D. Lecompte

Crack Detection in a Concrete Beam using Two Different Camera Techniques

The study presents an application of two different optical measurement techniques for the detection of cracks at the surface of a realistically sized concrete beam subjected to flexural loading conditions. Both techniques provide displacement measurements in a discrete number of points on the surface. Based on these displacement fields, deformations are calculated by means of the Green-Lagrange strain expression. The study deals with the relationship between cracks and the concept of deformation and it examines which of the two methods presented appears to be the most suitable for crack prediction or detection. The study shows that it is possible to detect the appearance and evolution of cracks, even before the cracks become visually detectable, with both methods and reveals their complementarities.

Analysis of speckle patterns for deformation measurements by digital image correlation

Digital Image Correlation (DIC) - also referred to as white light speckle technique - is an optical-numerical full-field measuring technique, which offers the possibility to determine in-plane displacement fields at the surface of objects under any kind of loading. For an optimal use of the method, the object of interest has to be covered with a speckle pattern. The present paper studies the efficiency of a random speckle pattern and its influence on the measured in-plane displacements with respect to the subset size. First a randomly sprayed speckle pattern is photographed three times. Each picture is taken with a different zoom, yielding three speckle patterns, which are different by the size of the speckles. Secondly a number of speckle patterns are generated numerically using a given speckle size and image coverage. Subsequently, each speckle pattern image undergoes a numerically controlled deformation, which is measured with digital image correlation software. Both imposed and measured displacements are compared and it is shown that the size of the speckles combined with the size of the used pixel subset, clearly influences the accuracy of the measured displacements. Furthermore it is shown that it is possible to create an optimal speckle pattern when a given subset size is chosen.

Biaxial testing of fibre-reinforced composite laminates:

Advanced composite material systems are increasingly used in almost every industrial branch. The structural components manufactured from these composite material systems are usually subjected to complex loading that leads to multi-axial stress and strain fields at critical surface locations. The current practice of using solely uniaxial test data to validate proposed material models is wholly inadequate. In order to test closer to reality, a biaxial test bench using four servo-hydraulic actuators with four load cells was developed. Besides the development of the test facility, a mixed numerical/experimental method was developed to determine the in-plane stiffness parameters from testing a single cruciform test specimen. To obtain the strength data an optimized geometry for the cruciform type specimen was designed. For the optimization procedure a full three-dimensional finite element model was used. The numerical results were validated with strain gauge, digital image correlation, and electronic speckle pattern interferometry data. The material system used for the experimental validation was glass fibre-reinforced epoxy with a lay-up [(+45°−45° 0°)4(+45°−45°)] typically used for wind turbine blades.

Parameter identification for anisotropic plasticity model using digital image correlation: Comparison between uni-axial and bi-axial tensile testing

The basic principle of the described procedure for plastic material identification is the generation of a complex and heterogeneous deformation field, which is measured by digital image correlation (DIC) and compared to Finite Element (FE) simulations. In this paper two tests for the identification of the hardening behaviour and the yield locus of DC06 steel are compared: a uni-axial test on a perforated rectangular specimen and a bi axial tensile test on a cruciform specimen. The work hardening of the material is assumed to be isotropic and the yield locus is modelled by the anisotropic Hill48 criterion. The identification results for the different material parameters, based on both the uni- and the bi-axial test, are discussed and show a significant agreement.

Deformation measurements and simulations of blast loaded plates

Three aluminum plates are subjected to a free-air explosive loading with 40g of C4. Combining two highspeed cameras in a stereoscopic setup and the digital image correlation technique, the full-field deformation is identified. Furthermore, the identified deformation fields are compared by data computed with an explicit finite element method. The Johnson-Cook material model is used to simulate the plastic behavior of the aluminum plate. A good agreement has been found between both experimental and numerical data.

Material Identification of Blast Loaded Aluminum Plates Through Inverse Modeling

This paper deals with full-field measurements of aluminum plates under free air blast loading conditions. A stereoscopic high-speed camera system is used to capture the plate response with an inter frame rate of 6,000 fps. The transient deformation fields are calculated using a three-dimensional digital image correlation technique. The deformation fields are compared by data computed with an explicit finite element method. A good agreement has been found between both experimental and numerical data. Furthermore, a mixed experimental-numerical method is proposed to determine the elasto-plastic material parameters during a free air blast event. The method is solely validated using a virtual experiment.

Inverse Method for Parameter Determination of Biaxially Loaded Cruciform Composite Specimens

This paper presents an inverse method for the identification of the in-plane orthotropic apparent engineering constants of bi-axially loaded composite materials using cruciform specimens. The full field displacements are identified by a digital image correlation technique. From the displacement field a strain field is computed and compared with finite element strain results of the experiment. The apparent engineering constants are unknown parameters in the finite element model. Starting from initial values, these parameters are updated till the computed strain field matches the experimental strain field, In a first stage, global apparent engineering constants were determined, In this stage a more local determination of the engineering constants is studied.

Plastic Material Model Identification Using Dic on Biaxial Tensile Test

The accuracy of a Finite Element Simulation for plastic deformation strongly depends on the chosen constitutive laws and the value of the material parameters within these laws. The identification of those mechanical parameters can be done based on homogeneous stress and strain fields such as those obtained in uni-axial tensile tests and simple shear tests performed in different plane material directions. Another way to identify plastic material parameters is by inverse modeling of an experiment exhibiting a heterogeneous stress and strain field. Material parameter identification methods, which integrate optimization techniques and numerical methods such as the finite element method (FEM), indeed offer an alternative tool. The most common approach is to determine the optimal estimates of the model parameters by minimizing a selected measure-of-fit between the responses of the system and the model, In the present study a method is proposed for the identification of the initial yield stress, the two parameters of a Swift isotropic hardening law and the four parameters of the Hil148 yield surface, based on the full-field surface measurements of a cruciform specimen subjected to biaxial tensile loading. Experimental forces and strains are in this case compared to the simulated values. A finite element model of the perforated specimen serves as numerical counterpart for the experimental set-up. The difference between the experimental and numerical strains (e x, e y and e xy) is minimized in a least squares sense by updating the values of the different parameters simultaneously. The sensitivities used to obtain the parameter updates are determined by finite differences, using small parameter perturbations. The optimization routine used, is based on a constrained Newton-type algorithm.

Computer simulation and experiments to evaluate digital image correlation methods

Digital image correlation is an optical-numerical full field displacement measurement technique, which is more and more widely used in experimental mechanics[l-3]. The technique is based on a numerical comparison of digitized images taken at different stages of deformation of an object. In principle, any imaging technique can be used, provided that the deformed image can be predicted starting from the undeformed image and the displacement field. The digital image correlation technique then consists of numerically searching the space of all possible displacement fields, to find the displacement field which gives the best correlation of the predicted image with the actual image of the deformed state.