Transcript Laser Speckle Extensometer

MESSPHYSIK
MATERIALS TESTING
Laser Speckle Extensometer
ME 53
Version 2.3
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© MESSPHYSIK Materials Testing
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Class 1 Device
Features:
Non-contacting measurement
Works with most materials
High temperature measurement
Micro Tensile Specimen measurement
1 Laser diodes
2 Cameras (CCD)
3 Speckle Patterns
4 Specimen
Principles of Displacement Measurement
with Laser Speckle Correlation
The optically rough surface of a specimen reflects coherent light.
Laser diode modules with a wave length of λ = 660 nm are directed
towards the specimen.
The reflected light passing through the laser beam creates so-called
„Speckle Patterns“.
The image size of the CCD camera is called „Field of View“.
Speckle = spot, dab or point
Speckle Pattern
Image Size = Field of View (FOV)
Principles of Displacement Measurement
with Laser Speckle Correlation
Requirements for displacement measurement with standard application:
2 Cameras = 2 Speckle Patterns
Distance between cameras = 50mm
Lenses … 75f x2 / 3,9 F
with 2x extender we receive
f=150mm
With these lenses a distance of 480 mm
between specimen and cameras is possible
Principles of Laser Speckle Correlation
While cameras and laser modules remain in a fixed position, the surface
of the specimen moves when extended.
Master
Slave
To measure the displacement of the surface, it‘s necessary to follow the
movements of the Speckle Patterns.
This is achieved by means of a cross-correlation algorithm comparing two
successive patterns.
This comparison is carried out up to twenty times per second.
Measuring rate = 20 Hz
The Speckle Pattern
The size of the evaluated patterns can be set to up to 128x128 pixels.
The cross-correlation (FFT) requires high computing capacity. Therefore
PCs with a minimum CPU speed of 500 MHz are needed to reach measuring
rates of 20 Hz.
In order to measure up to 200Hz a 3 GHz Processor and High Speed Digital
Cameras are required.
The Correlation Peak
The result of the cross-correlation
is a function, where the position
of the maximum (Correlation Peak)
corresponds with the
displacement of the pattern.
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Parameters for this interpolation
algorithm can be defined by the user.
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A special interpolation algorithm
determines the position of the
correlation peak in the subpixel
range and compensates for changes
of the surface in real-time.
The peak is intersected by several planes parallel to the
xy – plane.
The results are sectional planes
which are evaluated in two
dimensions.
The centres of gravity are regarded
as the position of the peak.
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With this procedure it is possible
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Calibration
Calibrating the LSE, the Speckle pattern must be moved for a
defined stretch. This way the size of the FOVs (Field of Views) and the
Calibration factors can be determined.
This stretch can be as long as needed and adjusted in the LSE Software.
The standard stretch for the calibration is 2 mm. The sample is mounted in one
grip only and shifted for exactly 2 mm.
The size of the FOVs is calculated, the result saved and the LSE is
ready for measuring.
If the distance between specimen and camera has changed, the
LSE has to be re-calibrated.
Features
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2 - dimensional strain measurement (longitudinal and transversal)
Features
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Displacement & 3 Point – Deflection Measurement
Features
Triangulation Sensor Head ME53-33
This application turning the cameras allows L0 setups from 0-250mm
Features
Triangulation Sensor Head ME53-33
applied to a Midi 10-20 Machine
Two dimensional strain, multiple strain
and deflection measurement
Testing all kind of materials:
Metals
Plastics
Textiles
Rubbers
Features
Works with most materials
Steel
Aloy
Rubber
Plastics
Features
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800° C high temperature measurement done in Australia, University of
Technology in Brisbane
Features
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compression test of all kind of
material
compression tests at high
temperatures up to 1600 °C
Features
Laser Speckle Extensometer looking through
temperature chamber glas window
illuminated specimen
Features
• 0.1 µm resolution measurement of micro specimen
• Calibration by marks on the surface of the specimen
Features
Micro Specimen curve
MESSPHYSIK
MATERIALS TESTING
Credits
Phillip Winkelmayer
Dipl. Ing. Oliver Spinka
© Messphysik Materials Testing, Oktober 2005