Full-field Displacement Measurements Research Articles

DIC measurement method for large rotation based on improved grid-based motion statistics.

As a noncontact optical measurement method, the digital image correlation (DIC) method can provide full-field displacement and strain measurement during object deformation. In the case of small rotation deformation, the traditional DIC method can obtain accurate deformation measurement results. However, when the object rotates at a large angle, the traditional DIC method cannot obtain the extreme value of the correlation function, resulting in the occurrence of decorrelation. In order to address the issue, a full-field deformation measurement DIC method based on improved grid-based motion statistics is proposed for large rotation angles. First, the speeded up robust features algorithm is applied to extract and match the feature point pairs between the reference image and the deformed image. Furthermore, an improved grid-based motion statistics algorithm is proposed to eliminate the wrong matching point pairs. Then, the deformation parameters of the feature point pairs obtained by the affine transformation are taken as the initial deformation value for DIC calculation. Finally, the intelligent gray-wolf optimization algorithm is used to obtain the accurate displacement field. The effectiveness of the proposed method is proved by simulation and practical experiments, and the comparative experiments show that the proposed method is faster and more robust.

Panoramic Digital Image Correlation for 360-Deg Full-Field Displacement Measurement

In full-field 3D displacement measurement, stereo digital image correlation (Stereo-DIC) has strong capabilities. However, as a result of difficulties with stereo camera calibration and surface merging, 360-deg panoramic displacement measurements remain a challenge. This paper proposes a panoramic displacement field measurement method in order to accurately measure the shape and panoramic displacement field of complex shaped objects with natural textures. The proposed method is based on the robust subset-based DIC algorithm and the well-known Zhang’s calibration method to reconstruct the 3D shape and estimate the full-field displacements of a complex surface from multi-view stereo camera pairs. The method is used in the determination of the scale factor of the 3D reconstructed surface and the stitching of multiple 3D reconstructed surfaces with the aid of the laser point cloud data of the object under test. Based on a discussion of the challenges faced by panoramic DIC, this paper details the proposed solution and describes the specific algorithms implemented. The paper tests the performance of the proposed method using an experimental system with a 360-deg six camera setup. The system was evaluated by measuring the rigid body motion of a cylindrical log sample with known 3D point cloud data. The results confirm that the proposed method is able to accurately measure the panoramic shape and full-field displacement of objects with complex morphologies.

High Dimensional Data Reduction in Modal Analysis with Stochastic Subspace Identification

Subspace system identification methods are widely used in output-only vibration analysis of civil structures, known as operational modal analysis. With the advent of new sensor technologies, such as video camera-based full field displacement or velocity measurements, the number of measured outputs is quickly increasing. In this paper, we propose principal component analysis-based data size reduction methods for efficient application of subspace methods, while preserving the high spatial resolution of the identified mode shapes for detailed modal analysis.

Characterization of shock-induced panel flutter with simultaneous use of DIC and PIV

In this experimental study, panel flutter induced by an impinging oblique shockwave is investigated at a freestream Mach number of 2, using the combination of planar particle image velocimetry (PIV) and stereographic digital image correlation (DIC) to obtain simultaneous full-field structural displacement and flow velocity measurements. High-speed cameras are employed to obtain a time-resolved description of the panel motion and the shockwave-boundary layer interaction (SWBLI). In order to prevent interference between the PIV and DIC systems, an optical isolation is implemented using fluorescent paint, dedicated light sources, and camera lens filters. The effect of the panel motion on the SWBLI behavior is assessed, by comparing it with the SWBLI on a rigid wall. The results show that panel oscillations occur with a maximum amplitude of ten times the panel thickness. The dominant frequencies observed in the panel oscillation (424 Hz and 1354 Hz) match the main spectral content of the reflected shockwave position. A further POD analysis of the panel displacement spatial distribution shows that these two frequency contributions are well captured by the first two POD modes, which correspond, respectively, to a first and a third bending mode shape and account for 92% of the total oscillation energy. The fluid-structure coupling is studied by identifying, in the flow, the regions of maximum correlation between the panel displacement and the flow velocity fluctuations. The results obtained prove that the inviscid flow region upstream of the SWBLI is perfectly in phase with the panel oscillation, while the downstream region has a delay of one quarter of the flutter cycle.

Super-Resolution and Accurate Full-Field Displacement Measurement With Millimeter-Wave Radars

Super-Resolution and Accurate Full-Field Displacement Measurement With Millimeter-Wave Radars

On the Fundamental Sensitivity Limit of Incoherent Optical Methods for Full-Field Displacement Measurements

On the Fundamental Sensitivity Limit of Incoherent Optical Methods for Full-Field Displacement Measurements

Real-Time Vibration Visualization Using GPU-Based High-Speed Vision

In this study, we developed a real-time vibration visualization system that can estimate and display vibration distributions at all frequencies in real time through parallel implementation of subpixel digital image correlation (DIC) computations with short-time Fourier transforms on a GPU-based high-speed vision platform. To help operators intuitively monitor high-speed motion, we introduced a two-step framework of high-speed video processing to obtain vibration distributions at hundreds of hertz and video conversion processing for the visualization of vibration distribution at dozens of hertz. The proposed system can estimate the full-field vibration displacements of 1920 × 1080 images in real time at 1000 fps and display their frequency responses in the range of 0–500 Hz on a computer at dozens of frames per second by accelerating phase-only DICs for full-field displacement measurement and video conversion. The effectiveness of this system for real-time vibration monitoring and visualization was demonstrated by conducting experiments on objects vibrating at dozens or hundreds of hertz.

Super-sensitivity incoherent optical methods for full-field displacement measurements.

The sensitivity of incoherent optical methods using video cameras (e.g., optical flow and digital image correlation) for full-field displacement measurements, defined by the minimum measurable displacements, is essentially limited by the finite bit depth of the digital camera due to the quantization with round-off error. Quantitatively, the theoretical sensitivity limit is determined by the bit depth B as δp = 1/(2B - 1) [pixel] which corresponds to a displacement causing an intensity change of one gray level. Fortunately, the random noise in the imaging system may be leveraged to perform a natural dithering to overcome the quantization, rendering the possibility of breaking the sensitivity limit. In this work we study such a theoretical sensitivity limit and present a spatiotemporal pixel-averaging method with dithering to achieve super-sensitivity. The numerical simulation results indicate that super-sensitivity can be achieved and is quantitatively determined by the total pixel number N for averaging and the noise level σn as δ p ∗∝(σ n /N)δ p.

Two-dimensional full-field displacement reconstruction of lattice towers using data fusion method: Theoretical study and experimental validation

Full-field displacement measurement of lattice tower is very important for assessing its service conditions. However, most of the existing measurement techniques or reconstruction methods only can obtain the point-type displacement or not suitable for lattice tower. To fill this gap, this paper proposes a data fusion method to reconstruct two-dimensional full-field displacement of lattice tower via strain and acceleration. First, lattice tower was simplified to thin-walled variable-section cantilever beam, then full-field displacement is calculated from the strain based on strain decomposition and modal conversion principles, and a novel data fusion algorithm is developed to fuse the strain-derived displacement and acceleration to acquire more accurate results. The effectiveness of the proposed method is verified by the numerical simulation of the transmission tower structure under wind loads, the results show that the full-field displacement can be calculated, and the reconstructed error of the highest point is only 1%. Then the parametric analysis illustrates that the proposed method has excellent performance under the influence of high sampling ratio or strong noise. The field measurements of the full-scale lattice tower are conducted to further verify the feasibility, and the results show that the reconstructed displacement value of the highest point differs from the observation value of the total station by a maximum of 5.4 mm, accounting for only 6% of the measured value.

In-situ noncontact measurement system for nozzle throat deformation in high-temperature gas heating via laser speckle digital image correlation with wavelet smoothing of displacement field

The shape of nozzle throat in an engine could greatly change the amount of thrust or heat transfer performance. To in-situ evaluate nozzle throat deformation in high-temperature gas heating tests, a non-contact full-field deformation measurement system via laser speckle Digital Image Correlation with wavelet smoothing of displacement field is proposed. For high-temperature thermal radiation suppression, high-power laser speckle pattern is used for digital image correlation calculation and multi-step filters with polarizers are used. A displacement field wavelet smoothing algorithm is proposed to mitigate imaging distortion caused by airflow disturbance. Gas heating experiments on a nozzle sample made of carbon fiber high silica phenolic resin composites were carried out. The full-field displacement measurement results show that compared with the commonly used air blade method, the proposed method can provide smoother displacement field for deformation evaluation, the displacement measurement difference of the proposed method and a dial indicator was within 0.01 mm.

Experimental Investigation Of Shock Induced Panel Flutter With Simultaneous Use Of DIC And PIV

In this experimental study, both classical and shock induced supersonic panel flutter are investigated at a freestream Mach number of 2, using a combination of planar particle image velocimetry (PIV) and stereographic digital image correlation (DIC) to obtain simultaneous full-field structural displacement and flow velocity measurements. Highspeed cameras are employed to obtain a time-resolved description of the panel motion and of the flow dynamics. In order to prevent interference between the PIV and DIC systems, an optical isolation is implemented using fluorescent paint, dedicated light sources, and camera lens filters. For the shock induced case, the effect of the panel motion on the SWBLI behavior is assessed, by comparing it with the SWBLI on a rigid wall. The results show that panel oscillations occur with an amplitude of ten times the panel thickness. The dominant frequencies observed in the panel oscillation (424 Hz and 1354 Hz) match the main spectral content of the reflected shockwave position. In absence of shock impingement, the oscillation amplitude is on the order of the panel thickness, while the dominant frequency contribution is at 730 Hz. The fluid-structure coupling is studied by identifying the flow regions of maximum correlation between the panel displacement and the flow velocity fluctuations. In presence of shock impingement the results obtained proved that the inviscid flow region upstream of the SWBLI is perfectly in phase with the panel oscillation, while the downstream region has a delay of one quarter of the flutter cycle. Instead, no delay is observed for the configuration in absence of shockwave impingement.

Application of Ultraviolet (UV) Radiation and Fluorescence for DIC Measurements - Quality Improvement

Digital image correlation (DIC) is a widely used method in materials science and experimental mechanics for full-field surface displacement or strain measurements. It is an optical method, which calculates displacement or strain fields from images taken during deformation of the test specimen and is able to achieve a sub-pixel accuracy in case of proper operation. However, the theoretical accuracy is strongly reduced by effects such as surface reflections or shadows resulting from a not ideal illumination or lighting system. This is especially the case for complex 3D structured specimen geometries such as for metal foams or other cellular materials.The present study investigates the usage of ultraviolet (UV) light illumination in combination with phosphorescent or fluorescent pigments used to apply the needed speckle pattern for high accuracy DIC. A method to optimise the lighting system in combination with the used background pigments is presented. Fluorescent pigments provide much better results than phosphorescent pigments. The application of UV lighting in combination with fluorescent background pigments strongly improves DIC accuracy not only for normal tensile specimens but also for complex 3D shaped metal foams by avoiding surface reflections and shadows.

Full-Field Displacement Measurement Method of Metal-As-Insulation Pancake at Liquid Nitrogen Temperature

In situ measurement of metal-as-insulation (MI) high temperature superconductor (HTS) pancake deformation gives insight to its actual mechanical state. Measurement of hoop strain for inner / outer turn of MI pancake is usually made by strain gauges. This measurement is however local and faces electromagnetic compatibility (EMC) and thermal issues. Non-contact full-field displacement measurement is an interesting alternative solution. This technique is based on digital image correlation (DIC), which derives the displacement field in specified region of interest (ROI) by correlation between the deformed state image and the reference state image. In this article, we present a novel DIC experiment setup for displacement measurement of MI pancake in cryogenic cooling. The setup is validated by a cooling test of 304 stainless steel disc. Then, it is used for cooling test of MI pancake made of co-wound copper beryllium - 304 stainless steel tapes. Both tests are done at 80 K by conduction cooling using liquid nitrogen. Finally, experimental results are compared to numerical results obtained from an analytical formulation of the mechanical equilibrium using generalized plane strain assumption.

In-situ full-field measurements for 3D printed polymers during mode I interface failure

The fracture behavior of parts manufactured by fused filament fabrication (FFF) is highly driven by their mesoscale architecture characterized by the presence of weak interfaces between layers and filaments, likely to fail under loading. We propose an in-situ characterization approach to study the local phenomena driving the mode I interlayer and interfilament failure of FFF polymer specimens and support the development of a constitutive model for the interface damage. We provide full-field displacement and strain measurements by microstereoscopic digital image correlation (DIC) over a region of interest of around 4 × 3 mm2 in the proximity of the propagating crack for differently oriented interfaces. The high-resolution camera framework promotes the crack propagation monitoring and the crack length tracking until the specimen’s brittle failure. The local strain distribution reveals the distinctive mesoscale failure and damage mechanisms of 0°−0° and 0°−90° polylactic acid interlayer and interfilament interfaces. Overall, the 0°−90° stacking ensures higher fracture properties when compared to the 0°−0° counterpart, in agreement with the specimens’ force–displacement response recorded by the electromechanical testing device. Additionally, we propose a correlation between the strain contour plot heterogeneities observed for 0°−90° interlayer interfaces and 0°−0° interfilament interfaces and the specimens’ architecture inspected by scanning electron microscopy or X-ray computed tomography. Ultimately, the full-field measurements promote the development and validation of a constitutive model explicitly capturing the mechanical behavior of the interface.

Optical coherence elastography of 3D bilayer soft solids using full-field and partial displacement measurements

Characterizing nonhomogeneous elastic property distribution of soft tissues plays a crucial role in disease diagnosis and treatment. In this paper, we will apply the optical coherence elastography to reconstruct the shear modulus elastic property distribution of a bilayer solid. In the computational aspect, we adopt a well-established inverse technique that solves for every nodal shear modulus in the problem domain (NO method). Additionally, we also propose a novel inverse method that assumes the shear moduli merely vary along the thickness of the bilayer solid (TO method). The inversion tests using simulated data demonstrate that TO method performs better in reconstructing the shear modulus distribution. Further, we utilize the experimental data obtained from the optical coherence tomography to reconstruct the shear modulus distribution of a bilayer phantom. We observe that the quality of the reconstructed shear modulus distribution obtained by the partial displacement measurement is better than that obtained by the full-field displacement measurement. Particularly, merely using the displacement component along the loading direction significantly improves the reconstructed results. This work is of great significance in applying optical coherence elastography (OCE) to characterize the elastic property distribution of layered soft tissues such as skins and corneas.

Lifetime measurement of a particular deep crack failure on a flat-type divertor mockup under cyclic high heat flux loading conditions

Plasma facing components are key to enduring high heat flux (HHF) loading from high-temperature plasma in nuclear fusion reactors. Understanding their thermal-mechanical behavior and cracking failure mechanisms related to structural designs and fabrication technologies during HHF loading is of great significance for improving their servicing performance and R&D (research and development) levels. In this study, a particular deep cracking failure process on the tungsten layer of a flat-type divertor mockup during 1800 cycles of 10 MW m−2 HHF loadings is completely monitored and measured with a special improved digital image correlation (DIC) technique. It is found that the DIC measurement under the HHF loading environment is improved successfully to capture fine deformation and strain fields with a spatial resolution less than 0.35 mm so that field strain on a 1 mm thick copper interlayer and deep crack initiation at several microns scale on the tungsten layer are measured out. Based on both full field and local strain and displacement measurements of the target divertor mockup, the thermal mechanical behaviors from deformation to crack initiation and propagation are successfully measured and traced. It is revealed that for the baseline copper interlayer design of a flat-type divertor mockup, the accumulation of plastic strain in the copper interlayer during ratcheting damage induces enough tensile stress on the tungsten layer during HHF cycles, leading to cracking and fracture failures even in its elastic state earlier than the copper LCF lifetime. Current Structural Design Criteria for ITER In-Vessel Components rules fail to cover this kind of ratcheting cracking failure mode in the design stage. New design models or mechanical validation rules to resolve this design blind spot should be established in the future.

2D full-field displacement and vibration measurements of specularly reflecting surfaces by two-beam common-path digital holography.

A new, to the best of our knowledge, configuration of common-path off-axis digital holography is proposed for simultaneous evaluation of out-of-plane and in-plane displacements of the vibrating object. The object is illuminated from two different directions, and each illumination interferes with its corresponding reference beam generated near the object, resulting in two independent holograms that are spatially multiplexed in a single camera image. Two multiplexed holograms, at undeformed and deformed states of the object, are recorded and processed to obtain the out-of-plane and in-plane displacements simultaneously. The proposed digital holographic system has the advantage of a simple and compact optical setup, is less sensitive to environmental disturbances, and has high temporal phase stability. The two-dimensional (z,x) full-field amplitude and phase vibration analysis of a perfect specularly reflecting surface are also demonstrated by the proposed holographic system. The experimental results authenticate the feasibility of the proposed system and reveal its unique advantages. The proposed digital holographic system, owing to simple and compact geometry and providing several advantages over other two-channel holographic systems, may find a wide range of applications in investigating real-time dynamic phenomena.

Experimental analysis of control technology and deformation failure mechanism of inclined coal seam roadway using non-contact DIC technique

The deformation and failure forms of inclined coal seam roadway under the joint action of dip angle and various geological conditions are complex, and there is a lack of targeted support measures, which brings great problems to the stability control of roadway surrounding rock. In order to safely and economically mine inclined coal seams, taking the engineering geology of Shitanjing No. 2 mining area as the background, and the physical similarity model of right-angle trapezoidal roadway in inclined coal seam, in which the non-contact digital image correlation (DIC) technology and the stress sensor is employed to provide full-field displacement and stress measurements. The deformation control technology of the roadway surrounding rock was proposed, verified by numerical simulation and applied to engineering practice. The research results show that the stress and deformation failure of surrounding rock in low sidewall of roadway are greater than those in high sidewall, showing asymmetric characteristics, and the maximum stress concentration coefficients of roadway sidewall, roof and floor are 4.1, 3.4 and 2.8, respectively. A concept of roadway "cyclic failure" mechanism is proposed that is, the cyclic interaction of the two sidewalls, the sharp angles and roof aggravated the failure of roadway, resulting in the overall instability of roadway. The roadway sidewall is serious rib spalling, the roof is asymmetric "Beret" type caving arch failure, and the floor is slightly bulging. On this basis, the principle of roadway deformation control is revealed and asymmetric support design is adopted, and the deformation of roadway is controlled, which support scheme is effective.

Perspective error reduction in 2D Digital Image Correlation measurements by combination with Defocused Speckle Imaging

2D Digital Image Correlation (2D DIC) enables full-field in-plane displacement measurement by tracking the physical surface texture in the recorded images. However, because camera magnification is inversely proportional to object distance, points along the imaged surface can have different scales and appear to move at different rates. Surface curvature and out-of-plane motions may thus lead to perspective errors that manifest as apparent strains, limiting the method suitable only for flat objects that move purely in-plane. Defocused Speckle Imaging (DSI) is a related optical method where the movements of a laser-illuminated surface are determined by tracking the scattered interference speckle pattern. When the camera is in focus, the speckles appear to move at the same rate with the physical surface. However, camera defocus causes the speckle motion to deviate from the actual surface motion. Interestingly, the observed speckle motion magnitude increases with positive defocus distance, which is contrary to the perspective effects inherent to 2D DIC. Remarkably, with optimized geometric arrangement, 2D DIC and DSI have perspective errors that are equal in magnitude but with opposite signs. It is thus possible to compensate perspective errors by performing DIC and DSI measurements simultaneously and simply calculating the average of the two signals. This article derives the proposed compensation principle analytically using simple geometric framework. An experimental demonstration shows that the combination of DIC and DSI reduces magnification-dependent perspective errors by 73%, and errors due to object rigid-body rotation by 86%.

Experimental study of adhesively bonded natural fibre composite – steel hybrid laminates

Natural fibre composites (NFC), such as flax fibre reinforced plastics are green material with good specific properties. Significant research is currently in progress to improve the mechanical properties and durability of these composites and to make them competitive to traditional synthetic composites. A multi-material design approach is proposed in this study to develop a novel hybrid structure with a thin metal layer adhesively bonded to NFC. Two adhesives; a commercial epoxy adhesive and natural rubber were used for the adhesion between a stainless steel layer and flax composite with thermoset and thermoplastic matrix. The performance of the hybrid joint was investigated using a single lap joint (SLJ) test supported by full-field displacement measurement methods. The manufacturing of the composite specimen and the bonding of the NFC – metal joint with rubber adhesive was demonstrated. The force displacement curves from the SLJ test showed that the bonded joint with epoxy adhesive was stronger in comparison to the bonding with natural rubber. It is supported by the adherend failure observed in the joint with epoxy adhesive, while the rubber joint showed adhesive failure. The key outcome of the research is that the viability of adhesively bonded NFC - metal hybrid is established.