Digital Volumetric Speckle Photography Research Articles

Interior Deformation and Failure of a Short Composite Sandwich Beam under Three-Point Bending

Composite sandwich plates and beams are increasingly employed as an engineering material in structures such as airplanes, ships, bridges, and vehicles because of their superb strength to weight ratio. Understanding a sandwich structure’s failure mechanism is a prerequisite for a safety design. In this paper, we employ a new experimental technique called DVSP (Digital Volumetric Speckle Photography) to map the interior deformation of a short composite sandwich beam under three-point bending. 3D displacement fields and shear strain fields of 5 transverse and 4 longitudinal sections of the beam are mapped quantitatively in detail as a function of increasing load. The beam fails in delamination of the interface between the face sheet and core.

Evaluation of 3D deformation field in siltstone with a pre-existing 3D surface flaw under uniaxial compression using X-ray computed tomography and digital volumetric speckle photography

The interior deformation or rupture of materials is usually derived from measuring the surface deformation of specimens. In this study, a set of uniaxial compression tests combined with in situ X-ray computed tomography (CT) was performed on siltstone specimens with a pre- existing three-dimensional (3D) surface flaw with various inclination angles (α = 30◦, 45◦, 60◦). The ratio of flaw depth to specimen thickness was 0.26. Based on the CT images of the specimens under different loads, the digital volumetric speckle photography procedure was applied to quantifying the 3D interior deformation of the specimens, and 3D displacement and major principal strain ε1 distribution in different sections were determined. The characteristics of internal strain before specimen fracturing indicated strain localization, which could describe the propagation mechanism of the single 3D surface flaw.

3-D strain estimation in sandstone using improved digital volumetric speckle photography algorithm

Accurately and stably estimating the strain in rocks is important to understand their failure mechanism. Digital volumetric speckle photography (DVSP) combined with X-ray computed tomography (CT) has been used to investigate the strain in rocks. This study introduced the inverse compositional Gauss–Newton algorithm into DVSP to improve the accuracy and precision of displacement and strain measurements. Granules with different particle sizes were used to simulate the mesostructure of sandstone, and a series of baseline experiments were conducted to evaluate the accuracy of the improved algorithm, which had a performance that was approximately three times better than DVSP alone. Subsequently, the algorithm was applied to red sandstone subjected to the Brazilian test and an indentation experiment. The interior deformation characteristics of the specimen were clearly depicted, which clarified the failure mechanism of the rock material.

An Improved Digital Volumetric Speckle Photography Technique with X-ray Microtomography and its Applications to Investigating Strain Localization in Red Sandstone

An Improved Digital Volumetric Speckle Photography Technique with X-ray Microtomography and its Applications to Investigating Strain Localization in Red Sandstone

3D Strain Mapping of Opaque Materials Using an Improved Digital Volumetric Speckle Photography Technique with X-Ray Microtomography

Digital volumetric speckle photography (DVSP) method has been used to strain investigation in opaque materials. In this paper, an improved DVSP algorithm is introduced, in which a multi-scale coarse–fine subset calculation process and a subvoxel shifting technique are applied to increase accuracy. We refer to the new algorithm as Multi-scale and Subvoxel shifting Digital Volumetric Speckle Photography (MS-DVSP). The displacement and strain fields of a red sandstone cylinder exposed to uniaxial compression and a woven composite beam under three-point bending are mapped in detail. The characteristics of the interior deformation of the specimens are clearly depicted, thus elucidating the failure mechanism of the materials.

3D strain evolution in concrete using in situ X-ray computed tomography testing and digital volumetric speckle photography

The fracture process for concrete materials under compression is a complex three-dimensional phenomenon that occurs as a result of material heterogeneity and complicated mixed-mode cracking mechanisms. To understand the failure mechanism of concrete, it is necessary to have a technique that can probe the interior of the material as it is being loaded. In this paper, we use an industrial computed tomography (CT) system to obtain the volumetric image of a concrete circular cylinder under compression by taking advantage of the ability of X-rays to penetrate opaque materials. In addition, we employ a newly developed 3D strain analysis technique called digital volumetric speckle photography (DVSP) to calculate the displacement and strain distribution in two longitudinal meridian sections and a midpoint transverse section of the specimen. We are able to reveal the regions of strain localization and their evolutions, which eventually lead to the failure of the specimen.

Digital volumetric speckle photography: a powerful experimental technique capable of quantifying interior deformation fields of composite materials

Due to the heterogeneity at different length scales, composite materials have a complex mechanical behavior. While many studies have been devoted to understanding their failure mechanism, much remain unknown. One of the main reasons is the lack of an effective experimental tool to monitor, in situ, the internal stress/strain field as the deformation progresses leading to failure. By taking advantage of the volumetric imaging capability of an X-ray CT (computed tomography), we have developed a new tool called digital volumetric speckle photography, whereby we can map quantitatively the interior deformation of almost any opaque material. In this paper, we described in detail the theory of this technique and its application to mapping the internal deformation fields of two different types of composites: fiberglass-reinforced woven composite and sandwich composite made of a polymeric foam core and two fiberglass face sheets.

Mapping interior deformation of a composite sandwich beam using Digital Volumetric Speckle Photography with X-ray computed tomography

Sandwich beams or plates with a soft core and stiff face sheets have been used extensively as a structural engineering material because of its high strength-to-weight ratio. While many studies have been devoted to understanding its failure mechanism, much remain unknown. For example while its main failure mode is a 45° shear crack in the core and delamination at the core/face sheet interface, its exact mechanism remains somewhat a mystery: where and how the crack is initiated and what effects its propagation. One of the main reasons is the lack of an effective experimental tool to monitor, in situ, the failure process and thereby quantify the stress/strain field as it progresses. In this paper we present a new tool called DVSP (Digital Volumetric Speckle Photography) that is capable of mapping quantitatively the interior deformation of sandwich beams/plates as it is being loaded. We have applied the technique to a sandwich beam under 3-point bending. Full field deformation in the form of displacement and shear strain contours of 6 transverse and 3 longitudinal sections of the beam are presented. It is believed that this technique will shed much light on the failure mechanism of sandwich beams/plates in the future.

3D strain mapping in rocks using digital volumetric speckle photography technique

Speckle photography technique is a versatile displacement/deformation mapping technique that can be applied to almost any material. It has its genesis in the laser speckle interferometry technique whereby the natural speckles created by illuminating an optically rough surface using a coherent laser beam are used as displacement gauging elements. It evolves into the white light speckle photography technique whereby a random pattern of any type is used as a displacement measurement transducer. With the advent of digital cameras and ubiquitous usage of computers, the digital version of the technique is developed. Up to now, the technique is essentially limited to 2D applications. Recently, we extended the technique into the 3D domain by using the volumetric imaging capabilities of CT or MRI. In this paper, we apply this technique to measuring the internal deformation of rocks. It so happens that certain rocks have natural density variation at various places such that when imaged by CT these impurities can be treated as 3D speckles. The elements of volumetric speckle photography technique are as follows. A reference volumetric image of the rock is recorded by a micro-CT scan and stored as a reference. Under load, the deformed CT image of the rock is also recorded. Both volume images are divided into subsets of certain voxel arrays. Each corresponding pair of the subsets is “compared” via a two-step 3D Fourier transform analysis. The result is a 3D map of displacement vectors representing the collective displacement experienced by all the speckles within the subset of voxels. The strain distribution of the entire rock specimen can then be calculated using appropriate displacement strain relations. The application of this technique to strain mapping of red sandstone and argillite rocks is presented.

Interior strain analysis of a woven composite beam using X-ray computed tomography and digital volumetric speckle photography

Materials such as fiber reinforced composite are inherently 3D in nature. Its internal stress/strain fields are very complex even under the simplest loading condition. There is no effective means of measuring interior strain other than placing isolated sensors. In this paper we introduce a new strain measuring technique called digital volumetric speckle photography whereby we can probe the interior strain field in a whole field manner. We apply it to the determination in a woven fiber composite beam under 3-point bend. The in-plane strain field of three transverse sections and two longitudinal sections are determined using the technique. In addition we show that one of the principal strain fields of the mid-longitudinal section tends to predict the impending failure of the beam.

3D Deformation and Strain Analysis in Uniaxial Compressive Coal Using Digital Volumetric Speckle Photography

Digital Volumetric Speckle Photography (DVSP), which is the extension of 2D digital speckle photography, is applied to measure the interior deformation and strain of the object by 2-step 3D FFT. For using FFT, DVSP has some advantages in computational efficiency. With the special loading setup for Micro-CT, the coal specimen under different uniaxial loadings is scanned, and the volumetric images are acquired. By using DVSP, the interior 3D displacements of the specimen are obtained, and then the equivalent strains and volumetric strains are estimated. Based on the interior displacement and strain mappings, the process of deformation or strain localization in the coal specimen is studied.

Numerical Model Building and Optimization of Concrete Based on the CT Images and Digital Volumetric Speckle Photography

In recent years, the research on the mesostructure of the concrete by using numerical simulation has been a hot topic. Mesoscopic research would contribute to reveal the concrete cracking failure mechanism. In this paper, the aggregate, the mortar , the interface and the pore are segmented based on CT images . With the aid of Mimics software, the numerical model for ANSYS is built. Besides this, the concrete specimen under uniaxial compression is scanned by using micro-CT in situ. The volumetric images under different loadings are acquired. By using digital volumetric speckle photography (DVSP) method, interior 3D deformation is obtained. Combined with the experiment results, the numerical model is optimized. Based on the optimized model, mesoscopic mechanics of concrete is analyzed by using numerical simulation.

Interior 3D-Strain Mapping in Opaque Materials Using Digital Volumetric Speckle Photography and X-Ray Micro Computer Tomography

Developing a true 3D experimental stress analysis technique has been the holy grail of solid mechanics researchers since the genesis of the field. Combined the high-resolution x-ray computer tomography (Micro CT) with the 2D speckle photography technique developed by one of us (Chiang), we recently proposed a new 3D strain analysis technique called Digital Volumetric Speckle Photography (DVSP). The elements of DVSP technique are as follows. A reference volumetric image of the specimen is recorded by a Micro-CT scan and stored as a reference. Under load the deformed CT image of the specimen is also recorded. Both volume images are divided into subimages of certain voxel arrays. Each corresponding pair of the subimages are ‘compared’ via a two-step 3D Fourier Transform analysis. The result is a 3D map of displacement vectors representing the collective displacement experienced by all the speckles within the subset of voxels. Strain distribution of the entire specimen can then be calculated using appropriate displacement strain relations. Application of this technique to strain mapping of red sandstone and composite are presented.

3D mapping of carbon dioxide-induced strain in coal using digital volumetric speckle photography technique and X-ray computer tomography

Evaluation of sorption-induced strain in coal by gases uptake is an important part of the research on CO2 sequestration and enhanced coalbed methane operations. In this paper a 3D strain mapping technique called digital volumetric speckle photography (DVSP) is introduced and applied to the investigation of sorption-induced strain in coal with CO2 uptake with the help of X-ray computer tomography scanning in situ. For comparison strain gauges are also used to measure the strain during the overall process. By using DVSP, 3D strain evolution maps in coal exposed to CO2 at pressure 1.5MPa as a function of sorption time are obtained. The results show that the strain distribution is localized and highly heterogeneous, and the swelling and compression regions are dependent on lithotype and microstructure. While the average volumetric strain from DVSP has a similar developmental trend as data from strain gauges, the magnitude is significantly less. This is attributed to the fact that the strain gauges' result is localized over the limited surface area covered by the gauges, whereas the DVSP result is the average of the entire volume.

Full-field mapping of internal strain distribution in red sandstone specimen under compression using digital volumetric speckle photography and X-ray computed tomography

It is always desirable to know the interior deformation pattern when a rock is subjected to mechanical load. Few experimental techniques exist that can represent full-field three-dimensional (3D) strain distribution inside a rock specimen. And yet it is crucial that this information is available for fully understanding the failure mechanism of rocks or other geomaterials. In this study, by using the newly developed digital volumetric speckle photography (DVSP) technique in conjunction with X-ray computed tomography (CT) and taking advantage of natural 3D speckles formed inside the rock due to material impurities and voids, we can probe the interior of a rock to map its deformation pattern under load and shed light on its failure mechanism. We apply this technique to the analysis of a red sandstone specimen under increasing uniaxial compressive load applied incrementally. The full-field 3D displacement fields are obtained in the specimen as a function of the load, from which both the volumetric and the deviatoric strain fields are calculated. Strain localization zones which lead to the eventual failure of the rock are identified. The results indicate that both shear and tension are contributing factors to the failure mechanism.

基于CT的数字体散斑法测量物体内部三维变形场

基于CT的数字体散斑法测量物体内部三维变形场

Development of interior strain measurement techniques using random speckle patterns

Experimental stress analysis is a well established mechanics discipline dealing with measuring stress/strain in solids under load. Most techniques can only be applied to measuring strain of the surface. In this paper we review the development of techniques of using a random speckle patterns as a quantitative tool to mapping interior strain field of solids. After some elementary treatment of the basic principles of the speckle photography technique we describe three approaches that can probe the interior strain field of a transparent object. They are followed by a newly developed 3D strain measurement technique called digital volumetric speckle photography which can probe the interior of opaque materials as well.

CO 2adsorption capacity of argonne premium coals : Ozdemir, E. et al. Fuel, 2004, 83, (7–8), 1085–1094

Evaluation of sorption-induced strain in coal by gases uptake is an important part of the research on CO2 sequestration and enhanced coalbed methane operations. In this paper a 3D strain mapping technique called digital volumetric speckle photography (DVSP) is introduced and applied to the investigation of sorption-induced strain in coal with CO2 uptake with the help of X-ray computer tomography scanning in situ. For comparison strain gauges are also used to measure the strain during the overall process. By using DVSP, 3D strain evolution maps in coal exposed to CO2 at pressure 1.5 MPa as a function of sorption time are obtained. The results show that the strain distribution is localized and highly heterogeneous, and the swelling and compression regions are dependent on lithotype and microstructure. While the average volumetric strain from DVSP has a similar developmental trend as data from strain gauges, the magnitude is significantly less. This is attributed to the fact that the strain gauges' result is localized over the limited surface area covered by the gauges, whereas the DVSP result is the average of the entire volume.