Advancements in Optical Methods, Digital Image Correlation & Micro-and Nanomechanics, Volume 4

Viruses are organisms that invade cells of living beings to reproduce. They consist of nucleic acids, RNA, underlying proteins, and a protective membrane. Their life cycle comprises three main stages: (1) penetration of a cell, (2) introduction of their genome generating new viruses, and (3) release of replicated viruses to the external cellular space for further infection propagation. Imaging techniques provide an important tool for understanding these mechanisms. Transmission electron microscopy (TEM) is one of the main tools utilized for this purpose. TEM investigations impose environmental limitations on the observation conditions. To get images of viruses, a TEM requires freezing the virus at extremely low temperatures. The bulk of TEM images are limited to 2D; for 3D images, TEM holography is available but poses additional difficulties and costs. A nano-microscope is being developed by the authors with resolution limits in the same range as a TEM. The nano-microscope can be utilized to observe viruses under environmental conditions in the range of biological entities and enables 3D dynamic observations.

A method for evaluating displacement and strain distributions around inclusion and matrix interface is studied in this chapter. Global digital image correlation (global DIC) is used for this method. Global DIC can obtain the displacement in inclusions and matrix parts separately. Strains are also computed using a mesh. The results show that strain distributions of rapid change around interface can be obtained by global DIC. Then, by comparing the measurement results of local DIC and global DIC, it is found that global DIC has the validity to measure strain around the interface.

A method for evaluating the stress state and fracture strain of the thin sheet of high-strength steel is studied in this chapter. Displacements and strains on the surface of high-strength steel plates are measured using stereo image correlation (DIC). The stress–strain relation after the necking is obtained from the load and strains by inverse problem analysis using the virtual field method. The stresses obtained by the proposed method are well balanced with the external forces.

In this work, we experimentally and numerically investigate the nonlinear dynamics of pairs of coupled opto-thermal micro-oscillators. The oscillators are driven into limit cycle oscillations using an external constant power laser source and coupled via mechanical linkages. As the input laser power is increased, the oscillators transition from a state of synchrony to a state of bistability that manifests as irregular oscillations in the experiments. The experiments are performed on two sets of devices with different coupling strengths, which reveal that the laser power required for the transition to irregular oscillations increases with an increase in the coupling strength. A numerical parameter sweep in the coupling strength and laser power parameter space reveals a similar trend.

Mammals possess an auditory system that carries out a number of functions that allow for sound perception from the surroundings. The hearing process is initiated when the tympanic membrane (TM) transfers acoustic waves from the air into mechanical vibration. The information pertaining to TM shape is essential to the expansion of our understanding of the middle ear. This will allow for better hearing protection, clinical diagnosis, and treatment of damaged or diseased ears. This chapter proposes a shape measurement approach with the use of a miniaturized fringe projection system having a scanning microelectromechanical systems (MEMS) mirror. This method utilizes a single fringe to scan the entire field of view while a series of images with spatially varied single fringe is captured by the high-speed camera. An individual multifringe image is then formed and reconstructed by summing up the single-fringe images numerically. Individually scanning with one fringe boosts optical efficiency and image quality during high-speed image capture. A National Institute of Standards and Technology (NIST)-traceable gauge helps validate shape measurements obtained using our methodologies. This shape measurement method is then applied to determine the shape of human eardrums.

This work studies the implementation of a high-speed linescan camera as a 1D high-speed optical extensometer in gathering strain histories during dynamic strain rate experiments. Aluminum 6061-T6 is tested in tension and compression and recorded using both the high-speed extensometer and a high-speed camera, and the resulting images are analyzed for sample strain and compared to strains found via 1D wave theory calculations. The results show good alignment between the two optical methods with the strain gathered from the wave calculations showing slightly higher strains in both tension and compression.

The Miura Ori auxetic geometry has been deeply studied for its capability to reach high negative Poisson coefficient values. This response is associated with some geometrical configuration, and there are several examples in specific literature. Miura Ori metamaterial has been analyzed with static loads, while from the dynamical side, the studies have been conducted with analytical/numerical techniques. The authors analyze this subject and propose an experimental setup (the most suitable technique appears to be an interferometric one, i.e., the time-averaged speckle approach) to get the modal response of a Miura Ori specimen. The results are validated by comparison with FEA simulation to state the reliability of experimental results.

Digital image correlation has recently seen a growing interest in both the research and industrial community thanks to the possibility to measure full-field information, with high confidence, and with a very limited instrumentation. Furthermore, advances in camera technology, particularly on resolution and data transfer rate, are now opening the door to new application, such as modal analysis and testing on rotating components. In this chapter, we give an overview of industrial applications of digital image correlation, ranging from the more classical characterization of material samples up to modal analysis and dynamic characterization of several components in stationary as well as operating conditions and show how the same instrumentation can be reused to cover multiple scenarios at the same time, without the need for changing instrumentation or data acquisition as is the case for other experimental techniques. We also show how DIC is used to characterize the static behavior of lattice structures, characterize both statically and dynamically mechanical components to validate numerical models, extract the modal behavior of rotating components, such as tires and fans, and finally understand the behavior of huge machines where the size and stiffness pose great limitations to the use of optical techniques.

Optical microscopy (OM) implementation of the digital image correlation (DIC) technique stands out with high practicality (e.g., no vacuum requirement and high amenability to be combined with other measurement channels). Despite its limited intragrain resolution compared to scanning electron microscopy (SEM) variants, OM-DIC provides critical identification at the interaction length scale of polycrystalline aggregates. The OM-DIC variant that is considered here (Shafaghi N, Kapan E, Aydıner CC, Exp Mech 60:735–751, 2020; Özdür NA, Üçel IB, Yang J, Aydıner CC, Exp Mech 61:499, 2020), however, further attempts to minimize the comparative intragrain resolution deficiency of OM-DIC by utilizing high-resolution [high numerical aperture (NA)] objectives. Images with high-NA objectives will typically immediately suffer defocusing for a deforming sample, given the extremely limited depth of fields (in the μm order). The technique employs continual automated working distance (WD) adjustment to fight off defocusing through custom instrumentation that also implements area scanning to expand field coverage to the mm scale. The precise WD adjustments also help to minimize WD error (a biaxial strain error in DIC measurements). While the technique has been formerly used to study highly strained polycrystalline fields (Shafaghi N, Kapan E, Aydıner CC, Exp Mech 60:735–751, 2020; Özdür NA, Üçel IB, Yang J, Aydıner CC, Exp Mech 61:499, 2020), the purpose of this study is to investigate and advance its accuracy limits. For this purpose, 40× microscopy with a high-NA objective (Özdür NA, Üçel IB, Yang J, Aydıner CC, Exp Mech 61:499, 2020) is utilized over a pure FCC nickel polycrystal with enlarged grains (average 70 μm), yielding about 5000 DIC grid points per grain. Regardless of the length scale, however, DIC offers limited sensitivity for pointwise strains (about 0.1%). While this is deemed sufficient for plasticity, elastic strains are not locally resolved. Here, we will employ small load increments in the initial elastic ramp of nickel to test the sensitivity limits of the method in a grain-resolved setting, i.e., the strain fields of individual grains are separately considered. The crystallographic orientations will be known thanks to pre-experiment electron backscatter diffraction. The trade-off between accuracy and resolution will be tested by local averages inside the grains to see whether regional DIC accuracy can be pushed toward elastic strain levels. Results of multiple grains will be compared and consistency with finite element predictions that account for the crystallite orientations will be presented.

One of the critical components of large-scale manufacturing of bioengineered tissues is the sensing of information for quality control and critical feedback of tissue growth. Modern sensors that measure mechanical qualities of tissues, however, are invasive and destructive. The goal of this project is to develop noninvasive methodologies to measure the mechanical properties of tissue engineering products. Our approach is to utilize acoustic waves to induce nanoscale level vibrations in the engineered tissues in which corresponding displacements are measured in full-field with quantitative optical techniques. A digital holographic system images the tissue’s vibration at significant modes and provides the displacement patterns of the tissue. These data are used to train a supervised learning classifier with a goal of using the comparisons between the experimental vibrational modes and the ones obtained by finite element simulation to estimate the physical properties of the tissue. This methodology has the promise of mechanical properties that would allow technicians to noninvasively determine when samples are ready to be packaged, if their growth deviates from expected time frames, or if there are defects in the tissue. It is expected that this approach will streamline several components of the quality control and production process.

Mechanical twinning underlies many of the challenges in the forming and structural utilization of magnesium alloys, the class of engineering metals with the lowest density. Intense research activity has been devoted to the understanding and modeling of the abrupt twinning phenomenon. A particularly challenging aspect of twinning is its abrupt coordinated proliferation across the polycrystalline aggregate. Macroscopically, these events are akin to Lüders banding while the geometry and compactness of the coordinated twinning bands show pronounced dependence on microstructure (most notably on crystallographic texture) in magnesium alloys. A series of systematic studies employed an in situ area scanning variant of digital image correlation with optical microscopy to quantitatively characterize these bands. While the length scale of optical microscopy is typically suited to investigate long-range strain structures over the polycrystalline aggregate, this instrument employs high optical resolution (high numerical aperture) objectives to also provide adequate intra-grain resolution even in medium-sized (~10 μm) grains (Özdür NA, Üçel IB, Yang J, Aydıner CC, Exp Mech 61:499, 2020).The utilization of these objectives that also possess extremely small depth of fields is only possible through continual automated working distance corrections to fight off defocusing. The sharpness of the consequent imaging has further been used to introduce a novel microscopy mode, called residual intensity (Özdür NA, Üçel IB, Yang J, Aydıner CC, Exp Mech 61:499, 2020). This mode isolates and presents the twins with an order higher resolution than the DIC strains, albeit using the same images. It has the further advantage of showing deformation structures (in this case, twins) that are activated in a specific load increment, namely, residual intensity is an imaging mode with a reference state.

Here, we will employ this OM-DIC technique over the sharp rolling texture (for which the band strain reaches about one-third of the twin transformation strain (Özdür NA, Üçel IB, Yang J, Aydıner CC, Exp Mech 61:499, 2020) in an unnotched sample), but in a notched sample to guide and overlap macroscopic twin bands. The notch locations will be specifically designed for this regime of extreme plastic anisotropy. Microscopic DIC is again implemented in situ to study formation, expansion, and overlap of the coordinated twin bands. Both strain and residual intensity calculations will be performed across loads to investigate the effect of stress raisers on the twin coordination.

Quasi-static tension testing of a single-layer composite reduces strain field complexity compared to multilayer composites in which strain fields are obscured by nesting, interpenetration, and overlapping of tows and laminas. Digital image correlation (DIC) may be used in single-layer tension tests to accurately reveal the complex strain fields of woven composites, and the time-resolved damage onset and evolution of transverse cracking. However, typical tensile specimens are 25.4 mm wide, which means only five yarns across the width for a standard 24 oz./yd² fabric composite. Free edge effects can cause shear strain concentrations, which leads to higher edge stresses than toward the specimen center. Also, free edges can result in periodic, out-of-plane warping. To ensure the area of interest for DIC analysis is adequate to capture the strain behavior without including strain concentrations at the free edges, three specimen widths were tested: 25.4, 50.8, and 76.2 mm (1, 2, and 3 inch). The 50.8 mm width was found to be optimal. A partial-coverage DIC speckle pattern was used to observe surface strain and transverse crack evolution simultaneously. Onset of transverse cracking was found to occur at a local strain of 0.45%, which is consistent with experiments using acoustic emission to identify transverse crack onset. During tensile testing, the number of cracks in a transverse yarn unit cell increases sequentially. The first three of these cracks are termed primary, secondary, and tertiary transverse cracks in a unit cell. The onset of the first secondary transverse crack occurred at a local strain of 0.50%, and the onset of the first tertiary transverse crack occurred at a local strain of 0.74%. This chapter reports on tensile testing of three specimen widths and on the local and global strains during the evolution of transverse cracks.