Internal Strain Fields Research Articles

Load-bearing behaviors of the composite gravity-type anchorage: Insights from physical model and numerical tests

Gravity-type anchorages (GTAs) have been extensively applied in large-span suspension bridges across the globe, but the load-bearing performance of composite GTA is not yet fully investigated. Here, the comprehensive load-bearing performance of GTA is investigated using a combination of physical modelling and numerical modelling methods, considering the impact of anchorage’s burial depth, notched sill, and pile. Six model tests are first conducted using a self-reversing force loading equipment, including flat bottom anchorage, notched sill anchorage, and notched sill anchorage with pile. Each type of anchorage is tested under two different burial depth conditions. The failure process, anchorage displacement, cable tension force, and the internal strain field of the foundation, were examined during the model tests. Additionally, through the numerical simulations, the evolution characteristics of contact stresses and load sharing ratio associated with various forms of anchorage are discussed. The results indicate that the anchorage’s burial depth and pile significantly impact on anchorage’s bearing capacity. The presence of the notched sill can change the distribution of contact stress between the anchorage and foundation, which can still resist the horizontal force until the foundation before the notched sill occurs shear failure. The failure process of the anchorage-foundation system was divided into four stages: stable bearing, sliding, sliding and overturning, and failure stage. The anchorage starts overturning with the applied load reaching to 3*Tm, except for the flat bottom anchorage of shallow burial depth condition. Under the same conditions, the greater the anchorage’s burial depth, the smaller the load shared by the friction between the anchorage bottom and foundation, the greater the load shared by the notched sill and foundation before the anchorage. In addition, the load shared by pile continuously increasing as the anchorage starts overturning. The findings from this study can provide a theoretical basis and valuable insights for the optimization design of GTA.

3D strain heterogeneity and fracture studied by X-ray tomography and crystal plasticity in an aluminium alloy

Strong correlations between measured strain fields and crystal plasticity finite element (CP-FE) predictions based on the real microstructure are found for a plane strain tensile specimen made of 6016 T4 aluminium alloy. This is achieved using multimodal X-ray lab tomography giving access to both the initial grain structure and the strain evolution. The real microstructure of the central region of interest (ROI) of the undeformed specimen is obtained non destructively using lab-based diffraction contrast tomography (DCT) and meshing. An in situ tensile test, using absorption contrast tomography (ACT) is then performed for twelve loading increments up to fracture. Taking advantage of the plane strain condition, the evolution of the internal strain field is measured by two-dimensional digital image correlation (DIC) in the material bulk using the natural speckle provided by intermetallic particles. Early strain heterogeneities in the form of slanted bands, that are spatially stable over time, are revealed and the fracture path – determined from the post mortem scan – is found to coincide with the bands exhibiting maximum strain. CP-FE simulations are performed on the meshed microstructure of the specimen acquired by DCT and are compared with image correlation measurements. The measured strain fields are well described by 3D CP-FE predictions, whilst it is shown that neither a macroscopic anisotropic plasticity model nor a CP-FE simulation with random grain orientations could reproduce the measurements.

Dynamic Coupling of Internal Strain Field and Lithium Pathway within Individual Battery Particles in Solid State Batteries

In all-solid-state batteries (ASSBs), (electro)chemo-mechanical aspects such as uneven interfaces, cathode-electrolyte interphase formation, delamination, fracture, defects, etc., are the major factors in capacity fade but remain largely unknown.Nonhomogeneous transfer of lithium ions can cause significant variations in strain and stress within battery electrodes, leading to degradation in battery performance. In all-solid-state batteries (ASSBs), the lithium pathway and the associated strain/stress field become more intricate due to the (electro)chemo-mechanical reaction at the electrode-electrolyte interfaces. The dynamic volume change in active particles are heavily influencing by strength of the solid electrolyte and interfacial conformality. This can continually alter the lithium pathway and the internal stress field, leading to recurrent redefinitions of (electro)chemo-mechanical environment.Here, we have developed an operando coherent X-ray imaging platform and associated analysis methodologies. This technology can track the nanoscale transport of lithium and the strain evolution of individual electrode particles in ASSBs. With this platform, we gained a comprehensive electrochemical and mechanical understanding of the cycling properties of single electrode particles in ASSBs.

Visualizing the Internal Nanocrystallinity of Calcite Due to Nonclassical Crystallization by 3D Coherent X-Ray Diffraction Imaging.

The internal crystallinity of calcite is investigated for samples synthesized using two approaches: precipitation from solution and the ammonium carbonate diffusion method. Scanning electron microscopy (SEM) analyses reveal that the calcite products precipitated using both approaches have a well-defined rhombohedron shape, consistent with the euhedral crystal habit of the mineral. The internal structure of these calcite crystals is characterized using Bragg coherent diffraction imaging (BCDI) to determine the 3D electron density and the atomic displacement field. BCDI reconstructions for crystals synthesized using the ammonium carbonate diffusion approach have the expected euhedral shape, with internal strain fields and few internal defects. In contrast, the crystals synthesized by precipitation from solution have very complex external shapes and defective internal structures, presenting null electron density regions and pronounced displacement field distributions. These heterogeneities are interpreted as multiple crystalline domains, created by a nonclassical crystallization mechanism, where smaller nanoparticles coalescence into the final euhedral particles. The combined use of SEM, X-ray diffraction (XRD), and BCDI allows for structurally differentiating calcite crystals grown with different approaches, opening new opportunities to understand how grain boundaries and internal defects alter calcite reactivity.

Initializing intragranular residual stresses within statistically equivalent microstructures for crystal plasticity simulations

We propose a mechanics framework to compute intragranular or type III residual stress within a polycrystalline aggregate for which experimental characterization of residual stress is unknown. The framework is built upon the inter-relationship between intragranular misorientation (IGM), geometrically necessary dislocation (GND), and long-range internal stress. Given a spatial distribution of IGM, we first calculate the spatial distribution of GND density; subsequently, we calculate the internal stress and strain fields due to the GND distribution. For demonstration, we consider a synthetic microstructure of a hexagonal close-packed material, namely Ti-7Al, wherein the grain size and orientation distributions are statistically equivalent to the distributions obtained via experimental characterization. We refer to this microstructure as a statistically equivalent microstructure (SEM). In an SEM, each grain is usually treated as a pristine crystal and therefore, the SEM contains no IGM. We propose a technique to introduce IGM within the SEM and, subsequently, calculate the GND density and residual stress distributions in a consistent manner. In the process, we ensure that statistical distributions of the IGM and residual strain within the SEM are statistically equivalent to the same obtained from the experimental characterization of Ti-7Al. While establishing statistically equivalent residual stress, the thermal effects during processing of Ti-7Al are implicitly taken into account. Next, we perform crystal plasticity finite element simulations to demonstrate the implications of the initialization of IGM and type III residual stress. Although the IGM and residual stress are related, the results suggest that the type III residual stress initialization has a much more pronounced effect on microplasticity-driven damage mechanisms, such as high cycle fatigue, even within a well-annealed material such as Ti-7Al, thereby emphasizing the need for its initialization for microstructure-sensitive fatigue analysis. Finally, we demonstrate that initializing thermally induced residual stress within Ti-7Al SEM through a cooling simulation, accounting for the anisotropic nature of the coefficient of thermal expansion, yields a grain-averaged residual stress distribution statistically similar to that obtained from our framework and is a special case of the type II residual stress implementation.

Corrosion damages of reinforced concrete characterized by X-ray CT and DVC techniques

X-ray microcomputed tomography (X-ray CT) technique was performed to monitor the whole corrosion cracking process of a cylinder reinforced concrete specimen non-destructively. Based on gray threshold separation and 3D reconstruction technology, the spatial distributions of each substance were obtained. Combined with Digital Volume Correlation (DVC) technique, the development of corrosion depth, generation and migration of corrosion products, variation of internal strain field and crack propagation during the corrosion process were all analyzed. The experiment results indicate that the electric current accelerated method can lead to a non-uniformity corrosion at the early age. The distribution fields of rusts and strain are affected by the micro- and meso- structure of concrete. A turning point of maximum principle strain along the cracking path can be identified as the critical point at which micro-cracks of concrete connect and propagate into macro-cracks. The statistical analysis reveals that the gray values of mortar matrix, corrosion products, steel bar and coarse aggregates in corroded concrete follow a Garrison distribution, while the pores do not.

Enhancing magnetocaloric properties of NiMnGa-based alloys via Dy micro-alloying and pseudoelastic cyclic training

Considerable interest to improve magnetic entropy change (ΔSm) and broaden working temperature interval (WTI) of NiMnGa ferromagnetic shape memory alloys (FESMAs) was stimulated by their applications as promising candidate materials for solid-state refrigeration. In the present study, we presented an approach to enhance the magnetocaloric properties of polycrystalline NiMnGa FESMAs via combining Dy micro-alloying and pseudoelastic cyclic training. The introduction of Dy elements established stable magneto-structural coupling transformation from the paramagnetic austenite to ferromagnetic martensite, accompanied by a large ΔSm [−16.42 J/(kg K)] and a widened WTI (∼15.98 K). Fascinatingly, it was demonstrated that the internal strain fields at phase interface between matrix and DyNi4Ga precipitates could assist the phase transformation nucleation, which significantly reduced the hysteresis loss from 20.84 J/kg of Ni54Mn25Ga21 alloy to 8.14 J/kg of Ni54Mn25Ga20.7Dy0.3 alloy. More importantly, the subsequent pseudoelastic cyclic training produced a strong ⟨110⟩NM preferred crystallographic orientation, which facilitated the magnetic alignment along easy magnetization axis. Consequently, the giant ΔSm value up to −24.25 J/(kg K) and effective refrigeration capacity RCeff of 198.77 J/kg were further achieved in the trained Ni54Mn25Ga20.7Dy0.3 alloy under an external magnetic-field change of 5.0 T.

Numerical and Experimental Investigation of Slope Deformation under Stepped Excavation Equipped with Fiber Optic Sensors

The real-time evaluation of slope stability is a crucial technical issue in foundation excavation and slope construction. However, conventional monitoring methods often fall short of achieving real-time and accurate measurements, which poses challenges to the timely assessment of slope stability. To address this problem, laboratory tests and numerical simulations were jointly used to evaluate slope stability in this paper. In numerical simulations, the finite element method (FEM) results clearly illustrate the distribution and evolution of internal strain during slope excavation, and the limit equilibrium method (LEM) calculates changes in the safety factor. In laboratory tests, the fiber Bragg grating (FBG) sensing technology was employed to monitor the internal strain of the slope in real time. The distribution characteristics of the slope internal strain field under the condition of stepped excavation were analyzed, and the feasibility of strain-based evaluation of slope stability was discussed. The measurements with FBG sensing technology agree well with the numerical simulation results, indicating that FBG can effectively monitor soil strain information. Of great significance is that the maximum horizontal strain of the slope is closely related to the safety factor and can be used to evaluate slope stability. Notably, the horizontal soil strain of the slope provides insight into both the formation and evolution of the critical sliding surface during excavation. The combination of numerical simulation and intelligent monitoring technology based on FBG proposed in this paper provides a reference for capturing strain information inside the slope and realizing real-time assessment and critical warning of slope stability.

3D deformation and strain fields in drying kaolinite obtained from tracking internal bubbles using X-ray CT and ANN

AbstractDrying fine-grained sediments experience shrinkage and desiccation cracking that may dramatically alter their mechanical and hydraulic properties. This study adopts X-ray computed tomography (CT) to monitor the three-dimensional (3D) internal deformation and strain fields, and their relationships with desiccation crack formation, for drying kaolinite samples contained in plastic containers. Two kaolinite samples, one dried at room temperature and the other oven-dried at 60 °C, were CT scanned at several intervals during the drying process. From sequential CT scans for the same sample, entrained gas bubbles were extracted and used as tracking markers for deformation and strain field measurements. Since the bubble morphology continuously changed during the drying process, an artificial neural network (ANN) model was developed to link bubbles in sequential scans for the same sample. The tracking algorithm was trained with manually linked bubbles and optimised by comparing different combinations of bubble information, e.g. bubble location, size and shape. The drying samples experienced primarily vertical displacement before the air-entry value, while horizontal displacement occurred during vertical crack formation. Internal vertical and horizontal strains were generally uniform, indicating a limited impact of non-uniform sample drying and substrate constraint.

Investigation of carbon-aramid hybrid braided composites using digital volume correlation

Hybrid braided composites combine two or more reinforcing fibres in a matrix. Limited research on hybrid braided composites has been conducted. The Digital Image Correlation (DIC) method has been used to understand hybrid braided composites' external deformation and strain. However, DIC measurements do not provide sufficient information on the internal behaviour of hybrid braids. Therefore, this study focuses on understanding the behaviour of hybrid braids Digital Volume Correlation (DVC). DVC is a micro-computed tomography (Micro-CT) based volumetric strain measurement technique that allows for the internal strain fields of an object. Tensile loading of the Carbon-Aramid hybrid braided samples is achieved using an integrated material testing stage. DVC is used to generate three-dimensional displacement and strain fields and understand the internal strain behaviour of hybrid braided composites. The peak and strain observed for the sample was around −0.065 (µm/µm) and 0.017 (µm/µm), respectively. The effect of contrast-enhancing tracking particles on the DVC resulting DVC strain fields were also examined. Samples with and without contrast particles were subjected to in-situ loading and then analyzed using the DVC process. From this comparison, it was found that the addition of contrast-enhancing particles improves the volumetric strain results from the DVC measurements.

Mapping internal strain fields of fused filament fabrication metal filled polylactic acid structure using digital volume correlation.

With the advancement in Fused Filament Fabrication (FFF), its application is increasing widely across different industries such as aeronautical, biomedical, robotics, etc. The internal structure is becoming more complex and intricate with varying materials of reinforcement which are used to improve mechanical properties. Current measurement techniques like Digital Image Correlation (DIC) are non-destructive testing methods that do not provide enough information on the behaviour of internal microstructure for anisotropic FFF materials. Digital Volume Correlation (DVC) is non-destructive testing technique which provides full field internal 3D deformation and strain fields. Copper particle filled PLA samples manufactured using FFF method with 20, 40, 60 and 80 infill percentages were loaded in tension inside Micro-CT. X-rays were passed through the sample to get a volumetric dataset for different loadings. Using DVC method on the dataset, internal displacement and strain fields were generated for 20, 40, 60 and 80 infill percentage FFF sample.

In situ micromechanical analysis of discontinuous fiber-reinforced composite material based on DVC strain and fiber orientation fields

The influence of fiber orientation and its distribution on the global and local mechanical behavior of a short fiber-reinforced thermoplastic (SFT) composite is studied based on an in situ tensile test using synchrotron X-ray tomography. The in situ test data are utilized to compute three-dimensional (3D) strain fields inside the material at various loading steps through an in-house digital volume correlation (DVC) code. The DVC analysis finds that, although the global strain fields seem relatively uniform, the internal strain fields are locally non-uniform because of the complex reinforcement architecture. The local non-uniformity is further investigated at a single fiber level to find a correlation between the orientation of fibers, fiber volume fraction, and strain field. The investigation is performed on micro-scale subvolumes, in which all the fibers are individually segmented from the 3D tomographic data. It is found that the fibers aligned in the direction transverse to loading impede transversal shrinkage, resulting in a locally small compressive strain in that direction. The in situ test has revealed that the average fiber orientation does not change noticeably even after the global strain reaches 18%. This small change implies that initially-determined fiber orientation is crucial to the locally anisotropic mechanical behavior of a SFT-like material even undergoing large deformation. On the other hand, the loading-directional strain is more affected by the local volume fraction than the fiber orientation. The local volume fraction also mainly determines the stiffness of the subvolume that can be estimated from an analytical homogenization model. The effect of the local volume fraction on the subvolume stiffness is identified from multivariate analysis.

Digital volume correlation for the characterization of musculoskeletal tissues: Current challenges and future developments.

Biological tissues are complex hierarchical materials, difficult to characterise due to the challenges associated to the separation of scale and heterogeneity of the mechanical properties at different dimensional levels.The Digital Volume Correlation approach is the only image-based experimental approach that can accurately measure internal strain field within biological tissues under complex loading scenarios. In this minireview examples of DVC applications to study the deformation of musculoskeletal tissues at different dimensional scales are reported, highlighting the potential and challenges of this relatively new technique.The manuscript aims at reporting the wide breath of DVC applications in the past 2 decades and discuss future perspective for this unique technique, including fast analysis, applications on soft tissues, high precision approaches, and clinical applications.

Analysis of The Working Performance of Large Curvature Prestressed Concrete Box Girder Bridges.

Based on numerical shape functions and the structural stressing state theory, the mechanical properties of the curved prestressed concrete box girder (CPCBG) bridge model under different loading cases are presented. First, the generalized strain energy density (GSED) obtained from the measured strain data is used to represent the stressing state pattern of the structure; then, the stressing state of the concrete section is analyzed by plotting the strain and stress fields of the bridge model. The stressing state pattern and strain fields of the CPCBG are shown to reveal its mechanical properties. In addition, the measured concrete strain data are interpolated by the non-sample point interpolation (NPI) method. The strain and stress fields of the bridge model have been plotted to analyze the stressing state of the concrete cross-section. The internal forces in the concrete sections are calculated by using interpolated strains. Finally, the torsional effects are simulated by measuring the displacements to show the torsional behavior of the cross-section. The analysis and comparison of the internal force and strain fields reveal the common and different mechanical properties of the bridge model. The results of the analysis of the curved bridge model provide a reference for the future rational design of bridge projects.

Cylindrical Bidirectional Strain Sensors Based on Fiber Bragg Grating.

To realize continuous real-time monitoring of the large-scale internal strain field of coal and rock mass, a bidirectional strain sensor based on FBGs encapsulated using a hollow cylindrical steel tube was designed. The sensor’s structural parameters were optimized through unidirectional loading, and the strain change laws of the sensor were analyzed under unidirectional and bidirectional loading conditions, in which the stress-strain fitting curves of the sensor and the relationships of the strain in the vertical and horizontal directions were obtained under different lateral pressure loading conditions. A similar theoretical model was established to verify the accuracy of the linear relationship between the surrounding rock stress and the strain measured by the sensor system.

Effect of the number of projections in X-ray CT imaging on image quality and digital volume correlation measurement

Digital volume correlation (DVC) quantifies internal 3D displacement and strain fields by correlating the volume images of a tested object acquired at different states. When using X-ray CT-based DVC, the number of projections is a key parameter that affects the acquisition time and quality of reconstructed volumetric images, and therefore the precision and temporal resolutions of DVC measurement. More projections result in high-quality volume images for DVC calculation but longer acquisition time and pronounced thermal drifts of the X-ray source. Few projections lead to quicker acquisition and fewer thermal drifts, but may degrade image quality and thus induce larger DVC measurement errors. Selecting an appropriate number of projections during CT imaging is therefore of practical significance for DVC measurement. To solve this dilemma, the effect of the number of projections on DVC measurements with X-ray CT is experimentally investigated in this work. First, numerically simulated speckle volume images with different numbers of projections were reconstructed by using FDK (Feldkamp) algorithm, and the influence of the number of projections on DVC measurement was analyzed. Then, real rescan and compression experiments performed on a copper foam sample were carried out to further study the effect of projection number on DVC measurements. Both simulation and real experiments show that more projections result in longer imaging time but higher quality volume image and DVC measurement. DVC measurement errors decrease with the increase of projections at different decline rates. Therefore, an appropriate number of projections can be specified based on the results according to the requirements of DVC measurement precision and temporal resolution. For the specific X-ray CT device used in this real compression experiment, 36 ∼ 60 projections are suggested to balance measurement precision and temporal resolution, and more than 720 projections are necessary for pursuing higher accuracy.

The volume strain of fiber reinforced concrete with three points bending during loading

In order to improve the tensile properties of concrete, fiber materials are usually added into concrete to improve its mechanical properties. The internal strain fields in the external loaded fiber reinforced concrete are very complex, and it is still difficult to test them currently. In this paper, the X-ray microcomputed tomography (XCT) in situ three-point bending test is used to track the loading process of fiber mortar. The digital volume correlation (DVC) is used to calculate the internal volume strain and the internal strain field of mortar with different fiber contents at different loading stages. The results show that adding fiber into concrete can improve the fracture performance of concrete, and the length, width and volume of crack are obviously reduced. The volumetric strain evolution of samples with different fiber content at different loading stages can be obtained by combining XCT with DVC. There is still non-negligible deformation energy consumption outside the fracture surface when samples fail. The internal deformation response of the specimen under load can be obtained by volume strain analysis.

Contribution of the nucleation and recovery of disconnections to shear viscosity in diffusional creep

According to the principles of diffusional creep, the normal and tangent components of the velocity jumps between adjacent grains arise from, respectively, the climbing and sliding of disconnections along grain boundaries. Stationary deformation thus implies a balance between nucleation and recovery of moving disconnections. The model considers a periodic lattice of hexagonal grains with nucleation of disconnection multipoles at triple junctions. The strain energy coupled to the population of climbing disconnections is calculated by inferring that the internal strain field associated to disconnection pile-ups brings a distribution of tractions along GBs that is consistent with the field of diffusion potential gradient that drives disconnection climb. It follows that the distribution of the density of climbing disconnections is parabolic and that the dissipation due to the nucleation and recovery of climbing disconnections is equal to 50% of the dissipation arising from diffusion fluxes. These results hold for both Nabarro-Herring creep and Coble creep. The analysis of the disconnection nucleation process highlights the sources of non-Newtonian behaviour and the existence of a threshold stress as an intrinsic feature of diffusional creep.

Fiber Bragg Grating–Based Flume Test to Study the Initiation of Landslide-Debris Flows Induced by Concentrated Runoff

Abstract In order to explore the initiation of landslide-debris flow caused by concentrated runoff, a flume test has been performed on soil samples taken from a debris flow site in Wenchuan earthquake area. The fiber Bragg grating (FBG) technology was used to measure the strain distribution information in the slope mass during erosion. Several hydrological sensors for monitoring pore water pressure and soil moisture were used to investigate their correlation with internal strain field. The test results show that the fiber optic sensing technique exhibited high sensitivity and precision in monitoring the slope deformation. The development of pore water pressures and soil moistures showed reasonable consistency with the dynamics of an infiltration. According to the strain measurements, there were four evolution stages during the initiation of landslide-debris induced by runoff, i.e., the water absorption stage, the deformation stage, the shear zone formation stage, and the fluidization stage. The results provide an improved insight into the mechanism of debris flow initiation and indicate the enormous potential of the FBG sensing technology in establishing an effective early warning system for landslides and debris flows.

Contribution of the Nucleation and Recovery of Disconnections to Shear Viscosity in Diffusional Creep

According to the principles of diffusional creep, the normal and tangent components of the velocity jumps between adjacent grains arise from, respectively, the climbing and sliding of disconnections along grain boundaries. Stationary deformation thus implies a balance between nucleation and recovery of moving disconnections. The model considers a periodic lattice of hexagonal grains with nucleation of disconnection multipoles at triple junctions. The strain energy coupled to the population of climbing disconnections is calculated by inferring that the internal strain field associated to disconnection pile-ups brings a distribution of tractions along GBs that is consistent with the field of diffusion potential gradient that drives disconnection climb. It follows that the distribution of the density of climbing disconnections is parabolic and that the contribution to dissipation is equal to 50% of the contribution arising from diffusion fluxes. These results hold for both Nabarro-Herring creep and Coble creep. The analysis of the disconnection nucleation process highlights the sources of non-Newtonian behaviour and the existence of a threshold stress as an intrinsic feature of diffusional creep.