Stress and strains

Experimental setup for a tensile test on an elastomer sample
Experimental setup for a tensile test on an elastomer sample

Stress and strains occur when a load is applied to a structure or component. The stresses and strains have a stress-strain relationship. This depends on the modulus of elasticity of the building material and is described by the Hooks law  (s = E · e).

These relationships are generally determined by a tensile test. This is a standardized method for material testing. This is one of the destructive test procedures in which a sample of the material is taken up to the breaking load. This enables the yield strength, tensile strength, elongation at break and other material properties to be determined.

The test setup for the tensile test on an elastomer specimen is shown in the figure on the right. You can see the specimen, which is clamped between 2 abutments. A laser optical measurement sensor is installed to measure the strain. During the test procedure, the specimen is pulled apart at a continuous velocity at the plates. The test is carried out up to the breaking load. The stress-strain relationship can then be determined using the stresses entered and the measured strains. 

Strain gages

So-called strain gauges can be used for the direct measurement of strain. These must be attached directly to the samples, which requires surface preparation. This is one of the reasons why they are used less frequently. 

Other direct measuring methods are:

  • Laser extensometer
  • Laser Speckle Extensometer
  • Videoextensometer
  • Gray value correlation 

Often, laser optical measurement sensors sensors are used to determine the strains, which offer a high resolution and can be used several times. 

Strain gages on a tensile test
Strain gages on a tensile test