Search

Damage due to MgO hydration in castables has been studied by bar resonance and recently by Digital Image Correlation (DIC). The hydration reaction kinetics was studied via principal component analyses (PCA) applied to DIC results of images acquired during 60 h of curing and drying of an MgO containing refractory castable. The experiment was carried out in a in-house climatic chamber at 50°C and 50% of relative air humidity. The displacement and maximum eigen strain fields were obtained via DIC. Their PCA revealed the crack network as the most relevant component, with a temporal development of a sigmoidal curve where a two-parameter Weibull law was satisfactorily fitted. The reaction duration was virtually identical for both fields, and only a time shift in reaction initiation and saturation was attributed to the choice of field. The approach allows the need for user-defined thresholds to be avoided for crack quantification.

Metamaterials are often studied for their peculiar mechanical properties. However, few 4D studies were conducted on 3D printed pantographs. This study aims at analyzing an in situ torsion test in a lab tomograph. The acquired scans were used to measure displacement fields via digital volume correlation. The final goal was to analyze the deformation mechanisms of an Inconel pantograph and to rationalize potential Poynting effects.

Multiview correlation is based on the use of explicitly parameterizing surfaces whose deformation is to be measured. In the present analyses, the surfaces are modeled as triangular facets (for local approaches), and with finite-element meshes (for global approaches) using a linear interpolation between nodal displacements. Displacement uncertainty due to the presence of noise in the images is considered. The noise floor levels of the latter are compared for both local and global cases with two different sets of pictures. A priori estimates based on the Hessian of the registration procedure are derived analytically, and a simplified expression is provided that is shown to be in very good agreement with their a posteriori evaluations. The uncertainty difference between local and global approaches is interpreted from their different spatial resolutions.

The present study aims at demonstrating the feasibility of performing a fracture test in less than 1 min in a laboratory CT scanner despite the severe time constraints of tomography acquisition. After introducing the basic concepts of projection-based digital volume correlation, the specific implementation of this methodology to a wedge splitting test on a refractory material is presented. The kinematics of the test is described over a mesh tailored to the sample geometry, and the elastic behavior of the sample is exploited through finite element computations to provide sensitivity fields of experimental boundary conditions to allow for their “measurements.” Enhancing the simulation to account for crack advance with extended finite element analyses allows the sensitivity of the procedure to the crack position to be assessed. A confidence interval for the refractory toughness is finally obtained.

This article discusses the dual use of digital image correlation and thermoelastic stress analyses for the study of propagating cracks. It is shown that a few critical parameters such as emissivity and the thermoelastic constant required for the latter can be calibrated with the former. A unified framework is introduced, which treats both experimental techniques equally to locate crack tips, and then evaluate stress intensity factors. This framework allows for a detailed and quantitative comparison between both methods. It is found that, for the case at hand, the thermoelastic stress analyses outputs were less noise sensitive while the digital image correlation method was less dependent on calibration. The proposed procedure was very robust for finding the crack tip location for both experimental methods.

A projection-based digital volume correlation method (presented in a companion paper) is extended to an integrated approach for the calibration of an elastoplastic law based on a single radiograph per loading step. Instead of following a two-step sequential procedure (i.e. first, measurement of the displacement field; second, identification), the integrated method aims at identifying few model parameters directly from the gray-level projections. The analysis of an in situ tensile test composed of 127 loading steps performed in 6 min is presented. An isotropic elastoplastic constitutive law with free-form hardening behavior (i.e. controlled by only eight parameters) is identified and shows a ductile behavior (up to 6.3% strain before failure). A large improvement on the residual quality is shown and validates the proposed model and procedure. The obtained displacement fields are similar to those measured with no mechanical integration. A different parameterization of the constitutive law provides a very close result, thereby assessing the robustness of the procedure.

The measurement of four-dimensional (i.e. three-dimensional space and time) displacement fields of in situ tests within X-ray computed tomography scanners (i.e. lab-scale X-ray computed tomography) is considered herein using projection-based digital volume correlation. With a singleprojection per loading (i.e. time) step, the developed method allows the loading not to be interrupted and to vary continuously during the scan rotation. As a result, huge gains in acquisition time (i.e. more than two orders of magnitude) need to be reached. The kinematic analysis is carried out using predefined space and time bases combined with model reduction techniques (i.e. proper generalized decomposition with space–time decomposition). The accuracy of the measured kinematic basis is assessed via gray-level residual fields. An application to an in situ tensile test composed of 127 time steps is performed. Because of the slender geometry of the sample, a specific beam space regularization is used, which is composed of a stack of rigid sections. Large improvements on the residual, the signal-to-noise ratio of which evolves from 9.9 to 26.7 dB, validate the procedure.

The effect of different data acquisition parameters on the displacement and strain uncertainties of kinematic fields is assessed via digital volume correlation applied to three-dimensional data sets imaged by synchrotron laminography. The measurement uncertainty is estimated for (1) different materials and associated varying contrast; (2) repeated scans, that is, uncertainty due to the imaging system and three-dimensional reconstruction; (3) rigid body motions between two subsequent scans; (4) rescaling of the gray level histogram during 32-bit floating point to 8-bit integer data conversion; (5) changes in the beam properties, that is, use of a monochromator or an undulator; and (6) changes in camera/detector characteristics. It is found that the amount of image contrast is not the only parameter that controls uncertainties for different materials. Furthermore, the rigid body motion procedure should be preferred over repeated scans as it provides more conservative measurement uncertainty values. The applied 32- to 8-bit conversion procedure, beam tuning and detector characteristics hardly affect the measurement uncertainty.

The interactions between plasticity and damage mechanisms are not clearly established concerning the fracture of ductile sheet materials (e.g., flat to slant transition). The question addressed herein is to elucidate which mechanism is responsible for localized phenomena leading to the final failure. A mechanical test carried out on a notched plate made of 2139-T3 aluminum alloy is imaged thanks to synchrotron laminography at micrometer resolution. Ductile damage (i.e., void nucleation, growth and coalescence) is analyzed via reconstructed volumes. Although the low volume fraction of secondary phases in the tested alloy is challenging, digital volume correlation is also utilized to measure displacement fields and estimate strain fields in the bulk of the alloy during the whole test. In the first part of this study, the resolution of the measurement technique is assessed under such conditions. Then strained bands are shown to occur very early on in what will be the slant region of the fracture path. Conversely, damage grows at very late loading steps.

The identification of the parameters of several constitutive laws is performed with the integrated digital image correlation (IDIC) technique in a biaxial experiment for a cruciform specimen made of stainless steel. The sought material parameters are assessed with the contribution of both reaction forces (from load sensors) and displacement fields (measured via digital image correlation). For each constitutive law a global residual quantifying the model error is assessed.

Thermal shocks are applied to a 304L austenitic stainless steel plate with a pulsed laser. A stroboscopic reconstruction is used for infrared (IR) and visible light camera measurements. The displacement fields are measured with a digital image correlation (DIC) technique. Different IR devices are used to measure the temperature variations (i.e.medium wave camera and short wave pyrometry). Several ways of determining the emissivity or absorptivity are discussed. The complete 3D thermal loading is numerically determined by minimising the difference between experimental measurements and finite element analyses of thermal fields. An elastoplastic model is then used to compute mechanical fields that are compared with DIC measurements.

It is proposed to develop a multiview framework to perform stereocorrelation by resorting to isogeometric descriptions of the observed three-dimensional surfaces. Once the three-dimensional surfaces have been reconstructed, the corresponding three-dimensional displacement fields are measured within the same space. These fields are subsequently utilized to update isogeometric analyses to calibrate the elastic properties in a tensile experiment. An alternative route consists of preforming such analyses within an integrated framework.

This paper deals with the characterization of crack networks on the surface of refractory castable slabs, which are induced by stresses arising during the curing and drying (CD) stages. Images of the surface of castable samples placed in a house-made climatic chamber were recorded during the CD stages with a digital camera and processed by Digital Image Correlation (DIC) tomeasure the displacement field. Post-processing of DIC results were performed to estimate the length, orientation and opening of cracks in networks. The methodology is detailed and first analyses of experimental results are shown.

We consider a composite medium which consists in resistance elements with a distibution of conductance, in a narrow range around a mean value. The question we address is the distibution of local power dissipation, and its correlation with the local conductance. Various discretizations of the problem are considered: regular networks of different types in two and three dimensions whose bonds are assigned conductances at random, finite element method with random distibution of conductances in each element. We also consider the case of elastic elements, and study the distibution of elastic energy stored in each element. It is shown that the correlation between the dissipated (or elastic) energy and the conductance (or elastic modulus) depends on the discretization. These correlations are analysed in an effective medium theory framework, and numerical simulations confirm the theoretical predictions. The distribution of local energy always tends towards a Gaussian distibution for all cases considered, in the limit of a small disorder.

By construction, digital image correlation is an ill-posed problem. To circumvent this difficulty, regularization is often performed implicitly through the kinematic basis chosen to express the sought displacement fields. Conversely, a priori information on the mechanical behavior of the studied material is often available. It is proposed to evaluate the gain to be expected from such a mechanical assistance, namely, the measured displacement not only satisfies as best as possible the gray-level conservation but also mechanical admissibility.

Most often, digital image correlation is used to analyse a sequence of images. Exploitation of an expected temporal regularity in the displacement fields can be used to enhance the performances of a digital image correlation analysis, either in terms of spatial resolution, or in terms of uncertainty. A general theoretical framework is presented, tested on artificial and experimental image series.

Global digital volume correlation is used to analyze a series of computed tomography images of a nodular graphite cast iron specimen subjected in situ to a fatigue test. From the obtained displacement field, a specific procedure is implemented to extract stress intensity factors all along the crack front. The proposed methodology allows one to measure key parameters in fatigue crack propagation directly from 3D images.

Abstract: Dynamic loadings produce high stress waves leading to the spallation of ductile materials such as aluminum, copper, magnesium or tantalum. The main mechanism used herein to explain the change of the number of cavities with the stress rate is nucleation inhibition, as induced by the growth of already nucleated cavities. The dependence of the spall strength and critical time with the loading rate is investigated in the framework of a probabilistic model. The present approach, which explains previous experimental findings on the strain-rate dependence of the spall strength, is applied to analyze experimental data on tantalum. [Copyright &y& Elsevier]

To reduce the measurement uncertainty, a measurement technique is proposed for estimating full displacement fields by complementing digital image correlation with an additional penalization on the distance between the estimated displacement field and its projection onto the space of elastic solutions. The extended finite element method is used for inserting discontinuities independently of the underlying mesh. An application to the brittle fracture of a silicon carbide specimen is used to illustrate the application. To complete the analysis, the crack tip location and the stress intensity factors are estimated. This allows for a characterization of the measurement and identification procedure in terms of uncertainty.

This paper deals with the characterization of crack networks on the surface of refractory castable slabs, which are induced by stresses arising during the curing and drying (CD) stages. Images of the surface of castable samples placed in a house-made climatic chamber were recorded during the CD stages with a digital camera and processed by Digital Image Correlation (DIC) tomeasure the displacement field. Post-processing of DIC results were performed to estimate the length, orientation and opening of cracks in networks. The methodology is detailed and first analyses of experimental results are shown.

Not Available

Une approche multi-échelles de la corrélation d'images numériques est présentée. Elle permet de résoudre précisément les fluctuations du champ de déplacement. Cette technique de mesure est ensuite appliquée à des essais de compression et d'arrachement réalisés sur laine de verre crêpée. Les champs de déplacement et de déformation sont alors déterminés. Cette méthode révèle, lors de la réalisation d'essais de compression, des hétérogénéités de déformation, prémices de la localisation, bien avant le plateau de contrainte. Les essais d'arrachement permettent d'identifier l'amorçage et la propagation de fissures. La fiabilité et la résolution des champs de déplacements et de déformations sont validées en utilisant différents paramètres lors de l'analyse par corrélation.

Lors de leur cycle de fabrication, des contraintes résiduelles sont générées notamment dans la partie fragile d'assemblages céramique/métal. Ces dernières sont responsables de l'amorçage et de l'arrêt des fissures générées par ce cycle, mettant en défaut les traitements probabilistes de la rupture basés sur l'hypothèse du maillon le plus faible. Un modèle probabiliste de fissuration est proposé et permet d'appréhender de tels arrêts. Une extension de ce modèle considère la fiabilité des assemblages soumis à de la fatigue statique. Une méthodologie d'identification des paramètres aléatoires microstructuraux est proposée. La démarche est ensuite appliquée à la prévision de la durée de vie d'un composant industriel.

Not Available

Not Available

ABSTRACTThe degradation and failure of brittle or quasi-brittle geomaterials are analyzed in this Lecture. The main emphasis is put on the discrimination between discrete and continuum approaches to describe damage and fracture. The materials studied herein are cement, high-performance concrete and rocks.

We propose a multiscale approach to determine the displacement field by digital image correlation. The displacement field is first estimated on a coarse resolution image and progressively finer details are introduced in the analysis as the displacement is more and more securely and accurately determined. Such a scheme has been developed to increase the robustness, accuracy, and reliability of the image-matching algorithm. The procedure is used on two different types of examples. The first one deals with a representative image that is deformed precisely and purposefully to assess the intrinsic performances. In particular, the maximum measurable strain is determined. The second case deals with a series of pictures taken during compression experiments on mineral-wool samples. The different steps of the procedure are analyzed and their respective role is assessed. Both reflection and transmission images are tested.

Abstract.:  Concentration profiles due to (inter)diffusion in materials may require high spatial resolution. These profiles may be measured by electron probe microanalysis, which allows one to determine the elemental composition with a good accuracy provided measurement ‘artefacts’ can be accounted for. Standard phenomena are usually corrected by commercial softwares that assume a homogeneous elemental composition in the analysed area. However, in the case of a diffusion process on a small scale, the composition is no longer homogeneous and the effect of the hemispherical volume of the X-ray emission on the spatial resolution of the concentration profiles, and consequently on the diffusion coefficients, has to be considered. Moreover, (secondary) fluorescence across interfaces or interphases has to be evaluated. A radial X-ray distribution associated with the characteristic depth distribution, φ(ρz), allows for the definition of a 2D X-ray emission function that enables the computation of the entire process for a given concentration profile.

Not Available

A method, believed to be new, is introduced to evaluate displacement fields from the analysis of a deformed image compared with a reference image. In contrast to standard methods, which determine a piecewise constant displacement field, the present method gives direct access to spectral decomposition of the displacement field. A minimization procedure is derived and used twice: first, to determine an affine displacement field and, then, the spectral components of the residual displacement. Although the method is applicable to any space dimension, only cases dealing with one-dimensional signals are reported: First, a purely synthetic example is discussed to estimate the intrinsic performance of the method, and a second case deals with a profile extracted from a sample of compressed glass wool.

Not Available

Fiber pull-out is one of the fracture features of fiber-reinforced ceramic matrix composites. The onset of this mechanism is predicted by using Continuum Damage Mechanics, and corresponds to a localization of the deformations. Alter deriving two damage models from a uniaxial bundle approach, different configurations are analysed through analytical and numerical (F.E. calculations) methods. For one model some very simple criteria can be derived, whereas for the second one none of these criteria can be derived and the general criterion of localization has to be used.

Not Available

A constitutive equation has been derived for fiber-reinforced ceramic-matrix composites, based on fiber breakage and distributed fiber pull-out. Length-dependent and length-independent regimes, governed by the size of the specimen, are differentiated. The constitutive equation is used to predict the ultimate strength of fiber-reinforced ceramic-matrix composites subjected to tensile and flexural loadings.

The effects of matrix cracking and debonding which occur in ceramic-matrix composites are described by a micromechanical model. The cracking and debonding processes induce loss of stiffness, inelastic strains, hysteresis loops and crack closure. These features are analysed within the framework of Continuum Mechanics by the introduction of internal variables identified in the micromechanical analysis. The evolution laws of the internal variables can be determined by combining the experimental data with micromechanical modeling. The influences of residual stress fields due to processing are also included. Comparisons are made between theoretical predictions and the results of experiments performed on layered materials.

In this paper, a constitutive law is presented to model the mechanical behaviour of ceramic matrix composites. It allows matrix-cracking, interfacial debonding, sliding and wear to be accounted for in the framework of continuum mechanics. Based upon micromechanical studies, a 1D and 2D model was derived. An application was performed on a [0,90] SiC/SiC composite.

Additively manufactured materials usually exhibit mesoscale heterogeneities. Mesoscale fluctuations of strain fields in notched samples made of 17-4PH Stainless steel and loaded in tension are investigated. Regularized digital image correlation enables for the analysis of strain fluctuations at different length scales. Five tests on specimen fabricated with different printing parameters are studied. It is shown that the strain fluctuations have no characteristic length scale and are essentially independent of the probed processing parameters.

The propagation of (planar) cracks in a heterogeneous brittle material characterized by a random field of toughness is considered, taking into account explicitly the effect of the crack front roughness on the local stress intensity factor. In the so-called strong-pinning regime, the onset of crack propagation appears to map onto a second-order phase transition characterized by universal critical exponents which are independent of the local characteristics of the medium. Propagation over large distances can be described by using a simple one-dimensional description, with a correlation length and an effective macroscopic toughness distribution that scale in a non-trivial fashion with the crack front length. As an application of the above concepts, the arrest of indentation cracks is addressed, and the analytical expression for the statistical distribution of the crack radius at arrest is derived. The analysis of indentation crack radii on alumina is shown to obey the predicted algebraic expression for the radius distribution and its dependence on the indentation load. [Copyright &y& Elsevier]

The present study aims at proposing a new generation of experimental protocol for analysing crack propagation in quasi brittle materials. The boundary conditions are controlled in real-time to conform to a predefined crack path. Servo-control is achieved through a full-field measurement technique to determine the pre-set fracture path and a simple predictor model based on linear elastic fracture mechanics to prescribe the boundary conditions on the fly so that the actual crack path follows at best the predefined trajectory. The final goal is to identify, for instance, non-local damage models involving internal lengths. The validation of this novel procedure is performed via a virtual test-case based on an enriched damage model with an internal length scale, a prior chosen sinusoidal crack path and a concrete sample. Notwithstanding the fact that the predictor model selected for monitoring the test is a highly simplified picture of the targeted constitutive law, the proposed protocol exhibits a much improved sensitivity to the sought parameters such as internal lengths as assessed from the comparison with other available experimental tests. [ABSTRACT FROM AUTHOR]

Strain and damage interactions during tearing of a ductile Al-alloy with high work hardening are assessed in situ and in 3D combining two recently developed experimental techniques, namely, synchrotron laminography and digital volume correlation. Digital volume correlation consists of registering 3D laminography images. Via simultaneous assessments of 3D strain and damage at a distance of 1-mm ahead of a notch root of a thin Compact Tension-like specimen, it is found that parallel crossing slant strained bands are active from the beginning of loading in a region where the crack will be slanted. These bands have an intermittent activity but are stable in space. Even at late stages of deformation strained bands can stop their activity highlighting the importance of plasticity on the failure process rather than damage softening. One void is followed over the loading history and seen to grow and orient along the slant strained band at very late stages of deformation. Void growth and strain are quantified. Gurson–Tvergaard–Needleman-type simulations using damage nucleation for shear, which is based on the Lode parameter, are performed and capture slant fracture but not the initial strain fields and in particular the experimentally found slant bands. The band formation and strain distribution inside and outside the bands are discussed further using plane strain simulations accounting for plastic material heterogeneity in soft zones. [ABSTRACT FROM AUTHOR]