Strain Investigations Around Circular Boundaries Based on Digital Image Correlation Data Using Parametrisation and Optimised Smoothing

Full‐field data from digital image correlation (DIC) provide rich information for finite‐element analysis (FEA) validation. However, there are several inherent inconsistencies between FEA and DIC data that must be rectified before meaningful, quantitative comparisons can be made, including strain formulations, coordinate systems, data locations, strain calculation algorithms, spatial resolutions and data filtering. In this paper, we investigate two full‐field validation approaches: (1) the direct interpolation approach, which addresses the first three inconsistencies by interpolating the quantity of interest from one mesh to the other, and (2) the proposed DIC‐levelling approach, which addresses all six inconsistencies simultaneously by processing the FEA data through a stereo‐DIC simulator to ‘level' the FEA data to the DIC data in a regularisation sense. Synthetic ‘experimental' DIC data were generated based on a reference FEA of an exemplar test specimen. The direct interpolation approach was applied, and significant strain errors were computed, even though there was no model form error, because the filtering effect of the DIC engine was neglected. In contrast, the levelling approach provided accurate validation results, with no strain error when no model form error was present. Next, model form error was purposefully introduced via a mismatch of boundary conditions. With the direct interpolation approach, the mismatch in boundary conditions was completely obfuscated, while with the levelling approach, it was clearly observed. Finally, the ‘experimental' DIC data were purposefully misaligned slightly from the FEA data. Both validation techniques suffered from the misalignment, thus motivating continued efforts to develop a robust alignment process. In summary, direct interpolation is insufficient, and the proposed levelling approach is required to ensure that the FEA and the DIC data have the same spatial resolution and data filtering. Only after the FEA data have been ‘levelled' to the DIC data can meaningful, quantitative error maps be computed.

Measurement and identification of the nonlinear dynamics of a jointed structure using full-field data, Part I: Measurement of nonlinear dynamics

Fatigue crack propagation studies based on the plastic component of the CTOD evaluated from Digital Image Correlation data

The paper addresses an experimental-calculational technique intended to determine geometrical parameters of invisible crack-like defects in thin-wall structural elements. Object deformation response to low trial loading is used as baseline data for assessment. The response is recorded using the digital image correlation method. Further analysis consists in minimization of differences between experimental findings and serial model calculations. The description of the approach is carried out on the example of a test specimen in the form of cylindrical shell (tube section) with a non-through crack simulator on the inner surface. The expected displacement fields for different particular cases are calculated using the finite element method.

Thermal to digital image correlation image to image translation with CycleGAN and Pix2Pix

Change in the mechanism of post corrosion bond behavior was investigated in a previous experimental study by analyzing the deformations in concrete using digital image correlation (DIC). Using their DIC data, this study focuses on the local strain non-uniformity in reinforcing bar during the uniaxial tensile test due to the effect of rib height reduction and corrosion layer. The results show that, the local strain below the rib tip is higher than that below the flat part between ribs, due to compressive stress resulting from mechanical interaction at the rib tip. Furthermore, the non-uniformity in local strain is less pronounced in highly corroded reinforcing bar because less compressive stress is transferred due to gentler rib slope. In highly corroded reinforcing bar, the appearance of non-uniformity in local strain is delayed since the accumulated corrosion product on the reinforcing bar surface delays the mechanical interaction and stress transfer between the concrete and the reinforcing bar.

This research provides a practical guideline for Digital Image Correlation (DIC) data variations minimization in structural engineering through simple image processing techniques. The main objective of this research is to investigate the Pixel Averaging (P.A.) effect on the differential strain Diff(εx) variations. Three concrete arches were tested with three-point bending using the DIC technique for strain measurements. The measured strains are obtained through two virtual horizontal extensometers in the middle of each arch. The Diff(εx) was selected to avoid other 2D-DIC issues, such as the sample-camera out-of-plane movement. Three image cases, namely, one, ten, and twenty averaged images, were used for DIC analysis of each arch. The conditions of each image case are assessed by computing the Diff(εx) variance and the linear least square criterion (R2) between the two extensometers. The second objective is to examine the speckles’ dilation effects on the speckle pattern density and surface component quality utilizing the Image Erode (I.E.) technique. The (P.A.) technique provided consistent differential strain Diff(εx) values with a variance reduction of up to (90%) when averaged images were used. The (R2) has considerably increased (from 0.46, 0.66, 0.91 to 0.90, 0.96, 0.99), respectively, for the three samples. Moreover, the (I.E.) technique provided qualitatively denser speckles with a highly consistent DIC surface component.

This work describes a method in which the digital image correlation (DIC) method and finite element analysis (FEA) were used to create a quasi-static mixed-mode fracture envelope for bonded joints consisting of 2024-T3 Al adherends and a tough structural epoxy adhesive. Symmetric and asymmetric versions of double cantilever beam, single-leg bend, and end-notched flexure tests are used to populate the mixed-mode fracture envelope with results at several mode mixities. Experiments are conducted in a universal testing machine while recording images for subsequent DIC analysis. Finite element analysis is used to implement cohesive zone models (CZMs) of the adhesive fracture and to account for plastic deformation of adherends. Mode I and mode II traction separation laws (TSLs) are determined from a property identification method with a Benzeggagh–Kenane mixed-mode coupling law used to model mixed-mode behavior. FEA results are shown to provide a good agreement to both the crosshead displacement and DIC data. The methods in this paper serve as a potential framework for future calibration of mixed-mode fracture envelopes for joints bonded with very tough adhesives that complicate assessment with traditional data analysis methods.

Fracture modes I and II interact in mixed-mode loading conditions, and the interaction is typically taken into account using a mixed-mode fracture criterion. In this work, a concept for defining the criterion is developed through an experimental-numerical simulation analysis. The mixed-mode behaviour for the criterion is tested and analysed with a single specimen design. The design of the specimen simplifies the measurement when complex test arrangements and preparations for various specimen geometries are excluded in practice. Here, the mixed-mode fracture behaviour of the specimen is analysed in detail using digital image correlation (DIC). The DIC data are used in collaboration with a finite element–based crack onset analysis, including the virtual crack closure technique, in order to consider the typical simplifications and their effects on the mixed-mode criterion. As a benchmark case, the fracture criterion is determined for an epoxy film adhesive FM 300-2 using the developed approach. The developed approach was shown to be feasible and effective for defining the mixed-mode fracture criterion. The determined Power law criterion’s exponents were below unity, which points out that the common (presumed) exponent values in the current literature are actually unconservative.

Optical measurement techniques have evolved since the mid-2000's due to the advancement in computer processing and digital camera sensors. These techniques allow for the digitization of full three-dimensional (3D) surfaces and provide time-domain surface information data with high details not previously possible with more traditional measurement techniques. A variety of industries are using these technologies with a focus on high accuracy industrial applications including aerospace and automotive. This paper introduces optical measurements techniques used to provide quantifiable data and visual information of a woman's abdomen during various stages of pregnancy. Two non-invasive techniques are used for this study including structured light (SL) scanning and digital image correlation (DIC). An overview of these techniques, the process for data collection, results of the data, and opportunities for future studies are presented in this paper. The structured light scanning was used to capture a 3D surface of the woman's abdomen at four to six week intervals starting at week 15 of maternity and ending six days prior to birth. The results quantify growth during each stage of pregnancy along with full surface data in addition to postnatal results. The DIC data was collected during week 30 when the baby was very active in the womb and external deformations on the woman's abdomen were observed visually. The results from these studies were not intended to be a formal medical study but to provide feasibility of the SL and DIC process during the stages of pregnancy and encourage unique uses of otherwise industrial measurements.

The fracture behavior of materials resulting from rapidly applied loads remains poorly understood, and requires consideration of the inertial forces and rapidly changing crack front. At the same time, with the advent of improved high-speed cameras and full-field optical techniques such as digital image correlation (DIC), the spatial and temporal resolution of measurable field data has increased. As such, this thesis presents a novel hybrid experimental-computational technique that extracts dynamic fracture criterion from DIC data using improved, iterative elastodynamic expressions relating stress intensity factors (SIFs) to displacement fields around the crack front. Two variations of the solution are explored: an over-deterministic least squares regression is used when the crack tip location is known, such as at initiation of fracture, and an iterative Newton-Raphson optimization is used when the crack tip location is unknown. Dynamic fracture experiments are conducted on single-edge-notch specimens, impacted on the opposite face by a striker in mode I (or crack opening), to determine the efficacy of the method. Three materials with varying microstructures are studied: brittle polymer polymethyl methacrylate (PMMA), nanolayered metal-ceramic MAX phase Ti₃SiC₂, and human femoral cortical bone, a natural hierarchical ceramic matrix composite. PMMA is a historically well-studied material in classical dynamic fracture mechanics and is chosen as a model material, and when combined with data from literature is used to conduct validation and uncertainty quantification arising from key experimental and numerical parameters. Material rate dependency and the effect of transient wave interactions are investigated in the dynamic fracture behavior of MAX phase Ti₃SiC₂, and found to hold little influence due to the energy absorbing mechanisms of kink banding and delamination unique to the layered microstructure. Mode-mixity arising from an orthotropic microstructure is investigated in human femoral cortical bone, and lead to significant mode II SIFs even in the absence of any applied mode II loading, though mode I fracture remains dominant. These collective results demonstrate the wide variety of material microstructures with varying deformation mechanisms to which the hybrid experimental-computational dynamic fracture analysis is well-suited to handle.

The purpose of this study was to investigate the applicability of the digital image correlation method, which uses image processing to obtain areal strain distributions, for the observation of rock fractures. In the results of the Brazilian tensile test, the strain increase process just prior to crack initiation was captured when the images were taken at 8,000 fps. The tensile testing of a rubber sheet coated with irregular spots confirmed that the accuracy of the analysis was sufficient, indicating that digital image correlation can be applied for the observation of crack propagation in rocks. By simulating the Brazilian tensile test with a manual mechanical jack, the experiment was conducted on gypsum specimens, and high-resolution images of the experiment were taken. The results of the analysis showed that the cracks started at the points of increased strain by comparing the normal strain distribution in the X direction and the shear strain distribution with the actual crack locations. INTRODUCTION It is important to evaluate the initiation and propagation of cracks in a rock mass during the cavity excavation. In the case of hard rock, the fracture initiation occurs in a short time, making it difficult to observe the process. However, recent advances in digital imaging devices have made it possible to capture high-speed, high-resolution images of rock fracture experiments. If the strain distribution in an area can be measured by image processing, it will be possible to track the initiation and progress of the fracture. Strain gauges have usually been used to measure the strain generated in structures. However, the strain value obtained from a strain gauge is the average strain at one point in one direction; and thus, many strain gauges need to be installed for multipoint measuring. This involves complicated wiring to the measurement device, which causes problems in terms of labor and cost. In addition, as cracks develop or crack widths increase, the strain gauges may break off, making measurement impossible. On the other hand, digital image correlation can obtain the strain distribution over a wider range than strain gauges. It is a simple method requiring only the image of the object to be captured. Digital image correlation (DIC) can measure complex cracks and large deformations. DIC uses a camera to capture the speckle pattern (irregular dots) on a sample surface and measures the displacement and strain distribution from the correlation of images before and after deformation.

A novel energetic approach for predicting fatigue crack growth in pseudoelastic NiTi alloys

The work is devoted to the testing of the algorithm for calculating J-integral based on the construction of vector fields by digital image correlation (DIC) method. A comparative analysis of J-integral values calculated using DIC and instrumental data obtained in accordance with ASTM E 1820 “Standard Test Method for Measurement of Fracture Toughness” has made. It is shown that this approach can be used for cases when the standard technique for measuring the J-integral cannot be applied, or the standard technique does not allow achieving the required accuracy for the integral determination in local areas of the loaded material.

The mechanical response of printed dog‐bone specimens of poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) (PHBH) in linear and nonlinear regimes is investigated. Fused filament fabrication (FFF), also regarded as fused deposition modeling (FDM), is adopted to manufacture the PHBH elements. A compressible Mooney–Rivlin constitutive law is back fitted onto digital image correlation (DIC) data, as well as uni‐axial tensile test data. Effective mechanical properties are given, which may be used to design and optimize the response of printed structures. Strain values at the verge of failure in tension are also experimentally obtained. Results are especially useful for modeling mechanical response of PHBH, which is an emerging biopolymer with promising application fields in biomedical engineering, through additive manufacturing techniques.