Propagation of Noise Uncertainty Through Virtual Strain Gauge Formulations for 2D Digital Image Correlation

The statistical gauge in geometrical verification. Part II. The virtual gauge and verification process

Due to the aging global civil infrastructure (e.g. bridges), there is a critical need for monitoring and assessing structural integrity of large scale structures. According to the ASCE, in 2008, the average bridge in the U.S.A. was 43 years old and 161,892 bridges were structurally deficient or obsolete. Currently, bridge health is assessed primarily using qualitative visual inspection, which is not always reliable because some damage is difficult to detect, quantify visually, or is subject to human interpretation. Traditional sensors such as strain gages, and displacement sensors, have been recently used to monitor bridges. These sensors only measure at discrete points or along a line, making it difficult to detect damage that is not in the immediate vicinity of the sensor or is difficult to interpret. To address these issues, this paper investigates the use of three-dimensional (3D) digital image correlation (DIC) as a sensing approach for improved bridge structural health monitoring. 3D DIC is a non-contact, full field, optical measuring technique that uses digital cameras to measure surface geometry, displacement, and strain. It is proposed that DIC can be used for monitoring by imaging a bridge periodically and computing strain and displacement from images recorded at different dates or operating conditions. In this paper, DIC is shown to locate non-visible cracks in concrete, quantify spalling, and measure bridge deformation. These techniques are first demonstrated in the laboratory. Field measurements are also made on three full-scale bridges. This paper discusses challenges and solutions to implementing DIC on large structures in the field. The results reveal that DIC is an effective approach to monitor the integrity of large scale civil infrastructure.

Propagation of uncertainties in deterministic systems

The decreasing size of solder joints leaves limited tolerance of allowable warpage range for successful assembly. Understanding the electronic packages' behavior during the reflow process becomes one of the most important tasks in developing assembly process and assuring reliability. Three-dimensional (3D) digital image correlation (DIC) plays an important role in quantifying warpage of BGA packages under thermal loading. To measure warpage of the substrate surface of a BGA package, conventional 3D DIC technique requires removing the solder balls, which takes the risk of damaging the solder mask surface and influences on measurement accuracy. Three parameters of 3D DIC (camera angle, facet size and facet step) are studied on the effects of data integrity and accuracy to realize warpage measurement while solder balls are attached to the packages. This method introduces novel settings of these parameters compared to current 3D DIC requirements, and summarizes a guidance for implementing nondestructive warpage measurement with different sizes of solder balls and camera resolutions.

2D and stereo Digital Image Correlation (DIC) allows to retrieve complex displacement and strain fields on a specimen’s surface. Although 2D DIC is strongly affected by out-of-plane motions, in many situations, it is preferred over stereo DIC because of its ease to use and because only one camera is required. The out-of-plane movements can be ascribed mainly to three causes: the camera positioning, the imperfections of the used test device, and the camera self-heating. These effects gain importance when the distance between the camera and the specimen is reduced. The positioning of the camera aims to have its optical axis perfectly perpendicular to the specimen to observe. Nevertheless small but effective misalignments can easily happen even if suitable devices are used for the alignment. This contribution concerns the experimental evaluation of these movements considering a cyclic uni-axial tensile test performed on an aluminium specimen. The study is particularly focused to the out-of-plane motions that occur at every cycle because of the tensile bench, which are the more critical ones. Finally a compensation method, based on fixed compensation plates, is presented. The method allows to properly correct the data coming from a 2D DIC set-up.

ABSTRACTA novel three-dimensional (3D) digital image correlation (DIC) approach using a stereovision system was developed to measure the evolution of strain and distortion near the fusion zone during the gas tungsten arc welding process. Unlike the previously reported two-dimensional (2D) DIC approach using a single camera, the 3D DIC method was immune to the out-of-plane displacement and was capable of measuring 3D deformation. Both 2D and 3D DIC approaches in welding applications were based on the utilisation of the novel high-temperature speckle and the pulsed laser illumination plus bandpass filters. However, the speckle pattern was partially specular reflective causing issues in subset matching in the 3D approach. A new algorithm and experimental procedure was incorporated to solve this problem.

During operation of vehicles and structures, excessive transient loading can lead to reduced fatigue life and even mechanical failure. It has been shown that when a structure undergoes a damaging sequence of events, such as those occurring during a helicopter hard landing, the structural health of a specimen can be severely affected. In order to effectively quantify damage and monitor the structural health of the specimen, experimental data is required across a wide area of the helicopter. Within this paper the use of three-dimensional (3D) digital image correlation (DIC) and dynamic photogrammetry (DP) is examined as a possible method to acquire the necessary data to perform structural health monitoring in a non-obtrusive manner. DIC and DP are a non-contacting measurement techniques that utilizes a stereo pair of digital cameras to track prescribed surface pattern or optical targets placed on the structure. The approaches can provide global information about changes to the structure over the entire field of view. A scale laboratory test is performed on a helicopter to simulate several loading scenarios. The changes in the structural shape and strain field of the model helicopter fuselage as a direct result of the loadings are identified. The tests demonstrate that this technique is a valid way to determine the damage inflicted on the structure due to an excessive applied loading or dynamic maneuver. Practical applications and common limitations of the technique are discussed.

Three-dimensional (3D) digital image correlation (DIC) is one of the most popular techniques used in engineering for strain and deformation measurement. However, the error analysis of 3D DIC, especially which kind of parameters dominates the error of 3D coordinate reconstruction in any kind of configuration, is still under study. In this paper, a technique that can be used for error determination of reconstruction is presented. The influence from the system calibration and image correlation to the error is theoretically analyzed. From numerical experiments of one-dimensional line and two-dimensional plane, the evaluation procedure is validated to be flexible. A typical test with standard objects is also conducted. With this technique, once a 3D DIC system is set up and images of objects with speckles and calibration boards are recorded, the error of the configuration can be immediately evaluated. The standard deviation of every point in the world coordinate can be determined by statistical analysis.

As civil infrastructure (i.e. bridges, railways, and tunnels) continues to age; the frequency and need to perform inspection more quickly on a broader scale increases. Traditional inspection and monitoring techniques (e.g., visual inspection, mechanical sounding, rebound hammer, cover meter, electrical potential measurements, ultrasound, and ground penetrating radar) may produce inconsistent results, require lane closure, are labor intensive and time-consuming. Therefore, new structural health monitoring systems must be developed that are automated, highly accurate, minimally invasive, and cost effective. Three-dimensional (3D) digital image correlation (DIC) systems have the merits of extracting full-field strain, deformation, and geometry profiles. These profiles can then be stitched together to generate a complete integrity map of the area of interest. Concurrently, unmanned aerial vehicles (UAVs) have emerged as valuable resources for positioning sensing equipment where it is either difficult to measure or poses a risk to human safety. UAVs have the capability to expedite the optical-based measurement process, offer increased accessibility, and reduce interference with local traffic. Within this work, an autonomous unmanned aerial vehicle in conjunction with 3D DIC was developed for monitoring bridges. The capabilities of the proposed system are demonstrated in both laboratory measurements and data collected from bridges currently in service. Potential measurement influences from platform instability, rotor vibration and positioning inaccuracy are also studied in a controlled environment. The results of these experiments show that the combination of autonomous flight with 3D DIC and other non-contact measurement systems provides a valuable and effective civil inspection platform.

Advances in computing and camera technologies have greatly enhanced the utility of three dimensional (3D) digital image correlation (DIC) in the last several years. For example, 3D DIC is now being used as a tool to measure operational deflection shapes, full-field dynamic strain and discrete point tracking. Due to the line of sight requirements of stereo photogrammetry it may not be possible to measure every location required on a surface within a field of view allowed by a single camera pair. Therefore, there is a need to have the capability to stitch several fields of view together, providing a continuous measurement over the entire surface of a structure. This paper presents an approach to stitch several fields of view measured using 3D DIC, to provide full-field information of a Sandia National Laboratories CX-100 nine meter wind turbine blade. A loading test was performed on the cantilevered turbine blade with 16 separate fields of view captured in both loaded and unloaded conditions. The processed results provide full surface displacement and strain information of the CX-100 nine meter blade. These results indicate the potential of utilizing stitching to perform full surface static and dynamic measurement of structures.

Three-dimensional digital image correlation with improved efficiency and accuracy

This study explores the effectiveness of 3D Digital Image Correlation (DIC) for measuring displacement and strain of a propeller undergoing angular motion. Traditional methods, such as strain gauges, face limitations including physical interference, technical difficulties in sensor connections, and restricted measurement points, leading to inaccuracies in capturing true conditions. To overcome these challenges, this research utilizes non-contact 3D DIC technology, enabling measurement of surface displacements and deformations without interfering with the tested component. Experiments were conducted using the model aircraft propellers mounted on a custom-built test stand for partial angular motion. The 1 Mpx high-speed cameras captured strain and displacement data across the propeller blades during motion. The DIC strain measurements were then compared to strain gauge data to evaluate their accuracy and reliability. The results demonstrate that 3D DIC enables precise displacement measurements, while strain measurements are subject to certain limitations. Displacement measurements were achieved with a noise level of ±10 μm, while strain measurement noise ranged from 26 to 174 µm/m depending on direction. Strain gauge measurements were also performed for verification of the DIC measurements and calibration of the filtering procedure. Two types of non-metallic materials were used in the study: Nylon LGF60 PA6 for the propeller and 3D-printed PC ABS for the cantilever beam used in strain measurement validation. This study underscores the potential of DIC for monitoring rotating components, with a particular focus on measuring strains that are often overlooked in publications addressing similar topics. Additionally, it focuses on comparing DIC strain measurements with strain gauge data on rotating components, addressing a critical gap in existing literature, as strain measurement in rotating structures remains underexplored in current research.

This thesis is concerned with the development of experimental diagnostic techniques for the study of nonlinear instabilities in annular liquid sheets. An axisymmetric annular sheet of varying thickness with no swirl and parallel injection of gas and liquid phases (i.e. no impingement) is considered. In this flow, break-up of the sheet is due to aerodynamic shear forces at the interfaces. Highly resolved velocity measurements of the phase interface from high speed digital images are achieved via a specialised sub-pixel image correlation algorithm. Metrics for image quality based on defocus and image noise are defined, and the effect on the uncertainty in the measured velocity is investigated. The image correlation approach is benchmarked against a number of alternative algorithms with regard to error sensitivity. Decomposition of spatial instability modes is achieved via a novel application of Koopman Mode analysis via the Dynamic Mode Decomposition (DMD). A synthetic error analysis is applied to the DMD to estimate the propagation of uncertainty as a function of data quantity and quality. DMD provides a spatially resolved amplification profile which yields a novel characterisation of the evolution of the non-linear instability. With these techniques, some new insight is obtained into the behaviour of the instability. An unexpectedly rapid onset of nonlinearity in the near-field is observed. In concert with DMD, a novel application of Hilbert-Huang decomposition allows a new theory of modulated nonlinear instability in the far-field of the spray to be proposed. A parametric analysis over an order of magnitude variation in Reynolds number, Weber number and gas momentum is undertaken. From this, a temporal scaling behaviour is observed which shows reasonable agreement with other studies. Using the DMD results, a new scaling behaviour for the effects of the sheet thickness on the convective nonlinear instability is proposed.

Discrepancies of speckle images under dynamic measurement due to the different viewing angles will deteriorate the correspondence in 3D digital image correlation (3D-DIC) for vibration measurement. Facing this kind of bottleneck, this paper presents two types of robust 3D-DIC methods for vibration measurement, SSD-robust and SWD-robust, which use a sum of square difference (SSD) estimator plus a Geman-McClure regulating term and a Welch estimator plus a Geman-McClure regulating term, respectively. Because the regulating term with an adaptive rejecting bound can lessen the influence of the abnormal pixel data in the dynamical measuring process, the robustness of the algorithm is enhanced. The robustness and precision evaluation experiments using a dual-frequency laser interferometer are implemented. The experimental results indicate that the two presented robust estimators can suppress the effects of the abnormality in the speckle images and, meanwhile, keep higher precision in vibration measurement in contrast with the traditional SSD method; thus, the SWD-robust and SSD-robust methods are suitable for weak image noise and strong image noise, respectively.

A coupled 3D laser scanning and digital image correlation system for geometry acquisition and deformation monitoring of a railway tunnel