Unlike most conventional videoextensometers that relied on the reading of gauge length marks at all times especially when working at high temperatures. The Messphsik overcame these difficulties and developed a laser based, contact-free system.
The unit is known as the Laser Speckle Extensometer and utilises full image instead of line cameras making handling much easier but more importantly enables simultaneous measurement in two axes
The Laser Speckle Extensometer comprises a PC based Video Processor, which continually measures the displacement of two speckle patterns, recorded by video cameras ( having CCD or CMOS Sensor Chips) in a master slave configuration.
The two displacements are converted into a strain signal, which can be sent to an external control or evaluation system for data evaluation and testing machine control
- Laser controllable up to 3 mW
- Wave length 650 nm
- CCD camera (detector)
- Imaging by frame grabber card
- Resolution of displacement 1µm
- Resolution of expansion 2 x E-5
Triangulation sensor head
The Triangulation Sensor Head ME 53-33 turns the cameras to vary the camera distance, called Lo or the measuring length.
While traditional extensometers work with marks, the Laser Speckle Extensometer works with virtual marks. These marks are measuring fields which detect surface movements. The marks can be placed anywhere in the field of the camera but mostly in the exact middle of the camera frame. Turning the camera means moving the virtual marks and places them in a defined distance on the specimen.
Gauge length usually between 0mm and 250mm
Non-contact measuring through glass window of a temperature chamber working completely contactless the Laser Speckle Extensometer is able to look through and measuring through glass windows. Almost every material that is invisible for the camera can be used to isolate the temperature chamber. The measurement won't be effected by it.
Vertical positioning unit for triangulation sensor head
When a typical machine grips at base and extends by pulling the specimen upwards, the permanent changing middle of the specimen will follow this direction. This middle is the most likely point where the specimen will burst.
While the Laser Speckle System detects movements of the surface of the specimen the cameras were fixed on the machine. Now the cameras move on half way with the crosshead to ensure that the breaking point of the specimen is between the two measuring cameras.