Digital Speckle Correlation Method Research Articles

Detection method for contact stress distribution of tapered roller bearings

The axle box of high-speed train adopts double row tapered roller bearings as transmission parts, and the reliability of bearings directly affects the safety of train operation. Tapered roller bearings can withstand axial and radial loads, their service life being closely related to the distribution of contact stress. The test object is the axle box bearing of high-speed train. According to bearing’s structural characteristics, based on the digital speckle correlation method (DSCM) and machine vision, the contact stress distribution detection devices for rollers and complete sets of bearings are developed respectively and an effective detection method is proposed. The contact stress data of the bearing which has reached the service life and the new bearing under the condition of no lubrication and grease lubrication are collected and normalized. Through the geometric relationship, based on the measured data of the selected detection points, the normal and tangential contact stress distribution of the roller and raceway under different contact conditions is obtained by data fitting. The test can be used as an evaluation basis for the effectiveness of bearing modification and provide reference for bearing design.

Preparation of alkali-activated nickel slag-based cemented tailings backfill: Workability, strength characteristics, localized deformation and hydration mechanism

To solve the problems of high magnesium nickel slag (HMNS) and tailing stacks with large storage and environmental pollution, alkali-activated HMNS-based cementitious materials were prepared by using NaOH, phosphogypsum (PG), HMNS and ground granulated blast furnace slag (GGBFS), and cemented tailings backfill (CTB) was prepared using tailings as aggregates. The workability, strength characteristics, localized deformation and hydration mechanism of CTB were studied. Based on the digital speckle correlation method (DSCM), the localized deformation of CTB was analyzed. The hydration mechanism of CTB was analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetry-differential scanning calorimetry (TGDSC), mercury intrusion porosimetry (MIP) and scanning electron microscopy and energy dispersive spectroscopy (SEMEDS). The results showed that the optimal mix proportions for the cementitious material were 8%, 30%, 6%, and 0.38 for PG content, GGBFS content, alkali dosage, and water/binder ratio, respectively. The best performance of CTB was achieved when the bone glue ratio (BGR) and mass concentration (MC) were 5:1 and 79%, respectively; the slump, slump flow, and consistency of CTB decreased continuously with increasing BGR and MC; the layering degree, bleeding rate, and linear shrinkage of CTB continuously increased with increasing BGR, and decreased with increasing MC; the unconfined compressive strength of CTB increased with increasing maintenance age and MC and decreased with increasing BGR; when the BGR was small (3:1), CTB exhibited shear failure, and when the BGR was large (6:1), CTB exhibited tensile failure. The main hydration products of CTB were C-S-H gel, M-S-H gel, ettringite (AFt), Ca(OH)2 and CaCO3. In the early stage of curing, CTB has more pores, fewer hydration products and a relatively loose internal structure, flocculation, sheet, network and needle rod hydration products coated tailing particles, porosity decreased, and CTB became dense.

Experimental and Simulation Study of the Fracture Instability Behavior in Polypropylene Fiber-Reinforced Concrete.

This study investigated the fracture characteristics of plain concrete and polypropylene fiber-reinforced concrete (PFRC) using pre-notched three-point bending beam tests with the digital speckle correlation method (DSCM). Then, the fracture instability behavior of the two types of beams was simulated in finite elements based on the plastic damage model and the cohesion model, for which the applicability was assessed. Furthermore, the stability of the Big Gang Mountain Dam made from plain concrete or PFRC subjected to the earth-quake loading was simulated with the plastic damage model. The results show that the limiting length of the non-local deformation zone can be used as an indicator of instability damage in a concrete structure. The simulation results of the plastic damage model agreed well with the local deformation in the pre-notched three-point bending beam test obtained from the DSCM. The plastic damage model was found to be capable of describing the residual strength phenomenon, which the cohesive model was not capable of. The damage evolution regions of the PFRC dam are strictly constrained in some regions without the occurrence of the local deformation band across the dam, and PFRC can dramatically reduce the failure risk under earthquake loading. The numerical solution proves that PFRC is an advisable material for avoiding failure in concrete dams.

Dynamic mechanical properties, interface structure evolution and deformation behaviors of PVA-carbon fiber reinforced concrete with negative Poisson’s ratio design

Based on the advantages of negative Poisson's ratio (NPR) materials in controlling deformation and absorbing energy, this paper combines NPR design with the preparation of fiber reinforced concrete (FRC) to optimize the anti-dynamic load performance of FRC. In this paper, a helical auxetic yarn (Hay) with NPR effect is obtained by wrapping carbon fiber (CF) on polyvinyl alcohol fiber (PVAF) at a certain angle, and it is mixed into concrete in different forms to obtain auxetic fiber concrete. In order to explore the influence of auxetic fiber on the anti-dynamic load performance of concrete, split Hopkinson pressure bar (SHPB) test and dropping ball impact test are carried out on different FRC. The impact resistance of FRC under different impact velocities is tested, and the impact resistance behavior of FRC is analyzed by digital speckle correlation method (DSCM). The results show that in SHPB test, the anti-impact strength of the layered auxetic fiber concrete is 19.1 % higher than that of the plain concrete (PC), and its fracture energy is increased by 25.8 %. In the dropping ball impact test, its fracture energy is 5 ∼ 9 times that of PC. In addition, the results of scanning electron microscopy (SEM) and DSCM test show that the incorporation of auxetic fiber has a positive effect on improving the generation and development of internal cracks in concrete, which is expected to provide a reference for the application of NPR materials in explosion resistance, penetration resistance and impact resistance of building structures.

Investigation of the crack evolution characteristics of coal and rock bodies around boreholes during progressive damage based on stress threshold values

The stress threshold is a key stress indicator of crack evolution of coal and rock bodies around boreholes during progressive damage. To investigate the crack evolution characteristics of coal and rock bodies around boreholes, digital speckle correlation method tests were conducted on intact specimens, specimens with boreholes, and specimens with grouted boreholes, and the characteristic parameters, such as the stress threshold, surface deformation field, crack opening and shear, and strain energy density, were quantitatively analyzed. The results show that (1) the distribution characteristics of surface cracks on the specimen are affected by the borehole structure and grouting; (2) the crack displacement evolution of coal and rock specimens remain basically stable at the initial stage of loading. In the middle and later stages, the cracks are continuously opening and shearing, which directly leads to the damage of the specimen; (3) the energy accumulation and release law of coal and rock bodies is related to the evolution of localization zone. In different areas outside the localization zone, the strain energy density shows different characteristics; (4) the tensile crack is the main reason for the destruction of coal and rock bodies around boreholes. During the progressive damage, the tensile cracks at the upper and lower ends of specimen experience the process of opening, closing and reopening. The presence of cohesion g(x) reduces the crack tensile stress intensity factor, so that the crack is in a balanced state before penetration, which is conducive to the connection and penetration of other cracks. The results provide theoretical guidance for the optimization design of gas extraction boreholes construction and grouting.

Lightweight and edge-preserving speckle matching network for precise single-shot 3D shape measurement

Single-shot three-dimensional (3D) shape measurement method based on digital speckle correlation (DSC) has high measurement precision and only requires one pair of speckle images. Nevertheless, the matching of speckles is time-consuming and may lead to some data defects at large gradient variations and edge regions. This paper proposes an end-to-end speckle matching network to achieve fast, precise, and edge-preserved 3D measurement. Compared with other networks, the proposed network reduces the redundant channels in the Siamese feature-extraction subnetwork and constructs cost volume by group-wise correlation, making it possible to provide efficient representations at low computational consumption, thus being suitable for high-precision 3D measurement in large resolution. A lightweight 3D stacked hourglass subnetwork is used to aggregate cost information and furthermore, an edge refinement module is integrated by leveraging the left speckle image as a guide to regular edge details. A reliable speckle binocular dataset is prepared with a specialized device in a simple procedure and the labels are produced from the DSC method. Experiments demonstrate that the proposed network can produce dense disparity maps with subpixel precision and reduce single-shot inference time to 0.13 s with less GPU occupation, which is a significant improvement compared with the traditional DSC method and other learning-based methods. Moreover, the experiment validates that measurement precision of the reconstructed plane can reach less than 0.03 mm, meeting the requirements of most industrial applications.

Fatigue Properties and Damage Characteristics of Polyurethane Mixtures under a Stress Control Mode

A polyurethane mixture (PUM) is an energy-saving and emission-reducing pavement material with excellent temperature stability; however, the fatigue properties and fatigue damage models of PUM still require further research. Therefore, four-point bending static load tests, fatigue tests, and digital speckle correlation method (DSCM) tests with different load levels of PUM and styrene butadiene styrene (SBS)-modified asphalt mixture (SMA) were carried out. The fatigue life, stiffness, midspan deflection, and maximum tensile strain were obtained and compared. The fatigue damage factor calculation method of PUM based on stiffness degradation was proposed, and the fatigue damage function of PUM at different load levels was fitted. The results show that the fatigue life of PUM was much larger than that of SMA, and the static loading failure and fatigue failure modes of PUM were both brittle. The fatigue damage of PUM exhibits an obvious three-stage damage law: the rapid development stage (accounting for about 10–20% of the fatigue life), the deformation stability expansion stage (accounting for about 70–80% of the fatigue life), and the instability development stage (accounting for about 10–20% of the fatigue life). The fatigue damage factors (DB) were calculated based on stiffness, according to DB=EI0−EInrEI0−EINr, and the fatigue damage functions of PUM were fitted based on the stiffness degradation, according to fnN=1−1−(n/N)a(1−n/N)b. The fatigue damage fitting curves have good correlation with the calculation results of the damage factor based on test data, which can predict the stiffness degradation of PUM at different load levels. The results can help further the understanding of the fatigue characteristics and damage mechanism of PUM, which will provide theoretical support for the application of PUM in pavement structures.

Study on dynamic splitting tensile damage characteristics of basalt fiber reinforced concrete based on AE and DSCM

This paper aims to study the action mechanism of basalt fiber on concrete and the internal damage evolution of basalt fiber reinforced concrete (BFRC) in Brazilian disc splitting tensile tests using acoustic emission (AE) technology and digital speckle correlation method (DSCM). The effects of loading rate and basalt fiber content on splitting tensile failure mode and deformation capacity of BFRC, as well as AE frequency domain characteristics were investigated herein. Then the strain field and local damage factor of BFRC under dynamic splitting tensile load were analyzed based on DSCM. The results indicate that the increase of loading rate and basalt fiber content can change the failure into a complex tensile-shear mixed failure. And the deformation capacity of BFRC can be considerably enhanced by adding a proper amount of basalt fibers. It is also found that the damage degree and damage mechanism of BFRC can be identified by analyzing the wavelet peak frequency of specific frequency range. Furthermore, the crack evolution of BFRC under dynamic splitting tensile load can be reflected well on the change of strain field nephogram of BFRC obtained by DSCM.

Mechanical behavior and self-healing mechanism of force-chloride ion triggered double-walled microcapsule/cement-based composites

The force-chloride ion triggered microcapsules with PbSO4 embedded on the wall material were successfully synthesized by a two-step interfacial polymerization method. The structure and properties of the microcapsules were characterized, and the self-healing properties of cementitious composites were studied from the aspect of mechanical properties. The results showed that the microcapsules synthesized are suitable in morphology, the average particle size being 53.8 μm. Fourier Transform Infrared Spectrometer (FT-IR) showed that the structure of the microcapsule with double-core and double-walled has been formed. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) confirmed that PbSO4 was successfully embedded on the capsule wall, and the microcapsules were suitable in the performance triggered by force-chloride ion. Moreover, the strength recovery rate of cementitious composites with 3% microcapsules could gain up to 84%. The microcapsules can survive in the cement mixing process and be evenly distributed in the matrix. Digital Speckle Correlation Method (DSCM) was used to track the deformation behaviors of self-healing cementitious composites during the loading procedure, obtaining the self-healing mechanism based on filling cracks, limiting cracks’ development, and healing cracks from the variation of stress–strain, distribution of strain field and eigenvalue StC and Sts.

Experimental study on the deformation localisation and acoustic emission characteristics of coal in Brazilian splitting tests

This paper presents an experimental study to assess the behaviour of coal samples under tensile loadings to better understand the failure mechanisms and the interactions with the coal characteristics. A set of Brazilian splitting tests were carried out using disk specimens obtained from Tashan Coal Mine in China. The digital speckle correlation method and acoustic emission (AE) were used to capture the deformation localisation and AE characteristics of each specimen during the loading process. The precursor characteristics of AE and the failure mechanism are discussed. It was found that the entire loading process mainly consists of compaction, elastic and post-peak dropping stage without an obvious yielding stage. Two kinds of deformation localisation were observed: central symmetry and axis symmetry. The corresponding AE evolution patterns have different phases, including gradual rise, step rise, transient rise and steady rise. During the subcritical failure stage, AE counts demonstrate a “rapidly increasing + flatten” intermittent feature. The results provide a reference for a better understanding of the damage process of the brittle coal material and its application in ground control design.

Mechanical properties of fibre reinforced seawater sea-sand recycled aggregate concrete under axial compression

This study investigates the mechanical properties of fibre reinforced seawater sea-sand recycled aggregate concrete (SSRAC). A total of 360 concrete specimens, considering four different variables—the type of concrete (SSRAC, recycled aggregate concrete and ordinary concrete), age (day 7, 14 and 28), fibre type (polypropylene and stainless steel fibres) and content—were fabricated. The macro properties and mechanical mechanism of discrete fibre reinforced SSRAC were systematically studied through a novel experimental system involving axial compression test and digital speckle correlation method (DSCM). The failure pattern, strength, ductility, stress–strain curve, deformation filed, crack initiation and propagation of fibre reinforced SSRAC were analysed. The major research findings indicate that the stainless steel fibres (SSF) could effectively offset the effects of seawater, sea-sand and recycled aggregates on the mechanical properties of concrete compared to polypropylene fibres (PF). The failure pattern of specimen slightly changed with the fibre type used. The compressive and splitting tensile strengths of SSRAC increased with an increase in PF content, while the optimal SSF content for SSRAC was 1.0% by volume content of concrete. Moreover, the typical influence of PF on the elastic modulus of SSRAC was negative but SSF could greatly improve that of specimen. The axial stress–strain curve of fibre reinforced SSRAC consisted of elastic, elastic–plastic ascending and declining stages. Increasing SSF and PF contents decreased the curve curvature while increased the ductility and peak strain, and the descending branch of curve tended to be smooth. Generally, the effects of fibres on the crack initiation of SSRAC and the distributions of deformation field and cracks were negligible compared to coarse aggregates, while the crack propagation of SSRAC significantly changed with the variations in fibre type and content. The cracks and high-value strain of SSF reinforced SSRAC developed slowly compared to plain SSRAC and PF reinforced SSRAC. Finally, an analytical stress–strain model of fibre reinforced SSRAC considering the coupled effects of various variables were proposed, which can be used to conduct a theoretical evaluation and design of the marine structures.

Improved Search Algorithm of Digital Speckle Pattern Based on PSO and IC-GN

Digital speckle correlation method has not only been widely used in a variety of photometric mechanical scenarios, but also integrated with multiple disciplines. In the future, it will even be inextricably linked to the Internet of Things, autonomous driving, deep learning and other fields. For a given hardware condition, it is of great significance to improve the efficiency of integer-pixel search and increase the accuracy and efficiency of the sub-pixel algorithm. In this paper, we propose an improved digital speckle correlation method, which consists of an integer-pixel search algorithm and a sub-pixel search algorithm. With respect to the integer-pixel search, aiming to address the two problems of uniqueness of maximum value and parameter setting of PSO-W algorithm, the algorithm PSO-1 is proposed, and the results of comparison experiments show that it has higher search efficiency. In terms of sub-pixels, based on IC-GN algorithm with the highest accuracy at present, the IV-ICGN algorithm is proposed, and the simulation experiment results show that the proposed algorithm has higher accuracy and higher efficiency than the comparison algorithm.

Research on fatigue performance of composite crumb rubber modified asphalt mixture under freeze thaw cycles

To study the effect of freeze thaw cycles on the fatigue damage characteristics of composite crumb rubber modified asphalt mixture (CCRMA), the semicircle bending fatigue test and the digital speckle correlation method (DSCM) were combined and the fatigue damage characteristics of CCRMA and styrene–butadienestyrene-modified asphalt mixture (SBSMA) under freeze thaw cycles were analyzed. The plateau valve (PV) of the stiffness modulus degradation ratio curve of the asphalt mixture is proposed as the analysis index to evaluate the macroscopic fatigue damage evolution of the asphalt mixture. The horizontal strain density (DE value) obtained through analyzing the horizontal strain characteristic cloud map of the main crack zone is identified as an index to evaluate the mesoscopic fatigue damage evolution of the asphalt mixture. The results show that after a freeze thaw cycle, the PV increases, DE decreases, fatigue damage rate accelerates, and fatigue resistance decreases. CCRMA has better freeze thaw resistance and fatigue resistance than SBSMA. The fatigue damage model established has high fitting accuracy and can effectively reflect the fatigue damage evolution degree of an asphalt mixture. A correlation analysis of PV and DE values reveals that there is a good linear correlation between them, indicating that applying the DSCM method to semicircle bending fatigue tests for studying the fatigue damage characteristics of asphalt mixtures is reliable.

A New Digital Speckle Correlation Method for Studying the Dynamic Fracture Parameters of Rocks under an Impact Load

A New Digital Speckle Correlation Method for Studying the Dynamic Fracture Parameters of Rocks under an Impact Load

Experimental Investigation on Uniaxial Compression Mechanical Behavior and Damage Evolution of Pre-Damaged Granite after Cyclic Loading

Failure behavior of pillars in deep mines is affected by various cyclic loads that cause initial pre-damage. Pillars will be further damaged and developed in the long-term compressive stress until they are destroyed. To reveal the strength characteristics and crack damage fracture laws after rock pre-damage, uniaxial compression tests were carried out on granite specimens damaged by cyclic loading using the digital speckle correlation method. The experimental results indicate that the mechanical properties of pre-damaged specimens show large damage differences for different cycles. The damage variable of the pre-damaged specimens increases with the increase of cycle number and confining pressure. The damage of specimens is primarily due to the strength weakening effect caused by cycle numbers, and the confining pressure restriction effect is not obvious. The evolution laws of uniaxial compression damage propagation in the pre-damaged specimens show differences and obvious localization phenomenon. Pre-damaged specimens experienced three failure modes in the uniaxial compression test, namely tensile shear failure (Mode I), quasi-coplanar shear failure (Mode II), and stepped path failure (Mode III), and under different pre-damage stress environments with high confining pressures, the failure modes are dominated by Mode II and Mode III, respectively.

Three-dimensional measurement method for pavement texture depth using an optimized digital speckle correlation method

Motivated by the low efficiency and poor accuracy of the existing techniques for measuring the depth of a pavement structure, we examine the characteristics of laser speckle and speckle measurement methods in relation to the depth of a pavement structure and propose a set of methods for the 3D measurement of such structures based on laser speckle and digital image processing. First, we analyze the relevant characteristics of laser speckle measurement, and then based on the relevant region and temporal sequence correlation, a faster digital speckle correlation method is used to establish a mathematical relationship between the 3D image and the structural depth. The erosion and dilation technique and mean optimization algorithm are proposed to eliminate the noise and burr of primeval 3D reconstruction. After this, a 3D point cloud of information on the depth of the pavement structure is obtained. Finally, through fitting, a good linear relationship was found between the texture depth obtained by the sand paving method and the system. From measurements using a laboratory calibration platform, it was found that the accuracy of measurement of our method can reach the subpixel level, and the running speed is high, meaning that it can be used for highly efficient and accurate detection of the depth of a road structure.

Kinetics of Plastic Deformation Localization Bands in Polycrystalline Nickel

Jerky flow has recently aroused interest as an example of complex spatiotemporal dynamics resulting from the collective behavior of defects in Al- and Mg-based alloys under loading. This paper presents the results of the study of the macroscopic strain localization kinetics in Nickel 200 (99.5 wt % purity). Uniaxial tension of flat samples is monitored at room temperature in the load–unload mode at a constant strain rate and total deformation increment up to 5%. The stress–strain curves reveal jerky flow from the yield point to the formation of the neck. The digital speckle correlation method evidences the movement of localized plastic deformation bands under the conditions of the Portevin–Le Chatelier effect (PLC). It is shown that stress drops during jerky flow in Ni are accompanied by the formation of morphologically simple single PLC bands. It is established that, with an increase in total deformation, the number of PLC bands and their velocity of motion along the sample decrease, while their time period increases. Moreover, an increase in total deformation leads to an increase in the parameters of the force response (i.e., time period and stress drop magnitude). It is found that the criterion of damage for PLC bands as a function of the total strain has a sigmoidal shape.

Correlation-Based Out-of-Plane Displacement Measurement for Optical Fiber Material

Due to the monitoring requirement of optical fiber industrial production, an out-of-plane displacement measurement method is proposed. Firstly, the in-plane displacements between two consecutive images, captured through a microscope with a CCD camera, are estimated by the Digital Speckle Correlation Method (DSCM). Subsequently, the out-of-plane displacement of optical fiber material can be obtained by the wedge model. Finally, the effectiveness of the method is verified experimentally by comparing the measurement data of routine practice with its theoretical values. Simulation and experimental results indicate that the absolute errors and the relative errors of the measurement by the optical microscope with a magnification of 50× are less than ±0.2 μm and 5%, respectively. The new method only needs two images obtained by the microscopic imaging system with a single camera to accomplish the measurement, which can significantly reduce the measurement time and complexity of the arrangement. Further, the method needs neither frequency domain conversion nor phase unwrapping operation, therefore, it is especially suitable for dynamic out-of-plane displacement measurement. The proposed method has been applied to the industrial uniformity and micro–nano-scale deformation monitoring of optical fiber image transmission materials.

Study on Determination Method of Rock Tensile Modulus of Brazilian Disc Splitting Test Based on Digital Speckle Correlation Method (DSCM)

Study on Determination Method of Rock Tensile Modulus of Brazilian Disc Splitting Test Based on Digital Speckle Correlation Method (DSCM)

Effects of bedding planes on the fracture characteristics of coal under dynamic loading

To investigate the influence of bedding planes on its fracture characteristics under dynamic loading, testing and numerical simulations are carried out on semi-circular bend specimens of coal. It is shown that the fracture load and dynamic initiation fracture toughness decrease as the increase of bedding-plane angle. The crack propagation direction is jointly controlled by the maximum principal stress and bedding planes. Based on the digital speckle correlation method, it is found out that, due to stress concentration, strain at the initial loading stage concentrates around crack tip. After crack initiation, there is a specific strain gradient with a candle flame-like shape on the surface of a specimen. The opening displacements at crack tip can be divided into stable and linearly increasing phases. Further, a continuum-based discrete element method is applied to virtually reproduce these fracture characteristics, which are instructive to study dynamic anisotropy in fracture of coal.