Conventional Uniaxial Tensile Tests Research Articles

Effects of pre-deformation and annealing on the mechanical properties and microstructure of miniaturized FeCrAl-Y-Si alloys

The evolution of mechanical properties and microstructure in FeCrAl alloys with added Y and Si was investigated using miniaturized specimens. The FeCrAl alloys underwent warm rolling, resulting in thicknesses of 1 mm and 2 mm, followed by annealing at temperatures ranging from 600 °C to 1050 °C. Mechanical properties were determined through three distinct methods: conventional uniaxial tensile testing (UTT), micron indentation hardness testing (MIHT), and small punch testing (SPT). Microstructure evolutions were characterized using various techniques, primarily focusing on electron backscatter diffraction (EBSD). Two types of precipitates containing Y, Si, and Al were observed during annealing. Fracture morphologies revealed that as the annealing temperature (AT) exceeded 900 °C, size effect and brittle failures became prominent in SPT specimens. Brittle failures in SPT can be attributed to the segregation of Y, Si, and Al along the grain boundaries, the size effect, and the dispersed distribution of grain sizes. The optimum inter-pass annealing temperature was identified as 700 °C based on grain orientation spread (GOS) maps. SPT and MIHT results were correlated with UTT results when conducted at room temperature (RT, 25 ℃), and a Hall-Petch equation was derived. It was observed that the correlation equations for SPT were significantly influenced by brittle failures, resulting in relatively larger fitting errors. However, these SPT correlation equations proved effective in predicting total elongation compared to MIHT correlation equations and the Hall-Petch equation. The Hall-Petch equation exhibited the lowest average fitting error though limited by deformed grains. MIHT exhibited the strongest correlation with UTT, particularly in terms of yield strength and ultimate tensile strength. The three fitting methods have distinct advantages and shortcomings in characterizing the mechanical properties of Y and Si-added FeCrAl alloys.

Mechanical properties of structural metallic alloys for nuclear applications deduced by small punch test

Small Punch Test (SPT) techniques are becoming a popular tool for the characterization of the irradiation damage effects in nuclear materials thanks to a number of advantages as compared to conventional test methods. As of today, there are two standards available (ASTM and EN) and several methods are developed to extract the tensile properties from the SPT force-displacement relationship. Here, we carry a set of benchmark tests using SPT and conventional uniaxial tensile tests on a number of metallic materials commonly applied in nuclear facilities. The investigation includes reactor pressure vessel steel, high-Cr ferritic martensitic steel, copper-chromium-zirconium alloy, nickel based super alloy, austenitic stainless steel and titanium. The SPT sample geometry and setup is designed according to the requirements of the EN10371 norm. As a result, we have deduced the tensile properties using both techniques and thus have revealed the accuracy/reliability of the currently available standards for the determination of the yield stress and ultimate tensile strength by the SPT.The study has revealed that in an overwhelming number of tests, the yield stress extracted from the SPT (for both EN and ASTM standards) is systematically overestimated as compared to the value measured from the uniaxial tensile test. However, the prediction for the ferritic and austenitic steels is close to the uniaxial stress, for both standards. In the case of Inconel material, the yield stress is underestimated. The ultimate tensile strength extracted by either of the existing methods is in a good agreement with the value otherwise measured from the uniaxial tests for all the materials except the titanium. Based on the obtained results and discussion, a number of recommendations for the evaluation of yield and ultimate tensile stress from the small punch test is given.

Region-wise evaluation of strength and creep behavior of AISI 904L weldments by small specimen testing techniques

Super austenitic stainless steel AISI 904L is a premium stainless steel renowned for its outstanding characteristics and the ability to be maintained even under harsh situations. This alloy is widely used in a variety of engineering applications, and hence generating creep and indentation data of the material will prove useful for those concerned. Our study investigates the impression creep (IC) and automated ball indentation (ABI) performance in activated tungsten inert gas (ATIG) and hot wire tungsten inert gas (HWTIG) weldments of 10 mm thick plates of AISI 904L. ABI-evaluated strength correlates well with the conventional uniaxial tensile test and hardness data. IC studies were performed on base metal and welds by applying a load of 60 kg on a 1 mm diameter flat-end cylindrical punch at 650 oC. Several cycles of loading followed by partial unloading were performed on ABI specimens at 25 °C using a 0.76 mm diameter tungsten carbide spherical indenter. The results indicated that the ATIG weld excelled over the HWTIG weld with higher creep resistance. During the ABI analysis, the heat-affected zone of HWTIG weldment displayed higher UTS and ABI hardness values of 499 MPa and 185 HB, respectively. Hardness values generated by the ABI analysis are 174 HB, 161 HB, and 177 HB for the base metal, ATIG weldment, and HWTIG weldment, respectively. This was found to be very much in agreement with conventional hardness test results. The results cemented the ability of IC and ABI analysis to precisely predict the creep and mechanical properties of the base metal and weldments. Data generated through this study will benefit industries and researchers associated with AISI 904L.

A Novel Method to Describe Large-Range Stress-Strain Relations of Elastic-Plastic Materials Based on Energy Equivalence Principle.

Due to the unique structure of tensile sheet specimens with a circular hole (CHS specimen), a novel method is proposed to predict the large-range uniaxial stress-strain relations of elastic-plastic materials analytically. Based on the energy equivalence principle, a load-displacement semi-analytical model of the CHS specimen is proposed. Subsequently, a semi-analytical model of constitutive parameters of elastic-plastic materials is developed by virtue of the load-displacement relation of the CHS specimen, and the prediction of the material's stress-strain relations is obtained. To examine the validity of the models, numerical simulations with a series of materials were performed. The results demonstrated that the dimensionless load-displacement curves and stress-strain relations obtained using the proposed models correspond well with those obtained using finite element analysis. In addition, tensile tests were performed on the CHS specimen for four elastic-plastic materials (T225 titanium alloy, 6061 aluminum alloy, Q345 steel, and 3Cr13 steel), and the validity of the models is also verified by the experimental results. Compared with the conventional uniaxial tensile tests, the stress-strain relation of elastic-plastic material captured by the novel method corresponds to a larger strain, which is of great importance for engineering design and safety assessment.

Hydrogen Embrittlement Behavior of Plastically Pre-Strained and Cathodically Hydrogen-Charged 316H Grade Austenitic Stainless Steel

In this work, the effects of electrochemical hydrogen charging of 316H grade austenitic stainless steel were investigated in order to characterize its hydrogen embrittlement (HE) resistance. The as-received 316H material was in a fully recrystallized (solution-annealed) material condition. The susceptibility to HE of the studied material was evaluated by determination of the embrittlement index from the results of conventional uniaxial tensile tests of nonhydrogenated and hydrogen-charged test specimens. The study was focused on the effects of two selected plastic pre-strain levels of tensile specimens on their resulting HE resistance. The selected pre-strains corresponded to the tensile stress conditions within the “yield stress–ultimate tensile strength” (YS–UTS) range and directly at the UTS point. The obtained embrittlement indices for the presently used pre-straining and hydrogen charging conditions indicated that the HE of the studied material states was small. However, it was revealed that the observed degradation of deformation properties of plastically pre-strained and hydrogen-charged materials was mainly caused by gradual plasticity exhaustion due to tensile straining, which well correlated with the observed effects indicated by electron backscatter diffraction analyses and indentation hardness measurements.

A simple apparatus for tensile testing of rock

Measuring tensile strength of a quasi-brittle material such as rock is a challenging task. Indirect tensile testing methods such as the Brazilian and flexural tests rely on the assumption of linear elastic behavior, which is not valid close to the peak stress. The direct tensile testing method is independent of the constitutive response of the material and should provide more realistic data. Nevertheless, the test set up in direct tensile testing is time consuming and the results can depend on the applied boundary conditions. In this work, a new apparatus is introduced that can measure tensile strength of quasi-brittle materials and provide information regarding the crack opening behavior. The apparatus, which is an aluminum or steel beam with a cut-out and central notch, acts as both the loading fixture and load cell. The specimen is attached to the central part of the apparatus using an epoxy adhesive . Strain gages mounted on the two sides of the apparatus provide the necessary information about the tensile behavior of the specimen. The device is also capable of detecting and quantifying the intensity of asymmetric failure (crack growth) in the specimen, as identified by digital image correlation . The apparatus, its calibration, and some tests results are presented and discussed. • A new apparatus for tensile testing of rock is introduced. • Uniaxial tensile strength and critical crack opening of Charcoal granite are measured. • Comparison is made between the data collected using the new and conventional uniaxial tensile testing devices. • Digital image correlation is used to study the failure process.

Brillouin confocal microscopy to determine biomechanical properties of SULEEI-treated bovine pericardium for application in cardiac surgery.

Heart valves are exposed to a highly dynamic environment and underlie high tensile and shear forces during opening and closing. Therefore, analysis of mechanical performance of novel heart valve bioprostheses materials, like SULEEI-treated bovine pericardium, is essential and usually carried out by uniaxial tensile tests. Nevertheless, major drawbacks are the unidirectional strain, which does not reflect the in vivo condition and the deformation of the sample material. An alternative approach for measurement of biomechanical properties is offered by Brillouin confocal microscopy (BCM), a novel, non-invasive and three-dimensional method based on the interaction of light with acoustic waves. BCM is a powerful tool to determine viscoelastic tissue properties and is, for the first time, applied to characterize novel biological graft materials, such as SULEEI-treated bovine pericardium. Therefore, the method has to be validated as a non-invasive alternative to conventional uniaxial tensile tests. Vibratome sections of SULEEI-treated bovine pericardium (decellularized, riboflavin/UV-cross-linked and low-energy electron irradiated) as well as native and GA-fixed controls (n = 3) were analyzed by BCM. In addition, uniaxial tensile tests were performed on equivalent tissue samples and Young's modulus as well as length of toe region were analyzed from stress-strain diagrams. The structure of the extracellular matrix (ECM), especially collagen and elastin, was investigated by multiphoton microscopy (MPM). SULEEI-treated pericardium exhibited a significantly higher Brillouin shift and hence higher tissue stiffness in comparison to native and GA-fixed controls (native: 5.6±0.2 GHz; GA: 5.5±0.1 GHz; SULEEI: 6.3±0.1 GHz; n = 3, p < 0.0001). Similarly, a significantly higher Young's modulus was detected in SULEEI-treated pericardia in comparison to native tissue (native: 30.0±10.4 MPa; GA: 31.8±10.7 MPa; SULEEI: 42.1±7.0 MPa; n = 3, p = 0.027). Native pericardia showed wavy and non-directional collagen fibers as well as thin, linear elastin fibers generating a loose matrix. The fibers of GA-fixed and SULEEI-treated pericardium were aligned in one direction, whereat the SULEEI-sample exhibited a much denser matrix. BCM is an innovative and non-invasive method to analyze elastic properties of novel pericardial graft materials with special mechanical requirements, like heart valve bioprostheses.

Stepwise modelling method for post necking characterisation of anisotropic sheet metal

Modelling and simulation are important tools during design and development processes. For accurate predictions of, e.g. manufacturing processes or final product performance, reliable material data is needed. Usually, the applied material models are calibrated by utilising direct methods such as conventional uniaxial tensile/compression tests but also inverse methods are occasionally applied. Recently, an effective inverse method, the stepwise modelling method (SMM), was presented. By using SMM, the flow stress from initial yielding, beyond necking to final fracture, can be determined. However, the method is developed for sheet materials having isotropic von Mises hardening. In this paper the SMM is extended for post necking characterisation of anisotropic sheet metals using the Barlat yield 2000 criterion. The novel method was applied to analyse the post necking plasticity of the widely used aluminium alloy AA6016 in T4 condition and the aluminium alloy AA5754 in H111 condition. The latter alloy has reported to show serrated yielding, also known as the Portevin–Le Chatelier effect. The obtained flow stress curves agree well with the curves form conventional uniaxial tensile tests up to the point of necking and show credible post necking predictions to final fracture. Furthermore, SMM showed that it could handle the effect of serrated yielding for AA5754-H111. Hence, the novel approach can be used to characterise the post necking hardening of a variety of anisotropic sheet metals and thereby contributes to efficient and reliable material model calibration.

Experimental prediction of lead failure under tensile load using scanning contact potentiometry technique

The issue of accurate identifying in advance structural nonhomogeneities of metals is one of the main factors in improving the manufacturing efficiency and safety during service. This paper illustrates the conventional uni-axial tensile test that is carried out on the diffuse and localized necking stages of dog-bone shape lead specimen. The scanning contact potentiometry method was used to locate the rupture zone. Infrared radiation was used to heat the specimen up to 40°C, in order to excite structural nonhomogeneities in the bulk of the tested specimen. It was found that the infrared radiation increases the predictable nonhomogeneity zone. A ductile rupture occurs in the same nonhomogeneity zone that was previously exactly identified using the scanning contact potentiometry method. The formed diffused necking was observed to initiate from the right where the rupture spread diagonally, at an angle of 70° between the tensile axis and the rupture plane. However, the angle between the tensile axis and the hexagonal singular reflex plane was 69°, which was slightly different from the rupture angle. Both results of the rupture angles of the tensile test and the SCP are relatively matching and diverse from the theoretical calculations. The results shown in this work highlight the importance and the efficiency of the experimental SCP method in prediction of the material failure behaviour.

Theoretical model and testing method for ball indentation based on the proportional superposition of energy in pure elasticity and pure plasticity

For a homogeneous, continuous, and isotropic material whose constitutive relationships meets with the Ramberg-Osgood law (R-O law), the energy in the elastoplastic indentation with a ball indenter was theoretically analyzed, and the proportional superposition of energy in pure elasticity and pure plasticity during indentation was considered based on the equivalence of energy density. Subsequently, a Proportional Superposition-based Elasto Plastic Model (PS-EPM) was developed to describe the relationships between the displacement and the load during the ball indentation. Furthermore, a new test method of Ball Indentation based on Elastoplastic Proportional Superposition (BI-EPS) was developed to obtain the constitutive relationships of R-O law materials. The load–displacement curves predicted using the PS-EPM model were found to agree closely with the Finite Element Analysis (FEA) results. Moreover, the stress vs. strain curves predicted using the BI-EPS method were in better agreement with those obtained by FEA. Additionally, ball indentation was performed on eleven types of metal materials including five types of aluminum alloys and six types of steel. The test results showed that the stress vs. strain relationships and the tensile strength values predicted using the proposed BI-EPS method agreed well with the results obtained using conventional uniaxial tensile tests.

Toughness improvement of LLDPE/PP blend by GTR incorporation

Introduction: Approximately 80 % of waste rubber comes from end of life tires. Land filling causes serious environmental troubles due to the impermeability and hollowed shape of tires. The global concern about the continue accumulation of discarded tires and the developing ecological awareness demand new ways and regulations to solve this problem, encouraging the scientific community to explore alternative solutions to reuse this waste material. However, the reuse or recycling of tires is not a simple task. Besides the several materials that tires are constructed from, rubbers used in tires are vulcanizated, having a cross-linked structure that makes them elastic, insoluble and infusible thermoset materials that cannot be reprocessed, as is the case of thermoplastic materials. It is therefore of critical importance to find technologically feasible and cost-effective methods for tires recycling. A possible industrial recycling method of tires is their downsizing to obtain a powder called ground tyre rubber (GTR). On the other side, a commonly used practice in industry to improve mechanical behavior and toughness of a polymer is to incorporate rubber particles into it (1). Then, adding GTR to a polymeric material, so high-quality products can be shaped at an acceptable cost, seems to be an attractive option. Compound of waste tire rubber with polymers allows to lower the cost of the final products while contributing to the world’s 3R (reduce, reuse, and recycle) notion: reducing the amount of virgin polymer used, reusing the tire rubber, and recycling the waste tires (2). In this work, we report results of adding GTR to a LLDPE/PP blend (the most abundant thermoplastics in municipal waste), with the aim of obtaining a completely recycled composite with improved toughness. Methods: GTR particles in different size ranges – lower than 300 µm, 300-500 µm and 500-800 µm – and in various percentages – 5, 10 and 15% – were incorporated to a 75% LLDPE / 25% PP blend in a twin screw extruder, and plaques obtained by injection molding. Materials were characterized by several techniques such as Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), conventional uniaxial tensile testing, and fracture tests. Results & Discussions: We found that GTR particles did not affect the microstructure of the thermoplastic matrix. The tensile behavior of the ternary materials was slightly affected by the composition of the mixture: the mechanical behavior became less rigid and more ductile; this being directly related to the elastic nature of the rubber particles. However, large differences were found in the fracture performance of the ternary blends, when compared with the matrix blend. In addition, different mechanical behaviors were observed that varied according to the relative percentage of the particles and their size. GTR greatly influenced both energies involved in crack initiation and in crack propagation, as well as mechanisms developed during deformation and crack evolution. Conclusions: Fracture tests seem to be a sensitive and adequate tool to estimate the influence of particles in polymer blends, much more than simple tensile tests. The fracture behavior of LLDPE/PP blends can be modified by the sole incorporation of GTR particles, leading to a toughness increase in the ternary blends, because of the diversification of deformation mechanisms. Keywords: Fracture toughness, fracture mechanisms, recycled tires, thermoplastic blend References Karger-Kocsis, J., L. Meszaros, and T. Barany. Ground Tyre Rubber (GTR) in Thermoplastics, Thermosets, and Rubbers. Journal of Materials Science. 2013; 48(1): 1-38. Ramarad, S., M. Khalid, C. T. Ratnam, A. Luqman Chuah, and W. Rashmi. Waste Tire Rubber in Polymer Blends: A Review on the Evolution, Properties and Future. 2015. Progress in Materials Science 72:100–140.

Construction of True Stress-Strain Curve of Metallic Material by Artificial Neural Network and Small Punch Test

Small punch test (SPT) is used to evaluate mechanical properties of metallic materials by a miniature specimen. A method combining SPT and artificial backpropagation neural network for determining the true stress-strain curve of metallic materials is proposed. The load-displacement curves of different hypothetical materials were obtained by the finite element model of SPT with considering Gurson-Tvergaard-Needleman (GTN) damage parameters and used to train a backpropagation neural network. The relationship between the load-displacement curve of SPT and the true stress-strain curve of the conventional uniaxial tensile test was established based on the trained neural network, which is validated by the experimental results of X80 pipeline steel. The results demonstrate that the established relationship can be used to predict the true stress-strain curve of the metallic materials and then to determine their elastoplastic properties by SPT.

Analytical and finite element modelling of hydraulic bulge test on extra deep drawing steel with fractography study

ABSTRACT In this paper, analytical, finite element modelling (FEM) and experimental studies are conducted on the biaxial stretching of extra deep drawing steel sheet using hydraulic bulge test, where the true plastic strain exceeds 70%, in contrast to the 20–30% strain achieved in the conventional uniaxial tensile test. The analytical modelling includes four different approaches, namely, Hill, Panknin, Chakrabarty and Kruglov. FEM is done using three yield criteria, Hill-48, Barlat-89 and Barlat-2000. Experimental results show that the stress-strain relationship using bulge radius based on Panknin approach and thickness based on Kruglov approach in the analytical study and using Barlat-2000 yield model in FEM study are in close agreement with the experimental curve. Fractography study, conducted on specimens from tensile and bulge test, reveals/confirms the presence of more deep elongated shear dimples in bulge test compared to equiaxed dimples in the tensile test. This corroborates with the experimental results of achieving the high level of plastic deformation till fracture in the hydraulic bulge test.

Application of Pre-Strained Steels in Empirical Correlation Between Small Punch Test and Uniaxial Tensile Test

In order to research the method of strength empirical correlation between conventional small punch test (SPT) and uniaxial tensile test, a series of austenitic stainless steel including pre-strained SUS304 have been tested in this study. The conventional SPT is conducted on a small disc-shaped specimen whose edge is firmly gripped by a die, and the specimen is deformed by a punch. The method of empirical correlation between SPT and uniaxial tensile test is a direct way to obtain the mechanical properties of materials. Through establishing the strength empirical correlation, it can achieve to calculate the strength of material by SPT which is nondestructive to equipments. However, the per-strained steels have never been tested in this method. This study is to fill that gap and to obtain the empirical correlation between SPT and uniaxial tensile test with pre-strained steel. In this study, a series of austenitic stainless steel including SUS304 after different levels of pre-strain were tested successively by uniaxial tensile test and SPT. It is found that the tensile strength obtained from uniaxial tensile test increases with the increasing levels of pre-strain. However, the maximum load obtained from prestrained SPT specimen does not increase with the increasing levels of pre-strain. It is contradictory to the linear relation between maximum load and tensile strength. According to the analysis of conventional discshaped SPT specimen, the directions of maximum load obtained from SPT and tensile strength from tensile test are not uniform. It results in the non-linearity between the maximum load and the tensile strength with pre-strained steel, and it indicates the pre-strained steel cannot be applied to the conventional disc-shaped SPT specimen. Furthermore, the prestrained steel is a typical kind of anisotropic material. Therefore, extending to anisotropic material, the conventional disc-shaped SPT specimen is not suitable for the method of strength empirical correlation.

Examining Failure Behaviour of Commercially Pure Titanium During Tensile Deformation and Hole Expansion Test

Hole expansion ratio is a material parameter which defines the extent to which sheet metals are formed. Research has shown that, the stress state observed at the hole edge after hole expansion test is similar to those observed during conventional uniaxial tensile test. However, conventional tensile test methods are not efficient in evaluating material edge formability. This work utilised optical non-contact measuring techniques to examine failure behaviour during tensile test and hole expansion test of commercially pure titanium sheet, fabricated with either abrasive water jet cutting or electric discharge machining. The work found that, the deformation mode in conventional tensile testing are governed by localised necking and subsequently diffused necking prior to failure. Deformation mode observed in hole expansion test is characterised by localised necking with no visible occurrence of diffused necking prior to failure. The highest strains are concentrated at the hole edge during hole expansion test due to their sensitivity to the hole preparation method with accompanying multiple localised necking sites resulting in non-uniform deformation. Strains become concentrated in the bulk material microstructure rather than the machined edge during tensile testing resulting in single localised deformation site and a more homogenous deformation.

A methodology for obtaining primary and secondary creep characteristics from indentation experiments, using a recess

A procedure is described for Indentation Creep Plastometery (using a spherical indenter), which is analogous to that developed previously for Indentation Plastometry. As in that case, it is based on iterative numerical simulation of the indentation process, with repeated comparison between an experimental outcome and the corresponding model prediction, systematically varying the values of parameters in a constitutive law until optimal agreement is achieved. The constitutive law used here is the Miller–Norton relationship, which covers both primary and secondary creep regimes (although the transition between them is not well-defined). The experimental outcome is the penetration depth as a function of time, under a constant applied load. An important feature of the procedure is the prior creation of a spherical recess in the sample, having a pre-selected depth and a curvature radius equal to that of the indenter. This allows control over the stress levels created during the indentation creep testing and can be used to ensure that no (time-independent) plastic deformation is stimulated during the test. In the absence of such a recess, this is virtually unavoidable, since the stress levels created during initial contact between a spherical indenter and a flat surface tend to be very high. Such plasticity introduces unwanted complications into creep testing. Confirmation of the viability of the procedure is provided via comparisons between the creep characteristics of pure nickel samples at 750 °C, obtained in this way and via conventional uniaxial tensile testing.

On the Constraint Factor Relating Uniaxial and Indentation Yield Strength of Polycrystalline Materials Using Spherical Microindentation

High-throughput (HT) material property characterization has become increasingly important for the rapid and cost-effective development of new material systems. In recent years, spherical indentation using the Pathak-Kalidindi (P-K) protocol has been shown to be a reliable and HT method for capturing the elastic-plastic properties of several different types of materials. However, because of the higher hydrostatic pressure under the indenter tip and the constraint of the surrounding elastically deforming material, the indentation yield strength is higher than the uniaxial yield strength and they can be related by a scaling factor, referred to as the constraint factor. For fully plastic indentation, the constraint factor was found by Tabor to be ~2.8. However, the constraint factor is not well defined for elastic-plastic indentation and depends on the strain induced. In this work, the application of the P-K indentation protocol is demonstrated on a set of aluminum 7050 samples that have undergone different solution and aging treatments. Conventional uniaxial tensile and compression tests were also conducted on the same material. The constraint factor at the 0.2 % offset yield point was determined from the indentation and uniaxial measurements to be ~1.4. Experimental observations from our work and a similar work on aluminum 6061 showed a dependence of the constraint factor at the 0.2 % offset yield point on the uniaxial plastic properties. To explain these experimental observations, further investigation was carried out using finite element simulations of spherical indentation on materials with different yield strengths and hardening parameters. New insights are provided on the constraint factor at the 0.2 % yield point, which show that it increases with the ratio of the plastic to elastic strain. It was also found that the effect of the hardening is much less than that of the yield strength on the constraint factor at the 0.2 % yield point.

On the inverse identification of Lankford coefficients using geometrical changes under quasi-biaxial loading

Finite element simulation has become an important tool of process and production design in various fields, especially in the automotive industry. The calculation of forming processes in the early concept phase of new cars allows virtual adaptions, which can reduce costs of later phases in the product development significantly. Therefore, the precise characterization and modelling of the material behavior is necessary to ensure a robust and reliable numerical process design. The mechanical properties of numerous materials are highly influenced by the rolling or extrusion direction in the production process. This necessitates the characterization of materials in different loading directions. However, depending on the dimensional aspects of the semi-finished product, the manufacturing of specimens can be challenging or even impossible. Thus, in this investigation, an innovative, indirect approach for the identification of the Lankford coefficient in transversal direction is presented. Based on numerical and experimental data of layer compression tests the Lankford coefficient is determined by inverse modelling of the resulting specimen contour. Due to the characteristics of the layer compression test, it can even be used for semi-finished products with small transversal dimensions like extruded profiles. The presented methodology is on the one hand verified by conventional uniaxial tensile tests for aluminum as well as steel blank material. On the other hand it is used to determine Lankford coefficients for an aluminum extrusion hollow profile and the inversely identified material model is validated by comparison of strain distributions of experimental and numerical square tube bending tests.

A Methodology for Obtaining Primary and Secondary Creep Characteristics from Indentation Experiments, Using a Recess

A procedure is described for Indentation Creep Plastometery (using a spherical indenter), which is analogous to that developed previously for Indentation Plastometry. As in that case, it is based on iterative numerical simulation of the indentation process, with repeated comparison between an experimental outcome and the corresponding model prediction, systematically varying the values of parameters in a constitutive law until optimal agreement is achieved. The constitutive law used here is the Miller-Norton relationship, which covers both primary and secondary creep regimes (although the transition between them is not well-defined). The experimental outcome is the penetration depth as a function of time, under a constant applied load. An important feature of the procedure is the prior creation of a spherical recess in the sample, having a pre-selected depth and a curvature radius equal to that of the indenter. This allows control over the stress levels created during the indentation creep testing and can be used to ensure that no (time-independent) plastic deformation is stimulated during the test. In the absence of such a recess, this is virtually unavoidable, since the stress levels created during initial contact between a spherical indenter and a flat surface tend to be very high. Such plasticity introduces unwanted complications into creep testing. Confirmation of the viability of the procedure is provided via comparisons between the creep characteristics of pure nickel samples at 750˚C, obtained in this way and via conventional uniaxial tensile testing.

Comparison between stress-strain plots obtained from indentation plastometry, based on residual indent profiles, and from uniaxial testing

This paper is focused on comparisons between stress-strain plots from conventional uniaxial (tensile or compressive) testing and those obtained from indentation experiments, via iterative FEM modeling of the process in which the plasticity is represented using a constitutive law. Both Ludwik-Hollomon and Voce equations are used in the current work. Advantages of a spherical indenter shape, and of using the residual indent profile as the main experimental outcome, are highlighted. It is shown via detailed study of two different materials, with low and high work hardening rates, that the methodology (here termed indentation plastometry) can be used to obtain (nominal) tensile stress-strain curves, which incorporate the onset of necking and the ultimate tensile strength. High levels of fidelity are observed between these and corresponding plots obtained by conventional tensile testing. It is noted that, while there is also excellent consistency with the outcomes of uniaxial compression tests, the latter inevitably involve some experimental complications that are best avoided. It is concluded that indentation plastometry has the potential to become a mainstream testing methodology in the near future.