Enhancing Standard GVT Measurements with Digital Image Correlation

Digital image correlation (DIC) measurements are affected by several sources of uncertainty. Motion blur is one of the most relevant problems in dynamic DIC applications. This work deals with the problem of compensating motion blur effects on DIC. Firstly, a robust motion blur estimation technique based on cepstral analysis is presented and validated. Secondly, the problem of image restoration has been tackled. Two image deconvolution techniques are presented: one based on cepstrum deconvolution and the other based on Wiener filter. The latter has shown better robustness in presence of noise. Each presented technique has been tested with synthetic DIC experiments. Results demonstrate that both the compensation algorithms are able to improve the accuracy of DIC measurement in presence of motion blur.

Welding is one of the most widely used joining techniques for manufacturing complex engineering components. However, due to transient temperature cycles in the adjacent materials being joined, the local microstructure is altered and plastic strain in the surrounding materials is introduced resulting in a variation of mechanical properties across the weldment. When an extracted cross-weld test specimen is subjected to an external uniaxial load, there is interaction between the various constituents of the weld (such as the weld fused zone, heat affected zone and parent metal) resulting in development of constraint in the weaker material due to the adjacent stronger material. This affects the distribution of stress along and within the test specimen. The level of constraint developed is dependent on the strength mismatch between adjacent materials which may be graded across the heat affected zone and the specimen cross-sectional geometry. With development of digital image correlation (DIC) and its application to measuring deformation of solids, it has become feasible to measure local stress-strain properties of cross-weld samples and also specimens of variable geometry at both room and high temperature. Local deformation measured by DIC is correlated with nominal stress based on an iso-stress assumption, where it is assumed that the various regions of the weld are arranged in series and that the stress is uniform over the cross-section area at any given location in the specimen. Thus, local mechanical properties are inferred by mapping the measured local strain against the global stress. In this context the effect of constraint is important because it introduces a local stress distribution that will differ from the global net section stress and can therefore introduce errors in determination of local stress-strain properties. The influence of these factors raises questions concerning the interpretation of apparent local mechanical properties of cross-welds measured using DIC. Specifically, there is uncertainty as to whether the measured variation in local strain versus average cross-sectional stress (average global stress) represents the local mechanical properties of the material with negligible error. The severity of potential constraint errors will be influenced by the specimen geometry (particularly the tensile specimen thickness to width ratio for 2D DIC strain measurements) and the gradient of material property inhomogeneity along the specimen gauge length. In the current study, measurement of deformation in varying stress fields using DIC was carried out. Two distinct methods of introducing varying stress fields were employed: in the first case, the stress field was varied by designing an hour-glass shaped specimen with the specimen width varying as a function of the gauge length. In the second case, stress fields were varied due to a change in the specimen thickness in mismatched dissimilar metal joints. To reduce the complexity in the developed stress fields in the region near the interface, two materials were joined using solid state diffusion bonding giving a step change in the material properties across a planar interface. Test specimens spanning the interface were studied by finite element modelling and experimental tests with DIC deformation monitoring. Using DIC and a single hour-glass shaped specimen, multiple creep curves at different stress levels along the specimen gauge length have been successfully extracted for a test temperature of 525oC. The creep data obtained at 525oC from the hour-glass specimen test have been fitted to RCC-MR, Garofalo and Graham-Walles creep deformation models. A better correlation is achieved between the experimental data and the RCC-MR model predicted values when the local deformations from the single specimen are plotted as a function of time and true stress at the end of the specimen’s initial loading. From both elastic-plastic FE models and experimental local stress-strain curves extracted from DIC measurements, it can be concluded that a thin (1mm thick or less) 6mm wide test specimen should be used to extract the local mechanical properties from tensile specimens that have varying material properties along the gauge length, such as found in cross-weld test specimens. This is because the thinner specimen reduces the size of the strain concentration region adjacent to the interface where errors in measured local mechanical properties can occur. As a ‘rule of thumb’, the intrinsic stress-strain properties of the material should not be inferred from regions within 1mm of the interface for these 1mm thick samples because of concentration errors. The results obtained demonstrate the capability of DIC as a measurement technique for characterising the local deformation of specimens with varying stress fields. The findings also highlight some aspects of the technique which need to be considered when interpreting data from DIC measurements of local deformation from specimens with varying stress fields.

The use of 2D ultrasound elastography for measuring tendon motion and strain

A direct crack sizing approach from DIC strain analyses under elasto-plastic and dynamic conditions

Repeatability of digital image correlation for measurement of surface strains in composite long bones

A detailed understanding of the changes in load transfer due to implantation is necessary to identify potential failure mechanisms of orthopedic implants. Computational finite element (FE) models provide full field data on intact and implanted bone structures, but their validity must be assessed for clinical relevance. The aim of this study was to test the validity of FE predicted strain distributions for the intact and implanted pelvis using the digital image correlation (DIC) strain measurement technique. FE models of an in vitro hemipelvis test setup were produced, both intact and implanted with an acetabular cup. Strain predictions were compared to DIC and strain rosette measurements. Regression analysis indicated a strong linear relationship between the measured and predicted strains, with a high correlation coefficient (R = 0.956 intact, 0.938 implanted) and a low standard error of the estimate (SE = 69.53 με, 75.09 με). Moreover, close agreement between the strain rosette and DIC measurements improved confidence in the validity of the DIC technique. The FE model therefore was supported as a valid predictor of the measured strain distribution in the intact and implanted composite pelvis models, confirming its suitability for further computational investigations.

Critical comparative assessment of digital image correlation vs contact extensometry: ensuring the fidelity and depth of mechanical properties determination

Infrastructure systems and key components such as bridges are usually considered for structural health monitoring especially with the increasing deterioration and rising need for repair or replacements. Moreover, in cases of extreme loading, e.g. earthquake, the dynamic response of bridges and their condition after the event need to be rapidly evaluated. This is to make decisions with respect to opening the bridge for traffic, for instance, if no immediate repair is needed. Using modern technologies and non-contact methods such as wireless sensing or vision- based tracking, e.g. target-tracking Digital Image Correlation (DIC), can provide to the means to monitor our infrastructure for cases of extreme events and advance the current state-of-practice in SHM. Vision-based monitoring systems still face several challenges such as the high cost of high-speed or monochrome cameras used for monitoring. Therefore, it is beneficial to assess the possibility of using commercial or lower-end cameras for SHM purpose. In this study, an application of DIC for monitoring the behavior and seismic response of a large-scale bridge model behavior was applied using commercial DSLR cameras and high-speed monochrome cameras. The bridge model was a two-span concrete bridge that was tested on multiple shake tables under white noise and incremental seismic excitations at the University of Nevada, Reno. The objectives of the tests were to: (1) compare the monitoring performance between commercial and highend monochrome cameras; (2) determine the modal properties of the bridge using system identification from DIC measurements and then compare it to the results from conventional contact sensors, e.g. accelerometers. The results show that feasible hardware such as commercial DSLR cameras can provide reasonable means to monitor and capture modal properties of bridges.

Abstract. The measurement of deformation and vibration of wind turbine rotor blades in field tests is a substantial part of the validation of aeroelastic codes. This becomes highly important for modern rotors as the rotor size increases, which comes along with structural changes, resulting in very high flexibility and coupling between different vibration modes. However, performing full-scale field measurements for rotor blade deformation is not trivial and requires high temporal and spatial resolution. A promising deformation measurement technique is based on an optical method called digital image correlation (DIC). Recently, DIC measurements on a Siemens Gamesa SWT-4.0-130 test turbine were performed on the tip of all blades in combination with marker tracking at the hub for the first time with synchronised measurement of the inflow conditions by a ground-based lidar. As the turbine was additionally equipped with strain gauges in the blade root of all blades, the DIC results can be directly compared to the actual prevailing loads to validate the measurement method. In the end, an example for a comparison of the measured deformations and torsion with aeroelastic simulations is shown in the time and frequency domain. All in all, DIC shows very good agreement with comparative measurements and simulations, which shows that it is a suitable method for measurement of deformation and torsion of multi-megawatt wind turbine rotor blades.

As a representative method of optical non-interference measurement, digital image correlation (DIC) technology is a non-contact optical mechanics method that can measure the displacement and deformation of the whole field. However, when the measurement range of the field is too large, the existing DIC method cannot measure the full-field strain accurately, which limits the application of the DIC measurement in the case of a large size and wide-field view. To address this issue, a DIC measurement method for large-scale structures based on adaptive warping image stitching is proposed in this paper. First, multiple adjacent high-resolution images are collected at different locations of large-scale structures. Secondly, the collected images are stitched by applying the adaptive warping image stitching algorithm to obtain a panoramic image. Finally, the DIC algorithm is applied to solve the whole deformation field. In the experiments, we first verify the feasibility of the proposed method for image matching and fusion through the numerical simulation of a rigid body translation experiment. Then the accuracy and robustness of the proposed method in practical application are verified by rigid body translation and a three-point bending experiment. The experimental results demonstrate that the measurement range of DIC is improved significantly with the adaptive warping image stitching algorithm.

ABSTRACTConstitutive behaviour of fusion‐relevant steels in the post‐necking regime is necessary for detailed design of future fusion device components. A finite element model updating (FEMU) procedure using digital image correlation (DIC), particle swarm optimisation (PSO) and a novel post‐necking constitutive law was developed. A DIC‐like strain filter was introduced within the optimisation loop to reduce cost function errors associated with spatial filtering of the DIC processing. The effects of the DIC process and experimental noise on the uncertainty of optimised parameters were quantified using synthetic image deformation techniques. For large plasticity problems, the effect of measurement system noise was found to be negligible. The FEMU method was applied to small‐scale specimens of Eurofer97 ferritic‐martensitic steel of two thicknesses. Strain rate sensitivity was found to be necessary to explain observed post‐UTS uniform strain due to delayed localisation onset. Local strain data from DIC measurements have been demonstrated to improve the accuracy of the identified constitutive model compared to global data alone. Additionally, DIC strain data can provide information to support the inclusion of additional model physics, such as rate sensitivity.

An experimental investigation was conducted to evaluate the permanent strain characteristics of a granular pavement foundation layer using Repeated Load Triaxial (RLT) tests, assisted by a non-contact strain measurement system based on a Digital Image Correlation (DIC) technique. Permanent strains are generally determined from laboratory triaxial tests using global measurement of axial strains. Under cyclic loading there may be formations of localized strains and instabilities, which may not be captured by conventional measuring methods, but can have significant influence on the performance of the constructed granular layer. Results are reported from a series of 40 consolidated drained triaxial tests, to develop a better understanding of the permanent strain response of a granular material under simulated traffic loadings. The DIC technology was employed to record deformation measurements, which included acquiring high-resolution images of the specimen at every 100 cycles of repeated deviatoric stresses, using a Q-Imaging / QICAM 1394 camera, and VIC-2D image correlation software. Results indicate that there is a reasonable match between the LVDT and DIC measurements when the entire specimen length was used as the gage length. However, localized strains at the top, middle, and bottom of the specimens computed from DIC measurements deviated from the LVDT measurements. This factor should be taken into account during the pavement design phase, or for the development of pavement rutting performance models.

Effects of Various Shape Functions and Subset Size in Local Deformation Measurements Using DIC

This study presents a novel method for no-contact detection of water leakage in pressurized pipelines using digital image correlation (DIC). This method focuses on pipe deformation caused by water hammers including leakage information, which combines the idea of transient test-based techniques (TTBTs) and non-destructive testing (NDT). Water leakage induces the hydraulic energy loss in pipelines and the damping of both water hammers and strain energy stored in pipe. We focus on detecting the leak-derived damping of this strain energy using DIC. In experiment, we measured pipe deformation due to water hammers with water leakage using DIC for an in-service agricultural pipeline system. Experimental cases included the conditions of no leak, one leak at downstream and one leak at upstream. As a result, DIC can detect the hoop and axial strain changes due to the water pressure changes in the in-service pipeline. Based on these DIC strain changes, we determine the strain energy of this pipe. The further downstream a leak location is, the greater the damping of the strain energy is. This study provides the non-contact method for detecting water leakage in the pipeline system, which enables us remotely and safely to inspect in-service pipelines that are difficult to access.

Analysis of mechanical behaviour and fracture interaction of multi-hole rock mass with DIC measurement