Biaxial Tensile Testing Machine Research Articles

Warp-knitted strain sensors: impact of loop parameters on sensing performance under biaxial stretching

This paper explores the sensing performance displayed by warp-knitted strain sensors under biaxial stretching. These sensors were knitted using silver-plated nylon to be interlooped on a tricot warp-knitting machine. Eight types of warp-knitted sensing fabrics with different loop parameters were prepared and, afterward, electro-mechanical tests were conducted on a biaxial tensile testing machine. These specimens offered similar ground structures but differed in conductive yarn configuration in terms of linear density, number of underlapping wales, open/closed loop type, and guide-bar lapping sequence. Experimental results showed that the loop parameters significantly played a fundamental role in determining sensing performance. It is therefore possible to improve the sensing performance of warp-knitted sensors and engineer them by differing the loop parameters based on certain applications.

Development of biaxial tensile testing for porous polymer membranes

Porous polymer membranes are typically polymeric materials that consist of many pores and complex network structures. As a result of these features, the membranes sometimes undergo anisotropic elastoplastic deformation. Hence, it is important to investigate the deformation behavior of these materials, including anisotropic deformation under multiaxial loading. In this study, we developed a new biaxial tensile testing machine for porous polymer membranes to evaluate their deformation behavior under biaxial tension and to understand the deformation mechanisms from their microstructure. First, uniaxial tensile tests were conducted to investigate changes in Young's modulus and yield strength for different tensile directions. These tests showed that the materials underwent anisotropic deformations. Next, we developed a novel biaxial tensile testing protocol for the membranes. It is usually difficult to grip the specimens and apply uniform deformations, as the membranes have low deformation rigidity and are very thin. Thus, a gripping component was developed to enable uniform tensile deformations to be applied. Stress–strain curves for each loading axis were obtained by changing the applied strain ratio. A yield surface based on the results was created to investigate the deformation behavior under biaxial tensile loading. We also investigated the deformation mechanisms of the porous microstructure via SEM observation.

Application and practical validation of topology optimization technology for the frame of biaxial tensile testing machine

Biaxial tensile testing machines are used to improve accuracy of yield loci and constitutive model for sheet materials. The frame of these machines is the main load-bearing structure significantly affecting the machine accuracy. However, the available loading frames are too heavy to be practical for popularization and application. To reduce both the total weight of the frame and install consumption, this study develops a design method of stepwise multi-stage topology optimization based on the solid isotropic microstructures with penalization (SIMP) method. First, the validity of the finite element model during the optimization is verified by experiments. Next, the design method of stepwise multi-stage topology optimization is proposed based on the results of conventional topology optimization and the influence law of key parameters. This method is then applied to the topological optimization design of frames for a biaxial tensile testing machine. Finally, the optimized frame is manufactured, and biaxial tensile tests are conducted on the designed machine. The measurement of frame deformations shows that the frame weight is decreased by 19.5% compared with that of the initial design and that the frame stiffness meets design requirements. These results prove the effectiveness of the proposed method.

Development of a biaxial tensile testing machine for pulsed neutron experiments

A biaxial tensile testing method has been used to get macroscopic information on elastoplastic deformation of a thin steel specimen and improve the accuracy of plastic processing of steel materials. We newly developed a biaxial tensile testing machine for pulsed neutron experiments (BTM-NEU) to provide the microscopic crystallographic information of steel materials under biaxial load and correlate it with the macroscopic mechanical properties of the materials. The performance of the BTM-NEU was experimentally evaluated with cold-rolled mild steel and hot-rolled high-tensile-strength steel materials and compared with that of a standard biaxial tensile testing machine (BTM-std) as follows.•The BTM-NEU can test an ISO-standardized cruciform specimen as the BTM-std and its performance is equivalent to that of the BTM-std.•The BTM-NEU has excellent long-time reliability and stability necessary for pulsed neutron experiments, especially Bragg-edge neutron imaging experiments.•The BTM-NEU can be applied to pulsed neutron experiments using a Bragg-edge imaging method.

Evaluation on Accuracies of Phenomenological Yield Criteria for Automotive Al sheets

The anisotropic behavior of two typical cold rolled Al alloy sheets, AA5182-O and AA6016-T4, were investigated and used to predictive the yield surface by the well-known yield criteria: Hill90, Barlat89 and Barlat2000. Uniaxial tensile tests with axial and wide extensometers were carried out at different angles to the rolling direction (RD). Then the yield stress σθ and anisotropic coefficient rθ were obtained. Biaxial tensile tests of cross-shaped specimens under certain liner loading paths were conducted on the well-designed biaxial tensile testing machine. The loading paths includes Fx:Fy = 1:4, 2:4, 3:4, 4:4, 4:3, 4:2, 4:1. Yield stresses points (σx, σy) and their strain ratios in the yield locus plane can be obtained from the strain-stress curves of the biaxial tensile tests. Parts of tested values were used to determinate the yield locus according to different yield functions. All experimental data was used to evaluate the precisions of the yield functions by taking into account of the accuracy of σθ and rθ and the yield stress and strain ratios of biaxial tensile tests.

DIRECTION-DEPENDENT AND MULTIAXIAL STRESS-SOFTENING BEHAVIOR OF CARBON BLACK–FILLED RUBBER

ABSTRACT The mechanical behavior of filled rubbers depends on the maximum stretch previously reached and consequently on the induced stress softening. This softening effect is referred to as the Mullins effect. Current investigations point out that the Mullins effect exhibits a significant directional dependence, which calls for an anisotropic material model. But for the formulation and validation of anisotropic material models, there is still a lack of suitable experimental data. For this the purpose, experiments based on chloroprene rubber (CR) are reported. To trace the anisotropic Mullins effect, the standard test method for characterization of the isotropic mechanical behavior must be extended. The appropriate type of specimen enables us to perform multiple load steps with alternating load directions. After repeated stretching in the same direction, a subsequent first uniaxial loading in any other direction is characterized by a stiffer stress–strain behavior compared with the stabilized curve of the previous primary load. Hence, the experimental results confirm the deformation-induced anisotropy. To identify the multiaxial material behavior after the prestretching in one direction, a biaxial tensile-testing machine is developed. A specific property of the biaxial tensile-testing machine is the independent control of both the loading axes. Thus, the rubber material can be subjected to arbitrary loading histories. Therefore, a cross-shaped specimen with four arms is used. Multiple slits parallel to the sides on each arm ensures the homogenous uniaxial load condition in the primary load. In the secondary load step, the loading axis, which was previously inactive, is moved in a uniform manner as the master axis or in any arbitrary defined ratio. The experimental results confirm the deformation-induced anisotropy of the Mullins effect. In summary, the material behavior significantly results from the deformation mode and the loading direction applied in the loading history.

Material modeling of 6000 series aluminum alloy sheets with different density cube textures and effect on the accuracy of finite element simulation

Biaxial tensile tests of 6000 series aluminum alloy sheet with different density cube textures were conducted using cruciform specimens. The specimens were loaded under linear stress paths in a servo-controlled biaxial tensile testing machine. The plastic orthotropy remained coaxial with the principal stresses throughout every experiment. Successive contours of plastic work in stress space and the directions of plastic strain rates were precisely measured and compared with those calculated using selected yield functions. The Yld2000-2d yield functions with exponents of 12 and 6 were capable of reproducing the general work contour trends and the directions of plastic strain rates observed for the test materials with high and low density cube textures, respectively. Hydraulic bulge tests were also conducted and the variation of thickness strain along the meridian directions of the bulged specimen was compared with that calculated using finite element analysis (FEA) based on selected yield functions. Differences in the cube texture density caused significant differences in the strain distribution of the bulged specimens, and were in good agreement with the FEA results obtained using the Yld2000-2d yield functions. It is thus concluded that the biaxial tensile testing method with a cruciform specimen is an effective material testing method for accurately detecting and modeling the deformation behavior of sheet metals under biaxial tension.

Hydraulic bulge forming simulation of 6000 series aluminum alloy sheets using anisotropic yield functions and experimental validation

Biaxial tensile tests of 6000 series aluminum alloy sheets with different density cube textures were carried out using cruciform specimens. The specimens were loaded under linear stress paths in a servo-controlled biaxial tensile testing machine. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. Successive contours of plastic work in stress space and the directions of plastic strain rates were precisely measured and compared with those calculated using selected yield functions: the von Mises, Hill's quadratic and Yld2000-2d [Barlat, F. et al., Int. J. Plasticity, 19 (2003), 1297–1319]. The Yld2000-2d yield functions with exponents of 12 and 6 are capable of reproducing the general trends of the work contours and the directions of plastic strain rates observed for test materials with high and low cube textures, respectively. Hydraulic bulge tests were also conducted and the variations of the thickness strain along the meridian directions of the bulged samples were compared with that calculated using finite element analysis (FEA) based on the Yld2000-2d yield functions with exponents of 12 and 6. The differences of cube texture density cause significant differences in the strain distributions of the bulged specimens, and the FEA results calculated using the Yld2000-2d yield functions show good agreement with the measurement results.

Measurement and Analysis of Ultra-Thin Austenitic Stainless Steel Sheet under Biaxial Tensile Loading and In-Plane Reverse Loading

Biaxial tensile tests of austenitic stainless steel sheet (SUS304) 0.2mm thick have been carried out using cruciform specimens. The specimens are loaded under linear stress paths in a servo-controlled biaxial tensile testing machine. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. The successive contours of plastic work in biaxial stress space changed their shapes progressively, exemplifying differential work hardening. The geometry of the entire family of the work contours and the directions of plastic strain rates have been precisely measured and compared with those calculated using conventional yield functions. Yld2000-2d [Barlat, F., Brem, J.C., Yoon, J.W., Chung, K., Dick, R.E., Lege, D.J., Pourboghrat, F., Choi, S.H. and Chu, E., International Journal of Plasticity, Vol. 19, (2003), pp. 1297-1319.] with an exponent of 6 was capable of reproducing the general trends of the work contours and the directions of plastic strain rates with good accuracy. Furthermore, in order to quantitatively evaluate the Bauschinger effect of the test material, in-plane tension/compression tests are conducted. It was found that the non-dimensional (σ /σu) - Δe /(σu/ E) curves measured during unloading almost fall on a single curve and are not affected by the amount of pre-strain, where σ is the current stress during unloading, σu is the stress immediately before unloading, Δe (< 0) is the total strain increment during unloading.

2241 2軸応力下における微細粒Al-Mg合金の高温変形と組織変化(S25 軽量化・高強度化材料とその加工)

To evaluate formability, deformation behavior can be examined under biaxial stress by using a biaxial tensile testing machine in combination with a cruciform specimen. Application of continuous biaxial stresses in the same plane makes it easier to correlate the mechanical properties and the structural changes that occur with increasing deformation than by using conventional methods. When an equivalent strain of 60% was applied at a temperature of 673K, the grain size was refined from 9.8μm to 5.0μm before and after tensioning. In this case, voids were connected in 45°directions to each tensile axis, and grain boundary separations were observed in the vicinity of the voids. Under the biaxial stress state, refining of grains, together with grain rotation and slip, occurs more readily than under a uniaxial stress state.

Measurement and analysis of yield locus and work hardening characteristics of steel sheets wtih different r-values

Biaxial tensile experiments of six kinds of steel sheet with different r-values are carried out using a servo-controlled biaxial tensile testing machine. Successive contours of plastic work in the biaxial stress space and the plastic strain rate vectors are precisely measured for linear loading paths. The measured data are compared with the theoretical predictions based on the Taylor–Bishop–Hill model with the full constraints and relaxed constraints assumptions using the experimentally determined crystallographic textures. Comparisons with the Hill quadratic and Hosford yield criteria are also made. It is found that (1) the TBH model with the full constraints assumption is superior to that with the relaxed constraints assumption in predicting the plastic deformation characteristics of steel sheets; (2) The Hosford yield criterion is an effective phenomenological model for predicting the plastic deformation characteristics of steel sheets.

６０００系アルミニウム合金板の２軸引張塑性変形特性の測定と降伏条件式の検証

Biaxial tensile tests of 6000–type sheet aluminum alloy were carried out using newly designed cruciform specimens. The specimens were deformed under linear loading paths in a servo-controlled biaxial tensile testing machine. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. On the other hand, the successive contours of equal plastic work in biaxial stress space changed their shapes progressively, exemplifying differential work hardening. The geometry of the entire family of the work contours was compared with the yield loci calculated from existing yield criteria. The work contour at 0.0005 equivalent plastic strain almost coincided with the von Mises ellipse, while its curvature decreased as work-hardening progressed and became close to the Tresca-type yield locus. Moreover, it was observed that the directions of the measured plastic strain vectors were almost coincided with the associated normals to the current work contour in stress space; accordingly it appears that the work contours act as instantaneous plastic potentials at least under linear loading paths. The effect of age-hardening on the geometry of work contours was small.

Measurement and analysis of differential work hardening in cold-rolled steel sheet under biaxial tension

Abstract Biaxial tensile tests of cold-rolled steel sheet were carried out using newly designed cruciform specimens. The specimens were deformed under linear loading paths in a servo-controlled biaxial tensile testing machine. The maximum equivalent strain attained was 0.04. Plastic orthotropy remained coaxial with the principal stresses throughout every experiment. However, the successive contours of plastic work in biaxial stress space changed their shapes progressively, exemplifying a phenomenon which has been termed differential work hardening by Hill and Hutchinson (Trans. ASME J. Appl. Mech. 59 (1992) 1) and by Hill et al. (Int. J. Solids Struct. 31 (1994) 2999). The geometry of the entire family of the work contours was compared with the yield loci calculated from several existing yield criteria. Hill’s quadratic yield criterion overestimated the measured work contours; in particular, in the neighborhood of balanced biaxial tension, the discrepancy was large, while the other yield criteria described the behavior of the work contours well. The only yield criterion that could describe the general trends of the work contours as well as the in-plane r-value distribution with good accuracy was Gotoh’s biquadratic yield criterion. Moreover, it was observed that the components of an increment of logarithmic plastic strain are proportional to the components of the associated normal to the current work contour in stress space. Accordingly, it appears that the work contours act instantaneously as plastic potentials, at least under linear loading paths.

Biaxial mechanics of excised canine pulmonary arteries

A new method has been developed for measuring the stress-strain relationship in excised canine pulmonary arteries. Segments of dog main right pulmonary arteries were isolated by making two transverse cuts at each end of a segment near the bifurcations, yielding short cylinders, which were then cut radially, relieving the residual stress, causing the cylindrical shells to spring open to approximately flat rectangular slabs with dimensions approximately 1.0 x 3.0 x 0.1 cm. The specimens were then tested using a biaxial tensile testing machine. The resulting data show an approximately linear relationship between Kirchhoff stress and Lagrangian strain with very little hysteresis. The following pseudostrain energy function serves as a practical approximation for pulmonary arteries subjected to physiological levels of stress and strain: rho 0W(2) = 1/2(a1E2xx + a2E2yy + 2 a4ExxEyy), where rho 0 is the density of the wall (mass per unit volume), W is the energy per unit mass [superscript "(2)" indicates this is a 2-dimensional strain energy function], E is strain, a1, a2, and a4 are material constants with units of stress, and the subscripts x and y refer to the circumferential and axial axes, respectively, of the artery. To assess the physiological level of strain in the main right pulmonary artery, vessels were perfused in situ at physiological pressure (26 cmH2O) with silicone elastomer. The arteries were then excised and marked with small ink spots. Photographs of the spots on four tangent planes of the excised artery indicate a maximum circumferential strain of 21.5% and a maximum axial strain of 36.5% relative to the zero-stress state. These values are within the range of strain used in the biaxial tests. The relationship between Kirchhoff stress and Green's strain is approximately linear within the physiological range. The stress levels required to cause tissue failure are at least 10 times greater than the estimated normal physiological level.