Modified Arcan Fixture Research Articles

Characterization of elastic-plastic fracture toughness of AM60 Mg alloy under mixed-mode loading conditions

In this paper, an estimation of the elastic-plastic fracture of the AM60 alloy under pure mode-I, mixed-mode, and pure mode-II loading conditions has been investigated. For this purpose, a modified Arcan-type device and a butterfly specimen were used. Fracture tests were conducted according to ASTM standard E1820; however, due to the standard being limited to some specific geometries and the mode-I loading, the relationships and parameters presented in the standard, including the non-dimensional stress intensity factor, the geometric work factor, and other parameters were extracted and analyzed using finite element analysis for the butterfly specimens and under different loading conditions and relationships developed for them. In the following, the fracture tests for the butterfly specimen were carried out under mode-I, 45° mixed-mode, and mode-II loading conditions and the material resistance curves against crack growth (J-R) were extracted. As a result, the material fracture toughness was extracted for each. Also, for the verification of the fixture used in this study, a beam-shaped specimen test was arranged, and its results were compared with the values of mode-I obtained from the modified Arcan fixture. The results indicated that, by changing the loading angle from mode-I to mode-II, the critical values of the fracture took on a decreasing trend, which indicated the greater resistance of the material to crack growth under the conditions of mode-I (tension) compared to mode-II (shear). On the other hand, the difference between the results obtained from the fixture used and the beam-shaped specimen was less than 9%, which indicates the correct performance of the fixture. Finally, in order to get a deeper understanding of the material fracture mechanism under different loading conditions, the fracture surfaces were examined using scanning electron microscopy (SEM).

Use of the modified Arcan fixture to study the strength of bonded assemblies for automotive applications

Adhesive joints are being increasingly used as a good solution to assemble primary structures of metallic and composite materials. Hence, the analysis of the behaviour of adhesively-bonded assemblies under mixed loadings is a critical issue for industry. This article discusses the use of the modified Arcan device to identify the strength of adhesively-bonded metal/metal and metal/composite assemblies. Different assemblies have been tested and their failure envelopes have been determined. Moreover, analyses have also been made of the influence of some crucial parameters on the behaviour of the assemblies in the study such as the composite surface state, the fabric orientation, the humidity and the presence of grease.

A novel fixture for determining the tension/compression-shear failure envelope of multidirectional composite laminates

A new approach for biaxial loading of fibre reinforced composites is presented, based on the use of a Modified Arcan Fixture (MAF). In addition to combined tension-shear loading which can be achieved using a conventional Arcan fixture, the MAF allows the application of pure compression and compression-shear loading. Digital Image Correlation is used to obtain the full-field deformations of the specimens to establish the load response and failure behaviour for a range of multiaxial stress-strain states. The approach is demonstrated on glass/epoxy prepreg laminates. For unidirectional matrix dominated laminate it is shown that the failure envelope is described accurately by Puck’s inter-fibre failure theory. For the multidirectional matrix-dominated laminate it is shown that the in-situ strength is much greater than that predicted using the unidirectional properties. The potential of the MAF for characterisation over the entire combined tension-shear and compression-shear domains using a single fixture is demonstrated.

Investigation of the effect of bonded composite patch on the mixed-mode fracture strength and stress intensity factors for an edge crack in aluminum alloy 2024-T3 plates

In this research, the fracture behavior of an edge crack in aluminum alloy 2024-T3 plate repaired with bonded composite patch has been investigated under mixed-mode loading condition, numerically and experimentally. For the experimental part, two batches of simple edge crack and repaired specimen have been tested by a tensile test machine at different loading levels by using a modified Arcan fixture. According to the experimental results, applying the repair patch improved the fracture strength even as much as 44%. Also, increasing the patch thickness enhances the repair efficiency; nevertheless, increasing the adhesive thickness decreases the strength as much as 3%. Finite element simulations were performed to model the specimens and establish the stress intensity geometry/loading factors. The results indicate that a significant reduction in the geometry/loading factors at the crack tip can be occurred in the presence of the patch.

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.

Decomposition method based on a modified Arcan fixture and its application in an in situ combined load tester

A miniature in situ tester integrated with a modified Arcan fixture was developed, providing simultaneous tensile and shear loading functions, to investigate the mechanical behaviors of bulk materials under combined loads or unidirectional load. Using the tester, a decomposition method was proposed to obtain the true tensile stress–strain and shear stress–strain relationships. Then, the feasibility of the decomposition method was verified via tensile loading, combined loading and shear loading tests on an extruded AZ31B magnesium alloy with initial gripping angles of 0°, 45° and 90°, respectively. The tensile stress–strain curve and shear stress–strain curve were extracted from the combined stress–strain curve and compared with the results obtained from the unidirectional load tests. Finally, metallographic microscope images of the magnesium alloy under combined load conditions were captured as an illustration of the in situ function, and the fracture morphologies of the magnesium alloy under various loading conditions were also analyzed via a scanning electron microscope.

Effects of 2D misalignment on tensile results and corresponding correction methods to obtain the true stress–strain curve

In miniature horizontal-type in situ tensile devices, misalignments on the horizontal and vertical plane cause shearing and bending effects in the specimen and further lead to incorrect testing results of mechanical parameters. To investigate the effects of misalignment between the tensile axis and the specimen’s geometrical axis on the testing parameters and obtain the true stress and strain relationship, two correction methods based on linear deformation and deflection curve deformation were proposed, and the effect laws of the horizontal deviation angle α and vertical deviation angle β on mechanical parameters were analyzed. The feasibility of the proposed correction methods was verified with the help of a miniature device integrating a modified Arcan fixture. The establishment of α was achieved by adjusting the relative angle between the device and Arcan fixture, and the establishment of β was achieved by adding thin copper pieces under one of the fixtures. The experimental tests indicated that when adopting the deflection curve deformation correction method, the obtained corrected mechanical parameters were similar to the results obtained from the uniaxial tensile test. The correction methods could be appropriate for the calculation of the true stress–strain curve on the condition of 2D misalignment, particularly for the correction of testing results obtained by in situ tensile devices with compact structure and miniature size.

Experimental analysis of the mechanical behaviour of a thick flexible adhesive under tensile/compression-shear loads

Flexible adhesives, characterised by low modulus, and large strains at failure in comparison with usual structural adhesives, present various advantages for assembling large scale parts and dissimilar materials while for instance limiting the influence of thermal stresses. Thus various applications can be found in automotive, aeronautic and building industries. The single-lap joint is widely used to characterise the behaviour of adhesively bonded assemblies, but it is associated with a complex tri-dimensional stress state especially close to the free edges of the adhesive. Moreover, in order to develop accurate numerical models for adhesively bonded assemblies, experimental results under various loading paths have to be analysed. The objective of this paper is to characterise the behaviour of a thick flexible adhesive (Sikaflex®-265) under different monotonic tensile/compression-shear loadings. For this purpose, a modified Arcan fixture has been designed to limit stress concentrations in order to characterise the mechanical behaviour of the adhesive up to failure. Results allow defining the load-displacement envelopes characterising the adhesive at failure under multi-axial loading conditions. Finally, the influence of some parameters on the mechanical behaviour of the assembly was analysed, such as the nature of the substrate material (ABS or aluminium), the geometry of the substrates and the strain rate.

Identification of Material Parameters of PVC Foams using Digital Image Correlation and the Virtual Fields Method

This paper presents an effective methodology to characterize all the constitutive (elastic) parameters of an orthotropic polymeric foam material (Divinycell H100) in one single test using Digital Image Correlation (DIC) in combination with the Virtual Fields Method (VFM). A modified Arcan fixture is used to induce various loading conditions ranging from pure shear or axial loading in tension or compression to bidirectional loading. A numerical optimization study was performed with different loading angles of the Arcan test fixture and off-axis angles of the principal material axes. The objective is to identify the configuration that gives the minimum sensitivity to noise and missing data on the specimen edges, which are the two major issues when identifying the stiffness components from actual DIC measurements. Two optimized Arcan test configurations were chosen. The experimental results obtained for these two optimized test configurations show a significant improvement of the measurement accuracy compared with a pure shear load configuration. The larger sensitivity of the pure shear test to missing data as opposed to the tensile test is also evident from the experimental data and confirms the analysis from the optimization study. The recovery of missing data along the specimen edges is a promising way to further improve the identification results.

Effect of bolt clamping force on the fracture strength of mixed mode fracture in an edge crack with different sizes: Experimental and numerical investigations

To investigate the effect of bolt clamping force, resulting from torque tightening, on the mixed mode fracture (I and II) strength and effective geometry/loading factor of an edge crack with different lengths, experimental and numerical studies have been carried out. In the experimental part fracture tests were conducted on three batches of simple edge crack and bolt torque tightened with 3.5 and 7Nm edge crack at three different crack sizes of Poly methyl-methacrylate (PMMA) rectangular plate. The specimens’ fracture strength was obtained using a tensile test machine at different loading angles by employing a modified Arcan fixture. In numerical part, finite element simulations were employed to model the three test specimen batches at the three crack lengths to obtain their stress intensity geometry/loading factors. The results show that the bolt tightening torque significantly reduces the effective geometry/loading factor, and thus increases the joint fracture strength compared to bolt-less simple edge crack specimens. However, the bolt clamping effect on increasing the fracture strength was almost the same for different crack lengths.

Determination of mechanical properties of PVC foam using a modified Arcan fixture

The design and development of a modified Arcan fixture (MAF) is described. The purpose of the fixture is to characterise polymer foam materials with respect to their tensile, compressive, shear and bidirectional mechanical properties. The MAF enables the application of pure compression or high compression to shear bidirectional loading conditions that is not possible with conventional Arcan fixtures. The tensile and shear behaviour to failure of a cross-linked Divinycell H100 PVC foam core material are studied using Digital Image Correlation (DIC). A detailed investigation of the parasitic effects of the fixture and misalignment of the fixture and loading machine are discussed. Thermoelastic Stress Analysis (TSA) is used to directly examine and validate the uniformity and symmetry of the stress fields obtained for both tensile and shear specimens. To account for the inhomogeneity of the strain field across the specimen cross sections, a “correction factor” for the measured “gauge section” surface strains has been determined using nonlinear finite element analysis (FEA). The outcome is a set of validated mechanical properties that are in excellent agreement with material property measurements conducted using conventional test tensile and shear test fixtures.

Investigating bolt clamping force effect on the mixed mode fracture strength and stress intensity factor for an edge crack in PMMA specimens

► The fracture strength of bolted specimens is greater than the simple edge crack specimens. ► The fracture strength of the PMMA specimens in mode II is bigger than mode I. ► The geometry factor of both modes I and II decreases when the specimen is bolt clamped. Experimental and numerical investigations have been conducted to evaluate the effect which the clamping force, resulting from torque tightening a nut and bolt, has on the mixed mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate having an edge crack. The experimental tests were carried out on three batches of simple edge crack and bolt torque tightened edge crack specimens made from poly methyl-methacrylate (PMMA) rectangular plate. The specimens’ fracture strengths were obtained using a tensile testing machine in different loading angles using a modified Arcan fixture. In numerical part, finite element simulations were used to model the three test specimens variants used and thereby establish their stress intensity geometry/loading factors. The results show that the bolt tightening torque, and hence the plate clamping force, has a significant effect on reducing the equivalent mixed mode stress intensity factor of all loading angles and the equivalent factor, and thus the joint fracture strength compared to bolt-less simple edge crack specimens.

Strategies for the Analysis of the Behavior of an Adhesive in Bonded Assemblies

Experimental and numerical analyses of the mechanical behavior of bonded joints can be made particularly difficult by the influence of edge effects. Therefore, understanding the stress distribution in an adhesive joint can lead to improvements in adhesively-bonded assemblies. Such an analysis is proposed in the case of usual single lap shear specimens. Stress singularities can contribute to the initiation and propagation of cracks in the adhesive. Thus, in order to obtain reliable experimental data to analyze the nonlinear behavior of an adhesive in an assembly, tests which strongly limit the influence of stress singularities must be proposed. The design and the abilities of such a device for shear tests are presented. Moreover, some experimental results obtained using a modified Arcan fixture, which has been designed to strongly limit edge effects, are presented in the case of monotonic and complex history loadings. Furthermore, a 2D non associated elasto-visco-plastic model is proposed to accurately describe the experimental behavior under tensile-shear monotonic loadings. An extension of this model is also proposed to represent relaxation type effects under shear loadings.

A new method to obtain shear modulus of solid propellant

The shear modulus G( t) is one of the most important mechanical parameters of viscoelastic materials, and it is widely used in 3-dimensional stress and strain calculation and strength analysis, but it is very difficult to be obtained directly from the experiment. In this article, the feasibility and process of shear relaxation experiments were discussed, and the Arcan fixture and butterfly specimen were considered; with the modified Arcan fixture, the shear relaxation experiments of solid propellant under different temperatures were carried out. The master curve of shear relaxation modulus and the parameters of W.L.F were obtained. This new method brings forward a new way to study the viscoelastic properties of solid propellant.

Experimental and Numerical Investigation of the Mixed-Mode Delamination of PAN Based Carbon/Epoxy Composite Using Modified Arcan Fixture

The present research examines analytically and experimentally the mode-I and mode-II and mixed mode-Interlaminar fracture toughness of PAN based carbon/epoxy composite. A modified Arcan fixture, well-suited for the study of the behavior of used composite assemblies, was developed in order to focus on the analysis of the fracture behavior of the material. The edge effects are minimized by using an appropriate design of the substrates so that experimental results give reliable data. Also the mode-I and mode-II stress intensity factors were computed for different crack lengths and load orientation angles using finite element analysis. The numerical results show that the modified Arcan specimen is able to provide pure mode-I, pure mode-II and any mixed mode loading conditions. It is shown that the results obtained from the fracture tests are consistent very well with mixed mode fracture theories. Obtained results indicated that fracture toughness and stress intensity factor for sliding mode enhanced up when the loading angle increased. Mechanism of fracture and toughening were examined by using scanning electron microscopy.

Characterization and Simulation of Mechanical Behavior of 6063 Aluminum Alloy Thin-Walled Tubes

The material characterization and mechanical behaviors of aluminum alloy (6063) were investigated using flat specimens and the butterfly specimens with a modified Arcan fixture. The butterfly specimens were tensile-loaded at various stress triaxiality by changing the loading angles of the Arcan fixture. The flat specimens were tensile-loaded at various strain rates. The results of tensile tests at various stress triaxiality showed that the curves of engineering stress-engineering strain are obviously different. The results of different strain rates tensile tests showed that the yield stress and fracture stress increased slightly with strain rates increasing, however the fracture strain decreased obviously with strain rates increasing, and the ultimate strength almost remained constant. The Johnson-cook constitutive model could be used to describe the dynamic axial crashing behavior of thin-walled 6063 aluminum alloy tubes under impact loading. The material constants of the Johnson-cook constitutive model and failure model were acquired to be used for FEM simulation through material characterization.

Mechanical Behavior of Syntactic Foams for Deep Sea Thermally Insulated Pipeline

Ultra Deep offshore oil exploitation (down to 3000 meters depth) presents new challenges to offshore engineering and operating companies. Flow assurance and particularly the selection of insulation materials to be applied to pipe lines are of primary importance, and are the focus of much industry interest for deepwater applications. Polymeric and composite materials, particularly syntactic foams, are now widely used for this application, so the understanding of their behavior under extreme conditions is essential. These materials, applied as a thick coating (up to 10-15 cm), are subjected in service to: - high hydrostatic compression (up to 30 MPa) - severe thermal gradients (from 4°C at the outer surface to 150°C at the inner wall), and to high bending and shear stresses during installation. Damageable behavior of syntactic foam under service conditions has been observed previously [1] and may strongly affect the long term reliability of the system (loss of thermal properties).This study is a part of a larger project aiming to model the in-service behavior of these structures. For this purpose it is important to identify the constituent mechanical properties correctly [2, 3]. A series of tests has been developed to address this point, which includes: - hydrostatic compression - shear loading using a modified Arcan fixture This paper will describe the different test methods and present results obtained for different types of syntactic foams.

Influence of adhesive bond line thickness on joint strength

While the geometry of aerospace assemblies is carefully controlled, for many industrial applications such as marine structures bond line thickness can vary significantly. In this study epoxy adhesive joints of different thicknesses between aluminium substrates have been characterized using physico-chemical analyses (differential scanning calorimetry, DSC; dynamic mechanical analysis, DMA; spectroscopy), nano-indentation and mechanical testing. Thermal analyses indicated no influence of thickness on structure. Nano-indentation revealed no evidence of an interphase at the metal/epoxy interface, nor any change in modulus for different thicknesses, though Raman spectroscopy suggested there may be slight variations in composition close to the substrates. However, mechanical testing using the modified Arcan fixture indicated a significant drop in strength and failure strain under pure tension and a smaller reduction for tension/shear and pure shear loads as thickness increased. Examination of sections through joints did not indicate any physical reason for this, but numerical analysis of the stress state revealed larger stress concentration factors for tensile loading in thick joints, which may explain the thickness effect. It is recommended that joint thickness should be kept below 0.8 mm to avoid obtaining artificially low values with the Arcan test.

Analysis of the nonlinear behavior of adhesives in bonded assemblies—Comparison of TAST and Arcan tests

This paper describes a study in which the shear behavior of a structural epoxy adhesive has been measured using the standard thick adherend shear test (TAST) specimen and a modified Arcan test. A numerical study of the TAST test taking into account the nonlinear behavior of the adhesive and the finite deformations of the adhesive joint, shows that there is a localization of plastic zones close to the adhesive–substrate interface near the free edge of the adhesive. Experimental tests carried out with steel and aluminum substrates and with various adhesives also show that failure initiates in this region. These edge effects in the TAST fixture can lead to an incorrect analysis of the behavior of the adhesive (for instance, underestimation of the shear stress in the joint at failure), particularly when an adhesive failure mode is dominating. The modified Arcan fixture provides a more homogeneous stress state. A similar improvement of the TAST fixture is proposed.

On Modelling the Non-linear Behaviour of Thin Adhesive Films in Bonded Assemblies With Interface Elements

This paper presents the development of a numerical model for an adhesive in an assembly, and compares results with experiments obtained from a modified Arcan fixture. The use of joint type elements (or interface elements) allows reducing the numerical cost of simulating adhesively-bonded assemblies. Under an elastic load, a non-uniform stress field is observed in the adhesive for the Arcan test and an inverse technique using simplified strategies has been developed to characterize the adhesive behaviour with a low-numerical cost. The study of monotonic radial loadings for a given displacement rate allows obtaining the elastic yield envelope in the normal stress–shear stress plane and pointing out the higher strength under a compression-shear mixed loading, compared to a tension-shear one. Moreover, the analysis of the adhesive deformations for tension-shear loadings shows larger displacements in shear than in peel. These effects must be considered in the numerical models. Besides, modelling of the viscoplastic behaviour of the adhesive in an assembly is proposed for a wide range of deformation rates under pure shear loadings. Numerical simulations show the usefulness of joint type elements, compared to traditional continuum elements in order to model the non-linear behaviour of the adhesive in bonded assemblies.