Standard Uniaxial Tensile Tests Research Articles

Design and validation of a new ring hoop plane strain test for characterizing the anisotropic plastic behavior of tubular materials

The accurate characterization of the plastic behavior of tubular materials is challenging due to the difficulties in acquiring proper specimens for mechanical testing. In this study, we propose a novel testing technique, namely the ring hoop plane strain Test (RHPST), as a supplementary test for characterizing materials experiencing distinct strain paths compared to standard uniaxial tensile tests. To design a suitable ring specimen’s geometry that satisfies the necessary plane strain conditions, we developed a meta-modelling approach merging response surface method (RSM) and the finite element method (FEM). Experimental tests are conducted on RHPST specimens, and 3D digital image correlation (DIC) technique is used to monitor the strain fields within the specimen's gauge section. However, observations indicate the presence of edge effects on the gauge specimen. These edge effects significantly influence and limit the homogeneity of the plane strain field. To tackle this limitation, a correction procedure is developed for separating the authentic plane strain area, constituting approximately 78% of the RHPST specimen's width. Furthermore, we successfully fabricated a novel grooved ring specimen (GRHPST), which exhibits a wider region of the plane strain state encompassing up to 90% of the gauge width, while minimizing the impact of edge effects. The true stress-true strain curve obtained from the GRHPST specimen exhibits exceptional agreement with the curve predicted using the Hill48 yield criterion. Notably, no additional corrections are required for the measured true stress-true strain curve, thereby affirming the efficacy of the GRHPST specimen as an adept test for characterizing the anisotropic plastic behavior of AA6063 tubes.

EXPERIMENTAL ASSESSMENT OF THE MECHANICAL PROPERTIES OF STEEL-FIBER COMPOSITE BAR (SFCB) REINFORCEMENT

The Steel-fiber reinforced polymer (FRP) composite bar (SFCB) is a recently proposed bar, it has a steel inner and an exterior layer of FRP, and an adhesive resin to bond the two materials. The SFCB bar used in this paper was made by using a handmade technique; this technique includes a manufacturing procedure that was described in this paper. SFCBs were tested using the standard uniaxial tensile test (ASTM A370) to figure out their mechanical properties and stress-strain relationship and compare them with the normal steel bars. Moreover, the bonding test was conducted to comprehend the bonding performance of SFCB in the concrete and to compare it with the normal steel performance, the bonding test was conducted according to ACI 440.3R-12. The experimental results illustrated that the values of yielding and ultimate tensile strengths of SFCB were slightly less than that of the conventional steel with percentages of 4% for both, the modulus of elasticity values which were found theoretically in good agreement with the test results values, the behavior of SFCB stress-strain curve was slightly different than the conventional steel behavior, with a stable post-yield modulus. Also, the results showed that the SFCB/Steel (SFCB/Steel) ratio between the bond strength of SFCB and the bond strength of steel bar was approximately 0.86, which means that the SFCB had lesser bond strengths than the normal steel with a percentage of 14%, the results also showed that the SFCB had less slip than the steel bar with a percentage of 10%.

A Statistical Analysis on Tensile behaviour of Single Edge Notched Jute Hybrid Composites

In this work, a statistical model was developed using Taguchi technique to study the factors influencing the tensile strength of single edge notched jute hybrid composites. Standard uniaxial tensile tests were conducted to evaluate tensile strength of single edge notched jute hybrid composite specimens, with different fiber orientations, glass volume fractions and notch sizes, in accordance with ASTM D3039 standards. It is observed that as glass volume fraction increases, tensile strength increases. As notch size increases, tensile strength decreases. The tensile strengths of specimens in fiber direction (0°/90°) were higher than those in off fiber direction (±45°). The forecast model indicates that the major parameters that impact the tensile strength were glass volume fraction and fiber orientation. Notch size had lesser impact on tensile strength. The optimum value of the tensile strength is observed for specimen with 0/90 fiber orientation, 45% glass volume fraction, 2mm notch size and from the confirmation test, the model results were observed to be near to the experimental values.

Vitamin D deficiency and anatomical region alters porcine growth plate properties

Ossification of growth plate cartilage mediates longitudinal extension of long bones. Biomechanical and biochemical disruptions of growth plate function may lead to abnormal bone growth. In humans and animals, severe dietary vitamin D deficiency can lead to rickets which features growth plate widening, resulting in abnormalities in growth. However, effects of marginal vitamin D deficiencies on growth plates are not well understood. The purpose of this study was to examine the effects of a vitamin D deficient diet in the 26-day nursery phase on mechanical properties (ultimate normal stress, ultimate shear stress, ultimate strain, and tangent modulus) of porcine growth plate. Standard uniaxial tensile tests were applied on bone-growth plate-bone sections and the total stress was decomposed into normal stress and shear stress. Ultimate shear stress and ultimate strain traits were lower in the vitamin D deficient group than in the control. Regional differences were observed in all four variables. Ultimate normal stress was higher in the anterior region, which was consistent with a previous study. Sex differences were detected in ultimate normal stress, which was higher in females than in males. Interestingly, the classical finding of growth plate widening seen in severe vitamin D deficiency was not observed in the pigs with marginal vitamin D deficiency utilized in this study.

Impact of the Elastane Percentage on the Elastic Properties of Knitted Fabrics under Cyclic Loading.

Elastic knitted fabrics find numerous applications in the industry for compression stockings, sports and leisure wear, swimwear, ballet wear, etc. During its use, knitwear is subjected to dynamic loading due to body movements. The loading and unloading of the knitted fabric affect the size of the elastic region in which unrecovered deformation completely disappears. This paper deals with the influence of the elastane percentage in the knitted fabric on the elastic properties of the knitted fabric under dynamic loading. For this experiment, three types of yarn were used in different combinations: polyamide (PA), wrapped elastane yarn and bare elastane. The mentioned yarns were used to knit three different groups of plated weft-knitted fabrics (two yarns in a knitted fabric row): without elastane, knitted fabric with a percentage of wrapped elastane, and knitted fabric with a percentage of bare elastane. The percentage of elastane ranged between 0% and 43%. First, standard uniaxial tensile tests were performed on knitted fabric samples until breakage under static load. The force–elongation diagrams obtained are used to determine the elastic limit up to which Hook’s law applies. All knitted fabrics were cyclically tested to the elastic limit. From the obtained loading and unloading curves, unrecovered deformation (unrecovered elongation), elastic elongation and hysteresis index were determined and calculated. The results showed that the percentage of elastane significantly affects the size of the elastic region of the knitted fabric and has no effect on the hysteresis index. Therefore, it is necessary to optimize the elastane percentage for different knitted fabric designs to achieve the best dynamic recovery of the knitted fabric and to design a more stretchable knitted garment that fits the body as well as possible.

An equivalent work-hardening description of an interstitial-free steel sheet based on uniaxial tensile and hydraulic bulge tests

In this work, the plastic behavior of an interstitial-free steel sheet was first evaluated by standard uniaxial tensile and hydraulic bulge tests to reproduce the intermediate and high straining levels commonly observed in sheet metal forming processes. Next, a simple procedure was proposed to obtain an equivalent work-hardening curve, representing the high strain domain instead of the conventional uniaxial tensile data. This procedure involves an anisotropy parameter calculated based on Hill's quadratic criterion using the measured r-values in the rolling and transverse sheet directions. Under the assumption of an isotropic work-hardening, the equivalent curve is defined from the true stress and true thickness plastic-strain results of the hydraulic bulge test. The transformed data were fitted to usual power-law type equations to obtain an adequate work-hardening description for finite element numerical simulation purposes. Based on the goodness-of-fit measures, namely, R-squared, root-mean-square error, and root-mean-square of residual percentage, the combined Swift–Hockett–Sherby equation has proved to be the most suitable choice to describe the plastic behavior of the investigated interstitial-free steel under both intermediate and high strains conditions.

Simplified method to identify full von Mises stress-strain curve of structural metals

The relationship between von Mises equivalent stress σ¯ and equivalent plastic strain ε¯ (PEEQ), termed as von Mises stress-strain, plays an important role in numerical simulations for ductile fracture of structural metallic materials with large plastic strain. The standard uniaxial tensile test on cylindrical specimens is the most straightforward method to derive the (σ¯−ε¯) data. However, the distribution of the stress and strain along the necking section of the tensile specimen is no longer uniform in the post-necking process, which induces difficulty in acquiring the (σ¯−ε¯) data. The existing correction methods to identify von Mises stress-strain in the post-necking regime need instantaneous geometry of the notched region and ignore influence of the initial geometry of the specimen, which induces these methods are inconvenient for application and inapplicable for circumferentially notched tensile specimens to determine (σ¯−ε¯) of each individual material zone in a hybrid coupon (e.g. weldment). In the present study, influences of initial geometry of the specimen and plastic hardening property of material on necking process are investigated firstly. It is found that the shrinking behavior of the minimum cross-section of the specimen is determined by the geometric parameter of specimen and plastic hardening exponent of materials. Based on an extensive parametric study a modified power law function to characterize the von Mises stress-strain curve from tensile tests on circumferentially notched specimens without measuring instantaneous geometry of cross-section is proposed.

New procedure for determining the strain hardening behavior of sheet metals at large strains using the curve fitting method

The stress–strain relationship of a sheet metal is conventionally evaluated from the experimental data obtained from the uniaxial tensile test before the onset of localized necking. The identification of the hardening behavior beyond the neck or post-necking behavior is essential for studying material fracture. This study presents a new procedure of identifying the flow stresses of sheet metals at large strains. In this procedure, the experimental data obtained from the uniaxial tensile test and the measured forming limit at a plane–strain tension mode FLC0 are adopted in the curve fitting procedure for calibrating the parameters of different hardening laws. The measured FLC0 is introduced in the cost function of the fitting to provide an additional constraint for the hardening parameters in large strain ranges. Using the uniaxial tensile test data ensures the accuracy of the pre-necking description of the hardening. The proposed method decreases the variation of the predicted flow stresses beyond the uniform elongation for different hardening laws. For validation purposes, the developed approach is applied to calibrate the parameters of the Kim–Tuan and the linear combined Swift/Voce hardening models for three sheet metals tested in Numisheet (2014) benchmarks. The hardening laws with newly identified parameters are implemented into the finite element software ABAQUS/Explicit to simulate the standard uniaxial tensile and reverse drawing tests. The comparative results show that the simulated tensile force in the uniaxial tensile tests and the simulated punch force in the reverse drawing tests agree well with the experiments, thereby validating the accuracy and the efficiency of the proposed identification procedure of large strain hardening in sheet metals.

Effect of surface roughness on tensile ductility of artificially corroded steel plates

Physically, corrosion damage results in a reduction in thickness and a change in surface geometry due to differential corrosion rates on the surface of a steel member. The effects of thickness reduction on the mechanical characteristics of structural steel have been investigated intensively, but the same cannot be said for the surface configuration. This study investigated the variation of corrosion thickness and surface roughness characteristics with increasing corrosion degradation and their contribution to a reduction in the tensile ductility of corroded steel. Twenty-two SS400 type steel plates were corroded by an underwater accelerated exposure (bubbling) test. Specimens were exposed for a varying number of days after which measurement and evaluation of thickness reduction and surface roughness characteristics were conducted. Then, the remaining load and elongation capacity were determined experimentally from the standard uniaxial tensile test. The distribution of the surface strains was studied employing digital image correlation techniques. Findings from this work revealed that surface unevenness due to corrosion causes stress concentration and uneven yielding, and strain localization after yielding during the tensile test. This translated to an early attainment of maximum load, resulting in reduced ductility in corroded specimens. A general trend is such that ductility decreases with increasing surface roughness. A plastic ductility loss of up to 80% is experienced when maximum surface height ratio, ζsz ≥ 0.9 and average surface height ratio, ζsa ≥ 0.15. Finally, empirical equations for estimating the remaining tensile ductility of corroded steel based on surface roughness characteristics were proposed.

Calibration of Advanced Yield Criteria Using Uniaxial and Heterogeneous Tensile Test Data

Conventionally, plastic anisotropy is calibrated by using standard uniaxial tensile and biaxial test results. Alternatively, heterogeneous strain field specimens in combination with full-field measurements can be used for this purpose. As reported by the literature, such an approach reduces the number of required tests enormously, but it is challenging to obtain reliable results. This paper presents an alternative methodology, which represents a compromise between the conventional and heterogeneous strain field calibration technique. The idea of the method is to use simple tests, which can be conducted on the uniaxial testing machine, and to avoid the use of advanced measuring equipment. The procedure is accomplished by conducting standard tensile tests, which are simple and reliable, and by a novel heterogeneous strain field tensile test, to calibrate the biaxial stress state. Moreover, only two of the parameters required for full characterisation need to be inversely identified from the test response; the other parameters are directly determined from the uniaxial tensile test results. This way, a dimension of optimization space is reduced substantially, which increases the robustness and effectiveness of the optimization algorithm.

Combined Effect of Pre-Strain at Elevated Temperature and Pre-Aging on Aging Strength and Microstructures of AA6082

With the increasingly serious energy and environmental issues, aluminum alloys are becoming increasingly desirable for transportation facilities due to their high specific strength. Their low formability results in the difficulty of forming complex-shaped panels and bodies. Hot stamping is a new technology developed to address this formability challenge by applying plastic deformation to the heated aluminum alloy sheet at the solution heat treatment temperature. The stamped panels gain high strength via the sequent aging treatment. Different panels, even different regions on the same panel, are subjected to different forming strains. The present paper is focused on the experimentally investigating the effects of the pre-strain at elevated temperature and pre-aging on the mechanical behavior and microstructures of AA6082 after the natural aging, paint-bake treatment, and artificial aging. Sample sheets of AA6082-T6 were pre-strained by uniaxial tension at different levels of strain, and then subjected to the pre-aging, natural aging, paint-bake treatment or artificial aging, respectively. The yield strength, tensile strength and elongation of post-aged samples were gained via the standard uniaxial tensile tests. The results show that both of the pre-strain at elevated temperature and pre-aging suppress the natural aging of AA6082. Pre-strain at elevated temperature induces weak bake softening, and has a detrimental effect on the artificial aging strengthening, while the sequent pre-aging could compensate the strength decrease.

Stress–strain curves of metallic materials and post‐necking strain hardening characterization: A review

AbstractFor metallic materials, standard uniaxial tensile tests with round bar specimens or flat specimens only provide accurate equivalent stress–strain curve before diffuse necking. However, for numerical modelling of problems where very large strains occur, such as plastic forming and ductile damage and fracture, understanding the post‐necking strain hardening behaviour is necessary. Also, welding is a highly complex metallurgical process, and therefore, weldments are susceptible to material discontinuities, flaws, and residual stresses. It becomes even more important to characterize the equivalent stress–strain curve in large strains of each material zone in weldments properly for structural integrity assessment. The aim of this paper is to provide a state‐of‐the‐art review on quasi‐static standard tensile test for stress–strain curves measurement of metallic materials. Meanwhile, methods available in literature for characterization of the equivalent stress–strain curve in the post‐necking regime are introduced. Novel methods with axisymmetric notched round bar specimens for accurately capturing the equivalent stress–strain curve of each material zone in weldment are presented as well. Advantages and limitations of these methods are briefly discussed.

Numerical and experimental study on ductile fracture of quenchable boron steels with different microstructures

Five sets of boron steel sheet blanks are heat treated and their microstructures are close to mixture of ferritic and pearlitic, upper bainitic, lower bainitic and fully martensitic, respectively. It shows that the micro-hardness of the blank that quenched in the water-cooled die is of 527 HV0.1, while intermediate hardness of 401 HV0.1, 322 HV0.1 and 280 HV0.1 maintained in the heated die as well as 181 HV0.1 cooled in the open furnace. Various constitutive models such as Ghosh, extend Swift, modified Voce, et al., are calibrated and compared using the stress-strain data of the five material grades. The constitutive parameters of each grade are calibrated using the standard uni-axial tensile tests at quasi-static strain rates and at room temperature. The inverse analysis method coupled with optimization algorithm is used to calibrate the parameters of the hardening laws. It is concluded that the Voce + Voce model can accurately predict the stress-strain relation of the five kinds of materials at the same time. The Gurson-Tvergaard-Needleman (GTN) model and the Johnson-Cook model are used to describe the damage behavior of the ductile boron steel B1500HS grades. The simulated force-displacement curves are compared with those of the experiments, showing considerable good coincidence.

An alternative procedure to identify stress-strain relation for DP980 sheet over a large strain range

In sheet metal forming, the standard uniaxial tensile test is a well-established testing method used to achieve stress-strain relation or flow curve for examined material. In agreement with the development of material science, the number of forming processes that exceeded the maximum uniform strain observed in a uniaxial tensile test is myriad. To obtain the relationship between stresses and strains over a large range of deformation, extrapolation of a common hardening model (Swift model or Voce model) is only accepted for some specific materials. Therefore, expensive calibrating methods, such as inverse finite element analysis and virtual field method, are widely used to identify the flow curve for recently developed sheet materials, for example, advanced high strength steels or aluminium alloys. This study presents an alternative procedure to identify the flow curve for sheet metals by using the extrapolation method. Key to the success of this procedure is using the Kim-Tuan hardening model, a newly developed hardening model proposed by the authors. The examined material is DP980 sheet that is tested material in Benchmark 01 of the NUMISHEET 2018 conference. To verify the accuracy of the calibrated flow curve, this curve is imported into a finite element analysis for a uniaxial tensile specimen and computational loading forces according to gauge length displacement are compared with experimental observation supported from the Benchmark’s data.

Round robin exercise on ball indentation technique in India: Indian nuclear reactor materials

This paper presents outcome of round robin exercise on usage of ball indentation technique to predict strength properties of three different materials which are used in Indian nuclear reactors. The main objectives were to evolve a standardized procedure of determining the mechanical properties by the Ball Indentation technique and quantification of the variation in strength and fracture properties due to ageing through such tests. Standard uniaxial tensile tests were also carried out to quantify the differences between strength properties predicted by ball indentation and those determined using conventional tests. Three different materials studied were, carbon-manganese steel (Grade: SA 333Gr.6), stainless steel (SA312 Type 304LN) and zirconium alloy (Zr-2.5Nb). The carbon-manganese steel and stainless steel materials were drawn from extruded pipes while Zr-2.5Nb was drawn from pressure tubes, used in Indian Pressurized Heavy Water Reactors. Ball indentation tests were carried out on materials in as received and in aged conditions. The ageing was simulated artificially. The carbon-manganese steel and stainless steels were subjected to cold work while Zr-2.5Nb alloy was subjected to heat treatment at different temperatures with varying hold times and in some cases with charged hydrogen. The round robin exercise was conducted within different research centers of Department of Atomic Energy (DAE), India. This was a blind exercise in which all the conventional tests were carried out by a group (who was not involved in Ball Indentation tests) after the completion of ball indentation tests by participants.The paper presents the prediction of strength properties, using ball indentation, by different participants. The reasons of scatter in the results among the various participants and differences with respect to conventional test results are discussed in the paper.

Effect of tempering temperature on strain hardening exponent and flow stress curve of 1000 MPa grade steel for construction machinery

To study the effect of tempering temperature on strain hardening exponent and flow stress curve, one kind of 1000 MPa grade low carbon bainitic steel for construction machinery was designed, and the standard uniaxial tensile tests were conducted at room temperature. A new flow stress model, which could predict the flow behavior of the tested steels at different tempering temperatures more efficiently, was established. The relationship between mobile dislocation density and strain hardening exponent was discussed based on the dislocation-stress relation. Arrhenius equation and an inverse proportional function were adopted to describe the mobile dislocation, and two mathematical models were established to describe the relationship between tempering temperature and strain hardening exponent. Nonlinear regression analysis was applied to the Arrhenius type model, hence, the activation energy was determined to be 37.6 kJ/mol. Moreover, the square of correlation coefficient was 0.985, which indicated a high reliability between the fitted curve and experimental data. By comparison with the Arrhenius type curve, the general trend of the inverse proportional fitting curve was coincided with the experimental data points except of some fitting errors. Thus, the Arrhenius type model can be adopted to predict the strain hardening exponent at different tempering temperatures.

An elasto-visco-plastic constitutive model of polypropylene incorporating craze damage behavior and its validation

An elasto-visco-plastic constitutive model incorporating the craze damage behavior was developed for the polypropylene (PP), by using the plastic failure model applied for the concrete, to capture the craze yielding and stress-whitening phenomena. In addition, the developed constitutive model was implemented into finite element codes in Abaqus to simulate the tensile deformation. The standard uniaxial tensile tests were carried out. The stress−strain curves from the uniaxial tensile tests show that the stress keeps decreasing after yielding and the yield stress rises with the increasing of the strain rate. It is worth noting that the craze damage is more visible with higher strain rate. The stress-whitening can be seen clearly around the fracture. The uniaxial tensile tests using specially designed specimen with circular holes weakening were performed for the validation of the developed model. The simulation results of the tensile deformation of the hole-weakened specimen suggest that the stress-whitening could be attributed to the equivalent visco-plastic strain. By comparing between the simulation analysis and the experimental results, the proposed model can describe the stress whitening phenomenon with good accuracy.

Ductile Behaviour Characterization of Low Carbon Steel: a CDM Approach

In this paper, the ductile behaviour of two different low carbon steels, C-Mn-440 and interstitial free high strength (IFHS), is presented using a continuum damage mechanics (CDM) approach. The damage growth law is adopted to predict the ductile response of the specified materials. Cyclic load-unload tensile tests in combination with standard uniaxial tensile tests helped to estimate the necessary parameters: damage variable, D, fracture stress, σf, threshold damage strain, e0, and strain hardening exponent, n, required to apply the model. The strain hardening exponent estimated from the cyclic test data is used to predict the damage variable, D. Increase of damage shows deterioration of the hardening exponent magnitudes varying nonlinearly. The simulated flow curve by the damage variable, D, corresponding to the loadunload test is observed to approximate the experimental true stress-true strain curve very closely up to the onset of necking for both the materials. The experimental values of D, as obtained for C-Mn-440 and IFHS steels, vary from 0.10 to 0.44 and 0.09 to 0.45, respectively. The critical damage parameters, DC, for the considered materials are 0.44 and 0.45, representing their good ductile response.

Uniaxial Pre-strain and Free Recovery (UPFR) as a Flexible Technique for Nitinol Characterization

The measurement of phase transformation temperatures of superelastic (SE) and shape memory (SM) NiTi alloy products and components was studied in this work. The transformation temperatures of a set of twenty different 300 μm NiTi superelastic wires were measured by two well-established and standardized techniques, namely differential scanning calorimetry (DSC) and bend and free recovery (BFR) and then compared with the results from the Uniaxial Pre-Strain and Free Recovery (UPFR) test. UPFR is a tension-based test, whose aim is to overcome the limitations associated with BFR testing. Within this work, a test procedure has been set up and validated. UPFR is found to be the only method showing a very strong correlation with the mechanical properties measured using the standard uniaxial tensile test method for superelastic NiTi alloy. Further, UPFR has been validated as a robust technique for measuring the R-phase and austenitic transformation temperatures in specimens of various sizes, composition, and of different geometries. This technique overcomes the limitations of BFR and DSC which cannot be used for testing products such as 25 μm SM wire, a 50 μm SE strip, and different springs and microsprings for actuation.

Investigation on Elastic Behaviour of DP800 Dual Phase Steel

The elastic relaxation behavior of dual phase steel DP800 is studied in this investigation, based on experimental and numerical methods the true stress-true strain curve obtained from a standard uniaxial tensile test differs according to angular rolling direction The relationship between true stress and true strain are presented in the form of power law equation. This form of material constitutive model shows that the strength coefficient and strain hardening exponent vary significantly in describing the nonlinear true stress-true strain relationship of the material. Finite Element (FE) calculations with Belytschko-Lin-Tsay shell element formulation are performed using the non-linear FE code Ls-Dyna to predict the plastic deformation of the material. Power Law Isotropic Plasticity criterion is adopted for these numerical analyses. The local strains in plastic deformations zone and true stress-strains characteristics obtained by experiment are compared. Using the same parameter the simulation was applied in different modes which are known as Isotropic Elastic-Plastic Model and Piecewise Linear Isotropic Plasticity Model providd in Ls-Dyna simulation for comparison. In general, good agreement in results is obtained between Power Law Isotropic Plasticity Model is obtained compared to Isotropic Elastic-Plastic Model and Piecewise Linear Isotropic Plasticity Model. It is demonstratedthat the behavior of the strain and the Power law criterion can be determined from uniaxial tensile test with the aid of non-linear FE analyses.