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 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 PatternImage 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 receive f=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 the movements of the Speckle Patterns. This is achieved by means of a cross-correlation algorithm comparing two successive patterns. MasterSlave 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 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. 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. Parameters for this interpolation algorithm can be defined by the user.
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. With this procedure it is possible to 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 the Calibration 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 is ready for measuring. If the distance between specimen and camera has changed, the LSE has to be re-calibrated.
Features 2 - dimensional strain measurement (longitudinal and transversal)
Features Displacement & 3 Point – Deflection Measurement Features
Triangulation Sensor Head ME53-33 This application turning the cameras allows L0 setups from 0-250mm 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 Machine Features
Works with most materials SteelAloy RubberPlastics Features
800° C high temperature measurement done in Australia, University of Technology in Brisbane Features
compression test of all kind of material compression tests at high temperatures up to 1600 °C Features
illuminated specimen Laser Speckle Extensometer looking through temperature chamber glas window Features
0.1 µm resolution measurement of micro specimen Calibration by marks on the surface of the specimen Features