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Understanding By Design

As a carrier of deformation information, the speckle pattern, or more exactly the random intensity distributions, which could be naturally occurred or artificially fabricated onto test samples’ surface, plays an indispensable role in digital image correlation (DIC). It is now well recognized that the accuracy and precision in DIC measurements not only rely on correlation algorithms, but also depend highly on the quality of the speckle pattern. Considering the huge diversity in test materials, spatial scales and experimental conditions, speckle pattern fabrication could be a challenging issue facing DIC practitioners. To obtain good speckle patterns suitable for DIC measurements, some key issues of fabrication methods and quality assessment of speckle patterns must be well addressed. To this end, this review systematically presents the speckle pattern classification and fabrication techniques for various samples and scales, as well as some typical quality assessment metrics.

A Review of Speckle Pattern Fabrication and Assessment for Digital Image Correlation

The analysis of adhesively bonded joints started in 1938 with the closed-form model of Volkersen. The equilibrium equation of a single lap joint led to a simple governing differential equation with a simple algebraic equation. However, if there is yielding of the adhesive and/or the adherends and substantial peeling is present, a more complex model is necessary. The more complete is an analysis, the more complicated it becomes and the more difficult it is to obtain a simple and effective solution. The finite element (FE) method, the boundary element (BE) method and the finite difference (FD) method are the three major numerical methods for solving differential equations in science and engineering. These methods have also been applied to adhesive joints, especially the FE method. This book deals with the most recent numerical modelling of adhesive joints. Advances in damage mechanics and extended finite element method are described in the context of the FE method with examples of application. The classical continuum mechanics and fracture mechanics approach are also introduced. The BE method and the FD method are also discussed with indication of the cases they are most adapted to. There is not at the moment a numerical technique that can solve any problem and the analyst needs to be aware of the limitations involved in each case.

Advances in Numerical Modelling of Adhesive Joints

Metal additive manufacturing techniques such as the powder-bed systems are developing as a novel method for producing complex components. This study uses synchrotron-based X-ray microtomography to investigate porosity in electron beam melted Ti-6Al-4V in the as-built and post-processed state for two different powders. The presence of gas porosity in the starting powder was shown to correlate to porosity in the as-built components. This porosity was observed to shrink after a hot isostatic press treatment, but grow following a subsequent heat treatment. Crystal plasticity simulations were used to observe the effects of various observed pore sizes on mechanical behavior under loading.

https://tandfonline.com/doi/pdf/10.1080/21663831.2017.1340911

Analyzing the effects of powder and post-processing on porosity and properties of electron beam melted Ti-6Al-4V

Direct extraction of rate-dependent traction–separation laws for polyurea/steel interfaces

A deformation mechanism map for a Ni-based superalloy is presented during cyclic loading at low (300 °C), intermediate (550 °C), and high (700 °C) temperatures for low (0.7%) and high (1.0%) applied strain amplitudes. Strain mapping is performed via digital image correlation (DIC) during interrupted fatigue experiments at elevated temperatures at 1, 10, 100 and 1000 cycles, for each specified loading and temperature condition. The DIC measurements are performed in a scanning electron microscope, which allows high-resolution measurements of heterogeneous slip events and a vacuum environment to ensure stability of the speckle pattern for DIC at high temperatures. The cumulative fatigue experiments show that the slip bands are present in the first cycle and intensify with number of cycles; resulting in highly localized strain accumulation. The strain mapping results are combined with microstructure characterization via electron backscatter diffraction. The combination of crystal orientations and high-resolution strain measurements was used to determine the active slip planes. At low temperatures, slip bands follow the {111} octahedral planes. However, as temperature increases, both the {111} octahedral and {100} cubic slip planes accommodate strain. The activation of cubic slip via cross-slip within the ordered intermetallic γ’ phase has been well documented in Ni-based superalloys and is generally accepted as the mechanism responsible for the anomalous yield phenomenon. The results in this paper represent an important quantifiable study of cubic slip system activity at the mesoscale in polycrystalline γ-γ’ Ni-based superalloys, which is a key advancement to calibrate the thermal activation components of polycrystalline deformation models.

/pdf/fatigue-strain-mapping-via-digital-image-correlation-for-ni-58z8jtjhcw.pdf

Fatigue strain mapping via digital image correlation for Ni-based superalloys: The role of thermal activation on cube slip

This paper describes a series of experiments and analyses that were used to examine crack growth near sapphire/epoxy interfaces. Adhesion of the epoxy to the sapphire was enhanced by coating the sapphire with mixtures of two silane coupling agents that form self-assembled monolayers. A new biaxial loading device was used to conduct a series of mixed-mode fracture experiments. Crack opening interferometry, atomic force microscopy, and angle-resolved X-ray photoelectron spectroscopy allowed cohesive zone sizes, fracture surface topographies, and loci of fracture to be established. The experiments were complemented by finite element analyses that accounted for the rate- and pressure-dependent yielding of the epoxy. The analyses also made use of traction-separation laws to represent the various interphases that were produced by the mixed monolayers. The intrinsic toughness (defined as the area underneath the traction-separation curve) of the bare sapphire interfaces was independent of mode-mix and lower than values from previous experiments with glass/epoxy and quartz/epoxy specimens. The increase in overall toughness with mode-mix was completely accounted for by viscoplastic dissipation in the epoxy outside the cohesive zone. The minimum toughness of the coated sapphire interfaces was about five times higher than the mode-mix independent intrinsic toughness of the uncoated specimens. The increase in overall toughness with mode-mix was almost completely accounted for by increases in the intrinsic toughness as the traction-separation law varied with mode-mix. As a result, viscoplastic dissipation outside the cohesive zone was minimal. Atomic force fractography and X-ray photoelectron spectroscopy indicated that the crack growth mechanisms and the loci of fracture in the coated and uncoated specimens were quite different.

The Effect of Self-Assembled Monolayers on Interfacial Fracture

Recently, scanning electron microscopy (SEM) has been used for digital image correlation (DIC), as micrographs can be acquired with high magnification, providing improved resolution to quantify strain heterogeneities. However, it has been shown by researchers that SEM images can be problematic due to inherent electromagnetic distortions that are not present in optical images. Drift, spatial distortions, and magnification uncertainties are the main issues that can seriously affect the accuracy of localized strain measurements. The present work focuses on long duration experiments, for which images are taken days or weeks apart. We have proposed a systematic procedure to reduce drift, correct spatial distortion, and account for magnification variations between pairs of acquired images. Additionally, SEM parameters are discussed and chosen to increase the signal-to-noise ratio and improve the accuracy of the DIC measurements. The spatial distortion correction increases the repeatability of the correlated values and the precision of the measurements. The implementation for this type of correction is done by applying the measured distortion gradient of a certified grid onto the DIC strain field. The magnification adjustment increases the reliability of the strain maps, ensuring the measurements are in agreement with the actual strain induced during the experiment. We have presented a systematic protocol for ex-situ DIC experiments within the SEM and some basic cross-check procedures that can be performed to evaluate the reliability of the reference grid and the precision of the final strain map.

Distortion Correction Protocol for Digital Image Correlation after Scanning Electron Microscopy: Emphasis on Long Duration and Ex-Situ Experiments

This paper describes a series of experiments and analyses that were used to examinecrack growth near sapphire/epoxy interfaces. Adhesion of the epoxy to the sapphire wasenhanced by coating the sapphire with mixtures of two silane coupling agents that formself-assembled monolayers. A new biaxial loading device was used to conduct a series ofmixed-mode fracture experiments. Crack opening interferometry, atomic force micros-copy, and angle-resolved X-ray photoelectron spectroscopy allowed cohesive zone sizes,fracture surface topographies, and loci of fracture to be established. The experimentswere complemented by ﬁnite element analyses that accounted for the rate- and pressure-dependent yielding of the epoxy. The analyses also made use of traction-separation lawsto represent the various interphases that were produced by the mixed monolayers. Theintrinsic toughness (deﬁned as the area underneath the traction-separation curve) of thebare sapphire interfaces was independent of mode-mix and lower than values from pre-vious experiments with glass/epoxy and quartz/epoxy specimens. The increase in overalltoughness with mode-mix was completely accounted for by viscoplastic dissipation in theepoxy outside the cohesive zone. The minimum toughness of the coated sapphire inter-faces was about ﬁve times higher than the mode-mix independent intrinsic toughness ofthe uncoated specimens. The increase in overall toughness with mode-mix was almostcompletely accounted for by increases in the intrinsic toughness as the traction-separation law varied with mode-mix. As a result, viscoplastic dissipation outside thecohesive zone was minimal. Atomic force fractography and X-ray photoelectron spectros-copy indicated that the crack growth mechanisms and the loci of fracture in the coatedand uncoated specimens were quite different.

The Effect of Self-Assembled Monolayers on Interfacial

Microstructure attributes are responsible for heterogeneous deformation and strain localization. In this study, the relation between residual strain fields and microstructure is examined and assessed by means of experiments and crystal plasticity modeling. The microstructure of rolled aluminum alloys (AA) in the 7050-T7451 condition was experimentally obtained with electron backscatter diffraction (EBSD) analysis along the rolling direction (L-T orientation), across the rolling direction (T-L orientation), and transverse to the rolling direction (T-S orientation). Each of these sections was also patterned using a novel microstamping procedure, to allow for strain mapping by digital image correlation (DIC). The measured microstructures were in turn used as input of an elasto-viscoplastic crystal plasticity formulation based on fast Fourier transforms (EVP-FFT). Comparisons between the strain maps obtained experimentally by the concurrent DIC-EBSD method and the EVP-FFT simulations were made for the three sections, corresponding to the initial textures. The comparisons showed that the predicted levels of strain concentration were reasonable for all three specimens from a statistical perspective, which is important to properly describe and predict the strains within an ensemble of components; however the spatial match with the actual strain fields needs improvement.

/pdf/effect-of-microstructure-on-strain-localization-in-a-7050-4s5gsi13tr.pdf

Alberto W. Mello

Papers

Fatigue strain mapping via digital image correlation for Ni-based superalloys: The role of thermal activation on cube slip

The Effect of Self-Assembled Monolayers on Interfacial Fracture

Distortion Correction Protocol for Digital Image Correlation after Scanning Electron Microscopy: Emphasis on Long Duration and Ex-Situ Experiments

The Effect of Self-Assembled Monolayers on Interfacial

Effect of microstructure on strain localization in a 7050 aluminum alloy: Comparison of experiments and modeling for various textures