Computer Vision-Based, Noncontacting Deformation Measurements in Mechanics: A Generational Transformation

DIC-based identification of the constitutive parameters of an elastoplastic law is addressed both from a general viewpoint, and applied to the particular case of dog-bone sample made of commercially pure titanium and subjected to tensile loading. A two-step procedure (Digital Image Correlation — DIC — followed by weighted Finite Element Method Updating — FEMU) is first presented. These two steps can be merged into a single-step procedure (i.e., Integrated-DIC or I-DIC). In both cases, the elastoplastic computations are performed with a commercial code (i.e., non-intrusive identification). When the suited weighting of FEMU is taken into account, which is based on DIC-processed image noise, both I-DIC and FEMU methods provide similar results. It is shown that the addressed experimental case requires the use of static (load) information to get precise estimates of the sought parameters.

/pdf/estimation-of-elastoplastic-parameters-via-weighted-femu-and-14hlyhygs6.pdf

Estimation of Elastoplastic Parameters via Weighted FEMU and Integrated-DIC

As any Digital Image Correlation (DIC) method, Finite-Element (FE) based DIC methods lead to uncertainties which are related to the spatial resolution (in pixel/element). To overcome the tricky and well-known compromise between spatial resolution and uncertainty, a multiscale approach to FE-DIC is proposed. Additional nearfield images are used to improve locally the resolution of the measurement for a given measurement mesh. An automatic and accurate estimation of the nearfield/farfield transformation is obtained by a dedicated DIC based method, in order to bridge precisely the measurement performed at both scales. This multiscale measurement is then associated to a multiscale Finite Element Model Updating (FEMU) identification technique. After being validated on synthetic test cases, the method is applied to a tensile test carried out on an open-hole specimen made of glass/epoxy laminate. The four in-plane orthotropic elastic parameters are identified at different levels of loading. Results show that the multiscale approach greatly improves the uncertainty of both the measured displacements and the identified material parameters.

/pdf/multiscale-displacement-field-measurement-using-digital-3axcd8qviu.pdf

Multiscale Displacement Field Measurement Using Digital Image Correlation: Application to the Identification of Elastic Properties

The goal of the present study is to illustrate the full integration of sensor and imaging data into numerical procedures for the purpose of identification of constitutive laws and their validation. The feasibility of such approaches is proven in the context of in situ tests monitored by tomography. The bridging tool consists of spatiotemporal (i.e., 4D) analyses with dedicated (integrated) correlation algorithms. A tensile test on nodular graphite cast iron sample is performed within a lab tomograph. The reconstructed volumes are registered via integrated digital volume correlation (DVC) that incorporates a finite element modeling of the test, thereby performing a mechanical integration in 4D registration of a series of 3D images. In the present case a non-intrusive procedure is developed in which the 4D sensitivity fields are obtained with a commercial finite element code, allowing for a large versatility in meshing and incorporation of complex constitutive laws. Convergence studies can thus be performed in which the quality of the discretization is controlled both for the simulation and the registration. Incremental DVC analyses are carried out with the scans acquired during the in situ mechanical test. For DVC, the mesh size results from a compromise between measurement uncertainties and its spatial resolution. Conversely, a numerically good mesh may reveal too fine for the considered material microstructure. With the integrated framework proposed herein, 4D registrations can be performed and missing boundary conditions of the reference state as well as mechanical parameters of an elastoplastic constitutive law are determined in fair condition both for DVC and simulation.

/pdf/toward-4d-mechanical-correlation-2xe6ii1fif.pdf

Toward 4D mechanical correlation

Subset-based local vs. finite element-based global digital image correlation: A comparison study

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.

https://hal.archives-ouvertes.fr/hal-00694332/document

Analysis of image series through global digital image correlation

A strain gauge includes a substrate (2) for mounting an element (3) to be reversibly lengthened by a force applied while displaying a variation in the resistance thereof, the element (3) lengthening itself along an axis for measurement by the gauge. The gauge includes at least one contrast target (5, 6) capable of reflecting an incident light beam, the at least one contrast target (5, 6) being placed on the gauge in a predetermined position that makes it possible to predetermine the center of the axis (4), for measurement by the strain gauge (1), by detecting the position of the at least one contrast target (5, 6).

Strain gauge, and system for spatially locating such gauges

https://hal.archives-ouvertes.fr/hal-01549237/document

Improving full-field identification using progressive model enrichments

The present paper is devoted to the identification of frictional properties in bolted assemblies. It is shown that kinematic data provided by digital image correlation can be used to analyze the change of the friction coefficient with the number of cycles. Two approaches are followed. The first one is based on the displacement jump between the assembled plates, and the second one relies on displacement fields measured on the same surface.

/pdf/identification-of-in-situ-frictional-properties-of-bolted-2tx555b04x.pdf

Identification of In Situ Frictional Properties of Bolted Assemblies with Digital Image Correlation

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...

Nicolas Swiergiel

Papers

Analysis of image series through global digital image correlation

Strain gauge, and system for spatially locating such gauges

Improving full-field identification using progressive model enrichments

Identification of In Situ Frictional Properties of Bolted Assemblies with Digital Image Correlation

Calibrating thermoelastic stress analysis with integrated digital image correlation: Application to fatigue cracks: