Determination of the elongation parameter εf in Small Punch tests on metallic materials using different experimental techniques

The Small Punch Test (SPT) has become an effective miniature testing technique for the mechanical characterisation of metallic materials when only very limited material volumes are available. Among the parameters derived from this test, the fracture equivalent strain εf (defined as the natural logarithm of the ratio between the initial thickness h₀ and the minimum remaining thickness hf after failure) is particularly relevant for assessing ductility and establishing correlations with properties obtained from conventional mechanical tests. However, the standard metallographic procedure for measuring εf shows a strong operator dependency and limited repeatability.

In this work, an alternative experimental methodology is proposed and validated for the robust determination of εf in 42CrMo4 steel (1.7225) in quenched and tempered conditions. The material is first characterised through chemical analysis, hardness measurements, microstructural observation, and uniaxial tensile testing, obtaining reference values for yield strength, ultimate tensile strength, and ductility. Subsequently, SPT discs extracted from tensile specimens are tested under standardised conditions, and εf is determined using three independent post-test techniques: X-ray computed tomography (with micro- and nano-detector), 3D optical digitalisation, and conventional metallography.

The different techniques are compared in terms of dispersion, repeatability, measurement uncertainty, and industrial applicability, supported by statistical analysis. The results highlight the advantages of non-destructive methods over classical metallography, particularly in terms of reproducibility and time efficiency.

The proposed methodology provides a robust basis for improving ductility assessment in SPT campaigns and supports the industrial implementation of miniature testing in high-strength steels such as 42CrMo4.