Method and system for direct strain imaging
Inventors
Iliopoulos, Athanasios • Michopoulos, John G.
Assignees
US Department of Navy • George Mason Research Foundation Inc
Publication Number
US-9311566-B2
Publication Date
2016-04-12
Expiration Date
2033-08-02
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Abstract
A method and system for measuring and determining the full-field spatial distributions of strain tensor field components in a two or three-dimensional space, as a consequence of deformation under generalized loading conditions. One or more digital cameras may be used to acquire successive images of a deforming body with optically distinctive features on its surface. A method for determining the location of characteristic points of the surface features and another one for tracking these points as deformation occurs. Elongations between neighboring points on the vicinity of a location of interest are computed. The elongation between points is calculated even though discontinuities may exist between them. Strain tensor fields are directly calculated as a tensor approximation from these elongations without determining or using the displacement vector distributions.
Core Innovation
The invention is a method and system for measuring and determining the full-field spatial distributions of strain tensor field components in two or three-dimensional space resulting from deformation under generalized loading conditions. This involves the use of one or more digital cameras to acquire successive images of a deforming body with distinctive surface features, determining the location of characteristic points on the features, tracking these points during deformation, and computing elongations between neighboring points at locations of interest. Strain tensor fields are directly calculated from these elongations without determining or using displacement vector distributions.
The problem being solved arises from limitations in existing full-field strain measurement methods, which rely on displacement field data and its differentiation to estimate strain fields. Such methods depend on the continuum hypothesis and strain compatibility equations, which fail in the presence of discontinuities, strain localization, or damage in the material. This leads to inaccurate or indeterminate measurements. Additionally, prior techniques suffer accuracy issues near boundaries, sensitivity to noise amplification through differentiation, and require specialized instrumentation or assumptions limiting their applicability. The proposed method overcomes these issues by directly approximating strain components from engineering strains measured between node pairs rather than displacement fields, allowing for better accuracy, noise robustness, and applicability to discontinuous or damaged media.
Claims Coverage
The patent includes three independent claims that cover methods and systems for determining strain characteristics through measuring engineering strains between node pairs and approximating strain fields directly using a spatial moving least squares approach.
Direct approximation of strain from engineering strains between node combinations
A computer-implemented method for determining strain characteristics at a point in a deformable domain by receiving captured data of features (nodes) in un-deformed and deformed configurations, measuring engineering strains between node combinations in the vicinity of the point, and applying an approximation or interpolation to these engineering strains to approximate the strain tensor components directly.
System for strain determination using multiple imaging devices and strain approximation
A system comprising at least two imaging devices arranged to capture images of features on a deformable domain in un-deformed and deformed states, with a computer processor configured to represent features as nodes, measure engineering strains between node combinations, and apply approximation or interpolation to directly determine strain components.
Method employing separation data from various imaging modalities and strain approximation
A method for measuring strain characteristics by receiving separation data of nodes in un-deformed and deformed configurations, measuring elongations (engineering strains) between node combinations, and approximating strain tensor components directly using a spatial moving least squares approximation, where separation data can be obtained from ultrasonography, X-ray tomography, or digital imaging.
Overall, the claims focus on a novel approach of directly determining strain tensor components from measured engineering strains between node pairs using spatial moving least squares approximation applied to imaging or separation data, avoiding displacement field computations and enabling improved accuracy and applicability.
Stated Advantages
Improved accuracy in strain measurements compared to prior meshless random grid (MRG) methods, with DSI outperforming MRG by factors up to 3.6 times, especially under noise.
Robustness to noise amplification issues common in displacement differentiation methods, leading to more reliable strain field estimations.
Capability to measure strain fields directly without relying on displacement vector fields or continuum hypothesis, allowing applicability to discontinuous or damaged media including cracks and strain localization regions.
Numerical stability and low variation in measurement error indicating consistent performance under repeated trials.
Applicability to complex specimen shapes and multidimensional spaces without requiring specialized grid patterns or assumptions.
Documented Applications
Full-field strain measurement of deformable bodies or specimens under generalized loads using digital imaging with patterns of dots or features.
Use with multiple digital imaging devices such as stereoscopic cameras to measure deformation in two or three dimensions.
Application in experimental mechanics for deformation analysis, including materials testing machines and specimens.
Extension to engineering strain data obtained from ultrasonography, X-ray tomography, or other imaging modalities for medical or diagnostic applications.
Potential application to diverse materials including metallic structures and biological tissues.
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