Bulge Test Technique Research Articles

Bulge test technique and digital image correlation for the determination of the biaxial behavior of polymeric films

Abstract Materials can display multiple mechanical behaviors when they are biaxially stressed. In this research work, films obtained through the blowing process were investigated in order to compare the mechanical properties and behavior of the film under uniaxial and biaxial stress conditions. Bulge test coupled with a continuous full-field strain measurement was undertaken in order to characterize polymeric film in a biaxial stress state. As far as this study is concerned, a laboratory-made pneumatic bulge test machine was used for the determination of the biaxial behavior of a 50 μm thickness PLSF (modified PLA) film. 3D-digital image correlation (3D-DIC) technique was used in order to compute the strain field continuously during the test. Tensile test was equally performed on the film along its machine direction (MD) and transverse direction (TD). Strain field distribution in the film sample was analyzed and compared in both tests. The tensile test results displayed an isotropic behavior of the film in both directions. However, a difference in the strain at break was noticed because of the thickness instability along the TD. The comparison between uniaxial and biaxial properties resulted in almost the same properties with respect to the Young Modulus as well as the tensile strength.

Numerical investigation on circular and elliptical bulge tests for inverse soft tissue characterization.

The acquisition of insights concerning the mechanobiology of aneurysmatic aortic tissues is an important field of investigation. The complete characterization of aneurysm mechanical behaviour can be carried out by biaxial experimental tests on ex vivo specimens. In literature, several works proposed bulge inflation tests as a valid method to analyse aneurysmatic tissue. Bulge test data processing requires the adoption of digital image correlation and inverse analysis approaches to estimate strain and stress distributions, respectively. In this context, however, the accuracy of inverse analysis method has not been evaluated yet. This aspect appears particularly interesting given the anisotropic behaviour of the soft tissue and the possibility to adopt different die geometries. The goal of this study is to provide an accuracy characterization of the inverse analysis applied to the bulge test technique using a numerical approach. In particular, different cases of bulge inflation were simulated in a finite element environment as a reference. To investigate the effect of tissue anisotropic degree and bulge die geometries (circular and elliptical), different input parameters were considered to obtain multiple test cases. The specimen deformed shapes, resulting from the reference finite element simulations, were then analysed through an inverse analysis approach to produce an estimation of stress distributions. The estimated stresses were, at last, compared with the values from the reference finite element simulations. The results demonstrated that the circular die geometry produces a satisfactory estimation accuracy only under certain conditions of material quasi-isotropy. On the other hand, the choice of an elliptical bulge die was proven to be more suitable for the analysis of anisotropic tissues.

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.

Flow-induced bending deformation of electrospun polymeric filtration membranes using the “leaky” bulge test

In service, hydraulic flow is usually perpendicular to the plane of electrospun fiber mats for water filtration, inducing an out-of-plane deformation. In this paper, we have investigated the out-of-plane deformation response of wet electrospun polyacrylonitrile fiber mats with distinct average constituent fiber diameters of 232 ± 36 nm, 727 ± 148 nm and 1017 ± 80 nm, but with nominally similar thicknesses and areal densities using the classical bulge testing technique. The resultant pressure-deflection relationships are incompatible with constitutive models for out-of-plane deformation of continuum materials. Rather, the fiber mats exhibited a one-half dependence (p∼h12) governed by fiber deflection, and pressure asymptote indicative of steady-state fluid percolation, but influenced by the mats areal densities. In addition, the apparent bending rigidity increased with increasing fiber diameter. In the range tested for fiber mats with average fiber size of 232 nm, increase in mat thickness did not noticeably alter measured rigidities, although evidence of slight increase in pressure asymptote is observed. Cyclic flow rate tests provided evidence of good recoverability as well as minimal topological modifications.

Elastic and fracture properties of free-standing amorphous ALD Al2O3 thin films measured with bulge test

We have investigated elastic and fracture properties of amorphous Al2O3 thin films deposited by atomic layer deposition (ALD) with bulge test technique using a free-standing thin film membrane and extended applicability of bulge test technique. Elastic modulus was determined to be 115 GPa for a 50 nm thick film and 170 GPa for a 15 nm thick film. Residual stress was 142 MPa in the 50 nm Al2O3 film while it was 116 MPa in the 15 nm Al2O3 film. Density was 3.11 g cm−3 for the 50 nm film and 3.28 g cm−3 for the 15 nm film. Fracture strength at 100 hPa s−1 pressure ramp rate was 1.72 GPa for the 50 nm film while for the 15 nm film it was 4.21 GPa, almost 2.5-fold. Fracture strength was observed to be positively strain-rate dependent. Weibull moduli of these films were very high being around 50. The effective volume of a circular film in bulge test was determined from a FEM model enabling future comparison of fracture strength data between different techniques.

A study on thermo-mechanical behavior of MCD through bulge test analysis

The Micro circular diaphragm (MCD) is the mechanical actuator part used in the micro electro-mechanical sensors (MEMS) that combine electrical and mechanical components. These actuators are working under harsh mechanical and thermal conditions, so it is very important to study the mechanical and thermal behaviors of these actuators, in order to do with its function successfully. The objective of this paper is to determine the thermo-mechanical behavior of MCD by developing the traditional bulge test technique to achieve the aims of this work. The specimen is first pre-stressed to ensure that is no initial deflection before applied the loads on diaphragm and then clamped between two plates, a differential pressure (P) and temperature (Tb) is leading to a deformation of the MCD. Analytical formulation of developed bulge test technique for MCD thermo-mechanical characterization was established with taking in-to account effect of the residual strength from pre-stressed loading. These makes the plane-strain bulge test ideal for studying the mechanical and thermal behavior of diaphragm in both the elastic and plastic regimes. The differential specimen thickness due to bulge effect to describe the mechanical behavior, and the temperature effect on the MCD material properties to study the thermal behavior under deformation were discussed. A finite element model (FEM) can be extended to apply for investigating the reliability of the proposed bulge test of MCD and compare between the FEM results and another one from analytical calculus. The results show that, the good convergence between the finite element model and analytical model.

Characterization Method of Thick Films Using the Bulge Test Technique

A new theoretical model considering the bending effect for the thick films (the ratio η of thickness to radius was larger than 1/80) using the bulge test method was established based on the energy method. The experiments of Ti films and PVDF films of different ratio η had validated the feasibility of the new theory. The universal applicability of the new theoretical model for different mechanical parameters (E and υ) was verified by FEM. The new expression of pressure-height could well predict the deformation of bulge test for the films with different thickness. As the ratio η increased, the competitive mechanism of bending energy gradually replacing stretching energy in the films was revealed. The modulus and the stress distribution of cross-section could be characterized correctly by the new theory for thick films using the bulge test technique. It will provide a powerful research tool to study the deformation mechanism of thick films in the bulge test.

Strain Evaluation and Fracture Properties of Hetero-Epitaxial Single Crystal 3C-SiC Squared Membrane

The following paper explores the development the bulge test technique combined with the micro-Raman analysis and a refined load-deflection model for high quality 3C-SiC squared-membranes. By the minimization of the total elastic energy, starting from the isotropic relation between the stress tensor and the strain tensor, it is possible to calculate the relationship between the maximum deflection and the applied pressure, in both regime of small and large deflection. From the measured breaking pressure through the refined model it is possible to evaluate the breaking strain of the membrane. Furthermore, the relationship between the measured shift of Raman Transverse Optical (TO) phonon modes and the total residual strain (Δa/a) within the epitaxial 3C-SiC layer was found.

Novel scanning electron microscope bulge test technique integrated with loading function.

Membranes and film-on-substrate structures are critical elements for some devices in electronics industry and for Micro Electro Mechanical Systems devices. These structures are normally at the scale of micrometer or even nanometer. Thus, the measurement for the mechanical property of these membranes poses a challenge over the conventional measurements at macro-scales. In this study, a novel bulge test method is presented for the evaluation of mechanical property of micro thin membranes. Three aspects are discussed in the study: (a) A novel bulge test with a Scanning Electron Microscope system realizing the function of loading and measuring simultaneously; (b) a simplified Digital Image Correlation method for a height measurement; and (c) an imaging distortion correction by the introduction of a scanning Moiré method. Combined with the above techniques, biaxial modulus as well as Young's modulus of the polyimide film can be determined. Besides, a standard tensile test is conducted as an auxiliary experiment to validate the feasibility of the proposed method.

Accuracy analysis of plane-strain bulge test for determining mechanical properties of thin films

The effect of a variety of geometrics, initial conditions and material properties on the deformation behavior of thin films in the plane-strain bulge test was systematically scrutinized by performing the finite element analysis, and then the accuracy of the plane-strain bulge test in determining the mechanical properties of thin films in terms of our finite element results was analyzed. The results indicate that although the determination of the plane-strain modulus in the light of the plane-strain bulge equation is fairly accurate, the calculation of the residual stress is not satisfied as expected, especially for low residual stress. Finally, an approach is proposed for analyzing bulge test data, which will improve the accuracy and reliability of this bulge test technique.

Evaluation of Mechanical and Optical Properties of Hetero-Epitaxial Single Crystal 3C-SiC Squared-Membrane

The aim of this work is to develop the bulge test technique combined with the micro-Raman analysis and a refined load-deflection model for high quality 3C-SiC squared-membranes. By the minimization of the total elastic energy, starting from the isotropic relation between the stress tensor and the strain tensor, it is possible to calculate the relationship between the maximum deflection and the applied pressure, in both regime of small and large deflection. Furthermore, the relationship between the measured shift of Raman Transverse Optical (TO) phonon modes and the total residual strain (Δa/a) within the epitaxial 3C-SiC layer was found and in order to understand the stress distribution within the membrane, TO Raman shift maps were performed along the corner and the border of the membrane.

On the validity regime of the bulge equations

Abstract

Effect of cation contamination and hydrated pressure loading on the mechanical properties of proton exchange membranes

Perfluorosulfonic acid (PFSA) polymer membranes are widely used as electrolyte thin films to transport protons in proton exchange membrane (PEM) fuel cells. The mechanical degradation of the membrane represents a common failure mode that limits the operational life of the fuel cells. In the present work, effect of contamination related to cation exchange on the mechanical reliability of PEMs was investigated. We applied the bulge test technique to assess the mechanical properties of Nafion ® PFSA membranes simulating pressure loading on hydrated PEMs in fuel cells. The corresponding elastic moduli of Nafion ® before and after cation exchange were analyzed and compared with the results measured by uniaxial tension experiments at selected humidity conditions, showing increasing stiffness with the increase of cation radius. We also used the out-of-plane tearing test method to characterize the fracture behaviors of PEMs. The effects of cation exchange and water absorption on mechanical and fracture properties of PEMs at different temperatures are discussed in terms of cation and water interactions with the molecular structure of PFSA polymers.

Strength and aging behavior of Mo/Pt multilayers

The strength of Mo/Pt multilayers as a function of layer thickness has been investigated using nanoindentation. Structures with layer thicknesses varying from 20 to 100 nm were sputter deposited and the hardness of the films was measured using nanoindentation. It is shown that at this length scale the strengthening behavior of the films can be described by the confined layer slip model which has been proposed for other multilayered systems. Of specific interest in this study was the increase in the strength of the films after aging in ambient conditions with no apparent change in modulus. X-ray photoelectron spectroscopy revealed partial oxidation of the Mo layers in the aged samples which was associated with an increase in the residual compressive stress of the films measured by the bulge testing technique. The formation of molybdenum oxides in the Mo layer could account for the increase in strength through a second phase precipitation strengthening mechanism.

Methods to improve reliability of bulge test technique to extract mechanical properties of thin films

Knowledge of thin films mechanical properties is strongly associated to the reliability and the performances of Micro Electro Mechanical Systems (MEMS). In the literature, there are several methods for micro materials’ characterization. Bulge test is an established non destructive technique for studying the mechanical properties of thin films. This technique is easy to implement and to use, but there are important issues that can distort the characterization results. In this paper, we set up a rigorous experimental protocol in order to improve the reliability of mechanical characterization by bulging test. First we analyse and examine the parameters involved in the test bench. Then we propose some methods to improve the accuracy of the measurement. Crystalline silicon free-standing membranes were fabricated and characterized in order to calibrate the test bench.

Investigation of mechanical properties of gold thin films using a bulge test technique

A bulge test device has been built, with the aim to perform mechanical tests on membranes with a thickness in the 100 nm to 10 μm ranges, between room temperature and 900 °C.Our set up a differential atmospheric pressure is applied across a membrane, while the deflection is measured using a laser interferometer. We show first results obtained from gold membranes with different thicknesses.

Combined experimental and numerical analysis of bulge test at high strain rates using split Hopkinson pressure bar apparatus

High strain rate bulge test technique which is introduced in this paper adopts a rubber-pad as pressure carrying medium to bulge a sheet metal at high velocity using split Hopkinson pressure bar (SHPB) system. The experimental set-up is based on conventional hydraulic bulge test which is modified to mount on SHPB. The thickness thinning of the sheet metal during the test will be considered as a measure of true strain of the bulged sheet. The theoretical approach is developed in this study to attain pressure–strain curves of sheet metals during high strain rate bulge forming process. This approach is followed by a finite element simulation of the process in ABAQUS/Explicit software. To verify the developed method, analytical and finite element methods are compared with experiments.

Plastic bulging of sheet metals at high strain rates

The biaxial bulge test is a material test for sheet metals to evaluate formability and determine the flow stress diagram. Due to the biaxial state of stress induced in this test, the maximum achievable strain before fracture is much larger than in the uniaxial tensile test. A new dynamic bulge testing technique is simulated and analyzed in this study which can be performed on a conventional split Hopkinson pressure bar (SHPB) system to evaluate the strain-rate dependent strength of material at high impact velocities. Polyurethane rubber as pressure carrying medium is used to bulge the OFHC copper sheet. The use of hyperelastic rubber instead of fluid as a pressure medium makes the bulge test simple and easy to perform. The input bar of SHPB is used to apply and measure the bulging pressure. The finite element simulation using ABAQUS/explicit and analytical analysis are compared and show good correlation with each other. The results clearly show that as the strain-rate increases, the strength of the OFHC copper increases. From the study, a robust method to determine the material behavior under dynamically biaxial deformation conditions has been developed.

Size effects on yield strength and strain hardening for ultra-thin Cu films with and without passivation: A study by synchrotron and bulge test techniques

We present a systematic study of the mechanical properties of different Cu, Ta/Cu and Ta/Cu/Ta films systems. By using a novel synchrotron-based tensile testing technique isothermal stress–strain curves for films as thin as 20 nm were obtained for the first time. In addition, freestanding Cu films with a minimum thickness of 80 nm were tested by a bulge testing technique. The effects of different surface and interface conditions, film thickness and grain size were investigated over a range of film thickness up to 1 μm. It is found that the plastic response scales strongly with film thickness but the effect of the interfacial structure is smaller than expected. By considering the complete grain size distribution and a change in deformation mechanism from full to partial dislocations in the smallest grains, the scaling behavior of all film systems can be described correctly by a modified dislocation source model. The nucleation of dissociated dislocations at the grain boundaries also explains the strongly reduced strain hardening for these films.

Biaxial Testing of Sheet Materials at High Strain Rates Using Viscoelastic Bars

A dynamic bulge testing technique is developed to perform biaxial tests on metals at high strain rates. The main component of the dynamic testing device is a movable bulge cell which is directly mounted on the measuring end of the input bar of a conventional split Hopkinson pressure bar system. The input bar is used to apply and measure the bulging pressure. The experimental system is analyzed in detail and the measurement accuracy is discussed. It is found that bars made of low impedance materials must be used to achieve a satisfactory pressure measurement accuracy. A series of dynamic experiments is performed on aluminum 6111-T4 sheets using viscoelastic nylon bars to demonstrate the capabilities of the proposed experimental technique. The parameters of the rate-dependent Hollomon–Cowper–Symonds J2 plasticity model of the aluminum are determined using an inverse analysis method in conjunction with finite element simulations.