Digital Image Correlation Strain Measurements Research Articles

A multi-level Stress–Strain relationship and hydraulic characteristics for supporting grain-porous rock fracture system

Fracturing technology, essential for enhancing permeability within oil and gas reservoirs, relies on supported particles to maintain fracture conductivity. A comprehensive understanding of the stress-strain relationships and hydraulic properties of Supported Grain-Porous Media Fracture System (SG-PMFS) is crucial for optimizing hydrocarbon extraction and advancing geomechanical modeling. In this study, we introduced a multilevel spring compression deformation model that categorized SG-PMFS deformation into an equivalent tensile component in matrix pores of the far-field region, equivalent compressive components in matrix pores of the near-field region and fractures, and multilevel compressive components in supporting grains. We derived distributed uniaxial/triaxial stress-strain relationships to quantitatively characterize these multilevel deformation behaviors across distinct characteristic regions. Digital image correlation strain measurement was employed for experimental validation, confirming the model's accuracy. The experimental results identified five distinct characteristic regions within SG-PMFS, with the distribution factors of each deformation component decreasing with distance from the fracture. Self-supported grains significantly enhanced the correlation coefficient of experimental data, reducing the fracture-related bulk modulus from 11.99 MPa to 1 MPa. The degree of grain crushing was directly proportional to the bulk modulus of the corresponding multilevel compressive component. Furthermore, we established constitutive relationships between stress and key hydraulic properties: fracture aperture, compressibility, and porosity. The model predictions and experimental results consistently characterized the relationship between hydraulic properties and stress across different regions. The hydraulic behavior of SG-PMFS under stress was segmented into three stages: porous rock fracture-dominated deformation (0–2.2 MPa), first-level particle crushing (2.2–4.6 MPa), and second-level particle crushing (4.6–25 MPa). The contribution of grain crushing to porosity changes consistently exceeded 50%, making it the dominant factor in the porosity variation of SG-PMFS.

Low-cycle fatigue properties and fracture location transition mechanism of dissimilar steel welded joints in towers of wind turbines

This study investigates the low-cycle fatigue (LCF) behavior of two types of dissimilar steel welded joints (DSWJs): AISI 1035 & S550Q and AISI 1020 & S550Q. Digital image correlation (DIC) and finite element analysis (FEA) were used to analyze the heterogeneous strain distribution. The results reveal that the location of maximum strain, which is load-dependent and related to strength mismatch, corresponds to the fracture location of the LCF specimens. Specifically, the crack initiation site shifts from the weld toe to the base metal as strain increases. Moreover, the weld reinforcement of the DSWJs significantly affects the failure site transition, introducing strain concentration at lower stress levels while enhancing deformation resistance at higher stress levels, as evidenced by DIC strain measurements and FEA. These findings highlight the importance of joint reinforcement profile, strength mismatch and external load level in designing DSWJs for wind turbine towers against LCF failure.

Low-cost two-dimensional digital image correlation system

A low-cost digital image correlation (DIC) system for measuring two-dimensional displacement fields is described. Using an off-the-shelf camera, lens, and open-source DIC analysis software, displacement fields in aluminum and steel dog-bone tensile specimens were measured. Correction for out-of-plane motion is used to obtain strains from the displacement fields. Proposed DIC strain measurement system is validated using conventional thin-film strain gauges for strains below 1%. Average strain measurements from the DIC system were found to be within 1% of the measurement obtained using strain gauges. For strains between 1% and 10%, strain histories from the DIC systems displayed excellent correlation with strain measurements obtained using an extensometer.

Strain Measurement during Quasi-Static and Cyclic Loads in AL-6XN Material Using Digital Image Correlation Technique.

A customized digital image correlation (DIC) system was implemented to monitor the strain produced in a cold-rolled AL-6XN stainless steel plate, 3.0 mm thick, subjected to quasi-static and cyclic loading tests. A comparison of the DIC strain measurements was made against those provided by conventional extensometers. Furthermore, the DIC system was used to monitor the fatigue crack initiation in low-cycle fatigue tests. The true stress-strain behavior for the AL-6XN material was properly captured by the DIC measurements. For low-cycle fatigue tests (strain control), the strain mapping generated by DIC allowed for identifying zones with higher strain than the nominal strain amplitude applied (εa) since the first stages of the fatigue life (FL). These zones become potential fatigue crack initiation sites.

Unveiling the enhancing mechanism of cyclic stability in Tb doped Ni-Mn-Sn Heusler alloys

Rare-earth (RE) doping has been demonstrated as a feasible approach to improve the cyclic performance of Ni-Mn-based Heusler alloys, but the current demonstration of underlying mechanism is simply ascribed to strengthening of the grain boundary cohesion. In this work, the effects brought by RE doping on microstructure, stress induced martensitic transformation behavior and cyclic stability of Ni-Mn-based alloys were systematically investigated. By Tb doping, a stable adiabatic temperature change (|ΔTad|) of ∼ 5.5 K over 1000 cycles with no significant degradation was obtained in (Ni43Mn47Sn10)99.5Tb0.5. (1) Through the in-situ digital image correlation strain measurement and in-situ SEM observation upon loading process, it revealed a multipoint nucleation of the martensitic transformation across the specimen arising from the intergranular NiSnTb secondary phase as nucleate sites, which weakens the strain concentration and suppresses the crack tendency. (2) The multipoint nucleation promotes the transformation volume fraction, making the large |ΔTad| could be driven by a small transformation strain (Δεtr) and (Ni43Mn47Sn10)99.5Tb0.5 yields a |ΔTad/Δεtr| up to 8.7 K %−1. Such reduction in applied strain further alleviates the extension of the cracks. Both the two factors brought by Tb doping in (Ni43Mn47Sn10)99.5Tb0.5 give rise to the enhancement in cyclic stability of the large elastocaloric effect.

Digital Image Correlation and Ultrasonic Lamb Waves for the Detection and Prediction of Crack-Type Damage in Fiber-Reinforced Polymer Composite Laminates.

The possibility of using the Digital Image Correlation (DIC) technique, along with Lamb wave analysis, was investigated in this study for damage detection and characterization of polymer carbon fiber (CFRP) composites with the help of numerical modeling. The finite element model (FEM) of the composite specimen with artificial damage was developed in ANSYS and validated by the results of full-field DIC strain measurements. A quantitative analysis of the damage detection capabilities of DIC structure surface strain measurements in the context of different defect sizes, depths, and orientation angles relative to the loading direction was conducted. For Lamb wave analysis, a 2D spatial-temporal spectrum analysis and FEM using ABAQUS software were conducted to investigate the interaction of Lamb waves with the different defects. It was demonstrated that the FEM updating procedure could be used to characterize damage shape and size from the composite structure surface strain field from DIC. DIC defect detection capabilities for different loadings are demonstrated for the CFRP composite. For the identification of any composite defect, its characterization, and possible further monitoring, a methodology based on initial Lamb wave analysis followed by DIC testing is proposed.

Experimental Examination of Additively Manufactured Patterns on Structural Nuclear Materials for Digital Image Correlation Strain Measurements

BackgroundThere are a limited number of commercially available sensors for monitoring the deformation of materials in-situ during harsh environment applications, such as those found in the nuclear and aerospace industries. Such sensing devices, including weldable strain gauges, extensometers, and linear variable differential transformers, can be destructive to material surfaces being investigated and typically require relatively large surface areas to attach (> 10 mm in length). Digital image correlation (DIC) is a viable, non-contact alternative to in-situ strain deformation. However, it often requires implementing artificial patterns using splattering techniques, which are difficult to reproduce.ObjectiveAdditive manufacturing capabilities offer consistent patterns using programmable fabrication methods.MethodsIn this work, a variety of small-scale periodic patterns with different geometries were printed directly on structural nuclear materials (i.e., stainless steel and aluminum tensile specimens) using an aerosol jet printer (AJP). Unlike other additive manufacturing techniques, AJP offers the advantage of materials selection. DIC was used to track and correlate strain to alternative measurement methods during cyclic loading, and tensile tests (up to 1100 µɛ) at room temperature.ResultsThe results confirmed AJP has better control of pattern parameters for small fields of view and facilitate the ability of DIC algorithms to adequately process patterns with periodicity. More specifically, the printed 100 μm spaced dot and 150 μm spaced line patterns provided accurate measurements with a maximum error of less than 2% and 4% on aluminum samples when compared to an extensometer and commercially available strain gauges.ConclusionOur results highlight a new pattern fabrication technique that is form factor friendly for digital image correlation in nuclear applications.

Quality assessment of laser speckle image sequences for digital image correlation by Sequence-image Quality Evaluation Index

Digital Image Correlation (DIC) with laser speckle is effective for measuring strain in extreme conditions, but its limited resistance to disturbances and decorrelation issues limit its broader use. Addressing this, the Sequence-image Quality Evaluation Index (SQEI) has been introduced. SQEI evaluates laser speckle image sequences by considering individual image quality and their correlation. It improves upon previous standards by confirming speckle image quality for DIC accuracy and removing decorrelated images, enhancing DIC's precision and durability. Verified through metal thermal strain experiments, SQEI assesses the impact of factors like laser wavefront, temperature, and CCD angle. The results confirm SQEI's efficacy in evaluating laser speckle quality and guiding the optimization of optical devices, thus significantly boosting the accuracy and efficiency of DIC strain measurement with laser speckles.

Experimental determination of strain in thin aluminum plate with central hole subjected to far-field tensile loading using digital image correlation (DIC)

A flat Aluminum specimen with a geometric discontinuity, which allows testing of the applicability of 2D and 3D Digital Image Correlation Strain measurements, has been considered for this research since it is prone to high stress concentration via Addis Ababa Institute of Technology research interest. Experimental strain using digital image correlation and geometry measurements should be measured with estimated material properties and compare the results with theoretical model predictions. Aluminum plate with central hole were subjected for far field stress in the Machine shop of School of Mechanical and Industrial Engineering at Addis Ababa University in order to test the agreement between DIC’s strain analysis, strain gauge strain analysis and calculated empirical formulas of strain analysis and for the stress distribution of the plate elastically deformed by using VIC-3D and strain gauge. The aim is to measure vertical strain field, Vertical strain along horizontal line through hole center as function of applied loading and vertical strain using theoretical formula along same line as measurements in a plate with central hole subjected to far field and near field tensile loading using visual image coloration (VIC-3D) software, Destensometere device and solid mechanics equations to compare strain results.

Strain evolution during hole expansion testing of 800 MPa tensile strength hot-rolled steels

One limitation of the standardized ISO 16630 hole expansion test is that it provides only one result: limiting hole expansion ratio (HER). In practice, steels with similar HER values can have different cut edge forming behavior due to possible differences in strain localization tendencies. Digital image correlation (DIC) strain measurement during formability testing allows more in-detail analysis of strain-state near the cut edge. In this paper, strain evolution during hole expansion testing was investigated for three 800 MPa tensile strength grade hot-rolled strip steels. The steels were selected to have differences in microstructures and anisotropies of mechanical properties. Two different hole expansion test methods with DIC strain measurement were utilized to investigate different edge loading scenarios: in-plane stretching with a flat-top punch and more out-of-plane stretching with a hemispherical punch. Test holes were prepared according to ISO 16630 standard. In order to examine strain evolution and localization during testing, strains were measured with circle-shape sections around the hole in various distances from the cut edge. Strain localization behavior was investigated in different sheet directions and the effect on the hole expansion ratio was evaluated. Results show considerable differences in the cut edge forming behavior between the investigated materials.

Remaining lifetime assessment of 141,000-hour service-aged forge 91-pipe 91 steel header welds with novel in-situ digital image correlation

Condition assessment of welded steam components is a crucial activity for safe power plant operations. In this work, remaining lifetime of a 141,000-hour service-aged large bore girth weld between an SA-182 forged 91 (F91) reducer and an SA-335 pipe 91 (P91) header was assessed with a specially designed in-situ high temperature digital image correlation (DIC) system. Nonuniform creep deformation/damages in this service-aged multi-pass F91-P91 welds were quantified and characterized. The in-situ DIC strain measurements reveal the nominal creep strain averaged over the gauge length was consistently below 3%, and the localized creep strain value at failure in the HAZ was ∼10%. Advanced metallurgical characterization was executed in the as-received and posttest conditions to determine the contributions of preexisting creep damage, the so-called “soft zone” in the HAZ, and microstructural variations in the HAZ to the observed Type IV failure. Creep failure occurred in the most predamaged region and this location is adjacent to, but not inside, the documented soft zone before the test. The higher fraction of inclusions in F91 steel contributed significantly to the lower creep resistance observed in both base metal and HAZ.

Development of a load frame for neutron diffraction and fluorescent thermometry at cryogenic temperature.

Over the years, rapidly rising interest in the mechanical properties of various materials at low temperatures has been simulated because of the growing cryogenic applications in modern engineering fields of space technology, environmental engineering, and superconductivity engineering. Realizing in situ measurement of the internal strain and the full-field strain and the temperature distribution of related materials in a cryogenic loading environment is a significant requirement for safety assessment and related research of some new large science facility projects. Here, we present a novel cryogenic load frame, which is suitable for neutron scattering measurements of internal stress at the temperature range of 6-300K. The loading capacity is 2500 N, and the slowest loading speed is 0.001 mm/s. By replacing the vacuum chamber sealing plate with a K9 glass window, the in situ digital image correlation strain measurement can be realized. Furthermore, fluorescence thermometry has also been investigated during a heating and cooling process without deformation. Using the present design, some typical results of the 316LN stainless steel and the YBCO tape at low temperatures were introduced.

Characterisation and modelling of micro- and macroscale creep and strain rate sensitivity in Zircaloy-4

Intrinsic crystal slip system properties which control stress, stress relaxation and strain rate sensitivity (SRS) are investigated using combined experimental bend testing, digital image correlation strain measurement and electron backscatter detection with crystal plasticity modelling. Intrinsic properties (activation energy and volume) have been determined from room temperature experiments on Zircaloy-4 which has been shown to creep at 20 °C with non-negligible SRS of ∼0.03. Polycrystal plasticity modelling has been shown to capture DIC-measured intra- and transgranular creep strain and SRS evolution, and intragranular stresses have been calculated and shown to quantitatively reproduce those estimated from differential DIC strain measurements. The modelling also reproduces polycrystal sample-measured stress, stress relaxation and SRS behaviour remarkably well demonstrating consistency from slip system properties to polycrystal strain rate sensitivity. Polycrystal plasticity finally predicts diffraction-measured effects of texture and slip system activation on lattice strain rate sensitivities.

Behavior of steel girder bridges damaged by vehicular collision

Vehicles often collide with girders of low-clearance bridges, yet there is a knowledge gap in understanding the behavior of damaged steel girders. This paper addresses this gap by performing non-destructive field testing on four damaged steel girder bridges. The focus is on two- and three-span continuous multi-girder steel bridges for which an exterior girder has sustained Category T damage, i.e., torsion about the longitudinal direction. The behavior of damaged girders, symmetric undamaged girders, and adjacent girders under quasi-static truck loading is monitored using digital image correlation (DIC). Specifically, the measured strains of damaged girders are compared to the strains of corresponding symmetric undamaged girders to investigate the effect of the damage on behavior for each bridge. Strain gauges were used to monitor the behavior of one bridge under quasi-static truck loading, as well as speed and crawl tests. The strain gauge readings are also used to validate DIC strain measurements for field monitoring of bridges. The measured strains are used to validate finite element (FE) numerical models. A parametric study investigates the effect of varying shapes and locations of damage on the load-deflection behavior of a single girder as well as two- and three-span continuous bridges. Overall, this study found that measured strains in all girders are less than 200 microstrain or approximately 40.0 MPa (5.80 ksi). Results indicate that vehicular damage may affect composite behavior, bridge railings participate in carrying live load, damaged girders may have increased live load amplification factors, and damage at the center of a span with a large rotation angle of the web results in the greatest loss of stiffness.

A novel specimen shape for measurement of linear strain fields by means of digital image correlation

Strains on the surface of engineering structures or biological tissues are non-homogeneous. These strain fields can be captured by means of Digital Image Correlation (DIC). However, DIC strain field measurements are prone to noise and filtering of these fields influences measured strain gradients. This study aims to design a novel tensile test specimen showing two linear gradients, to measure full-field linear strain measurements on the surface of test specimens, and to investigate the accuracy of DIC strain measurements globally (full-field) and locally (strain gauges’ positions), with and without filtering of the DIC strain fields. Three materials were employed for this study: aluminium, polymer, and bovine bone. Normalized strain gradients were introduced that are load independent and evaluated at two local positions showing 3.6 and 6.9% strain change per mm. Such levels are typically found in human bones. At these two positions, two strain gauges were applied to check the experimental strain magnitudes. A third strain gauge was applied to measure the strain in a neutral position showing no gradient. The accuracy of the DIC field measurement was evaluated at two deformation stages (at approx 500 and 1750 μstrain) using the root mean square error (RMSE). The RMSE over the two linear strain fields was less than 500 μstrain for both deformation stages and all materials. Gaussian low-pass filter (LPF) reduced the DIC noise between 25% and 64% on average. As well, filtering improved the accuracy of the local normalized strain gradients measurements with relative difference less than 20% and 12% for the high- and low-gradient, respectively. In summary, a novel specimen shape and methodological approach are presented which are useful for evaluating and improving the accuracy of the DIC measurement where non-homogeneous strain fields are expected such as on bone tissue due to their hierarchical structure.

Crystallography and elastic anisotropy in fatigue crack nucleation at nickel alloy twin boundaries

Fatigue crack nucleation at annealing twin boundaries (TBs) within polycrystal nickel-based superalloy René88DT is investigated with a microstructure-sensitive crystal plasticity (CP) model, digital image correlation strain measurements and experimental SEM crack nucleation observations. Strong slip localizations at TBs were experimentally observed and predicted by the CP model, which also showed high predicted geometrically necessary dislocation and corresponding stored energy densities, capturing experimental observations of crack nucleation. In a systematic study, elastic anisotropy was found to drive local elastic constraint and hence resolved shear stress, slip activation, GND density and stored energy density, demonstrating for this reason that TBs are preferential sites for crack nucleation in this alloy. The parent grain / twin pair crystallographic orientation with respect to remote loading was also demonstrated to be key to slip activation parallel to TBs and hence to stored energy density and fatigue crack nucleation, and the range of most damaging parent grain orientations has been identified.

High-temperature digital image correlation techniques for full-field strain and crack length measurement on ceramics at 1200°C: Optimization of speckle pattern and uncertainty assessment

High-temperature mechanical tests coupled with Digital Image Correlation (DIC) on ceramics which exhibit rather low level of strain require to overcome extreme experimental conditions that usually can reduce significantly measurement accuracy. Thermal resistance of speckle pattern, black body radiation and heat haze are three main concerns, which should thus be taken into account while designing a high-temperature image acquisition setup. In this aim, an experimental procedure has been specifically designed in order to minimize the three above-mentioned disturbances. The main objective of this study is to select a suitable high-temperature resistant speckle pattern for mechanical characterization at 1200 °C (or above) on refractory ceramics. Most of tested speckle patterns were performed with white alumina adhesive and dark ceramic grains (silicon carbide or brown fused alumina). Different grain sizes of silicon carbide were tested. At first, different speckle patterns are compared in terms of DIC strain measurement uncertainty by discussing speckle features, some main DIC parameters and two image pre-treatments (low pass filter, image size reduction). Then, these speckle patterns are tested to analyse fracture behaviour of refractories through a Brazilian test. An enhanced digital image correlation technique (2P-DIC), dedicated to monitor the fracture behaviour, is applied to study the evolution of crack length. The best representation of crack progression has been achieved for sample surface covered with a fine SiC powder ranging from 50 to 100 µm. It is then possible to compare the fracture behaviour between 1200 °C and 20 °C and to show that the refractory exhibits more crack branching at 1200 °C in comparison with behaviour at room temperature.

Speckle pattern creation methods for two‐dimensional digital image correlation strain measurements applied to mechanical tensile tests up to 700°C

AbstractThe purpose of this study is to develop novel speckle pattern techniques for digital image correlation (DIC) kinematic measurements of mechanical tests at high temperatures, typically from 400 to 700°C. In this context, the speckle pattern should not only meet morphological criteria (size, density, distance) in order to improve spatial resolution, but it should also present a high contrast and resist high temperature and strain levels. To find a speckle pattern matching these specifications, a comparison was performed on six types of speckle made using different techniques. First, a computer‐generated speckle pattern that meets DIC criteria was numerically designed to produce six types of speckle pattern. Next, the speckle patterns produced using these six techniques were compared in terms of speckle morphology, image quality and adherence to titanium alloy TA6V material at high temperatures. From 25 to 600°C, the speckle pattern made by the technique combining anodisation and laser engraving named M5 technique gave the best contrast (highest value of mean intensity gradient [MIG] and Shannon entropy value) and the adherence of 200% of strain measurements to the TA6V material. At 700°C, speckle image quality is considerably reduced due to oxidation of the titanium alloy, and this may not be suitable for DIC measurements. Only the speckles produced by painting in which the paint plays a protective role provide with a better speckle contrast compared with other techniques. However, these speckle patterns enable only a strain measurement of 22% by the DIC method. This article concludes with guidelines for producing a speckle pattern suitable for high‐temperature mechanical tests.

Super Resolution Digital Image Correlation (SR-DIC): an Alternative to Image Stitching at High Magnifications

BackgroundHigh-resolution Digital Image Correlation (DIC) measurements have previously been produced by stitching of neighboring images, which often requires short working distances. Separately, the image processing community has developed super resolution (SR) imaging techniques, which improve resolution by combining multiple overlapping images.ObjectiveThis work investigates the novel pairing of super resolution with digital image correlation, as an alternative method to produce high-resolution full-field strain measurements.MethodsFirst, an image reconstruction test is performed, comparing the ability of three previously published SR algorithms to replicate a high-resolution image. Second, an applied translation is compared against DIC measurement using both low- and super-resolution images. Third, a ring sample is mechanically deformed and DIC strain measurements from low- and super-resolution images are compared.ResultsSR measurements show improvements compared to low-resolution images, although they do not perfectly replicate the high-resolution image. SR-DIC demonstrates reduced error and improved confidence in measuring rigid body translation when compared to low resolution alternatives, and it also shows improvement in spatial resolution for strain measurements of ring deformation.ConclusionsSuper resolution imaging can be effectively paired with Digital Image Correlation, offering improved spatial resolution, reduced error, and increased measurement confidence.

Split-disk test with 3D Digital Image Correlation strain measurement for filament wound composites

The paper investigates the split-disk test methodology for measuring mechanical properties of a filament wound composite. Hoop strain is measured by three-dimensional (3D) Digital Image Correlation (DIC) on the outer surface of ring specimens. A finite element model (FEM) is employed to investigate the strain distribution along the circumference and the thickness of the ring and the effect of friction between the ring and the disk. The FE simulation shows that the effect of friction between the ring and the disk is significant, but precise quantification is difficult because of irregularities of the ring/disk contact. Both DIC strain map and FE simulation show a low strain area at the level of the split, caused by local bending of the ring. In conclusion of this work, the authors recommend: (1) to measure the strain, which is used in the stiffness calculation in the area with a 10° offset from the disk split, where the influence of the local bending becomes insignificant, (2) to use 3D DIC for precise hoop strain measurement, (2) to use concurrent FEM to estimate errors related to the difference between the surface strains and the average strains over the entire ring thickness.