Non-contact Deformation Measurement Research Articles

Efficient and robust deformation measurement based on unsupervised learning

Digital image correlation (DIC) as a non-contact deformation measurement technique is widely used in various engineering fields. Recently, satisfactory results have been achieved by deep learning-based DIC methods, which rely on synthetic datasets for supervised training due to the difficulty in obtaining the ground truth of deformation fields. However, generalization to real scenes remains challenging due to the inherent differences between synthetic and real images and the limited variability of synthetic datasets. To solve this problem, this paper proposes an unsupervised learning DIC method, referred as UnDIC-Net. UnDIC-Net proposes a Patch-based Zero-Normalized Sum of Squared Differences (Patch-ZNSSD) similarity measure as the loss function for unsupervised training, thereby eliminating the dependency on hard-to-obtain labeled data. Meanwhile, we also collected speckle image from real scenes and built an unlabeled dataset to train UnDIC-Net. For network structure, UnDIC-Net adopts an incremental deformation estimation strategy to compute the deformation field, and interestingly, this network can be applied to large deformation measurements. Adequate experiments show that UnDIC-Net performs well in small deformation measurements, and for larger deformation measurements (>100 pixels), some traditional methods fail while UnDIC-Net still performs well. The code and data of this paper is released at: https://github.com/LianpoWang/UnDICnet-master.

A multi-purpose prototype test system for mechanical behavior of tunnel supporting structure: Development and application

A multi-purpose prototype test system is developed to study the mechanical behavior of tunnel supporting structure, including a modular counterforce device, a powerful loading equipment, an advanced intelligent management system and an efficient noncontact deformation measurement system. The functions of the prototype test system are adjustable size and shape of the modular counterforce structure, sufficient load reserve and accurate loading, multi-connection linkage intelligent management, and high-precision and continuously positioned noncontact deformation measurement. The modular counterforce structure is currently the largest in the world, with an outer diameter of 20.5 m, an inner diameter of 16.5 m and a height of 6 m. The case application proves that the prototype test system can reproduce the mechanical behavior of the tunnel lining during load-bearing, deformation and failure processes in detail.

Investigation of the accuracy of Digital Image Correlation (DIC) in measuring full-field strain for timber materials

As one of the most successful techniques in experimental mechanics, digital image correlation (DIC) has been widely used to measure the motion and deformation of solid materials, especially in situations where non-contact measurement is required. This study describes and provides a comprehensive overview of the application of DIC using the open-source platform Ncorr on orthotropic materials. Orthotropic materials are a subset of anisotropic materials; their properties depend on the direction in which they are measured. Deformation displacement and in-plane strain in two directions are extracted from digital image correlation using a reference image recorded during the experiment. Tests are carried out on Laminated Veneer Lumber timber specimens according to the ASTM standard to determine the mechanical properties of materials under compression. DIC has proven to be a reliable, consistent, and cost-effective non-contact deformation measurement method that can assist in the extraction of mechanical properties of orthotropic materials. This study also showed that DIC analysis with a natural timber grain can be used to measure the displacement and strain fields of the material.

Deformation measurement in Al thin films at elevated temperatures by digital image correlation with speckles prepared by femtosecond laser

Deformation measurement in thin films at elevated temperatures is critical for assessing the reliability and performance of microelectronic devices. Digital image correlation (DIC) is a full-field, noncontact deformation measurement technique. However, the traditional DIC approach requires spraying paints for speckles, which is not suitable for high-temperature applications. The aim of this work is to propose a fully customizable speckle preparation method by pulsed laser that can deliver ideal speckle pattern, good contrast and consistency and controllable damage to sample for deformation measurement at elevated temperatures by DIC. Al thin films deposited on Si substrates were used for speckle preparation by FLs at different powers (10 mW, 8 mW, 6 mW and 4 mW). Serial images of speckles were captured as the temperature increased from room temperature to 325 °C, followed by data analysis. A finite element model was built to evaluate the deviation of measured strains in Al thin film from ideal values. To evaluate the damage of laser to sample surface during speckle preparation, a two-temperature model (TTM) was used to evaluate the ideal ablation depth at different laser fluences. Scanning electron microscopy and white light interferometry were also used to examine the surface damage and depth of holes. Based on our findings, speckles prepared by laser irradiation at 8 mW have the best speckle quality and measurement accuracy. The depth of speckles increased linearly with increasing laser power, ranging from 11.8 to 32.9 nm. The ideal depth calculated by the TTM was slightly lower than the experimental results, which can be attributed to the parameters of the model deviating from the actual sample (such as the reflectance of Al). DIC with speckles prepared by an FL can provide good measurement accuracy at elevated temperatures while causing minimal damage to the sample.

Sampling Moiré method for full-field deformation measurement: A brief review

The sampling Moiré (SM) method is one of the vision-based non-contact deformation measurement methods, which is a powerful tool for structural health monitoring and elucidation of damage mechanisms of materials. In this review, the basic principle of the SM method for measuring the two-dimensional displacement and strain distributions is introduced. When the grid is not a standard orthogonal grating and cracks exist on the specimen surface, the measurement methods are also stated. Two of the most typical application examples are described in detail. One is the dynamic deflection measurement of a large-scale concrete bridge, and the other is the residual thermal strain measurement of small-scale flip chip packages. Several further development points of this method are pointed out. The SM method is expected to be used for deformation measurement of various structures and materials for residual stress evaluation, crack location prediction, and crack growth evaluation on broad scales.

Development of Non-Contact Deformation Measurement System using Small Drone and Digital Image Correlation Method

Development of Non-Contact Deformation Measurement System using Small Drone and Digital Image Correlation Method

Mechanical properties characterization of fiber reinforced composites by nonlinear constitutive parameter optimization in short beam shear specimens

This work presents a novel approach to quantify the nonlinear shear stress-strain behavior of carbon fiber reinforced polymer composites without finite element stress calculation. Full-field noncontact deformation measurements using Digital Image Correlation are used to measure surface strains in Short Beam Shear IM7/8552 specimens. The presented spline-based optimization technique solves the inverse problem of generating both axial and shear stress-strain curves without ad hoc assumptions on the material model. Accuracy of the analysis method was verified by finite element analysis (FEA). This method improves on available closed-form and iterative FEA based solutions due to flexibility and accuracy without the computational expense.

Research on refraction error correction of digital image correlation technology in strain measurement of airship skin

Digital image correlation method is a widely used non-contact deformation measurement method. However, in low temperature environment the airships stay in, the camera needs to be protected by thermal insulation device, the optical glass of which introduces the refraction error to photogrammetry technology. To solve the influence of refraction error on the deformation measurement accuracy of digital image correlation method, the displacement deviation caused by refraction is theoretically deduced to obtain the deviation model based on the analysis of refraction light path and digital image correlation method. Moreover, the corresponding refraction error correction based on deformation function is proposed. Through the contrast experiment, it is found that the refraction offset error model is consistent with the actual displacement deviation. The digital image correlation method based on the refraction error correction proposed in this paper can effectively depress the refraction effect of glass, reduce the measurement error and improve the deformation measurement accuracy.

Model-Based Digital Image Correlation for Noncontact Deformation Measurement of Strain Field and Mechanical Property

This paper presents a new method that combines a physics-based model and finite-element (FE) based digital image correlation (DIC) with an interfacial continuity condition for simultaneous deformation measurements of strain field and material mechanical properties. The dual-domain global DIC method incorporates a model-based kernel that offers useful physical insights into the complex deformation and constrains the solutions to the iterative minimization of the DIC criterion, which leads to a faster convergence. Unlike local subset-based and global FE-based methods where the DIC is formulated to solve for the nodal displacements, the model-based DIC that accounts for complex deformation in the specific region is formulated to identify system parameters. Both domains, along with their interface and the continuity condition enforced on the global displacement field, are discretized and solved simultaneously in a single calculation. The dual-domain global DIC method has been investigated by comparing reconstructed strain fields and estimated parameters against targets simulated using the commercial FEA software COMSOL and published experimental data. Unlike FE-DIC which is noise sensitive, the model-based DIC is robust against the Gaussian noise effects.

Ultra-high temperature mechanical property test of C/C composites by a digital image correlation method based on an active laser illumination and background radiation suppressing method with multi-step filtering.

Carbon/carbon (C/C) composites have been widely used in aerospace engineering because of their high temperature resistance. However, the commonly used non-contact deformation measurement method based on digital image correlation (DIC) still has some problems of speckle preparation and background radiation in an ultra-high temperature environment above 2000°C. An ultra-high temperature mechanical property test method based on active laser illumination and a background radiation suppressing method with multi-step filtering is proposed. In the proposed method, a laser with good coherence is used as the active illumination source to generate laser speckles that will not fall off or discolor as temperature increases. Also, a multi-step filtering device is devised with the combination of a spatial filter, plano-convex lens, polarizing filter, and two different bandpass filters, which can successfully suppress the background radiation and retain the characteristics of laser speckle patterns at the same time. Tensile tests of C/C composite had been carried out at ultra-high temperatures of 2200°C, 2400°C, 2600°C, and 2800°C. The experimental results showed that high-quality laser speckle images were obtained and the deviations between the elastic moduli obtained by the proposed method and by the high temperature contact extensometer were all less than 7%.

A plane strain testing apparatus characterized by flexible loading and noncontact deformation measurement and its application to the study of shear band failure of sand

A new type of plane strain apparatus is developed to study the mechanical properties and shear band failure of soil, which possesses the advantages of flexible loading for lateral confining pressure and noncontact measurement and high measurement accuracy for surface deformation. In addition, the whole deformation procedure of the specimen can be recorded with images, which can be used to describe the development of strain localization and the shear band. It can be seen that the deformation process has three obvious stages, that is, the hardening stage, the softening stage, and the residual stage. The measured inclination angles of shear bands decrease as confining pressure or the mean size increases. In addition, it can be observed that the sand presents continuing growth of the unrecoverable plastic deformation inside the shear band and exhibits almost elastic deformation outside. From the detection results for local points in the specimen, the stress–strain relationships are different for different parts, and the sand sample behaves like an uneven structure instead of an even element, which means that the usual method of measuring the stress–strain relationship of the soil sample is only a macroscopic approximation.

Strain evaluation using a non-contact deformation measurement system in tensile tests of irradiated F82H and 9cr ODS steels

We developed a non-contact deformation measurement system to accurately evaluate strain for post irradiation tensile testing, since conventional strain gages cannot be used for small size specimens. The strain calculated from cross-head displacement generally includes deformation from specimen shoulders, fixtures, and the test frame in addition to the deformation from the specimen gauge section. In our system, the distance between painted marks within the specimen gauge section was measured using a high resolution video camera to evaluate the specimen deformation during room temperature tensile testing. The test materials were F82H and 9Cr ODS steels irradiated up to ≈71 displacements per atom (dpa) at about 573 K in High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). Our system yielded accurate stress strain curves without deformations other than the specimen gage section, and the elongation was less than that calculated from cross-head displacement. This system can contribute to expanding the technically reliable database for the design activity of fusion reactor blanket, including the effects of irradiation on tensile properties.

Optimized digital speckle patterns for digital image correlation by consideration of both accuracy and efficiency.

The technique of digital image correlation (DIC), which has been widely used for noncontact deformation measurements in both the scientific and engineering fields, is greatly affected by the quality of speckle patterns in terms of its performance. This study was concerned with the optimization of the digital speckle pattern (DSP) for DIC in consideration of both the accuracy and efficiency. The root-mean-square error of the inverse compositional Gauss-Newton algorithm and the average number of iterations were used as quality metrics. Moreover, the influence of subset sizes and the noise level of images, which are the basic parameters in the quality assessment formulations, were also considered. The simulated binary speckle patterns were first compared with the Gaussian speckle patterns and captured DSPs. Both the single-radius and multi-radius DSPs were optimized. Experimental tests and analyses were conducted to obtain the optimized and recommended DSP. The vector diagram of the optimized speckle pattern was also uploaded as reference.

Development of deformation criteria for predictive monitoring system for levees

Performance monitoring and real-time data analysis are essential tools to evaluate the integrity of infrastructure. Among other techniques, non-contact deformation measurement techniques such as satellite or drone-based methods are usually low-cost options, not requiring expensive sensors to be installed on target structures. The techniques are particularly economic for large structures such as levees, major slopes or dams. This study introduces lower bound and upper bound deformation concepts for levees. These two bounds are the deformation at which local failure is about to initiate and at which general failure is about to initiate, respectively. This study compared numerically computed lower and upper bounds deformation to field data from a full-scale loading test for a section of the levee at London Canal Avenue, New Orleans, and it showed that the two deformation criteria could be used to monitor and evaluate the integrity of levees.

In situ CTE measurements and damage detection using optical metrology

This paper presents a methodology to make coefficient of thermal expansion measurements through the combined use of two non-contact and full field optical metrology methods including digital image correlation and infrared thermography. In this context, active Infrared Thermography techniques combined with contact and non-contact deformation measurement methods have already been reported to measure materials’ thermal expansion. In addition, such techniques have been reported to be capable to detect surface and subsurface defects from changes in homogenous heat diffusion due to damage. Based on this knowledge, it is hypothesized in this article that the material response induced by thermal loading and quantified by coefficient of thermal expansion measurements could be further used as an indicator of damage. To validate the hypothesis three measurements were performed. The first established the effectiveness of using deformation and thermal full field data for coefficient of thermal expansion measurements. The second intended to demonstrate the advantage of using such full field data in order to provide site-specific measurements of thermal expansion. Finally damage was a priori induced to a metallic specimen, and the measured variations of local CTE confirmed the potential of using the described approach as a means of damage quantification in materials and structures.

Experimental characterization of the global and local behavior of multi-dowel LVL-connections under complex loading

The thorough experimental characterization of a dowel-type connection under various combinations of bending moments and normal forces is presented in this study. Double-shear steel-to-timber connections with 12 and 20 mm steel dowels were tested in a 4-point bending test set-up. The load, between the connected steel and wood beams, was transferred by the dowels themselves and also via an additional (passive) contact device, which introduced an eccentric normal force in the timber beam. The behavior of the connections was studied at the global scale of the connection and at the local scale of the individual dowels. A non-contact deformation measurement system was used to assess the changes of the location of the center of relative rotation over the entire loading. At the same time, the head deformations of the individual dowels could be measured, giving a direct indication about the force distribution among the dowels. Due to reinforcement, connections behaved distinctly ductile with a global relative rotation of up to 3°. Pre-stressing of the contact device by a force of 40 kN yielded an even stiffer behavior. For the particular configurations tested herein, the center of rotation was found to be close to the vertical axis of symmetry of the joint and close to the top row of the dowels. Moreover, the superimposed vertical shift of the center of relative rotation in case of a delayed normal force could be quantified.

Dowel deformations in multi-dowel LVL-connections under moment loading

The aim of the experimental study presented herein is the assessment and quantification of the behavior of individual dowels in multi-dowel connections loaded by a bending moment. For this purpose, double-shear, steel-to-timber connections with nine steel dowels arranged in different patterns and with different dowel diameters were tested in four-point bending. In order to achieve a ductile behavior with up to 7° relative rotation, the connections were partly reinforced with self-tapping screws. The reinforcement did not influence the global load–deformation behavior, neither for dowel diameters of 12 mm nor for 20 mm, as long as cracking was not decisive. The deformation of the individual dowels was studied by means of a non-contact deformation measurement system. Thus, the crushing deformation, that is, the deformation at the steel plate, and the bending deformation of the dowels could be quantified. In the case of 12 mm dowels, the bending deformation was larger than the crushing deformation, while it was smaller in the case of 20 mm dowels. Moreover, dowels loaded parallel to the grain showed larger bending deformations than dowels loaded perpendicular to the grain. This indicates that the loading of the individual dowels in the connection differs depending on their location.

Local variation of modulus of elasticity in timber determined on the basis of non-contact deformation measurement and scanned fibre orientation

During the last decade, the utilization of non-contact deformation measurement systems based on digital image correlation (DIC) has increased in wood related research. By measuring deformations with DIC systems, surface strain fields can be calculated. The first aim of this study concerns the possibility to detect detailed strain fields along the entire length of a wooden board subjected to pure bending and the potential of using such strain fields to determine a bending modulus of elasticity (MOE) profile along a board. Displacements were measured over 12 subareas along a flat surface of the board. For each such area, a separate local coordinate system was defined. After the transformation of locally measured coordinates to a global system, high resolution strain fields and a corresponding bending MOE profile were calculated. A second method in establishing bending MOE profiles is to use fibre angle information obtained from laser scanning and a calculation model based on integration of bending stiffness over board cross sections. Such profiles have recently been utilized for accurate strength grading. A second aim of this study was to investigate the accuracy of the bending MOE profiles determined using the latter method involving fibre angle information. Bending MOE profiles determined using the two described methods agree rather well. However, for some patterns of knot clusters, the local bending MOE, calculated on the basis of fibre angles and integration of bending stiffness, is overestimated. Hence, this research adds knowledge that may be utilized to improve the newly suggested strength grading method.

The Research for the 3D Morphology Measurement of Large-Scale Object by Using Digital Projection Speckle Correlation Method (DPSCM)

The Digital Projection Speckle Correlation Method (DPSCM) is a non-contact full-field optical 3D profile and deformation measurement technique, which combines with 2D Digital Image Correlation Method (DICM), using LCD projector to produce random speckle that is projected on the surface of the sample.In this paper,the 3D profile and deformation measurement experiment of large-scale objects is finished by DPSCM,and the factor which influences the result of experiment is also discussed

The Study of Non-Contact Measure Method for Deformation Based on the Digital Image Correlation

Based on the application of the CSI non-contact full field strains and 3D displacements gauge made in USA in the laboratory for wind engineering of Southwest Jiaotong University, the theory of DIC, a non-contact deformation measurement method, is studied in the paper. The reliability of the method is testified by contrast tests. The paper also offers some useful advice byanalysing the factors that affect the measurement accuracy.