Digital Image Correlation Measurements Research Articles

Improved image preprocessing method applicable to high-temperature digital image correlation measurement

Improved image preprocessing method applicable to high-temperature digital image correlation measurement

Stereo-DICNet: An efficient and unified speckle matching network for stereo digital image correlation measurement

Digital image correlation (DIC) is an image-based, non-contact optical measurement technique for measuring the deformation of objects. Recently, deep learning has been preliminarily used in monocular-DIC measurement and has brought significant advantages in efficiency and robustness. However, the deep learning-based stereo-DIC methods is still underexplored, although stereo-DIC can measure three-dimensional deformation and shape compared to monocular-DIC. Different from monocular-DIC, stereo-DIC not only requires temporal matching (inter-frame) but also stereo matching (inter-camera), and the difference in image deformation scale between the two matching tasks make it difficult to perform them using a unified network. To solve this problem, we propose a unified speckle matching network that can be used for deformation measurement at different scales, referred to as Stereo-DICNet. Stereo-DICNet consists of three modules: feature extraction, cost volume generation, and displacement map prediction. Firstly, the feature extraction module composed of the residual layer and the spatial pyramid pooling layer is designed to extract the features of different receptive fields. Then, an attention connect volume was proposed to represent the similarity information between pixels at different spatial scales. Finally, the displacement map is predicted by a stacked hourglass-shaped sub-net. Simulation and physical experiments verify that Stereo-DICNet can perform both temporal matching and stereo matching. Moreover, material tensile experiments show that the MAE of Stereo-DICNet in the U, V, and W directions are no more than 0.005 mm, 0.015 mm, 0.01 mm respectively.

An Optimization Approach for Creating Application-specific Ultrasound Speckle Tracking Algorithms

ObjectiveUltrasound speckle tracking enables in vivo measurement of soft tissue deformation or strain, providing a non-invasive diagnostic tool to quantify tissue health. However, adoption into new fields is challenging since algorithms need to be tuned with gold-standard reference data that are expensive or impractical to acquire. Here, we present a novel optimization approach that only requires repeated measurements, which can be acquired for new applications where reference data might not be readily available or difficult to get hold of. MethodsSoft tissue motion was captured using ultrasound for the medial collateral ligament (MCL) of three quasi-statically loaded porcine stifle joints, and medial ligamentous structures of a dynamically loaded human cadaveric knee joint. Using a training subset, custom speckle tracking algorithms were created for the porcine and human ligaments using surrogate optimization, which aimed to maximize repeatability by minimizing the normalized standard deviation of calculated strain maps for repeat measurements. An unseen test subset was then used to validate the tuned algorithms by comparing the ultrasound strains to digital image correlation (DIC) surface strains (porcine specimens) and length change values of the optically tracked ligament attachments (human specimens). ResultsAfter 1500 iterations, the optimization routine based on the porcine and human training data converged to similar values of normalized standard deviations of repeat strain maps (porcine: 0.19, human: 0.26). Ultrasound strains calculated for the independent test sets using the tuned algorithms closely matched the DIC measurements for the porcine quasi-static measurements (R > 0.99, RMSE < 0.59%) and the length change between the tracked ligament attachments for the dynamic human dataset (RMSE < 6.28%). Furthermore, strains in the medial ligamentous structures of the human specimen during flexion showed a strong correlation with anterior/posterior position on the ligaments (R > 0.91). ConclusionAdjusting ultrasound speckle tracking algorithms using an optimization routine based on repeatability led to robust and reliable results with low RMSE for the medial ligamentous structures of the knee. This tool may be equally beneficial in other soft-tissue displacement or strain measurement applications and can assist in the development of novel ultrasonic diagnostic tools to assess soft tissue biomechanics.

Nanoscale speckle patterning for combined high‐resolution strain and orientation mapping of environmentally sensitive materials

AbstractScanning electron microscopy‐based high‐resolution digital image correlation (HRDIC) is now an established technique, providing full‐field strain and displacement measurement at the microscale. Techniques for generating speckle patterns for sub‐micron strain mapping can often be either substrate dependent or rely on applying aggressive conditions which may alter the microstructure of interest or damage the substrate in highly sensitive materials. We detail a modification of a methodology successfully applied in the literature to allow its use with metallic materials that are particularly sensitive to the corrosive media, such as copper‐base alloys. Nanometre‐thick silver films, applied with physical vapour deposition, are remodelled using NaBr in non‐aqueous isopropanol, replacing the aqueous solution of NaCl in the original method, forming a uniform dispersion of silver islands highly suitable for digital image correlation (DIC) measurement. The entire procedure is performed at ambient temperature. We find that the DIC pattern is suitably electron transparent to allow electron backscatter diffraction (EBSD) measurements without pattern removal, producing diffraction patterns of sufficient quality for cross‐correlation based high‐angular resolution EBSD. This property facilitates simultaneous EBSD and DIC mapping experiments, providing deeper insights into the kinematics of plastic deformation in crystalline materials. Sub‐100 nm islands are achieved through control of the sputter coating parameters, resulting in DIC cross‐correlation subwindows of 140 nm with a 50% overlap. This resolution is sufficient to capture the fine detail of strain localisation phenomena during plastic deformation, demonstrated here with a case study in CuCrZr, a precipitation‐hardened heat sink material for application in nuclear fusion components. Here, we extract full‐field displacement data with DIC and corresponding orientation information using EBSD without the need for pattern removal.

Experimental investigation on axial compressive and splitting tensile behaviour of reinforced concrete with a low content of hybrid steel fibres

This study intends to explore how the hybrid steel fibre reinforcement with a low content affects the mechanical characteristics of concrete. The axial compression properties of hybrid steel fibre-reinforced concrete (HSRC) were systematically investigated regarding stress-strain curves, failure modes, elastic modulus, and toughness. The crack propagation and fracture process of HSRC were analyzed based on the cumulative crack width and horizontal strain field. The results showed that the micro-straight steel fibre (mSF) demonstrated a more pronounced influence on post-peak stress retention in the axial compression test, whereas macro hooked-end steel fibre (MHF) positively affected the peak strain enhancement. The average peak stress of the HSRC increased by 8.6 % higher than that of H40 and 16.4 % higher than that of S40. It is noteworthy that the mixing of steel fibres at a low fibre content was detrimental to the compressive toughness. The splitting tensile test showed a transitional stage before the HSRC reached its peak load, during which the load steadily increased and crack propagation slowly occurred. Moreover, the digital image correlation (DIC) measurement results suggested that mSF can enhance the internal quality of concrete. The study contributes to a deep understanding of the synergistic reinforcement mechanism and the fracture processes in HSRC.

A Comprehensive Review of In Situ Measurement Techniques for Evaluating the Electro-Chemo-Mechanical Behaviors of Battery Electrodes.

The global production landscape exhibits a substantial need for efficient and clean energy. Enhancing and advancing energy storage systems are a crucial avenue to optimize energy utilization and mitigate costs. Lithium batteries are the most effective and impressive energy utilization system at present, with good safety, high energy density, excellent cycle performance, and other advantages, occupying most of the market. However, due to the defects in the electrode material of the battery itself, the electrode will undergo the process of expansion, stress evolution, and electrode damage during electro-chemical cycling, which will degrade battery performance. Therefore, the detection of property changes in the electrode during electro-chemical cycling, such as the evolution of stress and the modulus change, are useful for preventing the degradation of lithium-ion batteries. This review presents a current overview of measurement systems applied to the performance detection of batteries' electrodes, including the multi-beam optical stress sensor (MOSS) measurement system, the digital image correlation (DIC) measurement system, and the bending curvature measurement system (BCMS), which aims to highlight the measurement principles and advantages of the different systems, summarizes a part of the research methods by using each system, and discusses an effective way to improve the battery performance.

An enhanced deep learning approach for vascular wall fracture analysis

AbstractThis work outlines an efficient deep learning approach for analyzing vascular wall fractures using experimental data with openly accessible source codes (https://doi.org/10.25835/weuhha72) for reproduction. Vascular disease remains the primary cause of death globally to this day. Tissue damage in these vascular disorders is closely tied to how the diseases develop, which requires careful study. Therefore, the scientific community has dedicated significant efforts to capture the properties of vessel wall fractures. The symmetry-constrained compact tension (symconCT) test combined with digital image correlation (DIC) enabled the study of tissue fracture in various aorta specimens under different conditions. Main purpose of the experiments was to investigate the displacement and strain field ahead of the crack tip. These experimental data were to support the development and verification of computational models. The FEM model used the DIC information for the material parameters identification. Traditionally, the analysis of fracture processes in biological tissues involves extensive computational and experimental efforts due to the complex nature of tissue behavior under stress. These high costs have posed significant challenges, demanding efficient solutions to accelerate research progress and reduce embedded costs. Deep learning techniques have shown promise in overcoming these challenges by learning to indicate patterns and relationships between the input and label data. In this study, we integrate deep learning methodologies with the attention residual U-Net architecture to predict fracture responses in porcine aorta specimens, enhanced with a Monte Carlo dropout technique. By training the network on a sufficient amount of data, the model learns to capture the features influencing fracture progression. These parameterized datasets consist of pictures describing the evolution of tissue fracture path along with the DIC measurements. The integration of deep learning should not only enhance the predictive accuracy, but also significantly reduce the computational and experimental burden, thereby enabling a more efficient analysis of fracture response.

Mode I fracture propagation and post-peak behavior of sandstone: Insight from AE and DIC observation

Microcracks generated at the crack tip, particularly in quasi-brittle materials such as sandstone, give rise to a non-linear region known as the fracture process zone (FPZ), which cannot be analyzed using linear elastic fracture mechanics (LEFM). To gain insights into the nonlinear fracturing behavior of rock, this study conducted three-point bending experiments on sandstone beams with prefabricated notch. Stable mode-Ⅰ fracture propagation during the post-peak stage was achieved by the monotonic increment of crack mouth opening displacement (CMOD) measured by a clip gauge. The acoustic emission (AE) and digital image correlation (DIC) techniques were utilized to visually track the detailed crack propagation process from both micro and macro perspectives. Nonlinear fracture parameters, including FPZ length, were identified by AE and DIC measurements and compared with the theoretical calculation. A nonlinear fracture criterion relating applied load, equivalent crack length, and stress intensity factor were established, from which the theoretical CMOD-P curve at the post-peak stage were obtained and validated with experimental measurement from MTS machine. The results reveal that microcracks formulating the FPZ materialize in the pre-peak stage, with the majority emerging post-peak to facilitate the propagation of the traction-free crack and FPZ in the loading direction, as evidenced by AE locations and DIC displacement gradient. Throughout the crack propagation process, over 65 % of microcracks are estimated to be tensile mode based on both AF/RA and polarity analysis, aligning with anticipated outcomes from macroscopic three-point bending fracture tests. Energy dissipation primarily occurs within the FPZ to facilitate separation of crack faces, with the dissipated energy region advancing towards the beam boundary. By integrating AE source locations and DIC displacement gradient, the FPZ length of the 3-point-bending sandstone specimens is determined to be lp = 8.1 mm, slightly exceeding the theoretical FPZ lengths calculated by the Irwin model (lp = 7.0 mm) and the Dugdale-Barenblatt model (lp = 7.6 mm). Using a non-linear fracture criterion that incorporates the FPZ and employing the in-time equivalent crack length measured by AE and DIC as an intermediate variable, the theoretical applied load-crack length curve during the post-peak stage is calculated, showing close agreement with load cell measurements with a negligible error of approximately 2 %. Furthermore, the theoretical post-peak CMOD vs. load curve aligns with experimentally obtained results, demonstrating a non-linear crack propagation speed at a constant CMOD rate, wherein crack propagation speed decreases as crack length increases.

Three-dimensional deformation measurement techniques in space multi-physical fields based on digital image correlation

Aiming at the three-dimensional deformation measurement requirement of the high-stability structure of spacecraft in space multi-physical fields, a comprehensive instrument protection device in a vacuum high-low temperature environment is developed using ultra-low temperature anti-condensation and vacuum heat insulation technology. This ensures that the digital image correlation (DIC) measurement camera always remains in a constant temperature and pressure environment and can stably exert its original efficiency in space multi-physical field environments. Subsequently, a path-dependent DIC method based on a graphics processing unit computing parallel acceleration is proposed by combining a pre-interpolation strategy and integrated image technology, which realizes high-precision matching of digital images before and after deformation in a spatial multi-physical field environment and improves the computing efficiency. To address the problem that a common reference point may deform significantly in a space environment with a large temperature change, which leads to inaccurate system accuracy, combined with a random sampling consistency algorithm and singular value decomposition method, the optimal registration of the coordinate system and the solution of deformation were realized by setting the threshold of the reference point margin screening. Finally, a three-dimensional deformation measurement system in high- and low-temperature vacuum environments was built, and the three-dimensional deformation measurement of the satellite antenna in high- and low-temperature vacuum environments was verified, which proves the effectiveness and accuracy of the above method.

Experimental investigations of dry-dry timber-concrete composite notched connections designed for disassembly

Timber-Concrete Composite (TCC) decks offer an efficient, and environmentally friendly alternative to conventional concrete slabs. Traditionally the concrete is cast directly onto the timber with a permanent connection arrangement that does not allow non-destructive disassembly at end-of-service-life. This will prevent the reuse of the materials and ultimately decrease the extent of the environmental benefit. This paper presents a deconstructable dry-dry notched connection made from fully prefabricated timber and concrete elements. The concept was investigated through 30 experimental pure-shear push-off tests, where various design parameters, such as prestressing degree, fastener placement, and notch length, were varied. The experimental results indicate that the new connection exhibits the same stiffness characteristics as a traditional wet-dry notched connection. Five different failure modes were observed in the series, and experimental measurements such as Digital Image Correlation and fastener force measurement provided insight into the connection behavior during failure. Two rigid-plastic upper bound solutions were established to estimate the load-carrying capacity for two of the observed concrete failures, and a satisfactory agreement with the test results was obtained. In conclusion, the experimental results indicated that dry-dry notched connections are feasible for achieving a more environmentally friendly TCC structure.

An Exploration of Grain-Averaged Stress Measurement Using Partial Unloads with In situ Microscopic Image Correlation

BackgroundIn polycrystal mechanics, determination of stress is associated with diffraction methods that measure (the inherently-related) elastic strain. Microscopic digital image correlation (DIC), while commanding much higher intragranular resolution, measures total strain, and its local accuracy is typically insufficient to evaluate elastic strain magnitudes.ObjectiveIn situ DIC measurements over a partial unload of the polycrystal, where strains are virtually elastic, are explored for grain-averaged elastic strains and then, through a posed formalism, the stresses at the point of unload. Grain averaging is functionally employed to improve the DIC accuracy. The large objective is to emulate in situ complementary diffraction.MethodsNickel with high elastic anisotropy is chosen. The utilized highly-automated instrument offers maximal resolution for DIC with optical microscopy over a gross grain field. Orientations are predetermined for the same grain layer via electron backscatter diffraction. High-accuracy grain masks are produced to isolate the strain fields of individual grains.ResultsVery promising results are shown over a number of grains with sensible apparent compliance and stress values as well as linear unload behavior. Grains with sane results are largely predicted by a posed objectivity test that relies on DIC repeated with multiple reference loads.ConclusionThough it will require extremely careful implementations of microscopic DIC with high intragranular resolution, the premise of measuring intergranular stress fields via partial unloads seems to be viable and worthy of further exploration and verification. This capability that is superposed over strain measurement offers a more stringent validation of high-fidelity crystal plasticity models.

I-section steel columns strengthened by wire arc additive manufacturing - concept and experiments

Wire arc additive manufacturing (WAAM) is a method of metal 3D printing which, when strategically combined with traditional methods of manufacture, has the potential to make a significant impact on the construction sector. To illustrate this potential, an experimental investigation into the flexural buckling response of 15 hot-rolled I-section columns, strengthened by WAAM, has been undertaken and is presented herein. The WAAM material was added at the flange tips and distributed non-uniformly along the member length. Complementary tensile coupon tests on the I-section and WAAM steel material were also carried out. 3D laser scanning was employed to determine the geometry and global geometric imperfections of all specimens, while digital image correlation measurements were taken to provide a detailed insight into the surface deformation characteristics of the specimens during testing. The WAAM stiffeners provided three mechanisms of strengthening: (1) increased cross-sectional area and second moment of area, (2) enhanced local buckling capacity and (3) a more favourable residual stress pattern. Disproportionately high increases in axial capacity of between 17% and 54% for relatively modest increases of mass between 2% and 26% (of the weight of the strengthened I-section column) were achieved, demonstrating, for the first time, the substantial benefits in terms of enhanced structural efficiency and material savings, that can be realised by the combination of WAAM with traditional manufacturing methods. In the best case, the efficiency of the WAAM material (in terms of increase in load-bearing capacity per unit mass) was almost eight times that of the bare steel section. The described hybrid manufacturing concept can be employed to improve the performance and reduce the environmental impact of structures and could be a game changer in both new construction and the rehabilitation of existing infrastructure.

Method of generating speckle patterns for digital image correlation based on modified Conway's Game of Life.

The measurement accuracy of digital image correlation (DIC) is influenced by the quality of the speckle pattern. Although various models for generating random speckle patterns have been well discussed, obtaining appropriate speckle images with isotropic quality and performance could be a challenging issue in DIC. In this paper, we propose a novel (to our knowledge) method for generating speckle patterns based on modified Conway's game of life (GoL). By sequentially assembling the speckle patterns generated from the modified GoL, we produced the GoL speckle image. Then, verification and comparison experiments were conducted through pure in-plane translations. The results show that the generated speckle image which was resized with k s=6& k r=2 processing and subsequently fuzzified using a Gaussian filter, produces the best accuracy for DIC measurement. Furthermore, based on the rigid body in-plane rotation displacement tests in the physical experimental results of three different speckle images, the GoL speckle generated from our proposed method shows the smallest measurement error. This indicates that the proposed speckle patterns generating method could provide a new type of speckle pattern with better quality and accuracy.

In-situ characterization of thermo-mechanical deformation fields based on DIC during orthogonal cutting of Ti6Al4V

Cutting phenomena containing abundant kinematic and thermo-mechanical information during the cutting process provide direct evidence for investigating machining mechanisms. In-situ digital image correlation (DIC) measurements are important methods for obtaining dynamic data such as strain and strain rate during the material deformation process. However, the secondary development of those deformation data is limited. The deformation information obtained by DIC is also limited to understanding complex thermo-mechanical deformation. In this study, an in-situ imaging system was developed to simultaneously acquire gray and infrared radiation (IR) images within the same deformation area during orthogonal cutting of Ti6Al4V. The deformation evolution and thermal history can be obtained simultaneously. In addition, a postprocessing methodology based on the DIC was proposed to derive thermo-mechanical information. Based on the postprocessing methodology, the stress fields were determined by the constitutive equation combined with the strain, strain rate, and temperature data. The stress propagation path during the formation of the chip segment was determined. Finally, the strain energy fields were derived to assess the potential energy of plastic deformation with the generation of the adiabatic shear band. The dependency between the machining phenomena and the cutting parameters was investigated. This work provides a novel understanding of the thermo-mechanical coupling behavior during the generation progress of serrated chips. The nature of the formation mechanisms and the deformation process involved for serrated chips are closely revealed.

Lagrangian operator inference enhanced with structure-preserving machine learning for nonintrusive model reduction of mechanical systems

Complex mechanical systems often exhibit strongly nonlinear behavior due to the presence of nonlinearities in the energy dissipation mechanisms, material constitutive relationships, or geometric/connectivity mechanics. Numerical modeling of these systems leads to nonlinear full-order models that possess an underlying Lagrangian structure. This work proposes a Lagrangian operator inference method enhanced with structure-preserving machine learning to learn nonlinear reduced-order models (ROMs) of nonlinear mechanical systems. This two-step approach first learns the best-fit linear Lagrangian ROM via Lagrangian operator inference and then presents a structure-preserving machine learning method to learn nonlinearities in the reduced space. The proposed approach can learn a structure-preserving nonlinear ROM purely from data, unlike the existing operator inference approaches that require knowledge about the mathematical form of nonlinear terms. From a machine learning perspective, it accelerates the training of the structure-preserving neural network by providing an informed prior (i.e., the linear Lagrangian ROM structure), and it reduces the computational cost of the network training by operating on the reduced space. The method is first demonstrated on two simulated examples: a conservative nonlinear rod model and a two-dimensional nonlinear membrane with nonlinear internal damping. Finally, the method is demonstrated on an experimental dataset consisting of digital image correlation measurements taken from a lap-joint beam structure from which a predictive model is learned that captures amplitude-dependent frequency and damping characteristics accurately. The numerical results demonstrate that the proposed approach yields generalizable nonlinear ROMs that exhibit bounded energy error, capture the nonlinear characteristics reliably, and provide accurate long-time predictions outside the training data regime.

Point-fixed connections in structural glass with injection mortar infill. experimental investigation and numerical simulations

This paper focuses on the experimental and numerical study of the performance of point-fixed connections in structural glass panels. For that purpose, an experimental program of in-plane uniaxial compressive tests was conducted on small-scale specimens. The specimens were manufactured using monolithic and laminated heat-strengthened glass (HSG). The point-fixed connections were realized using two infill products: (a) fischer FIS V Plus and (b) Hilti Hit HY270. A Digital Image Correlation (DIC) based measurement system in combination with strain gauges is used for tracking kinematics and deformations of the investigated specimens. It was found that the performance of the two infill products is comparable. The tests demonstrate that the failure mode of the monolithic panels is governed by infill failure, whereas the failure of the laminated panels is controlled by glass fracture. The numerical study was performed to assess the stress distribution around the hole in laminated glass. A 3D Finite Element (FE) model built in Abaqus is validated based on DIC and strain gauge measurements. A sensitivity analysis was performed on the eccentricity in the laminated glass panels and the friction coefficient between infill and glass/steel. It was observed that the eccentricity in the laminated panels has a significant influence on the stress amplification in the panels. Also, it was found that the value of the friction coefficient can significantly change the distribution of the principal stresses around the hole.

High-temperature thermal disturbance elimination method based on multi-channel extraction and correction.

Digital image correlation (DIC) technology has been widely used in high-temperature measurement fields. However, due to the complexity of high-temperature environments, there are many interference factors that limit the development of high-temperature DIC technology, among which thermal disturbance is one of the most significant factors that severely affects the measurement accuracy of high-temperature DIC. In this paper, a multi-channel separation technique combined with a low-cost laser speckle device is proposed to eliminate thermal disturbance errors in high-temperature DIC measurements. First, a blue laser speckle generation system is independently designed to produce the most suitable speckle particles, and the best laser speckle is determined and projected onto the blue background white spot pattern. Then a green LED illuminates the sample to provide illumination for the sample's own grayscale characteristics. A color camera collects photos, and the obtained images are processed with channel separation to extract and calculate the displacement of different channels. Finally, the displacement fields of the green and blue channels are subtracted to separate the thermal disturbance error and correct the measurement values. In this paper, a laser speckle projection system is first assembled, followed by a comprehensive evaluation of the projected speckle and, finally, a DIC experimental system is constructed for verification experiments at both room temperature and high temperature, and the corrected values are compared with the true values. The results show that the corrected values are highly consistent with the true values, which verifies the reliability of the proposed method.

Research on high-precision digital image correlation measurement techniques for highly stable structures

Abstract This study proposes a novel digital image processing system that combines a diffraction-limited resolution (DLRF)-based measurement technique with a windowed form-center tracking algorithm. To evaluate the accuracy of this system, this paper compares and analyzes the effectiveness of conventional digital image techniques and DLRF-based methods for deformation displacement measurements. In addition, the study includes thermal stability tests under ambient noise and uniform high temperature conditions to evaluate the stability performance of the system in a complex environment. The experimental results show that the DLRF-based digital image correlation method proposed in this study performs well in reducing the mean deviation (from a maximum of 5.17 × 10-3 to 1.73 × 10-3) and root-mean-square error (from a maximum of 5.14 × 10-3 to 0.75 × 10-3). It is worth noting that the DLRF method is faster in processing when using the single-precision format than the double-precision format, with a speedup of up to 1.05 times. In addition, the multiple displacement averaging processing method can effectively filter the noise in the test, and the noise effect is only in the range of 0 to 2 μm in most areas. In the analysis of test points 10-34 and 57-80, the displacement error is controlled within 5 μm, indicating that the modified structural analysis model can be used for on-orbit micrometer-scale thermal deformation analysis. The study proves the high accuracy and stability of the digital image system proposed in this paper in the measurement of deformation displacement, which provides adequate technical support for accurate measurement in related fields.

Experimental investigation of the rotational characteristics of a novel hybrid laminated glass beam-column connection prototype

This article presents an experimental investigation of the rotational characteristics of a novel hybrid laminated glass beam-column connection prototype. The testing of small-scale prototype specimens is conducted to assess the effect of the main structural parameters such as glass pane thickness, interlayer and reinforcement properties on the rotational characteristics. To explore the possibility of using different lamination technologies for the realization of the joints, two different bonding technologies are considered separately: (a) autoclave and (b) UV-curing lamination. The specimens are subjected to a pair of self-equilibrated compressive forces up to failure in a custom-made setup. Digital image correlation (DIC) measurements together with strain gauges are used for tracking the kinematics of the main components. It was found that the prototype is able to develop a significant post-fracture and rotational capacity in a properly balanced combination of structural and material parameters. The results are presented in terms of moment-rotation (M - θ) curves, strains evolution in the steel and evolution of the crack patterns. Experimental observations suggest that depending on the choice of structural parameters three failure modes are possible: (a) crushing failure of the joint core, (b) plastic failure of the adjoining sections and (c) failure due to debonding of one of the bars in tension. It is concluded that one can achieve a desirable post-fracture and ultimate behaviour by properly balancing the structural and material parameters. Additionally, pull-out opening and tensile stresses in steel are measured at the interrupted section, revealing a transition in sectional behaviour from fully interrupted to a hybrid section over a certain transition length.

Behavior of TRIP-aided medium Mn steels investigated by in situ synchrotron X-ray diffraction experiments and microstructure-based micromechanical modelling

Medium Mn steels belong to a new generation of advanced high-strength steels whose superior mechanical properties are explained by their ultrafine-grained ferrite/austenite/martensite microstructures and a possible transformation induced plasticity (TRIP) related to the stability of retained austenite. The mechanical behaviour of a set of model medium Mn steels is investigated during tensile testing using a combination of high-energy X-ray diffraction (HEXRD) and digital image correlation (DIC) measurements. HEXRD allows for the time-resolved determination of transformation kinetics and in situ stress partitioning among the different constituting phases. DIC provides precise spatiotemporal information on the strain evolution along the gauge length, particularly at the position of the diffracting volume. These experiments served to calibrate an innovative mean field micromechanical model which accounts for the local behaviour of each phase, as well as the strain-induced martensitic transformation (SIMT) of retained austenite. The work-hardening of both austenite and ferrite is modeled using a dislocation-based size-sensitive approach which includes kinematic hardening contributions. The behaviours of fresh and strain-induced martensite are predicted using a genuine model derived from the continuous composite approach. The model for SIMT is based on a thermodynamic assessment of the stability of retained austenite. The overall model is thus sensitive to the size of the microstructure components, their local chemistry, and their respective stability.