Bulge Test Research Articles

Inverse identification method for determining stress–strain curves of sheet metals at high temperatures

Because of the strong dynamic softening effect of sheet metals at high temperatures, necking occurs early and continues for longer periods. Therefore, obtaining a flow curve after necking has occurred is necessary. The high-temperature flow behavior of materials typically exhibits the coupled effects of work hardening, softening, strain-rate sensitivity, and damage, and using the traditional inverse identification method is quite challenging. This study presents a finite element (FE) inverse identification method that considers work hardening, softening, damage, and strain-rate sensitivity. The method uses a coupled viscoplastic-damage constitutive model as the modified object, and the parameters of the damage equation are iteratively modified by the FE method until the simulated force–displacement curve is consistent with that of the experiment. The identification procedure is demonstrated using 7075 aluminum alloy, and a complete high-temperature flow curve prior to fracture is obtained. The obtained flow curve is applied to analyze the strain field distribution in a hot tensile process and to predict the fracture occurrence and thickness distribution under an isothermal bulging test. Experimental and simulation results are shown to be in good agreement, thus proving the validity of the proposed method.

Effect of the crosslinking agent on the biorepulsive and mechanical properties of polyglycerol membranes

Polyglycerol (PG) based surfaces materials and surfaces are well-established bio-compatible materials. Crosslinking of the dendrimeric molecules via their OH groups improves their mechanical stability up to the point that free-standing materials can be attained. Here, we investigate the effect of different crosslinkers on PG films regarding their biorepulsivity and mechanical properties. For this purpose, PG films with different thicknesses (15, 50 and 100 nm) were prepared by polymerizing glycidol in a ring-opening polymerization onto hydroxyl-terminated Si substrates. These films were then crosslinked using ethylene glycol diglycidyl ether (EGDGE), divinyl sulfone (DVS), glutaraldehyde (GA), 1,11-di(mesyloxy)−3,6,9-trioxaundecane (TEG-Ms2) or 1,11-dibromo-3,6,9-trioxaundecane (TEG-Br2), respectively. While DVS, TEG-Ms2, and TEG-Br2 resulted in slightly thinned films, presumably due to loss of unbound material, increase of film thickness was observed with GA and, in particular, EDGDE, what can be explained by the different crosslinking mechanisms. The biorepulsive properties of the crosslinked PG films were characterized by water contact angle (WCA) goniometry and various adsorption assays involving proteins (serum albumine, fibrinogen, γ-globulin) and bacteria (E. coli), showing that some crosslinkers (EGDGE, DVS) improved the biorepulsive properties, while others deteriorated them (TEG-Ms2, TEG-Br2, GA). As the crosslinking stabilized the films, it was possible to use a lift-off procedure to obtain free-standing membranes if the thickness of the films was 50 nm or larger. Their mechanical properties were examined with a bulge test showing high elasticities, with the Young’s moduli increasing in the order GA ≈ EDGDE < TEG-Br2 ≈ TEG-Ms2 < DVS.

Formability and microstructure analysis of TiBw/TA15 composites sheet with superplastic tensile and gas bulging tests

Formability and microstructure analysis of TiBw/TA15 composites sheet with superplastic tensile and gas bulging tests

Hydraulic bulge testing to compare formability of continuous and stretch broken carbon fiber reinforced polymer composites

Hydraulic bulge testing to compare formability of continuous and stretch broken carbon fiber reinforced polymer composites

Hydraulic bulge testing to compare formability of continuous and stretch broken carbon fiber reinforced polymer composites

Hydraulic bulge testing to compare formability of continuous and stretch broken carbon fiber reinforced polymer composites

Using surround DIC to extract true stress–strain curve from uniaxial tension experiments

The exact stress–strain curve can be directly identified from uniaxial tension experiments up to the point of onset of diffuse necking. To gain insight into the post-necking hardening response of metals, other experimental methods such as the bulge, compression or torsion experiments are typically employed. Here, we introduce the idea of using surround DIC to obtain accurate specimen shape measurements of tensile specimens with rectangular gage section. Based on the results from a series of detailed three-dimensional finite element simulations of ASTM E8 type of uniaxial tension experiments on a wide spectrum of steel and aluminum behaviors, it is proposed to combine the history of the axial strain on the surface at the specimen center with the average axial stress to extract the stress–strain curve for strains of up to axial true strain of 1. The estimation uncertainty of this stress–strain curve estimation procedure is of the same order as that of the associated experimental uncertainties. This result is validated through an additional computational study for more than 100 distinct hardening behaviors. Furthermore, a surround DIC system composed of four stereo DIC systems is built and used to determine the stress–strain curve for a 1.5 mm thick DP780 steel. In addition, bulge experiments are performed on the same material revealing a good agreement of the post-necking hardening behavior determined through uniaxial and bulge testing.

Study on the Influence of Nickel Additions on AA7020 Formability Under Superplastic Forming Like Conditions

This study set out to look at the influence of nickel additions on a commercially available AA7020 to understand the impact of the resultant intermetallics on recrystallization, formability and material strength. Elevated temperature tensile testing across a range of strain rates (5 × 10−4 s−1 × 10−1 s−1) and three temperatures (450− 500 °C) to compare material ductility followed by gas bulge testing at 475 °C and two gas pressures to investigate formability in a test closer to industrial forming conditions. Material strength was established using standard tensile testing, and EBSD used to understand the microstructural evolution of the materials. It was seen that the nickel additions increased ductility of the material across all test conditions, coupled with increasing the material strength. This was achieved due to the formation of nickel rich intermetallics which refine the microstructure during pre-heating through particle stimulated nucleation and subsequently improve strength through precipitation hardening in aging treatments.Graphic

Anisotropic Hardening of TRIP780 Steel Sheet: Experiments and Analytical Modeling

By combining experimental and theoretical models, this research investigates the anisotropic hardening behaviors of TRIP780 steel. The specimens of TRIP780 steel were subjected to uniaxial tensile and bulging tests under different loading conditions to obtain hardening data. The experimental results show that the strength and plastic deformation of TRIP780 steel vary with the loading directions, which indicates that TRIP780 steel has anisotropy characteristics. In this paper, the dichotomous method is used to ensure the convexity of the Chen-coupled quadratic and non-quadratic (CQN) function. Comparing the predictions of the hardening behavior of the TRIP780 steel sheet by the Yld2000-2d, Stoughton-Yoon'2009 and Chen-CQN functions, the results show that the Chen-CQN function exhibits the advantages of simple numerical implementation and a more realistic prediction of yield stress compared to the former two, respectively. Comparing the prediction of Chen-CQN function with the experimental hardening data, the results show that the deviation between the experimental data and the experimental response given by the function is always within 3%, and this function maintains an accurate prediction under different stress states, indicating that the Chen-CQN yield function has accuracy and flexibility for the characterization of the yield surface of TRIP780 steel.

Size-dependent constitutive model for shape memory alloys based on couple stress elastoplasticity

A constitutive model for shape memory alloys considering size effects is developed based on thermodynamic framework. Couple stress elastoplasticity is utilized to simulate size effects. Three material length scale parameters, two for elastic size effect of the austenite and martensite phases and one for crystal structure transformation, are introduced as additional material properties. The developed model is formulated with three-dimensional equations and can be used for the analysis of structures having arbitrary shapes by being adopted into the finite element method. Two benchmark problems (cantilever and bulge test) are solved using the developed model, which shows significant increase in structural stiffness as the size of the structure decreases. The developed size-dependent constitutive model is expected to be utilized for the analysis of micro/nano-scale structures composed of shape memory alloys.

Mechanical properties characterization of 2D materials via pressure bulge testing

Pressure bulge testing is one of the standard techniques for characterizing the mechanical properties of thin films, whereas it is far less often used to characterize the mechanical properties of 2D materials. In the present work, the effectiveness of bulge testing on determining the mechanical properties of 2D materials is systematically investigated using numerical analysis. After assuming 2D materials as continuum thin films, we start by simulating the relationship between the pressure difference across the suspended film and the film deflection, and then fit the results with analytical models to extract the elastic modulus of the film. It should be noted that the clamped, pre-stretched film widely assumed in the existing analytical model is not appropriate for atomically thin materials; instead, freestanding 2D materials are actually slack in bulge testing due to the delamination of their adhesive boundary condition. Therefore, the initially unstressed flat films, pre-stretched films and slack films are investigated in the present work, and the Poisson’s ratio effect is considered to investigate the effectiveness of bulge testing for most 2D materials. It is found that bulge testing can be an effective approach to characterize the mechanical properties of 2D materials.

A Novel High-Speed Bulge Test to Identify the Large Deformation Behavior of Sheet Metals

A Novel High-Speed Bulge Test to Identify the Large Deformation Behavior of Sheet Metals

Anisotropic Mechanical Response of Bovine Pericardium Membrane Through Bulge Test and In-Situ Confocal-Laser Scanning.

In this work, we present a new experimental setup for the assessment of the anisotropic properties of Bovine Pericardium (BP) membranes. The chemically fixed BP samples have been subjected to a bulge test with in situ confocal laser scanning at increasing applied pressure. The high resolution topography provided by the confocal laser scanning has allowed to obtain a quantitative measure of the bulge displacement; after polynomial fitting, principal curvatures have been obtained and a degree of anisotropy (DA) has been defined as the normalized difference between the maximum and minimum principal curvatures. The experiments performed on the BP membranes have allowed us to obtain pressure-displacement data which clearly exhibit distinct principal curvatures indicating an anisotropic response. A comparison with curvatures data obtained on isotropic Nitrile Buthadiene Rubber (NBR) samples has confirmed the effectiveness of the experimental setup for this specific purpose. Numerical simulations of the bulge tests have been performed with the purpose of identifying a range of constitutive parameters which well describes the obtained range of DA on the BP membranes. The DA values have been partially validated with biaxial tests available in literature and with suitably performed uni-axial tensile tests.

BULGE TEST APPLICATION IN THE CHARACTERIZATION PROCESS OF ELASTOMER MATERIALS MEMBRANES

The mechanical characterization of elastomers is a complex task that requires several standardized tests to determine its behavior. When trying to character-ize membranes, the process requires sufficient material to be present. To en-sure that the obtained data is reliable, it is essential to validate the elastomer before using this information in a complex model simulation. This paper de-scribes the process of characterization using artificial intelligence techniques and the bulge test to determine the material constitutive parameters of elasto-mers. To achieve this goal, a test bench was designed. It includes an electronic control system to measure the bulge deformation through a laser sensor. Fi-nally, finite elements method (FEM) models were proposed. They replicate as much as possible the real test design. The Shell and Brick elements have been used to mesh the models’ membrane. The reliability of the characterization method has been proved by the results that were obtained. Deformation and stress values had fewer than 15% error when using Shell elements. This type of elements should be selected when developing future membrane models, thereby ensuring a fit between FEM and the problem under study. The small error obtained as well as the computational cost used suggest that the meth-odology proposed in this work could be a simpler and cheaper alternative to be used in the characterization of elastomers than other more sophisticated methods require

Deformation response research on aluminum sheets with different central pre-cracked defects under quasi-static hydraulic pressure loading

The crack behavior and bending deformation of pre-cracked sheet are coupled under the pressure loading. Two theoretical methods (SCMLD and PTELD) were used to compare and verify the bulge test data of intact aluminum alloy sheets, and the applicability of the two methods in equilibrium bulge test were discussed. ABAQUS extended finite element method (XFEM) and the parameters of fracture energy based on experiments were used to simulate the bulging deformation and failure behavior of pre-cracked sheet, the results agreed with the experimental test data well. What is more, the slope of h−p curve reflects the equivalent bending stiffness, which is affected by the crack shape length rather but not the crack shape.

Mechanism of considerable strain hardening in ultrafine-grained pure aluminum sheets at cryogenic temperatures

Research on the deformation behavior of ultrafine-grained (UFG) pure aluminum (Al) sheets was conducted at a temperature of −196 °C to provide a new strategy for forming complex components with UFG sheets. In this work, a UFG pure Al sheet with an average grain size of 0.9 μm was prepared by cold rolling (CR) and recovery annealing. Then, the mechanical behaviors of the UFG pure Al sheet were evaluated by uniaxial tensile tests and bulging tests. The fracture morphology, microstructure evolution and surface topography were observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The UFG pure Al exhibits considerable strain hardening and superior ductility at −196 °C, significantly different from the obvious strain softening behavior that occurs at room temperature (RT). As the temperature is decreased from RT to −196 °C, the uniform elongation (UEL) is improved 24.2 times from 1.2% to 30.2%, and the total elongation (TEL) is improved 3.3 times from 11.8% to 51.2%. The limiting dome height (LDH) and the ultimate bulging rate, K, of the UFG pure Al sheet at −196 °C are 1.6 times and 2.9 times greater than those at RT, respectively. Microstructure observations show that many dislocation substructures such as dislocation pile-ups, dislocation cells and dislocation networks are formed in UFG pure Al deformed at −196 °C, that would otherwise be absent in its counterpart deformed at RT. This unusual phenomenon is attributed to the remarkable transition of the dominant deformation mechanism of UFG pure Al from significant dynamic recovery accompanied by grain boundary sliding (GBS) to dislocation multiplication and accumulation with decreasing temperature.

Origins of improved elongation to fracture in cyclic bending under tension of AA6022-T4 sheets as revealed using crystal plasticity modeling

Plastic instabilities can be suppressed by imposing cyclic bending during tension to increase elongation-to-fracture (ETF) of metallic sheets. This work is a combined experimental and modeling study into the origins of the improved ETF in cyclic bending during tension facilitated by a continuous-bending-under-tension (CBT) test. Load-displacement is measured during the CBT test under an optimized combination of band depth and pull speed process parameters for aluminum alloy (AA) 6022-T4. Monotonic and load reversal response of the alloy is also characterized under strain histories that resemble the history in CBT. Electron-backscattered diffraction and neutron diffraction are employed to characterize the initial microstructure and texture evolution of the alloy after CBT and simple tension (ST). Monotonic, load reversal, and texture data are used to adjust and verify the parameters of an elasto-plastic self-consistent crystal plasticity-based finite element model for simulating CBT. Additionally, a flow stress curve based on a bulge test is used to calibrate the model to capture the large strain hardening behavior of the alloy. The model is then used to simulate the CBT test while predicting the load-displacement and texture evolution data. The model successfully reproduced the succession of spikes and plateaus during CBT validating the prediction of the extrapolated hardening. After the validation, the development of stress and strain is probed through the sheet thickness to determine the underlying profiles as well as along the sheet to elucidate the location and onset of failure axially. The model reveals the development of more strain and dislocation density at the surface then at the center of the sheet and correctly predicts the strain localizations. Comparisons of texture evolution and mechanical fields developed in ST and CBT reveal very similar mechanical states in the sheet. These results in combination with experimental results suggest that the origins of improved ETF during CBT pertain primarily to the incremental deformation structural effects and secondary to the microstructural evolution effects.

Direct stress computations in arbitrarily shaped thin shells and elliptic bulge tests

Most of engineering and biological thin shell structures are characterized by non-axisymmetric and statically indeterminate configurations, which often require the knowledge of the constitutive material model to determine the stress state. In this work, a forward elastostatic method is introduced for the direct stress measurements in elastic homogeneous thin shells of arbitrary shape. The stress distribution is proven to be independent of the material properties for incompressible solids, while in compressible materials it depends only on the Poisson’s ratio, which is shown to have a negligible influence on the stress state. Hence, the proposed technique enables the direct assessment of the stress field in statically indeterminate thin shells without a known material model. The shell formulation is implemented using the finite difference method to independently measure the stresses during finite inflation of planar elliptical membranes and from the deformed shapes obtained through digital image correlation during bulge tests on a hyperelastic material, showing very good agreement with finite-element predictions and the applicability of the method to nonlinear elastic materials. Therefore, the procedure can be coupled with imaging techniques for the direct assessment of stresses in thin shell structures and in the identification of material parameters through non-axisymmetric bulge tests.

Effectiveness of the analytical models used in standard techniques for mechanical property characterization of 2D materials

Two dimensional (2D) materials are usually assumed to be continuum thin films to study their mechanical behavior. Freestanding indentation testing and pressure bulge testing are two standard techniques for characterizing the mechanical properties of thin films. Compared with the former, the latter is far less often used to characterize the elastic moduli of 2D materials, even though there is no solid evidence to show that the former is more effective than the latter. In the present work, the effectiveness of the analytical models adopted in both approaches on determining the elastic moduli of 2D materials are investigated using numerical analysis, which can be evaluated based upon the accuracy of the determined elastic moduli, and the effects of Poisson’s ratio and the applied loading range are also considered. To account for the effect of the adhesive boundary condition of freestanding 2D materials in the testing, the numerical simulations of the initially unstressed samples, pre-stretched samples and slack samples are all analyzed in the present work. It is found that the bulge testing model is actually more effective than the indentation model on determining the mechanical properties of 2D materials.

Numerical simulation of sheet metal forming of an Al-Mg alloy incorporating plane strain properties in the yield criterion

In this work, numerical simulation of AA5083-O aluminum alloy sheets is carried out using Yld2000-2d yield criterion to predict thinning and variation of load with displacement in sheet metal forming. It has been found that the accuracy of predictions in numerical simulation depends on the coefficients and the stress exponent used in the yield criterion. Hydraulic bulge tests with circular and elliptical die cavities are performed to characterize the material in equi-biaxial and plane strain conditions. An improved stress exponent is determined by incorporating the material properties obtained in plane strain in the existing criterion. The improved stress exponent is validated by using it in the numerical simulations of cylindrical cup deep drawing and stretch forming at room temperature. The same exponent is also used in the numerical simulations carried out for the case of experiments conducted at 250°C. The predicted thinning and load-displacement variations from the numerical simulations are validated with the experimental measurements. A considerable improvement in the results predicted using the improved stress exponent is observed at both the temperatures.

In-situ irradiation creep behavior of dilute Al-Sc alloys: Using a thin film bulge test method

• In situ thin film bulge test on nc-AlSc is effective method for irradiation induced creep. • Preexisting Al 3 Sc precipitates suppress irradiation induced creep deformation. • Transition regime of AlSc binary system is located between room temperature and 75 °C. In-situ irradiation induced creep(IIC) measurements on a nanocrystalline AlSc 1.1 binary alloy were performed using a thin-film bulge test technique. A thin film bulge technique was employed since any substrate effect can be excluded, and hence is beneficial to analyze the intrinsic properties of specimen. Here, an as-deposited film and a pre-annealed film, both underwent heavy ion irradition with 1.8 MeV Kr ions. Al 3 Sc precipitates in the pre-annealed samples appear to reduce the irradiation creep compliance. Dissolution of these precipitates during irradiation led to increased irradiation creep rates. Microstructural evolution during irradiation was investigated by transmission electron microscopy and high resolution transmission electron microscopy.