Showing papers in "Strain in 2014"

An Evaluation of Convergence Criteria for Digital Image Correlation Using Inverse Compositional Gauss–Newton Algorithm

Abstract: A fast, robust and accurate digital image correlation (DIC) method, which uses a robust zero-mean normalized sum of squared difference correlation criterion, a sophisticated reliability-guided displacement tracking strategy and an efficient inverse compositional Gauss–Newton (IC-GN) algorithm, was recently proposed for full-field deformation measurement As an iterative local optimization algorithm, IC-GN algorithm iteratively solves for the incremental warp assumed on the reference subset until the preset convergence criteria are satisfied In the literature, different convergence criteria have been set for iterative optimization algorithms However, on the one hand, stringent convergence criteria lead to increased number of iterations and lessen the computational efficiency On the other hand, too loose convergence conditions enhance the computational efficiency but may decrease the registration accuracy Understanding the impact of prescribed convergence criteria on DIC measurement and how to choose proper convergence criteria are therefore fundamental problems in realizing high-efficiency yet high-accuracy DIC analysis In this paper, the convergence characteristics of IC-GN algorithm are investigated in terms of convergence speed and radius of convergence using real experimental images The effect of various convergence criteria on the efficiency and accuracy of IC-GN algorithm are carefully examined Recommendations are given to select proper convergence criteria for more efficient implement of IC-GN algorithm

50th Anniversary Article: Comparison Studies of Full Wavefield Signal Processing for Crack Detection

Abstract: The signals acquired by measurements of elastic wave propagation have been used for damage detection since the 1970s. The measurements have been carried out mostly by using piezoelectric transducers of various types. Many different sensor configurations and data processing have been proposed to detect and localise structural defects, both for real-time and off-line testing. Nevertheless, in the last decade, significant progress in the measurement techniques such as scanning laser Doppler vibrometry and shearographic interferometry has been made. These techniques enable measurement of a full wavefield of elastic waves. This opens up new possibilities and solutions for the problems of the damage detection in structures. Many researchers successfully applied this type of measurements for the damage detection and localisation in thin-walled structures. Moreover, advanced signal processing techniques, such as wavenumber filtering, give the possibility of damage size estimation, by filling the gap between damage detection and damage prognosis. The aim of this paper is to give a comprehensive review of methods used for the full wavefield measurement. It also describes and compares selected signal processing algorithms developed for damage detection and visualisation based on these measuring techniques. Criticism aspects, as well as advantages of each algorithm, are denoted based on the authors' expertise in the field.

Digital Volume Correlation Applied to X-ray Tomography Images from Spherical Indentation Tests on Lightweight Gypsum

Abstract: Cylinders made of lightweight gypsum are extracted from industrial plasterboard and then indented in-situ in an X-ray tomograph. The results from the in-situ experiment show that a compacted zone develops under the indenter, displaying a very sharp boundary with the undamaged material. Tomographic imaging during the mechanical load associated with digital volume correlation enable the displacement fields to be measured during the test. However, because of the inhomogeneous nature of the indentation test, a high spatial resolution for the displacement is called for, and because the range of displacement amplitudes is small, uncertainties on the measured displacement and strain fields are large. In this study, a new methodology is presented to address integrated digital volume correlation based on a library of fields computed from a commercial finite element software. It allows many fluctuations in the estimated displacement fields to be filtered out and the measurement to be much more robust and reliable. This opens new pathways for the identification of mechanical properties.

Effect of Sensor Noise on the Resolution and Spatial Resolution of Displacement and Strain Maps Estimated with the Grid Method

Abstract: This paper deals with noise propagation from camera sensor to displacement and strain maps when the grid method is employed to estimate these quantities. It is shown that closed-form equations can be employed to predict the link between metrological characteristics such as resolution and spatial resolution in displacement and strain maps on the one hand and various quantities characterising grid images such as brightness, contrast and standard deviation of noise on the other hand. Various numerical simulations confirm first the relevance of this approach in the case of an idealised camera sensor impaired by a homoscedastic Gaussian white noise. Actual CCD or CMOS sensors exhibit, however, a heteroscedastic noise. A pre-processing step is therefore proposed to first stabilise noise variance prior to employing the predictive equations, which provide the resolution in strain and displacement maps due to sensor noise. This step is based on both a modelling of sensor noise and the use of the generalised Anscombe transform to stabilise noise variance. Applying this procedure in the case of a translation test confirms that it is possible to model correctly noise propagation from sensor to displacement and strain maps, and thus also to predict the actual link between resolution, spatial resolution and standard deviation of noise in grid images.

Dynamic Brazilian Test of Brittle Materials Using the Split Hopkinson Pressure Bar and Digital Image Correlation

Abstract: In this paper, the dynamic tensile properties of brittle materials were studied by using the dynamic Brazilian tests. Combined with a high-speed photographic system, a split Hopkinson pressure bar (SHPB) was used to conduct dynamic Brazilian tests on three brittle materials: Al2O3 ceramic, granite and poly(methyl methacrylate). Based on the images recorded by a high-speed camera, the displacement and strain fields were obtained by the digital image correlation method. The dynamic deformation and failure of the brittle materials were analysed. The rate-related dynamic tensile strength of the three brittle materials was also determined and analysed. The results show that the dynamic Brazilian test is an effective method to study the dynamic tensile properties of brittle materials.

Evaluating J-integral from Displacement Fields Measured by Digital Image Correlation

Abstract: Path integral, domain integral and least squares methods for evaluating J-integral from measured displacement fields for a power-law hardening material are described in this paper. The values of the J-integral are evaluated by applying the path and domain integral methods to the displacement fields obtained by elastoplastic finite element analysis and the displacement fields obtained through the measurement using digital image correlation. Results show that the values obtained by the domain integral method are slightly better than those by the path integral method, because the domain integral method efficiently uses the full-field measurement data. The values of the J-integral are also evaluated by the least squares method with the Hutchinson, Rice, and, Rosengren displacement fields. Results show that the J-integral can be obtained by the least squares method simply and easily without any calculation of the integration. The J-integral values obtained by the least squares method agree well with the values obtained using other methods. Because J-integral can be evaluated easily by any method described in this paper, it is expected that these methods are applicable to various fracture problems during experimental evaluation of structural components.

Comparison of Different Biaxial Tests for the Inverse Identification of Sheet Steel Material Parameters

Abstract: The aim of this work is to compare different biaxial specimen geometries and loading conditions concerning their applicability as experimental database for an inverse finite element model updating procedure to identify the material parameters of sheet steel Therefore, the deformation of the specimens is recorded with an optical, three-dimensional full-field deformation measurement system, and the utilised displacement data at the surface of the specimens are calculated via digital image correlation The numerical material model for the simulations is based on a three-dimensional, anisotropic elasto-plastic ansatz and is implemented into a commercial finite element software code The material parameters that are identified with the different specimen geometries are the hardening variables and the anisotropic plastic values Based on the identification results, a selection criterion for the evaluation of specimen geometries for the inverse parameter identification is presented

50th Anniversary Article: The Origin and Management of Residual Stress in Heat-treatable Aluminium Alloys

Abstract: The presence of macroscopic residual stresses in heat-treatable aluminium alloys can give rise to machining distortion, dimensional instability and increased susceptibility to in-service fatigue and stress corrosion cracking. This paper presents and reviews details about the residual stress magnitudes and distributions introduced into wrought aluminium alloys by the thermal operations associated with heat treatment. Experimental measurement data and the results of finite element analysis are presented and discussed. The available technologies by which residual stresses in aluminium alloys can be relieved are reviewed. The limitations of these techniques are described, and recommendations are made as to selecting the most appropriate technique to manage residual stresses. Opportunities for the future optimisation of these techniques are also presented.

Automated Fast Initial Guess in Digital Image Correlation

Abstract: A challenging task that has hampered the fully automatic processing of the digital image correlation (DIC) technique is the initial guess when large deformation and rotation are present. In this paper, a robust scheme combining the concepts of a scale-invariant feature transform (SIFT) algorithm and an improved random sample consensus (iRANSAC) algorithm is employed to conduct an automated fast initial guess for the DIC technique. The scale-invariant feature transform algorithm can detect a certain number of matching points from two images even though the corresponding deformation and rotation are large or the images have periodic and identical patterns. After removing the wrong matches with the improved random sample consensus algorithm, the three pairs of closest and non-collinear matching points serve for the purpose of initial guess calculation. The validity of the technique is demonstrated by both computer simulation and real experiment.

Image correlation to evaluate influence of hygrothermal loading on wood

Abstract: Many wooden objects from cultural heritage consist in wooden panels, painted on one face Some of these panels show permanent cupping, micro-cracks of the painted layer, and cracks of the painted support itself Different physical and mechanical phenomena are at the origin of these damages: wood is a hygroscopic material (its dimensions vary with humidity), it is highly anisotropic, the paint layer on one face has properties of permeability different from those of raw wood of the back face, and a rigid frame possibly restrained the deformation of the panel Experimentations on our mock-up panels combined with numerical simulations of these panels in real situations of hygrothermal fluctuations will allow us to test specific situations and eventually to make suggestions to conservators and restorers and guide them in their interventions Hygrothermal treatments are often used to improve wood durability thanks a reduction of its hygroscopicity They have been considered as means to reproduce the physical properties of ancient wood We intend to model the mechanisms involved in mechanical and chemical effects of wooden painted panels exposed to climatic variations To develop such conservation tool, we need to work on mock-up, which replicate panel painting So we will investigate the process of hygroscopic ageing and compression set generation of the back of painted panels to explain their permanent cupping and replicate their ageing state For this purpose, digital image correlation is used to evaluate the strain field of the section of a wood piece submitted to variations of relative humidity

Cross‐laminated Timber Plates Subjected to Concentrated Loading

Abstract: Since wood products for structural elements, especially cross-laminated timber (CLT), have gained importance in the building sector, the need for appropriate and reliable design codes for such wood products has become essential. For the improvement and development of design concepts, a profound knowledge about the mechanical behaviour of these products is necessary. For this reason, this work focuses on global failure mechanisms and the corresponding evolution of different crack modes in CLT plates, depending on geometric and/or material related properties. Therefore, plate-bending experiments on three- and five-layered CLT plates were carried out. In addition to standard evaluation methods, each specimen was cut into small cubes to identify the failure modes inside the plates. Regions with dominant shear failure, tensile failure, delamination and mixed failure modes could be captured and connected to geometry and loading situation. Based on this evaluation, well-known but not yet in detail described effects, such as the ductile structural behaviour of CLT plates, can be explained. Moreover, the evolution of rolling shear failure modes as well as from which point the progressive failure highly affects the plate stiffness is investigated and analysed in detail.

An Introduction to the Properties of Silica Glass in Ballistic Applications

Abstract: The purpose of this review is to introduce the interested reader to the literature on the use of silica glasses in ballistic applications. These applications include optically transparent windows (where their amorphous nature, appropriate band gap, and cheapness make them ideal window materials) as well as layers in opaque armours designed to be resistant to shaped charges. In the latter application, their increase in volume (bulking) on fracture disrupts the shaped charge jet. Other topics covered in this review include low-velocity damage by windborne debris, liquid and solid impact, dynamic methods of testing (Hopkinson pressure bar, Taylor impact, and plate impact shock loading), and constitutive models. The use of glass as a shaped charge is also discussed.

Local Measurement of Stress–Strain Behaviour of Ductile Materials at Elevated Temperatures in a Split-Hopkinson Tension Bar System

Abstract: A split-Hopkinson tension bar system is modified to allow for measuring the stress-strain behaviour of ductile materials at large strains, high strain rates and elevated temperatures. The specimen is heated by induction, and a pyrometer provides a laser-based temperature measurement that controls the testing temperature in a feed-back loop. A high-speed digital camera system and an edge detection algorithm are used to obtain local measures of strain after necking of the axisymmetric specimens. Using the local strain measurements and Bridgman's analytical formulas, it is feasible to find the equivalent stress-strain curve to fracture for different levels of strain rate and temperature. Thermal and thermo-mechanical finite element simulations of the test set-up are used to evaluate the validity of the proposed experimental method.

A multigrid PGD-based algorithm for volumetric displacement fields measurements

Abstract: The use of a finite elements-based Digital Volume Correlation (FE-DVC) leads to lower measurement uncertainties in comparison to subset-based approaches. However, the associated computing time may become prohibitive when dealing with high-resolution measurements. To overcome this limitation, a Proper Generalised Decomposition solver was recently applied to 2D digital image correlation. In this paper, this method is extended to measure volumetric displacements from 3D digital images. In addition, a multigrid Proper Generalised Decomposition algorithm is developed, which allows to use different discretisations in each term of the decomposition. Associated to a coarse graining of the digital images, this allows to avoid local minima, especially in presence of large displacements. Synthetic and practical cases are analysed with the present approach, and measurement uncertainties are compared with standard FE-DVC. Results show that such an approach reduces the computational cost (when compared to FE-DVC) whilst maintaining lower measurement uncertainties than standard subset-based DVC.

Full‐Field Measurements on Low‐Strained Geomaterials Using Environmental Scanning Electron Microscopy and Digital Image Correlation: Improved Imaging Conditions

Abstract: Full-field strain measurement at microscale on geomaterials by means of hydromechanical in situ testing and imaging in an environmental scanning electron microscope and digital image correlation techniques is a challenging task because of both low magnitude of investigated strains (typically of the order of 10−3) and unfavourable imaging conditions. In view of improving strain measurement accuracy, three major sources of measurement errors are evaluated, and methods to minimise their effects are proposed. First, a fast and simple procedure to accurately quantify image noise is proposed and subsequently used to optimise various environmental scanning electron microscope setting parameters, such as dwell time, spot size, working distance and chamber pressure. Second, a specific procedure to limit magnification fluctuations to a sufficiently low level is described. Finally, digital image correlation systematic errors are quantified on real images, and several ways to reduce their amplitude are compared. The combination of these improvements finally allows us to reach an appropriate accuracy for overall strain measurements and characterisation of microscale heterogeneities.

Anti‐buckling Fixture for Large Deformation Tension–Compression Cyclic Loading of Thin Metal Sheets

Abstract: Determination of cyclic properties of a material used for many engineering structures, e.g. airframes, requires preparation of specimens from two-dimensional, shell structures. A specimen must be cut out from the structure component in the way allowing application of a load corresponding to that reflecting service cases. In order to fulfil this requirement, tension–compression testing of the flat specimen is necessary to obtain credible data. Problems associated with such kind of testing are discussed in this paper. In the first part, an extensive review of the anti-buckling fixtures developed up to now is given. Several representative solutions are discussed, with special emphasis on the technique limitations. In the second part of the paper, detailed description of the proposed design is given. Finally, the results of preliminary tests carried out using new fixture to mount the flat specimens cut from steel sheets are presented. Application of the proposed testing technique allowed tension–compression tests to be performed at the displacement amplitude within the range ±5 mm what corresponds to the maximum strain amplitude of ±0.4 for the specimen gauge length to be equal 12.5 mm. Taking into account all data captured by means of new fixture, one can conclude that the technique is promising with respect to providing data for modelling of cyclic deformation behaviour for shell structures.

Influence of Wooden Board Strength Class on the Performance of Cross-laminated Timber Plates Investigated by Means of Full-field Deformation Measurements

Abstract: Although cross-laminated timber (CLT) plates are increasingly used in high-performance building structures, a tailored composition of them or, at least, a performance-based classification scheme is not available. Especially, the influence of the quality of the ‘raw’ material (wooden boards) on the load carrying capacity of CLT elements is hardly investigated yet. For this reason, within this work, bending tests on 24 CLT plates consisting of wooden boards from three different strength classes have been carried out. The global mechanical response as well as the formation of failure mechanisms were investigated, including a full-field deformation measurement system, which allowed for a qualitatively as well as quantitatively identification of board failure modes. Interestingly, no influence of the board strength class on the elastic limit load of the CLT plates was observed, but the situation was different for the load displacement history beyond the elastic regime, where basically, two different global failure mechanisms could be distinguished. The obtained knowledge about the ‘post-elastic’ behaviour of CLT plates may serve as a basis for the optimisation of CLT products and the development or improvement of design concepts, respectively. Moreover, the obtained large ‘post-elastic’ capacity reserve of CLT consisting of high quality boards could lead to a better utilisation of the raw material.

Development of a High‐fidelity Experimental Substructure Test Rig for Grid‐scored Sandwich Panels in Wind Turbine Blades

Abstract: This paper outlines high-fidelity experimental substructure testing of sandwich panels which constitute the aerodynamic outer shell of modern wind turbine blades. A full-scale structural experimental and numerical characterisation of a composite wind turbine blade has been conducted. The development of a full-scale numerical model is detailed, and the necessary experimental set-up is described. Further, the numerical and experimental results obtained are compared, and an idealised set of boundary conditions for a chosen blade substructure is presented. From this, the development of a test rig suitable for representing the established loading and boundary conditions is presented, and some preliminary experimental results are discussed. The work provides a road map for developing high-fidelity experimental substructure tests, which in more generic terms are applicable to similar developments of substructure tests for composite wind turbine blades. Furthermore, recommendations on the use of grid-scored sandwich structures in wind turbine blades are presented, which outline the sensitivity in terms of quasi-static strength to the established loading conditions.

Dynamic High-temperature Testing of an Iridium Alloy in Compression at High-strain Rates: Dynamic High-temperature Testing

Abstract: Iridium alloys have superior strength and ductility at elevated temperatures, making them useful as structural materials for certain high-temperature applications. However, experimental data on their high-strain -rate performance are needed for understanding high-speed impacts in severe environments. Kolsky bars (also called split Hopkinson bars) have been extensively employed for high-strain -rate characterization of materials at room temperature, but it has been challenging to adapt them for the measurement of dynamic properties at high temperatures. In our study, we analyzed the difficulties encountered in high-temperature Kolsky bar testing of thin iridium alloy specimens in compression. We made appropriate modifications using the current high-temperature Kolsky bar technique in order to obtain reliable compressive stress–strain response of an iridium alloy at high-strain rates (300–10 000 s-1) and temperatures (750 and 1030°C). The compressive stress–strain response of the iridium alloy showed significant sensitivity to both strain rate and temperature.

50th Anniversary Article: Review on Interface Fracture and Delamination of Composites

Abstract: In this review paper, a methodology for measuring the interface fracture toughness of a crack between two isotropic, homogeneous materials and a delamination between two laminae of unidirectional composite materials of differing directions is presented. Four cases are considered. Two isotropic material pairs are described: glass/epoxy and two ceramic clays. Similar studies are presented for two cross-ply laminates: 0°/90° and +45°/−45. The Brazilian disk specimen was used to carry out mixed mode fracture tests. The load and crack or delamination length at fracture were measured and used in a finite element analysis to determine the displacement field. An interaction energy or M-integral was used to obtain the stress intensity factors at failure. These in turn were employed to calculate the critical interface energy release rate and two phase angles ψ and φ, which measure the mode mixity. For the M-integral and for each interface crack or delamination, the first term of the asymptotic solution of the fields is required. For two isotropic materials, these solutions are well known. For the laminates described here, they were determined by the Stroh and Lekhnittski formalisms. A failure criterion determined from first principles is presented. The values of , ψ and φ are used to specify the criterion for each material pair. A statistical analysis is presented, which predicts a 5% probability of failure.

Optical Coherence Tomography for the Study of Polymer and Polymer Matrix Composites

Abstract: Based on low-coherence interferometry, a hybrid optical coherence tomography (OCT) system has been built. It coupled time-domain OCT (TD-OCT) and Fourier-domain OCT (FD-OCT) into one system. TD-OCT can take the advantage of a large axial scan range while FD-OCT has superior performance in fast imaging as no axial scan is needed. The two imaging modalities shared a broad bandwidth light source with a centre wavelength of 1550 nm, which is less scattering and can give better penetration depth in the polymer-based material than the shorter wavelength used for biomedical applications. 2D translation stages were incorporated in the system to make cross-sectional and volume imaging available. It can provide larger scan range as well as less image distortion compared with galvo scanners. Finally, we reported on successfully characterized specimens such as polymer coatings and glass-fibre composites. The cross-sectional and volumetric images obtained clearly show the microstructure of the materials. The thickness as well as the defects, e.g. microcrack and delamination can be determined.

Comparison of Shearography to Scanning Laser Vibrometry as Methods for Local Stiffness Identification of Beams

Abstract: Local stiffness of Euler–Bernoulli beams can be identified by dividing the bending moment of a deformed beam by the local curvature. Curvature and moment distributions can be derived from the modal shape of a beam vibrating at resonance. In this article, the modal shape of test beams is measured by both scanning laser vibrometry (SLV) and shearography. Shearography is an interferometric optical method that produces full-field displacement gradients of the inspected surface. Curvature can be obtained by two steps of derivation of the modal amplitude (in the case of SLV) or one step of derivation of the modal shape slope (in the case of shearography). Three specially prepared aluminium beams with a known stiffness distribution are used for the validation of both techniques. The uncertainty of the identified stiffness distributions with both techniques is compared and related to their signal-to-noise ratios. A strength and weakness overview at the end of the article reveals that the shearography is the technique that shows the most advantages.

Inverse Identification at Very High Strain Rate of the Johnson–Cook Constitutive Model on the Ti‐6Al‐4 V Alloy With a Specially Designed Direct‐impact Kolsky Bar Device

Abstract: In the present work, an inverse identification of the Johnson–Cook constitutive model is performed on the titanium alloy Ti-6Al-4 V at three strain rates until about 2·10 4 s-1 (till about 1.1·10 4 s-1 of plastic strain rate) on a specially designed direct impact Kolsky bar device. First, the design of such a device must meet several criteria, and is shown to be the solution of an optimization problem. A systematic design procedure for such a device is then introduced. Second, an inverse analysis using the finite element code ABAQUS is carried out to identify the Johnson–Cook parameters on experimental data obtained with the designed system.

An Efficient Technique for Surface Strain Recovery from Photogrammetric Data using Meshless Interpolation

Abstract: In recent years, techniques for monitoring displacements and surface strains in laboratorial tests have been introduced. Some of those methods are based on photogrammetry and image post-processing, allowing both displacement and strain fields to be monitored at a significant number of target points at any stage. This manuscript aims at contributing by proposing a simple and comprehensive approach for monitoring experimental tests using photogrammetry. An innovative strategy is introduced to efficiently recover the strain fields at the surface of a loaded specimen. This technique is based on a meshless approach and does not require any triangulation of data points. In this approach, shape functions are constructed from radial basis function interpolation in local subdomains, for which different numbers of data points may be included. To allow accounting for the presence of generic geometries, which may include concavities or convexities, the selection of interpolating points is performed considering a line-of-sight algorithm, avoiding inappropriate selection of neighbouring points. A number of practical examples are here presented for different concrete specimens tested in the laboratory. The presented results indicate that the proposed method can provide smooth strain fields, which allow an easy identification of microcracking development at early stages of loading.

A Fourier-series-based virtual fields method for the identification of 2-D stiffness and traction distributions

Abstract: The virtual fields method (VFM) allows spatial distributions of material properties to be calculated from experimentally determined strain fields. A numerically efficient Fourier-series-based extension to the VFM (the F-VFM) has recently been developed, in which the unknown stiffness distribution is parameterised in the spatial frequency domain rather than in the spatial domain as used in the classical VFM. However, the boundary conditions for the F-VFM are assumed to be well-defined, whereas in practice, the traction distributions on the perimeter of the region of interest are rarely known to any degree of accuracy. In the current paper, we therefore consider how the F-VFM theory can be extended to deal with the case of unknown boundary conditions. Three different approaches are proposed; their ability to reconstruct normalised stiffness distributions and traction distributions around the perimeter from noisy input strain fields is assessed through simulations based on a forward finite element analysis. Finally, a practical example is given involving experimental strain fields from a diametral compression test on an aluminium disc.

Performance Assessment of a Numerical Simulation Tool for Wooden Boards with Knots by Means of Full-Field Deformation Measurements

Abstract: In wooden boards, knots and the resulting fibre deviations in their vicinities are mainly responsible for qualitative downgrading of timber elements. Thus, the development of reliable numerical simulation tools for the determination of effective strength and stiffness properties of timber elements and, in a next step, for the development and evaluation of grading criteria is highly desirable. Due to the complexity of such tools, a comprehensive validation is required. Within this work, the suitability of full-field deformation measurements for four-point bending tests on wooden boards with knots is evaluated first. Next, the test series is used to validate a previously developed three-dimensional numerical simulation tool, which combines a geometrical model for the grain course and a micromechanical model for a density and moisture dependent characterisation of the clear-wood material. The digital image correlation technique proved to be capable to reproduce the strain fields in the vicinity of knots under bending load. Moreover, a very good correlation between numerical and experimental results was obtained.

High-Speed Stereo Digital Image Correlation: Application to Biaxial Fatigue

Abstract: To study the behaviour of a carbon-fibre reinforced-plastic laminate under a biaxial stress state, a new type of cruciform specimen has been developed. This specimen is loaded biaxially under static and cyclic fatigue conditions. The experiments are monitored simultaneously using digital image correlation with two high-speed cameras and infrared thermography. A comparison between the measurements and a finite element model is used to validate the design of the sample.

Experimental testing and full and homogenized numerical models of the low velocity and dynamic deformation of the trapezoidal aluminium corrugated core sandwich

Abstract: The simulations of the low velocity and dynamic deformation of a multi-layer 1050-H14 Al trapezoidal zig-zag corrugated core sandwich were investigated using the homogenized models (solid models) of a single core layer (without face sheets). In the first part of the study, the LS-DYNA MAT-26 material model parameters of a single core layer were developed through experimental and numerical compression tests on the single core layer. In the second part, the fidelities of the developed numerical models were checked by the split-Hopkinson pressure bar direct impact, low velocity compression and indentation and projectile impact tests. The results indicated that the element size had a significant effect on the initial peak and post-peak stresses of the homogenized models of the direct impact testing of the single-layer corrugated sandwich. This was attributed to the lack of the inertial effects in the homogenized models, which resulted in reduced initial peak stresses as compared with the full model and experiment. However, the homogenized models based on the experimental stress–strain curve of the single core layer predicted the low velocity compression and indentation and projectile impact tests of the multi-layer corrugated sandwich with an acceptable accuracy and reduced the computational time of the models significantly.

Strain and Displacement Controls by Fibre Bragg Grating and Digital Image Correlation

Abstract: Test control is traditionally performed by a feedback signal from a displacement transducer or force gauge positioned inside the actuator of a test machine. For highly compliant test rigs, this is a problem since the response of the rig influences the results. It is therefore beneficial to control the test based on measurements performed directly on the test specimen. In this paper, fibre Bragg grating (FBG) and Digital Image Correlation (DIC) are used to control a test. The FBG sensors offer the possibility of measuring strains inside the specimen, while the DIC system measures strains and displacement on the surface of the specimen. In this paper, a three-point bending test is used to demonstrate the functionality of a control loop, where the FBG and DIC signals are used as control channels. The FBG strain control was capable of controlling the test within an error tolerance of 20 µm m−1. However, the measurement uncertainty offered by the FBG system allowed a tolerance of 8.3 µm m−1. The DIC displacement control proved capable of controlling the displacement within an accuracy of 0.01 mm.

A Drop-Bar Setup for the Compressive Testing of Rubber-Like Materials in the Intermediate Strain Rate Range

Abstract: Only few works have been focused on soft materials at medium strain rates. In the present paper, we propose a new experimental setup to investigate the compressive response of rubber-like materials in the range of intermediate strain rates, such experiments being difficult to conduct with conventional mechanical frames or traditional Hopkinson bar technique. The new apparatus consists in a hybrid technique that takes advantage of the assets of the Hopkinson bar and drop-weight techniques. It involves an instrumented tubular aluminium bar connected to a steel bar, both being accelerated by gravity. The new device, associated with a classical servo-hydraulic machine, permits to characterise natural rubber material at strain rate ranging from 0.01 to 100 s−1, highlighting the sensitivity of this material to strain rate.