Showing papers in "Strain in 2020"

An experimental study of the feasibility of phase-based video magnification for damage detection and localisation in operational deflection shapes

Abstract: Optical measurements from high-speed, high-definition video recordings can be used to define the full-field dynamics of a structure. By comparing the dynamic responses resulting from both damaged and undamaged elements, Structural Health Monitoring (SHM) can be carried out, similarly as with mounted transducers. Unlike the physical sensors, which provide point-wise measurements and a limited number of output channels, high-quality video recording allows very spatially dense information. Moreover, video acquisition is a non-contact technique. This guarantees that any anomaly in the dynamic behaviour can be more easily correlated to damage and not to added mass or stiffness due to the installed sensors. However, in real-life scenarios, the vibrations due to environmental input are often so small that they are indistinguishable from measurement noise if conventional image-based techniques are applied. In order to improve the signal-to-noise ratio (SNR) in lowamplitude measurements, Phase-Based Motion Magnification (PBMM) has been recently proposed. This study intends to show that model-based SHM can be performed on modal data and time histories processed with PBMM, whereas unamplified vibrations would be too small for being successfully exploited. All the experiments were performed on a multidamaged box beam with different damage sizes and angles.

Inverse identification of constitutive parameters from heat source fields: A local approach applied to hyperelasticity

Abstract: In this paper, a new inverse identification method of constitutive parameters is developed from full kinematic and thermal field measurements. It consists in reconstructing the heat source field from two different approaches by using the heat diffusion equation. The first one requires the temperature field measurement and the value of the thermophysical parameters. The second one is based on the kinematic field measurement and the choice of a thermo-hyperelastic model that contains the parameters to be identified. The identification is carried out at the local scale, ie, at any point of the heat source field, without using the boundary conditions. In the present work, the method is applied to the challenging case of hyperelasticity from a heterogeneous test. Due to large deformations undergone by the rubber specimen tested, a motion compensation technique is developed to plot the kinematic and the thermal fields at the same points before reconstructing the heterogeneous heat source field. In the present case, the constitutive parameter of the Neo-Hookean model has been identified, and its distribution has been characterized with respect to the strain state at the surface of a cross-shaped specimen.

A Bayesian approach to triaxial strain tomography from high‐energy X‐ray diffraction

Abstract: Diffraction of high-energy X-rays produced at synchrotron sources can provide rapid strain measurements, with high spatial resolution, and good penetrating power. With an uncollimated diffracted beam, through thickness averages of strain can be measured using this technique, which poses an associated rich tomography problem. This paper proposes a Gaussian process (GP) model for three-dimensional strain fields satisfying static equilibrium and an accompanying algorithm for tomographic reconstruction of strain fields from high-energy X-ray diffraction. We present numerical evidence that this method can achieve triaxial strain tomography in three-dimensions using only a single axis of rotation. The method builds upon recent work where the GP approach was used to reconstruct two-dimensional strain fields from neutron based measurements. A demonstration is provided from simulated data, showing the method is capable of rejecting realistic levels of Gaussian noise.

On the identification of cohesive zone model for curved crack in mortar

Abstract: This paper proposes an approach to defining the path of a curved crack in a single edge notched specimen with gray level residuals extracted from digital image correlation, followed by the calibration of the parameters of a cohesive zone model. Only the experimental force is used in the cost function minimized via finite element model updating. The displacement and gray level residual fields allow for the validation of the calibrated parameters. \revision{Last, the results are confronted with those given by a straight crack to highlight the benefits of accounting for the actual crack path.

Three‐dimensional physics‐based registration of pelvic system using 2D dynamic magnetic resonance imaging slices

Abstract: This paper introduces a method for dynamic 3D registration of female pelvic organs using 2D dynamic magnetic-resonance images (MRI). The aim is to provide a better knowledge and understanding of pathologies such as prolapsus or abnormal mobility of tissues. 2D dynamic MRI sequences are commonly used in nowadays clinical routines in order to evaluate the dynamic of organs, but due to the limited view, subjectivity related to human perception cannot be avoided in the diagnoses. A novel method for 2D/3D registration is proposed combining 3D Finite Element models with a priori knowledge of boundary conditions, in order to provide a 3D extrapolation of the dynamic of the organs observed in a single 2D MRI slice. The method is applied to the 4 main structures of the female pelvic floor (bladder, vagina, uterus and rectum), providing a full 3D visualization of the organs' displacements. The methodology is evaluated with two patient-specific data sets of volunteers presenting no pelvic pathology, and a sensitivity study is performed using synthetic data. The resulting simulations provide an estimation of the dynamic 3D shape of the organs facilitating diagnosis compared to 2D sequences. Moreover, the method follows a protocol compatible with current clinical constraints presenting this way potential short term medical applications.

Determination of effective stress intensity factors under mixed-mode from digital image correlation fields in presence of contact stresses and plasticity

Abstract: Digital image correlation (DIC) is more and more popular to monitor fatigue crack growth and to determine the stress intensity factors. However, the posttreatment of the recorded displacement fields becomes tricky when the crack faces are not stress‐free and when crack tip plasticity becomes significant. Several posttreatment methods to locate the crack tip and measure the effective stress intensity factors in such cases are compared, using finite element method‐computed displacement fields, and then used on real DIC fields. An approach coupling DIC and finite element method is proposed to estimate the contact stresses along the crack.

Analysis of the accuracy on computing nominal stress in a biaxial test for arteries

Abstract: Strain. 2020;56:e12331. https://doi.org/10.1111/str.12331 Abstract Biaxial tests are commonly used to investigate the mechanical behaviour of anisotropic soft biological tissues such as cardiovascular tissues. However, there is still no clear understanding of the influence that the biaxial test set‐ up conditions may have on the computing material stress of the experimental results. The aim of the present study is to further investigate the accuracy of calculated material stress from measured force during biaxial tests using finite element methods (FEM). The biaxial mechanical response of ascending aorta and pulmonary artery tissue samples was obtained by FEM simulation under two different gripping methods: (a) a system with noodle clamps and (b) a clamped system with needles which leave the specimen's edges free to expand laterally. The results show that the clamped method whose joints allow free movement in the lateral direction produces stresses closer to the universally accepted formulation of biaxial material stress in the central region. However, the system with noodle clamps, commonly used to grip the sample, produces an alteration of the measurement stresses. Our simulations show results giving an inaccurate estimation of the stress at the centre of the sample. In some cases, the stresses are overestimated and in others underestimated depending on the anisotropy of the sample. We can conclude that the clamped system with needles which leave the specimen's edges free to expand laterally should be used as an efficient methodology to other commonly used gripping methods for biological tissues with anisotropic materials.

Improvement of the digital image correlation close to the borders of an object

Abstract: The measurement error of the finite-element-based Digital Image Correlation is reduced along the borders of an object by using the precise measurement, obtained by Virtual Image Correlation (VIC). The proposed method is called Virtual and Digital Image Correlation (VDIC). The boundary, identified by the VIC with a subpixel precision, is used firstly to create an adapted mesh and secondly to create a pixel mask. Futhermore, the VDIC also uses the VIC measurement of the boundary in the deformed state as a constraint on the radial displacement field along the border. The optimal values of the parameters of the VDIC are discussed throughout a sensitivity analysis. The compared performances of the constrained and unconstrained VDIC are obtained thanks to a synthetic test of a plate with a hole in tension. Finally, the method is checked on a sample geometry which includes holes and a U-shaped notch.

X‐ray tomographic observations of microcracking patterns in fibre‐reinforced mortar during tension stiffening tests

Abstract: For the last decades, new reparation or fabrication processes have been studied to replace traditional rebar by roving of different mineral or organic fibres to avoid corrosion issues. Such materials refer to the family of cementitious composite. Their tensile strength would directly depend on the proportion of reinforcement but also strongly on the interfacial mechanical properties between fibres and cementitious matrix. From now, evaluation of interfacial properties was mostly limited to the use of force-displacement curves obtained from mechanical experiments. This work presents a new methodology using micromechanical tension stiffening tests combined with X-ray computed tomography (XRCT) observations, performed at the Anatomix beamline at Synchrotron Soleil, and specific image processing procedures. Multi XRCT acquisitions with suitable scanning strategy are used to image the whole fibre-matrix interface along centimetric samples at four to five different levels of loading magnitude. Intensive image processing is then performed on tomographic images including digital volume correlation (DVC), image subtraction and Hessian-based filtering. This experiment allows to study damage mechanisms at small scale. The proposed methodology shows great potential to provide both qualitative and quantitative elements on interfacial mechanical behaviour such as crack growth and crack orientation. The interface between mortar and sufficiently small multi-fibre yarn used in this paper is shown to behave in certain condition as traditional rebar interface producing conical cracks in the surrounding matrix rather than debonding in mode 2, permitting a much higher energy dissipation during debonding. According to this study, conical cracks repartition and geometry are mostly influenced by the cementitious matrix. The spacing between cracks goes from 50 to 100 μm and the angle between crack normal vector and yarn orientation goes from 35 to 50 degrees.