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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 paper, a novel encoding feature based on speckle patterns. It comprises three components: a pre-positioning ring, localization speckle area, and encoding speckle area. The pre-positioning ...ring identifies potential regions of interest. Shape function of the localization speckle area can be estimated by formulating the elliptical equation of the pre-positioning ring. Next, digital image correlation method is employed to achieve precise positioning and refine the shape function. Through the high-precision shape function, the encoded speckle area can be determined. Encoding technique utilizes a reference encoding scheme, in which each coded value region either same or is inverse to the reference region. Zero-mean normalized cross correlation relative to the reference region can be used to obtain a highly robust decoding value. Experimental results demonstrate that, despite challenges such as noise, defocus, variations in lighting intensity, and lighting uneven, the coded speckle target maintains high robustness and precise positioning accuracy. Moreover, the structure from motion and displacement measurement experiments further verifies its high accuracy in practical applications.
•Novel encoding feature based on speckle patterns.•Coding and decoding method based on zero-mean normalized cross-correlation.•High robustness and accuracy to noise, defocus, lighting variations through simulation and experiments.
In the last few decades, there has been a surge of research in the area of non-contact measurement techniques. Photogrammetry has received considerable attention due to its ability to achieve ...full-field measurement and its robustness to work in testing environments and on testing articles in which using other measurement techniques may not be practical. More recently, researchers have used this technique to study transient phenomena and to perform measurements on vibrating structures. The current paper reviews the most current trends in the photogrammetry technique (point tracking, digital image correlation, and target-less approaches) and compares the applications of photogrammetry to other measurement techniques used in structural dynamics (e.g. laser Doppler vibrometry and interferometry techniques). The paper does not present the theoretical background of the optical techniques, but instead presents the general principles of each approach and highlights the novel structural dynamic measurement concepts and applications that are enhanced by utilizing optical techniques.
•Photogrammetry measures displacements to monitor dynamics of structures.•The paper reviews the most current trends in photogrammetry.•The paper compares the applications of photogrammetry to conventional approaches.•Photogrammetry effectively measures high-displacement and low-frequency vibrations.•Photogrammetry can be applied to measure dynamics of rotating structures.
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.
Kink band formation is a common deformation mode for anisotropic materials and has been observed in polymer matrix fiber composites, single crystals, geological formations, and recently in metallic ...nanolaminates. While numerous studies have been devoted to kink band formation, the majority do not consider the often rapid and unstable process of kink band propagation. Here we take advantage of stable kink band formation in Cu-Nb nanolaminates to quantitatively map the local strain fields surrounding a propagating kink band during uniaxial compression. Kink bands are observed to initiate at specimen edges, propagate across the sample during a rising global stress, and induce extended strain fields in the non-kinked material surrounding the propagating kink band. It is proposed that these stress/strain fields significantly contribute to the total energy dissipated during kinking and, analogous to crack tip stress/strain fields, influence the direction of kink propagation and therefore the kink band inclination angle.
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A review of the extensive studies on the feasibility and practicality of utilizing high-speed 3 dimensional digital image correlation (3D-DIC) for various random vibration measurement applications is ...presented. Demonstrated capabilities include finite element model updating utilizing full-field 3D-DIC static displacements, modal survey natural frequencies, damping, and mode shape results from 3D-DIC are baselined against laser Doppler vibrometry (LDV), a comparison between foil strain gage and 3D-DIC strain, and finally the unique application to a high-speed wind tunnel fluid–structure interaction study. Results show good agreement between 3D-DIC and more traditional vibration measurement techniques. Unfortunately, 3D-DIC vibration measurement is not without its limitations, which are also identified and explored in this study. The out-of-plane sensitivity required for vibration measurement for 3D-DIC is orders of magnitude less than LDV making higher frequency displacements difficult to sense. Furthermore, the digital cameras used to capture the DIC images have no filter to eliminate temporal aliasing of the digitized signal. Ultimately DIC is demonstrated as a valid alternative means to measure structural vibrations while one unique application achieves success where more traditional methods would fail.
•Modal survey with high-speed 3D DIC of all modes below 1200Hz in single measurement.•Full-field operational deflected shapes of thin panel subjected to Mach 2 shock wave.•Identified measurement noise sources for high-speed 3D DIC vibration measurement.
This paper presents a verification and validation analysis of Finite Element (FE) models predicting the mechanical response of linearly elastic honeycomb structures. We have studied three main ...models, namely: analytical models based on beam theories, explicit FE models where the cell geometry is explicitly meshed with 3D elements, and numerical homogenized FE models where a plate made of a honeycomb structure is meshed with 2D elements whose mechanical properties were predicted from numerical homogenization. We compared the predictions of these simulations against experimental uni-axial tensile tests where we mechanically tested 3D printed honeycomb specimens having a relative density of 40% and made of 13 and 37 cells, respectively. Comparison of the experimentally measured axial stiffness to the numerical predictions revealed that the numerical homogenization models can predict the apparent in-plane stiffness in structures made of 37 cells within 4%, while the discrepancy increases to 70% when 13 cells are considered. To quantify this discrepancy, we also provided a relationship between the number of represented cells and the discrepancy of the numerical homogenized model against the explicit FE models to predict the in-plane stiffness. We believe that these results could be important for the application of the homogenized models in optimization of honeycomb lattice structures whose relative density varies with space.
•We validated the numerical and analytical homogenizations of honeycomb structures.•The honeycombs had a relative density of 40% and featured 13 and 37 cells.•The honeycomb with 37 cells had a response 76.6% stiffer than the one with 13 cells.
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