Digital image correlation

About: Digital image correlation is a research topic. Over the lifetime, 7842 publications have been published within this topic receiving 132166 citations.

Experimental investigation of microstructural changes in deforming granular media using x-ray tomography

Abstract: This doctoral thesis presents an experimental investigation into the mechanics of granular media.The novelty that this work brings is that the specimens of sand tested in this work are systematicallyand non-destructively imaged using x-ray tomography. Sample size is considerably reducedfrom standard (specimens measure approximately 22 mm height by 11 mm diameter), allowingentire specimens to be scanned at a sufficiently high resolution to identify all the grains (morethan fifty thousand) in each specimen.A campaign of triaxial compression tests has been run on a series of three different naturalsands with different grain shapes (Hostun sand, Ottawa sand and Caicos ooids – all prepared atrelatively dense initial states), and tested at 100 or 300 kPa cell pressure. In each test around 15x-ray scans are performed. In the 3D images resulting from the reconstruction of the x-ray scansperformed, grains are identified each state using a standard watershed algorithm. Starting fromthese discretised data, techniques are developed in order characterise grain-to-grain contacts,as well as to measure the kinematics of all the identified grains between imaged states. Grainkinematics are measured with two specifically-developed tools: “ID-Track” to track grains yieldingtheir displacements, and a discrete image correlation technique to measure grain rotations.Grain-scale measurements are reported in detail for one test, and are then compared to testsin different conditions, in order to highlight the micro-mechanisms responsible for the observedmacroscopic behaviour. This comparison highlights some important micro-scale mechanisms suchas the increasing rotational frustration of more angular grains when the sample’s deformation isconcentrated in a fully developed shear band; this is used to explain to some extent the highervalue of their residual stress for these materials. Signs of localised deformation are seen to occurwell before the peak in many samples, and complex patterns of rotating grains (which match alocal, grain-based measurement of strain) are noticed around the peak of each sample’s response.

Damage law identification of a quasi brittle ceramic from a bending test using digital image correlation

Abstract: Although ceramics are generally considered to be elastic brittle solids, some of them are quasi brittle. These ceramics show a non-linear mechanical behaviour resulting most of the time in a difference between their tensile and compressive stress–strain laws. The characterization of their fracture strengths might be biased if linear elastic formulae are used to analyze classical tests like bending tests. Based on Digital Image Correlation (DIC), an efficient technique to measure full field displacements, a methodology is proposed to characterize and model materials with dissymmetric behaviours between tension and compression. Applying specific basis functions for DIC displacement decompositions for bending, compressive and tensile tests, a stress–strain model and its damage law are identified and then validated for aluminium titanate, a damageable micro-cracked ceramic at room temperature. This identification method using DIC can obviously be applied to other quasi brittle materials.

Probing the micromechanics of a multi-contact interface at the onset of frictional sliding

Abstract: Digital Image Correlation is used to study the micromechanics of a multi-contact interface formed between a rough elastomer and a smooth glass surface. The in-plane elastomer deformation is monitored during the incipient sliding regime, i.e. the transition between static and sliding contact. As the shear load is increased, an annular slip region, in coexistence with a central stick region, is found to progressively invade the contact. From the interfacial displacement field, the tangential stress field can be further computed using a numerical inversion procedure. These local mechanical measurements are found to be correctly captured by Cattaneo and Mindlin (CM)'s model. However, close comparison reveals significant discrepancies in both the displacements and stress fields that reflect the oversimplifying hypothesis underlying CM's scenario. In particular, our optical measurements allow us to exhibit an elasto-plastic like friction constitutive equation that differs from the rigid-plastic behavior assumed in CM's model. This local constitutive law, which involves a roughness-related length scale, is consistent with the model of Bureau \textit{et al.} [Proc. R. Soc. London A \textbf{459}, 2787 (2003)] derived for homogeneously loaded macroscopic multi-contact interfaces, thus extending its validity to mesoscopic scales.measurements allow for the first quantitative test of Cattaneo and Mindlin (CM) classical model of the incipient sliding of a smooth interface. Small deviations are observed and interpreted as a result of the finite compliance of the rough interface, a behavior which contrasts with Amontons' law of friction assumed to be valid locally in CM's model. We illustrate how these measurements actually provide a method for probing the rheology of the rough interface, which we find to be of the elasto-plastic type.

Modal Analysis Using Digital Image Correlation Technique: An Application to Artificial Wing Mimicking Beetle’s Hind Wing

Abstract: Understanding the dynamic behavior of structures has become increasingly important in the design process of any mechanical system. Therefore, the demands for improved structural performance have motivated the search for an effective method of structural dynamics testing, since the conventional methods are limited to the location of relatively few applied sensors. Due to the state-of-the-art optical technologies, the shape and deformation of a vibrating structure have been measured using 3-dimensional digital image correlation (DIC) technique. Although DIC has been used widely to construct the mode shape of the structure in modal measurement, it has rarely been applied to determine the full modal parameters (natural frequencies, mode shapes, damping factors). Therefore, this study presents an effective method to measure the full modal parameters of an artificial wing that mimics a beetle’s hind wing using the DIC technique. In our measurement, the artificial wing was mounted on a shaker, which was vibrated with a white noise signal. The full-field result as well as the displacement of a single point on the wing over time was then obtained using ARAMIS® software, a DIC technique-based software. From the temporal displacement of a single point signal in the time domain, we performed fast Fourier transform to obtain the frequency response function (FRF). The spectrum averaging technique and Savitzky-Golay filter were used to reduce the noise. Also, the natural frequencies and damping factors were determined from smoothed FRF. Finally, the mode shapes were measured using DIC at the pre-measured natural frequency.