Digital Image Correlation Results Research Articles

Study on the effect of heating rate and temperature on the fracture properties of flawed granite

The effects of heating rate and temperature on fracture characteristics and mechanical properties of flawed granite were studied by using the cracked straight-through Brazilian disc specimen. Four temperature groups were established, and the specimens were heated in a furnace with different heating rates. After thermal treatment, the specimens were conducted to static compression tests, the compression process was recorded by a camera and the deformation on the specimen surface was calculated by the digital image correlation (DIC) technology. A microscope which can magnify nine times was employed to observe the surface of specimen and the transgranular or intergranular fracture. The results show that: at the same temperature, the thermal cracks increase as the heating rate increases. Within the same temperature group, the mode-I fracture toughness of granite shows a gradual decay characteristic as the rate of heating increases. Due to the DIC results, at the same temperature, when the heating rate increases, the maximum principal strain and the crack tip opening displacement (CTOD) of crack tip show an increasing trend at fracture. At 200 ℃, the maximum principal strain in the range of 0.0781% ∼ 0.237 %, the effect of heating rate is distinct. At 200–400 ℃, the input energy and release energy by fracture decrease with the heating rate. In the same temperature group, the specimen heating rate of 20 ℃/min has more distinct ductile characteristics.

Aggregate interlock and effective strain of FRP-strengthened flanged RC members subjected to torsion: Experimental evaluation and analytical modeling

The effective strain of externally bonded FRP strengthening systems and the loss of concrete aggregate interlock are critical factors for predicting strength and developing design models for structural strengthening. Current research on FRP strengthening of reinforced concrete (RC) members primarily focuses on flexural, shear, and axial loading, neglecting torsional systems. This study introduces an innovative analytical-experimental model/formulation predicting FRP effective strain and concrete strain, based on Digital Image Correlation (DIC) analysis outcomes of an experimental study on RC beams strengthened with FRP sheets under torsion. Eight flanged RC beams, strengthened with various FRP ratios, configurations, and installation methods were also systematically tested. The findings emphasize the need for reevaluation of strain limits in existing guidelines. It points out that the shear integrity of concrete is influenced by concrete compressive strength and strengthening scheme. The DIC results suggest that the strain limit for concrete surfaces between FRP strips should be in the range of 0.0055–0.008, as opposed to the limit of 0.004 set by ACI 440.2R-17. The use of the grooving method in certain systems also requires an increase factor of 1.65 and 1.45 for the effective strain of transverse and longitudinal FRP sheets.

Comparative study on fracture evolution in steel fibre and bar reinforced concrete beams using acoustic emission and digital image correlation techniques

In recent decades, the demand for sustainable construction practices has increased, but raw materials such as reinforcing steel remain scarce. Therefore, steel fibres have emerged as a popular and sustainable choice in the construction industry, offering a cost-effective alternative to traditional steel bar reinforcement for both flatwork and elevated structures. The purpose of this study is therefore to compare the performance of fracture behaviour between steel fibre-reinforced concrete (SFRC) and steel bar-reinforced concrete (SBRC) beams subjected to three-point bending. The fracture process was monitored by using two non-invasive techniques: acoustic emission (AE) and digital image correlation (DIC). The damage level was identified by characterizing the parameter-based AE data such as hit rate, energy release, count, rise time, amplitude, and signal strength. DIC images were employed to visualise the crack propagation in parallel with the AE data. To further understand the fracture characteristics, the integration of 2D source localization of AE events (based on local AE fracture energies) with DIC results was investigated. The parameter-based AE results showed that SBRC beam experienced a high density of AE hits with large peak amplitude events that were accelerated during the pre-peak loading phase. The Ib-value analysis revealed that SBRC beam exhibited a higher degree of fracture magnitude during the primary crack development process than SFRC beam. Following the main cracking stage, SFRC beam demonstrated an improved post-cracking softening behavior and superior ability to arrest crack propagation compared to SBRC beam. The integration of local AE fracture energy and DIC results provided a novel approach for a better understanding of the fracture behaviour in both SFRC and SBRC beams. This study’s findings contribute to more precise monitoring of fracture evolution in SFRC and SBRC beams, ultimately improving the selection process for primary reinforcement in flatwork and elevated structures.

Dynamic tensile mechanical properties of red sandstone under different pulse widths and amplitudes: Brazilian disk experiment and macroscopic and microscopic analysis

To investigate the dynamic tensile mechanical properties of red sandstone under varying impact pulse widths and amplitudes, we employed the response surface methodology to examine the dynamic response characteristics of sandstone under different bullet lengths and impact velocities. Subsequently, a dynamic damage constitutive model for sandstone was developed. Macroscopic and microscopic features of sandstone were analyzed through digital image correlation (DIC) and scanning electron microscopy (SEM) experiments. Additionally, ANSYS software was utilized to analyze stress wave characteristics under controlled shock waves and the impact of geological factors on rock fracturing effects. The findings revealed the following. First, the length and impact velocity of bullets exhibit an interactive effect on the tensile response characteristics of sandstone. Peak load, energy consumption rate, and energy density display a positive correlation with bullet velocity. Second, according to the DIC results, the fractal dimension of the crack is negatively correlated with the length of the bullet during equal energy impacts. Third, microscopic failure modes included fractures along particle cementation and through particles, with section roughness decreasing as bullet length increased under equivalent energy impacts. Fourth, a dynamic damage constitutive model (R2 ≥ 0.84) was established based on the Zhu-Wang-Tang (Z-W-T) model and Drucker-Prager (D-P) criterion, clarifying model parameters and influence rules. Fifth, under controlled shock waves, the pulse width facilitates crack propagation, while the pulse amplitude initiates crack formation. Optimal rock breaking efficiency is achieved when stress waves exhibit a large pulse width and low radiation values, meeting specific threshold conditions.

Influence of Aging on the Flexural Strength of PLA and PLA-X 3D-Printed Materials.

The three-point bending test is a valuable method for evaluating the mechanical properties of 3D-printed biomaterials, which can be used in various applications. The use of 3D printing in specimen preparation enables precise control over material composition and microstructure, facilitating the investigation of different printing parameters and advanced materials. The traditional approach to analyzing the mechanical properties of a material using a three-point bending test has the disadvantage that it provides only global information about the material's behavior. This means that it does not provide detailed insight into the local strain distribution within the material. However, the 2D Digital Image Correlation (DIC) method offers additional insight, especially in terms of strain localization. DIC is an optical technique that measures full-field displacements and strains on the surface of a sample. PLA and enhanced PLA-X material were utilized to create three-point bending samples. The aim of this paper was to analyze and compare the influence of aging on the mechanical properties of PLA and enhanced PLA-X materials using three-point bending coupled with the DIC method. The results showed statistically significant differences between the PLA and PLA-X, for both the new and aged materials. The aged PLA samples had the highest average value of maximal force around 68 N, which was an increase of 8.8% compared to the new PLA samples. On the other hand, the aged PLA-X material had an increase of 7.7% in the average maximal force compared to the new PLA-X samples. When comparing the two materials, the PLA samples had higher maximal force values, 6.2% for the new samples, and 7.3% for the aged samples. The DIC results showed that both the new PLA and PLA-X samples endured higher strain values at Points 1 and 2 than the aged ones, except for the aged PLA-X sample at Point 2, where the new sample had higher strain values. However, for the first 5 min of the experiment, both materials exhibited identical behavior, after which point significant differences started to occur for both materials, as well as at Points 1 and 2. A more profound comprehension of the biomechanical characteristics of both PLA and PLA-X material is essential to enhance the knowledge for potential biomedical applications. The DIC method was found to be a powerful tool for analyzing the deformation and failure behavior of samples and for complementing the traditional approach to material testing.

Evaluation of Damage Stress Thresholds and Mechanical Properties of Granite: New Insights from Digital Image Correlation and GB-FDEM

We employed a novel combination of digital image correlation (DIC) and grain-based hybrid finite–discrete element method (GB-FDEM) to improve the comprehension of the relationships between microstructural features and the mechanical properties of granitic rocks. DIC and numerical results showed that macrocracks initiated and propagated along grain boundaries among different minerals driven by the high stiffness contrast between the compliant biotite and the stiffer feldspar/quartz grains. Surface deformation analyses revealed that tensile-dominated macrocracks open at monotonically increased rates before the crack damage threshold, and the opening accelerated afterwards with the increased shear component. The onset of the acceleration of the opening rate of macrocracks can be used to infer the crack damage threshold. Both strain and acoustic emission were used to infer damage stress thresholds in the synthetic numerical samples. Numerical results showed that the damage stress thresholds and uniaxial compressive strength decrease with increasing grain size following log-linear relations. Coarse-grained samples tend to fail by axial splitting, while fine-grained samples fail by shear zone formation. Biotite and quartz contents significantly affect mechanical properties, while quartz to feldspar ratio is positively related to the mechanical properties. Our study demonstrates the capacities of DIC and GB-FDEM in inferring damage conditions in granitic rocks and clarifies the microstructural control of the macroscopic mechanical behaviors. Our results also provide a comprehensive understanding of the systematics of strain localization, crack development, and acoustic emission during the rock progressive failure process.

Damage monitoring of masonry structures using the acoustic emission technique – From tensile and shear characterization tests to shear wall tests

This study evaluated the ability of the acoustic emission technique (AET) to monitor the mechanical damage in masonry structures through multiscale experimental tests ranging from the mesoscopic scale (unit–mortar–unit samples) to the macroscopic scale (shear wall).At the mesoscopic scale, the AET was applied to a series of stone–assembly laboratory specimens subjected to tensile and shear loading. Initially, a mechanical analysis was conducted to characterize two types of cracking, namely mode I (tensile) and mode II (shear). Various acoustic emission (AE) parameters were analyzed in relation to damage accumulation to understand their responses to each type of loading. The representation of AE amplitude versus duration was distinctive in discriminating signals originating from different sources.To validate these findings on the macroscopic scale, we conducted a full-scale test for a shear wall subjected to compression, using both the AET and digital image correlation (DIC). Compared with the DIC results, AET demonstrated a strong performance in damage evaluation and source distinguishment of damage in masonry at the macroscopic scale, which involves more complex failure mechanisms.

Numerical and digital image correlation study of the flexural behavior of prestressed ultra-high-performance concrete beams made from locally available materials

This paper presents finite element simulations of the flexural behavior of three prestressed ultra-high-performance concrete (UHPC) beams based on the concrete damaged plasticity model (CDPM) available within Abaqus. A novel numerical model that applies the prestressing force through surface traction is proposed. The flexural deformation of the prestressed UHPC beams is simulated from the application of the prestressing force through failure under flexural loading. Material properties were measured and informed values for CDPM input parameters. The model was compared with four-point loading tests previously conducted to study the flexural behavior of large-scale prestressed UHPC beams. Digital image correlation (DIC), supplemented with traditional sensors, was used during the test to monitor behavior. The primary results based on the numerical modeling are as follows: • The model is validated to accurately simulate the flexural behavior of prestressed UHPC beams, both in global deflection and local strain. • The prestressing process, including camber deformations, is effectively simulated. • The new approach to apply prestressing force in the model can be applied in scenarios requiring large prestressing forces. • The numerical results are also validated through comparison with DIC results. The new numerical modeling approach will be effective for guiding experimental studies and design of prestressed UHPC beam elements.

Tensile behaviour of pitting corroded steel bars: Laboratory investigation and probabilistic-based analysis

Localised corrosion in steel bars has been a long-standing issue in the durability of reinforced concrete structures, but a comprehensive scheme for the analysis of pitting corroded steel bars, especially with respect to the deformation capacity, is not currently available. In this study, the morphological characteristics of 27 pitting steel bars were captured using a 3D scanner. The measured data were used to establish the probability distribution model of the cross-sectional areas of the corroded bars. Uniaxial tensile tests were conducted, and the evolving deformation field of the corroded bars was recorded through Digital Image Correlation (DIC). Based on the 3D reconstructed model and DIC results, an analytical method for evaluating the mechanical properties of pitting steel bars was developed and validated. The results show that the two-component Gaussian mixture distribution model outperforms conventional unimodal distribution models. Comparison of the analytical results with experimental data demonstrates that the proposed procedure is capable of predicting not only the ultimate strength but also the gauge length-dependent ultimate strain of corroded bars. Additionally, there exists a strengthening effect in the ultimate stress at the critical sections and this effect should not be ignored for accurate predictions.

Full‐Field Analyses of Density‐Graded Elastomeric Foams Under Quasistatic and Impact Loadings

Density‐graded elastomeric foams are emerging as effective protective structures to guard humans against mechanical loading. This research investigates the deformation of ungraded and graded foams under quasistatic and impact scenarios using digital image correlation (DIC). The graded samples are assembled using two interfacing strategies (seamless and adhered), leveraging the adhesiveness of the foam slurry and bulk polyurea, respectively. Deformation mechanisms, including the effect of the interface type on strain transduction and localization in density‐graded structures, are imperative for improving the impact efficacy of protective paddings. Cuboid foam plugs are subjected to quasistatic and impact loading while recording the corresponding deformation for DIC analysis. The DIC results are separated into three case studies based on the number of layers (1, 2, and 3). The interface effect on the overall mechanical performance of polyurea foam is revealed from the bilayer, monodensity samples, showing drastic differences between the deformations within each layer. Seamless interface samples exhibit greater compliance than their adhered counterparts in the bilayer density‐graded configurations. Trilayer‐graded foams broaden strain–time history, extend the impact duration, and reduce strains. This research substantiates the importance of interfacing and gradation strategies on the mechanical response of elastomeric foams as a function of strain rate.

Self-healing performance of reinforced concrete beams using engineered aggregates

This paper presents a study on the self-healing performance of reinforced concrete (RC) beams using a new approach called engineered aggregates (EA). EA are a type of capsules that release healing agents when the concrete is cracked, without requiring any changes to the conventional concrete preparation methods. In this study, polyurethane (PU) filled EA were added to RC beams with dimensions of 127 mm × 203 mm × 1448 mm and three-point bending tests were conducted. Companion control tests were also conducted on identical beams to compare the self-healing performance of the concrete with and without EA. The development of cracks during the first (initial cracking) and second (after healing) stages was monitored using the digital image correlation (DIC) technique. The addition of EA did not have any impact on the load carrying capacity of the beams, and the DIC results showed that the cracks closed during healing reopened in subsequent testing. Ultrasonic measurements were taken before and after healing to determine the extent of crack healing. The ultrasonic pulse velocity (UPV) measurements and diffusion coefficient calculations showed that the RC beams with PU-filled EA healed the cracks. Water absorption tests were performed to determine the recovery of the original permeability after healing. The study demonstrated the effectiveness of EA in improving the self-healing performance of RC beams and its potential for practical application in the construction industry. This paper is the first member scale study for self-healing concrete with EA, which demonstrated successful filling and bonding of flexural cracks.

Study on the semi-circular bend method for characterizing the mixed mode I/II fracture toughness of sandstone: A micro-perspective

Manufacturing and precracking specimens for semi-circular bend (SCB) tests are simple processes and do not require complicated loading devices. They have been used to conduct mode I, mode II and mixed mode I/II fracture tests on brittle materials. However, characterizing mode I and mode II fractures from a microcosmic and quantitative perspective has never been thoroughly evaluated. In this paper, high-resolution digital image correlation (DIC) and moment tensor inversion are first used to quantitatively measure micron-level displacement and analyse microfailure types. The DIC results indicate that SCB specimens with a notch inclination angle exhibit both opening displacement and sliding displacement, resulting in a mixed mode I/II fracture. A quantitative analysis of three typical classification methods based on moment tensor inversion suggests that SCB specimens with mixed loads are mainly tensile mechanisms. Some tensile sources are not pure tensile cracks, and shear components also play an important role. It is also observed that the moment tensor components, nature of sources and Hudson k value are associated with the notch inclination angle, which sheds new light on the mechanism of mixed mode I/II fractures.

A comparison between FEM predictions and DIC results of crack tip displacement field in CT specimens made of titanium

In the context of fatigue crack growth (FCG), the validation of FEM predictions with experimental results has been made in terms of crack closure and da/dN. The objective here is to compare the prediction of the displacement field, which is the primary output of FEM, with experimental results obtained with Digital Image Correlation (DIC). The qualitative comparison indicates that DIC and FEM results are very similar. The comparison of CTOD curves obtained behind the crack tip showed a good agreement between the DIC and numerical predictions obtained under plane strain conditions. This indicates the dominance of the interior regions over the surface displacements, although the small thickness of the specimen (1 mm). The more detailed analysis of the CTOD loops, through the values of plastic CTOD ranges, showed a significant difference between FEM predictions and DIC results. The robustness of the numerical model was checked considering a mesh sensibility analysis, as well as considering different sets of material parameters in the simulation.

Microcrack monitoring and fracture evolution of polyolefin and steel fibre concrete beams using integrated acoustic emission and digital image correlation techniques

The use of polymer and steel fibres in plain concrete appears to be an excellent solution for limiting crack propagation and improving the post-ductility performance of concrete structures. Based on this premise, this study investigated the fracture evolution of polyolefin fibre-reinforced concrete (PFRC) and steel fibre-reinforced concrete (SFRC) specimens through the integrated application of two diagnostic techniques, acoustic emission (AE) and digital image correlation (DIC), under three-point bending tests. Based on the processing of AE signals, different AE statistical parameters such as the cumulative number of hits, amplitude distribution, and some representative analysis methods including the b-value method, Ib-value method, and AE intensity analysis methods were selected to analyse the early detection of cracking and post-cracking behaviour in PFRC vs SFRC specimens during mechanical degradation. Simultaneously, the DIC technique was used to validate the fracture evolution of the AE results. Furthermore, to verify the reliability of the AE and DIC results, the damage localisation and fracture evolution of the PFRC versus SFRC specimens were confirmed by integrating the AE fracture energies and DIC outcomes. The tests and analysed results showed that the addition of steel fibres to plain concrete significantly improved the ability to restrict crack propagation and provided higher post-cracking resistance compared to PFRC specimens owing to their stronger fibre–matrix bonding, effective fibre bridging, and crack-arresting mechanism. The present study indicates that the combined AE and DIC techniques are highly effective for the early detection of damage and ductility performance in fibre-reinforced concrete structures.

Digital Image Correlation Analysis of Strain Fields in Fibre-Reinforced Polymer–Matrix Composite under ±45° Off-Axis Tensile Testing

This study presents an experimental investigation of an in-plane shear of a glass lamina composite using a ±45° off-axis tension test. Typically, the shear stress curve, shear modulus, and in-plane shear strength for composite lamina-type materials are identified. Previous research indicated that a loading rate affects the strength of this composite. This study extends the existing literature by utilising a non-contact optical digital image correlation (DIC) method to measure strain distribution during the test. Two cross-head displacement rates were examined. The obtained strain maps reveal an uneven distribution resembling fabric texture. As the deformation progresses, the differences in the strain pattern increase. Subsequently, a quantitative analysis of the differences between regions with extreme (minimum and maximum) strain values and regions with average values was conducted. Based on these measurements, shear stress–strain curves, indicating variations in their courses, were constructed. These differences may reach several percent and may influence the analysis of numerical simulations. The DIC results were validated using strain gauge measurements, a commonly utilised method in this test. It was demonstrated that the location of the strain gauge installation impacts the results. During the tests, the occurrence of multiple microcracks in the resin was observed, which can contribute to the nonlinearity observed in the shear stress–shear strain curve.

In-situ SEM investigation on the damage behavior of an interpenetrating metal ceramic composite

For the investigation of interpenetrating materials, the microstructure and the associated damage mechanism are crucial for the understanding of its properties. In this work, an aluminum-alumina interpenetrating composite is investigated and characterized regarding the microstructure and the tensile and compression properties during in-situ SEM experiments. The composite is fabricated by gas pressure infiltration and consists of a highly homogeneous and fine-pored Al2O3 preform and an AlSi10Mg cast alloy. Due to the fine-pored ceramic phase, building a strong contrast to the metallic phase, digital image correlation (DIC) could be applied directly on the SEM images, captured during the in-situ experiments, without the need of a speckle pattern on the surface. The microstructure analysis and DIC results were correlated with the load level, derived from stress-strain curves, to achieve deeper understanding from the damage behavior in interpenetrating metal ceramic composites.

Experimental investigation of the air blast performance of hybrid composite skinned sandwich panels with X-ray micro-CT damage assessment

This research investigates the performance of interlaminar hybrid composites as the skins of composite sandwich panels under blast loading with the aim of promoting delamination between dissimilar plies for energy absorption. The deformation of the composite panels was captured using high-speed digital image correlation (DIC). High-speed full-field DIC enables failure to be captured at the moment it occurs across the entire panel. X-ray micro-CT imaging was used to assess the post-blast damage sustained by particular areas of interest from each panel, which were selected based on DIC results. The combination of full-field DIC and detailed X-ray micro-CT scanning enabled a unique comparison of both the global and localised blast resilience of hybrid and conventional composite sandwich panels to be performed.Following a single blast load, the extent of damage to the Hybrid-3B skinned sandwich panel was found to lie between that of GFRP and CFRP skinned sandwich panels. X-ray micro-CT scanning of these panels reveals that there is no continuous damage path through the skin thickness of Hybrid-3B, whereas the GFRP and CFRP panels sustain damage in every ply.Following repeat blast loading, the Hybrid-4 skinned sandwich panel suffered from a front skin crack spanning the length of the panel. Post-blast compressive strength testing reveals that this skin crack and resulting core crack acted as a stress relief, limiting the damage sustained elsewhere in the panel.It was concluded that Hybrid-3B results in a good trade-off between strength and stiffness and is advantageous over conventional CFRP and GFRP panels under a single blast load. Under repeated loading Hybrid-4 offers advantages over Hybrid-3B. Finally, the design of the support structure can significantly aid in blast resilience, and, a holistic approach considering both panels and support should be taken when designing for blast resilience.

The effect of alternating the sequence of variable‐energy repeated impact on the residual strength and damage evolution of composite laminates

AbstractThis article investigates the effect of alternating the sequence of variable‐energy repeated low‐velocity impact on the residual strength of carbon fiber reinforced polymer (CFRP) laminates. Quasi‐isotropic CFRP specimens were impacted using three impact energy levels, that is, Low (L), Medium (M), and High (H). Every group, out of three groups in total, was impacted consecutively using one of three sequences: LMH, HML, and MLH. Delamination and its growth after each impact was quantified using a phased‐array ultrasonic probe. The impacted specimens were then subjected to compression after impact loading with in situ acoustic emission (AE) and digital image correlation (DIC) measurement. Mechanical results showed that the residual strength of the specimens was not significantly affected by alternating the impacts sequence. However, the AE, sentry function and DIC results revealed that the impact‐induced damage in HML configuration was activated much earlier than MLH, and the damage induced in LMH configuration was activated later than both.

Nanoscale digital image correlation at elementary fibre/matrix level in polymer–based composites

Multiscale mechanical modelling aims at predicting the failure of composites from the fibre/matrix level up to the component scale. Existing frameworks are limited by the lack of reliable experimental data and by an incomplete understanding of the submicron deformation and failure mechanisms. A novel digital image correlation (DIC) method has been developed for the characterisation of the nanoscale mechanical response in composites, based on latest advances in surface patterning. Indium has been deposited on unidirectional composites leading to a dense, homogeneous speckle with particle diameter around 15 nm. The specimens were subjected to transverse compression in a scanning electron microscope, while minimising distortion effects. Strain concentration areas, like submicron shear bands and fibre–matrix interphases, were successfully captured for two systems: a carbon fibre-reinforced thermoset and a glass fibre-reinforced thermoplastic. DIC results were compared with alternative experimental data, obtained by atomic force microscopy, and with finite element simulations based on a conventional elastoplastic model.

Investigating the Fracture Process and Tensile Mechanical Behaviours of Brittle Materials under Concentrated and Distributed Boundary Conditions

In this study, concrete was selected to investigate the real-time splitting tensile mechanical and fracture behaviours of brittle materials using the Brazilian test under concentrated and distributed boundary conditions. The digital image correlation (DIC) method was adopted to evaluate the tensile strength and failure process in Brazilian tests using a high-resolution camera. The DIC results showed that the position of the crack initiation randomly occurred at the centre of a disc and at the boundary in Brazilian tests with concentrated loads (BTC). Comparatively, the crack initiated at the centre of discs in most Brazilian tests with distributed loads (BTD), as validated by the DIC results. Our results indicated that the average nominal tensile strength of specimens cracking at the boundary was larger than that of specimens cracking at the centre in the BTC, suggesting that the nominal tensile strength measured by the Brazilian test was related to the failure process of the discs. Moreover, this study demonstrated that the tensile strength measured by the Brazilian test was dependent on the boundary conditions. The equation recommended by the ISRM (International Society for Rock Mechanics) might slightly overestimate the tensile strength of brittle materials based on the pure elastic theory and hyperbolic-distributed loading conditions at the boundaries.