The Laser Extensometer of the type W-500 was used for strain measurement on a rubber specimen. The system is especially suitable to measure large strains up to several 100 % because of its large scanning range.
Before the experiment two markers were placed at the specimen. A mask with holes in a defined distance was sticked at the specimen and serigraph color was brushed over. After the mask was removed, two measuring markers remain on the specimen. The screen printing color stays wet for several days and remains flexible. Furthermore its is able to follow the deformation of the specimen, therefore the measuring point for strain determination is always correctly defined.
The markers were placed in a distance of 20 mm. At the beginning of the measurement the Laser Extensometer automatically determines the exact reference length, therefore small deviations can be adjusted instantly.

The photos show the specimen at first in the unstrained condition, second shortly before and finally after rupture. The deformation of the markers caused by the strain of the specimen is easily seen.

The figure on the left hand side shows the stress-strain-diagram. The specimen with a length of 76 mm, a cross-section of 4 x 2.5 mm was deformed with a transformation speed of 50 mm/min. The strain at rupture was about 470 %.

The Laser Extensometer W-500 is an optimum solution for measuring materials with large strain. The system works with a resolution of 3 ppm of the scanning range - about 1.6 µm. Therewith it is able to accurately track small strain at the beginning of a measurement as well as large strain until rupture with a single sensor. The contact free measuring principle guaranties an unaffected strain measurement of the specimen throughout the whole strain range.