Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics

A method to diagnose the stages of deformation nondestructively, quickly and as full-field information is discussed. In-plane sensitive Electronic Speckle-Pattern Interferometry is used in the subtraction mode to form the fringe pattern representing the differential displacement occurring during a short-time interval. The dark fringes in a fringe pattern exhibit the contours of the differential displacement field. The interferometer keeps forming such fringe patterns continuously. The change in the fringe pattern with the development of the deformation is interpreted based on a field theory of deformation and fracture. Based on a fundamental principle of physics, this theory describes all stages of deformation and fracture on the same theoretical basis. It does not need to use phenomenology or empirical formulation of the phenomenon. The transition from one stage to another, e.g., the elastic to plastic stage or the plastic to fracturing stage, of given deformation is diagnosed based on specific features of the fringe patterns and the field theoretical interpretation of the features. The transition from the elastic to plastic stage is characterized by the generation of shear instability that triggers the initiation of a large-scale rotation wave. The transition from the plastic to fracturing stage is characterized by the immobility of the rotation wave that causes the generation of material discontinuity.

The pressure tubes for CANDU® (CANDU® (CANada Deuterium Uranium) is a registered trademark of Atomic Energy of Canada Limited.) reactor fuel channels are made of Zr-2.5Nb alloy material. The modelling of fuel channel deformation behavior during accident scenarios, for example, a large break loss-of-coolant accident (LBLOCA), requires knowledge of the high-temperature properties of the pressure tubes. Uniaxial flat specimens are commonly used for tests to obtain the mechanical properties of the material for their response to various types of loads that simulate accident conditions. For CANDU fuel sheathing, made from Zircaloy-4 material, biaxial closed-end burst tests are usually conducted to evaluate their creep and ballooning deformation behavior at high temperatures under high heating rates. Since the Zr-alloy materials oxidize readily at elevated temperatures and their metal properties can be drastically affected, the high-temperature testing of samples from these materials should be conducted in a controlled environment, either in vacuum or surrounded by an inert gas atmosphere. By testing inside such an environment, representative mechanical properties of the metal can, therefore, be obtained with minimal effects from chemical interaction during the test.

At Canadian Nuclear Laboratories (CNL), we have developed a specially designed facility for high-temperature testing of uniaxial tensile specimens and biaxial burst specimens inside a sealed chamber. The sample is joule-heated at high heating rates with alternating current. Spot-welded thermocouple on the specimen and a PID controller are used for temperature control. Non-contact measurement of strains on the sample is made continuously, using two orthogonal sets of twin “blue” lasers. The use of blue lasers with a shorter wavelength (UV) than infrared emissions has been demonstrated to allow discernable deformations to be measured while the sample is heated to 1000 °C or higher. This paper will briefly describe the novel four-laser measurement technique with two examples to demonstrate strain measurements for the uniaxial test case and the biaxial burst test case both heated to high temperatures.

This paper deals with the onset of plasticity and the transition to fracture in metallic rectangular tensile specimens. The plastic instability manifests itself by the propagation of wave fronts that sweep the specimen and finally localize at a given site during the fracture process. The plastic instability is associated with changes in specimen geometry. In optical recordings, propagation and localization of structural instability manifest in the form of wide bands. The configuration of these bands is influenced by degradation of properties of a metal under analysis. Understanding the meaning of band configurations is vital for evaluating mechanical properties of a metal and its effective use. In this paper, a family of fringes called iso-derivatives is utilized to analyze the configuration and properties of the wide bands. The retrieval of the information contained in the wide bands depends on the spatial and temporal resolution of the recordings.

A digital method is proposed whereby spatial speckles created when an optically rough surface is illuminated by a diffuse coherent laser beam are used to generate slope contour fringes. This is done by photographing the speckles contained in a parallel plane in front of the plate before and after deformation. The resulting photographs are then fast Fourier transformed twice and numerically compared with the digital image correlation (DIC) software computer-aided speckle interferometry (CASI) to produce slope contours of the plate. The slope contour data is then stored in an array. The curvature contours are generated by numeric differentiation of the slope data; similarly the deflection contour is generated by numeric integration of the slope data. The curvature data can then be further used in conjunction with plate theory to obtain the bending and twisting moments, the plate’s experiences, as well as the fiber stress on the surface of the plate.

A new method of performing holography and holographic interferometry is proposed, whereby the film does not have to be developed in the manner that traditional film needs to be developed. This is done by implementing a photopolymer film, a film that develops via a reaction diffusion-driven photo-polymerization process. The utilization of photopolymer film rapidly simplifies the process of performing holography and holographic interferometry as it removes the wet development process that was previously required in holography.

This study proposes a method for computing stresses from measured values of in-plane strains in viscoelastic body under plane stress condition. Since Poisson’s ratio depends on time and temperature, it is difficult to evaluate stresses from in-plane strains unless Poisson’s ratio is treated as a constant. This research focuses on pseudoelasticity in the linear viscoelasticity, and the stresses are computed using a numerical Laplace transformation. Since the relation between a through-thickness strain and in-plane strains is expressed in Laplace domain, Poisson’s ratio can be treated as time- and temperature-dependence. The Laplace transformation is performed numerically using a method developed by the authors. The stresses are computed from the through-thickness strains and in-plane strains by numerical integration. Therefore, the stresses are computed ignoring the effect caused by using the constitutive equations in Laplace domain. The effectiveness of the proposed method is demonstrated by computing stresses from strains. Results show that the stresses can be evaluated from in-plane strains even if the Poisson’s ratio exhibits time- and temperature-dependence more accurate than the developed method.

In this study, dynamic compressive behaviors of resin-based CFRP were measured by using split Hopkinson pressure bar tests to consider the effect of water absorption on dynamic mechanical properties. The split Hopkinson pressure bar test method (SHPB method) is used to provide high strain rate. In order to satisfy the experimental equipment needs of SHPB method, cylindrical CFRP material was used for impact compression test.

Similar sample with different water absorption rates after several days of continuous water absorption under hygrothermal conditions were used in some of the experiments. This fiber interlayer and resin matrix water absorption will pose a great influence on the strength and strain rate of the material and even the load rate dependency under the compression of the vertical fiber direction. The process of destruction was observed using the experimental mechanics, and the experimental data was evaluated.

The full-scale performance of modern materials, such as composites and additively manufactured parts, is dependent on their characteristic substructure. An understanding of the material interactions within these repeating substructures can improve the understanding of the material’s bulk properties. Taking measurements within these substructures is made difficult by their small size and potentially non-homogenous nature. There are some techniques that already exist for making small-scale strain measurements, but these techniques can require extensive specimen preparation and expensive specialty equipment and can place significant limitations on the specimen and test design. Southern Research is working to develop a method to measure strain in these substructures using Digital Image Correlation (DIC). This approach relies on a well-designed experimental setup to apply thermal loads and observe the resulting strains. The project design has moved through a number of iterations to address the various difficulties in capturing high magnification, high-resolution images with the correct quality for use with DIC.

Two-dimensional Digital Image Correlation (2D-DIC) is a popular method for determining object in-plane motion and deformation by tracing surface texture. However, the applicability of the method is limited by perspective errors that may be generated by object surface curvature, out-of-plane motions and non-perpendicular camera alignment. The resulting variations in camera magnification lead to errors in the measured surface motion signals.

Speckle imaging is a closely related DIC method where the motions of a laser-illuminated surface are tracked by analyzing the scattered interference speckle pattern. When the camera is in focus, the speckles move at the same rate as the surface. However, if the camera is defocused, the speckle motion rate deviates from the actual surface motion. Speckle imaging measurement sensitivity can thus be controlled by varying the amount of defocus. Interestingly, speckle imaging sensitivity increases with near-focus, in contrast to sensitivity reduction in 2D-DIC caused by perspective change. Therefore, if the surface motions are analyzed using both DIC and speckle imaging, the resulting sensitivity difference can be utilized to compensate any perspective errors present.

The amount of defocus must be moderated in order to achieve a balance between sensitivity and sufficient spatial resolution for full-field measurements. Therefore, it is important to study how speckle imaging behavior changes as a function of defocus. Here, the speckle imaging characteristics are experimentally investigated by recording speckle patterns at varying defocus distances and with different lens aperture sizes (f-numbers). The resulting speckle patterns are characterized by computing the average speckle diameters. The results reveal how at small defocus distances, speckle size is linearly dependent on lens f-number and inversely proportional to defocus distance. At large defocus distances, on the other hand, speckle size is independent on f-number and linearly proportional to defocus distance.

The shape of the tympanic membrane (TM) plays an important role in conducting acoustic energy from the environment to the ear for hearing. Changes of the TM shape also have the diagnostic value, e.g., bulging of the TM is a sign of middle ear effusion. Previously we developed a high-speed holographic system employing a tunable wavelength laser for rapid TM shape measurement. However, because of the semi-transparency of the TM and the short-exposure time required for the high-speed acquisition, the illumination intensity (0.01 mW/mm²) of the tunable laser was insufficient for measurements on the semi-transparent TM surface. Here, we describe a multi-angle illumination technique that allows us to use a single wavelength (532 nm) laser with higher illumination intensity (0.06 mW/mm²) to perform shape measurements of the semi-transparent TM. The accuracy of the proposed method is demonstrated by measurements of a stepwise gauge provided by the National Institute of Standards and Technology. We successfully applied the abovementioned method to resolve the shape of the fresh postmortem human TM. We see a potential for miniaturization of the apparatus into a holographic otoscope for in-vivo measurements.

We present an in situ Digital Image Correlation (DIC) technique to characterize the interface debond behavior in a Sylgard 184-based particulate composite under quasi-static tensile loading. We construct tensile specimens with a single 650 μm glass bead inclusion at the center of the sample along with an embedded DIC speckle pattern at the midplane to enable measurement of the strain field in the region surrounding the embedded glass bead. The planar nature of the speckle pattern paired with the transparent binder allows for measurement of the sub-surface strain with simple camera diagnostics. Symmetry of the tensile specimen enables straightforward interpretation of the results. By capturing the evolution of the strain field from the unloaded state up to, and through, the delamination, we obtain a rich data set describing the debond process. Through repetition of the experiment, the debond behavior can be described in a statistically meaningful manner. This approach allows for calibration of a targeted constitutive model and serves as a tool to probe the effect of material or surface modification to promote adhesion.

Precise surgical procedures such as deep brain tumor ablation may benefit from intra-operative image guidance using magnetic resonance imaging (MRI). However, the MRI’s strong magnetic fields and constrained space pose the need for robotic devices to assist the surgeon. Piezoelectric motors are often used to actuate these robots. The piezoelectric resonant motor (PRM) is a class of such motors that consist of a bonded piezoelectric ring stator and a frictionally coupled rotor. Steady-state excitation at certain frequencies leads to specific mode shapes on the stator with surface waves having both in-plane and out-of-plane displacement components that cause the coupled rotor to spin. High-speed digital holography (HDH) can be used to measure time variant displacements with nanometer and microsecond resolution. We present initial measurements acquired at 67 k frame per second (fps) of two custom-designed plastic stator components operating at drive frequencies of 6.788 kHz and 6.980 kHz, respectively. Circumferential motion of the traveling surface waves with out-of-plane peak-to-peak displacement of approximately 100 nm peak-to-peak amplitude and a settling time of 2.99 ms was observed. Results demonstrate that plastic stators may be a promising alternative to metallic stators for use in the MRI environment.

The present research examines the deformation-induced anisotropy in the necking part of thermoplastic. Firstly, a uniaxial tensile force is applied to the large polycarbonate specimen to create the necking part. Then, small specimens are cut off from the large deformed specimen with various cutting angle to the initial tensile direction. The small specimens are used to evaluate the stress-strain curves. The results show that the elastic modulus and the maximum stress depend on the cutting angle to the initial tensile direction. Therefore, it is confirmed that the plastic deformation-induced anisotropy. Moreover, it is revealed that the dependence of the cutting angle on the elastic modulus and the maximum stress can be expressed by the anisotropic elastic constitutive equation and Tsai-Hill criterion, respectively.