Biaxial Tensile Tests Research Articles

3D printed ascending aortic simulators with physiological fidelity for surgical simulation

IntroductionThree-dimensional (3D) printed multimaterial ascending aortic simulators were created to evaluate the ability of polyjet technology to replicate the distensibility of human aortic tissue when perfused at physiological pressures.MethodsSimulators were...

Effect of fibre content on the mechanical properties of hemp fibre woven fabrics/polypropylene composite laminates

Nowadays natural fibre composites have gained great significance as reinforcements in polymer matrix composites. Composite material based on a polymer matrix reinforced with natural fibres is extensively used in industry due to their biodegradability, recyclability, low density and high specific properties. A study has been carried out here to investigate the fibre volume fraction effect of hemp fibre woven fabrics/PolyPropylene (PP) composite laminates on the tensile properties and impact hammer impact test. Initially, composite sheets were fabricated by the thermal-compression process with desired number of fabric layers to obtain composite laminates with different fibre volume fraction. Uniaxial, shear and biaxial tensile tests were performed and mechanical properties were calculated. Impact hammer test was also carried out to estimate the frequency and damping parameters of stratified composite plates. Scanning Electron Microscope (SEM) analysis was performed to observe the matrix and fibre constituent defects. Hemp fabrics/PP composite laminates exhibits viscoelastic behaviour and as the fibre volume fraction increases, the viscoelastic behaviour decreases to elastic behaviour. Due to this, the tensile strength increases as the fibre content increases. On the other hand, the natural frequency increases and damping ratio decrease as the fibre volume fraction increases.

Influence of the strain dependent material behaviour under plane strain on the yield locus modelling

Modern lightweight materials mostly exhibit a low formability especially in the plane strain state. Having in mind that over 80% of the occurred failures in the press shop in the automotive sector can be related to this strain state, the characterization under plane strain load should be further intensified. The evaluation of the plane strain flow curve is commonly done by notched tensile tests or biaxial tensile tests with optimized strain path. Both tests have some disadvantages in the flow curve evaluation especially at a high degree of deformation. With an elliptic hydraulic bulge test, the material behaviour under near plane strain condition can be evaluated up to fracture. The aim in this investigation is the analysis of the resulting material behaviour under plane strain of commonly used alloys AA5182, DC06 and DP600 with respect to the material anisotropy. Therefore, hydraulic bulge tests with a plane strain load path are performed in rolling direction and transversal direction. The resulting flow curves are used to analyse the material hardening with respect to the anisotropy. The results show differing hardening mechanisms for the investigated materials which will lead to different strategies for the material modelling.

Analysis of the Cruciform Sample Shapes for Bi-Axial Tensile Tests Based on the Geometries Currently Presented in the Literature

Analysis of the Cruciform Sample Shapes for Bi-Axial Tensile Tests Based on the Geometries Currently Presented in the Literature

Comprehensive study on mechanical properties of coated biaxial warp-knitted fabrics

In recent years, coated fabrics have become the major material used in membrane structures. Due to the special structure of base layer and mechanical properties, coated biaxial warp-knitted fabrics are increasingly applied in pneumatic structures. In this article, the mechanical properties of coated biaxial warp-knitted fabrics are investigated comprehensively. First, off-axial tensile tests are carried out in seven in-plane directions: 0°, 15°, 30°, 45°, 60°, 75°, and 90°. Based on the stress–strain relationship, tensile strengths are obtained and failure modes are studied. The adaptability of Tsai–Hill criterion is analyzed. Then, the uniaxial tensile creep test is performed under 24-h sustained load and the creep elongation is calculated. Besides, tearing strengths in warp and weft directions are obtained by tearing tests. Finally, the biaxial tensile tests under five different load ratios of 1:1, 2:1, 1:2, 1:0, and 0:1 are carried out, and the elastic constants and Poisson’s ratio are calculated using the least squares method based on linear orthotropic assumption. Moreover, biaxial specimens under four load ratios of 3:1, 1:3, 5:1, and 1:5 are further tensile tested to verify the adaptability of linear orthotropic model. These experimental data offer a deeper and comprehensive understanding of mechanical properties of coated biaxial warp-knitted fabrics and could be conveniently adopted in structural design.

Experimental and Numerical Investigation of PA66 Fabrics Coated/Uncoated PVC by Biaxial Tensile Tests

Automobile airbags have become mandatory safety components in many countries. In this study, the PA66 (polyamide 66) airbag fabric was tested with and without a surface coating of PVC (polyvinyl chloride). Cyclic loading and unloading experimental conditions were defined according to the loading characteristics of the airbag deployment process. To determine mechanical properties such as elastic modulus and stress-strain curves for the two materials, biaxial loading/unloading cyclic tensile tests were carried out using a DIC (Digital Inspection Camera) to monitor the field strain process. Quadratic polynomial regression was used to fit material constitutive models of each loading and unloading process to the test data. The material subroutine VUMAT was encoded and applied to the simulation of the biaxial tensile testing of the fabrics. There was good agreement between the experimental and simulation results, which verifies that the quadratic polynomial constitutive material model is suitable for the study of coated and uncoated PA66 automobile airbag fabric. Hence, the material behaviour of automobile airbag fabrics can be modelled accurately.

Material parameter identification using finite elements with time-adaptive higher-order time integration and experimental full-field strain information

In this article, we follow a thorough matrix presentation of material parameter identification using a least-square approach, where the model is given by non-linear finite elements, and the experimental data is provided by both force data as well as full-field strain measurement data based on digital image correlation. First, the rigorous concept of semi-discretization for the direct problem is chosen, where—in the first step—the spatial discretization yields a large system of differential-algebraic equation (DAE-system). This is solved using a time-adaptive, high-order, singly diagonally-implicit Runge–Kutta method. Second, to study the fully analytical versus fully numerical determination of the sensitivities, required in a gradient-based optimization scheme, the force determination using the Lagrange-multiplier method and the strain computation must be provided explicitly. The consideration of the strains is necessary to circumvent the influence of rigid body motions occurring in the experimental data. This is done by applying an external strain determination tool which is based on the nodal displacements of the finite element program. Third, we apply the concept of local identifiability on the entire parameter identification procedure and show its influence on the choice of the parameters of the rate-type constitutive model. As a test example, a finite strain viscoelasticity model and biaxial tensile tests applied to a rubber-like material are chosen.

Comparative Study of Aortic Wall Stress According to Geometric Parameters in Abdominal Aortic Aneurysms

In abdominal aortic aneurysm (AAA), the rupture of the aortic tissue is directly related to wall stress. Thus, the investigation of maximum wall stress is a necessary procedure to predict the aortic rupture in AAA. In this study, computational simulations were performed to investigate the correlation between peak wall stress (PWS) and AAA geometry. The Holzapfel model and various orientations of the collagen fibers and thicknesses of the layers of the aorta were employed in the simulation. The material constants used in the Holzapfel model were estimated from the examination and analysis of the biaxial tensile test results of the normal abdominal aorta and AAA. The aneurysm diameter, height, neck angle, and iliac angle were selected as geometric factors affecting the AAA rupture. In addition, a simulation scenario was conceived and created based on the measurement results using the computed tomography data of patients with AAA. Accordingly, the correlation between the PWS and AAA geometry was estimated.

Mechanics of Uterosacral Ligaments: Current Knowledge, Existing Gaps, and Future Directions.

The uterosacral ligaments (USLs) are important anatomical structures that support the uterus and apical vagina within the pelvis. As these structures are over-stretched, become weak, and exhibit laxity, pelvic floor disorders such as pelvic organ prolapse occur. Although several surgical procedures to treat pelvic floor disorders are directed toward the USLs, there is still a lot that is unknown about their function. This manuscript presents a review of the current knowledge on the mechanical properties of the USLs. The anatomy, microstructure, and clinical significance of the USLs are first reviewed. Then, the results of published experimental studies on the in vivo and ex vivo, uniaxial and biaxial tensile tests are compiled. Based on the existing findings, research gaps are identified and future research directions are discussed. The purpose of this exhaustive review is to help new researchers navigate scientific literature on the mechanical properties of the USLs. The use of these structures remains very popular in reconstructive surgeries that restore and augment the support of pelvic organs, especially as synthetic surgical mesh implants continue to be highly controversial.

Biaxial tensile failure of short carbon-fibre-reinforced PEEK composites

The biaxial tensile failure of short carbon fibre reinforced polyether-ether-ketone (SCFR-PEEK) composites is characterized in this paper. An improved cruciform specimen was proposed to apply biaxial stresses, and five different stress ratios were employed in experiments. The stress distribution within the gauge section is not measurable across the varying stress ratios, therefore it is not trivial to obtain stress-strain data in the experiments. In this work, a combined experimental and numerical approach was proposed to deduce the true material behaviour during biaxial tensile tests. A numerical model with consideration of anisotropy and plasticity was proposed, it was further calibrated with uniaxial experiments and then validated with biaxial tests. Through this approach, the failure stress states at different stress ratios were successfully obtained. Compared with the numerical results based on constitutive relations, numerical results with linear elastic simplification overestimate the biaxial tensile strength, and are not suitable for material with an apparent nonlinear segment. The biaxial tensile failure envelope matches the Tsai-Hill criterion, and equal biaxial tensile results can be used for determining interaction coefficient F12 in the Tsai-Wu criterion. This research approach offers an accurate and reliable solution for biaxial tensile testing of materials with consideration of anisotropic and nonlinear behaviour.

Effects of Enzyme-Based Removal of Collagen and Elastin Constituents on the Biaxial Mechanical Responses of Porcine Atrioventricular Heart Valve Anterior Leaflets

The leaflets of the atrioventricular heart valves (AHVs) regulate the one-directional flow of blood through a subtle coordination of the tissue's extracellular matrix (ECM) components, including collagen fibers, elastin fibers, and glycosaminoglycans. Dysfunction of the AHVs, such as those caused by unfavorable microstructural remodeling, results in valvular heart diseases and improper blood flow that can eventually lead to heart failure. To better understand mechanics and remodeling of the AHV leaflets and how therapeutics may inadvertantly cause adverse microstructural changes, a systematic characterization of the role of each ECM constituent in the biomechanical properties is warranted. Previous studies have quantified the contributions of the microstructural components to the tissue-level behavior for the semilunar valve leaflets, but not for the AHV leaflets. In this study, for the first time, we quantify the AHV leaflet microstructure-mechanics relationships, by using a three-step experimental procedure: (i) biaxial tensile and stress relaxation testing of the control (untreated) porcine AHV anterior leaflet specimens; (ii) enzyme treatment for removing a portion of either the collagen or elastin constituent; (iii) biaxial tensile and stress relaxation testing of the constituent-removed (treated) specimens. We observed that removing about 25-50% collagen from the leaflets resulted in a ~3% increase in the extensibility of the tissue and a ~50% increase in the stress relaxation behavior. Meanwhile, the removal of approximately 90-99% elastin led to a ~10% decrease in the extensibility of the leaflets and a ~75% increase in the stress relaxation behavior. These findings provide insight into the microstructure-mechanics relationship of the AHVs, and will be beneficial in the future developments and refinements of microstructurally informed constitutive models to simulate diseased and surgically-intervened AHV function.

Predicting muscle tissue response from calibrated component models and histology-based finite element models

Skeletal muscle is an anisotropic soft biological tissue composed of muscle fibres embedded in a structurally complex, hierarchically organised extracellular matrix. In a recent work (Kuravi et al., 2021) we have developed 3D finite element models from series of histological sections. Moreover, based on decellularisation of fresh tissue samples, a novel set of experimental data on the direction dependent mechanical properties of collagenous ECM was established (Kohn et al., 2021). Together with existing information on the material properties of single muscle fibres, the combination of these techniques allows computing predictions of the composite tissue response. To this end, an inverse finite element procedure is proposed in the present work to calibrate a constitutive model of the extracellular matrix, and supplementary biaxial tensile tests on fresh and decellularised tissues are performed for model validation. The results of this rigorously predictive and thus unforgiving strategy suggest that the prediction of the tissue response from the individual characteristics of muscle cells and decellularised tissue is only possible within clear limits. While orders of magnitude are well matched, and the qualitative behaviour in a wide range of load cases is largely captured, the existing deviations point at potentially missing components of the model and highlight the incomplete experimental information in bottom-up multiscale approaches to model skeletal muscle tissue.

Study on patterned photodynamic cross-linking for keratoconus

PurposeThis study examined the patterned treatment of corneal collagen cross-linking (CXL) for keratoconus to reduce the complications caused by ultraviolet (UV) irradiation. By modifying the method of UV irradiation during the cross-linking process, cross-linking with a special structure is achieved, and the cross-linking effect is analyzed and compared to that of traditional cross-linking. By constructing an animal model of keratoconus, the process and effect of corneal cross-linking can be investigated more fundamentally. These studies provide valuable references for future cross-linking precision improvement and specialization. MethodBy injecting exogenous collagenase into the corneal stroma of rabbits, the balance between collagenase and collagenase inhibitor in the corneal stroma was disrupted, the collagen fiber structure of the cornea was broken to simulate the pathogenesis of keratoconus, and an animal model of keratoconus was thus constructed. Two custom cross-linking patterns were designed with reference to the cable dome structure, and these two special patterns were irradiated and cross-linked by a DMD chip. The cross-linking effect was evaluated by optical coherence tomography (OCT), corneal topography and corneal biaxial tensile tests. The experimental rabbits were divided into four groups: group A, cross-linking of the bird's nest structure; group B, cross-linking of the honeycomb structure; group C, cross-linking of the traditional spot structure; and group D, normal (without modeling and cross-linking). ResultFollowing collagenase treatment, the collagen fiber structure of the rabbit cornea was destroyed, the central thickness of the cornea was reduced, the mechanical properties of the cornea were weakened, and no keratitis, ulcers or haze occurred. After the three cross-linking treatments, the morphology of the cornea improved, the density of the stromal layer increased, and the mechanical properties were enhanced. For the improvement of keratoconus mechanical properties, the average relative difference (Δ) of the four outcome measures was 61.98% for bird's nest cross-linking versus keratoconus (Wilcoxon rank sum test, P = 0.024), 16.13% for honeycomb cross-linking versus keratoconus (Wilcoxon rank sum test, P = 0.025), and 21.07% for traditional spot cross-linking versus keratoconus (Wilcoxon rank sum test, P = 0.014). All these differences are statistically significant. ConclusionAll three methods of cross-linking can improve the morphology and tissue structure of keratoconus and significantly improve the biomechanical properties of the cornea. Among them, the corneal cross-linking of the bird's nest structure attains the best biomechanical properties, followed by the corneal cross-linking of the traditional spot structure and of the honeycomb structure. This suggests that similar or better cross-linking effects can be achieved by designing custom structures with less UV exposure. This provides a direction for future research on better and more accurate pattern cross-linking treatments.

An effective method for determining necking and fracture strains of sheet metals

Biaxial tensile testing methods using cruciform specimens have been developed in the last few decades for the determination of forming limit diagrams (FLDs) and fracture forming limit diagrams (FFLDs) for sheet metals. One of the difficulties associated with this test geometry is the lack of a widely accepted method to determine the necking and fracture strains which are necessary to construct these diagrams. In this study, a novel spatio-temporal method has been proposed for the determination of necking and fracture strains. In the method, two rectangular zones: the base zone (BZ) and the reference zone (RZ) are selected at the location where fracture initiates. The zone RZ includes the zone BZ and both zones have the same side length in the direction parallel to the necking band but different side length in the perpendicular direction. By plotting the thickness reduction within RZ against that in BZ, the onset of localised necking can be determined by finding the intersection of the two straight lines fitted separately using the data in the initial and final stages of deformation. The corresponding limit strains are then determined using the strains within the zone BZ. The method has been successfully applied to uniaxial tensile tests on AA6082 and boron steel dog-bone specimens, and to equi-biaxial tensile tests on AA5754 cruciform specimens.• Compared to widely used existing methods, the novel spatio-temporal method has greater simplicity, stability and accuracy with regard to the determination of localised necking strains.• The spatio-temporal method has good potential to become a standard method for the determination of limit strains for sheet metals.

Investigating an Inverse Finite Element Approach for Characterising Soft Materials

ABSTRACT Soft materials, such as soft biological tissue and soft silicone rubber, are non-linear materials which require the classical uniaxial and biaxial tensile testing methods for characterisation. Unfortunately, in special cases, such as for soft biological tissue, the samples are smaller than 10 mm χ 10 mm in size and these classical tensile testing methods produce unwanted stress and strain gradients due to the fastening techniques associated with these methods. Micro-indentation is proposed as an alternative method for characterising soft materials. Using inverse Finite Element (FE) analysis and a known Mooney-Rivlin three parameter material model, six different micro-indentation tests were proposed. A theoretical approach was used to determine which indentation test best characterised a silicone sample, by using two FE models. The results showed that microindentation is capable of characterising a soft material in ideal conditions with a cylindrical indenter applied in a diagonal orientation over the sample, as the best indentation method. Finally, it was observed that the material model can either match the displacements with the smallest objective function or the stress vs. stretch curve can be matched to 99 % over the whole stretch range but not both simultaneously. Additional keywords: Inverse Finite Elements analysis, Micro-indentation, Gradient optimisation, Mooney-Rivlin hyper-elastic material model.

Progressive damage characterization of biaxial carbon fiber Reinforced epoxy composites

The objective of the work was to investigate bi-axial characteristics of composite materials with cruciform geometry in both experimental and numerical techniques. The carbon reinforced fibers with epoxy composites prepared by hand lay-up technique with various stacking sequences such as 0°/90°, 45°/45°, 15°/75° and 30°/60°. The fabricated composites were machined in cruciform shape, having different notches namely circular, square and rhomboidal according to ASTM – D6856 standard by abrasive water jet machining to ensure dimensional accuracy of ± 0.1 mm. The prepared cruciform specimens were tested using a biaxial fixture developed indigenously by taking the specifications of universal testing machine. The Young’s modulus, proof stress, ultimate tensile strength and fracture toughness were evaluated for 16 specimens of different fibre orientations and notches using software. The response from the biaxial tensile test showed highest stress developing in [0°/90°] Carbon Fibre Reinforced Plastic (CFRP) composites of 61.37 MPa with highest ultimate tensile strength of 71.93 MPa experimentally. Numerical simulation with ANSYS was showing maximum stresses developing in [0°/90°] Carbon Fibre Reinforced Plastic (CFRP) composites 65.37 MPa.

Mechanical characterization and viscoelastic model of the ovine temporomandibular joint Disc in indentation, uniaxial tension, and biaxial tension

There have been recent investigations into developing disc replacements and regenerative medicine to treat internal derangements of the temporomandibular joint (TMJ) disc. Previous attempts at disc replacements have faced challenges related in part to a limited understanding of the TMJ's complex mechanical environment. The purpose of this study was to characterize the mechanical behavior of the ovine TMJ disc and to derive viscoelastic constitutive models from the experimental data. Fresh ovine TMJ discs were tested in indentation stress-relaxation tests on the inferior surface, uniaxial tension tests to failure, and dynamic biaxial tensile tests. Results showed an order of magnitude stiffer behavior in tension in the anteroposterior (primary fiber) direction compared to the mediolateral direction. The stiffness in tension was much greater than in compression. Regional comparisons showed greater elastic moduli in indentation in the posterior and anterior bands compared to the central region. A hyper-viscoelastic constitutive model captured the dynamic stress-stretch behavior in both indentation and biaxial tension with good agreement. These data will support ongoing and future computational modeling of local TMJ mechanics, aid in biomaterials identification, and ultimately enhance development of implant designs for TMJ disc replacement.

Formability prediction of tailor-welded blanks in hydraulic bulging using flow curves from biaxial tensile tests

In this work, the formability of laser-welded tailored blanks of low carbon steel of two different thickness combinations in hydraulic bulging has been studied by numerical simulation. For material modeling, flow curves of the parent sheets were obtained in biaxial stress condition by conducting hydraulic bulge tests. These curves were used to extrapolate the uniaxial tensile curves up to large strains using the work equivalence principle. The limiting dome height in conventional forming and hydraulic bulging of tailor-welded blanks has been predicted in finite element simulations using the flow curves obtained directly from the hydraulic bulge tests and the extrapolated uniaxial tensile curves. Hydraulic bulging and conventional forming experiments on tailor-welded blanks have also been conducted to validate the predicted results. It has been found out that the predicted limiting dome height of the tailor-welded blanks in conventional forming and hydraulic bulging using extrapolated uniaxial tensile curves is closer to the experimental values when compared to the results obtained by using stress–strain curves obtained from hydraulic bulge tests. It has also been found that using an extrapolated uniaxial tensile curve it is also possible to predict strain distribution and percentage thinning more accurately. It has been observed that with an increase in thickness ratio, the peak pressure increased but the predicted values of peak pressure using flow curves obtained directly from hydraulic bulge tests are closer to the experimental values.

Investigation on Yield Behavior of 7075-T6 Aluminum Alloy at Elevated Temperatures

Aluminum alloys have drawn considerable attention in the area of automotive lightweight. High strength aluminum alloys are usually deformed at elevated temperatures due to their poor formability at room temperature. In this work, the yield behavior of 7075 aluminum alloy in T6 temper (AA7075-T6) within the temperature ranging from 25 °C to 230 °C was investigated. Uniaxial and biaxial tensile tests with the aid of induction heating system were performed to determine the stress vs. strain curves and the yield loci of AA7075-T6 at elevated temperatures, respectively. Von Mises, Hill48 and Yld2000-2d yield criteria were applied to predicting yield loci which were compared with experimentally measured yield loci of the AA7075-T6. Results show that yield stress corresponding to the same equivalent plastic strain decreases with increasing temperature within the investigated temperature range and the shape of yield loci evolves nearly negligibly. The experimental yield locus expands with an increase of equivalent plastic strain at the same temperature and the work hardening rate of AA7075-T6 exhibits obvious stress-state-dependency. The non-quadratic Yld2000-2d yield criterion describes the yield surfaces of AA7075-T6 more accurately than the quadratic von Mises and Hill48 yield criteria, and an exponent of 14 in the Yld2000-2d yield function gives the optimal predictions for the AA7075-T6 at all investigated temperatures.

A new technique for characterising mechanical properties of materials under hot stamping conditions

In order to characterise mechanical properties of materials (e.g. formability) under hot stamping conditions, significant efforts have been made to the development of the biaxial tensile testing method using cruciform specimens. However, no method for necking strain determination and no cruciform specimen design have been widely accepted. In this study, a new technique for characterising mechanical behaviour of materials under hot stamping conditions has been proposed. It includes two main parts: 1) a novel spatio-temporal method for determining necking and fracture strains, and 2) a cruciform specimen design for formability evaluation using biaxial testing method. In the first part, the theoretical base of the novel spatio-temporal method has been discussed, and the method has been validated by applying to uniaxial tensile tests on AA6082 specimens. The method has also been compared with several existing popular methods, in the determination of limit strain at onset of localised necking. It is found that the novel method has greater simplicity, stability and accuracy for the determination of localised necking strain. In the second part, a proposed cruciform specimen of AA5754 has been tested under the equi-biaxial tension, and both the necking initiation location and the strain path at the location where necking initiates, have been analysed. Furthermore, the novel spatio-temporal method has been applied to the biaxial tensile test for the determination of necking and fracture strains. The results show that the designed cruciform specimen enables to initiate fracture at the centre of the specimen and realisation of linear strain path under equi-biaxial tension.