Health Checking Samples for PIP Testing

Published on 16 August 2022

There are a few characteristics of every sample that must be probed to confirm it is suitable for PIP testing. These are anisotropy, grain size and porosity, each of which can act as artefacts that affect accuracy of the inferred properties. As part of the health check, PIP can be used to rapidly detect if anisotropy is present and if present, give a qualitative indication of the strength of anisotropy.

When a plastically anisotropic material is indented, the residual profile produced will not be radially symmetric. This is often the case for strongly textured materials, see Figure 1 for an example. During a health check, a residual indent will be scanned in at least two orthogonal directions. Plastic anisotropy in the indented plane can be identified by comparing the pileup heights of these scan profiles, which is to say the highest point in the residual profile where material has been pushed above the original sample surface. Microstructure can also be revealed in this region as persistent slip bands form, and grains rotate with heavy plastic deformation.

Graph showing properties of anistropic sample
Figure 1 - Residual indent profile of an anisotropic indent scanned in three directions; (inset) micrograph of the indent with texture visible in the piled-up grains

When the sample is indented the effect of plastic anisotropy on the stress and strain fields is quite complex. It’s not straightforward to infer mechanical properties from a non-radially symmetric indent, so if plastic anisotropy is detected, indentation will need to be performed on a cross-section plane.

Grain size is implicated in the idea of representative volume. The region deformed when indenting needs to include a suitable number of grains such that the behaviour is representative of the bulk material response. This means the grains need to be of a suitably small size relative to the radius of the indenter. 

Micrograph of a residual indent
Figure 2 Micrograph of a residual indent on a cast sample with large piled-up grains visible and some porosity visible in the top left of image

In the largely compressive stress state while indenting, compaction of pores invalidates some of the assumptions underlying the PIP methodology. Properties inferred from indenting a material with >1% porosity will not be accurate.

Both porosity and large grain sizes may be identified in an optical micrograph, see Figure 2 for an example. The polished sample surface may reveal pores intersecting the surface, and large grains will be apparent in the pileup region.

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