Biaxial Tensile Tests Research Articles

結晶塑性有限要素法によるアルミニウム合金板の数値二軸引張試験と成形シミュレーション

結晶塑性有限要素法によるアルミニウム合金板の数値二軸引張試験と成形シミュレーション

Synthetic surgical meshes used in abdominal wall surgery: Part II-Biomechanical aspects.

This work reports the second part of a review on synthetic surgical meshes used for abdominal hernia repair. While material and structural characteristics, together with mesh-tissue interaction, were considered in a previous article (Part I), biomechanical behavior is described here in more detail. The role of the prosthesis is to strengthen the impaired abdominal wall, mimicking autologous tissue without reducing its compliance. Consequently, mesh mechanical properties play a crucial role in a successful surgical repair. The main available techniques for mechanical testing, such as uniaxial and biaxial tensile testing, ball burst, suture retention strength, and tear resistance testing, are described in depth. Among these methods, the biaxial tensile test is the one that can more faithfully reproduce the physiological loading condition. An outline of the most significant results documented in the literature is reported, showing the variety of data on mesh mechanical properties. Synthetic surgical meshes generally follow a non-linear stress-strain behavior, with mechanical characteristics dependant on test direction due to mesh anisotropy. Ex-vivo tests revealed an increased stiffness in mesh explants due to the gradual ingrowth of the host tissue after implant. In general, the absence of standardization in test methods and terminology makes it difficult to compare results from different studies. Numerical models of the abdominal wall interacting with surgical meshes were also discussed representing a potential tool for the selection of suitable prostheses. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 892-903, 2017.

Inflation and Bi-Axial Tensile Testing of Healthy Porcine Carotid Arteries

Knowledge of the intrinsic material properties of healthy and diseased arterial tissue components is of great importance in diagnostics. This study describes an in vitro comparison of 13 porcine carotid arteries using inflation testing combined with functional ultrasound and bi-axial tensile testing. The measured tissue behavior was described using both a linear, but geometrically non-linear, one-parameter (neo-Hookean) model and a two-parameter non-linear (Demiray) model. The shear modulus estimated using the linear model resulted in good agreement between the ultrasound and tensile testing methods, GUS = 25 ± 5.7 kPa and GTT = 23 ± 5.4 kPa. No significant correspondence was observed for the non-linear model aUS = 1.0 ± 2.7 kPa vs. aTT = 17 ± 8.8 kPa, p ∼ 0); however, the exponential parameters were in correspondence (bUS = 12 ± 4.2 vs. bTT = 10 ± 1.7, p > 0.05). Estimation of more complex models in vivo is cumbersome considering the sensitivity of the model parameters to small changes in measurement data and the absence of intraluminal pressure data, endorsing the use of a simple, linear model in vivo.

A Study on Yield Function for Ti–6Al–4V Titanium Alloy Sheets at Elevated Temperatures

Currently, the finite element (FE) method simulation technique is being widely applied to optimize the forming process. To achieve high-accuracy FE simulation results, the identification of material properties and deformation characteristics such as yield criteria is important. In this study, characterisation of plastic deformation behavior of Ti–6Al–4V titanium alloy using anisotropic sheets were carried out using, biaxial tensile tests at 600 °C with a cruciform specimen. The test was performed until the specimen was broken down. The experimental results for the yield locus were first compared with other theoretical models to show the mismatch of the conventional function. A new yield function was then proposed in this study to describe not only the anisotropic but also the asymmetric behavior between the tension and compression of Ti–6Al–4V titanium alloy sheets using a fitting equation to determine the material constants of the yield function. Finally, yield loci at other elevated temperatures were obtained based on the relationship between tension yield stress and temperature.

Design of a Testing Device for Cruciform Specimens Subjected to Planar Biaxial Tension

Multiaxial experiments are necessary to determine materials behavior subjected to complex stress state corresponding to real operating conditions under complex loadings. Stresses applied in biaxial experiments are closely to the stresses that materials experience during their function life. Because of reduced acquisition cost, operation cost and maintenance costs devices attached to the universal testing machine are beginning to be used in ever more applications. The present paper examines a new type of mechanism used to test biaxial cruciform specimens in order to evaluate stress state in planar biaxial tensile testing.

Design of a Flat Plate Specimen Suitable for Biaxial Tensile Tests on Polymer Materials

Since the introduction of fibre-reinforced polymer composites there has been a surge in the use of adhesives for joints and repairs, and polymer resins as the matrix material for fibre-reinforced composites. The failure mechanisms of these materials have been studied by many researchers; however, there is little accurate experimental data under tension-tension loading published. This is due to the lack of a standard specimen design to perform these tests. The authors propose a flat plate specimen design that has been shown to overcome some of the difficulties presented in literature. A series of tests with this specimen configuration were conducted under varying biaxial loading conditions. The results are plotted in the tension-tension quadrant of the materials failure envelope. For the two polymer materials tested; a linear truncation within the first quadrant of the failure envelope was found.

Ultraviolet Radiation Affects Thoratec HeartMate II Driveline Mechanical Properties: A Pilot Experiment.

Longevity and quality of life for left ventricular assist device (LVAD) patients are plagued by driveline exit site infections. Ultraviolet (UV) radiation, a current treatment in wound healing clinics, could potentially treat LVAD exit site infections. However, the effect of UV radiation on the tensile properties of HeartMate II (HMII) driveline material is unknown. The sleeve of a single HMII driveline was distributed into six exposure groups (n = 10/group). The six groups were further divided into two treatment cohorts designed to replicate wound treatment schedules of postimplant LVAD patients. Strip biaxial tensile tests were performed on both unexposed and exposed samples to analyze changes in material elasticity (Young's modulus), point of deformation (yield strength), and breaking point. Our data suggest that UV exposure changes the elasticity of the HMII driveline. However, the material endured aberrantly large forces and the properties remained within the safety threshold of device performance. This study warrants further examination of the effect of UV light on driveline material, to determine safety, reliability, and efficacy of UV treatment on exit site infections.

Characterization of short fiber reinforced polymers

AbstractThis contribution presents ideas, how composite materials can be characterized with respect to experimental testing. The material properties of the investigated short glass fiber reinforced polymer are obtained by providing results from the experiment in order to seperate different material effects, such as elasticity, plasticity, damage, viscoelasticity, compressibility and anisotropy. Therefore, at first, linear uniaxial tensile tests with cyclic loadings have been realized. The application of the material in this work is the machining by a three‐dimensional forming process. Hence, multiaxial loadings have to be additionally taken into account matching these conditions. In order to provide more information, biaxial tensile tests have to be realized using a testing device supplying the additional necessary experimental data [1, 2]. A final aim of this work is to develope a verification experiment representing the three‐dimensional forming process as realistically as possible, e. g. a Nakajima test [4]. For this case, a three‐dimensional optical analysis in order to get the necessary measurement data, is indispensable to realize an inverse method [3] by comparing the information of the complete deformation field as well as the force data of the experiment using a given material model. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Parameter Identification by Inverse Modelling of Biaxial Tensile Tests for Discontinous Fiber Reinforced Polymers

AbstractIn the present article we discuss the characterization of the mechanical properties of long fiber reinforced thermoplastics (LFT) and sheet molding compounds (SMC) with biaxial tensile tests and the inverse parameter identification. The full 3D strain field is measured via digital image correlation (DIC). The anisotropic viscoelastic material properties are identified through inverse modelling by comparison of the heterogeneous experimental and simulated strain fields. A Gauss‐Newton type algorithm is used to identify the optimal parameter set [1]. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

THE DIRECTIONAL DEPENDENCY OF ASCENDING AORTA BIOMECHANICS DISAPPEARS WITH SEVERE MEDIAL DEGENERATION

The study of biomechanics can potentially provide insight into the pathogenesis of aortic complications and improve risk assessment. Aortas are described to have anisotropy, the directional dependency of mechanical properties. We hypothesize that disease-related remodelling of the aortic wall microstructure found in ascending aortic aneurysms results in changes in anisotropy. Excised aneurysmal ascending aortic tissue was obtained at the time of surgery and control tissue was obtained at organ donation or autopsy. The tissue was subjected to biaxial tensile testing. Stress-strain relationships were collected in both the circumferential and axial directions; from this data, both the apparent modulus of elasticity and energy loss were derived. Energy loss describes normalized hysteresis between aortic loading and unloading curves. Movat pentachrome histological staining was performed on each sample and aortic wall composition was quantified colorimetrically. Energy loss was greater in the circumferential direction than in the axial direction for both aneurysmal and control aortas (p<0.0001), while the modulus of elasticity was the same in both directions. This directional dependency in energy loss became diminished in larger aortas (r2=0.15, p=0.01), especially when indexed to body surface area (r2=0.29, p=0.002), aortas with greater energy loss (r2=0.44, p<0.0001), and aortas associated with tricuspid valves (p=0.004). An inflection point was noted in disease severity separating two populations: aortas without any directional dependency in energy loss (isotropic), and those with varying degrees of anisotropy. Aortas with indexed aortic size greater than 3.5 cm/m2 or energy loss greater than 35% were uniformly isotropic. Evaluating the microstructure, aortas with elevated collagen-to-elastin ratios (greater than 2) reflecting severe medial degeneration, were also uniformly isotropic. Aneurysmal aortas with higher collagen-to-elastin ratios tended towards isotropy (r2=0.29, p=0.001). Energy loss detects the anisotropic nature of aortic tissue unlike the apparent modulus of elasticity; this is further evidence that it may be a more sensitive and physiologically relevant metric in the evaluation of aneurysmal disease. Isotropy is associated with severe medial degeneration indicating that destruction of microstructure can be captured by global biomechanics, thereby identifying energy loss isotropy as a marker of disease severity.

Standardized test methods for mechanical properties of dental prosthetic/restorative materials and their applications

,4) A number of mechanical test protocols have been developed to ev aluate the mechanical properties for dental prosthetic and conservative materials. This review article presents and discusses internationally standardized test methods including uniaxial and biaxial flexural test, compressive and diametral tensile tests, Weibull reliability test, fracture toughness tests, and their applications. There has been little information on the relationship between the results from standardized tests and clinical performance for dental materials. Thus, the mechanical properties resulting from standard tests can merely be used in evaluation of mechanical quality among various candidates. For more practical evaluation of mechanical performance of dental materials, there are needs for developing a quasi-clinical test protocol under considerations with stress concentration and distribution and clinical complexity. Furthermore, those results should be compared with the mechanical properties resulting from standardized tests.

Identification of sheet metal hardening for large strains with an in-plane biaxial tensile test and a dedicated cross specimen

In this work, an in-plane biaxial tensile test of cruciform specimen is performed to identify the hardening behaviour of metallic sheets under large strains. Firstly, an optimal shape of the specimen is suggested. Then, a biaxial tensile test is carried out for an aluminium alloy AA5086. Experimental forces on the two axes of the specimen are measured during the test and strains in the central area of the specimen are post-treated by means of Digital Image Correlation (DIC) technique. Finally, by considering different yield criteria, the associated hardening laws are identified thanks to an inverse procedure based on a Finite Element (FE) modelling of the biaxial tensile test and on the experimental data mentioned above. The identified biaxial flow curves are then compared with the ones from the classical uniaxial tensile test.

Mechanical characterization of aortic valve tissues using an inverse analysis approach

The use of numerical simulation to investigate heart and valvular mechanics is becoming increasingly popular. In particular, finite element analysis is often used to support the operation planning procedure as well as the design of new prostheses with mechanical properties as close as possible to those of natural tissues and an even better biocompatibility. With one of the highest prevalence of cardiovascular degenerative diseases [1], aortic valves (AV) have been widely studied during the last decades. The elastic [2] and time-dependent [3] behaviors of the AV leaflets under physiological biaxial loading states have been previously investigated in the literature over a wide range of loading conditions.. As most soft tissues, AV has an oriented network of collagen fibers embedded in an elastin matrix, which is responsible for their hyperelastic and anisotropic behaviors. Accordingly, non-linear transverse isotropic constitutive equations are often used assuming a macroscopically-identifiable preferred fiber direction. In this study a new method is proposed in order to estimate relevant material and structural properties of AV while reducing at the same time the number of complex and time-consuming experiments. An inverse analysis procedure based on the finite element computation of planar biaxial tensile tests was used to set-up a reduced protocol. This protocol was then experimentally reproduced to identify real material parameters. The obtained material parameters will be later used to model heart valve tissues.

J‐Integral from Full Field Kinematic Data for Natural Rubber Compounds

AbstractThe application of the digital image correlation technique to the determination of the J‐integral at fracture initiation for carbon black‐filled natural rubber compounds is discussed. Three different compounds with varying carbon black content were tested, using two different test configurations: pure shear and biaxial tensile test. Digital image correlation was used to measure the displacement field around the crack tip in the tested specimens. From the displacement field, which was interpolated using a finite element scheme, the stresses were evaluated by using Ogden's hyperelastic model, and the J‐integral could be calculated. The results compare well with both theoretical and finite element results.

Design of Bioprosthetic Aortic Valves using biaxial test data.

Bioprosthetic Aortic Valves (BAVs) do not have the serious limitations of mechanical aortic valves in terms of thrombosis. However, the lifetime of BAVs is too short, often requiring repeated surgeries. The lifetime of BAVs might be improved by using computer simulations of the structural behavior of the leaflets. The goal of this study was to develop a numerical model applicable to the optimization of durability of BAVs. The constitutive equations were derived using biaxial tensile tests. Using a Fung model, stress and strain data were computed from biaxial test data. SolidWorks was used to develop the geometry of the leaflets, and ABAQUS finite element software package was used for finite element calculations. Results showed the model is consistent with experimental observations. Reaction forces computed by the model corresponded with experimental measurements when the biaxial test was simulated. As well, the location of maximum stresses corresponded to the locations of frequent tearing of BAV leaflets. Results suggest that BAV design can be optimized with respect to durability.

Cruciform specimen to obtain higher plastic deformation in a gauge region

The objective of this study was to develop a new cruciform sample for a biaxial tensile test. The goal was to concentrate the plastic deformation in a gauge region. To achieve such a behaviour, arms of the cruciform sample were initially reinforced. To verify the proposed idea, a test rig was built, this allowed for the bi-axial tensile tests. The research was carried out on a cold-rolled DC5 steel specimen cut with a water jet. The sample, in its proposed shape, reaches almost 8% plastic strain before fracture and seems to be an interesting solution for further investigation.

Development of a Global Indicator to Assess Mechanical Tests

Full-field measurement methods have emerged in the last years and these methods are characterized by directly providing displacement and strain fields for all points over the specimen surface. Thus, the design of heterogeneous tests can be performed for material parameter identification purposes since the inhomogeneous strain fields can be measured. However, (i) no defined criterion yet exists for designing new heterogeneous tests, (ii) it is rather difficult to compare and rate different tests and (iii) a quantitative way to define the best test for material behavior characterization of sheet metals has yet to be proposed. Due to this, the goal of this work is the development of a global indicator able to assess mechanical tests. The proposed indicator quantifies the strain state range, the deformation heterogeneity and the strain level achieved in the test, based on a continuous evaluation of the strain field up to rupture. This global indicator was applied to rank some classical tests, such as uniaxial tensile, simple shear, plane strain and biaxial tensile tests. These tests were carried out numerically by reproducing the virtual behavior of DC04 mild steel. A constitutive model composed by the non-quadratic Yld2004-18p yield criterion combined with a mixed isotropic-kinematic hardening law and a macroscopic rupture criterion was used. The performance of the tests was compared with the indicator and a ranking was established. The results obtained show that biaxial tension is the test providing more information for the mechanical behavior characterization of the material. It was also verified that plane strain test presents a better performance than simple shear and uniaxial tensile tests.

A generalized thermodynamically consistent distortional hardening model for Mg alloys

In the current work, a generalized distortional hardening model is developed by assuming each part of dissipation non-negative. More precisely, quadratic forms are assumed for dissipation and thermodynamical consistency is fulfilled naturally. In order to balance the contributions of isotropic and distortional hardening into dissipation, a weighting coefficient w is introduced in contrast to the work in Feigenbaum and Dafalias (2007). It is found that the saturation directions of distortional hardening are governed by this coefficient w, which is related with deformation modes. In particular, by setting the coefficient w = 2, the current generalized model recover to the one based on convex plastic potential, cf. Shi and Mosler (2013), and thus Armstrong–Frederick type distortional hardening law is obtained. The predictive ability of the current distortional hardening model is verified by comparison with experimental observations in three cases: stress-strain behaviours along different directions, biaxial tensile test and biaxial compression test. The evolution of normalized yield surfaces for the three cases is briefly discussed.

Biomechanical properties and microstructure of human ventricular myocardium

In the multidisciplinary field of heart research it is of utmost importance to identify accurate myocardium material properties for the description of phenomena such as mechano-electric feedback or heart wall thickening. A rationally-based material model is required to understand the highly nonlinear mechanics of complex structures such as the passive myocardium under different loading conditions. Unfortunately, to date there are no experimental data of human heart tissues available to estimate material parameters and to develop adequate material models. This study aimed to determine biaxial extension and triaxial shear properties and the underlying microstructure of the passive human ventricular myocardium. Using new state-of-the-art equipment, planar biaxial extension tests were performed to determine the biaxial extension properties of the passive ventricular human myocardium. Shear properties of the myocardium were examined by triaxial simple shear tests performed on small cubic specimens excised from an adjacent region of the biaxial extension specimens. The three-dimensional microstructure was investigated through second-harmonic generation (SHG) microscopy on optically cleared tissues, which emphasized the 3D orientation and dispersion of the myofibers and adjacent collagen fabrics. The results suggest that the passive human LV myocardium under quasi-static and dynamic multiaxial loadings is a nonlinear, anisotropic (orthotropic), viscoelastic and history-dependent soft biological material undergoing large deformations. Material properties of the tissue components along local microstructural axes drive the nonlinear and orthotropic features of the myocardium. SHG microscopy investigation revealed detailed information about the myocardial microstructure due to its high resolution. It enabled the identification of structural parameters such as the fiber and the sheet orientations and corresponding dispersions. With this complete set of material data, a sophisticated material model and associated material parameters can be defined for a better description of the biomechanical response of the ventricular myocardium in humans. Such a model will lead to more accurate computational simulations to better understand the fundamental underlying ventricular mechanics, a step needed in the improvement of medical treatment of heart diseases. Statement of SignificanceUnfortunately, to date there are no experimental data of human heart tissues available for material parameter estimation and the development of adequate material models. In this manuscript novel biaxial tensile and shear test data at different specimen orientations are presented, which allowed to adequately capture the direction-dependent material response. With these complete sets of mechanical data, combined with their underlying microstructural data (also presented herein), sophisticated material models and associated material parameters can be defined for the description of the mechanical behavior of the ventricular myocardium in humans. Such models will lead to accurate computational simulations to better understand the fundamental underlying ventricular mechanics, a step needed in the improvement of medical treatment of heart diseases.

Identification of material parameters for thin sheets from single biaxial tensile test using a sequential inverse identification strategy

An inverse analysis methodology to simultaneously identify the parameters of various anisotropic yield criteria together with isotropic work-hardening models of metal sheets is outlined. This identification makes use of results of the cruciform biaxial test, i.e., the evolution of the force during the test, for the two axes of the sample, and the major and minor strain distributions along both axes, at a given moment during the test. Based on a study of the sensitivity of the constitutive parameters to the biaxial tensile test results, the inverse identification consists on a procedure that sequentially minimises the gap between experimental and numerical results. Each step of the sequence uses a distinct cost function according to the type of results to be minimised, using a gradient-based optimisation algorithm, the Levenberg-Marquardt method. The inverse methodology allows for the identification of constitutive parameters of complex constitutive models. This sequential identification strategy is compared to a strategy based on a single cost function, involving all parameters and type of results, which has lower performance.