With this deflection principle there is a two-step force gradient, resulting in a specimen break at gage length without sliding of the specimen on the lower roller and without jaw breaks. A simple tensile test is used for calculating the maximum force and maximum strain. The results are shown in a typical force-strain diagram.
With the double capstan grip the specimen is placed on the roller connected to the load cell in the form of a sling. The specimen is then guided over a lower roller and again over a height-adjustable third roller. After the defection of the double thread, a defined pretension weight is attached. During this phase all three rollers are moving in order to balance the extensions which are created by the attachment of the pretension weight.
The three lengths are adjusted to the material being tested. Finally, the double thread is clamped in the height-adjustable specimen grip.
After the specimen grip closes the deflection rollers are locked, thereby creating frictional forces between the rollers and the specimen preventing the specimen from slipping. The roller diameter is dimensioned in such a way that the occurring friction does not influence the test results.
Since the materials to be tested often slip out of the clamping area, a specimen grip with constant gripping force is used.