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David Nguyen
David Nguyen

Optical Methods For Solid Mechanics A Full Field Approach


This work presents an optimized implementation of the Computer-Aided Speckle Interferometry algorithm which enables full-field determination of displacements and strains on commodity Graphics Processing Units at high resolution and frame rates. By combining careful control of the average speckle size in a laser speckle pattern with a simple sampling rate conversion scheme, a compact representation of the optical speckle is achieved. This allows for optimal use of Graphics Processing Unit architecture with robust range extension. The optimal mapping of the Computer-Aided Speckle Interferometry algorithm to Graphics Processing Unit architecture is shown in detail, and a straightforward method for disambiguating large displacements is illustrated. Lastly, this paper demonstrates a two-step subimage-tapering modification to the original algorithm that enables robust range enhancement while maintaining resolution. Results from numerical simulations on synthetic speckle patterns are shown, and runtime performance metrics are provided, with performance ranging up to 60 frames per second in some cases. The method is suitable for interactive experimental mechanics research, process and testing or any application where real-time high-resolution displacement-strain monitoring is needed. A .NET Framework class library enabling the incorporation of the algorithm into 3rd -party applications is available for download.




optical methods for solid mechanics a full field approach


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In the field of fluid mechanics, the primary goal is often characterizing the two-dimensional flow field [4,5,6]. This typically involves seeding a fluid with tracer particles and then illuminating it with a laser beam expanded in one direction (usually parallel to the dominant flow direction). Particle image pairs - made visible by their scattering of the laser illumination - are then analyzed either by Laser Speckle Velocimetry (LSV, an interferometric approach leveraging a speckle diffraction effect) if high fluid-particle concentrations exist, or the more general Particle Image Velocimetry (PIV, a statistical analysis approach) for use with either dense or sparse particle concentrations [5]. Both cases fundamentally generate two-dimensional displacement fields as their dataset outputs, which are then optionally post-processed via numerical differentiation to estimate velocity vector fields.


In conclusion, a series of enhancements to the original CASI II algorithm for simulating speckle interferometry have been detailed, the goal being to amplify both the usable range of the technique as well as its computational performance. Results for representative data sets have shown the efficacy of these modifications. This new version of CASI II generates high-resolution full-field displacement fields at common camera frame rates and is suitable for use in many non-contact industrial inspection applications, as well as classical experimental mechanics studies.


Abstract:This paper is a critical review of in situ full-field measurements provided by digital image correlation (DIC) for inspecting and enhancing additive manufacturing (AM) processes. The principle of DIC is firstly recalled and its applicability during different AM processes systematically addressed. Relevant customisations of DIC in AM processes are highlighted regarding optical system, lighting and speckled pattern procedures. A perspective is given in view of the impact of in situ monitoring regarding AM processes based on target subjects concerning defect characterisation, evaluation of residual stresses, geometric distortions, strain measurements, numerical modelling validation and material characterisation. Finally, a case study on in situ measurements with DIC for wire and arc additive manufacturing (WAAM) is presented emphasizing opportunities, challenges and solutions.Keywords: digital image correlation; in situ; monitoring; additive manufacturing


Approaches vary from continuum to discrete description of material responses. Deterministic and stochastic approaches are used to understand interactions between material elements are different length and time scales. Analytical and computational methods include finite element calculations, molecular dynamics, influence functions, ab-initio calculation, Monte Carlo simulation. These methods are used to link different length and time scales that allow investigators to develop physically based material models, including state variable approaches. Experimental approaches include x-ray diffraction, scanning probe microscopy, optical methods and mechanical testing under a wide range of environmental conditions and size scales from macro to nano. Mechanics of solids is currently one of the most viable areas of mechanical engineering - from intellectual, technological and funding perspectives. There is new pressure on designers to improve efficiency, lower cost and improve safety and reliability. Invariably such advances occur through improved microstructural design and mechanical characterization of materials and structures that lead to new applications and improve performance. Accurate mathematical representation of the structure-property relations is at the heart of these efforts. To develop these representations faculty and students in solid mechanics work collaboratively to advance the necessary theory, experiments, modeling and computational mechanics and to apply their results to societal needs.


MECH 7390 VARIATIONAL MECHANICS (3) LEC. 3. Energy methods in solid mechanics. Virtual work, stationary potential energy, and variational calculus. Elastic strain energy. Applications to bars, trusses, beams, frames, and plates. Castigliano's Theorem and the Ritz Method.


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