the AMCs. Nickel particulate reinforced aluminium matrix composite (AMC) showed superior resistance to deformation in comparison with silicon carbide reinforced AMC under the different testing conditions. In both AMCs, work hardening, dynamic recovery and dynamic recrystallisation influenced their response to imposed parameters. The signature of dynamic recrystallisation was very apparent in aluminium matrix composite reinforced with nickel particulates.

Texture development in 304LN austenitic stainless steel during post-hot-axisymmetric compression

In the present work, 304LN austenitic stainless steel has been considered for the texture evolution study after the hot-deformation process. The axi-symmetric compression tests, with post-deformation isothermal holding at the same temperature, were performed at 900°C, 1000°C and 1100°C with a strain rate of 0.1 s−1. Texture evolution during the post-hot-deformation was studied through electron back scattered diffraction. Effect of temperature and holding time on texture evolution were studied. At low deformation temperature and lesser holding time 〈100〉 and 〈110〉 fibre textures were observed. At high deformation temperature and/or high holding time the texture becomes random. Texture intensity along α fibre, for all the temperatures, is chaotic whereas along β and τ fibres it is uniform at low temperature and becomes chaotic with the increase in deformation temperature. Goss component was found to be a major texture component with significant amount of ND and RD rotated cube components.

Simulation of aggregate behavior in low confinement geotechnical applications

Abstract Isotropic compression and drained axisymmetric triaxial compression tests were performed on six structural backfill aggregates commonly used in the construction of geosynthetic-reinforced soil (GRS) systems to characterize their shear behavior. The behavior of these aggregates was then simulated using the Single Hardening Model (SHM) for frictional materials. The confining pressures that were tested and analyzed in the current study (e.g., 34 to 413 kPa) are generally consistent with those encountered with various GRS applications. The pressures at the bottom end of this range are relatively low for typical geotechnical engineering testing and simulation approaches. Certain modifications to the SHM were required in order to more accurately simulate the volume change response that occurs under drained loading conditions at these lower confining pressures. The model simulation results showed good agreement with the results from the triaxial compression tests. These findings are important, as accurately simulating the volumetric response during shear is very important for GRS systems because, although initially compacted, such systems are subjected to relatively low confining pressures during service conditions.

On the prediction of hot deformation mechanisms and workability in Al6063/Nip and Al6063/steelp composites using hyperbolic-sine constitutive equation

The present study investigates the hot deformation mechanisms and workability in novel AA6063-Nip and AA6063-Steelp composites using hyperbolic-sine constitutive equation. The AA6063-Nip and AA6063-Steelp composites were produced with AA 6063 as matrix, and 6 wt% Ni or 6 wt% steel particles as reinforcement for each of the composite, with the adoption of double stir casting method for the composite production. Axisymmetric compression testing was performed using Gleeble 3500 thermomechanical simulator at temperature and strain rate of 200–400 °C and 0.01–10 s−1 respectively to a global strain of 0.5, while constitutive model was used to study the hot working characteristics of AA6063-Nip and AA6063-Steelp composites. The results showed that anomalous flow stress oscillations and partial insensitivity to strain rate, characterised the flow stress patterns displayed by both composites. The activation energies (QHW) values derived at incremental strain, were within the range of 83.8 to 218.4 kJ/mol for AA 6063/Nip and 14.2 to 271.9 kJ/mol for AA 6063/Steelp composites. The highest values of the QHW, for AA 6063/Nip was 53% and for AA 6063/Steelp was 91% higher than that for self-diffusion of Aluminium (QSD ∼142 kJ/mol), which suggest that the deformation controlling mechanism was work hardening facilitated by dispersion strengthening. This was supported by the stress exponent values which exceeded 5 for both composites – affirming dispersion strengthening to dominate the deformation process. Also, the QHW of the composites was intermediate in comparison with the range of 111 – 509 kJ/mol reported for AMCs of similar reinforcement composition in literature, indicating that both composites have intermediate workability.

Effect of initial grain size on the mechanical behaviour of austenite during deformation under hot-working conditions

The present investigation has been conducted to study the effect of the austenitic grain size (AGS) on the flow stress behaviour of austenite, deformed in a wide range of temperatures and strain rates, employing samples of a commercial ER7 steel grade. To accomplish this objective, a novel general methodology for the analysis of the austenitic grain growth kinetics under combined arbitrary isothermal and non-isothermal austenitizing treatments has been developed. In this way, it has been possible to characterize the AGS effect when this parameter varies between approximately 135 to 260 μm. For this purpose, axisymmetric compression tests have been conducted in the temperature range of 850 °C–1250 °C, at strain rates in the range of 0.005–10 s−1. The experimental results show that the constant associated with the strength of grain boundaries in the Hall-Petch relationship, is approximately 57 MPa μm1/2. Accordingly, the athermal stress values are in the range of 3–5 MPa. Also, the exponent which characterizes the effect of the AGS on the time required to achieve 50% DRX, was found to be approximately 0.1, which also reflects a rather weak effect of this parameter on the evolution of the volume fraction recrystallized dynamically. The relevance of analyzing the DRX kinetics of the alloy based on the recrystallization time rather than on the applied strain is also discussed. The prediction of the change in the volume fraction recrystallized dynamically during deformation under variable strain rate conditions is analyzed and the verification of the conditions which promote DRX is highlighted. A set of sequential equations encompassing both a physically-based constitutive description, as well as a simple parametric relationship for the computation of the flow stress of the alloy, are proposed.

Numerical Verification of the Schroeder–Webster Surface Types and Friction Compensation Models for a Metallic Specimen in Axisymmetric Compression Test

Three types of surfaces in the Schroeder–Webster (SW) theory, i.e., sliding, mixed, and sticking surfaces, have been verified via finite element analysis of an axisymmetric compression test for a metallic specimen. Judging from (i) the radial profile of the pressure at the top elements and (ii) the radial displacement at the top nodes, the three types of SW surfaces are not manifested in the numerical simulation. However, the SW friction compensation model developed for the SW-sliding surface is remarkably reliable in predicting the measured stress–strain curve of the barreled specimen down to the height-to-diameter ratio of 0.1. The origin of this reliability is discussed along with recommendations for using the SW friction compensation model for the SW-sliding surface.

Hardness prediction of a cold rolled Nimonic 80A exhaust valve spindle

Purpose: of this paper is to predict the hardness of cold rolled exhausts valve spindle fabricated of Nimonic 80A via axisymmetric finite element analysis, compression testing, and hardness inspection. Design/methodology/approach: The stress-strain relationship of Nimonic 80A was obtained via compression testing with deformation ratios of 10%, 20%, and 30%. Hardness changes caused by the strain hardening effect were measured in cut specimens in both the axial and circumferential directions following compression testing. The effective strain at the measurement position was calculated via finite element analysis. The regression equation for hardness changes caused by work hardening was derived from analysed strain and inspected hardness. The cold-rolling deformation of an exhaust valve spindle was analysed using axisymmetric finite element analysis. Findings: The stress-strain relationship calculated from compression testing was well expressed using the Holloman equation and the strain-hardness relationship by strain hardening was successfully regressed using the shifted power law model for Nimonic 80A, Nickel-Chromium based super alloy. Research limitations/implications: This research focused hardness prediction of spindle after ring rolling operation for generating beneficial compressive surface residual stresses for enhancing fatigue life. Further research to quantify compressive residual stress after rolling shall be followed to increase fatigue life. Practical implications: The cold rolling process is a typical incremental forming method and should be analysed under three-dimensional conditions. However, it takes lots of time to solve incremental forming analysis. To predict hardness distribution after rolling in the manufacturing field, FE analysis was performed under two-dimensional axisymmetric conditions based on the assumption of no friction generated by the rolling tool. The deformed shapes and hardness distribution from the inspection quality standard and two-dimensional FE analysis showed very similar results. Simplified finite element analysis method for ring rolling process for local area could be very effective method in the industrial field. Originality/value: The stress-strain relationship and the hardness and strain relationship were derived by compression test and hardness measurement for compressed specimen for Nimonic 80A, Nickel-Chromium based super alloy. And simplified finite element analysis method was suggested to predict deformed shape and hardness distribution of locally cold rolled region and achieved similar result between FE analysis result and Quality standard. Suggested method would be very effective method to engine spindle manufacture to predict hardness of different size of product.

Friction Correction of Austenite Flow Stress Curves Determined under Axisymmetric Compression Conditions

The experimental flow stress curves of structural steels obtained from axisymmetric compression tests conducted under hot-working conditions very often include the frictional effects present at the tool/specimen interface. Such effects have a significant influence on the flow stress and therefore, should be corrected prior to any quantitative analysis aimed at determining the constitutive description of these materials. Commonly, such a correction is carried out by assuming a constant friction coefficient (μ) or friction factor (m) independent of deformation conditions, which is an unrealistic approach. The present investigation analyzes experimentally the frictional effects that occur when steel is deformed under axisymmetric compression conditions in the temperature range of 850 to 1200 °C at a strain rate of 0.1 s−1 and applied effective strains of 1, employing cylindrical samples with an initial diameter to initial height ratio (d0/h0) in the range of 0.5 to 2. Finite element modeling (FEM), as well as element-free Galerkin modeling (EFGM), have been employed for the analysis and prediction of the von Mises stress distribution, barreling and amount of metal folding undergone by the compression specimens. It has been shown that the increase in flow stress due to frictional effects can be corrected on the basis of either μ or m, by assuming that these parameters vary in the course of plastic deformation and are strongly dependent on deformation temperature. A novel procedure for the systematic correction of the flow stress curves, taking into consideration the changes in friction conditions during plastic deformation, has been proposed.

Hot Deformation Behavior and Dynamic Recrystallization Characteristics of 12Cr Ultra-Super-Critical Rotor Steel

In this paper, the constitutive model and dynamic recrystallization characteristics of 12Cr ultra-super-critical rotor steel were investigated quantitatively during hot deformation. A series of axisymmetric hot compression tests at temperatures from 900 to 1200 °C under strain rates of 0.001–1 s−1 were conducted on a Gleeble−1500D thermal simulator. Based on the experimental true stress–strain curves varying with temperature and strain rates, a complete constitutive model was established and all material parameters in the model could be expressed as a function of strain using a fifth order polynomial fit. The proposed model was verified so as to have the capability of accurately predicting the flow behaviour with an average absolute relative error of < 2.82%. Meanwhile, after hot deformation the microstructure was observed via electron backscatter diffraction technology. Then, the dependence of the characteristic parameters on the Zener–Hollomon parameter were confirmed. Furthermore, the kinetics equation of dynamic recrystallization was obtained, which included the flow stress calculated based on the evolution equation of the dislocation density during the work hardening-dynamic recovery stage. The result indicated that the predicted values for dynamic recrystallization volume fraction and flow stress were in line with the experimental values.

Initiation and Evolution of a Network of Deformation Bands in a Rock Analogue Material at Brittle–Ductile Transition

We investigate the deformation localization at brittle–ductile transition in axisymmetric compression tests of rock analogue material GRAM1 made of bonded rigid particles. This transition in rocks (as well as in GRAM1) is recognized by the formation of a network of conjugate deformation localization bands in the postmortem rock samples and by a shallow stress reduction followed by a stress plateau in stress–strain curves. GRAM1 is much weaker than hard rocks. Therefore, brittle–ductile transition occurs in GRAM1 at a low confining pressure, 0.3 MPa, which is much smaller than that for real rocks. This allows using a transparent pressure cell and applying the digital image correlation technique to visualize the deformation evolution. Taking advantage of this technique and of the detailed characterization of GRAM1’s constitutive properties in the previous studies, we show that the initiation of deformation localization bands occurs in the dilatant (positive dilatancy factor $$\beta$$ ) and strain-softening (negative hardening modulus $$h$$ ) deformation regime. During the band evolution, the deformation within it becomes compactive ( $$\beta$$ < 0) and is accompanied by the material hardening ( $$h$$ > 0), which causes the band to widen and new bands to form successively resulting in a progressively densified band network. The formation of a conjugate band network at brittle–ductile transition is thus due to the transition from dilatancy to compaction and from softening to hardening during the inelastic straining.

A Correction to the Stress–Strain Curve During Multistage Hot Deformation of 7150 Aluminum Alloy Using Instantaneous Friction Factors

Multistage stress–strain curve correction based on an instantaneous friction factor was studied for axisymmetric uniaxial hot compression of 7150 aluminum alloy. Experimental friction factors were calculated based on continuous isothermal axisymmetric uniaxial compression tests at various deformation parameters. Then, an instantaneous friction factor equation was fitted by mathematic analysis. After verification by comparing single-pass flow stress correction with traditional average friction factor correction, the instantaneous friction factor equation was applied to correct multistage stress–strain curves. The corrected results were reasonable and validated by multistage relative softening calculations. This research provides a broad potential for implementing axisymmetric uniaxial compression in multistage physical simulations and friction optimization in finite element analysis.

Plausible extension of Anand's model to metals exhibiting dynamic recrystallization and its experimental validation

Modeling and optimization of industrial hot-working operations by means of commercial finite element computer codes require robust constitutive descriptions, which could be easily implemented. Also, it is expected that such formulations are able to describe as accurate as possible the flow stress, work-hardening and work-softening rate of the material, as a function of microstructure and deformation conditions. The present communication describes the development of a novel constitutive description of metals which exhibit dynamic recrystallization during deformation under hot-working conditions. The formulation is founded on the extension of the model earlier developed by Anand and co-workers for the constitutive description of metals which exhibit dynamic recovery as the only dynamic restoration mechanism during hot deformation. The advanced constitutive formulation has been validated by conducting axisymmetric compression tests with samples of a R260 steel employed for the manufacture of rail tracks. The specimens were deformed in a wide range of temperatures and strain rates, both under constant and variable strain rate conditions. The latter involve either an increase or a decrease in the Zener-Hollomon parameter as plastic deformation is applied. The results obtained indicate that the proposed constitutive description is able to reproduce quite accurately the experimental flow stress data. It has also been shown that the correct computation of the recrystallization time and volume fraction recrystallized dynamically, by means of an appropriate differential or incremental formulation of the Avrami equation, constitutes an essential aspect of the model. Such an approach is particularly important for predicting correctly the changes in flow stress under variable strain rate conditions.

Modeling the viscoplastic flow behavior of a 20MnCr5 steel grade deformed under hot-working conditions, employing a meshless technique

The present work has been conducted in order to develop a novel approach to predict the inhomogeneous flow of a 20MnCr5 steel during an axisymmetric hot compression test, by using the element-free Galerkin (EFG) method under a differential constitutive description. The governing equations have been solved on the basis of the EFG global weak formulation. A detailed explanation concerning the characteristics inherent to the application of the EFG method to this problem, has also been provided. Furthermore, a return mapping algorithm for the solution of associative von Mises inelastic problems, has been formulated for the differential constitutive description employed in this communication, namely, a differential return mapping algorithm (DRMA). The feasibility and suitability of the EFG method for solving the axial compression problem has been shown by comparing its results with a FEM based solution employing a simple constitutive description. The reliability of the proposed DRMA has been demonstrated by a comparison of a homogeneous deformation numerical test with the experimental and direct integration results reported in a previous communication. Finally, the EFG model has been used to predict the stress, strain and volume fraction recrystallized distributions under steady and transient nominal strain rate and temperature deformation conditions. These parametric studies have been carried out by considering the differential constitutive description, but also a conventional integrated constitutive model of a 20MnCr5 steel. The results have revealed the suitability of the EFG formulation under the proposed DRMA, for predicting the performance of an axisymmetric hot compression test. The differences between the use of differential and integrated constitutive descriptions on the performance of hot-working processes under inhomogeneous deformation conditions, have also been evidenced in this research work.

Application of a flexible membrane to DEM modelling of axisymmetric triaxial compression tests on sands

An innovative flexible membrane algorithm was developed and applied to the 3-D DEM modelling of axisymmetric triaxial compression tests on sands. This proposed flexible membrane was made of simply packed single spheres. The vertical and side extensions of the membrane were bounded by a series of boundary walls. These measures equipped the membrane with enough flexibility to contain local deformations of the cylindrical specimen which was comprised of un-bonded particles. With the application of the flexible membrane, the specimens failed with bulging shapes which were typical of axisymmetric triaxial tests. A comprehensive parametric sensitivity study indicated that the normal stiffness and particle size of the membrane spheres had limited influence on the test results, while the shear stiffness of the membrane spheres substantially enhanced the residual strength of the specimen. To examine the effectiveness of the flexible membrane algorithm, the numerical specimen was characterised on the macroscopic, mesoscopic and microscopic scales. The stress and strain response of the specimen well matched the laboratory test, and the volumetric strain presented apparent contraction and expansion stage. More importantly, the characterisation on the local void ratio clearly indicated the flexible membrane enabled full development of strain localisation inside the specimen.