International standardisation, ISO:

ISO 527-1: Determination of tensile properties – Part 1: General principles
ISO 527-2: Determination of tensile properties – Part 2: Test conditions for form and extrusion masses
ISO 527-3: Determination of tensile properties – Part 3: Test conditions for film and sheets.

American standardisation, ASTM:

ASTM D 638: Standard Test Method for Tensile Properties of Plastics
ASTM D 882: Test Methods for Tensile Properties of Thin Plastic Sheeting

Comparability between ISO and ASTM Standards.

The ASTM D 638 states that ISO 527-1 is ”technically equivalent” to itself. This is in reference to the basic
definitions of both Standards. In detail, there are significant differences, which is why the ASTM cannot be considered as an „identical“ Standard.

The specimen in ISO 527-2

In addition to the already mentioned, multi-purpose specimen, which are described in this Standard as preferred specimen shape type 1A and 1B, the Standard also makes other specimen available.

Type Description of the specimen shape in ISO 527-2

1A – Identical to ISO 3167 Type A, except for variable specimen thickness, preferred shape

1B – Identical to ISO 3167 Type B, except for variable specimen thickness, preferred shape

1BA – Reduction 1:2 to type 1B

1BB – Reduction 1:5 to type 1B

5A – Identical to type 2 to ISO 37 (rubber)

5B – Identical to type 4 to ISO 37 (rubber)

 

The miniaturized specimen are used according to the corresponding conditions present, if it is not possible to use the preferred specimen. This can be the case, e.g. if specimen must be taken from finished components or semi-finished products. These specimen are, however, not suitable for determination of comparable single-point data, since they lead to different test results.

Specimen in ASTM D 638

The ASTM also has various specimen geometry and dimensions standardised. The preferred specimen is Type I, which can be used for material thicknesses up to 7 mm. Various requirements can make the use of a different specimen necessary.

Starting point of the test

During clamping of the specimen, tensile or compressive strain can occur, depending on the type of grips. These strains damage the specimen prematurely and they must be kept to a minimum. The correction of the extension zero point is especially important for the determination of the modulus of elasticity.

Method of ISO 527:

After the specimen has been clamped in the grips, a small, positive stress is initially placed on the specimen. This pre-stress is designated σ0 in the ISO.

 

At the pre-stress, the value of the specimen´s extension is, by definition, zero. In order to keep the error in specimen extension, which is related to this „cut-off“, to a minimum, two definitions have been defined in the ISO:

  1. i) σ0 ≤ 0.0005 E

Means that the „cut-off“ initial extension may not be larger than 0.05 % (E = Modulus of elasticity)

  1. ii) σ0 ≤ 0.01 σ

Means that the pre-stress must be smaller than 1% of the stress value to be determined (σY, σM or σB)

As long as the material displays a linear slope in the stress-strain diagram, the straight line should be applied to this area. The crossing point of this line with the strain axis corresponds to the corrected zero point. If the material does not show a linear area, the straight line is applied, as a tangent, to the point of maximum slope.

Summary

Both Standards give definitions for determination of the strain zero point. These definitions, in reference to the ISO, are mainly relevant for the determination of the modulus, since this normally decreases with increasing elongation. This observation is less important for further strain values such as strain at yield εY, since the measurement error is little compared to larger measured strains