Experimental Identification of Orthotropic In-Plane Elastic Properties Using Whole-Field Optical Measurements and the Virtual Fields Method

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INTRODUCTION ON IDENTIFICATION OF MATERIAL
PROPERTIES
Professor Fabrice PIERRON LMPF Research Group,
ENSAM Châlons en Champagne, France
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Introduction
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• Optical full-field measurements
• Interferometry techniques
• Moiré interferometry
• Speckle interferometry
• White light techniques
• Digital image correlation
• Grid method
• Features
• Provide displacement, strain, slope maps (more
  than 100.000s measurement points) field
  information
• Digital technology (easy to process)

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• Qualitative / quantitative ?
• Metrological properties not fully assessed yet
• Often reduced to visual information
• Very few quantitative use of the data!
• International effort
• SPOTS programme (http//www.opticalstrain.com)
• French research network (DIC, image simulation)
• Extraordinary potential
• From a few measurements points to a few
  100.000s!!
• Extensive research required to take full
  advantage
• Eventually new standard test procedures
• Extensive training needed (undergrads, postgrads,
  engineers etc.)

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• A few key issues
• Extensive literature on inverse problems
  (mathematical)
• Very few tools targeted at full-field processing
• Very few groups with expertise in measurements
  and inverse problems at the same time
• Inversion closely related to measurement
  performance!
• Huge investigation area, critically unexplored

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• Identification of the mechanical properties of
  materials
• Usual strategy for in-plane orthotropic elastic
  moduli measurements

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• Novel strategy for orthotropic elastic moduli
  measurements

Full-field kinematic measurements
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• Basic ideas
• Uniform tests (tension, compression) or simple
  tests (bending, torsion)
• ?
• Data reduction easy
• No a priori model
• ?
• Boring (very limited information)
• Coupling effects difficult to address (load path
  sensitivity)
• Hard to achieve parasitic effects

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Tensile test on a PMMA specimen
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(No Transcript)
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Faulty gripping
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Grid method
Long. disp. in microns
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More recent example
Longitudinal strain
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• Perform heterogeneous (non uniform) tests
• ?
• No a priori information on stress distribution
  (statically undetermined test)
• More complex data processing, a priori model(s)
• ?
• Much more information available
• Reduced sensitivity to B.C.
• Issues to be addressed
• Field processing how many images, noise
  filtering, strain calculations, artifacts
• Inversion procedure
• Design of new tests adapted to field processing

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Statement of the problem
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• Basic equations

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• Direct problem

• Tools for solving this problem
• Direct integration (closed-form solution)
• Approximate solutions
• Galerkin, Ritz
• Finite elements, boundary elements
• etc

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• Inverse problem material identification

Known
Unknown
Geometry Some information on the boundary
conditions (load cell)

• Tools for solving this problem
• Statically determined testsClosed form solution
  of Eq. I (uncoupled system)Force BC, simple
  geometryEx. tensile test, bending tests (on
  rect. beams) etc

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• Tools for solving this problem
• Model updatingIdea iterative use of tool for
  direct problem (analytical or approximate)

Exemple
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• Model updating
• Advantages
• General method (full-field measurements not
  compulsory)
• Tools already developped
• Shortcomings
• Sensitive to boundary conditions (generally badly
  known)
• CPU intensive (for numerical approximations and
  non-linear equations)
• FE package needed to process test results
• Not fully dedicated to full-field measurements