Full-field Surface Deformation Research Articles

Full field and wideband surface deformation reconstruction from the Fourier domain by spatio-temporal heterodyne interferometry (STHI)

Full field and wideband surface deformation reconstruction from the Fourier domain by spatio-temporal heterodyne interferometry (STHI)

Progressive failure of asphalt shingles under high winds: Assessment via full-field deformation measurements on full-scale roof panels

Failures of roof asphalt shingles under high winds have been reported to occur at wind speeds lower than the shingle performance classification. The understanding of the progressive failure mechanisms of shingles subjected to high-wind pressures has been hindered by difficulty in accurately quantifying uplifts and wind-induced pressures with high spatial resolution, especially for large-scale wind test setups that simulate realistic scenarios. This paper reports on an experimental study to assess the use of three-dimensional digital image correlation (‘StereoDIC’) to accurately measure full-field surface deformations of shingles installed on full-scale roof structures and subjected to realistic high winds. Deformation measurements were acquired on shingles installed on typical wood-frame roof panels and subjected to winds with speed up to 257 km/h (160 mph) using an outdoor wind tunnel. The specimens were designed to study the proposed test setup and StereoDIC measurement method for two different combinations of shingle surface coloration (and thus speckle pattern) and field of view. The experiments produced accurate local and full-field shingle deformation measurements. Different datasets were used to gain new insight into the influence of salient aspects of the wind test setup and deformation measurement method on the understanding of the temporal evolution of shingle uplift and failure mechanisms.

Bi-prism-based single-bilateral-telecentric-camera stereo-DIC for accurate underwater 3D deformation measurement: Implementation of a parametric model

Full-field surface shape and deformation measurements of submerged objects using stereo-digital image correlation (stereo-DIC) remains challenging due to the light ray refraction occurred on the interface of two different transparent media (e.g., air, glass and water). In this work, we proposed an underwater full-field 3D deformation measurement method based on the bi-prism-based single-bilateral-telecentric-camera (SBTC) stereo-DIC, which adopts a parametric model to establish the mathematical relationship between the image points and their corresponding object points by taking the refraction into account in the ray-tracing process. The key parameters (e.g., bi-prism/glass orientation and position) of the parametric model were calibrated by a one-step calibration. To verify the correctness and effectiveness of the proposed method, translation tests with a rotated bi-prism and underwater translation tests were performed. Experimental results indicate that the proposed parametric model can effectively handle the imperfect system configuration and the unavoidable light ray refraction that occurred on the optical path, thus allowing high-accuracy underwater deformation measurement. Compared with the existing underwater deformation measurement approaches, the proposed method can not only deliver displacement measurement with higher accuracy, but also is free of any assumptions on the configuration of the camera system or the refractive interface.

Dual beam-shear differential interference microscopy for full-field surface deformation gradient characterization

We propose a new experimental mechanics method: dual beam-shear differential interference microscopy (DInM) for full-field surface deformation measurement. The method integrates the principles of differential interference contrast, photoelasticity, digital image correlation and the theories of continuum mechanics for a 4D quantification of the surface topography. Our first DInM prototype provides the lateral resolution of 787nm for up to 12% measurable out-of-plane strains. The resolution can be improved by using the shorter spectrum light and higher magnification objective lens. We use our system to characterize the buckling profile of Si microribbon on an elastomer substrate, which was verified by atomic force microscopy. We also demonstrate the stress-induced phase transformation in NiTi alloy by our system. The evolution of the surface topographies and the full-field deformation gradients are successfully captured and quantified. The dynamic measurement provides the information to calculate the relative transformation strains between phases of different symmetries. The establishment of dual beam-shear DInM opens a new avenue in the field of experimental meso/micromechanics.

Viscoplastic constitutive model for Ni-based directionally solidified superalloy: Experimental validation on notched specimen

This paper aims to validate a viscoplastic constitutive model and investigate the local and the full-field strain of notched specimen. A high temperature digital image correlation (DIC)-based test system was constructed to measure the full-field surface deformation. Surface strain fields of notched DZ125 specimens were measured under monotonic tensile loadings at 20 °C, 650 °C, 760 °C, and 850 °C. Taking the effect of anisotropy, applied stress, and temperature into account, numerical simulations were conducted using a developed viscoplastic constitutive model. DIC-based measurements agreed well with the simulated strains, with the minimum correlation coefficient of 95.47%. Both the numerical and experimental results show that the area of notch plasticity increases with temperature and applied stress, and the high strain zone nearly remains constant under various tensile loadings. For a given notch geometry, the strain mutation in high strain zone nearly remains constant no matter how the temperature and the applied stress change.

Complex deformation of naturally fractured rocks

Strain is a key parameter in the evaluation of rock mechanical properties and failure behavior. Strain is usually measured using strain gauges, however, this laboratory method has inherent uncertainty in the attachment of sensitive gauges to the rock surface and it is limited to one-point strain measurement. In complex geometries such as rock samples with natural fractures or lamination, that classical approach cannot provide the whole picture of strain development in such complex geometries. To overcome these challenges, the application of optical technique with image processing is proposed. The optical technique has the advantage of being a non-contact method and it provides quantitative visualization of full-field strain measurement.To examine quantitative visualization of strain development, DIC technique was first conducted on a homogeneous sandstone sample as a benchmark. The results verify that the maximum tensile strain is developed at the central vertical line of the sample as it was expected from theoretical solutions. This DIC measurement matches the post-failure fracture pattern of the specimen. Then the technique was applied on the Buda Limestone samples with horizontal, sub-horizontal or multiple complex natural fractures (sealed or partially open) and inclusions while the samples were under diametrical compression experiment. The results of this study provide further insight into the role of natural fractures on induced fracture initiation and propagation under this specific testing condition. Horizontal natural fractures are planes of maximum compressive strain which can delay the propagation of vertical tensile fractures. The maximum amount of tensile strain usually initiates at the boundary of these natural fractures and vertical tensile fractures usually deviate toward horizontal fracture for a short distance before turning back to propagate vertically following the maximum stress path. Full-field surface deformations are mapped optically in the crack–inclusion vicinity where DIC results showed that crack front is arrested by the inclusion for the complete fracture event. In contrast to the failure pattern in homogenous sandstone specimen, complex fracture pattern was observed on several specimens with multiple fracture network. Fracture slippages along the natural fracture, fracture deviation was observed in samples with steep inclined natural fractures.The results of this study can help us to better understand the complex failure mechanism in naturally fractured media and elaborate on various mechanisms of failure under diametrical compression tests. DIC is a powerful technique in providing the dynamic strain development over the surface of the sample in real-time, thus it can provide the fracture initiation and propagation in such heterogeneous rock media.

On the link between fracture toughness, tensile strength, and fracture process zone in anisotropic rocks

This paper presents experimental results on the anisotropy of the fracture toughness, Brazilian tensile strength, and the fracture process zone (FPZ) in granodiorite samples. The fracture toughness is measured using semi-circular bending tests, while Brazilian disk tests were conducted to measure the tensile strength indirectly. Digital image correlation (DIC) was employed to obtain full-field surface deformation associated with the fracture propagation and identify the FPZ. An averaging scheme is proposed to determine the length and width of the FPZ from the strain field. The DIC results confirm a semi-elliptical FPZ developing ahead of the crack tip, with an average length-to-width ratio of approximately two. The results also indicate that the theoretical models such as Irwin and strip-yield with uniform traction, which are based on plastic deformation near the crack tip, underestimate the extent of the inelastic zone, while the strip-yield model with a linear cohesion stress distribution overestimate the length of the process zone. The anisotropy ratio of the FPZ length obtained from the models, however, agrees very well with the ratio obtained from the DIC measurements. This evidence supports the basis of the theoretical models that predict the FPZ length to be proportional to the square of fracture toughness over tensile strength.

Noninvasive, three-dimensional full-field body sensor for surface deformation monitoring of human body in vivo.

Noninvasive, three-dimensional (3-D), full-field surface deformation measurements of the human body are important for biomedical investigations. We proposed a 3-D noninvasive, full-field body sensor based on stereo digital image correlation (stereo-DIC) for surface deformation monitoring of the human body in vivo. First, by applying an improved water-transfer printing (WTP) technique to transfer optimized speckle patterns onto the skin, the body sensor was conveniently and harmlessly fabricated directly onto the human body. Then, stereo-DIC was used to achieve 3-D noncontact and noninvasive surface deformation measurements. The accuracy and efficiency of the proposed body sensor were verified and discussed by considering different complexions. Moreover, the fabrication of speckle patterns on human skin, which has always been considered a challenging problem, was shown to be feasible, effective, and harmless as a result of the improved WTP technique. An application of the proposed stereo-DIC-based body sensor was demonstrated by measuring the pulse wave velocity of human carotid artery.

Note: Dynamic meso-scale full field surface deformation measurement of heterogeneous materials.

A dynamic experiment at mesoscale is developed to measure local deformation and strain in granular materials at high temporal and spatial resolutions. The experimental setup is comprised of a high-speed camera along with a high magnification extension tube. The method is demonstrated by measuring the full field strain across and in the boundary of the crystals at a high temporal resolution in polymer bonded sugar crystals and glass beads filled epoxy particulate composite specimens under dynamic loading. In both cases, the local strain heterogeneity is captured successfully. The measured strain and deformation field can be further used to obtain the relative motion of each crystal, crystal rotation, and the relative displacement between the polymer interface and the crystal, which are very critical to understand the local failure mechanisms in heterogeneous materials.

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.

Multimodal optical measurement in vitro of surface deformations and wall thickness of the pressurized aortic arch.

Computational modeling of arterial mechanics continues to progress, even to the point of allowing the study of complex regions such as the aortic arch. Nevertheless, most prior studies assign homogeneous and isotropic material properties and constant wall thickness even when implementing patient-specific luminal geometries obtained from medical imaging. These assumptions are not due to computational limitations, but rather to the lack of spatially dense sets of experimental data that describe regional variations in mechanical properties and wall thickness in such complex arterial regions. In this work, we addressed technical challenges associated with in vitro measurement of overall geometry, full-field surface deformations, and regional wall thickness of the porcine aortic arch in its native anatomical configuration. Specifically, we combined two digital image correlation-based approaches, standard and panoramic, to track surface geometry and finite deformations during pressurization, with a 360-deg fringe projection system to contour the outer and inner geometry. The latter provided, for the first time, information on heterogeneous distributions of wall thickness of the arch and associated branches in the unloaded state. Results showed that mechanical responses vary significantly with orientation and location (e.g., less extensible in the circumferential direction and with increasing distance from the heart) and that the arch exhibits a nearly linear increase in pressure-induced strain up to 40%, consistent with other findings on proximal porcine aortas. Thickness measurements revealed strong regional differences, thus emphasizing the need to include nonuniform thicknesses in theoretical and computational studies of complex arterial geometries.

Digital image correlation-based point-wise inverse characterization of heterogeneous material properties of gallbladder in vitro

Continuing advances in mechanobiology reveal more and more that many cell types, especially those responsible for establishing, maintaining, remodelling or repairing extracellular matrix, are extremely sensitive to their local mechanical environment. Indeed, it appears that they fashion the extracellular matrix so as to promote a ‘mechanical homeostasis’. A natural corollary, therefore, is that cells will try to offset complexities in geometry and applied loads with heterogeneous material properties in order to render their local environment mechanobiologically favourable. There is a pressing need, therefore, for hybrid experimental–computational methods in biomechanics that can quantify such heterogeneities. In this paper, we present an approach that combines experimental information on full-field surface geometry and deformations with a membrane-based point-wise inverse method to infer full-field mechanical properties for soft tissues that exhibit nonlinear behaviours under finite deformations. To illustrate the potential utility of this new approach, we present the first quantification of regional mechanical properties of an excised but intact gallbladder, a thin-walled, sac-like organ that plays a fundamental role in normal digestion. The gallbladder was inflated to a maximum local stretch of 120% in eight pressure increments; at each pressure pause, the entire three-dimensional surface was optically extracted, and from which the surface strains were computed. Wall stresses in each state were predicted from the deformed geometry and the applied pressure using an inverse elastostatic method. The elastic properties of the gallbladder tissue were then characterized locally using point-wise stress–strain data. The gallbladder was found to be highly heterogeneous, with drastically different stiffness between the hepatic and the serosal sides. The identified material model was validated through forward finite-element analysis; both the configurations and the local stress–strain patterns were well reproduced.

Experimental investigation and numerical modeling for elasto-plastic notch-root stress/strain analysis under monotonic loadings

Experimental investigation and numerical modeling on elasto-plastic notch-root stress/strain distributions under monotonic loadings of both the Ni-based directionally solidified (DS) superalloy and Titanium alloy were carried out simultaneously. For measuring inhomogeneous deformation fields at notch roots, an optical-numerical full-field surface deformation measurement system was developed based on the digital image correlation (DIC) method. The obtained strain distributions were then verified with reasonable accuracy by finite element simulation, where an anisotropic elastic-viscoplastic constitutive model was developed for DS superalloy and a simple isotropic stress-strain relationship was adopted for Titanium alloy. Meanwhile, factors affecting elasto-plastic notch-root stress/strain distributions were systematically investigated numerically, where the emphasis was placed on temperature, loading stress rate, sample shape, anisotropy and notch features. The results show that stress/strain behavior at notch root is significantly affected by the mentioned factors, which are concretely embodied in the distribution of tensile stress/strain, equivalent stress and accumulative equivalent plastic strain.

Methods to measure interlaminar tensile modulus of composites

This work expands a recently developed short-beam method coupled with the Digital Image Correlation full-field surface deformation measurement technique to enable assessment of the interlaminar tensile stress–strain constitutive properties of polymer–matrix composite materials. This work also expands the American Society for Testing and Materials Standard D 6415 curved-beam method as another means for measurement of the interlaminar tensile stress–strain constitutive behavior. The interlaminar tensile modulus values resulting from both methods are compared for Hexcel IM7/8552 carbon/epoxy tape composite material system.

Fluorescent stereo microscopy for 3D surface profilometry and deformation mapping.

Recently, mechanobiology has received increased attention. For investigation of biofilm and cellular tissue, measurements of the surface topography and deformation in real-time are a pre-requisite for understanding the growth mechanisms. In this paper, a novel three-dimensional (3D) fluorescent microscopic method for surface profilometry and deformation measurements is developed. In this technique a pair of cameras are connected to a binocular fluorescent microscope to acquire micrographs from two different viewing angles of a sample surface doped or sprayed with fluorescent microparticles. Digital image correlation technique is used to search for matching points in the pairing fluorescence micrographs. After calibration of the system, the 3D surface topography is reconstructed from the pair of planar images. When the deformed surface topography is compared with undeformed topography using fluorescent microparticles for movement tracking of individual material points, the full field deformation of the surface is determined. The technique is demonstrated on topography measurement of a biofilm, and also on surface deformation measurement of the biofilm during growth. The use of 3D imaging of the fluorescent microparticles eliminates the formation of bright parts in an image caused by specular reflections. The technique is appropriate for non-contact, full-field and real-time 3D surface profilometry and deformation measurements of materials and structures at the microscale.

Quantifying Full Field Surface Deformation in Organic Insulators under Electrical Stress with DIC

Applications of the digital image correlation method (DIC) for the determination of the full filed surface deformation, induced by a DC electrical field in organic insulator thin films, are investigated in this paper. Digital images of the film sample at different voltages were recorded and processed to obtain the full-field surface deformation associated with electrical stress changes. Considerable work has been carried out to describe the various parameters at play during the electrical ageing process of organic insulators from displacement field measurements. The result shows that the approximation method can smoothly reconstruct the experimentally obtained displacement field.

Role of inclusion stiffness and interfacial strength on dynamic matrix crack growth: An experimental study

Experimental simulations of dynamic crack growth past inclusions of two different elastic moduli, stiff (glass) and compliant (polyurethane) relative to the matrix (epoxy), are carried out in a 2D setting. Full-field surface deformations are mapped in the crack–inclusion vicinity optically. The crack growth behavior as a function of inclusion–matrix interfacial strength and the inclusion location relative to the crack is studied under stress-wave loading conditions. An ultra high-speed rotating mirror-type digital camera is used to record random speckle patterns in the crack–inclusion vicinity to quantify in-plane displacement fields. The crack-tip deformation histories from the time of impact until complete fracture are mapped and fracture parameters are extracted. The crack front is arrested by the symmetrically located compliant inclusion for about half the duration needed for complete fracture event. The dynamically propagating crack is attracted and trapped by the weakly bonded inclusion interface for both stiff and compliant symmetrically located inclusion cases, whereas it is deflected away by the strongly bonded stiff inclusion and attracted by strongly bonded compliant inclusion when located eccentrically. The crack is arrested by a strongly bonded compliant inclusion for a significant fraction of the total dynamic event and is longer than the one for the weakly bonded counterpart. The compliant inclusion cases show higher fracture toughness than the stiff inclusion cases. Measured crack-tip mode-mixities correlate well with the observed crack attraction and repulsion mechanisms. Macroscopic examination of fracture surfaces reveals much higher surface roughness and ruggedness after crack–inclusion interaction for compliant inclusion than the stiff one. Implications of these observations on the dynamic fracture behavior of micron size A-glass and polyamide (PA6) particle filled epoxy is demonstrated. Filled-epoxy with 3% Vf of PA6 filler is shown to produce the same dynamic fracture toughness enhancement as the one due to 10% Vf glass.

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.

Modeling of mixed-mode crack growth in ductile thin sheets under combined in-plane and out-of-plane loading

This paper describes a modeling approach for analyzing mixed-mode crack growth events in ductile thin-sheet materials under large deformation and combined in-plane and out-of-plane loading conditions. The remote mixed-mode I/III loading leads to local mixed-mode I/II/III fields near the crack front. Making use of full-field surface deformation measurements, finite element models of mixed-mode I/III stable tearing events in thin-sheet specimens have been developed. Model predictions have been compared with experimental measurements (a) just prior to initial crack growth and (b) during stable tearing crack growth. Analyses of curvilinear crack growth events are carried out using a nodal release option or a local re-meshing option and using a generalized CTOD parameter with experimentally measured critical CTOD values. Results of this study suggest that the modeling approach can be employed to numerically re-construct experimental crack growth events in thin plate specimens. This offers a viable means of analyzing and understanding the mixed-mode crack growth events and provides a tool for further investigations of 3D crack front fields (which are otherwise unavailable experimentally) and for the study of fracture criteria for stable tearing events.

Error Assessment in Stereo-based Deformation Measurements

Increasing interest in the use of digital image correlation (DIC) for full-field surface shape and deformation measurements has led to an on-going need for both the development of theoretical formulae capable of providing quantitative confidence margins and controlled experiments for validation of the theoretical predictions. In the enclosed work, a series of stereo vision experiments are performed in a manner that provides sufficient information for direct comparison with theoretical predictions using formulae developed in Part I. Specifically, experiments are performed to obtain appropriate optimal estimates and the uncertainty margins for the image locations/displacements, 3-D locations/displacements and strains when using the method of subset-based digital image correlation for image matching. The uncertainty of locating the 3-D space points using subset-based pattern matching is estimated by using theoretical formulae developed in Part I and the experimentally defined confidence margins for image locations. Finally, the uncertainty in strains is predicted using formulae that involves both the variance and covariance of intermediate variables during the strain calculation process. Results from both theoretical predictions and the experimental work show the feasibility and accuracy of the predictive formulae for estimating the uncertainty in the stereo-based deformation measurements.