The use of 3D imaging is becoming increasingly common, so too is the use of fiducial markers to identify/track regions of interest and assess material deformation. While many different materials have been used as fiducials, they are often used in isolation, with little comparison to one another.
ObjectiveIn the current study, we aim to directly compare different Computed Tomography (CT and μCT) fiducial materials, both metallic and nonmetallic.
MethodsμCT imaging was performed on a soft-tissue structure, in this case heart valve tissue, with markers from eight different materials attached. Additionally, we evaluated the same markers with DiceCT stained tissue in a fluid medium.
ResultsAll of the metallic markers generated significant artifacts, and were found unsuitable for soft-tissue μCT imaging, whereas alumina markers were found to perform the best, with excellent contrast and consistency.
ConclusionsThese findings support the further use of alumina as fiducial markers for soft material and tissue studies that utilize CT and μCT imaging.
相似文献Images from scanning electron microscopes, transmission electron microscopes and atomic force microscopes have been widely used in digital image correlation methods to obtain accurate full-field deformation profiles of tested objects and investigate the object’s deformation mechanism. However, because of the raster-scanning imaging mode used in microscopic observation equipment, the images obtained from these instruments can only be used for quasi-static displacement measurements; otherwise, spurious displacements and strains may be introduced into the deformation results if these scanning microscopic images are used directly in general digital image correlation calculations for moving and temporally deformed surfaces.
ObjectiveRealizing kinematic parameter and dynamic deformation measurements on a scanning electron microscope platform.
MethodsEstablishing a scanning imaging model of moving and temporally deformed objects that contains motion and deformation equations, a scanning equation and an intensity invariance assumption for small deformations. Then proposing a scanning-digital image correlation (S-DIC) method based on combing the characteristics of the scanning imaging mode with digital image correlation.
ResultsQuantitatively investigating the effects of the spurious displacements and strains introduced when using scanning images to represent moving and temporally deformed surfaces in the measurement results. Numerical simulations verify that the accuracy of the S-DIC method is 10?2pix for the displacement, 10?4 for the strain, 10?4pix/s for the velocity and 10?6s?1 for the strain rate. Experiments also show that the proposed S-DIC method is effective. Conclusions: The results of this work demonstrate the utility of S-DIC on the field of microscopic dynamic measurement.
相似文献Digital image correlation (DIC) has advanced to become a flexible, reliable and fast optical method for the measurement of non-contact and full-field surface deformation. However, the accuracy of existing methods in measuring heterogeneous deformation fields—especially for the high gradient strain field – can be improved.
ObjectiveIn state-of-art local DIC applications, several methods have been put forward to adapt a subset to unknown deformation. Although improvements in performance using these methods are obtained, results are still ungratified for severely heterogeneous deformation such as the Star 2 and Star 5 images from DIC Challenge 2.0.
MethodsIn this paper, a rotated Gaussian weighted zero-mean normalized sum of squared difference (RGW-ZNSSD) criterion function is proposed as the basis for RGW-DIC subset size adaptation. RGW-DIC can automatically determine the optimum weight distribution, hence self-adaptivity in subset size and orientation are achieved simultaneously.
ResultsThe effectiveness of the proposed RGW-DIC is verified using DIC-challenge 2.0 images and simulated sinusoidal deformation images. Results reveal that the adaptively determined subset weight distribution can significantly improve the accuracy of heterogeneous deformation measurement compared with traditional DIC and DIC with isotropic Gaussian weight functions.
ConclusionsThe proposed RGW-DIC can be applied to unknown severely heterogeneous deformation measurement.
相似文献Measuring true stress–strain curve over a large-strain-range is essential to understand mechanical behavior and simulate non-linear plastic deformation. The digital image correlation (DIC) technique, a non-contact full-field optical measurement technique, is a promising candidate to obtain a long-range true stress–strain curve experimentally.
ObjectiveThis paper proposes a method for measuring true stress–strain curves over a large-strain-range during tensile testing using DIC.
MethodsThe wide-strain-range true stress–strain curves of dual-phase and low carbon steels were extracted on the transverse direction in the neck region. The axial strain on the neck section was estimated by averaging the inhomogeneous deformation on the cross-section of the tensile specimen. The true stress was calculated from the engineering stress and the cross-sectional area of the neck.
ResultsThe validity of the proposed method was assessed by comparing the experimental load–displacement responses during tensile testing with the finite element method (FEM) simulation results. The stress and strain on the neck section estimated using the FEM and DIC, respectively, were proven to satisfy the uniaxial condition and successfully obtained.
ConclusionsThe experimental results agree well with the FEM results. The proposed concept can be applied to various deformation modes for accurately measuring long-range true stress–strain curves.
相似文献Digital Image Correlation (DIC) is a length scale independent surface pattern matching and tracking algorithm capable of providing full field deformation measurements. The confident registration of this pattern within the imaging system becomes key to the derived results. Practically, conventional speckling methods use non-reliable, non-repeatable patterning methodologies including spray paints and air brushing leading to increased systematic and random errors based on the user’s experience.
ObjectiveA methodology to develop a speckle pattern tailored to the imaging and experimental conditions of the given system is developed in this paper.
MethodsIn this context, a novel bio-inspired speckle pattern development technique is introduced, leveraging spatial imaging parameters in addition to frequency characteristics of speckle patterns, enhancing the results obtained through DIC. This novel technique leverages gradient parameters in the frequency spectrum obtained from patterns fabricated using a bio-templating manufacturing technique.
ResultsThe analysis presented shows that optimized gradient features alongside tailored spatial characteristics reduce errors while increasing the usefulness of DIC results across the entire region of interest. With this new approach, gradient information is derived from the bio-templated pattern, extracted, optimized and then convolved with spatial properties of a numerically generated 2D point clouds which can then be transferred onto actual specimens. Numerical error analysis shows that the optimized patterns result in significant reduction in root mean square error compared to conventional speckling methods.
ConclusionsPhysical experiments show the scalability of this optimized pattern to allow for varying working distances while consistently maintaining a lower error threshold compared to conventional speckling techniques.
相似文献Developments in digital image correlation (DIC) in the last decade have made it a practical and effective optical technique for displacement and strain measurement at high temperatures.
ObjectiveThis overview aims to review the research progress, summarize the experience and provide valuable references for the high-temperature deformation measurement using DIC.
MethodsWe comprehensively summarize challenges and recent advances in high-temperature DIC techniques.
ResultsFundamental principles of high-temperature DIC and various approaches to generate thermal environment or apply thermal loading are briefly introduced first. Then, the three primary challenges presented in performing high-temperature DIC measurements, i.e., 1). image saturation caused by intensified thermal radiation of heated sample and surrounding heating elements, 2) image contrast reduction due to surface oxidation of the heated sample and speckle pattern debonding, and 3) image distortion due to heat haze between the sample and the heating source, and corresponding countermeasures (i.e., the suppression of thermal radiation, fabrication of high-temperature speckle pattern and mitigation of heat haze) are discussed in detail. Next, typical applications of high-temperature DIC at various spatial scales are briefly described. Finally, remaining unsolved problems and future goals in high-temperature deformation measurements using DIC are also provided.
ConclusionsWe expect this review can guide to build a suitable DIC system for kinematic field measurements at high temperatures and solve the challenging problems that may be encountered during real tests.
相似文献Optical metrology is widely used to measure materials’ deformation and mechanical properties but current fundamental research requires more precise measurement of microstructure and deformation in internal materials. Electron backscattered diffraction (EBSD) technique measures crystal orientation in individual grain and high resolution EBSD (HR-EBSD) method provides information about residual strain and GND density.
ObjectiveDeformation of two stainless steels Nitronic 60 and Tristelle 5183 with different proportions of ferrite and carbides are characterised.
MethodsPush-release bend testing was used to provide progressive increasing bending stress in two iron-based material samples. HR-EBSD and high resolution digital image correlation (HR-DIC) methods characterised residual strain, GND density and plastic strain distributions in each sample.
ResultsNitronic 60 and Tristelle 5183 were deformed and obtained 3.8% and 0.9% plastic strain ?xx. High GND densities distributed neighbouring grain boundaries in Nitronic 60 while high GND densities distributed around carbides, especially intragranular carbides in Tristelle 5183.
ConclusionsHR-EBSD and HR-DIC quantitative characterised deformation in two iron-based alloys, grain/twin boundaries and carbides resulted in GND density increase, promoted work hardening and accumulated high residual elastic strain. Heterogeneous grain/carbide size distribution leaded to stress concentration and cause carbide decohesion and brittle fracture of sample.
相似文献High-velocity oxy-fuel (HVOF)-sprayed metallic coating can be used to create a surface layer that plays a significant role in enhancing the overall strength, stiffness, and fatigue life of the treated material. The micro-deformation around a single impacted particle is a critical factor that must be considered for the optimization of the HVOF process.
ObjectiveIn this study, the micro-deformation field of stainless steel impacted by a ceramic particle was characterized at the micro-scale.
MethodA grid with a frequency of 1200 lines/mm was fabricated on the surface of stainless steel specimen. The microscopic deformation field formed on the substreate surface, induced by the impact of micro-particles with a diameter of 18 µm, was determined using the electron moiré method and numerical simulations.
ResultsThe in-plane plastic strain around the impacted particle was found to be as high as 9.1%, and the value sharply decreased with the increase of the distance to the edge of the impacted particle. The diameter of the plastic area was about 40 µm, which was approximately 2.2 times larger than the particle size. The experimental results were compared with numerical simulation results, and good agreement between the results was found.
ConclusionsThe electron moiré technique can be a useful tool for the measurement of the deformation field induced by an impacted particle in a very local area with a size on the order of microns.
相似文献Digital Image and Volume Correlation (DIC and DVC) are non-contact measurement techniques that are used during mechanical testing for quantitative mapping of full-field displacements. The relatively high noise floor of DIC and DVC, which is exasperated when differentiated to obtain strain fields, often requires some form of filtering. Techniques such as median filters or least-squares fitting perform poorly over high displacement gradients, such as the strain localisation near a crack tip, discontinuities across crack flanks or large pores. As such, filtering does not always effectively remove outliers in the displacement field.
ObjectiveThis work proposes a robust finite element-based filter that detects and replaces outliers in the displacement data using a finite element method-based approximation.
MethodsA method is formulated for surface (2D and Stereo DIC) and volumetric (DVC) measurements. Its validity is demonstrated using analytical and experimental displacement data around cracks, obtained from surface and full volume measurements.
ResultsIt is shown that the displacement data can be filtered in such a way that outliers are identified and replaced. Moreover, data can be smoothed whilst maintaining the nature of the underlying displacement field such as steep displacement gradients or discontinuities.
ConclusionsThe method can be used as a post-processing tool for DIC and DVC data and will support the use of the finite element method as an experimental–numerical technique.
相似文献Surface topography strongly modifies adhesion of hard-material contacts, yet roughness of real surfaces typically exists over many length scales, and it is not clear which of these scales has the strongest effect. Objective: This investigation aims to determine which scales of topography have the strongest effect on macroscopic adhesion.
MethodsAdhesion measurements were performed on technology-relevant diamond coatings of varying roughness using spherical ruby probes that are large enough (0.5-mm-diameter) to sample all length scales of topography. For each material, more than 2000 measurements of pull-off force were performed in order to investigate the magnitude and statistical distribution of adhesion. Using sphere-contact models, the roughness-dependent effective values of work of adhesion were measured, ranging from 0.08 to 7.15 mJ/m2 across the four surfaces. The data was more accurately fit using numerical analysis, where an interaction potential was integrated over the AFM-measured topography of all contacting surfaces.
ResultsThese calculations revealed that consideration of nanometer-scale plasticity in the materials was crucial for a good quantitative fit of the measurements, and the presence of such plasticity was confirmed with AFM measurements of the probe after testing. This analysis enabled the extraction of geometry-independent material parameters; the intrinsic work of adhesion between ruby and diamond was determined to be 46.3 mJ/m2. The range of adhesion was 5.6 nm, which is longer than is typically assumed for atomic interactions, but is in agreement with other recent investigations. Finally, the numerical analysis was repeated for the same surfaces but this time with different length-scales of roughness included or filtered out.
ConclusionsThe results demonstrate a critical band of length-scales—between 43 nm and 1.8 µm in lateral size—that has the strongest effect on the total adhesive force for these hard, rough contacts.
相似文献In-plane vibration is significant to a structure and has been accurately solved by many numerical methods; however, there are still not enough studies on its experimental measurement.
ObjectiveThis work aims to propose a non-contact and fast way to measure dense full-field in-plane vibration of a plate structure, which has high frequencies and low response magnitudes.
MethodsA novel three-dimensional (3D) continuously scanning laser Doppler vibrometer (CSLDV) system that contains three CSLDVs is developed to conduct full-field scanning of a plate with free boundary conditions under sinusoidal excitation to measure its 3D vibrations. Calibration among the three CSLDVs in the 3D CSLDV system based on the geometrical model of its scan mirrors is conducted to adjust their rotational angles to ensure that three laser spots can continuously and synchronously move along the same two-dimensional scan trajectory on the plate. The demodulation method is used to process the measured response to obtain in-plane operating deflection shapes (ODSs) of the plate.
ResultsFour in-plane ODSs are obtained in the frequency range of 0–5000 Hz. Modal assurance criterion (MAC) values between in-plane ODSs from 3D CSLDV and step-wise scanning laser Doppler vibrometer (SLDV) measurements are larger than 95%. MAC values between ODSs from 3D CSLDV measurements and corresponding mode shapes from the finite element model of the plate are larger than 91%.
ConclusionsResults from 3D CSLDV measurements have good accuracy compared to those from SLDV measurements and numerical calculation, and the 3D CSLDV system can scan much more measurement points in much less time than the SLDV system.
相似文献Measurement precision and uncertainty estimation are important factors for all residual stress measurement techniques. The values of these quantities can help to determine whether a particular measurement technique would be viable option.
ObjectiveThis paper determines the precision of hole-drilling residual stress measurement using repeatability studies and develops an updated uncertainty estimator.
MethodsTwo repeatability studies were performed on test specimens extracted from aluminum and titanium shot peened plates. Each repeatability study included 12 hole-drilling measurements performed using a bespoke automated milling machine. Repeatability standard deviations were determined for each population. The repeatability studies were replicated using a commercially available manual hole-drilling milling machine (RS-200, Micro-Measurements). An updated uncertainty estimator was developed and was assessed using an acceptance criterion. The acceptance criterion compared an expected percentage of points (68%) to the fraction of points in the stress versus depth profile where the measured stresses ± its total uncertainty contained the mean stress of the repeatability studies.
ResultsBoth repeatability studies showed larger repeatability standard deviations at the surface that decay quickly (over about 0.3 mm). The repeatability standard deviation was significantly smaller in the aluminum plate (max ≈ 15 MPa, RMS?≈?6.4 MPa) than in the titanium plate (max ≈ 60 MPa, RMS?≈?21.0 MPa). The repeatability standard deviations were significantly larger when using the manual milling machine in the aluminum plate (RMS?≈?21.7 MPa), and for the titanium plate (RMS?≈?18.9 MPa).
ConclusionsThe single measurement uncertainty estimate met a defined acceptance criterion based on the confidence interval of the uncertainty estimate.
相似文献