Print attempt

In terms of the direction of force, the compression test is the reverse of the tensile test and is mainly used to determine the flow curves of brittle building materials and metals - for example, cast iron .

Another application of compression tests is the measurement of the explosiveness of explosives according to Heß / Kast by means of a compression test.

Test types

Cylinder compression test

Schematic printing test

→ Main article : Cylinder compression test

To test metallic materials, cylindrical samples with a ratio of height (h ₀ ) and diameter (d ₀ ) of 1 to 2 are used in the cylinder compression test:

${\displaystyle 1\leq {\frac {h_{0}}{d_{0}}}\leq 2}$.

For bearing metals , a diameter of 20 mm is used, the ratio must be 1:

${\displaystyle {\frac {h_{0}}{d_{0}}}=1,0\qquad h_{0}=20\,{\text{mm}}}$

For steels , a diameter of 10-30 mm is used, the ratio must be 1.5:

${\displaystyle {\frac {h_{0}}{d_{0}}}=1,5\qquad h_{0}=15\dots 45\,{\text{mm}}}$

Cube-shaped samples are used for building materials .

Strength test

In the force test, the samples ( cross-sectional area ) are loaded with a steadily increasing force between two parallel pressure plates . The load is increased until it ${\displaystyle S_{0}}$

• For brittle materials, the break comes
• a crack on the surface occurs in ductile materials
• when an agreed total compression is achieved (see below). ${\displaystyle \varepsilon _{dt}}$

The force applied is determined and the compressive strength in N / mm² is determined from this : ${\displaystyle F_{B}}$

${\displaystyle \sigma _{dB}={\frac {F_{B}}{S_{0}}}.}$

Has a deformable material been compressed by 50%

${\displaystyle \varepsilon _{dt}={\frac {\Delta L}{L_{0}}}=50\,\%,}$

the attempt is terminated. The compressive strength is then given as

${\displaystyle \sigma _{d50}={\frac {F_{50}}{S_{0}}}.}$

In the case of building materials and wood , it is mostly determined. ${\displaystyle \sigma _{d10}}$

The stress-strain diagram of the compression test of metallic materials usually shows a discontinuity at the transition from predominantly elastic deformation to predominantly plastic deformation . This is called the crush limit and corresponds to the yield point in the tensile test. If the curve does not have this discontinuity, the technical compression limit can be determined if necessary . ${\displaystyle \sigma _{dF}}$ ${\displaystyle \sigma _{0,2}}$

However, the opposite direction of force to the tensile test must not hide the principle differences. Thus, the apparent between the sample and the pressure plates hindered friction , the deformation of the samples, therefore, there is a barrel-shaped bulge (engl .: barreling). There is no uniform compression and no bulging analogous to the constriction in the tensile test . The deformation hindrance continues conically into the interior of the sample, which is why only part of the sample volume is plastically deformed (see figure).

Pressure test according to Rastegayev

The pressure test according to Rastegajew, sometimes also written in English Rastegaev, is a method of material testing . Here, a compression test is provided with a small cup hole on both end faces. This hole is  filled with a lubricant - for example a grease or commercially available Vaseline . If the sample is compressed in the test, the lubricant reduces the friction between the sample and the contact surfaces of the testing machine, stamp and contact surface (the pressure plates in the picture) to a minimum. Since the contact surfaces of the sample can enlarge almost unhindered, a "bulging" of the sample is almost prevented and it is possible to measure the true compressive stress during the test.

See also

• Flat compression test

Web links

• Pressure testing of polystyrene samples Youtube video: fully automated pressure and bending tests

Individual evidence

1. ^ Josef Köhler, Rudolf Meyer, Axel Homburg: Explosivstoffe . 10th edition. John Wiley & Sons, February 28, 2012.