Questions and answers UTM

High frame rigidity ensures that the testing machine delivers precise and repeatable results. It also helps to minimise unwanted side effects that could falsify the test result. This leads to better control and accuracy of test conditions. Stiff frames tend to be more durable as they are less susceptible to damage from repetitive loading and mechanical stresses.

Axial stiffness is a measurement in the unit kN/mm. To measure it, a distance and a force must therefore be measured.

Determining the frame stiffness

In spindle-driven machines, the displacement is usually measured using a rotary encoder that measures the rotation of the spindles and calculates the displacement of the crosshead using the spindle pitch. The easiest way to do this in the compression area is with compression plates that are in contact. Without a sample, the sample stiffness is infinite and the entire measured displacement is due to the machine deformation.

In order to be able to measure the force, a load cell is required, which in turn has a stiffness and thus contributes to the overall stiffness of the machine. A good 100kN testing machine has a stiffness of approx. 300kN/mm, which corresponds to a deformation of 0.33mm at 100kN tensile or compressive force.

A 100kN load cell deforms by approximately 0.1 mm at nominal load. The rest of the machine therefore deforms by about 0.23 mm.

The greater the specimen stiffness, the greater the influence of the machine stiffness on the axial strain. For stiff specimens, the strain measurement must therefore be carried out using an extensometer.

If a zero adjustment cannot be performed or a strain gauge load cell can no longer be calibrated, this is unfortunately often a sign of overloading.

• Is there no signal at all from the load cell?
  Switch off the testing machine, check the cables and plugs, and switch the machine on again with the sensor cable plugged in.
• Is a heavy clamping device attached to the load cell?
  Remove this and try again with a lighter fixture.

Today’s testing machines accept a maximum of 10-20% of the nominal force as a deviation of a load cell in order to be able to perform a zero adjustment. This deviation can come from the fixture hanging from the loadcell during zeroing, or it can come from plastic deformation of the loadcell. In the former case, the remedy is to reduce the weight, while in the latter case, unfortunately, the load cell often has to be replaced.