Full-field Deformation Measurement Research Articles

Model test on the collapse mechanism of subway tunnels in the soil-sand-rock composite strata

Collapse is a typical type of accident during subway tunnel construction. Complex geological conditions, particularly soil-sand-rock composite strata, significantly contribute to strata instability, posing a serious threat to tunnel safety. Indeed, the progressive failure leading to the collapse of subway tunnels in these composite strata exhibits distinct characteristics from that of single strata and warrants further research. In this paper, model tests were conducted to investigate the collapse process of the soil-sand-rock composite strata under different overlayer rock thicknesses. Soil pressure sensors monitored the mechanical response of the strata, while a two-dimensional full-field deformation measurement and analysis system (XTDIC-2D) provided displacement and strain fields. Additionally, an industrial camera captured video footage of the failure process. The results demonstrated that the collapse characteristics of the soil-sand-rock composite strata were significantly impacted by overlayer rock layer thickness. As the thickness of the rock layer decreased, the collapse easily expanded to the sand layer under a slight loading. The sand layer exhibited distinct behaviors with high compressibility, thixotropy, and flowability during the collapse process. The high compressibility of the sand layer before collapse resulted in strain concentration within it, thereby resisting the deformation of the rock strata. After the collapse of the rock strata, the thixotropic and flowability properties caused the collapse surface to expand in a funnel shape towards both sides. A stabilized stratigraphic arch could not be formed due to the arch foot being located above the sand layer, which cannot provide stable support. Overall, the results of this research offer valuable guidance for the prevention and control of tunnel collapse in soil-sand-rock composite strata.

Shack–Hartmann wavefront sensing: A new approach to time-resolved measurement of the stress intensity factor during dynamic fracture

The stress intensity factor describes the stress state around a crack tip in a solid material and is important for understanding crack initiation and propagation. Because stresses cannot be measured directly, the characterization of the stress intensity factor relies on the measurement of deformation around a crack tip. Such measurements are challenging for dynamic fracture of brittle materials where the deformation is small and the crack tip velocity can be high. Digital gradient sensing (DGS) is capable of full-field measurement of surface deformation with a sub-micrometer sensitivity and a sub-microsecond temporal resolution, but it has only been demonstrated on centimeter-scale specimens with a spatial resolution of ∼ 1 mm. This makes it challenging to measure deformations close to the crack tip. Here, we demonstrate the potential of Shack–Hartmann wavefront sensing (SHWFS), as an alternative to DGS, for measuring surface deformation during dynamic brittle fracture of millimeter-scale specimens. Using an opaque commercial glass ceramic as an example material, we demonstrate the capability of SHWFS to measure the surface slope evolution induced by a propagating crack with a micrometer spatial resolution and a sub-microsecond temporal resolution. The SHWFS apparatus has the additional advantage of being physically more compact than a typical DGS apparatus. We interpret our SHWFS measurements of the surface slope by comparing them with 2D analytical predictions and phase-field simulations as well as 3D linear-elastic FEM simulations, based on which we discuss the relevance of 3D effects around the crack tip. Then, we introduce our procedure for extracting the apparent stress intensity factor associated with the propagating crack tip using SHWFS measurements. We conclude by discussing potential future enhancements of this technique and how its compactness could enable the integration of SHWFS with other characterization techniques including x-ray phase-contrast imaging (XPCI) for multi-modal characterization of dynamic fracture.

Rapid full-field deformation measurements of tall buildings using UAV videos and deep learning

Full-field deformation of tall buildings is informative and imperative for structural health condition assessments. This study explores the feasibility of using videos recorded by an unmanned aerial vehicle (UAV) to measure the vibration profiles of tall buildings with the aid of deep neural networks. First, a Transformer network with a self-attention mechanism is used, and its outcome is employed for the homography transformation to minimize the drifting problems induced by the UAV. Second, a boundary-aware salient object detection network is adopted to extract the boundary of the buildings in the stabilized videos for further full-field deformation measurements. The accuracy of the proposed method is experimentally validated using a wood frame structure under harmonic excitations. Under a measuring distance of approximately 4.2 m, the proposed method achieves averages of root-mean-square-errors (RMSEs) for the displacement and rotation measurements of 2.13 mm and 1.74 × 10−3 rad, respectively, as compared to that determined by an infrared light tracking system. The study further analyzes the field-test data recorded from the vibration of a 247-m tower. Under a measuring distance of about 55 m, the proposed method is comparable to the vibration measurement at a specific story and estimates the full-field deformation within the region of interest. Overall, the proposed method with rapid UAV deployment is feasible and accurate for monitoring the full-field deformation using the UAV-recorded videos.

2D full-field deformation measurement at grain level using optical flow with deep networks

2D full-field deformation measurement at grain level using optical flow with deep networks

Measuring 3D shape and deformation in the presence of extremely strong ambient light and thermal radiation with a single time-gated camera.

Monochromatic light-illuminated active-imaging stereo-digital image correlation (stereo-DIC) has been extensively used for measuring the surface deformation of materials and structures at elevated temperatures. Despite the improvements in the image acquisition techniques or devices, it is still challenging to measure the 3D deformation of materials and structures in the presence of strong, time-varying ambient light and thermal radiation. In this study, we present what we believe to be a novel dual-filtering single-camera stereo-DIC technique for full-field 3D high-temperature deformation measurement, even in the case of extremely intense ambient light and thermal radiation. In contrast to conventional active-imaging stereo-DIC that only suppresses the thermal radiations in the spectral domain, the proposed technique utilized a dual-filtering strategy (i.e., narrow bandpass optical filtering and ultrashort exposing) to suppress the strong ambient light and thermal radiation in both time and spectral domains. Besides, a four-mirror adapter is adopted to realize 3D shape and deformation measurement using a compact single time-gated camera. Experimental verifications, including assessments with laboratory experiments and validations on real thermal deformation tests under transient aerodynamic heating and direct ohmic heating, convincingly demonstrated that the proposed single-camera dual-filtering stereo-DIC method can achieve accurate 3D shape, motion and deformation measurement, even with strong light and thermal radiation from the quartz lamps and the heated sample.

Full-field deformation measurement and cracks detection in speckle scene using the deep learning-aided digital image correlation method

An enhanced full-field deformation and crack measurement method for oblique optical-axis conditions was proposed. First, the thickness of the calibration plate causes an error in the homography conversion (H matrix) and the final displacement results. To solve this problem, a Perspective-n-Point (PnP) based thickness compensation method for H calibration was proposed. Second, for better measurement of the full-field deformation, the measured surface is mapped with random speckles and markers, and the complex background and covering problems create obstacles for crack skeleton detection. To overcome these interferences, a strain-guided two-phase detection method based on the deep learning model was innovatively designed. Third, the traditional crack width calculation method is susceptible to the binarization threshold and the connection to the speckles, and a crack width calculation method using the displacement of control points was suggested. During a bending test, the above methods were applied to a basalt fiber-reinforced polymer (BFRP) beam. The enhancement in the displacement measurement accuracy and the robustness of crack detection, particularly the potential of detecting hidden cracks, were verified by comparing the performance of the proposed method to that of traditional contact sensors.

Mechanical and thermo-physical properties of heat and energy storage backfill based on MicroPCMs

Combining backfill mining with geothermal resource exploitation, geothermal extraction is achieved by using heat and energy storage functional backfill to achieve a win-win situation of mineral and geothermal co-mining. In this study, MicroPCMs were partially substituted for tailings to prepare phase change backfill material. The influence of different MicroPCMs contents (0, 5, 10, 15, 20 wt%) on the mechanical and thermophysical properties of backfill was investigated to determine the optimal MicroPCMs contents. The results showed that with the increase of MicroPCMs content, the strength exhibited a cliff-like drop at 5% followed by a gradual decline, as confirmed by the XTDIC full-field strain deformation measurement and analysis system. The thermal conductivity and thermal diffusivity decreased with the increase of MicroPCMs content, while the volumetric specific heat capacity and enthalpy showed an increasing trend. SEM scanning results indicated that MicroPCMs would fracture under mechanical action, and the fracture proportion increased with the increase in MicroPCMs content. The recommended MicroPCMs content was around 15 wt%, where the compressive strength, volumetric specific heat capacity, and enthalpy reached 5.2 MPa, 1.6 MJ/(m3·K) and 6.16 J/g, respectively. This study provides valuable insights into the influence of MicroPCMs on the strength and thermal properties of backfill materials.

A Unified Full-Field Deformation Measurement Approach for Plates With Different Thickness

A Unified Full-Field Deformation Measurement Approach for Plates With Different Thickness

Progressive failure of asphalt shingles under high winds: Assessment via full-field deformation measurements on full-scale roof panels

Failures of roof asphalt shingles under high winds have been reported to occur at wind speeds lower than the shingle performance classification. The understanding of the progressive failure mechanisms of shingles subjected to high-wind pressures has been hindered by difficulty in accurately quantifying uplifts and wind-induced pressures with high spatial resolution, especially for large-scale wind test setups that simulate realistic scenarios. This paper reports on an experimental study to assess the use of three-dimensional digital image correlation (‘StereoDIC’) to accurately measure full-field surface deformations of shingles installed on full-scale roof structures and subjected to realistic high winds. Deformation measurements were acquired on shingles installed on typical wood-frame roof panels and subjected to winds with speed up to 257 km/h (160 mph) using an outdoor wind tunnel. The specimens were designed to study the proposed test setup and StereoDIC measurement method for two different combinations of shingle surface coloration (and thus speckle pattern) and field of view. The experiments produced accurate local and full-field shingle deformation measurements. Different datasets were used to gain new insight into the influence of salient aspects of the wind test setup and deformation measurement method on the understanding of the temporal evolution of shingle uplift and failure mechanisms.

Direct spatio-temporal stress field determination combining full-field deformation measurements and explicit finite element method: Concept verification

High strain rate loading of materials with spatially heterogeneous properties produces non-homogeneous and transient stress-strain states. Such stress-strain states are atypical of classical split Hopkinson pressure bar (SHPB) analysis that assumes homogeneous deformation and dynamic force equilibrium. To determine spatio-temporal stress fields in heterogeneous materials, a novel concept combining digital image correlation (DIC) full-field deformation measurements and explicit dynamic finite element method (EFEM) is described in this paper. The concept utilizes measured boundary forces and DIC measurements including both displacements and accelerations as input. The measured input is used to calculate internal forces, and hence the stress fields, within each continuum finite element by solving the governing equations of motion using the framework of EFEM. The DIC+EFEM concept procedure is verified using simulated synthetic data considering one-dimensional stress wave propagation in a bar. The reconstructed axial stress fields match identically with the exact EFEM solution for both homogeneous and heterogeneous materials that are elastic or elastic-plastic. The influence of DIC measurement noise on the reconstructed stresses is also studied.

Compressive response of GFRP and BFRP bars at different temperatures and quasi-static rates

The adoption of fiber-reinforced polymer (FRP) bars in concrete structures has increased due to their superior mechanical properties and durability. However, the compressive response of FRP bars at high temperatures remains unclear. This article presents an experimental study on the behavior of glass FRP (GFRP) and basalt FRP (BFRP) bars subjected to compressive loads at different elevated temperatures and deformation rates. The study considers four different temperature conditions: ambient temperature (25 °C), 50 °C, 100 °C, and 150 °C, and two different deformation rates: 0.5 mm/min and 1000 mm/min. Digital Image Correlation (DIC) was used to accurately measure strains at high temperatures by generating a suitable speckle pattern over the specimen surface for full-field deformation measurements. The study results revealed that the modulus of elasticity and compressive strength of both GFRP and BFRP bars degraded as the temperature increased from 25 °C to 150 °C. The compressive strength reduction was higher at 150 °C compared to 100 °C, and the reduction was more significant for BFRP bars than GFRP bars. The compressive strength of GFRP bars was comparable under both strain rates for all temperature conditions. However, the elastic modulus of the bars showed a slight variation between the different load rates, especially at an elevated temperature of 150 °C. For the BFRP bars, the study found that different strain rates at ambient temperature resulted in significant deviation in the compressive strength and modulus of elasticity results. At higher temperatures, the reduction in mechanical properties was more pronounced. In summary, the study provides valuable insights into the behavior of GFRP and BFRP bars under compressive loads at elevated temperatures, which is useful for designing and assessing the performance of FRP-reinforced structures under fire conditions.

Valorization of Air-Cooled EAF Manganese Slag in Comminution Processes: an Investigation into the Breakage Characterization

In recent years, slag, a residue from pyrometallurgical processes, has become more attractive in circular economy frameworks to increase the efficient use of resources throughout the life cycle of steel products and help in the reduction of carbon emissions. Its applicability is strongly dependent on the particle size, and therefore, the optimization of breaking processes should be approached by increasing the knowledge of the dynamics of slag to promote fracture. Increasing the knowledge on the mechanical response of manganese slag opens up the potential for the development of cost-effective numerical models, e.g., constitutive models based on inverse engineering calibration frameworks or digital twins. In this study, rate-dependent tests of manganese slag have been performed using a split Hopkinson pressure bar device for testing its dynamic mechanical response. In order to obtain information about the crack initiation and fracture process, 2D ultra-high speed imaging was implemented with a sampling frequency of 663,200 fps for diametrically loaded specimens. Full-field deformation measurements using digital image correlation (DIC) techniques showed a staggered fracture process where failure points on mechanical response curves vary due to the internal events happening in the material. Localized frictional occurrences and inertial effects acting inside the pre-cracked matrix have a strong effect on the global mechanical response, and therefore, a great variability of strengths was obtained.

Making use of Digital Image Correlation to identify the true character of the applied load

Digital Image Correlation became one of the dominant optical full-field techniques used in experimental mechanics. It does not require sophisticated equipment to evaluate the full displacement vector field of the specimen under test with great accuracy. In this paper, we would like to put the reader's attention to the DIC technique as a solution that may identify the genuine character of the load applied to the object under test. Our experience with testing wind turbine blades using DIC proves that possibility. Full Text: PDF References E.M.C Jones, M.A. Iadicola, "A good practises guide for digital image correlation" International Digital Image Correlation Society, (2018), CrossRef B. Pan, "Digital image correlation for surface deformation measurement: historical developments, recent advances and future goals", Meas. Sci. Technol. 29, 082001 (2018), CrossRef B. Pan, L.P. Yu, Q.B. Zhang, "Review of single-camera stereo-digital image correlation techniques for full-field 3D shape and deformation measurement", Sci. China Technol. Sci. 61, 2 (2018), CrossRef S. Sharma, N. Iniyan Thiruselvam, S.J. Subramanian, G.S. Kumar, "Computation of strains from stereo digital image correlation using principal component analysis", Meas. Sci. Technol. 32, 105201 (2021), CrossRef IEC 61400-5:2020 Wind energy generation systems - Part 5: Wind turbine, DirectLink IEC 61400-23:2014 Wind turbines - Part 23: Full-scale structural testing of rotor blades, DirectLink

Impact-dynamic properties of aromatic hyperbranched polyester/RTM6 epoxy nanocomposites

Present study investigates the influence of aromatic hyperbranched polyester (AHBP) nanofillers on the impact-dynamic properties of RTM6 epoxy. The strain rate dependence of the energy absorption capacity of neat RTM6 epoxy and RTM6 nanocomposites with filler weight ratios of 0.1%, 1%, and 5% was determined through compression testing. Additionally, static and dynamic tensile tests were performed on these materials. Universal testing machines were used for the quasi-static tests, split Hopkinson bar setups for the high-strain rate experiments. Furthermore, dynamic single-edge notch bending (SENB) tests were conducted to measure fracture toughness using an adapted split Hopkinson compression bar setup. In addition to global deformation measurements, full-field deformation measurements were also performed in all experiments using the digital image correlation (DIC) technique. The results indicate that the energy absorption capacity, tensile strength, fracture strain, and toughness are significantly influenced by both strain rate and filler content of the RTM6 epoxy.

Fabrication of Ultra-Stable and Customized High-Temperature Speckle Patterns Using Air Plasma Spraying and Flexible Speckle Templates.

Reliable and accurate full-field deformation measurements at elevated temperatures using digital image correlation (DIC) require stable and high-contrast high-temperature speckle patterns to be prepared on the sample surface. However, conventional high-temperature speckle patterns fabricated by the existing methods possess several limitations, e.g., easily fail to preserve original pattern features due to the harsh environment and heavily dependent on the operator's experience. In this study, we propose a reliable and reproducible high-temperature speckle fabrication method based on air plasma spraying (APS) and flexible speckle templates. This method involves covering the sample surface with pre-designed speckle templates and then spraying the melted speckle powders onto the specimen surface using an air plasma spray technique to obtain customized speckle patterns. The validity of the proposed method was verified by the speckle fabrication on both planar and curved samples and heating tests with these samples. Experimental results demonstrate that the speckle patterns made by the proposed method adhere well to the sample surface, remain stable during the heating process, and exhibit excellent agreement with the reference values in terms of the thermal expansion coefficients. The proposed method provides a reliable and efficient way to create customized and stable speckle patterns for accurate high-temperature DIC measurements.

In situ measurement of full-field deformation for arc-based directed energy deposition via digital image correlation technology

Deformation measurement and control is a critical challenge in metal additive manufacturing. The metal deposition and solidification occur under the condition of high temperature gradient and large restriction accompany with complex metallurgical phenomena and stress evolutions. High stresses may result in deformation over yield strength of material, or even crack and fracture over the ultimate strength. Thus, it is necessary to monitor the deformation during additive manufacturing process. This paper focus on an in situ measurement of full-field deformation for arc-based directed energy deposition process via digital image correlation (DIC) technology. The measurement qualities were evaluated from the following aspects including system error, speckle pattern, influence of arc light and comparison in 2D-DIC and 3D-DIC. Results indicated that the relative system error of the measured deformation is no more than 0.12 % within a total deformation of 12 mm. Both the artificial speckle and natural speckle can meet the requirement of 2D-DIC system. The image qualities for artificial speckle is higher than that of natural speckle. For 3D-DIC, however, artificial speckle must be applied. The moving arc light will weaken the image qualities, which must be shielded in order to prevent the interference of intense light and the damage of speckle. An L-type shield was proposed and proved to be practical. Finally, DIC measuring was applied in both single-wall and cylindrical AM parts, comparing with numerical simulation. Results indicated that the evolution of the measured deformations showed good agreement with that of numerical simulation. Deformation evolution mechanism was revealed through numerical simulation and the evolution process was divided into three stages. For the cylindrical part, the final outer contour measured by DIC was consistent with that of 3D laser scanning and numerical simulation. As a conclusion, DIC technology was proved to be a feasible approach for in situ monitoring of full-field deformation of arc-based directed energy deposition process.

Leveraging Full-Field Deformation Measurements in Computational Modeling of Damage

Abstract Advances in sensing and nondestructive evaluation methods have increased the interest in developing data-driven modeling and associated computational workflows for model-updating, in relation also to a variety of emerging digital twin applications. In this context, of particular interest in this investigation are transient effects that lead to or are caused by deformation instabilities, typically found in the cases of complex material behavior or interactions between material and geometry. In both cases, deformation localizations are observed which are typically also related to damage effects. This paper describes a novel framework to incorporate deformation data into a finite element model (FEM) that has been formulated using non-local mechanics and is capable of receiving such data and using it to describe the development of localizations. Specifically, experimentally measured full-field displacement data is used as an input in FEM as an ad-hoc boundary condition at any or every node in the body. To achieve this goal, a plasticity model which includes a spatially averaged non-local hardening parameter in the yield criterion is used to account for associated numerical instabilities and mesh dependence. Furthermore, the introduction of a length scale parameter into the constitutive law allows the connection between material behavior, geometry, and localizations. Additional steps remove the experimental data and evolve the computational predictions forward in time. Both one and three-dimensional boundary value problems are used to present results obtained by the proposed framework, while comments are made in terms of its potential uses.

A novel DIC-based methodology for crack identification in a jointed rock mass

Crack identification in brittle rocks is a challenging problem in rock mechanics. To represent the details of jointed rock masses, rock-like specimens containing inclined joint sets were generally prepared by three-dimensional (3D) printing using sand. Digital image correlation (DIC) was employed for full-field deformation measurement during loading. The results show that the sand 3D-printed specimen has distinct advantages in the preparation of jointed rock masses. DIC was extended to measure the displacement vector field around newly propagated cracks. Two types of cracks are recognized: tensile and shear cracks. Two different coalescence patterns are classified: step-path failure and planar failure. A covariance matrix-based multivariate measure, the rate of effective variance change (REVC), was proposed to quantify the dispersion of strain data. The variation in the strain dispersion measure was found to be closely related to the crack type. For the first occurrence of tensile and shear cracks, the rates of mutation (RMs) are within the ranges of 1.56–3.42 and 14.54–86.44, respectively, which can be considered a new crack identification criterion. A virtual extensometer method based on DIC was established to interpret the variation mechanism of the REVC associated with different crack types.

Mirror-assisted multi-view high-speed digital image correlation for dual-surface dynamic deformation measurement

Based on the recently proposed mirror-assisted multi-view digital image correlation (MV-DIC), we establish a cost-effective and easy-to-implement mirror-assisted multi-view high-speed digital image correlation (MVHS-DIC) method and explore its applications for dual-surface full-field dynamic deformation measurement. In contrast to the general requirement of four expensive high-speed cameras for dual-surface dynamic deformation field measurement, the established mirror-assisted MVHS-DIC halves the cost by involving only two synchronized high-speed cameras and two planar mirrors. The two synchronized highspeed cameras can dynamically measure the front and rear surfaces of a sheet sample simultaneously through the reflection of the two mirrors The results on the two surfaces are then transformed into the same coordinate system, leading to the required dual-surface 3D dynamical deformation fields. The effectiveness and accuracy of the established system are validated through modal tests of a cantilever aluminum sheet. The vibration measurement of a drum and dual-surface transient deformation measurement of a smartphone in the drop-collision process further prove its practicability. Benefiting from the attractive advantages of multiview dynamic deformation measurement in a cost-efficient way, the established mirror-assisted MVHS-DIC is expected to encourage more comprehensive dynamic mechanical behavior characterization of regular-sized materials and structures in vibration and impact engineering fields.

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

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