Showing papers on "Digital image correlation published in 2014"

An Experimental Investigation into Additive Manufacturing-Induced Residual Stresses in 316L Stainless Steel

Abstract: Additive manufacturing (AM) technology provides unique opportunities for producing net-shape geometries at the macroscale through microscale processing. This level of control presents inherent trade-offs necessitating the establishment of quality controls aimed at minimizing undesirable properties, such as porosity and residual stresses. Here, we perform a parametric study into the effects of laser scanning pattern, power, speed, and build direction in powder bed fusion AM on residual stress. In an effort to better understand the factors influencing macroscale residual stresses, a destructive surface residual stress measurement technique (digital image correlation in conjunction with build plate removal and sectioning) has been coupled with a nondestructive volumetric evaluation method (i.e., neutron diffraction). Good agreement between the two measurement techniques is observed. Furthermore, a reduction in residual stress is obtained by decreasing scan island size, increasing island to wall rotation to 45 deg, and increasing applied energy per unit length (laser power/speed). Neutron diffraction measurements reveal that, while in-plane residual stresses are affected by scan island rotation, axial residual stresses are unchanged. We attribute this in-plane behavior to misalignment between the greatest thermal stresses (scan direction) and largest part dimension.

Vibration Monitoring of Multiple Bridge Points by Means of a Unique Vision-Based Measuring System

Abstract: Bridge static and dynamic vibration monitoring is a key activity for both safety and maintenance purposes. The development of vision-based systems allows to use this type of devices for remote estimation of a bridge vibration, simplifying the measuring system installation. The uncertainty of this type of measurements is strongly related to the experimental conditions (mainly the pixel-to-millimeters conversion, the target texture, the camera characteristics and the image processing technique). In this paper two different types of cameras are used to monitor the response of a bridge to a train pass-by. The acquired images are analyzed using three different image processing techniques (Pattern Matching, Edge Detection and Digital Image Correlation) and the results are compared with a reference measurement, obtained by a laser interferometer providing single point measurements. Tests with different zoom levels are shown and the corresponding uncertainty values are estimated. As the zoom level decreases it is possible not only to measure the displacement of one point of the bridge, but also to grab images from a wide structure portion in order to recover displacements of a large number of points in the field of view. The extreme final solution would be having wide area measurements with no targets, to make measurements really easy, with clear advantages, but also with some drawbacks in terms of uncertainty to be fully comprehended.

Tension, compression, and bending of superelastic shape memory alloy tubes

Abstract: While many uniaxial tension experiments of shape memory alloys (SMAs) have been published in the literature, relatively few experimental studies address their behavior in compression or bending, despite the prevalence of this latter deformation mode in applications. In this study, superelastic NiTi tubes from a single lot of material were characterized in tension, compression, and pure bending, which allowed us to make direct comparisons between the deformation modes for the first time. Custom built fixtures were used to overcome some long-standing experimental difficulties with performing well-controlled loading and accurate measurements during uniaxial compression (avoiding buckling) and large-rotation bending. In all experiments, the isothermal, global, mechanical responses were measured, and stereo digital image correlation (DIC) was used to measure the evolution of the strain fields on the tube's outer surface. As is characteristic of textured NiTi, our tubes exhibited significant tension–compression asymmetry in their uniaxial responses. Stress-induced transformations in tension exhibited flat force plateaus accompanied by strain localization and propagation. No such localization, however, was observed in compression, and the stress “plateaus” during compression always maintained a positive tangent modulus. While our uniaxial results are similar to the observations of previous researchers, the DIC strain measurements provided details of localized strain behavior with more clarity and allowed more quantitative measurements to be made. Consistent with the tension–compression asymmetry, our bending experiments showed a significant shift of the neutral axis towards the compression side. Furthermore, the tube exhibited strain localization on the tension side, but no localization on the compression side during bending. This is a new observation that has not been explored before. Detailed analysis of the strain distribution across the tube diameter revealed that the traditional assumption of elementary beam theory, that plane sections remain plane, does not hold. Yet when the strain was averaged over a few diameters of axial length, the tensile and compressive responses input into elementary beam theory predicted the global bending response with reasonable accuracy. While it is encouraging that a simple model could predict the moment–curvature response, we recommend that beam theory be used with caution. The averaged strain field can under/over predict local strains by as much as two-fold due to the localized deformation morphology.

Surface reconstruction and landslide displacement measurements with Pléiades satellite images

Abstract: Recent advances in image-matching techniques and VHR satellite imaging at submeter resolution theoretically offer the possibility to measure Earth surface displacements with decimetric precision. However, this possibility has yet not been explored and requirements of ground control and external topographic datasets are considered as important bottlenecks that hinder a more common application of optical image correlation for displacement measurements. This article describes an approach combining spaceborne stereo-photogrammetry, orthorectification and sub-pixel image correlation to measure the horizontal surface displacement of landslides from Pleiades satellite images. The influence of the number of ground-control points on the accuracy of the image orientation, the extracted surface models and the estimated displacement rates is quantified through comparisons with airborne laser scan and in situ global navigation satellite measurements at permanent stations. The comparison shows a maximum error of 0.13 m which is one order of magnitude more accurate than what has been previously reported with spaceborne optical images from other sensors. The obtained results indicate that the approach can be applied without significant loss in accuracy when no ground control points are available. It could, therefore, greatly facilitate displacement measurements for a broad range of applications.

Experimental study of cone penetration in silica sand using digital image correlation

Abstract: The problem of cone penetration, particularly deep penetration, remains one of the most challenging in geotechnical engineering. It involves large displacements, rotations and deformation of soil elements in the path of the cone as well as complex response of the soil, including crushing and the development of large mean stresses, to the displacements imposed by the penetration process. As a result, rigorous theoretical solutions are not available for this problem, and experimental simulations of penetration provide insights that would not otherwise be available. This paper presents the results of a series of cone penetration tests performed in a half-circular chamber in sand samples prepared with three silica sands with different crushability. Cone resistance was measured, and digital images of the cone penetrating into the sand samples were acquired simultaneously during the entire penetration process. The digital image correlation (DIC) technique was then used to process these images to obtain the soil...

Study of dynamic underwater implosion mechanics using digital image correlation

Abstract: The physical processes associated with the implosion of cylindrical tubes in a hydrostatic underwater environment were investigated using high-speed three-dimensional digital image correlation (3D ...

An effective procedure to create a speckle pattern on biological soft tissue for digital image correlation measurements.

Abstract: Creating a speckle pattern on biological soft tissue, which would be suitable for digital image correlation measurements, is challenging. Speckle patterns should neither cause or require sample dehydration, nor alter the mechanical response, but they should adhere to the tissue surface and withstand large deformations. A two-step procedure has been implemented to create a highly-contrasted pattern. It requires staining of the tissue with methylene blue solution to obtain a dark background and airbrushing the surface with paint to create white speckles. This study evaluated the effectiveness of the proposed procedure and whether the pattern creation had any effect on the elastic response of soft tissue. Forty porcine collateral ligaments underwent three series of cyclic tensile tests to a nominal elongation of 10% for 30 cycles. The specimen stiffness was calculated from the load-elongation curve collected during the last 10 cycles. One side of 20 ligaments was blue stained between the first and second test series, and white patterned between the second and third test series. During the last series, ligament surface images were also acquired and elaborated using the digital image correlation technique. The other 20 ligaments were untreated. The data show a small non-significant upward trend in stiffness in treated as well as in untreated ligaments (maximum increase of 1.7%). The 'successfully-correlated area' of the stereo-visible ligament surface was on average 96%, i.e. small parts of the 'stereo-visible area' were lost during computation. The described procedure is an effective method to create a pattern on biological soft tissues.

Microscale plastic strain heterogeneity in slip dominated deformation of magnesium alloy containing rare earth

Abstract: The propensity for the magnesium alloy ZEK100 to develop large microscale plastic strain heterogeneity has been quantified and correlated to microstructure using a combination of experiments and simulations. Conditions were specifically selected where deformation is dominated by slip rather than twinning. Digital image correlation measurements of intragranular plastic strain heterogeneity have revealed plastic strains as large as 5 times the macroscopic tensile strain. This strain amplification was found to be neither spatially correlated nor correlated with crystal orientation. Large local strains were, however, found to be statistically linked to proximity to grain boundaries. This suggests the importance of interactions between neighboring grains. To investigate this further full-field viscoplastic Fast Fourier Transform (VPFFT) crystal plasticity simulations were performed on synthetic microstructures representative of the material studied experimentally. It was found that both the macroscopic stress–strain response as well as the local plastic strain distribution could be well reproduced when the anisotropy in ‘hardness’ of the basal and non-basal slip systems was sufficiently high. Strain amplification was found to occur, in this case, to slip in regions of high basal slip activity adjacent to regions of high non-basal slip activity.

High-accuracy 2D digital image correlation measurements using low-cost imaging lenses: implementation of a generalized compensation method

Abstract: The ideal pinhole imaging model commonly assumed for an ordinary two-dimensional digital image correlation (2D-DIC) system is neither perfect nor stable because of the existence of small out-of-plane motion of the test sample surface that occurred after loading, small out-of-plane motion of the sensor target due to temperature variation of a camera and unavoidable geometric distortion of an imaging lens. In certain cases, these disadvantages can lead to significant errors in the measured displacements and strains. Although a high-quality bilateral telecentric lens has been strongly recommended to be used in the 2D-DIC system as an essential optical component to achieve high-accuracy measurement, it is not generally applicable due to its fixed field of view, limited depth of focus and high cost. To minimize the errors associated with the imperfectness and instability of a common 2D-DIC system using a low-cost imaging lens, a generalized compensation method using a non-deformable reference sample is proposed in this work. With the proposed method, the displacement of the reference sample rigidly attached behind the test sample is first measured using 2D-DIC, and then it is fitted using a parametric model. The fitted parametric model is then used to correct the displacements of the deformed sample to remove the influences of these unfavorable factors. The validity of the proposed compensation method is first verified using out-of-plane translation, out-of-plane rotation, in-plane translation tests and their combinations. Uniaxial tensile tests of an aluminum specimen were also performed to quantitatively examine the strain accuracy of the proposed compensation method. Experiments show that the proposed compensation method is an easy-to-implement yet effective technique for achieving high-accuracy deformation measurement using an ordinary 2D-DIC system.

High-temperature digital image correlation method for full-field deformation measurement captured with filters at 2600°C using spraying to form speckle patterns

Abstract: A method is presented for obtaining good images of a sprayed speckle pattern on specimen surfaces at high temperatures, suitable for strain measurement, by digital image correlation (DIC) using plasma spray for speckle preparation in which a bandpass filter, neutral density filters, and a linear polarizing filter are used to reduce intensity and noise in images. This is accomplished by speckle preparation through the use of plasma spray and suppression of black-body radiation through the use of filters. By using plasma spray for speckle preparation and the filters for image acquisition, the method was demonstrated to be capable of providing accurate DIC measurements up to 2600°C. The full-field stretching deformation of the specimen was determined using the DIC technique. Experimental results indicate that the proposed high-temperature DIC method is easy to implement and can be applied to practical, full-field, high-temperature deformation measurements with high accuracy.

Determination of Anisotropic Plastic Constitutive Parameters Using the Virtual Fields Method

Abstract: The aim of the present study is to retrieve all the anisotropic plastic constitutive parameters from uniaxial loading. A complex geometry which can provide very heterogeneous stress states in a uniaxial tensile test was chosen for steel sheet specimens. A digital image correlation technique was used for the full-field heterogeneous strain measurement. The orthotropic Hill1948 yield criterion with Swift isotropic hardening was adopted as an elasto-plastic constitutive model. The virtual fields method (VFM) was employed as an inverse analytical tool to determine the constitutive parameters. All the parameters were successfully identified using the VFM by combining two tensile test results obtained in rolling and transverse directions.

Direct Stress-Strain Measurements from Bulged Membranes Using Topography Image Correlation

Abstract: This paper discusses an experimental method to characterize thin films as they are encountered in micro-electronic devices. The method enables the measurement of the stress and strain of pressure deflected bulged membranes without using a priori defined bulge equations. An enrichment to the Global Digital Image Correlation method is detailed to capture the membrane strain and curvature while robustly dealing with acquisition noise. The accuracy of the method is analyzed and compared to the standard bulge test method. The method is applied to a proof of principle experiment to investigate its applicability and accuracy. Additionally, it is shown for two experimental cases that the method provides accurate results, although the bulge equations do not hold.

A DIC-based study of in-plane mechanical response and fracture of orthotropic carbon fiber reinforced composite

Abstract: The in-plane elastic properties and quasi-static fracture response of an orthotropically woven carbon fiber reinforced composite are investigated using 3D digital image correlation. The elastic properties are determined as a function of fiber orientation, and the effect of fiber angles on the crack extension direction is investigated. The modified maximum hoop stress criterion is used to predict the crack extension angle of the examined material. The predicted angles are in very good agreement with those found experimentally. Optical observations and local strain data from the quasi-static fracture studies reveal that crack initiation occurs well before the maximum far-field load is reached.

Crack-tip thermal and mechanical hysteresis in Shape Memory Alloys under fatigue loading

Abstract: Crack tip stress-induced phase transformation mechanisms in nickel–titanium alloys (NiTi), subjected to fatigue mechanical loads, have been analyzed by full field measurement techniques. In particular, Infrared thermography (IR) and Digital Image Correlation (DIC), have been applied to analyze the cyclic temperature and displacement evolutions in the crack tip region of a commercial pseudoelastic alloy, together with the associated thermal and mechanical hysteresis, by using Single Edge Crack (SEC) specimens. IR investigations revealed a global temperature variation of the specimen due to crack formation and propagation mechanisms, which is similar to common engineering metals, i.e. surface temperature rises quickly in an initial phase, then it reaches an almost constant value, and finally it increases rapidly as a consequence of the fatigue crack growth. In addition, cyclic thermal variation in the crack tip region and related hysteresis (temperature vs load) has been measured, which can been directly related to the thermal effects of the reversible stress-induced phase transformations. Furthermore, a proper experimental setup has been made, based on a reflection microscope, for direct measurements of the crack tip displacement field by the DIC technique. Furthermore, a fitting procedure has been developed to calculate the mode I Stress Intensity Factor (SIF), starting from the displacement field, and the related mechanical hysteresis (SIF vs load).

Mechanical Behavior and Anisotropy of Spin-Coated SU-8 Thin Films for MEMS

Abstract: Spin-coated SU-8 thin films used in Microelectromechanical Systems (MEMS) devices are investigated using tensile experiments on freestanding microscale specimens to understand their mechanical response. Modified in situ optical microscope experiments are performed on 2 μm thick transparent SU-8 films in the strain rate range of 2×10-5/s to 2×10-2/s and accurate strain measurements are extracted using Digital Image Correlation. The effects of strain rate and anisotropy, on mechanical properties, in hard baked and non-hard baked films were studied in conjunction with Fourier Transform Infrared Spectroscopy. The Young's modulus and tensile strength were found to increase with strain rate significantly, with the stress vs. strain behavior changing from viscoelastic to linear elastic. Hard baking of the films resulted in better mechanical properties, viz., elastic modulus and tensile strength, due to increased cross-linking density. The average moduli for hard baked and non-hard baked films were 3.48±0.57 GPa and 2.92± 0.43 GPa, respectively, obtained from 30 experiments each, over the entire strain rate range. Furthermore, the spin-coat process used to make the films developed anisotropy in the plane of the films due to molecular orientation, which was independent of hard baking temperature and duration.

Cluster approach based multi-camera digital image correlation: Methodology and its application in large area high temperature measurement

Abstract: A cluster approach based multi-camera Digital Image Correlation (DIC) system has been developed to quantify dynamic material response at temperature up to 1200 °C The Monochromatic Light Illuminated Stereo DIC technique was embedded to eliminate surface radiance at high temperature. The employed measurement system not only takes advantage of a conventional 3D DIC system, but also provides a feasible way to enlarge the measurement field without losing effective resolution in the area of interest. Two pairs of pre-calibrated CCD cameras are used to measure a piece of sheet nickel alloy. The view of each pair of cameras covers about half of the specimen. To guarantee the continuity of the evaluation result, an overlapped area that is covered by all four cameras is used in the setup. Unlike the conventional data stitching technique which stitches data from different pairs of cameras, our system with the cluster approach technique, maps all data points into a universal world coordinate system before evaluating the contour, displacement, and strain. To evaluate our system, a specimen was loaded with infrared heaters, and the dynamic contour, displacement, and strain field was evaluated. The methodology of the employed system is introduced in this paper. The system has the potential to be expanded with more cameras to measure a very large surface with one shot.