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The high source stability and brightness of field emission gun equipped scanning electron microscopes (SEM) makes them ideal for high‐resolution digital image correlation (HRDIC). However, their high initial capital cost can be prohibitive for research organisations and groups. Conventional thermionic SEMs using either a tungsten hairpin or LaB6 filament are far more widespread due to their lower cost. Whilst it is understood that overall performance and ultimate resolution are lower than field emission SEMs, we propose that there is no fundamental reason why these instruments are unsuitable for HRDIC. We investigate the use of a LaB6 SEM as a viable tool for HRDIC. We detail the subtleties of performing HRDIC using a LaB6 thermionic source SEM, providing technical recommendations for best practices in using these instruments for strain mapping. The effects of instrument parameters on strain measurement noise are examined, with a focus on parameters of key relevance to in situ and ex situ mechanical testing. Errors in focus and image pixel size are found to be the primary contributors to the strain noise floor values, with stage accuracy being of secondary importance. We present a case study in oxygen‐free high‐conductivity copper, OFHC‐Cu, which is used in the designs of nuclear fusion components as a heat sink interlayer. Heterogeneous strain patterns are observed in this material, with high levels of strain localisation at grain boundaries. Active slip systems are identified using the relative displacement ratio method, demonstrating the quality of these data and the suitability of LaB6 instruments for HRDIC strain mapping, achieving performance approaching that expected of a field emission SEM. [ABSTRACT FROM AUTHOR]

This paper details the utilization of the focused ion beam digital image correlation (FIB-DIC) technique for measuring in-plane displacements and the employment of the height digital image correlation (hDIC) technique as a two-step DIC method for determining displacements without an out-of-plane component within the region of interest. Consideration is given to the microscopy data's measurement scale and resolution to confirm the capability of both techniques to conduct micro-scale correlations with nano-scale sensitivity, making them suitable for investigating the residual elastic strains formed due to processing. The sequential correlation procedure of the FIB-DIC technique has been optimized to balance accuracy and performance for correlating sequential scanning electron microscope (SEM) images. Conversely, the hDIC technique prioritizes the accurate correlation of SEM images directly with the reference state without a sequential procedure, offering optimal computational performance through advanced parallel computing tools, particularly suited for correlating profilometry data related to large-scale displacements. In this study, the algorithm of the hDIC technique is applied as a two-step DIC to evaluate the elastic strain relaxation on the surface of a ring core drilled using a focused ion beam. Both techniques are utilized to correlate the same SEM images collected during the monitoring of the ring drilling process. A comparison of the correlation results of both techniques is undertaken to quantify the near-surface residual elastic strains, with an analysis conducted to discern the accuracy of the hDIC algorithm. Furthermore, the distinctions between the two techniques are delineated and discussed. [ABSTRACT FROM AUTHOR]

Performing in situ scanning electron microscope (SEM) tests is an interesting way to visualise strain heterogeneities under mechanical loading. An essential step before performing the tests is to define the acquisition conditions. The aim of this paper is to propose a classification of the acquisition conditions that are most important for the accuracy of strain measurements using digital image correlation (DIC) in in situ SEM tests. More than 200 image pairs were acquired using a field emission gun SEM. The influence of different acquisition conditions was investigated: acceleration voltage, probe current, working distance, magnification, number of integrated images, image resolution, integration and number of integrated images, scan speed, contrast, brightness and exposure time of the sample in a given area. The methodology implemented in this work is an interesting tool for detecting scan line shift, drift distortion, spatial distortion and rastering artefacts. It allows the optimization of SEM acquisition conditions for strain measurements. Finally, optimal acquisition conditions for in situ testing are proposed and used to perform a tensile test on pure copper. The main factors highlighted include the size of the subset used in the DIC, the beam stabilisation time before image acquisition and the size of the images, which play a significant role in the results. It is recommended to apply the methodology to each device to optimise the acquisition conditions. [ABSTRACT FROM AUTHOR]

A miniature pneumatic bellows device was designed for in-situ testing inside the scanning electron microscope. The device uses a pneumatic actuator to load the specimen and displacement is directly monitored with a linear variable differential transformer sensor. Its application allowed the direct monitoring of microstructural and defect evolution in materials at the micro scale. The data produced by this testing device, in combination with measurements from micro digital image correlation, were successfully used to model the crystal plasticity of a martensitic/bainitic steel at the microstructural length scale. [ABSTRACT FROM AUTHOR]

As the aerospace industry is increasingly demanding stronger, lightweight materials, ultra-strong carbon nanotube (CNT) composites with highly aligned CNT network structures could be the answer. In this work, a novel methodology applying topological data analysis (TDA) to scanning electron microscope (SEM) images was developed to detect CNT orientation. The CNT bundle extensions in certain directions were summarized algebraically and expressed as visible barcodes. The barcodes were then calculated and converted into the total spread function, V(X, θ), from which the alignment fraction and the preferred direction could be determined. For validation purposes, the random CNT sheets were mechanically stretched at various strain ratios ranging from 0 to 40%, and quantitative TDA was conducted based on the SEM images taken at random positions. The results showed high consistency (R2 = 0.972) compared to Herman's orientation factors derived from polarized Raman spectroscopy and wide-angle X-ray scattering analysis. Additionally, the TDA method presented great robustness with varying SEM acceleration voltages and magnifications, which might alter the scope of alignment detection. With potential applications in nanofiber systems, this study offers a rapid and simple way to quantify CNT alignment, which plays a crucial role in transferring the CNT properties into engineering products. [ABSTRACT FROM AUTHOR]

Digital image correlation of scanning electron microscope images is a powerful technique for measuring full-field deformation at microstructural length scales. A major challenge in applying this technique is the fabrication of speckle patterns small enough to facilitate full-field measurements with high spatial resolutions and at high magnifications. Current approaches are inconsistent, damaging to the substrate, or highly substrate dependent, which requires researchers to recalibrate or develop new patterning approaches when changing materials systems. Here, multi-layered Au, Ti, and Ag sputtered coatings are reconfigured in a NaCl solution to quickly form DIC-appropriate speckle patterns. Our proposed technique is shown to be substrate independent, as demonstrated on neat epoxy, Ti-6Al-4V titanium alloy, and tetragonal zirconia polycrystal samples, and allows for controllable particle distributions by varying the sputtered Ag layer thickness. Patterns produced by the proposed technique enable the use of correlation window (subset) sizes smaller than 1 μm, small enough to capture highly localized deformation gradients at material discontinuities areas. Capabilities of this method in characterizing highly heterogeneous deformation conditions at sub-micron scales are demonstrated by measuring localized deformations in a single fiber model composite system. [ABSTRACT FROM AUTHOR]

• Calculated the correction of nonlinear fracture toughness based on FPZ. • Scanning electron microscopy reveals the connection between macroscopic fracture properties and mesoscopic mechanism. • Digital image correlation method divides the fracture evolution law into three stages. • The mesoscopic fracture characteristics of the FPZ are discussed. Cyclic disturbance is common in engineering which accounts for the main reason for fractures and plays an important role in the fracture process zone (FPZ), leading to unclear characteristics at the macro-mesoscopic level. It is thus essential to focus on the macro-mesoscopic analysis of fracture characteristics in FPZ subjected to cyclic loading. In this paper, a series of tests are conducted on sandstone to quantify the macro-mesoscopic features of FPZ. The experimental results indicate that the cyclic loading has an important effect on fracture toughness, the correction of nonlinear fracture toughness based on FPZ is reduced at different amplitude levels. Detailed scanning electron microscope (SEM) observation revealed the scenario of the damage zone indicating the connection between macroscopic fracture properties and mesoscopic mechanism. Additionally, it is found that more intergranular cracks were produced on the cycling failed surface, which is opposite to static rupture. A cohesive zone model (CZM) was introduced to characterize the constitutive relationship of tensile fracture, combined with digital image correlation (DIC) method, the evolution law of FPZ can be divided into three stages. Moreover, 3D laser scanning technology was applied to calculate and analyze the parameters of failed surfaces, namely, fractal dimension D , asperity height, and slope angle which were positively correlated with surface roughness. Furthermore, the joint roughness coefficient (JRC) increases almost linearly with decreasing amplitudes. [ABSTRACT FROM AUTHOR]

The representative volume element (RVE) in fiber composites refers to the smallest volume of material that contains all constituents of the composite, i.e. , fiber and matrix, and is a domain in which the homogenization of microscale response will lead to the macroscale behavior. Therefore, the RVE size in fiber composites is the length scale at which micro to macroscale transition occurs. In this work, a multiscale experimental approach is developed to experimentally characterize the length scale of RVE in a carbon fiber composite with a random fiber distribution. The approach is based on combining optical and scanning electron microscope (SEM) digital image correlation (DIC) measurements. A statistical procedure based on spatial averaging of local strain data is applied to the strain fields obtained from optical and SEM DIC measurements. The convergence between local strains averaged over a carefully selected domain and global strains is used as the criterion for identifying the RVE size in the examined composite. The RVE sizes determined using this approach are stress dependent and increase with the evolution of internal damage in the composite. [Display omitted] [ABSTRACT FROM AUTHOR]

A self-assembled gold nanoparticle surface patterning technique is presented that enables nanometer spatial resolution for digital image correlation (DIC) experiments in a scanning electron microscope. This technique, originally developed for surface-enhanced Raman scattering substrates, results in the assembly of individual 15-136 nm diameter gold nanoparticles over the surface of the test sample. The resulting dense, randomly isotropic, and high contrast pattern enables DIC down to an unprecedented image resolution of approximately 4 nm/pixel. The technique is inexpensive, fast, results in even coverage over the entire surface of the test sample, and can be applied to metallic and non-metallic substrates as well as curved or delicate specimens. In addition, the pattern is appropriate for multi-scale experimental investigations through the utilization of nanoparticle aggregates that collect on the surface in combination with the pattern formed by individual nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract: Large tensile deformation behavior of the alloy of polycarbonate (PC) and acrylonitrile butadiene styrene (ABS) are experimentally investigated in this paper. Based on the adaptations of digital image correlation (DIC) method suitable for large inhomogeneous deformation in three dimensions, a new displacement measurement system is developed. Using the new displacement measurement system, we obtain the displacement fields in PC/ABS alloy. Such that variations of the strain and strain rate during tension process, and the true stress strain curves for PC/ABS alloy are derived. Also, volume variation of PC/ABS alloy is also discussed. Moreover, in situ scanning electron microscopic (SEM) observation is carried out to study the micro-mechanism of large inhomogeneous deformation of PC/ABS alloy. It is seen that contraction rate of specimen thickness is bigger than that of the width of specimen during plastic deformation. The axial tensile strain rate is much higher in the necking region. It should be noted that the local volume increases with the increase of axial true strain to a certain value, after that it decreases. Initiation and growth of crazes are the main mechanism of tensile deformation of PC/ABS alloy after yielding. [Copyright &y& Elsevier]

The application of machine learning and computer vision approaches to microscale deformation data for the automated identification of deformation twinning systems, and their associated twin area, is introduced. Deformation data was obtained during in-situ SEM compression testing of a WE43 Mg alloy using digital image correlation (DIC) modified for use with electron microscopy. A 5.7 mm × 3.4 mm area of interest was analyzed, generating ~100 million data points of deformation. Experimental twin trace directions were determined by applying k-means clustering, morphological thinning, and Hough transforms to the deformation data. The identification of deformation twinning was achieved by consideration of the twin trace direction, its strain value, and its evolution. The deformation map was divided into areas corresponding to individual active twin systems, enabling the analysis of microstructure dependence of deformation behavior and twinning activity. The performance of the proposed twinning identification approach was evaluated by accuracy, precision, and sensitivity metrics. The effect of the tuning parameters used in the algorithm on the performance is also discussed. • An automated method for deformation twinning identification was created. • Twin trace directions from full-field deformation data were extracted by morphological thinning and Hough transform. • Tuning parameters for the identification can be determined from small initial datasets. • Evaluation of the identification performance shows high accuracy (>95%). [ABSTRACT FROM AUTHOR]

The residual stress of thin films during the deposition process can cause the components to have unpredictable deformation and damage, which could affect the service life and reliability of the microsystems. Developing an accurate and reliable method for measuring the residual stress of thin films at the micrometer and nanometer scale is a great challenge. To analyze the residual stress regarding factors such as the mechanical anisotropy and preferred orientation of the materials, information related to the in-depth lattice strain function is required when calculating the depth profiles of the residual strain. For depth-resolved measurements of residual stress, it is strategically advantageous to develop a measurement procedure that is microstructurally independent. Here, by performing an incremental focused ion beam (FIB) ring-core drilling experiment with various depth steps, the digital image correlation (DIC) of the specimen images was obtained. The feasibility of DIC to FIB images was evaluated after the translation test, and an appropriate procedure for reliable results was established. Furthermore, the condition of the film in the function of residual stress was assessed and compared to elucidate the applicability of this technology. [ABSTRACT FROM AUTHOR]

Knowledge of deformation and failure mechanisms at micro- to nano-length scales is important for the prediction of material behavior as well as the development of new materials with desired properties. In situ multiaxial testing with scanning electron microscopes (SEM) can reveal physical deformation mechanisms under realistic multiaxial loading conditions. Although in situ SEM testing has gained traction in recent years, there is currently no multiaxial in situ SEM testing stage available with axial-torsional loading capabilities which can generally be used in any SEM. In this study, we report the development of a multiaxial miniature testing system (MMTS) with a unique capability for performing axial-torsional testing of a tubular specimen with a 1–2 mm outer diameter, inside most SEMs. The different challenges of developing a multiaxial in situ SEM testing stage, such as small load frame size, appropriate specimen position, high vacuum compatibility of MMTS load frame components, as well as the development of installation accessories, were addressed. A custom SEM stage door was developed for the MMTS load frame. Verification tests have confirmed the successful development of the MMTS for in situ SEM testing. In addition, digital image correlation was used with recorded SEM images during the test to determine the surface strain. [ABSTRACT FROM AUTHOR]

• Shear-tension and shear-compression experiments with biaxially loaded specimens have been performed. • Results for proportional and non-proportional loading paths are compared. • Strain fields are analyzed by digital image correlation technique. • Damage and fracture mechanisms are visualized by scanning electron microscopy. • Corresponding numerical simulations reveal stress states leading to different fracture processes. The paper deals with a series of new experiments and corresponding numerical simulations to study the effect of non-proportional loading paths on damage and fracture behavior of ductile metals. In this context, a thermodynamically consistent anisotropic continuum damage model is presented. It takes into account the effect of stress state on damage conditions as well as on evolution equations of damage strains. Different branches of the respective criteria are considered corresponding to various ductile damage and fracture mechanisms depending on stress state. New experiments with the two-dimensionally loaded H-specimen have been developed covering a wide range of stress triaxialities and Lode parameters in the tension, shear and compression domains. These tests are driven under different non-proportional loading paths. Formation of strain fields in critical regions of the specimens is recorded by digital image correlation technique. In addition, scanning electron microscope analysis of the fracture surfaces clearly shows various failure modes corresponding to these loading conditions. Furthermore, results of corresponding numerical simulations are discussed showing good agreement with available experimental data and are used to reveal stress states in critical specimens areas leading to different fracture modes. [ABSTRACT FROM AUTHOR]

A ferrite-pearlite two-phase steel was investigated using in situ scanning electron microscope (SEM) tensile testing combined with digital image correlation (DIC). Two different speckled patterns were used and compared. The first pattern was achieved by etching a polished surface in order to reveal the microstructural features. Second, a gold speckled pattern was obtained. Here, a continuous layer of gold was applied to a polished surface. This continuous layer was remodeled into gold nanoparticles by keeping the specimen at 180 °C for 96 h with an Ar/Styrene mixture flowing across the specimen surface. The result is randomly distributed gold nanoparticles on the surface. These particles and the etched microstructure were then used by the DIC software to correlate an image series to obtain the local strain field of the material. The differences between the two techniques are numerous. Considering the etched surface, most microstructural features were grain boundaries and pearlite lamellas. As a consequence, large areas within grains did not provide sufficient contrast for DIC, thus restricting maximum resolution. However, the technique is fast and does not expose the material to any elevated temperatures. In contrast, the gold remodeling method provides a finely dispersed gold speckle pattern on the surface, giving excellent contrast across the recorded area. DIC with gold particles achieved a spatial resolution of 0.096 µm, compared to 2.24 µm in the DIC for the etched specimen. As a result, DIC with gold speckles can resolve slip lines. Conversely, DIC with etched microstructure resolves local strains on grain level. However, it is less cumbersome and faster to perform the test on the etched specimen. [ABSTRACT FROM AUTHOR]