Laser Speckle Extensometer ME 53 Class 1 Device MESSPHYSIK MATERIALS TESTING © MESSPHYSIK Materials Testing Version 2.3.

Laser Speckle ExtensometerME 53

Class 1 Device

MESSPHYSIKMATERIALS TESTING

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© MESSPHYSIK Materials Testing

Version 2.3

Features:Non-contacting measurement

Works with most materials

High temperature measurement

1 Laser diodes

2 Cameras (CCD)

3 Speckle Patterns

4 Specimen

Micro Tensile Specimen measurement

Principles of Displacement Measurement with Laser Speckle CorrelationThe optically rough surface of a specimen reflects coherent light.Laser diode modules with a wave length of λ = 660 nm are directedtowards 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)

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 receivef=150mm

With these lenses a distance of 480 mm between specimen and cameras is possible

Principles of Displacement Measurement with Laser Speckle Correlation

Principles of Laser Speckle Correlation

To measure the displacement of the surface, it‘s necessary to follow themovements of the Speckle Patterns.

This is achieved by means of a cross-correlation algorithm comparing two successive patterns.

Master Slave

While cameras and laser modules remain in a fixed position, the surface of the specimen moves when extended.

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 DigitalCameras are required.

The Correlation Peak

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The result of the cross-correlation is a function, where the position of the maximum (Correlation Peak) corresponds with the displacement of the pattern.

A special interpolation algorithmdetermines the position of thecorrelation peak in the subpixel range and compensates for changesof the surface in real-time.

Parameters for this interpolationalgorithm can be defined by the user.

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The peak is intersected by several planes parallel to thexy – plane.

The results are sectional planes which are evaluated in two dimensions.The centres of gravity are regarded as the position of the peak.

With this procedure it is possibleto reach a very accurate displacement measurements while saving a lot of calculation time.

S

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 theCalibration factors can be determined.

The standard stretch for the calibration is 2 mm. The sample is mounted in one grip only and shifted for exactly 2 mm.

This stretch can be as long as needed and adjusted in the LSE Software.

The size of the FOVs is calculated, the result saved and the LSE isready for measuring.

If the distance between specimen and camera has changed, theLSE has to be re-calibrated.

Features• 2 - dimensional strain measurement (longitudinal and transversal)

Features• Displacement & 3 Point – Deflection Measurement

Features

FeaturesTriangulation Sensor Head ME53-33

This application turning the cameras allows L0 setups from 0-250mm

Features

Features

Two dimensional strain, multiple strain

and deflection measurement

Testing all kind of materials:

Metals

Plastics

Textiles

Rubbers

Triangulation Sensor Head ME53-33

applied to a Midi 10-20 Machine

Features

Features

Works with most materials

Steel Aloy

Rubber Plastics

Features

Features• 800° C high temperature measurement done in Australia, University of

Technology in Brisbane

Features

Features

• compression test of all kind of material

• compression tests at high temperatures up to 1600 °C

Features

Features

illuminated specimenLaser Speckle Extensometer looking throughtemperature chamber glas window

Features

Features

• 0.1 µm resolution measurement of micro specimen• Calibration by marks on the surface of the specimen

Features

FeaturesMicro Specimen curve

Features

MESSPHYSIKMATERIALS TESTING

Credits

Phillip WinkelmayerDipl. Ing. Oliver Spinka

© Messphysik Materials Testing, Oktober 2005