G. Chiappini

Superimposed fringe projection for three-dimensional shape acquisition by image analysis

The aim in this work is the development of an image analysis technique for 3D shape acquisition, based on luminous fringe projections. In more detail, the method is based on the simultaneous use of several projectors, which is desirable whenever the surface under inspection has a complex geometry, with undercuts or shadow areas. In these cases, the usual fringe projection technique needs to perform several acquisitions, each time moving the projector or using several projectors alternately. Besides the procedure of fringe projection and phase calculation, an unwrap algorithm has been developed in order to obtain continuous phase maps needed in following calculations for shape extraction. With the technique of simultaneous projections, oriented in such a way to cover all of the surface, it is possible to increase the speed of the acquisition process and avoid the postprocessing problems related to the matching of different point clouds.

Characterization of aluminum alloys using a 3D full field measurement

In the 2009 SEM Conference in Albuquerque, a measurement technique that combines the digital image correlation and the fringe projection was proposed by the authors to evaluate the 3D displacement of a specimen during a tensile test: the DIC is used to measure the in-plane displacement on both faces of the specimen while the fringe projection and phase shifting is used to reconstruct the shape with a high spatial resolution. In this work, the mentioned experimental technique was employed to characterize the plastic behavior of an aluminum alloy. First a 3D mesh was utilized to regularize the measured data and to get the strain field inside the specimen, in such way the full strain history of the test was reconstructed until failure. Then the Virtual Fields Method was adopted to identify the parameters of an anisotropic plasticity model. A comparison with FEM was also made to assess the correctness of the identified parameters.

Advanced Test Simulator to Reproduce Experiments at Small and Large Deformations

Full field measurements and inverse methods can be conveniently used to identify the constitutive properties of materials. Several methods are available in the literature which can be applied to many different types of materials and constitutive models (linear elasticity, elasto-plasticity, hyper-elasticity, etc.). The effectiveness of the identification procedure is related to the specimen geometry and the quality of the optical measurement technique. A method to improve and optimize the identification procedure is to numerically simulate the whole process. In such a way it is possible to compare different configurations and chose the one that shows the lowest identification error.

Out-of-Plane Motion Evaluation and Correction in 2D DIC

2D and stereo Digital Image Correlation (DIC) allows to retrieve complex displacement and strain fields on a specimen’s surface. Although 2D DIC is strongly affected by out-of-plane motions, in many situations, it is preferred over stereo DIC because of its ease to use and because only one camera is required. The out-of-plane movements can be ascribed mainly to three causes: the camera positioning, the imperfections of the used test device, and the camera self-heating. These effects gain importance when the distance between the camera and the specimen is reduced. The positioning of the camera aims to have its optical axis perfectly perpendicular to the specimen to observe. Nevertheless small but effective misalignments can easily happen even if suitable devices are used for the alignment. This contribution concerns the experimental evaluation of these movements considering a cyclic uni-axial tensile test performed on an aluminium specimen. The study is particularly focused to the out-of-plane motions that occur at every cycle because of the tensile bench, which are the more critical ones. Finally a compensation method, based on fixed compensation plates, is presented. The method allows to properly correct the data coming from a 2D DIC set-up.

Strain Assessment in Cracked Sheet Metals by Optical Grid Method

The present work is an extension of the optical grid method for the experimental investigation of deformation in stamped sheet metals. The classical method presents difficulty or is inapplicable where the deformation has resulted in the tearing of the sheet. In these cases, the measurement result is not available right where the most interesting data are expected.

Assessment of inverse procedures for the identification of hyperelastic material parameters

This work aimed to implement and compare two competitive procedures for the identification of hyperelastic material parameters. Formerly, experimental tests have been conducted on fluorosilicone rubber specimens in equal-biaxial tension; the cruciform shaped-specimens underwent heterogeneous large strain distributions, which were captured by Digital Image Correlation technique; while load cells grabbed the force signals. The experimental data have been used in two different inverse techniques for material parameters estimation: the first method was based on “classic” FE model updating, which uses only the global quantities measured during the experiments (i.e. forces and boundary displacements) to define the error function to be minimized; the second method was still based on FE model updating, but experimentally determined strain fields was compared with the numerical ones in order to define a more adequate cost function; third technique was based on the Virtual Fields Method, which naturally takes into account the real strain distributions and permits to overcome the experimental difficulties represented by non-symmetry of the test/specimen, non-uniform boundary conditions, friction. The results of the three procedures are showed and compared in terms of accuracy, transferability, computational efficiency and practicability.

Identification of Constitutive Model Parameters in Hopkinson Bar Tests

In this work, tension and compression tests have been carried out on aluminium samples at low and high strain rate, the latter performed by means of a direct tension Hopkinson bar equipment. The parameters of the Johnson-Cook constitutive model have been identified using different approaches; the first method consists in the classical Finite Element Model Updating, where numerical simulations are repeated with different material parameters until the mismatch between the experimental and numerical load–displacement curves falls below an acceptable threshold.

Performance Assessment of Inverse Methods in Large Strain Plasticity

The Virtual Fields Method (VFM) is an inverse method which allows to identify the constitutive parameters of materials from full-field measurements. The method relies on the principle of virtual work and, traditionally, is mainly used to characterize the material properties. Nonetheless the VFM can also be used as an inspection tool to compare constitutive models or experimental configurations. For instance, it can be adopted to compare different constitutive models in order to find out which one is more suitable to describe the mechanical behaviour observed during an experiment. In this paper this idea is applied in the case of large strain plasticity. A sensitivity study was conducted to evaluate how the specimen geometry and the texture orientation influence the identification of the constitutive parameters. Then the method was used to compare the performances of three constitutive models in reproducing the plastic behaviour of a given material. The studied models are isotropic von Mises, Hill48 for normal and planar anisotropy. The work was conducted on simulated data.

Identification of the plastic zone using digital image correlation

In this paper Digital Image Correlation (DIC) is used to study the evolution of the plastic zone close to a crack tip. A modified CT-specimen was used in order to fulfill the plane stress condition. The strain field around the crack tip was measured using two cameras and stereo DIC, so that out-of-plane movements are taken into account. Then, the Virtual Fields Method was used to identify the plastic zone, looking at the parts of the specimen which deviates from the linear elastic behavior. With such approach, it was possible to individuate the onset of plasticity close to the crack tip and follow its evolution. A comparison with FEM results is also provided.

Evaluation of Stress Equilibrium in Dynamic Tests on Agglomerated Cork

In this work, experimental compression tests have been performed on rectangular specimens cut from a cork slab. The tests have been performed both using a quasi-static testing machine and a Split Hopkinson Bar. The dynamic tests revealed a high sensitivity of the material to the strain rate, with stress strain curves significantly higher than in quasi-static condition. Given the low density of the cork, the specimen material may suffer from non-equilibrium; for this reason, a high speed camera was used to frame acquire pictures of the sample during the deformation, which have been used to perform DIC analyses. Indeed, inhomogeneous strain distributions were found, especially due to the low density of samples tested at the higher speed. Moreover, the low impedance of the tested material also determines difficulties in signal synchronization and, hence, in the correct calculation of the stress in the sample. Data from DIC analyses were then used to calculate the true strain in different portions of the specimen and to evaluate the stress due to inertia effect.