Results Of Biaxial Tests Research Articles

Load history dependency of stiffness properties of PTFE-coated glass fibre membranes

The aim of this contribution is to investigate whether the order of load ratios applied to a textile structure impacts the biaxial stiffness behaviour of the structural textile itself. These investigations are performed based on biaxial tests on an architectural woven glass fibre fabric, a representative of such membranes. The first part of the paper investigates the existence and extent of load history dependency in the material response. Biaxial test results show that effects of a previous load history on the weave geometry cannot be eliminated by further load cycling in all biaxial stress ratios. Moderate to strong effects are recognized in two stress ratios, although not in both weave directions.The second part of the paper examines how these effects influence material stiffness parameters and structural calculations. Two boundary value problems are investigated: a pressure chamber test and a roof structure. Parameters obtained by fitting biaxial tests with or without history are considered. By incorporating an adaptive load-ratio dependent scheme, highly predictive numerical results are obtained, showing that while the effect of load history may appear small in the biaxial tests and pressure chamber test, it can be significant in complex structural problems like roof structures.

Analysis of multiaxial fatigue models based on the results of biaxial tests for aluminum alloys

During operation, modern structures and their parts are subjected to complex cyclic loads. In this case, a complex stress-strain state can arise in products, which can lead to a significant decrease in their fatigue life. In this regard, the development of methods for predicting fatigue fracture of critical structures becomes urgent. The purpose of this work is to test a simple approach for describing multiaxial fatigue datasets presented in the literature and compare it with known multiaxial fatigue models. Within the framework of the study, actual experimental data was considered, containing experimental data on the multiaxial fatigue of aluminum alloys 2024-T3 and 2024-T4. The alloys are actively used in the designs of aircraft, ships and spacecraft. As a result, the Sines model and its modernized version were considered (taking into account the first invariant of the stress tensor with respect to amplitude values). At the same time, an approach was developed to determine the constants of the models based on the results of two variants of training experiments. The paper presents the results in the form of relationships of the predicted and experimental fatigue life, obtained using the Sines model and its modification for two types of training tests. In the final part of the work, a comparative analysis of the Sines model and its modification, Smith-Watson-Topper, Fatemi-Socie and its modification is carried out. The analysis showed that the modified Sines model significantly improves the predictive ability, the proposed model better described the experimental data than the traditional Sines model and the Smith-Watson-Topper approach. At the same time, the descriptive ability of the model is comparable to the Fatemi-Socie one.

Behavior of recycled steel fiber reinforced concrete under uniaxial cyclic compression and biaxial tests

In this study, the behavior of recycled steel fiber reinforced concrete (RSFRC) is investigated through uniaxial cyclic compression and biaxial tests conducted on 56 cubic specimens with four admixtures of RSF-content. The results obtained from cyclic compression tests show that the presence of RSFs reduces the damage of specimens in the post-peak regime which leads to an increase of residual strength. Furthermore, the biaxial failure-stress envelopes constructed based on the biaxial testing results indicate that an increase in RSF-content expands the failure envelope. Consistently, maximum biaxial strength for all specimens is found to occur at a stress confinement ratio α=~0.44-0.48. Moreover, measurements show that the failure biaxial strain significantly increases with increasing the RSF-content. The biaxial test results are found in good agreement with the literature on steel fiber reinforced concretes (SFRCs) indicating that industrial steel fibers can be substituted by RSFs which are environment-friendly. Employing these results, a model for the biaxial behavior of RSFRCs is proposed which can be employed in the constitutive relations required for the modeling of RSFRCs as well as SFRCs.

Calibration of Advanced Yield Criteria Using Uniaxial and Heterogeneous Tensile Test Data

Conventionally, plastic anisotropy is calibrated by using standard uniaxial tensile and biaxial test results. Alternatively, heterogeneous strain field specimens in combination with full-field measurements can be used for this purpose. As reported by the literature, such an approach reduces the number of required tests enormously, but it is challenging to obtain reliable results. This paper presents an alternative methodology, which represents a compromise between the conventional and heterogeneous strain field calibration technique. The idea of the method is to use simple tests, which can be conducted on the uniaxial testing machine, and to avoid the use of advanced measuring equipment. The procedure is accomplished by conducting standard tensile tests, which are simple and reliable, and by a novel heterogeneous strain field tensile test, to calibrate the biaxial stress state. Moreover, only two of the parameters required for full characterisation need to be inversely identified from the test response; the other parameters are directly determined from the uniaxial tensile test results. This way, a dimension of optimization space is reduced substantially, which increases the robustness and effectiveness of the optimization algorithm.

Simulation of the Injection Stretch Blow Molding Process: An Anisotropic Visco‐Hyperelastic Model for Polyethylene Terephthalate Behavior

The mechanical behavior of polyethylene terephthalate (PET) under the severe loading conditions of the injection stretch blow molding process is strongly dependent on strain rate, strain, and temperature. In this process, the PET near the glass‐transition temperature (Tg) highlights a strongly nonlinear elastic and viscous behavior. In the author's previous works, a nonlinear visco‐hyperelastic model has been build and identified from equi‐biaxial tensile experimental results. Despite the good agreement with biaxial test results, the model fails to reproduce the sequential biaxial test (with constant width during first step) and the shape evolution during the free blowing of preforms. In this work, an anisotropic version of this visco‐hyperelastic model is proposed and identified form both equi and constant width results. The new version of the nonlinear visco‐hyperelastic model is implemented into the Abaqus environment and used to simulate the free‐blowing process. The comparison with the experimental results managed in Queen's University of Belfast validates the approach in terms of bubble shape and thickness distribution. POLYM. ENG. SCI., 60:823–831, 2020. © 2020 Society of Plastics Engineers

An Analytical Study on Mechanical Behavior of Human Arteries – A Nonlinear Elastic Double Layer Model

The focus of this article is on analytical solution for stress and deformation of human arteries. The artery is considered as a long homogeneous isotropic cylinder. Hyperelastic, incompressible stress-strain behavior is used by adopting a classical Mooney-Rivlin material model. The elastic constants of the arteries are calculated by using the reported results of biaxial test. The analysis is based on both single and double layer arterial wall models and. Radial and circumferential stress distribution on the minimum and maximum blood pressure is calculated. Variation of radii due to internal pressure within the arteries is found which is in a good accordance with the experimental results. The results containing the changes in diameter and thickness together with the stress distribution for both single and double layer models have been plotted. It is shown that the major difference between the single and double layer models is in their stress distributions. The circumferential stress distribution for different human’s ages is plotted which shows that the stress increases by increasing the age due to decreasing the flexibility of the artery. It is also shown that despite the artery’s inner layer is softer than the outer layer, the maximum stresses occur at the inner layer.

Structure, Mechanical Behavior and Puncture Resistance of Grass Carp Scales

This work is conducted to investigate the hierarchical structure, mechanical behavior and fracture resistance of grass carp scales with different water contents (hydrated and dehydrated) and load conditions (uniaxial, biaxial and punch tests). The whole cross-section of scales is investigated, and it is found that the bony layer displays discontinuity and partly embeds in collagen layer. Four different locations are considered under both tensile and punch tests. The results of the uniaxial tensile test show a correlation between the failure mode and the distribution of surface morphology on scales. The biaxial test results show that there are minor differences in the tensile strength and the Young’ modulus compared with those of the uniaxial tests, but the ultimate strain is about 20% – 50%. Puncture tests are also conducted with different size of needles and different hardness silicon rubbers as substrate. The results show that the puncture force and deformation are dependent on the size of needle and the hardness of substrate. The failure pattern of scales is related to the water content. Radial cracks occur in the bony layer of hydrated scale, and the collagen fibers twist around the puncture site. However, the shear failure occurs in the bony layer of dehydrated scale.

Integrated Experimental and Numerical Comparison of Different Approaches for Planar Biaxial Testing of a Hyperelastic Material

Planar biaxial testing has been applied to a variety of materials to obtain relevant information for mechanical characterization and constitutive modeling in presence of complex stress states. Despite its diffusion, there is currently no standardized testing procedure or a unique specimen design of common use. Consequently, comparison of results obtained with different configurations is not always straightforward and several types of optimized shapes have been proposed. The purpose of the present work is to develop a procedure for comprehensive comparison of results of biaxial tests carried out on the same soft hyperelastic material, using different types of gripping methods and specimen shapes (i.e., cruciform and square). Five configurations were investigated experimentally using a biaxial test rig designed and built by the authors, using digital imaging techniques to track the displacements of markers apposed in selected positions on the surfaces. Then, material parameters for a suitable hyperelastic law were determined for each configuration examined, employing an inverse method which combines numerical simulations with the finite element method (FEM) and optimization algorithms. Finally, efficiency of examined biaxial configurations was assessed comparing stress reductions factor, degree and uniformity of biaxial deformation, and operative strain ranges.

Random gravel model and particle flow based numerical biaxial test of solid backfill materials

Fully mechanized coal mining with backfilling (FMCMB) has become a major solution of “green mining”. The mechanical properties of backfill materials are key factors of ground control in FMCMB face. To study the strength behaviors of the solid backfill materials, we developed a method of generating random gravel model by using computational geometry algorithms, then coded the generating method into a program with MATLAB software, and implemented several numerical biaxial tests under different confining stresses by using a generated random gravel model and the Particle Flow Code PFC2d. Peak deviatoric stress, post-peak softening, initial contraction and dilation and the relations between them and confining stress can be observed. And the initial elastic modulus of the samples, Poisson’s ratio, frictional angle and cohesion can be derived from the numerical biaxial test results. The results indicate a good feasibility study of mechanical properties of the solid backfill materials by random gravel model and particle flow based numerical biaxial test.

A comparative analysis of energy dissipation and equivalent viscous damping of RC columns subjected to uniaxial and biaxial loading

The hysteretic behaviour of RC columns has been object of many experimental studies over the past years. However, the majority of these studies are focused on unidirectional loading. An experimental program was carried out where 24 columns were tested for different loading histories, under uniaxial and biaxial conditions. The experimental results are presented in this paper and are discussed in terms of global column behaviour, and particularly with regards to energy dissipation and damping capacity. The energy dissipation capacity of the columns was evaluated in terms of cumulative dissipated energy, comparing uniaxial and biaxial test results, and individual cycle dissipated energy. Ultimately, an equation relating the normalised dissipated energy with the displacement ductility is proposed. The equivalent viscous damping was analysed by comparing the uniaxial with biaxial test results, demonstrating the high influence of the load path in the biaxial response of RC columns. Proposals for estimating the equivalent viscous damping given by other authors are compared with the experimental results. Finally, simplified expressions are proposed to estimate equivalent viscous damping in RC columns under biaxial loading.

Strength and failure modes of rock mass models with non-persistent joints

Most problems faced by the practicing rock engineer involve the evaluation of rock mass strength and deformability. The theoretical evaluation of the mechanical properties of fractured rock masses has no satisfactory answer because of the great number of variables involved. One of these variables, the influence of which over rock mass behavior is poorly documented, is the degree of fracture persistence. This paper presents the results of biaxial tests performed on physical models of rock with non-persistent joints. The failure modes and maximum strengths developed were found to depend on, among other variables, the geometry of the joint systems, the orientation of the principal stresses, and the ratio between intermediate stress and intact material compressive strength ( σ 2 /σ c). Tests showed three basic failure modes: failure through a planar surface, stepped failure, and failure by rotation of new blocks. Planar failure and stepped failure are associated with high strength behavior, and small failure strains, whereas rotational failure is associated with a very low strength, ductile behavior, and large deformation.

Design of a cruciform specimen for biaxial testing of fibre reinforced composite laminates

To study the behaviour of fibre reinforced composite laminates under static and cyclic in-plane complex stress states, a biaxial loading frame has been developed. A cruciform type specimen is biaxially loaded in its plane using four independent servo-hydraulic actuators. An appropriate control unit keeps the centre of the specimen at the same position during the test. For obtaining reliable biaxial failure data the design of the cruciform specimen is of paramount importance. Finite element simulations of the cruciform specimen to study the influence of various geometrical parameters and experiments on nine selected cruciform specimen types using both strain gages and an optical-numerical full field method for strain determination over the entire biaxially loaded test zone have been carried out. In the present paper, the four most important geometries will be discussed. Since stresses in the biaxially loaded zone cannot be calculated from the experimentally applied loads due to the ill definition of the load bearing area, strain values – the only measurable quantity – are used for the comparison of the different geometry types. For the glass fibre reinforced composite material with a [(±45/0) 4/±45] T lay-up this led to the proposal of an optimized geometry: a plane cruciform type specimen with a circular central region of reduced thickness and an adapted corner fillet at the intersection of the arms. Using this geometry failure will occur in the biaxially loaded test zone, rather than in the uniaxially loaded arms, at failure strains in the centre of the specimen comparable for the uniaxial load ratios to strains obtained on standard beam specimens. Tests at different load ratios were performed to obtain data as input for failure criteria. This paper focuses on the design of the specimen. In the near future, the biaxial test results will be compared with existing failure criteria.

Comparison of non‐local and polar modelling of softening in hypoplasticity

AbstractThe paper deals with the comparison of a non‐local and a polar (Cosserat) hypoplastic model. The hypoplastic constitutive law in the version of von Wolffersdorff is chosen as local reference model. For the comparison the results of biaxial tests on dense Hostun Rf sand are predicted with both enhanced models. The comparison is based on a strict separation of input data from triaxial‐ and oedometer tests and output data from biaxial tests. The comparison is drawn in terms of the shear band width, the load–displacement curves and the influence of the pressure level. Finally, the non‐local and the polar hypoplastic model are applied on the strip foundation problem. Copyright © 2004 John Wiley & Sons, Ltd.

Nonlocal modeling of softening in hypoplasticity

The present paper deals with a nonlocal hypoplastic model for granular materials like e.g. sands. The nonlocal formulation is based on a scalar variable which is a certain weighted average of the local variable taken from all material points under consideration. An internal length scale enters the constitutive equation as an additional material parameter of the Gaussian weighting function. A relation between the internal length scale and the mean grain diameter is derived for dense sands from numerical calculations and experimental results of biaxial tests. The proposed nonlocal theory preserves well-posedness of the governing equations in the post-localization regime and prevents pathological mesh sensitivity of the numerical results e.g. in biaxial tests and the strip foundation problem. The ability of the presented model to describe the behaviour of granular materials is demonstrated by comparisons of the results of numerical calculations and laboratory tests.

516 The effect of heavy metal ions on the expression of MRPS and UMAT in relalation to intracellular glutathione level

A nonlinear viscoelastic constitutive model for the solid propellant is proposed. In their earlier work, the authors have developed an isotropic constitutive model and verified it for one-dimensional case. In the present work, the validity of the model is extended to three-dimensional cases. Also, implementation of the model into a commercial code for the use of the constitutive model in the finite element analysis of solid propellant is discussed. Large deformation, dewetting and cyclic loading effects are treated as main sources of nonlinear behavior of the solid propellant. The softening of the solid propellant due to dewetting is treated by the modulus decrease. The nonlinearities during cyclic loading are accounted for by the functions of the octahedral shear strain measure. The constitutive equation is implemented into a finite element code for the analysis of propellant grains. A commercial finite element package ‘ABAQUS’ is used for the analysis and the model is introduced into the code through a user subroutine. The model is evaluated with different loading conditions and the predicted values are in good agreement with the biaxial test results.

Biaxial test results for strength and deformation of a range of E-glass and carbon fibre reinforced composite laminates: failure exercise benchmark data

In Part A of the World Wide Failure Exercise (published in Composite Science and Technology, Vol 58, No 7, 1998), all contributors were given exactly the same set of material properties and were asked to predict the strength and deformation of the same set of laminates under a range of specified loading conditions. In this part (Part B) of the exercise, available experimental results are superimposed on the theoretical predictions and returned to the contributors for comment. The test data were for (a) 0° unidirectional laminae under biaxial direct and shear loads (b) (90°/±30°) s, (0°/±45°/90°) s, (±55°) s multi-directional laminates under biaxial loads and (c) stress strain curves for (0°/±45°/90°) s, (±55°) s, (±45°) s and (0°/90°) s laminates under uniaxial and biaxial loads. This paper briefly describes the experimental results issued in Part B of the Exercise and their origin and limitations. Comments are made on the material properties given for the unidirectional fibre reinforced layers and constituents used in Part A of the Exercise and on approximations in the laminate models specified in Part A. © QinetiQ Ltd. 2002.

A three-dimensional nonlinear viscoelastic constitutive model of solid propellant

A nonlinear viscoelastic constitutive model for the solid propellant is proposed. In their earlier work, the authors have developed an isotropic constitutive model and verified it for one-dimensional case. In the present work, the validity of the model is extended to three-dimensional cases. Also, implementation of the model into a commercial code for the use of the constitutive model in the finite element analysis of solid propellant is discussed. Large deformation, dewetting and cyclic loading effects are treated as main sources of nonlinear behavior of the solid propellant. The softening of the solid propellant due to dewetting is treated by the modulus decrease. The nonlinearities during cyclic loading are accounted for by the functions of the octahedral shear strain measure. The constitutive equation is implemented into a finite element code for the analysis of propellant grains. A commercial finite element package ‘ABAQUS’ is used for the analysis and the model is introduced into the code through a user subroutine. The model is evaluated with different loading conditions and the predicted values are in good agreement with the biaxial test results.

コンクリートの破壊進行領域における圧縮せん断挙動に関する実験的研究

Most cracks in concrete structures are initiated first by the largest principle stress, and propagate under subsequent shear and transverse compression or tension. A biaxial testing device, with which the vertical and horizontal load as well as displacement can be independently controlled by closed-loop servo systems, has been developed in order to establish the shear behavior in cracked concrete. The shear tests on specimens containing narrow cracks (fracture process zone introduced by the tensile test in displacement control) have been carried out maintaining either normal displacement or load across the crack surface constant. The relation of normal and shear stresses to crack opening and sliding displacements was discussed and distinguished by the boundary conditions of the normal direction across the crack surface. Several expressions which give shear retention factor, β, have been derived based on biaxial test results.

Design of a Cylindrical Specimen for Biaxial Testing of Composite Materials

The design and analysis results of a specimen and fixture to study the failure of composite materials under biaxial loading environments are presented. The objective of the current study was to achieve a capability of transmitting higher axial loads than had been achieved previously. The magnitude of the axial load is limited by failure and debonding of the end-tab which is bonded to the composite. Higher axial loads were achieved primarily by designing an end-tab geometry to put the bondline normal stress in compression, which was found to increase the bond shear strength, and by careful material selection for the end grip region. The end-tab was analyzed using finite elements to verify a smooth introduction of load in both the axial and hoop directions. Experiments showed that the grip and apparatus design was successful in achieving gage section failures over a wide range of biaxial test conditions. Biaxial test results are presented for tension-tension and compression-compression tests of carbon/epoxy braid specimens to illustrate the present test methods.

A biaxial thermomechanical disk test for glassy polymers

Failure criteria for polymers need to include effects from the stress state. For this reason, biaxial test results are of interest. However, biaxial test methods usually require expensive equipment. In the test method presented here, a disk of epoxy is bonded between a steel ring and a steel disk. The temperature is then lowered until fracture is observed. Experiments were performed on three different glassy epoxy polymers. The biaxial stress state was analyzed by finite element analysis and by an approximate analytical model. Experimental observations support the ability of the method to provide material property data. An approximate analytical model was found sufficiently accurate for stress analysis and determination of the stress state at failure.