This part of ISO 3386 specifies a method for the determination of the compression stress-strain characteristics of flexible cellular polymeric materials of density greater than 250 kg/m³.

The compression stress-strain characteristic is a measure of the load-bearing properties of the material, though not necessarily of its capacity to sustain a long-term load.

The compression stress-strain characteristic differs from the indentation hardness characteristics (as determined in accordance with ISO 2439) which are known to be influenced by the thickness and the tensile properties of the flexible cellular material under test, the shape of the compression piate, and the shape and size of the test piece.

ISO 3386-1 specifies a method for low-density flexible materials, and differs from Part 2 in the following ways:

• Part 1 is concerned with materials of density up to 250 kg/m³, whilst Part 2 is mainly concerned with materials of density above 250 kg/m³;
• compression stress values have been deleted from Part 2;
• Part 2 does not allow the use of a cylindrical test piece.

This part of ISO 3386 is a general method for testing denser flexible cellular materials (i.e. expanded cellular rubbers), measurements being made on one of more points on the steeply rising part of the stress-strain curve. The shape factor of the test piece is important and comparative test results can only be obtained on test pieces having the same shape factor.