Calculations and Results
When presenting test results, it is important to ensure that the terms are properly defined in order to comply with the ISO standard and facilitate data comparison between different laboratories.
The most common mistake in data reporting is the reporting of strain values using an incorrect source. For plastics, percent elongation at break often cannot be measured exclusively by an extensometer because plastic does not break down homogenously and strain is often focused on a disproportionately small part of the sample, a property known as "necking." Because necking may occur outside of the extensometer's gauge length, a term called "nominal strain" must be used to report percent elongation at any points after yield. Using an extensometer for strain at break is only acceptable if the strain is homogenous throughout the specimen and does not exhibit necking or yield.
Nominal strain is defined differently depending on which test method is being used. For ISO 527-2, nominal strain can be measured in two different ways: Method A measures nominal strain purely by crosshead displacement, but for multipurpose specimens, Method B is preferred. Method B measures nominal strain as the strain measured from the extensometer until yield and and from crosshead displacement after yield, which ensures that necking behavior outside of the extensometer gauge length is taken into account.
ISO 527-2 defines modulus as the slope of the curve between 0.05% and 0.25% strain using either a chord or a linear regression slope calculation. Because the modulus calculation starts at 0.05% strain, it is extremely important that appropriate pre-stresses are applied to the material to remove any slack or compressive forces induced from gripping the specimen. It shall not exceed 0.05% strain or 1% of the tensile strength of the material.
In the 2012 update to the standard, a change was made to the definition of tensile strength. In previous versions, tensile strength was defined as the maximum stress at any time throughout the test. In the latest version of ISO 527-2, tensile strength is taken at the first local maximum exhibited. This change is particularly critical for customers testing materials which have yield points such as polypropylene, polyethylene, and nylons.