Local Strain Distribution Research Articles

Experimental and numerical investigations on the tensile response of pin-loaded carbon fibre reinforced polymer straps

Carbon fibre reinforced polymer (CFRP) pin-loaded looped straps are increasingly being used in a range of structural load-bearing applications, notably for bridge hanger cables in network arch rail and highway bridges. The static performance of such CFRP straps is investigated through experimental and numerical analyses. Finite element (FE) models based on both one-eighth and half pin-strap assembly geometries were modelled. The resulting strains, stresses, and applied loads were compared against experimental data obtained using Digital Image Correlation, Distributed Fibre Optic Sensing (DFOS), and Fibre Bragg Grating (FBG) Sensing. The FE models effectively captured local strain distributions around the vertex area, close to the pin ends of the straps, as well as in the mid-shaft region, and aligned reasonably with experimental observations. The half FE model accurately predicted the overall strain distribution when compared to DFOS data; however, higher strain magnitudes (by 0.45–10.2 %) and larger strain reductions were observed in some locations. Regarding failure loads, the FE models agreed well with Schürmann's analytical solution and the maximum stress criterion, exhibiting less than 2.5 % deviations from the experimental data. Furthermore, the predicted onset of strap failure (by delamination) in the half model agreed with experimental values, with a maximum variance of 9.2 %.

Quantitative evaluation of local strain distribution induced by deformation twins and triggering effect of deformation twinning on neighboring grains in polycrystalline AZ31 magnesium alloys

{101‾2}<1‾011> deformation twinning commonly occurs in Mg/Mg alloys. In their polycrystals, however, twinning is not always activated at locations with high Schmid factor for {101‾2}<1‾011> twinning. It is believed that twinning behavior in polycrystalline Mg/Mg alloys can be understood with a consideration of interaction between adjacent twins as well as local deformation behavior on a micro- and meso‑structural scale. In this study, we theoretically estimate local plastic strain within twins by transforming coordinates and then compare it with that experimentally obtained from digital image correlation (DIC) analysis for understanding the reason for non-Schmid twinning in polycrystalline-Mg alloys. The theoretically estimated strain of twins was found to be in good agreement with the DIC-strain value. Furthermore, it was observed that the variant selection of twin can be explained by the compatibility parameter m’, rather than Schmid factor, suggesting that twin activation in polycrystalline-Mg is strongly influenced by adjacently appeared twins.

Multimodal deep learning framework to predict strain localization of Mg/LPSO two-phase alloys

This study proposes a method for predicting three-dimensional (3D) local strain distribution under compressive deformation of as-cast Mg/LPSO two-phase alloys from 3D microstructure images. The 3D local strain distribution was obtained by applying the digital volume correlation method to X-ray CT images before and after compression tests. Three microstructure descriptors were extracted from the 3D microstructure images around each strain measurement point: volume fractions of the phases, persistent diagrams that can express the connectivity of the phases, and two-phase spatial correlation that can express the spatial distribution of the phases. A deep learning model was then constructed to predict local strain from the three microstructure descriptors. Since two types of descriptors were used in this study, numerical data and image data, multimodal deep learning was employed to make predictions. Thus, the use of multiple microstructure descriptors enabled predictions to be made with higher accuracy than when predictions were made from a single descriptor. Feature importance of the descriptors was assessed through correlation analysis and occlusion sensitivity analysis. The results revealed that high strain tended to occur in the region where the hard phase, LPSO phase, had a large elongated phase oriented at a 45° direction to the loading direction. This result is consistent with other previous studies and indicates that the proposed method is effective in elucidating the relationship between the microstructure and the deformation behavior of the material.

Damage Detection and Localization Methodology Based on Strain Measurements and Finite Element Analysis: Structural Health Monitoring in the Context of Industry 4.0

This paper investigates the integration of Structural Health Monitoring (SHM) within the frame of Industry 4.0 (I4.0) technologies, highlighting the potential for intelligent infrastructure management through the utilization of big data analytics, machine learning (ML), and the Internet of Things (IoT). This study presents a success case focused on a novel SHM methodology for detecting and locating damages in metallic aircraft structures, employing dimensional reduction techniques such as Principal Component Analysis (PCA). By analyzing strain data collected from a network of sensors and comparing it to a baseline pristine condition, the methodology aims to identify subtle changes in local strain distribution indicative of damage. Through extensive Finite Element Analysis (FEA) simulations and a PCA contribution analysis, the research explores the influence of various factors on damage detection, including sensor placement, noise levels, and damage size and type. The findings demonstrate the effectiveness of the proposed methodology in detecting cracks and holes as small as 2 mm in length, showcasing the potential for early damage identification and targeted interventions in diverse sectors such as aerospace, civil engineering, and manufacturing. Ultimately, this paper underscores the synergistic relationship between SHM and I4.0, paving the way for a future of intelligent, resilient, and sustainable infrastructure.

Modeling local deformation, damage distribution, and phase transformation in zirconia particle-reinforced TRIP steel composites

This study investigates tensile deformation, damage analysis, and the effect of microtexture on a 10 % vol. zirconia particle-reinforced TRIP steel composite. In situ tensile tests and simulations were conducted, with sequential SEM images captured during tensile loading tests and electron backscatter diffraction (EBSD) providing initial crystal orientation data. SEM images were utilized in digital image correlation (DIC) analysis to calculate the local plastic strain distribution using VEDDAC software. Initial micrographs and EBSD data were transformed into geometry files for simulations, employing crystal plasticity with a dislocation density-based model and a newly proposed phase field approach-based ductile-brittle damage criterion for both austenite and ceramic particles. MATLAB's image processing functions were used to compare and validate experimentally observed damage instances with simulation predictions. The effect of microtexture combined with various physical quantities was explored using MTEX. The results show that the simulations accurately predict strain concentration around ceramic particles, with a higher strain observed in the middle region compared to the DIC observations. The simulated strain in the austenitic matrix is found to be 1.4 times higher than the DIC measurements. Quantitative analysis of damage pixels reveals that the proposed critical plastic strain value aligns closely with experimental results, showing an error of - 0.25 % compared to 1.121 % for a smaller strain case. This comprehensive analysis provides insight into the effect of microstructural attributes on material performance and behavior under tensile deformation, leading to a validated microstructural-informed damage-inclusive crystal plasticity model.

Shear deformation behavior of additively manufactured 316L stainless steel lattice structures

With the development of additive manufacturing, lattice structures have emerged as a disruptive technology across various fields, including aerospace, automotive, and defense. Lattice structures exhibit exceptional properties for lightweight and energy absorption capacities, especially under compressive loads. However, the lack of experimental data on complex loads, such as shear properties of lattice structures, poses challenges to their broader application across different load types. This study aims to analyze the shear deformation behavior of lattice structures and compare them with their compressive properties, enabling the safe design of lattice structures for various load applications. To achieve this, 316 L stainless steel lattice structures were fabricated using additive manufacturing in three different geometries: body-centered cubic (BCC), face-centered cubic (FCC), and octet truss (OCT), all at the same relative density. The lattice geometries were compared for their mechanical properties and deformation mechanisms under the two types of loading using DIC and FEM analyses, compression and shear tests. It was found that the mechanical properties of the lattice structures under compression and shear loads varied depending on the geometry, showing that each lattice geometry has a unique load-bearing capacity that exhibits superior mechanical properties. Furthermore, under shear loading, the struts of the BCC lattice structure showed a bending-dominated deformation mode due to the rotation of the nodes, whereas the struts of the FCC and OCT structures presented a compression and tension-dominated deformation mode. As a result, the BCC lattice specimens showed a higher shear energy absorption capacity than the other specimens and were capable of bearing higher loads. In addition, the FCC specimens showed lower shear energy absorption. The OCT specimens presented a uniform strain distribution under both compression and shear loading and showed compliant performance. The shear deformation behavior of various lattice structures has been evaluated by considering both geometrical and local strain distribution aspects. Implications for optimization guidelines and potential improvements were also discussed. These results can be used for research and development of the shear properties of lattice structures, the safe design of lattice structures, and industrial applications.

Application of the digital image correlation technique in wide width tensile test of geogrids

This paper aims to explore the DIC technique for use in wide width tensile testing of geogrids, from specimen preparation to selection of DIC parameters required for analysis to provide a guide for a proper use. A series of monotonically loaded wide width tensile tests were conducted on a PET geogrid to investigate the effects of specimen surface preparation methods and the user-defined DIC parameters. An additional set of tests under cyclic loading was conducted to investigate the effect of the image sampling rate. The results indicate that the speckle pattern (image texture) has a significant effect on the DIC results such that the larger is the speckle size, the greater is the uncertainty level, eventually leading to increased nonuniformity in the calculated strains. Also, it was revealed that a subset size smaller than optimal, typically 20–30 pixels, results in highly localized strain distribution. A similar trend was observed in step size. In addition, the image sampling rate was found to have a significant effect on the DIC-calculated cyclic strains, such that the lower is the image sampling rate, the lower is the calculated cyclic strain.

Strain distribution of a fir‐tree tenon/mortise structure under combined high and low cycle fatigue loads

AbstractThe fir‐tree tenon/mortise structure in areo‐engine suffers from the combined high and low cycle fatigue (CCF) loads during service. The structural integrity of the tenon/mortise structure is significantly affected by the local strain distribution, while the strain value under the CCF loads is difficult to acquire. In this study, a simulated specimen of tenon/mortise structure is designed, and the CCF test is carried out using a divided‐path loading fixture to avoid interference between the high cycle fatigue (HCF) loads and the low cycle fatigue (LCF) loads. The high speed digital image correlation (DIC) method is adopted to acquire the real‐time strain distribution during CCF test. According to the strain results, the critical position of the mortise and its strain variation are determined. The high cycle strain of the mortise is proportional to the vibration amplitude of the tenon. The experimental results can provide basis for strength and life assessment.

Shock response of gradient nanocrystalline CoCrNi medium entropy alloy

A comprehensive investigation of shock-induced deformation and spallation mechanisms in nanocrystalline CoCrNi medium-entropy alloys (MEAs) is conducted by large-scale molecular dynamics simulations, encompassing both gradient and homogeneous nanocrystalline structures. Under the shock velocity ranging from 0.2 km/s to 1 km/s, the effects of grain heterogeneity on shock wave propagation, spallation and defect evolution are examined. An inverse Hall–Petch relationship, where the Hugoniot elastic limit (HEL) increases with larger grain sizes is observed within the grain size range of 3 to 12 nm for homogeneous nanocrystalline samples. Compared to the localized strain distribution either within grain interior (GI) or at grain boundaries (GB) in homogeneous nanocrystalline structures, gradient-grained counterparts exhibit compatible deformation along shock direction, which coordinates the load partitioning between GB and GI. As a result, lower densities of dislocation and stacking fault are triggered within the GI, while shear localization is mitigated at GB, leading to higher spall strength observed in the gradient nanocrystalline MEA samples at the same shock velocity. The results shed light on the insights of deformation and spallation mechanisms in gradient-grained MEAs under dynamic loading, which may open up a new avenue for the design of advanced structural MEAs with high strength and toughness.

In Situ Analysis of Local Strain Distribution of Amorphous Polyrotaxane Adhesives Constrained by Metal Substrates

In Situ Analysis of Local Strain Distribution of Amorphous Polyrotaxane Adhesives Constrained by Metal Substrates

FeSiAl soft magnetic composite with double Al2O3 insulation layers for simultaneous high mechanical and magnetic properties

FeSiAl-based soft magnetic composites (SMCs), prepared from insulated FeSiAl powders, are widely applied in electronic devices. However, it is still challenging to achieve high magnetic and mechanical properties simultaneously due to the undesirable insulation layer. Here, double Al2O3 insulation layers are prepared for FeSiAl SMC. Atomic-scale characterizations reveal an in-situ epitaxial Al2O3 layer at FeSiAl surface under the catalysis of NaAlO2, and an outer amorphous Al2O3 layer by subsequent NaAlO2 hydrolysis. The above structure ensures effective insulation of FeSiAl powders and excellent magnetic properties of the FeSiAl/NaAlO2 SMC, with permeability of 101 and power loss of 128 mW/cm3 (50 mT, 100 kHz) respectively. Moreover, in-situ Al2O3/amorphous Al2O3 on FeSiAl matrix also leads to distinguished crush strength of 36.5 MPa for the core sample, which is ascribed to the enhanced adhesion at different interfaces as evidenced by similar local oxygen coordination and low strain distribution. This work provides a novel method to fabricate high-performance FeSiAl SMCs.

Local ratcheting and stress relaxation at notch root of CP-Ti specimen under cyclic loading: Experiment and simulation

Damage and crack always origin from notch, and it is necessary to understand the local ratcheting and stress relaxation at notch root for commercially pure titanium (CP-Ti). Symmetric strain-controlled cyclic test for smooth CP-Ti specimen is conducted to determine the cyclic constitutive relationship based on the combined hardening model. Asymmetric force-controlled cyclic test combined with Digital Image Correlation (DIC) technology is conducted to reveal local ratcheting strain distribution at notch root of CP-Ti specimen, while the distributions of stress, local stress ratio, and local hysteresis curve at notch root are obtained by finite element (FE) simulation. It is found that plastic zone, local ratcheting and stress relaxation at notch root area change significantly with cyclic load levels. In the low stress amplitude region with the mean nominal stress lower than 87 MPa, the small-scale yield occurs, and the notch root is in a state of plastic shakedown. With the mean nominal stress increasing to 96 MPa in the transition region, the notch root is in a state that exceeds the plastic shakedown limit but the continuous ratcheting progress is not found. In the high stress amplitude region with the mean nominal stress higher than 104 MPa, the plastic zone significantly expends, and the notch root is in a continuous ratcheting progress state, accompanied by stress relaxation. For a specific structural configuration, the local ratcheting at notch root is affected by both the load level and the cyclic deformation characteristics of the material. In the high stress amplitude region, the reverse yield occurs at the notch root during unloading, which promotes continuous ratcheting progress and stress redistribution, resulting in stress relaxation. The higher the load level, the larger the ratcheting strain rate, and the lower the stress relaxation rate.

Origins of HDI stress in copper–brass laminates with dual-heterostructured interfaces

In this work, a copper–brass hetero-laminate with dual-heterostructured interfaces was fabricated by diffusion welding in combination with cold deformation and annealing. These dual-heterostructured interfaces possess the characteristics of both gradient interface and sharp interface. The evolutions of geometrically necessary dislocations (GNDs) and local strains near these dual-heterostructured interfaces were investigated by in-situ and quasi in-situ tests. By employing a constitutive model, the evolutions of GND density and the HDI synergistic strengthening mechanism were systematically explored. The results demonstrated that the distribution of GNDs and local strains near the coarse/fine-grained interface exhibited a gradient feature, without any obvious concentrations of GNDs and strains, which can be attributed to the transition effect of the gradient interface. Furthermore, the density of sample-scale GNDs accumulated near the coarse/fine-grained interface was found to be at least one order of magnitude lower than that of grain-scale GNDs accumulated near grain boundaries. Both the constitutive model and experimental results confirmed that grain-scale GNDs serve as the primary origins of significant HDI stress, rather than the sample-scale GNDs.

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.

In-situ analysis of SCC initiation on Alloy 600 surface in primary water environment using digital image correlation with electron backscatter diffraction

This study examined the stress corrosion cracking (SCC) phenomenon of Alloy 600 in a high-temperature primary water environment. The investigation involved the observation of local plastic deformation and SCC initiation on the specimen surface using an observation apparatus, comprising a laser confocal microscope (LCM) and an autoclave equipped with a diamond window view-port. Prior to conducting the SCC initiation test, the crystalline structure of the specimen, including grain boundary (GB) characteristics, Schmid factor, and trace inclination angle, underwent examination using an electron backscatter diffraction (EBSD) technique. The local strain distribution was subsequently analyzed using a digital image correlation (DIC) method, employing LCM images obtained during the SCC experiments. The ex-situ EBSD analysis indicated that SCC mainly initiated at random high-angle grain boundaries (HAGB) with trace inclination angles close to 90° relative to the tensile axis. DIC analysis further revealed that most GBs in the vicinity of the highly strained area cracked among random HAGBs with a high trace inclination angle to the tensile axis. This confirms that the local strain gradient at GBs plays an important role in SCC initiation, serving as a more suitable indicator of an additional stress component to effective normal stress acting on GBs compared to the Schmid factor mismatch.

Measurement of residual elastic strain in rolled-up amorphous nanomembranes using nanobeam electron diffraction

Amorphous nanomembranes play a crucial role in flexible electronics due to their ability to create intricate 3D structures through strain engineering. To better understand the formation of these structures, accurately mapping the local elastic strain distribution is essential. In this study, we conducted position-sensitive nanobeam electron diffraction investigations on various rolled-up amorphous nanomembranes. By analyzing the diffraction rings obtained from different locations on the amorphous samples, we extracted anisotropic structure information in reciprocal space and determined the local strain distributions in real space. Our analysis revealed that particle-assisted dry-released samples exhibited higher strain values than pure amorphous samples. This suggests that nanoparticles introduce additional strain through dewetting effects, thereby facilitating the formation of self-rolling 3D structures.

Effects of mechanical heterogeneity and dislocation storage capacity on the strength–ductility synergy of heterostructured laminates

Heterostructured laminates have attracted significant attention in the materials community owing to their exceptional combination of strength and ductility. However, long-standing questions remain regarding the impact of the strength difference between the soft and hard layers on the synergistic strengthening in hetero-laminates with weak mechanical heterogeneity; the role played by soft/hard layers with various dislocation storage capacities on the strength–ductility synergy is also poorly understood. In this study, a series of typical heterostructured copper–brass laminates were fabricated using the diffusion welding + forging + rolling + annealing technique. The results demonstrated that larger microstructural and mechanical heterogeneities improved the strength–ductility combination of the hetero-laminates. The relatively weak mechanical heterogeneity and superior dislocation storage capacity of the soft copper and hard brass layers facilitated inter-layer synergistic deformation. This induced a gradual distribution of local strains and geometrically necessary dislocations (GNDs) near the hetero-interface. Thus, when evaluating the role of soft layers in synergistic strengthening, it is crucial to consider their generation and storage capabilities for dislocations.

Implications of the micromechanical Taylor Factor on work hardening parameters: New perspectives from FFT simulations in DAMASK

This study uses Fast Fourier Transform (FFT) simulations in the Düsseldorf Advanced Material Simulation Kit (DAMASK) to investigate the influence of the micromechanical Taylor Factor on the work hardening parameters. Departing from the assumption of a homogeneous deformation in the classical Taylor model, we use the micromechanical Taylor Factor, which takes better account of local grain interactions and strain distributions. Our comparison of simulations using both classical and micromechanical Taylor factors reveal crucial differences in the predicted dislocation mechanisms, leading to different interpretations of work hardening behavior. Our results question the adequacy of conventional models and suggest a more sophisticated understanding of the deformation of polycrystalline materials. The study underscores the importance of an improved modeling approach to characterize deformation and leads to a re-assessment of existing predictive mechanisms in materials science with possible implications for optimizing of material performance.

On the reduced damage tolerance of fine-grained nuclear graphite at elevated temperatures using in situ 4D tomographic imaging

Fine-grained nuclear graphite is one of the key structural materials for high temperature gas-cooled reactors as well as several Generation IV nuclear fission reactor designs. However, its deformation and fracture behaviours at elevated temperatures are not well understood. In light of this, the current study focused on investigating the flexural strength and fracture toughness of two fine-grained graphite (SNG623 and T220) using real-time X-ray computed micro-tomography imaging at room temperature, 750 °C and 1100 °C. Specifically, nonlinear-elastic fracture mechanics-based JR(Δa) R-curves at these temperatures were presented with evolution of damage and failure micro-mechanisms, local strain distributions and J-integral fracture analysis, purveying notable findings. Compared to the coarser-grained Gilsocarbon nuclear graphites used in the current UK Advanced Gas-cooled Reactors (AGRs), these modern fine-grained graphites display deficient fracture resistance in the form of far less stable crack growth prior to catastrophic fracture and reduced failure strain at 1100 °C. Moreover, their elevation in strength and toughness at high temperatures is remarkably lower than that of Gilsocarbon graphite. Based on in situ high-temperature Raman spectroscopy mapping, we believe that one of the major causes of this behaviour can be attributed to the smaller magnitude of ‘frozen-in’ residual stress relaxed at elevated temperatures compared with Gilsocarbon graphite.

Study on local strain distribution and grain boundary characteristics of tensile-deformed TRIP-assisted medium manganese steel

The current study emphasizes the local strain distribution and grain boundary characteristics of tensile-deformed Fe-6.72Mn-3.92Al-0.18C medium manganese steel (MMnS), which has undergone thermomechanical processing followed by intercritical annealing at various temperatures (700, 750 and 800 °C for 1 h). Microstructural examination through electron backscattered diffraction (EBSD) analysis is executed to evaluate the local strain distribution, and grain boundary misorientation before and after the tensile test. In the annealed samples, the Kernel average misorientation (KAM) of the face-centered cubic (FCC) phase is close to 0°, indicating strain-free austenite grain formation. Before the tensile test, the body-centered cubic (BCC) phase is relatively free from local strain; however, as the misorientation increases due to tensile deformation, the BCC/FCC interphase boundaries become highly strained. The sum of the fraction of low-angle grain boundaries (ΣLAGBs) decreases with an increase in annealing temperature, while the sum of fractions of misorientation greater than 15°, excluding coincident site lattice (CSL) boundaries, increases moderately. With an increase in annealing temperature, the sum of the CSL special boundary fractions (Σ3 and Σ9) increases considerably. The weighted average density of LAGBs in the BCC phase is higher in the sample annealed at 700 °C compared to the other annealed samples. The increase in the fraction of LAGBs in the FCC phase during tensile deformation is due to the significant transformation-induced plasticity (TRIP) effect and high austenite transformation ratio. Crack formation, propagation, and branching and blunting in the tensile-tested 750 °C-annealed sample are examined by the EBSD analysis. Crack propagation is hindered by the existence of γ-fiber texture components and high-angle grain boundaries in the sample annealed at 750 °C.