Detecting Inhomogeneity and Using PIP for Mapping

Detecting Inhomogeneity

Differences in processing conditions such as variable heating conditions or deformation processes can result in parts having varied mechanical properties across a fine scale. Therefore, many metallic parts show inhomogeneity.

Thin sheet samples are often softer in the centre and harder on their surface, due to surface hardening from the processing conditions. Therefore, PIP results will differ depending on whether they are carried out on a sheet’s surface or in the centre. Additionally, sheet samples will often be anisotropic and be softest in the through thickness, further impacting the PIP results as a multi-directional test.

Standard tensile testing probes a relatively large volume of material during testing. It is possible (in some cases likely) that the mechanical properties will vary across the scale of the gauge length. However, the tensile test results will not identify this inhomogeneity, rather the generated results will be an approximately average response across the inhomogeneous volume.

PIP tests are carried out on a finer scale than tensile tests, allowing variations in mechanical properties across a part to be easily identified. However, it is possible that this could cause some discrepancies with tensile results where a tensile gauge length is inhomogeneous.

Using PIP for Mapping

For parts which are known to be inhomogeneous, being able to map the mechanical properties across the part is of particular importance, whether that is for further modelling of the part in its end use or identifying the most likely failure regions. Hardness testing is sometimes used for this due to the smaller scale of the region probed during the test. However, hardness numbers offer limited information about the plastic behaviour of a material and can be misleading. Using PIP offers the significant advantage over conventional hardness testing that the fine spatial resolution is retained but a full stress-strain curve can be derived for each test, yielding significantly more information than a single hardness number. This makes PIP a powerful technique for mapping the plasticity properties across a part.