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Most methodologies to measure the moisture‐induced deformation (hygro‐expansion) of paper microconstituents, including fibres and interfibre bonds, are low resolution or time‐consuming. Hence, here, a novel method is proposed and validated to measure high‐resolution full‐field strain maps of paper microconstituents during hygro‐expansion, based on environmental scanning electron microscopy (ESEM). To this end, a novel climate stage enables accurate control of the relative humidity (RH) near the specimen in the ESEM from 0%–100%. The fibre surface, which is decorated a priori with a microparticle pattern, is captured during RH change. Subsequently, correlating the fibre surface using a dedicated global digital image correlation algorithm enables high‐resolution hygro‐expansion strain maps. Method optimisation involved performing contrast enhancement, scan‐correction to reduce ESEM artefacts and a background correction, resulting in a strain resolution of 6·10−4. Method validation revealed that the fibres' crystallinity is affected by the electron beam, even for minimal invasive electron beam settings. Interestingly, however, the fibres consistently exhibit conventional hygro‐expansion behaviour during the drying slopes. Using the optimised procedure, hygro‐expansion characterisation of two interfibre bonds and four interfibre bond cross‐sections revealed the competition between the low longitudinal and large transverse fibre hygro‐expansion in the bonded area. [ABSTRACT FROM AUTHOR]

The article discusses a study on the mechanical characteristics of paper packaging. The ecological aspects of paper packaging, such as biodegradability and its low production costs have made these type of packaging widely used. The study used computational modeling, and possible mathematical models, to determine the strength of paper packaging under compression.

The checkerboard constitutes the best pattern for full‐field strain measurement because it maximizes image gradient. In the experimental mechanics community, employing this pattern is currently strongly limited because depositing it on the surface of specimens raises practical difficulties. A recent study shows, however, that it is technically possible by using a laser engraver. The present paper aims to push this solution forward by printing a checkerboard pattern on a thin polymeric film and then gluing the resulting laser‐engraved film on the specimen surface. The underlying idea is to separate the manufacturing process of this optical strain gauge on the one hand and its use on the other hand to help spread this strain measuring tool in the experimental mechanics community. The polymeric film employed here is the same as that used in the manufacturing process of classic electrical gauges, so one can rely on the know‐how of classic strain gauge bonding to glue this optical strain gauge on the specimen surface. The main difference between the proposed tool and classic electrical gauges is that the strain field beneath the polymeric support is measured instead of localized strain values. The paper is a proof of concept for this strain field measuring tool. The manufacturing and bonding processes are described in the paper. The localized spectrum analysis, a spectral technique developed for processing images of periodic patterns, is used to retrieve the strain fields from checkerboard images. Through two complementary examples, we show the ability of this new type of strain gauge to detect and quantify local details in the strain field beneath. A simplified 1D model is also proposed to assess the minimum width of the strain peak that can reliably be measured with this technique. [ABSTRACT FROM AUTHOR]

A better physical understanding of mesoscale and microscale mechanisms behind deformation and failure of paperboard material is important to optimize industrial packaging converting processes and decrease waste. In this study, these mechanisms were investigated using synchrotron X‐ray tomography during in situ continuous uniaxial tensile loading. High spatial and temporal data resolution enabled quantification of rapid changes in the material occurring before, during and after material failure. The evolution of 3D strain fields, fibre orientations and sample thickness showed that deformation and failure mechanisms differ significantly between samples tested in machine direction (MD), cross direction (CD) and 45° from the loading direction. In 45° and CD, gradual failure processes could be followed across several load steps. Immediately after failure, the in‐plane fracture region was significantly larger in both 45° and CD compared to MD. Both fracture characteristics and strain field distributions differed between the three material directions. Significant fibre reorientation was an active deformation mechanism in 45° already from the beginning of the loading, also present in CD after peak load but absent in MD. The MD‐dependent mechanisms interpreted and quantified at the scale of the fibre network in this study can help guide model development and likely have wider applicability to other paper‐based materials. [ABSTRACT FROM AUTHOR]

This study presents the basic ingredients needed to model the out-of-plane compression behaviour of a folded core up to the global peak load. This core is made of an aramid paper. The proposed approach is both experimental and numerical and handles the problem at different scales: fibre, paper and core scales. To have a good representation of the physics, two key points are essential. The first is the mechanical behaviour of the paper. The aramid paper can be modelled as a laminate composite. An experimental study of the paper behaviour in bending has been conducted to build a damage model for the paper. The second key point concerns the geometrical defects of the folded core. The defects, arising from the manufacturing process, are built by simulation of the folding process. The macroscopic behaviour of the core under out-of-plane compression loading is shown to be very sensitive to these defects. The results of a simulated crushing test are compared to experimental ones. A good agreement is shown. [ABSTRACT FROM AUTHOR]

As a non‐destructive inspection method, micro‐computed tomography has been employed for determining local properties of a cellulose‐based product, specifically pressboard. Furthermore, by utilizing the determined properties in a detailed numerical model, by means of a finite element analysis, we demonstrate a continuum anisotropic viscoelastic‐viscoplastic model. Through such a combination of non‐invasive experiments with accurate computations in mechanics, we attain a better understanding of materials and its structural integrity at a pre‐production stage increasing the success of the first prototype. In detail, this combination of micro‐computed tomography and finite element analysis improves accuracy in predicting materials response by taking into account the local material variations. Specifically, we have performed indentation tests and scanned the internal structure of the specimen for analysing the densification patterns within the material. Subsequently, we have used a developed material model for predicting the response of material to indentation. We have computed the indentation test itself by simulating the mechanical response of high‐density cellulose‐based materials. In the end, we have observed that pressboard, having initially a heterogeneous density distribution through the thickness, shows a shift in the densification to the more porous part after indentation. The densification maps of the simulated results are presented by comparing with the experimental results. A reasonable agreement is observed between the experimental and the simulated densifications patterns, which suggests that the proposed methodology can be used to predict densification also for other fibre‐based materials during manufacturing or in service loading. [ABSTRACT FROM AUTHOR]

ABSTRACT The measurement of early shrinkage cracking in concrete is important to prevent aesthetic issues and avoid surface cracking that could lead to reinforcement corrosion and reduce the durability, long-term service life and integrity of a structure. Moreover, the lack of standards and subjectivity of the very few methodologies proposed so far complicate its estimation. This research presents a new imaging methodology for evaluating and quantifying early shrinkage cracking patterns. The methodology was developed testing highly restrained square concrete slabs subjected to severe conditions of restraint and moisture loss. Its quantification consisted of photographing, processing the pictures and highlighting the cracks. For the first time, early shrinkage cracking in concrete can be measured through an experimental technique and quantified by means of geometric figures. In this way, more precise and automatic results are achieved, as flat figures adapt to the shape of cracks and store their properties. Therefore, parameters such as the total cracked area, total crack length, maximum crack width or average crack width were easily calculated. The results demonstrated the suitability of the wind tunnel test to produce significant cracking patterns, as well as the great capacity of the imaging methodology to identify and characterize the cracking pattern. [ABSTRACT FROM AUTHOR]

The paper provides a picture of the changes and advances in the field of strain measurement and related topics, as represented by the contents of the journal Strain, from its inception in 1965 to the present day. [ABSTRACT FROM AUTHOR]

With the advent of camera‐based full‐field measurement techniques such as digital image correlation (DIC), researchers have been trying to exploit such rich data sets through the use of more complex test configurations than the standard ones (uniaxial tension/compression, bending etc.). This new paradigm in mechanical testing of materials has recently been christened 'Material Testing 2.0'. This paper provides a brief overview of this field, which is currently seeing a large increase in research effort. [ABSTRACT FROM AUTHOR]

ABSTRACT In 'Geomechanical and Geochemical Evidence of Piezonuclear Fission Reactions in the Earth's Crust' [Carpinteri and Manuello, Strain (2011) 47(2), 267], the authors claim that the composition of the Earth's crust and of its atmosphere can be explained by hypothetical piezonuclear reactions. We argue that these claims are contrary to the known experimental evidence of the energy balance of the nuclear reactions involved, and we report a detailed criticism of the claims of the paper. [ABSTRACT FROM AUTHOR]

The calibration of solid constitutive models with full‐field experimental data is a long‐standing challenge, especially in materials that undergo large deformations. In this paper, we propose a physics‐informed deep‐learning framework for the discovery of hyperelastic constitutive model parameterizations given full‐field surface displacement data and global force‐displacement data. Contrary to the majority of recent literature in this field, we work with the weak form of the governing equations rather than the strong form to impose physical constraints upon the neural network predictions. The approach presented in this paper is computationally efficient, suitable for irregular geometric domains, and readily ingests displacement data without the need for interpolation onto a computational grid. A selection of canonical hyperelastic material models suitable for different material classes is considered including the Neo–Hookean, Gent, and Blatz–Ko constitutive models as exemplars for general non‐linear elastic behaviour, elastomer behaviour with finite strain lock‐up, and compressible foam behaviour, respectively. We demonstrate that physics informed machine learning is an enabling technology and may shift the paradigm of how full‐field experimental data are utilized to calibrate constitutive models under finite deformations. [ABSTRACT FROM AUTHOR]

The article discusses various papers dicussed at the symposium on "Digital Holography: Exploring Possibility of Using Digital Holography for Experimental Mechanics," including the paper about the development of digital holography, and another one on the history of holographic interferometry.

Even though the simulations used to describe the failure of laminates are becoming more and more predictive, complex testing under multiaxial loadings is still required to validate the design of structural parts in a wide range of industrial domains. It is thus essential to assess the actual boundary conditions to allow for an objective comparison between testing and calculations, in particular since the structural tests are complex and often leads to buckling. Therefore, accurate estimation of force and moment fluxes applied to the specimen is critical. In this context, stereo digital image correlation (SDIC) has proven to be an important measurement tool and provides very well‐resolved surface displacement fields, but the exploitation of such measurements to calculate fluxes remains problematic when testing composites. The first objective of this study is both to reduce the uncertainty associated with fluxes determination on a complex test and to simplify the extraction process with respect to existing procedures. The second objective is to make this methodology robust to geometrically non‐linear deformations. In this paper, we propose a new methodology that extracts minimal boundary conditions in the form of 3D mechanically admissible displacements fields. The approach developed uses a finite element SDIC (FE‐SDIC) method regularized by means of mechanical behaviour admissibility equations. Results show that the new methodology outputs much more accurate fluxes than classical data generated from multiple differentiations of the displacement fields. Excellent noise robustness is obtained and quantified. Numerical predictions have been satisfactorily compared with experimental data from one structural‐scale composite specimen under complex testing. [ABSTRACT FROM AUTHOR]

Biphasic flow propagating through a granular medium can be established as an unstable infiltration and can be precursors of nonnegligible deformations. Capturing such infiltration through the lens of a high‐resolution camera, full‐field strain maps are determined via digital image correlation (DIC). However, this image analysis technique is based on the fundamental assumption of grey level conservation between the reference and deformed images. In this paper, DIC algorithm is recalled, and suitable controlling parameters are introduced to improve the accuracy of the output displacement fields and to remove the grey level variations induced by an air‐water flow percolating through a Fontainebleau sand. In addition, an in‐depth investigation on the full‐field volumetric strain maps is conducted in order to extract the hidden relationship between the front propagation and the strains evolution. [ABSTRACT FROM AUTHOR]

Even though the simulations used to predict failure are becoming increasingly predictive, complex multiaxial loading tests are still required to validate the design of structural components in a wide range of industries. Large specimen testing often requires two different scales. A global Far Field to obtain boundary conditions and a local Near Field to evaluate strain gradients around discontinuities such as bolts, notches… The main goal of this study is to provide a continuous displacement over the whole specimen surface integrating data from multiple cameras. In this paper, we propose a new methodology that generates 3D displacements determined by finite‐element stereo digital image correlation in the Near Field and in the Far Field using a unique fractal speckle pattern and an off‐line determined texture. The displacements are obtained in the same coordinate system and on the same mesh. Satisfactory data fusion from both Near Field and Far Field images of a biaxial test on a notched laminate composite was obtained with a refined mesh at the notch tip. This methodology can be applied to any tests requiring multiple camera systems and will support the use of the finite‐element digital image correlation framework as an experimental‐numerical efficient technique. [ABSTRACT FROM AUTHOR]

Single‐reed musical instruments, such as the saxophone, generate sound through a complex interplay between the mechanics of the reed and the hydrodynamic and acoustic pressure in the instrument mouthpiece. To understand this complex mechanism, experimental data are lacking. This paper presents full‐field, time‐resolved measurements of strain and displacement of a vibrating saxophone reed, measured under mimicked realistic playing conditions. It is found that strain along the length axis of the reed is mainly expansive, except in a small zone near the tip where it becomes compressive when the reed touches the front edge of the mouthpiece. At the instant in the vibration phase where the reed touches the mouthpiece, significant bending and compressive strain appear along the direction perpendicular to the reed axis. Strain magnitudes in both directions are similar, with absolute values of 0.1%. Full‐field strain maps reveal subtle characteristics which are not revealed by displacement measurements. Bi‐axial bending and strain may be an essential component in reed mechanics, which up till now has been fully neglected in modelling. [ABSTRACT FROM AUTHOR]

Biphasic hyperelastic models have become popular for soft hydrated tissues, and there is a pressing need for appropriate identification methods using full‐field measurement techniques such as digital volume correlation. This paper proposes to address this need with the virtual fields method (VFM). The main asset of the proposed approach is that it avoids the repeated resolution of complex nonlinear finite element models. By choosing special virtual fields, the VFM approach can extract hyperelastic parameters of the solid part of the biphasic medium without resorting to identifying the model parameters driving the osmotic effects in the interstitial fluid. The proposed approach is verified and validated through three different examples: the first and second using simulated data and then the third using experimental data obtained from porcine descending thoracic aortas samples in osmotically active solution. [ABSTRACT FROM AUTHOR]

Reports developments related to engineering as of May 1, 2004. Selection of professors Robert Cripps and Brian Day to the editorial board of the journal "Strain"; Solicitation of research papers by the American Society for Engineering Education and Tsinghua University for an electronic conference on engineering education; Joint membership arrangement between the British Society for Strain Management and Society of Environmental Engineers.

Although full‐field measurement techniques have been well established, material characterization from these data remains challenging. Often, no closed‐form solution exists between measured quantities and sought material parameters. In this paper, a novel approach to determine the stiffness of thin curved membranes is proposed, based on the virtual fields method (VFM). Utilizing Kirchhoff‐Love shell theory, we show that the displacements can be decomposed into an in‐plane displacement and a rotation of the mid‐surface of the shell. Consequently, the strain tensor at the outer surface of the shell can then be decomposed into a membrane and a bending part. This allows for the VFM to be applied based only on data of the outer surface and on surfaces of arbitrary curvature. The method is first applied to simulated data. It is shown that the elastic modulus can be identified with less than 5% error if the thickness and Poisson ratio are known accurately. A 5% uncertainty in either the Poisson ratio or the thickness changes the identified value by 5%. Then, the method is applied on experimental data acquired on rubber samples having a dome‐like shape. Tensile tests are performed on the same samples, which permits to assess the linearized Young's modulus of this material for moderate strains (0–2.1%). Using regression analysis, a Young's modulus of 1.21 ± 0.08 MPa is found. Next, we performed pressurization tests on eight dome‐like shapes with pressures up to 4 kPa. The average Young's modulus obtained with the novel virtual fields method is 1.20 ± 0.13 MPa. The results are in good agreement with the ones from the tensile test. Future applications could benefit from this method to analyse more complex shapes, for example those found in biological structures like arteries or eardrums. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue including one by H. Aben and Anton Errapart on the photoelasticity for residual stress measurement in glass and another by J. M. Dulieu-Barton, J. Eaton-Evans, E. G. Little, and I. A. Brown on thermoelastic stress analysis of vascular devices.

The strength and deformation of full‐scale adhesively bonded multi‐material joints is studied in this paper. Four joints with a thick layer of methyl methacrylate adhesive (MMA) have been manufactured in shipyard conditions. In two specimens, cracks have been introduced at steel–adhesive and composite–adhesive interfaces. One cracked and one un‐cracked specimen were subjected to quasi‐static tensile testing; the two remaining specimens were stepwise loaded/unloaded with increasing load until failure. The strain in the adhesive layers was measured with digital image correlation (DIC). This showed a predominant shear deformation and dissimilar shear strain patterns for different bond lines. Fibre Bragg (FBG) sensors were used to monitor strains at steel and composite constituents and to detect the onset and evolution of damage in the un‐cracked specimen. Strains measured by FBG sensors correspond well with DIC results at nearby regions. All specimens failed by delamination of the composite panel near the composite–adhesive interface. [ABSTRACT FROM AUTHOR]

Many destructive methods for measuring residual stresses such as the slitting method require an inverse analysis to solve the problem. The accuracy of the result as well as an uncertainty component (the model uncertainty) depends on the basis functions used in the inverse solution. The use of a series expansion as the basis functions for the inverse solution was analysed in a previous work for the particular case where functions orders grew consecutively. The present work presents a new estimation of the model uncertainty and a new improved methodology to select the final basis functions for the case where the basis is composed of polynomials. Including nonconsecutive polynomial orders in the basis generates a larger space of possible solutions to be evaluated and allows the possibility to include higher‐order polynomials. The paper includes a comparison with two other inverse analyses methodologies applied to synthetically generated data. With the new methodology, the final error is reduced and the uncertainty estimation improved. [ABSTRACT FROM AUTHOR]

Fibre woven thermoplastic composites (FWTC) are widely used in aerospace and other fields because of their excellent performance. During service, FWTC structures are inevitably subjected to low‐velocity impact (LVI), which can cause invisible damage and eventual failure of the material. At the moment, studies on FWTC mostly focused on the orthogonal woven yarns while there's few reports about the effect of the yarn angle changing on the woven material's LVI damage response. This study aims at the effect of yarn angle changing on the damage behaviour of FWTC. A method for preparation of nonorthogonal prepregs was proposed, by which FWTC laminates with different yarn angles (60°, 75°, and 90°) were prepared for LVI tests. The results show that the maximum impact displacement and the impact duration of the impactor decrease with the decrease of the yarn angle when the FWTC laminate is subjected to LVI, while the maximum impact force shows an increasing trend. This indicates that the smaller yarn angle causes the better load‐bearing capacity of the FWTC laminate under LVI conditions, while the orthogonal FWTC laminate is more ductile. The damage morphology indicated by the impact of the FWTC laminate are matrix cracks and yarn breaks, and the damage area increases with the decrease of yarn angle, where the damage of orthogonal laminate is more serious more concentrated. The results found in this paper can provide useful guidance for engineering applications and failure analysis of FWTC. [ABSTRACT FROM AUTHOR]

Performing in situ scanning electron microscope (SEM) tests is an interesting way to visualise strain heterogeneities under mechanical loading. An essential step before performing the tests is to define the acquisition conditions. The aim of this paper is to propose a classification of the acquisition conditions that are most important for the accuracy of strain measurements using digital image correlation (DIC) in in situ SEM tests. More than 200 image pairs were acquired using a field emission gun SEM. The influence of different acquisition conditions was investigated: acceleration voltage, probe current, working distance, magnification, number of integrated images, image resolution, integration and number of integrated images, scan speed, contrast, brightness and exposure time of the sample in a given area. The methodology implemented in this work is an interesting tool for detecting scan line shift, drift distortion, spatial distortion and rastering artefacts. It allows the optimization of SEM acquisition conditions for strain measurements. Finally, optimal acquisition conditions for in situ testing are proposed and used to perform a tensile test on pure copper. The main factors highlighted include the size of the subset used in the DIC, the beam stabilisation time before image acquisition and the size of the images, which play a significant role in the results. It is recommended to apply the methodology to each device to optimise the acquisition conditions. [ABSTRACT FROM AUTHOR]

The implementation of digital image correlation (DIC) involves several different problems, such as image prefiltering, image intensity gradient calculation, image intensity interpolation at subpixel positions, shape function construction and strain calculation. This paper offers a unified insight into the nature of several key problems in DIC technique. We treat all the problems involved in the former mentioned key steps as fundamentally the same problem, that is, reconstructing an analytical description from a discrete and noisy sampling of a signal, such as discrete image intensity and displacement at some scattered nodes. From the reconstructed analytical description, the gradient and physical value at subpixel or integral pixel position can be analytically calculated without extra error. Here, we solve all these problems using the same mathematical tool, the meshfree method, leading to a unified DIC (U‐DIC) method. This method introduces errors solely during the steps involving the determination of the continuous description. However, it effectively avoids errors in the remaining steps. It holds the best balance between spatial resolution and measurement resolution for both displacement and strain measurements compared to 28 state‐of‐the‐art DIC algorithms based on the benchmark tests on DIC Challenge 2.0. It also holds all the unique advantages of the advanced meshfree DIC (MF‐DIC) compared to conventional local DIC and global DIC. The novel concept and excellent balance between spatial resolution and measurement resolution make U‐DIC an attractive replacement for conventional DIC methods. Additionally, the consistency of the implementation procedures in U‐DIC can simplify the parameter selection in DIC, which is of potential for the standardization of DIC technique. [ABSTRACT FROM AUTHOR]

Information about the topics discussed at the 13th International Conference on Experimental Mechanics (ICEM 13) which was held in Alexandroupolis, Greece from July 1-6, 2007 is presented. The event emphasizes the novel application of various techniques of experimental mechanics. The nine papers of the present issue, were peer reviewed according to the standard review policy of the journal. A paper by P.-F. Lou and C.-H. Wang presents a stereo vision method to measure the crack tip parameters.

Numerical simulations have become essential in engineering and manufacturing processes involving plasticity. The reliability and effectiveness of the simulations depend strongly on the accuracy of the adopted constitutive model. Accordingly, in recent years, an increasing interest is pointed towards experimental procedures and characterization methods that can be used to identify the constitutive parameters of advanced plasticity models, which allow to simulate properly the plastic behaviour of complex materials like, for instance, high strength steel. This paper provides a thorough review of the current state‐of‐the‐art, looking at both academia and industry. The available methodologies can be subdivided in two main areas: quasi‐homogeneous material tests with analytical or numerical post‐treatment of the experimental data and heterogeneous tests coupled with inverse methods for parameter identification. For each method, a brief description and references to norms and articles is provided, illustrating the advantages and the disadvantages. [ABSTRACT FROM AUTHOR]

Abstract: When a crack initiates and grows in a metal or composite structure, for example, due to high cycle fatigue, the crack propagation gives rise to acoustic emissions (AE)—ultrasonic waves travelling through the structure. Because the presence and rate of growth of any cracks are important pieces of information about the condition or health of the structure, the monitoring of AE activity using sensors mounted on its surface is a potentially useful technique of structural health monitoring. In tests, acoustic emissions are often simulated by breaking a pencil lead against the surface of the structure in a standardised way (a "Hsu‐Nielsen" source), but the forces that this imparts are not well understood at present. The current paper proposes a new evolutionary optimisation‐based approach to source characterisation. The principle is to introduce a parametrised representation of a general source and then identify the parameters that allow the source to best match responses measured elsewhere on the structure. The predicted responses are modelled using a local interaction simulation approach (LISA) algorithm to simulate the propagation of the ultrasonic waves. The approach is validated here using experiments on AE propagation in thin plate‐like structures, where the ultrasound propagates as Lamb waves. Three separate case studies are proposed here. In the first case, an idealised point source is simulated using laser‐generated ultrasound, and the optimisation algorithm uses a two‐dimensional LISA model. A differential evolution optimisation scheme is used to find the optimal profile of forcing to match the simulation with experiment. In the second case, the two‐dimensional LISA approach is used to characterise the forces associated with standard pencil lead breaks. The final study addresses the full three‐dimensional wave propagation. Because of the computational expense of the latter calculation, the LISA algorithm is implemented using a CUDA graphics card computer system. [ABSTRACT FROM AUTHOR]

Abstract: This paper uses the Continuous Wavelet Transform Analysis on mode shapes for damage identification. The wavelet analysis is applied to the difference in the mode shapes between a healthy and a damaged state. The paper also includes a novel methodology for estimating the level of noise of the experimental mode shapes based on a standard Signal to Noise Ratio (SNR). The estimated SNRs are used for identifying and making emphasis on the less noisy data. Moreover, a mass attached to the structure is considered to enhance the sensitivity of the structure to damage. Modal analysis is performed for different positions of the mass along the beam. The results obtained for all the positions of the mass are combined so an averaging process is implicitly applied. The paper presents the results from an experimental test of a cantilever steel beam with different severity levels of damage at the same location. The results show that the use of the attached mass reduces the effect of noise and increases the sensitivity to damage. Little damage can be identified with the proposed methodology even using a small number of sensors and only the first five bending modes. [ABSTRACT FROM AUTHOR]

Estimating stress in statically undetermined tests remains an issue in experimental mechanics. Most estimation methods rely on the a priori choice of a behaviour equation leading to an unavoidable model bias. Recently, efforts have been made to propose methods circumventing the parametric description of constitutive behaviours. In particular, Leygue et al. (2018) proposed a new paradigm called data‐driven identification (DDI). An extension of Leygue's method to history‐dependent materials is proposed in this paper. The formulation of the problem and its resolution are presented with emphasis on boundary conditions. The method is tested on real experimental data where the elasto‐plastic material is subjected to the formation of Piobert–Lüders bands. We finally show that the DDI allows to obtain balanced fields that are closer (more consistent) to the field measurements than the fields obtained by parametric identification strategies, even more in the presence of strain localization bands whose kinematics are usually not described by a standard constitutive model. [ABSTRACT FROM AUTHOR]

The author reflects on his goals and objectives after being appointed as the new editor of the journal "Strains." The author gives an assurance that as an editor he will strive hard for the success of the journal. The author informs that the submission and review of papers will be electronically processed. The author also opines that the journal with dedicated editorial council, board members and reviewers will serve the experimental mechanics community in the best way.

Deformable components such as sandwich structures possess promising properties for use in protection systems. Detailed studies on energy absorption and fluid–structure interaction effects are necessary for the application of deformable sandwich structures in blast resistant design. In this paper, an existing shock tube facility has been extended with a transparent section to observe and measure fluid flow and the structural response of deformable components during transient dynamic loading. The extension was instrumented with pressure sensors and load cells to measure the pressure and force transmitted through the component during testing. The transparent design allows the use of optical measurement techniques. Here, high‐speed cameras were used both for digital image correlation and background‐oriented schlieren imaging. Tests with free‐standing plates and sandwich components were performed. A strong dependency was observed between the plate mass, and thus the velocity of the plates, and the pressure measured upstream and downstream of the components. The tests were simulated with a one‐dimensional numerical model for compressible shock flow with fluid–structure interaction. The numerical model accurately reproduced the shock flow and component displacements measured experimentally. Overall, the experimental set‐up presented in this study proved to be suitable for the detailed examination of deformable components subjected to airblast loading. [ABSTRACT FROM AUTHOR]

The present paper describes a novel experimental method for characterising the confined shear strength of concrete or rock‐like materials based on the use of pre‐stressed sample. This method, called PS‐PTS (Pre‐Stressed Punch‐Through‐Shear), employs a metallic confinement cell that is first quasi‐statically deformed so the concrete sample can be introduced in between the cell jaws. The confinement force is transmitted to the sample in the unloading stage. It is shown that this initial pre‐load level can be predicted by an elastic closed‐form solution. Strain‐gauges glued on the cell allow the confinement level applied to the sample to be experimentally measured during the pre‐stressing stage and the shear stage. In the next stage, the central part of the sample is subjected to a differential displacement towards the lateral parts by means of a hydraulic press so the confined shear strength of the tested material can be deduced. The results are compared to the data previously obtained with an "oedometric" confinement cell in terms of confinement stresses and shear strength. Highlights: A Pre‐Stressed Punch‐Through‐Shear technique is proposed based on pre‐loaded cell.A close‐form solution predicts the level of pre‐load applied to the sample.The cell is instrumented with strain gauges, so the applied forces are measured.The technique is applied to characterise the confined shear strength of concrete.Results are compared to those obtained with the 'Oedo‐PTS' testing method. [ABSTRACT FROM AUTHOR]

The checkerboard constitutes the best pattern for in‐plane displacement and strain measurement because it maximizes image gradient. The use of laser marking to deposit such a pattern on flat surfaces is investigated in this paper. Optimal settings for the parameters influencing the quality of the pattern are given. This pattern being periodic, the images are processed with the localized spectrum analysis, which is one of the spectral techniques used to process such images. It is shown that the metrological performance in terms of measurement resolution is equivalent to the one obtained with transferred checkerboards. Compared to the classic transfer technique generally used to deposit such patterns, the benefit of using laser marking is its ease of use, a much higher print speed and the fact that a thin coat of white paint instead of a thicker layer of white adhesive is employed to ensure a good contrast in the images. Various examples of strain measurements are given, in particular the measurement of the strain field around a knot embedded in a wood specimen subjected to a tensile test. [ABSTRACT FROM AUTHOR]

This paper proposes to measure true stress–strain curves at the necking region of cylindrical bar samples by using an advanced mirror‐assisted multi‐view digital image correlation (MV–DIC) technique. The mirror‐assisted MV–DIC can measure the panoramic shape and deformation fields of a cylindrical bar sample with only a single two‐camera stereo‐DIC system and two planar mirrors. The measured panoramic shape enables accurate determination of the cross‐section area and radius of curvature at the necking region by using cylindrical or torus fitting. The retrieved cross‐section area and radius of curvature at the necking region further enable the measurement of the true stress–strain curves and even the Bridgman's correction of the true stress. [ABSTRACT FROM AUTHOR]

Deterioration assessment of materials is essential to the continued effective operation of critical components in thermal power plants. Establishing the degree of creep exhaustion of power engineering alloys operating at high temperatures and stresses guides maintenance strategies to ensure reliable plant operation. Within progressive inspection philosophies, traditional laboratory‐based creep testing is often difficult to conduct on ex‐service steel due to the limited material availability from which to machine standard specimen geometries. This work investigates a technique for the measurement of creep strain curves at several stresses and at 600°C from a single test using a nontraditional specimen geometry together with full‐field strain measurement through digital image correlation (DIC). Of interest is ex‐service X20CrMoV12‐1 (X20) which is widely used in older, subcritical thermal power plants. The paper aims to show that multiple creep curves can be resolved over a spatially varying stress field using DIC whilst preserving material economy. Differences in creep behaviour between ex‐service X20 with varying levels of service exposure are evident from quantitative comparisons of the creep strain and rate curves through threshold stress computation which agrees with hardness measurements and microstructural observation of subgrains and precipitates using electron microscopy. These single‐specimen tests yield high densities of creep data which can be used in the calibration of creep damage models for characterisation of ex‐service X20. [ABSTRACT FROM AUTHOR]

We introduce a novel diagnostic approach for the uncovering of hidden content in layered documents, based on a photoacoustic imaging set‐up with optimised acoustic detection parameters. By exploiting the laser‐induced ultrasound following the intense absorption of pulsed optical radiation by printed ink, this traditional biomedical imaging method is proven capable of revealing text characters buried under several layers of stacked paper sheets at high spatial resolution and excellent contrast levels. Such a remarkable performance benefits from the effective combination of strong light scattering by paper fibrils and the virtually unobstructed propagation of the generated ultrasonic waves through multiple layers, enabling thus the accurate recording of text, even in double‐sided prints, with minimum shadowing artefacts. The imaging effectiveness, the implementation simplicity, the robustness, and the relatively low‐cost features offered by the proposed photoacoustic modality are anticipated to contribute in its wide adoption by the preservation of cultural heritage community, as a powerful diagnostic tool for the in‐depth investigation of complex multilayered objects. [ABSTRACT FROM AUTHOR]

This paper considers the main developments of RGB photoelasticity with reference to the maximum measurable retardation. In this paper, a new procedure based on the standard error function evaluated on a subset of the calibration array is also proposed and experimentally tested. The experiments show that the filament lamp makes it possible to find retardations until approximately 4 fringe orders while the fluorescent lamp makes it possible to determine higher fringe orders (12 fringe orders in this paper) owing to the discrete spectrum of the source. The paper shows that, by using the incandescent lamp, the primary limiting factor is the lack of modulation of the R, G and B signals whereas, by using the fluorescent lamp, the limitation of the maximum fringe order derives mainly from the gradient of the fringes and the procedure of search of the retardation. [ABSTRACT FROM AUTHOR]

Comments on the growth of the publication "Strain" since its inception in 1964. Increase in input of international papers for publication in the journal; Citation of papers previously published in the journal; Topics in engineering discussed in the papers submitted for publication.

The article discusses various reports published within the issue, including the application of fracture mechanics to quantify the ageing of bone and measurements for extracting constitutive parameters,

Editorial. Discusses the author's plans for the periodical "Strain." Well-defined identity of the journal; Need for the journal to grow in terms of the number of good quality research papers published; Continued expansion into technology areas such as biotechnology, advanced materials and computer-based data analysis; Need for the journal to increase it international profile.