Showing papers in "Strain in 2010"

Multiscale Full‐Field Strain Measurements for Micromechanical Investigations of the Hydromechanical Behaviour of Clayey Rocks

Abstract: Digital image correlation techniques (DIC) are applied to sequences of optical images of argillaceous rock samples submitted to uniaxial compression at various saturation states at both the global centimetric scale of the samples and the local scale of their composite microstructure, made of a water-sensitive clay matrix and other mineral inclusions with a typical size of 50 μm. Various scales of heterogeneities are revealed by the optical technique. Not only is it confirmed that the clay matrix deforms much more than the other mineral inclusions, but it also appears that the deformation is very inhomogeneous in the matrix, with some areas almost not deformed, while others exhibit deformation twice the average overall strain (for a gauge length of 45 μm), depending on the local distribution of the inclusions. In almost-saturated rocks, overall heterogeneities are also linked to the presence of a network of cracks, induced by the preliminary hydric load. On such wet samples, DIC analysis shows that the overall strain results both from the bulk deformation of the sound rock, with deformation levels similar to those in dry samples, and the closing or opening of these mesoscopic cracks.

Full Three‐Dimensional Strain Measurements on Wood Exposed to Three‐Point Bending: Analysis by Use of Digital Volume Correlation Applied to Synchrotron Radiation Micro‐Computed Tomography Image Data

Abstract: A microscale three-point bend experiment on wood has been carried out. The full 3D strain field of the microscale wood structure has been determined by use of digital volume correlation, based on reconstructed 3D image data acquired with synchrotron radiation micro-computed tomography. The wood specimen, which measures 1.57 × 3.42 × 0.75 mm3, was scanned in different load states along the three-point bend load cycle, from unloaded state to failure. The correlation algorithm is based on a Chebyshev polynomial description of the displacements, which allows a continuous representation of the 3D deformation fields. The methodology of the correlation algorithm is described thoroughly and its performance is tested for a 3D structure that is exposed to a virtual pre-defined deformation. The performance is tested both for noise free volume data as well as for structures with additive noise content. The performance test shows that the correlation algorithm resolves the applied deformation satisfyingly well. In the real experiment, on wood microstructure, the displacement fields show a structural behaviour that is consistent with what is expected for a specimen exposed to three-point bend. However, there are also anomalous effects present in the displacement fields that can be coupled to characteristic features in the cellular structure of the wood. Furthermore, 3D strain calculations based on the obtained displacement data shows a concentration of tensile strain in the region where the specimen eventually collapses. The experimental results show that the use of X-ray-based tomography with high spatial resolution in combination with digital volume correlation can successfully be used to perform 3D strain measurements on wood, at the microscale.

Use of Full‐Field Digital Image Correlation and Infrared Thermography Measurements for the Thermomechanical Analysis of Material Behaviour

Abstract: The paper aims to highlight the advantages of using data supplied by digital image correlation (DIC) and infrared thermography (IRT) to study the thermomechanical behavior of materials. It describes an experimental procedure for the determination of mechanical energy and heat sources involved locally during a heterogeneous tensile test. This procedure involves two complementary imaging techniques: DIC provides in-plane displacement fields while IRT enables the temperature distribution at the specimen surface to be monitored. Numerous different application examples are successively proposed to underline the promising potential of this experimental approach. Kinematical assessments can reveal the extent of homogeneity of the deformation state for a given gauge length. They can also help to determine the relevance of the variables and/or material parameters introduced in the behavioral description at the length-scale imposed by the spatial resolution of optical systems (typically 0.1 mm). Moreover, infrared and kinematical data can be used to derive heat source fields induced by the specimen loading and then to generate information on the dissipative or coupled nature of the deformation mechanisms.

Adaptive Quality Guided Phase Unwrapping Algorithm for Whole-Field Digital Photoelastic Parameter Estimation of Complex Models

Abstract: With advancements in digital image processing and data acquisition, a separate branch of photoelasticity namely digital photoelasticity came into existence. Here, intensity information of the acquired image is used for the evaluation of whole-field photoelastic parameters. Digital photoelasticity provides only wrapped phasemaps of isoclinics and isochromatics and they have to be unwrapped in different ways for getting the continuous-phase values. In the case of the isochromatic phasemap, ambiguity removal prior to unwrapping is essential. In this paper, a 10-step phase-shifting methodology is proposed and a new strategy for obtaining the isochromatic phasemap free of ambiguity is demonstrated. Isoclinic unwrapping is performed by a new adaptive quality guided algorithm. Adaptive in the sense that isoclinic phase unwrapping is done autonomously even in the presence of isotropic points/π jumps occurring in the isoclinic phasemap. The isochromatic phasemap is also unwrapped using the quality guided path follower. The methodology is validated for the problem of a ring under diametral compression and later shown for three other models which have complex stress fields. Wherever possible, the parameters obtained by the new methodology are compared with analytical or numerical methods and the comparison is quite good.

A Comparison Between the EN 383 and ASTM D5764 Test Methods for Dowel‐Bearing Strength Assessment of Wood: Experimental and Numerical Investigations

Abstract: The purpose of this paper is to provide a comparison between embedding tests covered by the EN383 and ASTM D5764 standards, highlighting some difficulties regarding the application of the referred standards. These embedding tests are essential to evaluate the embedding strength of wood, which is required for the assessment of joints strength. The proposed comparison is based either on experimental data and numerical simulations through the finite element (FE) method. Tests were performed on maritime pine wood (Pinus pinaster Ait. species) according to the longitudinal and radial directions, allowing the comparison of the embedding strength and elastic foundation modulus. Three dimensional FE models of the tests were built using contact elements technology and assuming the steel dowel and wood as linear elastic isotropic and orthotropic materials, respectively. The contact modelling is a challenging topic for which this paper also proposes some guidance. The test configuration proposed in the EN383 standard for assessment of the embedment strength in compression is more susceptible to the dowel bending than the half-hole test configuration proposed in the ASTM D5764 standard. The numerical simulation of the EN383 embedding test raises some additional difficulties regarding the dowel boundary conditions.

RGB Photoelasticity: Review and Improvements

Abstract: 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.

Flexural Behaviour of Concrete Beams Reinforced With GFRP Bars

Abstract: The paper presents some of the results from a large experimental program undertaken at the Department of Civil Engineering of Salerno University. The static behaviour under service conditions of concrete beams reinforced with GFRP (Glass Fibre-Reinforced Polymer) as well as CFRP (Carbon Fibre-Reinforced Polymer) bars and stirrups is examined. Within the whole experimental program concerning forty beams, two different concrete strengths and two different percentages of reinforcement are taken into consideration. The final aim is to investigate both deflections at midspan and crack widths. Moreover, the ultimate behaviour up to failure is also investigated. A part of this experimental program, concerning ten prototypes reinforced with GFRP bars, is here presented and discussed. Finally, comparisons with analytical predictions given by CNR-DT 203/2006 are also shown.

Measurement of Steep Surfaces Using White Light Interferometry

Abstract: Scanning white light interferometry (SWLI) is an increasingly popular method to measure the surface profile of miniature components. Although it is tolerant to step changes in profile, its capability to measure the large surface gradients that are characteristic of high-aspect-ratio surfaces is limited. This is in part due to the numerical aperture of the objective lens which restricts the spatial frequency content of both the illumination and recorded fields. More fundamentally, though, SWLI instrumentation neglects the effects of multiple scattering and assumes that the field which illuminates the object is that which would be present if the object were absent. Although this is a reasonable approximation for slowly varying surfaces, it is generally not true for those with steep gradients. In this paper the 3D theory of SWLI is presented and the approximations made by current instrumentation are discussed in this context. Using finite element methods (FEM), SWLI interferograms are calculated, for the cases of 2D Silicon V-grooves and step artefacts, and the effects of multiple scattering are illustrated. Methods to improve the capability of SWLI to measure large surface gradients, first by tilting the sample and subsequently by using an iterative FEM model to provide improved estimates of the illuminating conditions are introduced.

Fatigue on Shot‐Peened Gears: Experimentation, Simulation and Sensitivity Analyses

Abstract: The present paper deals with fatigue experimentation and with the application and improvement of predictive models; in addition, a sensitive analysis is performed on the main factors related to the shot-peening treatment and on the efficiency of the aforementioned models. The research involved gears, made of high-strength steel and carburised, quenched, ground, shot-peened and superfinished. The experimental campaign initially dealt with the investigation into the influence of isotropic superfinishing; the attention was then focused on shot peening and how to optimise the fatigue limit, by a suitable choice of operative parameters. The option of duplex peening for further fatigue improvement was also considered. Results concerning component residual stress distributions and fatigue limits were then processed by investigating their sensitivity with respect to driving factors, namely the shot diameter and the Almen intensity. Two theories for fatigue prediction (the method of the relative stress gradient and the theory of critical distances) were reviewed for application to shot-peened components, with a comparison between experimental and numerical results. A comparative analysis was then performed on the two theories, on the basis of the number of data inputs, advantages and drawbacks, while sensitivity analyses focused on how uncertainties affecting input data propagate to predictive results.

Smoothing Measured Displacements and Computing Strains Utilizing Finite Element Method

Abstract: A method for smoothing measured displacements and computing strains utilizing a finite element method is proposed. Nodal displacement values of a finite element model are determined by fitting the interpolation functions of finite elements to measured displacement values using the method of least-squares. Then, the smoothed displacement distributions are obtained. The displacements in the region where the measurement values are not obtained or unreliable are determined by solving finite element equations. The validity is demonstrated by applying the proposed method to displacements of a plate with a hole obtained by finite element method. Results show that the displacements and the strains can be determined accurately by the proposed method. Furthermore, the strains near free boundaries can be determined easily. As strains can be evaluated easily and accurately, it is expected that the proposed method can be applied to various problems in solid mechanics.

Processing, Microstructure and Mechanical Properties of Air Plasma-Sprayed Ceria–Yttria Co-stabilized Zirconia Coatings

Abstract: Thermal barrier coatings (TBCs) are widely adopted to protect mechanical components in gas turbine engines operating at high temperature. Basically, the surface temperature of these components must be low enough to retain material properties within acceptable bounds and to extend component life. From this standpoint, air plasma-sprayed (APS) ceria and yttria co-stabilized zirconia (CYSZ) is particularly promising because it provides enhanced thermal insulation capabilities and resistance to hot corrosion. However, essential mechanical properties, such as hardness and Young’s modulus, have been less thoroughly investigated. Knowledge of Young’s modulus is of concern because it has a significant effect on strain tolerance and stress level and, hence, on durability. The focus of the present study was to determine the mechanical properties of APS CYSZ coatings. In particular, X-ray diffraction (XRD) is adopted for phase analysis of powders and as-sprayed coatings. In addition, scanning electron microscopy (SEM) and image analysis (IA) are employed to explore coating microstructure and porosity. Finally, the Young’s modulus of the coating is determined using nanoindentation and a resonant method. The results obtained are then discussed and a cross-check on their consistency is carried out by resorting to a micromechanical model.

Combined Effect of Notches and Fibre Orientation on Fatigue Behaviour of Short Fibre Reinforced Polyamide

Abstract: The combined effect of notches and fibre orientation on static and fatigue strength of short glass fibre reinforced polyamide 6 was studied using notched specimens that, by proper design of the mould, could be injection moulded through gates of two different sizes and locations. The results showed that the values of the tensile and fatigue notch factors for a particular geometry were strongly influenced by varying the way specimens were injected. A new experimental method was applied to investigate whether the observed differences in mechanical behaviour of specimens having the same geometry were caused by different fibre orientation distributions. Synchrotron light micro- tomography allowed for visualisation of local fibre orientation at notches and the obtained images of the internal structure of the samples were subsequently analyzed by a global morphological parameter, the Mean Intercept Length. Results were also discussed in light of the full displacement field measured by Digital Image Correlation.

Evaluating Non-Uniform Residual Stress by the Hole-Drilling Method With Concentric and Eccentric Holes. Part II: Application of the Influence Functions to the Inverse Problem

Abstract: This part deals with the applications of the influence functions developed in part I to solve the inverse problem of determining residual stresses from the measured relaxed strains. The effects of the hole eccentricity and the number of strain gauges on the accuracy of the reconstructed residual stresses are considered for typical experimental conditions. Influence functions for eccentric holes reduce the error in the reconstructed residual stress for typical experimental conditions below 1%, instead of 10% which would be obtained neglecting the eccentricity. The analysis was also applied for comparing the performance of three- and six-element rosettes for compensating eccentricities. When the eccentricity is not included in the elaboration, six-element rosettes with couples of opposite strain gauges halve the error committed by using the equivalent three-element rosette. However, if the eccentricity is included in the influence functions, the three element rosette can predict with a smaller error.

On the Shear Behaviour of Concrete Beams Reinforced by Carbon Fibre-Reinforced Polymer Bars: An Experimental Investigation by Means of Acoustic Emission Technique

Abstract: This paper reports test results of 20 concrete beams reinforced by carbon fibre-reinforced polymer (CFRP) bars, without any shear reinforcement, subjected to three-point bending tests. The tests were conducted at the Official Laboratory of the Department of Structural Engineering of the University of Calabria with the aim of both analysing and improving the knowledge of the mechanical behaviour of these new structural elements and to validate, from an experimental point of view, some theoretical relationships suggested in the recent Italian Guidelines CNR-DT 203/2006. All tested specimens were classified into four groups according to the different concrete compressive strength and to CFRP-bar reinforcement. Mid-span deflection, moment-curvature and bar strains were investigated by means of linear inductive displacement transducers, centesimal dial gauges and strain gauges directly applied onto CFRP bars. In almost all cases, shear failure mode was observed. A major diagonal crack within the beam shear span extended horizontally at the level of the CFRP bars indicating bond failure. During the experimental study, the acoustic emission technique was used to find out the beams cracking moment and to investigate the damage phenomena occurred inside the beams during the experimental tests, by means of a monitoring system set up.

Evaluating Non-Uniform Residual Stress by the Hole-Drilling Method with Concentric and Eccentric Holes. Part I. Definition and Validation of the Influence Functions

Abstract: Residual stresses play a fundamental role in predicting the resistance of several structural components, in particular when operating under alternating loads or in aggressive environments. Among the measurement techniques, the hole-drilling method is unrivalled in terms of cost and flexibility. The currently available data reduction techniques do not account for some important parameters, in particular the hole eccentricity, and can be directly used only with certain rosettes or require the partial holes to be drilled at fixed depths. This paper presents a general approach for interpreting hole-drilling measurements that includes the hole eccentricity and several other geometric and material parameters. The method is based on analytical influence functions relating the measured relieved strains to the residual stress by means of integral equations. By elaborating the results of accurate finite-element simulations, continuous analytical influence functions are produced, which can be used to reduce the error in the reconstructed residual stress. We focus on the definition of the influence functions, their evaluation and assessment.

A Quantitative Analysis of the Thermoelastic Effect in CFRP Composite Materials

Abstract: In this study the thermoelastic signal from carbon fibre-reinforced plastic (CFRP) laminates is investigated. A comparison between the theoretical and experimental values of the thermoelastic signal is reported, with the theoretical predictions obtained from two different quantitative models. These models are based on the classic thermoelastic effect law extended to the case of orthotropic materials (by using the mesomechanical or bulk approach), and the modified law assuming that the surface resin-rich layer behaves as a strain witness of the laminate. It is found that the theoretical predictions of the two models can be strongly and differently influenced by the intrinsic orthotropy of carbon fibres. Some effects are highlighted in particular such as the influence of the laminate lay-up and the strong mismatch between the thermal expansion coefficients of the polymer matrix and the fibres. These influences are investigated analytically, predicting the thermoelastic signal from various lay-ups and using strain-based and stress-based analytical models. Experimental evidence of some theoretical findings is provided by reporting on tests performed on CFRP tensile samples manufactured from low-crimp unidirectional fabrics.

Assessing the Feasibility of Monitoring Strain in Historical Tapestries Using Digital Image Correlation

Abstract: Digital image correlation (DIC) is used to monitor strain in a representative textile material and an historic tapestry. The validity of a ‘map function’ that allows 3D DIC displacement measurements to be obtained when the reference data are collected with a camera set-up different from that of the deformed data is assessed. An experiment was devised to study the effects of DIC processing parameters (interrogation cell size and overlap) on strain measurements, and to investigate if the textile contains adequate contrast for DIC to operate. The study shows that the textile’s weave pattern can be used as the device for correlation. Long-term tests for monitoring creep strain using DIC both in the laboratory and in situ are presented. The results show good correspondence between strain changes in the tapestry and relative humidity.

Spatially Resolved Stress Measurements in Materials With Polarisation‐Sensitive Optical Coherence Tomography: Image Acquisition and Processing Aspects

Abstract: We demonstrate that polarisation-sensitive optical coherence tomography (PS-OCT) is suitable for mapping the stress distribution within materials in a contact-free and non-destructive way. In contrast to transmission photoelasticity measurements, the samples do not have to be transparent but can be of scattering nature. Denoising and analysis of fringe patterns in single PS-OCT retardation images are demonstrated to be the bases for a quantitative whole-field evaluation of the internal stress state of samples under investigation.

An Advanced Creep Model Allowing for Hardening and Damage Effects

Abstract: In this work an original creep prediction model is presented with a unified comprehensive formulation with primary, secondary and tertiary stages for variable stresses and temperature fields. The creep model is implemented in the ANSYS FEM code, using the Fortran subroutine usercreep, specifically suited for the cases examined, in order to assess the material behaviour more accurately than that provided by the mono-dimensional approach. Furthermore, the residual stresses that arise from thermo-mechanical loading and unloading cycles, under the temperature-dependent bilinear elastic–plastic hypothesis, are considered. In order to validate the model proposed, an investigation into the final stages of turbine blade is presented and discussed. The nonlinear properties of the material are taken from an industrial context and the literature. In addition to the temperature dependence of the properties of the material, geometric nonlinearities are also considered. The material adopted for model calibration is a polycrystalline Ni-based superalloy. Load conditions used in the simulations are obtained using a computational fluid dynamics (CFD) analysis under working conditions provided by industrial research and the constraints modelled are representative of the real joining condition of the blade. Finally, the simulation results are compared with those obtained by the Norton–Bailey prediction model.

Phase Extraction in Dynamic Speckle Interferometry with Empirical Mode Decomposition and Hilbert Transform

Abstract: In many respects, speckle interferometry (SI) techniques are being considered as mature tools in the experimental mechanics circles. These techniques have enlarged considerably the field of optical metrology, featuring nanometric sensitivities in whole-field measurements of profile, shape and deformation of mechanical rough surfaces. Nonetheless, when we consider classical fringe processing techniques, e.g. phase-shifting methods, the deformation range is intrinsically limited to the correlation volume of the speckle field. In addition, the phase evaluation from such patterns is still computationally intensive, especially in the characterisation of dynamic regimes, for which there is a growing interest in a wide range of research and engineering activities. A promising approach lies in the pixel history analysis. We propose in this paper to implement the empirical mode decomposition (EMD) algorithm in a fast way, to put the pixel signal in an appropriate shape for accurate phase computation with the Hilbert transform. © 2008 Blackwell Publishing Ltd.

Application of Plasmons to the Determination of Surface Profile and Contact Strain Distribution

Abstract: Free electrons in the surface of metals can produce surface plasmons if they are excited by evanescent waves. The surface plasmons are electromagnetic waves that by decaying can generate photons. These photons produce propagating waves that feed energy to an oscillating cavity. From this effect a Fabry–Perot like interferometer is generated. In view of this basic knowledge a truly novel application on how these plasmons can be used experimentally is introduced in this article. Essentially, by using the transformation of evanescent waves into propagating waves via the plasmon generation it is possible to perform high accuracy contouring of any metal surface as well as the determination of its contact stresses. A laser beam illuminates a glass-air-metal interface and through a proper optical setup it will produce double evanescent illumination. This illumination generates interference fringes that contain surface depth information as well as in-plane strain information. In order to show the robustness of the proposed technique, three different applications are presented in the paper. First, the surface roughness mapping of a copper plate is determined and the obtained values are in statistical agreement with the data measured by a mechanical profilometer. Second, the surface profiles of nickel alloy surfaces used as calibrated standards for the determination of Ra values are obtained. The measured Ra values statistically agree with the values of the different standards. Finally, the contact strains of a copper cylinder on glass are also measured. The contact strains follow the trend predicted by the K.L. Johnson’s model of contact. Local regions (asperities) experiment large plastic deformations.

Longitudinal Strain Solitary Wave in a Two‐Layered Polymeric Bar

Abstract: Both theoretical investigations and successful experimental research were performed recently, confirming the existence and demonstrating the main properties of bulk strain solitary waves in nonlinearly elastic solid wave guides Our current research is devoted to nonlinear wave processes in layered elastic wave guides with inhomogeneities modelling damage/delamination Here, we present first experimental and theoretical results, demonstrating the change in the amplitude and width of a strain solitary wave propagating in an inhomogeneous two-layered wave guide made of Polymethyl Methacrylate (PMMA) Parameters of such waves in layered structures may be useful for the assessment of their operational integrity and robustness

Study of the Mechanical Behaviour of a Clarinet Reed Under Forced and Auto‐oscillations With Digital Fresnel Holography

Abstract: This paper presents methods for vibration analysis using digital Fresnel holography. Methods are based on time averaging for forced oscillations and pulsed recording devoted to auto-oscillations. The two methods are applied to a clarinet reed. In the forced oscillation regime, the reed is excited by an acoustic wave with controlled frequency and amplitude such that the reconstructed holograms exhibit resolvable Bessel fringes. In the case of the auto-oscillation regime, the reed is placed in an artificial mouth. Deformation of the clarinet reed can be extracted with the recording of 3150 pulsed digital holograms. Experimental results show the vibration behaviour of the clarinet reed under forced and auto-oscillation regimes, exhibiting the modal structures and high amplitude shocks.

Displacement measurement through digital image correlation and digital speckle pattern interferometry techniques in cold-expanded holes

Abstract: In this paper, the displacement field induced by the split-sleeve cold expansion of holes was measured using both digital image correlation (DIC) and digital speckle pattern interferometry (DSPI) techniques. Thus, the experimental results, which were evaluated on the inlet surface of a 6082-T6 aluminium plate, were compared with those from theoretical prediction. DIC provided accurate measurements up to the elastic–plastic boundary, whereas the DSPI technique highlighted the changes of displacement in the elastic domain. Prediction of the displacement based on the existing analytical model agreed with the experimental results achieved with both techniques. Possible explanations for the differences are discussed.

An Integrated Data Acquisition System for on‐Water Measurement of Performance in Rowing

Abstract: The present paper reports on an experimental activity carried out to develop a new on-water data acquisition system suitable for measuring performance in sculling/sweep rowing (i.e. to gather the most significant forces applied by the rowers to the shell as well as the relative displacement of the seat and the rotation of the oars). A preliminary study was carried out to single out the effective components of those forces resulting in the displacement of rowing shells, locating also the positions of the corresponding application points. According to the outcomes of this preliminary investigation different sensors were designed to gather, from any rowing station, the following pieces of information: effective magnitude of the propulsive forces parallel to the shell axis, oar rotation angle, vertical force applied to the seat by the rower, the relative position of the seat and, finally, the forces applied by the rower’s legs to the footstretcher. Further, two accelerometers were used to measure acceleration and pitch of the shell. The accuracy and repeatability of the developed system and sensors were checked by carrying out several on-water acquisitions not only on standard shells (considering different crews), but also through an on-purpose built dry-land rowing station. To clearly show features and potentialities of such a data-acquisition system, the present paper reports on a series of tests carried out considering two different coxless pair crews, i.e. professional athletes and amateurs. The obtained results are definitely encouraging, proving that the pieces of information which can be gathered through our data acquisition system could be really helpful not only in evaluating rowers’ characteristics in terms of athletic performance and technique, but also in designing innovative rowing shells meeting the specific requirements of a crew.

Experimental Strain Analysis of the Mast of a 420 Sailboat During Sailing

Abstract: Aim of the work was the experimental strain analysis of a 420 sailboat mast during sailing: this type of data is of great interest in the structural design of sailboat masts and hulls as well as in the sail-making. The work involved the design, calibration and installation of strain gauge bridges at seven sections of an instrumented mast and at the shrouds: this was followed by the collection of strain data during simulated laboratory rigging tests and real sailing sessions with medium/strong wind. Compression loads at the mast step were recorded with a customised load cell calibrated on a force platform. On the basis of laboratory calibration constants, the strain measurements were converted into structural loads and averaged over steady states during rigging and sailing either close to the wind or beam reach: the longitudinal bending moment, the lateral bending moment and the axial loads were analysed and plotted along the 6.20 m mast length. Highest values of lateral bending (146 ± 14.5 Nm) were recorded at the mast head, whereas longitudinal bending showed highest values at the vang connection to the mast (229 ± 29 Nm). Tension loads acting on the shrouds were also measured with highest values of 2887 ± 167 N. The knowledge of loads acting along the mast will support the designers in improving the mast’s cross section profile, the material selection and the validation of numerical structural analysis. Moreover, experimental data about the loads acting on the standing rigging and on the mast-step during sailing will support the optimised design of the boat shell.

The Enhanced Digital Image Correlation Technique for Feature Tracking During Drying of Wood

Abstract: The enhanced digital image correlation (EDIC) technique is proposed as an improvement of the digital image correlation (DIC) technique in that it utilises monogenic filtering as a prefilter for extracting intrinsic local phase information from the low-contrast images and normalized cross-correlation (NCC) as a feature-tracking algorithm. The monogenic filtering separates local structural information that is the local phase of an image, which is invariant with respect to the local energy of the image. Therefore, it improves the image, permitting the DIC technique to produce stable and accurate measurements of deformation for a heterogeneous and hygroscopic material like wood during drying.

Dissipation measurements in steel sheets under cyclic loading by use of infrared microthermography

Abstract: Heterogeneous dissipation in steel sheets due to cyclic loading is difficult to measure, especially in the transverse direction because of the high conductivity and low thickness of the sheets. The goal of this article is thus to develop an experimental protocol allowing for the dissipation field determination from infrared thermography. The protocol is based on a specific differential measurement and an asynchronous acquisition. It reduces measurement artefacts due to coating, rigid body motion, convection, and optical deleterious effects. It is eventually applied to different specimens.

A Particular Instability of Unilaterally Supported Thin Plates Under Transversal Load: Effect of the Residual Stresses Induced by Vitreous Enameling

Abstract: Simply supported rectangular thin plates under transversal load, concentrated at the centre of the plate, can show a particular kind of instability at one or two edges depending on their dimensions. If only small displacements are considered, the problem is one of pure bending and instability does not occur; on the contrary this instability is induced by the compressive membrane stresses due to large displacements. Vitreous enamel coatings, generally used to improve anti corrosion properties, increment the load value required to induce the instability by inducing residual tensile stresses on the plate. This paper compares and discusses experimental tests on steel and enamelled steel plates and the results obtained by a non linear numerical model taking into account residual stresses. By both experimental and numerical analysis it was possible to conclude that the presence of residual stresses caused an increment of the values of force and work at which the instability took place with respect to the case where residual stresses were not present.