Deformation Measurement Method Research Articles

A novel deformation measurement method for ablation materials in combustion and ablation process

Ablation material is at the base of the entire aerospace industry. The ablation process is the primary mechanism of thermal protection provided by the ablation material. The mechanical performance of ablation materials across the ablation process is a significant problem for the ablation material research. The combustion phenomenon in the ablation process leads to the failure of the measurement of mechanical properties of materials in the ablation process, which limits the study of mechanical properties of ablation materials. In this paper, a method for the thermo-mechanical coupled deformation measurement is presented. Through the use of filter and light source with a specific wavelength, the high-temperature deformation process of the ablation material specimen from room temperature to 2300K is measured accurately. A new method of speckle feature change matching is presented. The convolution neural network is used to establish the corresponding matching and parameter model between the feature changed speckle feature and the initial speckle feature. The DoG values replace the traditional gray-scale feature for the correlation matching, and the continuous thermo-mechanical coupling deformation of ablation materials is carried out through simulation experiments and actual experiments. The experimental results show that the proposed method can be used to measure and analyze the strain of ablation materials in the process of high-temperature continuous thermo-mechanical coupling deformation.

A novel static deformation measurement and visualization method for wind turbine blades using home-made LiDAR and processing program

Wind turbine blades play a key role in the wind energy collection system. Deformation measurement for turbine blades is an important and efficient way to prevent blade damages. Here, a novel and low-cost measurement method of static deformation for wind turbine blades using home-made LiDAR was proposed, and then verified by measuring the flap-wise load bending of a wind turbine blade model. The horizontal and vertical angular resolution of the proposed system are 0.09° and 0.225°, meaning a high resolution of 500 × 200 distance points in a 45° × 45° field of view of each depth image. The visualization and processing of depth images was realized using Point Cloud Library. To fast segment the 3D wind turbine blade model from background data, we also proposed a segmentation approach based on Euclidean clustering and union-find algorithm. The experimental results show that this method can segment the blade model from acquired data effectively, reducing the time-cost by nearly 50%. Furthermore, the bending degrees and curvatures of the blade in different buckling loads can be calculated using processed data. The proposed method can realize low-cost, on-site, portable and visualized deformation measurement for wind turbine blades and would be a promising way to test blades and prevent blade damages.

Design of and Research of Vertical Deformation Measuring Instrument based on Synthetic Material

With the publication and implementation of the national standard GB 36246 “Sports Ground for Synthetic Materials in Primary and Secondary Schools”, relevant testing activities have come in droves. However, the smooth development of testing requires not only standards, but also corresponding testing equipment. Therefore, in accordance with the requirements of the national standard GB 36246 for vertical deformation measurement indicators, this paper carried out the design and research work of related testing equipment, and successfully developed a synthetic material-based surface Layer vertical deformation measuring instrument, which connects the electromechanical control system composed of electromagnetic control system, main structure, etc. and the data storage and analysis system, realizes the systemization and automatic control of the synthetic material surface vertical deformation measuring device, and meets the national standard GB 36246 and European standard EN14809 “Measurement method of vertical deformation of ground layer of sports field”.

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.

Measurement of tensile mechanical properties of fiber reinforced plastic rebars by 3D digital image correlation

Abstract Fiber reinforced plastic (FRP) plays an increasingly important role in many engineering fields due to its advantages of light weight, low cost, simple maintenance, and excellent corrosion resistance. The conventional contact methods for deformation measurement, such as strain gauge and mechanical extensometer, have some shortcomings in measuring the strain of FRP rebar specimens with spiral curved surfaces. Therefore, the non-contact three-dimensional digital image correlation (3D-DIC) technique was employed to determine the tensile mechanical properties of two types of FRP rebar specimens, i. e. glass FRP (GFRP) and carbon FRP (CFRP) specimens. Uniaxial tensile tests with three different loading rates were conducted to obtain the stress-strain curves, elastic modulus, tensile strength and percentage total extension at maximum force of these two FRP specimens. Experimental results indicate that the axial-strain field of all FRP rebar specimens present nonuniform distribution and that the stress of GFRP and CFRP rebars varies linearly with the strain. Moreover, no yielding phenomenon was observed from the stress-strain curves, which indicates that both GFRP and CFRP rebars belong to the group of typical brittle materials. The dispersion of elastic moduli obtained using 3D-DIC is better than that using a clip-on extensometer, which demonstrates the validity of 3D-DIC for the determination of tensile mechanical properties of FRP rebars with spiral curved surfaces.

Global-local deformation measurement of stress concentration structures using a multi-digital image correlation system

The digital image correlation (DIC) technique is a method of measuring the displacement and strain of the entire area of a structure by comparing surface images taken by digital cameras before and after deformation of the structure. The DIC technique is currently being spotlighted because it can overcome the weakness of conventional contact measurement methods. So far a single DIC system with one camera or two cameras has been used for most applications. However, many structures need two or more DIC systems in order to measure their deformation efficiently and precisely. For example, the stress concentration problem and the cylindrical shell buckling problem are typical applications for a multi-DIC system. This study focuses on the stress concentration problem. The paper presents a globallocal deformation measurement method with a multi-DIC system. First, a tensile test of an aluminum beam was performed and compared with the strain gage method to validate the accuracy of DIC method. Then the tensile test specimens with a hole or a notch were prepared, and one DIC system measured the local deformation near a hole or a notch and the other DIC system measured the global deformation of the specimen. The results confirmed the proposed multi-DIC technique can be applied to the stress concentration problem.

Continuous measurement of tire deformation using long-gauge strain sensors

Measurement of tire deformation is a hot spot in the research of intelligent tire, which not only provides deep insights into the mechanism of generating tire forces and moments, but also has a significant influence on assessing the health condition of bridges. This paper proposed a tire deformation measurement method by monitoring tire hoop strain with long-gauge Fiber Bragg Grating (FBG) sensors, which can achieve real-time continuous measurement of the tire deformation by the modified conjugate beam method. Taking advantage of the FBG sensor’s high sensitivity, high durability and macro–micro, the modified conjugate beam method transforms the problem of solving the tire deformation into a more convenient solution to the bending moment of the conjugate structure. Intelligent bridge impact vehicle experiments and high-speed experiment were carried out to validate the proposed method, from which the estimated tire deformations under different wheel loads and tire pressures show good agreements with reference measurements.

An advanced vision-based deformation measurement method and application on a long-span cable-stayed bridge

An advanced vision-based method focusing on fast and accurate deformation measurement was developed. First, computing speed and sub-pixel matching accuracy were improved by modifying the feature extraction process and optimizing the elimination mechanism of random sample consensus. An impact vibration test on a beam was designed to verify this improvement. Second, a new displacement transformation equation that considers the location change of measuring points for oblique optical axis measurement was proposed, with its advantage verified through a specially designed translation test. Based on the above, software for synchronous multi-region tracing was developed and applied to monitor the mid-span deflection and multi-cable tensile force of the Su Tong Bridge, the second longest cable-stayed bridge in the world. The test results show the great application prospects of the proposed optical method for remote and non-target deformation measurement.

Second-order moiré method for accurate deformation measurement with a large field of view.

In this study, we propose a second-order moiré method by performing digital sampling at two stages to realize high-accuracy deformation measurement in a wide field of view, where a grid image is recorded at a low magnification. Simulations have verified that this method has high strain measurement accuracy when the grid pitch is close to or even smaller than two pixels for both parallel and oblique grids with random noise. As an application, the two-dimensional microscale strain distributions of a carbon fiber reinforced plastic specimen when the grid pitch was about 2.1 pixels were presented. Shear strain concentration was detected before an interlaminar crack emerged, and tensile strain concentration was found prior to the occurrence of a transverse crack. The proposed method makes the two-step phase-shifting technique achieved indirectly, not only enlarging the field of view, but also maintaining the measurement accuracy.

Dynamic Monitoring of RS and GIS Resources and Ecological Environment Based on High Temperature Materials

The dynamic monitoring and analysis of regional ecological environment is an important part of land use research, which can provide support for regional ecological environment and macro regulation of government departments. With the support of remote sensing technology (RS) and geographic information system (GIS), a land use dynamic monitoring system is established to obtain land use change information quickly, accurately and economically. In order to reduce the influence of thermal radiation, noise and vibration of high temperature material machinery on the measurement results in high temperature measurement environment, the paper proposed the measurement method of ultraviolet light high humidity thermal deformation and gray average method. In this paper, the algorithm of digital image correlation method and the establishment of high temperature measurement system are studied. A set of high temperature material mechanical digital image correlation system suitable for high humidity environment measurement is independently established and applied to the high humidity test of materials. The results show that the above comprehensive method can effectively improve the influence of thermal flow disturbance on system imaging in high humidity environment. Comprehensive analysis of regional ecological environment data shows that environmental quality is closely related to regional ecological environment. Meteorological conditions are one of the important factors affecting the environment, and the project of returning farmland to forest plays a vital role in improving the environment.

Real-time subtraction-based calibration methods for deformation measurement using structured light techniques.

Calibration of the optical trigonometric relationships in structured light is essential for the accuracy of out-of-plane displacement measurement. A simple calibration mechanism based on real-time image subtraction is proposed. Details of calibrating the height-to-phase ratio of the digital fringe projection method, the relationship between out-of-plane and in-plane displacements of the digital projection-speckle correlation method, and the scale factor of the optical system are described. The calibration methods are applied to the deformation measurement of a clamped Plexiglas plate, which demonstrates the effectiveness of the calibration methods. The real-time subtraction-based calibration methods provide alternatives for calibrating structured light techniques used for out-of-plane displacement measurement.

Fast bridge deflection monitoring through an improved feature tracing algorithm

AbstractCompletely non‐contact and fast measurements are still the focus of vision‐based monitoring in real‐bridge applications. Overall, a fast deflection measurement method, that traces the natural features of structures instead of artificial targets, was developed in this study. More specifically, due to the tiny scale change between images and the small deformation level of small and medium bridges, a simplified fast‐Hessian detector and a pre‐purification‐based RANdom Sampling Consistency (RANSAC) were proposed and verified by performing experiments under different illuminations. The non‐target deformation measurement method based on an improved algorithm was applied to field testing of an arch bridge with 100 m main girder and showed better processing speed and sub‐pixel accuracy than their originals. The optical results obtained by an industrial camera were consistent with other contact‐type sensors, such as level gauges and inclinometers.

Overcoming high luminance gradient using serial exposure time method for synchronous full-field measurement of temperature and deformation.

The high-temperature optical method has been widely used for evaluating structural materials subjected to high temperature. Obtaining high-quality images of a specimen surface in such a harsh environment is detrimental for the accurate measurement of temperature and strain field. However, the high-temperature environment poses many challenges on these measurements, e.g., the large luminance gradient on the sample surface would jeopardize the image quality when using the full-field optical method. In order to overcome this issue, we propose here a simple and effective algorithm to obtain image sequences with serial exposure times. This algorithm incorporates exponentially decreasing exposure times to successfully reduce the disturbance caused by large luminance gradient, as will be shown by the verification on samples tested both in arc wind tunnel and oxy-propane torch flame. In comparison to the images with single exposure time, further experiment carried out on C/SiC sample up to 1100°C shows that image sequences with different exposure times can be effectively obtained by the image fusion technique. The calculation of the deformation and temperature fields using the image sequence method gives more accurate and reasonable results.

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%.

Dynamic Displacement of an Aluminum Frame Using Close Range Photogrammetry

Dynamic displacement measurement of objects can be challenging due to the limitations of conventional methods and pricey instrumentation of unconventional methods, such as laser scanners. In this research, Close Range Photogrammetry (CRP) is used as an affordable non-contact method to measure 3D dynamic displacements. It is proposed as a reliable alternative to traditional dynamic deformation measurement methods such as displacement sensors or accelerometers. For this purpose, dynamic displacements of a three-dimensional one-story building frame model on a one-dimensional shake table are determined by using the traditional method of attached accelerometer and CRP. The results of the CRP method are compared with the results of the traditional methods as well as numerical models. The results show a good agreement which evidences the reliability of the CRP with regular cameras.

MEMS cytometer for porcine oocyte deformation measurement

Mechanical properties of cells are indicators of biological potential, quality and presumptive pathological status. Mammalian oocytes are particularly important cells from the point of view of testing their fertilizing potential and performing in vitro fertilization. However, only occasionally are their mechanical parameters measured.A new method for deformation measurement of in vitro-matured porcine oocytes has been developed and presented in this study. The method was used for multiparametric characterization of gametes using a MEMS-type microcytometer and a new methodology for determining cell deformability. The novel chip design, principle of operation, and fabrication processes are described in the study. Using original analysis of microscopic images with the usage of dedicated software and data analysis algorithms, deformation parameters were determined. Results indicate the relationships between parameters under the influence of cell compression and show that the behaviour of oocytes during measurements depends on the morphological quality of the cumulus-oocyte complexes they originated from.

Real-Time GPU-Based Digital Image Correlation Sensor for Marker-Free Strain-Controlled Fatigue Testing

Digital image correlation (DIC) is a highly accurate image-based deformation measurement method achieving a repeatability in the range of σ = 10−5 relative to the field-of-view. The method is well accepted in material testing for non-contact strain measurement. However, the correlation makes it computationally slow on conventional, CPU-based computers. Recently, there have been DIC implementations based on graphics processing units (GPU) for strain-field evaluations with numerous templates per image at rather low image rates, but there are no real-time implementations for fast strain measurements with sampling rates above 1 kHz. In this article, a GPU-based 2D-DIC system is described achieving a strain sampling rate of 1.2 kHz with a latency of less than 2 milliseconds. In addition, the system uses the incidental, characteristic microstructure of the specimen surface for marker-free correlation, without need for any surface preparation—even on polished hourglass specimen. The system generates an elongation signal for standard PID-controllers of testing machines so that it directly replaces mechanical extensometers. Strain-controlled LCF measurements of steel, aluminum, and nickel-based superalloys at temperatures of up to 1000 °C are reported and the performance is compared to other path-dependent and path-independent DIC systems. According to our knowledge, this is one of the first GPU-based image processing systems for real-time closed-loop applications.

Application of Digital Image Correlation (DIC) to the Measurement of Strain Concentration of a PVA Dual-Crosslink Hydrogel Under Large Deformation

Hydrogels are a class of soft, highly deformable materials formed by swelling a network of polymer chains in water. With mechanical properties that mimic biological materials, hydrogels are often proposed for load bearing biomedical or other applications in which their deformation and failure properties will be important. To study the failure of such materials a means for the measurement of deformation fields beyond simple uniaxial tension tests is required. As a non-contact, full-field deformation measurement method, Digital Image Correlation (DIC) is a good candidate for such studies. The application of DIC to hydrogels is studied here with the goal of establishing the accuracy of DIC when applied to hydrogels in the presence of large strains and large strain gradients. Experimental details such as how to form a durable speckle pattern on a material that is 90% water are discussed. DIC is used to measure the strain field in tension loaded samples containing a central hole, a circular edge notch and a sharp crack. Using a nonlinear, large deformation constitutive model, these experiments are modeled using the finite element method (FEM). Excellent agreement between FEM and DIC results for all three geometries shows that the DIC measurements are accurate up to strains of over 10, even in the presence of very high strain gradients near a crack tip. The method is then applied to verify a theoretical prediction that the deformation field in a cracked sample under relaxation loading, i.e. constant applied boundary displacement, is stationary in time even as the stress relaxes by a factor of three.

Non-landing vehicle-mounted electro-optical theodolite deformation measurement method using inertial sensors

Working on a vehicle platform, an electro-optical (EO) theodolite’s measurement accuracy deteriorates due to platform deformation caused by external forces, such as the acceleration and deceleration of the EO theodolite, wind and people walking on the platform. This paper presents a vehicle platform deformation measurement method using inertial sensors. The necessity of the vehicle platform’s deformation measurement is thoroughly analyzed. An inertial sensor calibration method is then proposed so that the platform’s three-dimensional deformation can be calculated by the inertial sensors’ axes outputs. Experiments show that the measurement resolution reaches 2” and the measurement error is less than 4”. The proposed method does not require a reference device, and hence largely improves the measurement system’s stability and adaptability.

Importance of Exposure Time on DIC Measurement Uncertainty at Extreme Temperatures

Digital Image Correlation (DIC) is a popular optical method for deformation and strain measurement. At extreme temperatures, it is known that materials emit light in addition to reflecting the light supplied by a light source, and the emitted light can saturate a camera sensor. More recently, a novel variation of DIC, named ultraviolet (UV) DIC, extended the range of temperature further by using a UV bandpass filter to screen out some of the brightest glowing and external UV illumination to provide additional reflected lighting. In principle, for a given optical set-up the temperature range can be extended further by reducing the camera’s sensitivity to light, and exposure time is an instrumental parameter when setting such camera configurations. In this paper, we examine the influence of multiple exposure times on the uncertainty of UV-DIC correlation measurements. Rigid-motion experiments were performed at four different temperatures: room temperature, 1300oC, 1450oC, and 1600oC. At each temperature level, UV images were recorded for DIC at exposure times ranging from 500 μs to 61,000 μs – a range of over two orders of magnitude. The results showed abrupt increases of error at extremely dark or bright exposure times, but at intermediate exposure times the errors of UV-DIC were minimal. A normalized metric was presented in order to give a general guideline when choosing exposure time for camera sensitivity. It is recommended that cameras should be set at a suitable range of exposure time (between 10,000 μs and 40,000 μs for the camera used in this paper) in order to perform meaningful DIC up to 1600oC.