Accuracy Of Deformation Measurement Research Articles

Material flow analysis in indentation process by 3D Digital Image Correlation

Focusing on the application of the 3D Digital Image Correlation technique, this work proposes a material flow analysis of an indentation process. The study establishes the methodology for the calibration and implementation of the 3D image sensing technology for deformation measurements. The purpose is to continue with the validation of DIC application on the indentation processes, where a deep penetration is achieved and extensive material flow is produced. With the 3D DIC technique is possible to perform accurate deformation measurements in non planar specimens and study the material emerging towards the exterior of the tested specimen, which is not possible with the 2D DIC technique. Although previous 2D studies were efficient detecting the flow field and von Mises strains on the specimens tested, the bulge emerging under the punch on the front surface (dead zone) could not be studied due to its predominantly 3D character. Therefore, present work implements a 3D methodology that carries out a complete deformation study, including the material flow that occurs on the Z axis, towards the exterior of the tested specimen, optimizing previous analyses.

Improvement of high-temperature deformation measurement accuracy based on image restoration method

Digital image correlation (DIC) is a full-field and non-contact technique based on white-light illumination for displacement and strain measurement. Because of these advantages, it can be used to measure the mechanical properties of materials at high temperature. However, there are still many urgent matters to be solved in using the DIC method in high-temperature measurement, such as the heat flow disturbance. This can warp the images acquired in high temperatures and cause a tiny move of the images acquired at the same temperature, even making the gray value of the images change, so the results of the measurement will not be guaranteed. This paper proposes a method to reduce these influences and improve the accuracy of high-temperature measurement results. Degraded images can be processed by using a combination of the image inverse filter method and an image averaging algorithm. Then the processed images can be used to calculate the displacement and strain. The experimental results show that using an image inverse filter and image averaging algorithm to process images can produce smaller RMS errors and more stable results than the values calculated from the original images with no processing. Using this method can improve the accuracy of high-temperature measurement.

Full-field deformation and strain measurement of vehicle body under high-speed impact

A crash test is a common method to evaluate passive vehicle safety, but its full-field deformation and strain is difficult to measure due to instantaneous large deformation during the intense impact process. By means of high-speed cameras, a full-field deformation and strain measurement under high-speed impact is presented based on digital image correlation and binocular stereo vision. To deal with weak correlation in image matching caused by optical noise and large deformation, a controllable Gaussian filter and adaptive light intensity coefficients are used to suppress image intensity interference introduced by optical noise. In addition, based on deformation continuity of the neighboring states, images before and after large deformation are matched by the minimization of reference datum and transfer of deformation coefficients. Finally, full-field deformation and strain data are obtained from an impact test on an engine cover made from DC04 steel. Results show that the deformation measurement accuracy of the method can achieve 0.03 mm within a 500 × 400 mm2 field of view. This study solves the problems of limited measurement points and low efficiency both in the contact measurement method by displacement sensors and traditional optical measurement by the detection of artificial target points, and provides a scientific basis for quantitative analysis of vehicle impact safety, vehicle body structure and material improvement, and finite element simulation of impact theory.

Factors influencing the accuracy of the photogrammetry-based deformation measurement method

Triaxial test has been widely used to investigate the stress–strain relationship of unsaturated soils. During triaxial testing, soil volume is an essential parameter to be measured. For an unsaturated soil, due to the presence of air phase, accurate volume/deformation measurement during triaxial testing was a great challenge for researchers. Recently, a photogrammetry-based method has been developed to measure the soil volume/deformation during triaxial testing. Preliminary triaxial test results indicate the new method is simple, accurate, and cost- and time-effective. However, some concerns regarding its measurement accuracy and applicability, which are critical for the dissemination of the photogrammetry-based method, have been raised by other researchers. These concerns were addressed in details in this study. The factors concerning the deformation measurement accuracy were systematically evaluated through a series of triaxial tests on an aluminum cylinder with different confining media and chamber pressures. A sensitivity analysis was carried out to investigate the impact of the system parameters on the volume measurement accuracy of the photogrammetry-based method. In addition, a triaxial test on a saturated sand specimen was conducted to evaluate the influences of mesh density, mesh pattern, and interpolation technique on the volume change measurement accuracy. Finally, some suggestions were provided to improve the accuracy of the photogrammetry-based measurement method.

GROUND SUBSIDENCE ALONG SHANGHAI METRO LINE 6 BY PS-InSAR METHOD

Abstract. With the rapid development of urban economy, convenient, safe, and efficient urban rail transit has become the preferred method for people to travel. In order to ensure the safety and sustainable development of urban rail transit, the PS-InSAR technology with millimeter deformation measurement accuracy has been widely applied to monitor the deformation of urban rail transit. In this paper, 32 scenes of COSMO-SkyMed descending images and 23 scenes of Envisat ASAR images covering the Shanghai Metro Line 6 acquired from 2008 to 2010 are used to estimate the average deformation rate along line-of-sight (LOS) direction by PS-InSAR method. The experimental results show that there are two main subsidence areas along the Shanghai Metro Line 6, which are located between Wuzhou Avenue Station to Wulian Road Station and West Gaoke Road Station to Gaoqing Road Station. Between Wuzhou Avenue Station and Wulian Road Station, the maximum displacement rate in the vertical direction of COSMO-SkyMed images is −9.92 mm/year, and the maximum displacement rate in the vertical direction of Envisat ASAR images is −8.53 mm/year. From the West Gaoke Road Station to the Gaoqing Road Station, the maximum displacement rate in the vertical direction of COSMO-SkyMed images is −15.53 mm/year, and the maximum displacement rate in the vertical direction of Envisat ASAR images is −17.9 mm/year. The results show that the ground deformation rates obtained by two SAR platforms with different wavelengths, different sensors and different incident angles have good consistence with each other, and also that of spirit leveling.

Sensitivity of in-Plane Strain Measurement to Calibration Parameter for out-of-Plane Specimen Rotations

In practice, out-of-plane motions usually are not avoidable during experiments. Since 2D–DIC measurements are vulnerable to parasitic deformations due to out-of-plane specimen motions, three-dimensional digital image correlation (StereoDIC or 3D–DIC) oftentimes is employed. The StereoDIC method is known to be capable of accurate deformation measurements for specimens subjected to general three-dimensional motions, including out-of-plane rotations and displacements. As a result, there has been limited study of the deformation measurements obtained when using StereoDIC to measure the displacement and strain fields for a specimen subjected only to out-of-plane rotation. To assess the accuracy of strain measurements obtained using stereovision systems and StereoDIC when a specimen undergoes appreciable out of plane rotation, rigid body out-of-plane rotation experiments are performed in the range −400 ≤ θ ≤ 400 using a two-camera stereovision system. Results indicate that (a) for what would normally be considered “small angle” calibration processes, the measured normal strain in the foreshortened specimen direction due to specimen rotation increases in a non-linear manner with rotation angle, with measurement errors exceeding ±1400μe and (b) for what would normally be considered “large angle” calibration processes, the magnitude of the errors in the strain are reduced to ±300μe. To theoretically assess the effect of calibration parameters on the measurements, two separate analyses are performed. First, theoretical strains due to out-of-plane rigid body rotation are determined using a pinhole camera model to project a series of three-dimensional object points into the image plane using large angle calibration parameters and then re-project the corresponding sensor plane coordinates back into the plane using small angle calibration parameters. Secondly, the entire imaging process is also simulated in order to remove experimental error sources and to further validate the theory. Results from both approaches confirmed the same strain error trends as the experimental strain measurements, providing confidence that the source of the errors is the calibration process. Finally, variance based sensitivity analyses show that inaccuracy in the calibrated stereo angle parameter is the most significant factor affecting the accuracy of the measured strain.

Accuracy-enhanced constitutive parameter identification using virtual fields method and special stereo-digital image correlation

The virtual fields method (VFM) is generally used with two-dimensional digital image correlation (2D-DIC) or grid method (GM) for identifying constitutive parameters. However, when small out-of-plane translation/rotation occurs to the test specimen, 2D-DIC and GM are prone to yield inaccurate measurements, which further lessen the accuracy of the parameter identification using VFM. In this work, an easy-to-implement but effective “special” stereo-DIC (SS-DIC) method is proposed for accuracy-enhanced VFM identification. The SS-DIC can not only deliver accurate deformation measurement without being affected by unavoidable out-of-plane movement/rotation of a test specimen, but can also ensure evenly distributed calculation data in space, which leads to simple data processing. Based on the accurate kinematics fields with evenly distributed measured points determined by SS-DIC method, constitutive parameters can be identified by VFM with enhanced accuracy. Uniaxial tensile tests of a perforated aluminum plate and pure shear tests of a prismatic aluminum specimen verified the effectiveness and accuracy of the proposed method. Experimental results show that the constitutive parameters identified by VFM using SS-DIC are more accurate and stable than those identified by VFM using 2D-DIC. It is suggested that the proposed SS-DIC can be used as a standard measuring tool for mechanical identification using VFM.

Three-Dimensional Deformation Retrieval in Geosynchronous SAR by Multiple-Aperture Interferometry Processing: Theory and Performance Analysis

The 3-D deformation retrieval is significant for the accurate evaluation of geologic disasters (e.g., earthquakes and landslides). Multiple-aperture interferometry (MAI) is an effective method to obtain 3-D deformation, combined with the cross-heading tracks synthetic aperture radar (SAR) data. However, because of the limitations of the low earth orbit SAR, a long satellite revisit time, small common areas of the cross-heading tracks data, and the unsatisfied along-track deformation measurement accuracy usually exist in the traditional MAI 3-D deformation retrieval. Geosynchronous SAR (GEO SAR) runs in the geosynchronous orbit, which has the advantages of a large observation area and a short revisit time. This paper focuses on 3-D deformation retrieval by GEO SAR MAI processing. Aiming at the high orbit and the squint looking of GEO SAR, the accurate expressions of the along-track deformation, 3-D deformation, and the errors in GEO SAR MAI processing are given. The distortions and their correction in the MAI interferogram brought by the geometrical difference between the forward- and backward-looking interferograms and the multicycles flat-earth and topographic phases are given. Moreover, an optimal subaperture selection method based on minimum position dilution of precision is proposed. Finally, the effectiveness of the proposed method is validated by simulations and the experiment of BeiDou-2 inclined geosynchronous orbit navigation satellite. The theoretical analysis and the experimental results suggest centimeter-level and even millimeter-level deformation measurement accuracy could be obtained in 3-D by GEO SAR MAI processing.

Two-Dimensional Deformation Measurement Based on Multiple Aperture Interferometry in GB-SAR

Ground-based synthetic aperture radar (GB-SAR) technique has been widely applied for the deformation monitoring and measurement of the natural and engineered slopes. To extend the 2-D deformation measurement from the conventional 1-D measurement along the radar-target line of sight (LOS), multiple aperture interferometry (MAI) techniques based on phase differences between interferograms of the forward-looking and backward-looking subapertures are tackled in this letter. The optimal subaperture selection is analyzed considering the typical signal-to-noise ratios and correlations in GB-SAR applications. Simulations prove that the coherent integration (CIM) can be utilized to improve the measurement accuracy of the MAI method. Besides, GB-SAR experiments are carried out to validate the feasibility and effectiveness of the 2-D deformation measurement method based on MAI. Accuracy comparison of deformation measurement with the MAI and cross correlation methods is also taken. Experimental results show that the accuracy of deformation measurement along the perpendicular direction to LOS based on MAI and CIM can reach millimeter level for displaceable corner reflector.

A persistent scatterer interpolation for retrieving accurate ground deformation over InSAR‐decorrelated agricultural fields

AbstractInterferometric synthetic aperture radar (InSAR) is a radar remote sensing technique for measuring surface deformation to millimeter‐level accuracy at meter‐scale resolution. Obtaining accurate deformation measurements in agricultural regions is difficult because the signal is often decorrelated due to vegetation growth. We present here a new algorithm for retrieving InSAR deformation measurements over areas with severe vegetation decorrelation using adaptive phase interpolation between persistent scatterer (PS) pixels, those points at which surface scattering properties do not change much over time and thus decorrelation artifacts are minimal. We apply this algorithm to L‐band ALOS interferograms acquired over the San Luis Valley, Colorado, and the Tulare Basin, California. In both areas, the pumping of groundwater for irrigation results in deformation of the land that can be detected using InSAR. We show that the PS‐based algorithm can significantly reduce the artifacts due to vegetation decorrelation while preserving the deformation signature.

Mechanical testing of internal fixation devices: A theoretical and practical examination of current methods

Successful healing of long bone fractures is dependent on the mechanical environment created within the fracture, which in turn is dependent on the fixation strategy. Recent literature reports have suggested that locked plating devices are too stiff to reliably promote healing. However, in vitro testing of these devices has been inconsistent in both method of constraint and reported outcomes, making comparisons between studies and the assessment of construct stiffness problematic. Each of the methods previously used in the literature were assessed for their effect on the bending of the sample and concordant stiffness. The choice of outcome measures used in in vitro fracture studies was also assessed. Mechanical testing was conducted on seven hole locked plated constructs in each method for comparison. Based on the assessment of each method the use of spherical bearings, ball joints or similar is suggested at both ends of the sample. The use of near and far cortex movement was found to be more comprehensive and more accurate than traditional centrally calculated interfragmentary movement values; stiffness was found to be highly susceptible to the accuracy of deformation measurements and constraint method, and should only be used as a within study comparison method. The reported stiffness values of locked plate constructs from in vitro mechanical testing is highly susceptible to testing constraints and output measures, with many standard techniques overestimating the stiffness of the construct. This raises the need for further investigation into the actual mechanical behaviour within the fracture gap of these devices.

Accurate ex situ deformation measurement using an ultra-stable two-dimensional digital image correlation system.

In situ planar deformation measurement using two-dimensional digital image correlation (2D-DIC) and a fixed camera has been fully investigated in many published works and widely used for various applications. However, in certain special cases (e.g., long-term deformation monitoring of engineering structures, or deformation measurement of a specimen subjected to uncommon loading), it is very difficult or impossible to carry out in situ deformation measurement using 2D-DIC, and necessitates ex situ 2D-DIC measurements for a repositioned specimen or using a relocated camera. To achieve accurate measurements, the error sources of ex situ 2D-DIC measurements should be identified and minimized, and the strain accuracy of ex situ 2D-DIC measurements should be quantified. In this work, the potential error sources of ex situ 2D-DIC measurements are first discussed in detail. Then, to mitigate the errors associated with these issues, an ultra-stable 2D-DIC system combining the idea of active imaging and a well-designed bilateral telecentric lens is established, which is invariant to the potential variations in ambient lighting and the possible small out-of-plane motions in object surface and/or image plane. The established ultra-stable 2D-DIC system is first compared with the regular 2D-DIC setup in determining the surface strains of an unstrained sample. Then, ex situ residual stress measurement using the hole-drilling technique and the established 2D-DIC system was conducted and compared with the applied ones. The results of this work confirm that the accuracy of ex situ 2D-DIC deformation measurements using the proposed system is of high fidelity, and can be used for accurate deformation measurement in practical ex situ tests.

TU‐A‐12A‐06: Intra‐Observer Variability in Delineation of Target Volumes in Breast Radiotherapy and Its Effect On Accuracy of Deformation Measurements

Purpose:In breast radiotherapy, the target volume may change during treatment and need adaptation of the treatment plan. This is possible for both tumour bed (TB) and whole breast (WB) target volumes. Delineation of the target (to detect changes) is also subject to uncertainty due to intra‐ and inter‐observer variability. This work measured the uncertainty, due to intraobserver variability, in the quantification of tissue deformation.Methods:Datasets consisting of paired prone and supine CT scans of three patients were used. Significant deformation in target volumes is expected between prone and supine patient positions. The selected cases had 1) no seroma, 2) some seroma, and 3) large seroma. The TB and WB were outlined on each dataset three times by one clinician. Delineation variability was defined as the standard deviations of the distances between observer outlines. For each target volume and each case, tissue deformation between prone and supine delineations was quantified using the Dice similarity coefficient (DSC) and the average surface distance (ASD). The uncertainty in the tissue deformation (due to delineation variability) was quantified by measuring the ranges of DSC and ASD using all combinations of pairs of outlines (9 pairs).Results:For the TB, the range of delineation variability was 0.44‐1.16 mm. The deformation, DSC and ASD, (and uncertainty in measurement) of the TB between prone and supine position of the cases were: 1) 0.21 (0.17‐0.28) and 12.4 mm (11.8‐13 mm); 2) 0.54 (0.51‐0.57) and 3.3 mm (3.1‐3.5 mm); 3) 0.62 (0.61‐0.64) and 4.9 mm (4.6‐5.2 mm). WB deformation measurements were subject to less uncertainty due to delineation variability than TB deformation measurements.Conclusion:For the first time, the uncertainty, due to observer variability, in the measurement of the deformation of breast target volumes was investigated. Deformations in these ranges would be difficult to detect.This work was supported in part by Cancer Research‐UK under Programme Grant C46/A10588 and in part by the National Institute for Health Research (NIHR) through funding of the biomedical research imaging centre. P. Juneja is supported by the EPSRC Platform Grant EP/H046526/1.

A robust regression-based linear model adjustment method for SAR interferogram stacking techniques

Differential interferometric synthetic aperture radar (DInSAR) is recognized as an effective remote-sensing technique for a variety of ground deformation mapping applications. Centimetre-level measurement accuracy can be achieved with the DInSAR technique. However, two key limitations – temporal decorrelation and phase delay due to atmospheric inhomogeneities – might decrease the accuracy of deformation measurements. To overcome such problems, interferogram stacking techniques, which extend the DInSAR technique, have been developed in recent years. In most implementations of such techniques, a so-called ‘linear model adjustment’ step is required to obtain the relative linear deformation rate and digital elevation model error from the double-differenced phase observations along the stack. In this step, since a non-linear system has to be resolved, the traditional least squares method cannot be directly applied. In order to overcome this problem, several methods have been developed in recent years. In this article, a new method has been developed to deal with the problem of linear model adjustment. This method repeatedly uses robust regression to resolve the non-linear system and is much easier to implement compared with other methods. This method is applied to both simulated and real data, and the results demonstrate that it can be efficiently used for linear model adjustment.

Diffraction Assisted Image Correlation: A Novel Method for Measuring Three-Dimensional Deformation using Two-Dimensional Digital Image Correlation

Digital Image Correlation (DIC) provides a full-field non-contact optical method for accurate deformation measurement of materials, devices and structures. The measurement of three-dimensional (3D) deformation using DIC in general requires imaging with two cameras and a 3D-DIC code. In the present work, a new experimental technique, namely, Diffraction Assisted Image Correlation (DAIC) for 3D displacement measurement using a single camera and 2D-DIC algorithm is presented. A transmission diffraction grating is placed between the specimen and the camera, resulting in multiple images which are then used to obtain apparent in-plane displacements using 2D-DIC. The true in-plane and out-of-plane displacements of the specimen are obtained from the apparent in-plane displacements and the diffraction angle of the grating. The validity and accuracy of the DAIC method are demonstrated through 3D displacement measurement of a small thin membrane. This technique provides new avenues for performing 3D deformation measurements at small length scales and/or dynamic loading conditions.

Digital Image Correlation with Self-Adaptive Gaussian Windows

A novel subpixel registration algorithm with Gaussian windows is put forward for accurate deformation measurement in digital image correlation technique. Based on speckle image quality and potential deformation states, this algorithm can automatically minimize the influence of subset sizes by self-adaptively tuning the Gaussian window shapes with the aid of a so-called weighted sum-of-squared difference correlation criterion. Numerical results of synthetic speckle images undergoing in-plane sinusoidal displacement fields demonstrate that the proposed algorithm can significantly improve displacement and strain measurement accuracy especially in the case with relatively large deformation.

Application of videometric technique to deformation measurement for large-scale composite wind turbine blade

The size of wind turbine blades is expected to increase considerably in the future. Since the modern wind turbine blade is generally large-scale, faulty design production results in high expense. To achieve a desired wind turbine blade, a better understanding of the structural behavior on different scale is necessary. Although the structural deformation of the full-scale blade under various loads is an important parameter in the analysis of structural behavior and the blade’s performance, there are many difficulties in an accurate deformation measurement of the large-scale wind turbine blade. A videometric technique reported here was developed to determine the large-scale blade deformations. The technique principle and methodology will be presented in the current paper, which involves the detail of implementation in the large-scale blade deformation measurement. Some application examples with the obtained data and their analysis are presented, which demonstrates the technique’s application on large-scale wind turbine blade, and which is relevant to understanding the structural behavior of wind turbine blade in the full-scale tests and during operation. The videometric technique will be useful for structural investigation of composite wind turbine blades. A wide variety of other applications, especially those involving fieldwork, could exploit this implementation of videometric.

Displacement Measurement of Bending Tests Using Digital Image Analysis Method

Over the last few years due to sharp increase in experimental investigations in the field of structural engineering, the necessity for reliable and precise deformation measurement is felt more than ever. The most common experimental test in the field of structural engineering is bending test in which beam's deformation is evaluated in order to calculate other structural parameters. One of the most usual techniques for obtaining beam's deformation is utilizing Linear Variable Differential Transformer (LVDT) sensors. However, non-mechanical procedures for displacement measurement have shown much interest between researchers. Photogrammetry, Digital Image Correlation (DIC) method and Particle Image Velocimetry (PIV) are some of the new image-based methods for displacement measurement. Due to the advantages of PIV method in the field of experimental fluid mechanics and Geotechnics, the current study has been focused on the performance of PIV method for displacement measurement of bending test in structural engineering. To reach the aforementioned goal, one steel I-beam was subjected to four-point flexural loading and the load-displacement curve was plotted using PIV method. The experimental results obtained from the PIV method showed excellent agreement with LVDT's data. It is concluded that due to the lower cost and potential of obtaining whole displacement field on common bending tests, PIV method could be widely used as an alternative to conventional measurement techniques in structural engineering. Over the last few years due to extreme increase in experimental investigations in the field of structural engineering, the necessity for accurate deformation measurement is felt more than ever. It is obvious that further calculations could be seriously affected by accuracy of the evaluated displacement fields. Bending test is the most common test in the field of structural engineering in which beam's deformation should be evaluated in order to calculate any other structural parameters. Utilizing Linear Variable Differential Transformer (LVDT) is a primitive technique for obtaining deformation of bending tests. Although there are certain benefits of LVDTs such as high accuracy and simple installation, there are also some disadvantages such as high cost, inability to obtain the whole displacement field and extra charges for data acquisition. Photogrammetry is another technique which is applied to

A New Piezoelectric Ceramic Electronic Voltage Transducer and its Principle Design

In this paper, the piezoelectric ceramic electronic voltage transducer principles is researched, a new internal structure of the measurement system is put forward and the signal processing methods are discussed. On the basis of the analysis of freedom cantilever optical measurement methods for the longitudinal piezoelectric deformation, we proposes the piezoelectric resonance method to detect amplitude of piezoelectric ceramic and cleverly designed mechanical structure, in which the lightweight elastic material is used to support the resonance thin film. This structure overcomes the impact of cantilever longitudinal self-gravity and horizontal friction between cantilever and piezoelectric ceramic, and can improve the deformation measurement accuracy. 10kV 0.05 class transducer voltage measurement accuracy is achieved theoretically.

Study of optimal subset size in digital image correlation of speckle pattern images

This paper investigates the effect of subset size, associated with image pattern quality and subset displacement functions, on the accuracy of deformation measurements by digital image correlation(DIC). A concept of subset entropy is introduced in this work to quantify the subset image pattern quality for DIC analysis and its effectiveness was demonstrated by experimental studies. By employing white-light images with almost uniform subset entropy and first-order displacement functions, the effect of subset size on DIC analysis was investigated for the deformation cases of translation, uniform deformation, and simulated quadratic deformation, respectively. The results show that the chosen subset size must be large enough for precise displacement measurements when subset displacement functions match underlying actual deformation. On the other hand, optimal subset size in DIC for nonhomgeneous deformation measurements appears as a result of a tradeoff between the influence of random errors and systematic errors.