Compression Test

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Compression test

Compression test arrangement and loading scheme

The compression test is used to assess the material behaviour under uniaxial compression loading, whereby rectangular prisms, cylinders or tubes can be used as test specimens.

Figure 1 schematically illustrates a compression test facility and the test procedure.

Fig. 1:

Compression test arrangement and schematic of the loading of a compression test specimen

The measurement requirements for the exact performance of a compression test are:

Compression plates and material testing machine must generate a uniaxial load and stress condition in the test specimen
Point M must lie centrically in the compression area and the load line of the material testing machine (independent parallel setting of the compression plates)

Technical significance of the compression test

Although many different, material-oriented standards exist for testing the mechanical properties under uniaxial compressive stress, the compression test – apart from a few special cases – has basically not been able to attain the importance as, for example, the tensile test or bending test or the test of hardness.

This situation is due to the relatively low practical relevance of compressive load and the test-related problems, so that the use of the compression test is limited to special applications and/or selected materials. These include building materials (concrete, polymer concrete, bricks, wood, foams, etc.), materials used in dampers, sliding bearings and seals, etc. (copper alloys, polyamide, polyethylene, rubber, etc.) and packaging materials (cardboard, foams, etc.).

Testing standards for different material groups

The harmonisation of standards at international level has not progressed as far as can be observed, for example, with regard to hardness testing or tensile testing.

The following test standards (see also references) are used for the various types of plastics

Plastics in general: ISO 604
Elastomers: ISO 7743
Polymer concrete: DIN 51290-3
Foams: ISO 844, ISO 3386
Fibre-reinforced plastics: EN 2850, ASTM D 5467/D5467M, DIN V 65380, DIN 65375

are applied, whereby ISO 604 has certainly found the widest distribution.

ISO 604 is used to investigate the compressive deformation behaviour of test specimens under an external uniaxial compressive load and to determine the compressive strength, the compressive modulus and other aspects of the compressive stress–compression strain relationship under specified conditions.

ISO 604 is applicable for:

rigid and semi-rigid thermoplastic injection and extrusion moulding compounds, including filled and reinforced moulding compounds
rigid and semi-rigid thermoset moulding compounds, including filled and reinforced compounds, and
thermotropic liquid crystalline polymers.

The method is generally not suitable for use with textile fibre reinforced materials, rigid foams and foam-core laminates, for some of which specific standards have been developed.

The compression test for plastics

Removal of test specimen

To carry out the procedure, test specimens are used which are either

have been formed in the selected dimensions
from the centre part of the multipurpose test specimen according to ISO 3167 (Fig. 2) or
from finished parts or semi-finished products, such as moulded parts, laminates, extruded or cast sheets.

Fig. 2:

Removal of test specimens for the compression test from the multipurpose test specimen type 1A

The use of multipurpose test specimens according to ISO 3167 for the production of three-point bending test specimens (see SENB-specimens) according to ISO 179 and test specimens for the compression test according to ISO 604 ensures that test specimens with a uniform internal material condition are tested with regard to the formation of morphological structures and orientations under tensile, bending and compression loading and that the results can be compared with certain restrictions.

Determination of the material parameter of the compression test

The compression modulus is also determined as secant modulus between 0.05 % and 0.25 % compression according to the following equation:

E_{c}\,=\,{\frac {\sigma }{\varepsilon }}\,=\,{\frac {\sigma _{2}-\sigma _{1}}{\varepsilon _{2}-\varepsilon _{1}}}\,=\,{\frac {\Delta \sigma }{\Delta \varepsilon }}\,=\,{\frac {\Delta \sigma }{0.002}}

.

Measured parameters:

Force in [N]

Compression strain in [mm]

Fig. 3:

Determination of compression modulus according to the secant method

Brittle polymer materials such as polystyrene ( abbreviation: PS) and polymethyl methacrylate ( abbreviation: PMMA) and tough polymer materials polyamide ( abbreviation: PA) show clear differences in the compression stress–compression strain diagrams due to different force–deformation characteristics. While PS and PMMA have a compressive stress flow range, the fracture of brittle epoxy resin materials occurs at the maximum force F_max; for PA, a substitute characteristic value must be determined at x % compression (crushing).

Fig. 4:

Compressive stress–compressive strain diagrams for different plastics

a – brittle plastics (epoxy resin, abbreviation: EP)
b – ductile plastics with compressive stress at yield (PS)
c – ductile plastics without compressive stress at yield (PMMA)
d – ductile plastics without fracture (PA)

The following material parameters, among others, can be derived from the compression stress (σ)-compression strain (ε_c)-diagram:

Compression Strength $\sigma$ _M:

\sigma _{M}\,=\,{\frac {F_{max}}{A_{0}}}\left[MPa\right]

Standard: maximum compressive stress that can be borne by the test specimen during the compression test and can usually only be determined on brittle materials.

It is not permissible to use the tensile strength as a substitute for the compressive strength, as polymeric materials show completely different deformation mechanisms under tensile and compressive load and thus different force–deformation characteristics.

Compressive Stress at Yield $\sigma$ _y:

\sigma _{y}\,=\,{\frac {F_{y}}{A_{0}}}\left[MPa\right]

Stress at which the slope of the compressive stress–compressive strain curve becomes zero for the first time.

Standard: The stress at which the slope in compressive strain occurs for the first time without an increase in stress.

Compressive Stress at Break $\sigma$ _B:

\sigma _{B}\,=\,{\frac {F_{B}}{A_{0}}}\left[MPa\right]

Compressive stress at fracture of the test specimen

Compressive Stress at x % Strain $\sigma$ _x:

\sigma _{x}\,=\,{\frac {F_{x}}{A_{0}}}\left[MPa\right]

Compressive stress at which the compression strain reaches the specified value of compression in x % (if compressive stress–compression strain curve does not reach a yield point)

Nominal Compressive Yield Strain $\epsilon$ _cy:

\varepsilon _{cy}\,=\,{\frac {\Delta l_{y}}{l}}\cdot 100\left[\%\right]

The compression at which the compressive yield strength $\sigma$ _y is reached.

Nominal Strain at Compressive Strength $\epsilon$ _cM:

\varepsilon _{cM}\,=\,{\frac {\Delta l_{M}}{l}}\cdot 100\left[\%\right]

compression strain at compressive strength

Nominal Compressive Strain at Break $\epsilon$ _cB:

\varepsilon _{cB}\,=\,{\frac {\Delta l_{B}}{l}}\cdot 100\left[\%\right]

compression strain at break

Since the test specimen have a comparatively short length, the nominal compressive strain ε_c, which results from the movement of the compression plates and corresponds to the crosshead path, is mostly used in practise. The compression strain values are given dimensionless in %.

Compression strength values for plastics

The following table contains compressive strength values for selected plastics.

**Table**: Compressive strength at 23 °C of selected plastics
Material	$\sigma$ _M (MPa)
Thermoplastics unreinforced
PMMA	110
PTFE	12
Thermoplastics reinforced
PP + 30% GF	60
PA 6 + 30% GF	160
PA 66 + 30% GF	170
Duroplaste
Phenolic resin	170
Harnstoffharz	200
Melamine resin	200
unsaturated epoxy resins	150
epoxy resins	150
PUR	110

A comprehensive literature review of the mechanical values for compressive strength is compiled in [1].

[1]	Bierögel, C., Grellmann, W.: Compression Loading. In: Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein. Volume VIII/6A3, Springer Berlin (2014) 150–163 (ISBN 978-3-642-55165-9; see AMK-Library under A 16)
[2]	ISO 604 (2002-03): Plastics – Determination of Compressive Properties
[3]	ISO 844 (2021-03): Rigid Cellular Plastics – Determination of Compressive Properties
[4]	ASTM D 695 (2023): Standard Test Method for Compresssive Properties of Rigid Plastics
[5]	ISO 3386: Polymeric Materials, Celluar Flexible – Determination of Stress-Strain Characteristics in Compression Part 1 (2025-07): Low-density Materials Part 2 (1997-06): High-density Materials (AMD1: 2010-04)
[6]	ISO 7743 (2017-10): Rubber, Vulcanized or Thermoplastic – Determination of Compressive Stress-strain Properties
[7]	DIN EN 2850 (2018-01):Aerospace Series – Carbon Fibre Thermosetting Resin – Unidirectional Laminates – Compression Test Parallel to Fiber Direction (German and English Version EN 2850: 2017)
[8]	DIN V 65380 (1987-04): Aerospace – Fiber-reinforced Plastics – Testing of unidirectional Laminates – Compression Test, Parallel and Transverse to Fiber Direction (withdrawn)
[9]	DIN 65375 (1989-11): Aerospace – Fiber Reinforced Plastics – Testing of Unidirectional Laminates – Compression Test Transverse to Fiber Direction
[10]	ASTM D 5467/D 5467M (1997, reapproved: 2017): Standard Test Method for Compressive Properties of Unidirectional Polymer Matrix Composites Using a Sandwich Beam