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Fatigue crack nucleation in crystalline materials typically develops due to highly localized cyclic slip. During a fatigue cycle, reverse slip differs locally from slip in the forward ...direction particularly in precipitate-containing materials such as superalloys. In this paper we report the first direct measurements of irreversibility at the scale of individual slip bands by high-resolution digital image correlation (DIC) in a polycrystalline nickel-based superalloy. Quantitative measurements of the slip irreversibility are challenging for regions of material that have a size that captures the microstructure and its variability. High spatial resolution at the nanometer scale during experimental measurements is needed to observe slip localization during deformation. Moreover, large fields are also needed to obtain the material response over statistically representative populations of microstructural configurations. Recently, high resolution scanning electron microscope (SEM) digital image correlation (DIC) has been extended for quantitative analysis of discontinuities induced by slip events using the Heaviside-DIC method. This novel method provides quantitative measurements of slip localization at the specimen surface. In this paper, the Heaviside-DIC method is used to measure slip irreversibility and plastic strain accumulation in a nickel-based superalloy. The method detects bands with high levels of irreversibility early in cycling that ultimately form fatigue cracks upon further cycling. The local microstructural configurations that induce large amounts of plasticity and slip irreversibility are correlated to crack nucleation locations.
Jointed structures are ubiquitous constituents of engineering systems; however, their dynamic properties (e.g., natural frequencies and damping ratios) are challenging to identify correctly due to ...the complex, nonlinear nature of interfaces. This research seeks to extend the efficacy of traditional experimental methods for linear system identification (such as impact testing, shaker ringdown testing, random excitation, and force or amplitude-control stepped sine testing) on nonlinear jointed systems, e.g., the half Brake–Reußbeam, by augmenting them with full-field data collected by high-speed videography. The full-field response is acquired using high-speed cameras combined with Digital Image Correlation (DIC), which enables studying the spatial–temporal dynamic characteristics of the system. As this is a video-based experiment, additional constraints are attached to the beam at the node points to remove the rigid body motion, which ensures that the beam is in the view of the camera during the entire test. The use of a video-based method introduces new sources of experimental error, such as noise from the high-speed camera’s fan and electrical noise, and so the measurement accuracy of DIC is validated using accelerometer data. After validating the DIC data, the measurements are recorded for several types of excitation, including hammer testing, shaker ringdown testing, fixed sine testing, and stepped sine testing. Using the DIC data to augment standard nonlinear system identification techniques, modal coupling and the mode shapes’ evolution are investigated. The suitability of videography methods for nonlinear system identification of nonlinear beams is explored for the first time in this paper, and recommendations for techniques to facilitate this process are made. This article focuses on developing an accurate data collection methodology as well as recommendations for nonlinear testing with DIC, which paves the way for video-based investigation of nonlinear system identification. In Part-II (Jin et al., 2021) of this work, the same data set is used for a rigorous assessment of nonlinear system identification with full-field DIC data.
•Nonlinear system identification strategies are applied to a jointed structure.•The use of Digital Image Correlation for nonlinear system identification is assessed.•A novel shaker ringdown excitation method is proposed.•Hardening-softening behavior is observed in the frequency response of the structure.•Modal interactions between the first and third mode are observed.
The use of Digital Image Correlation (DIC) to reveal microstructural damage in cross-ply laminates was investigated. Matrix toughness plays a key role in governing microcracking at the tow level in ...near-surface plies. Experiments revealed that using a tough epoxy polymer as the matrix of the laminate resulted in increased laminate moduli in the principal directions. DIC provides insights into cross-ply laminate failure; the increase in modulus is attributed to microcrack formation in transverse plies. Early onset of matrix cracking around the tows is revealed by variations in the strain along the gauge length. The use of a tough epoxy polymer delays the load at which this cracking occurs. When an untoughened epoxy polymer is used as the matrix, microcracking can be observed at the beginning of the test, suggesting processing induced damage. The use of toughened polymers as the matrix of composite laminates is recommended to mitigate against this.
The dynamic responses of assembled structures are greatly affected by the mechanical joints, which are often the cause of nonlinear behavior. To better understand and, in the future, tailor the ...nonlinearities, accurate methods are needed to characterize the dynamic properties of jointed structures. In this paper, the nonlinear characteristics of a jointed beam is studied with the help of multiple identification methods, including the Hilbert Transform method, Peak Finding and Fitting method, Dynamic Mode Decomposition method, State-Space Spectral Submanifold, and Wavelet-Bounded Empirical Mode Decomposition method. The nonlinearities are identified by the responses that are measured via accelerometers in a series of experiments that consist of hammer testing, shaker ringdown testing, and response/force-control stepped sine testing. In addition to accelerometers, two high-speed cameras are used to capture the motion of the whole structure during the shaker ringdown testing. Digital Image Correlation (DIC) is then adopted to obtain the displacement responses and used to determine the mode shapes of the jointed beam. The accuracy of the DIC data is validated by the comparison between the identification results of acceleration and displacement signals. As enabled by full-field data, the energy-dependent characteristics of the structure are also presented. The setup of the different experiments is described in detail in Part I (Chen et al., 2021) of this research. The focus of this paper is to compare nonlinear system identification methods applied to different measurement techniques and to exploit the use of high spatial resolution data.
•The results of several nonlinear system identification methods are compared.•Backbone and damping curves of a jointed beam are identified.•Free decay and forced responses data reveal consistent nonlinear characteristics.•The amplitude-dependent mode shape is reconstructed using digital image correlation.•Digital image correlation allows the computation of the kinetic energy of the beam.
In this study, we present a distinctive phenomenon of strain localization observed in pre-strained AZ31 alloy during tensile deformation. The strain localization behavior involves the repetitive ...formation, propagation, and subsequent annihilation of localized deformation bands within the gage section of the specimen. Although this strain localization behavior bears resemblance to the Portevin–Le Chatelier (PLC) effect commonly observed in steels and Al-Mg alloys, it is driven by a fundamentally different mechanism. Through detailed microstructural analysis, we reveal that this unique strain localization is closely associated with the process of de-twinning. Our findings contribute to a deeper understanding of the deformation behavior in magnesium alloys and offer new insights for the strain localization in metals.
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A novel non-contact, full-field, three dimensional, multi-camera (N cameras N=2, 3 ,…) Digital Image Correlation (DIC) measurement system is proposed in this work. In the proposed system, multiple ...cameras are calibrated as a single system. In this system, any two arbitrary cameras can be grouped into pairs, and each pair of cameras measures a part of a 3D object based on the fundamentals of triangulation. The measured data from different pairs of cameras can be mapped into a universal coordinate system based on the calibration data. A 3D contour of the object can be extracted. Further data, such as deformation, can be obtained based on the contour of the object at a different time. The methodology of the proposed system is introduced. Four synchronized Charged Couple Device (CCD) cameras are employed in the experimental setup, and the performance of the setup is tested in both static and dynamic cases to show the potential of the system.
•A non-contact, multi-camera deformation measuring system is established based on 3D DIC.•The proposed method will map all measured points to a world coordinate system without stitching.•A particular case of a four-camera system is built based on the methodology.
Additively manufactured 316L stainless steels display significantly higher yield strength than their as-cast or wrought counterparts. This is associated with the micro-scale cellular structure and ...complex grain and sub-grain structure, resulting from high cooling rates occurring during the additive manufacturing process. The consequences of these peculiar microstructural features on plastic localization early in the plastic regime at the sub-grain scale are investigated. The plastic localization involved during monotonic deformation of conventional and additive manufactured 316L stainless steels is investigated using high-resolution digital image correlation. Significant heterogeneous slip localization is observed in the additively manufactured 316L stainless steels compared to the wrought 316L stainless steels. The cellular structure and low-angle grain boundaries are observed to control the incipient plasticity. In addition, slip localization characteristics indicate that the additional strengthening in the AM material is mainly related to the cellular structure acting as a dislocation forest-type obstacle.
•Slip localization amplitude in AM 316L SS is higher on average than in wrought 316L.•Heterogeneity of sub-grain structures controls slip deformation in the AM Material.•Strengthening in the AM SS is due to the cell structure acting as a dislocation forest.•Schmid Factor is a poor indicator of incipient plasticity in the AM Material.
•Grain size Effect: larger grain sizes in graphite lead to higher fracture toughness values under quasi-static loading, but the trend is opposite under dynamic loading.•Loading-Rate Effect: the ...dynamic fracture toughness exhibits a near-linear correlation with the impact velocity.•Fracture mode Transition: the fracture mode changes from intergranular to transgranular fracture as the impact velocity increases.
Graphite materials play critical roles as moderators, reflectors, and core structural components in high-temperature gas-cooled nuclear reactors. During the reactor operation, graphite materials may experience a variety of loads, including thermal, radiation, fatigue, and dynamic loads, potentially leading to crack initiation and propagation. Therefore, it is imperative to investigate their fracture properties. Despite this, there remains a paucity of comprehensive studies on the fracture toughness of graphite materials with varying grain sizes, particularly concerning dynamic fracture toughness. This study addresses this gap by employing a digital-image-correlation-based virtual extensometer to analyze crack propagation in graphite materials of different grain sizes, enabling precise measurement of crack propagation length and fracture toughness. Findings reveal that static fracture toughness increases with larger grain sizes, while under dynamic loading, smaller grain sizes exhibit greater fracture toughness. Additionally, dynamic fracture toughness shows a near-linear increase with impact speed. Scanning electron microscopy analysis of fracture surface morphology highlights the impact of grain size and impact speed on fracture toughness. Nuclear graphite specimens with larger grain sizes have more irregular grain and pore distributions, enhancing crack deflection and propagation resistance, thereby increasing fracture toughness. The observed loading rate dependence of dynamic fracture toughness is attributed to a gradual transition from intergranular to transgranular fracture modes with increasing impact speed.
Tensile strength is one of the most important mechanical parameters controlling the development of cracks in soil. However, it is frequently neglected in conventional geotechnical practice, because ...its magnitude is small and difficult to measure relative to other soil strength parameters. In this paper, a newly designed direct tensile test apparatus was employed to measure the tensile strength of an unsaturated clayey soil. A digital image acquisition and analysis system was developed for tensile strain analysis with the help of Particle Image Velocimetry (PIV) and Digital Image Correlation (DIC) techniques. Six groups of samples were compacted at a dry density of 1.7 Mg/m3 and different water contents (6.5%, 8.5%, 10.5%, 12.5%, 16.5% and 20.5%). Test results show that the tensile strength characteristic curve (tensile strength versus water content) of the compacted unsaturated soil exhibits mono-peak feature. When water content is relatively low, the tensile strength increases with increasing water content and reaches the maximum value at a critical water content of about 9.3%. Then, it declines with further increase in water content. The evolution of tensile strength with water content depends on both suction and microstructure. Based on plotted tensile load-displacement curves, the tensile failure process can be divided into three typical stages which are: stress increasing stage (I), failure developing stage (II) and post-failure stage (III). It is found that the overall tensile failure process presents different patterns controlled by water content. Generally, the failure developing stage (II) lasts longer and the failure ductility is more pronounced when the sample is compacted at higher water content. Using PIV and DIC techniques, the development of displacement direction and strain concentration during tension can be well captured for appreciation of the soil failure mechanism. Based on the strain concentration information, the tensile fracture location and direction can be pre-determined for soil samples.
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•New direct tensile test apparatus with image capture system is designed.•Tensile curve shows three typical stages which strongly depend on water content.•TSCC of compacted soil shows mono-peak feature related to both suction and fabric.•Tensile stiffness and failure brittleness decrease with increasing water content.•PIV/DIC is valuable to get deformation and strain concentration characteristics.
An essential theoretical foundation for preventing coal and rock dynamic disasters is understanding the propagation of stress waves within coal and dynamic monitoring of the stress conditions of ...coal. A split Hopkinson pressure bar and electric potential (EP) test system is utilized in this work to conduct the EP test experiment. The EP response and surface displacement characteristics of the stress wave propagation process were investigated, the evolution law between the EP parameters and displacement parameters was analysed, and the multifractal characteristics of the EP, surface displacement and stress wave were revealed. The findings demonstrate how the EP can accurately depict the propagation process of the stress wave, and both show significant attenuation characteristics. The multi parameter evolution of the EP is highly consistent with the variations in the surface displacement of the specimen. The multifractal characteristics of the EP response can be used to quantitatively characterize the stress anomalies within the specimen, in which the abrupt increase and decrease in Δα and Δf(α) can be used as the key indicators for identifying the stress anomalies within the coal. The findings of this work have important theoretical implications for the stability and safety of deep coal mining since they offer fresh ideas for real-time dynamic monitoring of the stress conditions of the coal seams utilizing EP test techniques.
•Experiments with electric potential (EP) testing under stress wave loading were conducted using the SHPB-DIC-EP setup.•The EP response and surface displacement characteristics of the stress wave propagation process were investigated.•The evolution law between the EP parameters and displacement parameters was analysed.•The multifractal characteristics of the EP, surface displacement and stress wave were revealed.•The evolution of multifractal parameters of EP can be used as key indicators for identifying coal stress anomalies.
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