Samantha Daly

Small-scale patterning methods for digital image correlation under scanning electron microscopy

Digital image correlation (DIC) is a powerful, length-scale-independent methodology for examining full-field surface deformations. Recently, it has become possible to combine DIC with scanning electron microscopy (SEM), enabling the investigation of small-scale deformation mechanisms such as the strains accommodated within grains in polycrystalline metals, or around micro-scale constituents in composite materials. However, there exist significant challenges that need to be surmounted before the combination of DIC and SEM (here termed SEM-DIC) can be fully exploited. One of the primary challenges is the ability to pattern specimens at microstructural length scales with a random, isotropic and high contrast pattern needed for DIC. This paper provides a thorough survey of small-scale patterning methods for SEM-DIC and discusses their advantages and disadvantages for different applications.

Tips and Tricks for Characterizing Shape Memory Wire Part 5: Full-Field Strain Measurement by Digital Image Correlation

This is the fifth paper in a series on the experimental characterization of shape memory alloy (SMA) wires. In this installment we focus on the use of digital image correlation (DIC) to measure the strain field on the surface of the wire. After a brief overview of the principles and mathematics behind DIC, two different thermo-mechanical tension tests using DIC are presented to demonstrate the technique. The first experiment consists of Joule heating a shape memory (SM) wire to induce the shape memory effect, using 2-D DIC to measure the strain field. The second experiment measures the response of a superelastic (SE) wire to mechanical cycling at room temperature, using 3-D DIC to measure the strain field and an infrared camera to measure the temperature field. In addition to describing the experimental results, attention is paid to specimen preparation and the two experimental setups. Many of the challenges and precautions associated with using DIC are discussed, along with practical recommendations for specimen speckle patterns, digital photography, and data post processing.

The effect of texture on stress-induced martensite formation in nickel–titanium

An experimental study was performed to investigate the effect of texture on stress-induced martensitic phase transformation in the shape memory alloy (SMA) nickel?titanium (Nitinol). Thin sheet specimens of Nitinol were examined under uniaxial tensile loading using three-dimensional digital image correlation in order to spatially and temporally track strain localization indicative of martensitic transformation. Tensile specimens were fabricated along directions oriented 0??(RD), 45?, and 90??(TD) to the rolling direction of the sheet and subjected to 50 cycles at prescribed strain rates of , and 10?2?s?1. It was found that upon loading, specimens unfavorably oriented for transformation (TD specimens) nucleated a greater number of deformation bands due to a smaller difference between nucleation and propagation stresses, and also accommodated less axial strain inside the band and more axial strain outside the band. The unfavorable (TD) specimens also exhibited a stronger cycle-to-cycle similarity in the strain accommodated inside the deformation band, which has important implications for the design of SMA structures for fatigue applications. Finally, the (primarily martensite) region of the deformation band(s) consistently showed significantly stronger cycle-to-cycle similarity than the (primarily austenite) region outside the band(s), regardless of specimen texture.

Multiscale damage characterization in continuous fiber ceramic matrix composites using digital image correlation

Damage in continuous fiber CMCs with weakly bonded fiber–matrix interfaces evolves in several stages that span multiple length scales. Comprehensive damage characterization necessitates identifying how damage initiates as well as how it accumulates (through final failure), which is not feasible with information gathered from a single length scale. Using digital image correlation to measure full-field surface deformations, damage evolutions in continuous fiber SiC/SiC laminates were analyzed at three distinct length scales: constituent, lamina, and laminate. Constituent scale analyses indicated that fine matrix cracks initiated in localized regions of transverse fiber coatings at low stresses. Investigations at the larger lamina scale revealed that some, but not all, of the coating cracks evolved into matrix cracks, the propagation of which was dependent on the state of stress near the crack tip and the local microstructure. Many of these matrix cracks morphed into cracks large enough to be detected at the (largest) laminate scale. The density of the large matrix cracks increased with load, reaching saturation prior to failure. While the constituent scale identifies when (with respect to stress state) and where (with respect to local microstructure) damage initiates, the lamina scale elucidates damage progression between neighboring constituents. Only laminate scale fields of view are large enough to capture the accumulation of the large matrix cracks that ultimately lead to final fracture. However, as spatial resolution is reduced at this scale, finer cracks (which may provide pathways for environmental ingress) go undetected. As each length scale provides a unique perspective of damage evolution in CMCs, multiscale analysis is essential for comprehensive damage characterization.

Experimental assessment of toughness in ceramic matrix composites using the J-integral with digital image correlation part II: application to ceramic matrix composites

This paper is the second of a two-part series assessing the feasibility of using the J-integral with full-field deformation data from digital image correlation (DIC) to characterize toughness in continuous fiber, SiCf/SiCc ceramic matrix composites (CMCs). In part I, line and area integral adaptations of the J-integral were validated with analytical and experimental deformation fields. It was found that variability in experimental data and contour truncation at the crack wake introduced error and path-dependency, but this could be mitigated through data filtering. In this paper, the line and area integral methods were evaluated for cross-ply and longitudinal CMC tensile specimens. It is demonstrated that these approaches cannot be used for quantitative characterization, but can be successfully used to examine qualitative trends. For both architectures, area integral measurements of the potential energy release rate, J, were larger (and arguably more accurate) than line integral measurements. Resistance curves were characteristic of tough materials, but the stress intensity magnitudes were larger in the longitudinal laminates than the cross-ply (for equivalent crack extension). Experimentally derived bridging laws showed that fiber bridging tractions were largest in the cross-ply laminates, which was attributed to a lower volume fraction of longitudinal fibers. Quantitative, single-value measurements of fracture toughness could not be established since (1) noise in the DIC data masked strain signals indicative of first matrix cracking, and (2) widespread matrix cracking rendered the J-integral invalid (regardless of size and position, the boundaries of the line and area contours inevitably intercepted microcracks). Rather, gross estimates of toughness and stress intensity factors were made under the assumption that the limitations in the spatial resolution of the DIC data essentially transforms regions with fine microcracking along the contour into continuous regions of elastic deformation. When evaluated with the J-integral, the smoothed deformation fields manifest as resistance curves indicative of a tough material. Thus, the J-integral was valuable for qualitative assessments of toughening mechanisms in the CMCs.

Experimental Studies of Phase Transformation in Shape Memory Alloys

This paper presents experimental studies to examine stress-induced martensitic phase transformation during the superelastic deformation of the shape memory alloy Nickel-Titanium. The phase transformation, which is solid-to-solid and diffusionless, occurs between austenite, a B2 cubic crystal structure, and martensite, a monoclinic crystal structure during loading and unloading at constant ambient temperature. To examine the complex local thermo-mechanical interactions that affect transformation behavior, we utilize a temporally- and spatially- simultaneous combination of strain and thermal mapping using three-dimensional digital image correlation and infrared thermography, respectively. This combined experimental approach enables full-field, quantitative maps of strain and temperature fields over the specimen surface, allowing the investigation of factors including cycling, strain rate, texture, and local temperature variations. The effects of these factors on fundamental transformation properties, such as the stresses required for phase nucleation and propagation, accumulated residual plastic strain, total strain accommodated by phase transformation, the evolution of martensitic volume fraction, and the amount of hysteresis, are discussed.

Optimum Paint Sequence for Speckle Patterns in Digital Image Correlation

This note answers whether it is better to use black-on-white or white-on-black painted speckle patterns, and provides recommendations for optimum painted patterns for digital image correlation (DIC). Although DIC algorithms have no preference between tracking the patterns of either bright features or dark features on contrasting backgrounds, we show that paint sequence can be important due to fundamental differences in paint pigments. If the sample and experiment conditions call for two painting steps (basecoat and speckles), then applying a basecoat of white paint followed by black speckles is better than the converse. Black speckles are preferred because they increase contrast and provide a higher mean intensity gradient, lower correlation confidence interval, and lower displacement error. The primary reason for the increased contrast is the greater hiding power of black paint versus white paint. A secondary effect that also reduces contrast in white speckles is undertone, or a slight blue hue shift from Rayleigh scattering.

Experimental assessment of toughness in ceramic matrix composites using the J-integral with digital image correlation part I: methodology and validation

The complex nature of damage in ceramic matrix composites (CMCs) renders conventional methods of measuring quantitative fracture properties impractical. This paper is the first of two-part series that assesses the feasibility of using the J-integral with full-field deformation data from digital image correlation (DIC) to characterize toughness in continuous fiber SiC/SiC CMC laminates. It provides a resource for best practices when incorporating experimentally measured, full-field deformation data into J-integral evaluations of toughness. The techniques discussed are important for researchers attempting to measure fracture properties in advanced materials with damage mechanisms that have yet to be well characterized. Two methods for evaluating potential energy release rates J are presented: (1) numerical integration over a line contour and (2) Gaussian integration over an area contour. Accuracy and path independency for both methods were verified using analytically derived deformation fields for a center-cracked, infinite plate of isotropic material under equi-biaxial tension. Inherent noise in the deformation data and necessary contour truncation at the crack surfaces reduced accuracy and introduced path dependency. However, this was mitigated by careful noise filtering of the deformation data prior to the evaluation of J. Applying the line and area integrals to DIC data from tapered, double-cantilever beam, acrylic compact tension specimens resulted in the findings that (1) both integrals measured fracture toughness within a range of published values; (2) both integrals captured the constant stress intensity factor behavior that is characteristic of the tapered beam geometry; and (3) the area integral measurements were consistently larger than line integral measurements. The area integral is more accurate than the line integral as it samples more data points, reducing its sensitivity to experimental noise. Although variability in experimental data can be minimized, it cannot be eliminated. Measurement error is inevitable; thus, the line and area integrals must be used with caution when characterizing quantitative properties including fracture toughness.

Pseudoelastic shape memory alloy cables

Conventional structural cables (wire ropes) are composed of steel wires helically wound into strands that are then wound around a core. Cables made from shape memory alloy (SMA) wires are a new structural element with promising properties for a broad range of applications. Among the many potential advantages of this form are increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and significant design flexibility. Currently there are few studies of SMA cables in the literature. This paper describes exploratory thermomechanical experiments that were performed on two commercially available cable designs.

Shape Memory Alloy Cables: Exploratory Experiments

Conventional structural cables (or wire ropes) are composed of steel wires helically wound into strands, which, in turn, are wound around a core. Cables made from NiTi shape memory alloy (SMA) wires are a new structural element with promising adaptive and enhanced structural properties for a broad range of applications. Potential advantages of this form include increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and significant design flexibility. Exploratory thermomechanical experiments were performed on a conventional cable construction: the right regular lay 7 × 7, consisting of 7 strands with 7 wires per strand. Uniaxial tension experiments characterize the cables’ sensitivity to strain rate, temperature, and lubrication. Experiments were also performed on individual strands and wires from the cable to study the hierarchical nature of the cable construction. Special attention was paid to the propagation of phase transformation fronts, similar to that seen previously in uniaxial tension of NiTi wire.© 2009 ASME