What is currently available for sale by the Company?
The basic Indentation Plastometer comprises a loading frame, with integrated profilometer and control software. Usage requires a standard laptop that is normally supplied by the user, although a suitable one can be provided by PX at the RRP. The control software is supplied on subscription, with that for the first year included in the basic price.
How much does the base product cost?
Details of the cost model, which includes differential arrangements for commercial industry and educational establishments, will be supplied on request. As an initial guideline, however, the purchase price falls within a similar range to that covered by hardness machines to basic tensile testing facilities.
Can the test be applied to components in the field?
A plastometer suitable for in situ testing of (shaped) components is envisaged at some point, but the base version available at present is for use in the laboratory only.
Is the firm prepared to carry out trial tests for a potential customer?
Indeed it is, although this will be via individual arrangement, and is not guaranteed. In general, PX is receptive to the idea of samples being sent by a potential customer, so that indentation plastometry can be carried out by the firm and the results sent back. It might be requested that such information could be used for other purposes, such as the compilation of case studies.
Is there any capability for the measurement of other types of mechanical characteristics?
The machine is designed for plasticity characterisation and provides no information about elastic properties. In fact, the approximate Young's modulus of the material is supplied by the user. There is the prospect of machines becoming available in due course for measurement of creep characteristics (primary and secondary), for the strain rate sensitivity of plastic deformation (at high strain rates) and for measurement of residual stresses, but these products have not yet been fully developed.
To which direction within the sample does the inferred stress-strain curve relate, and can the test be used to obtain information about anisotropy?
During an indentation test, the deformation being imposed is multi-directional, such that the inferred stress-strain curve is a direction-averaged one. The standard procedure therefore provides no information about anisotropy in the sample. Of course, most (polycrystalline) metals do not exhibit strong (plastic) anisotropy. Nevertheless, there is potential interest in the area and a variant of the machine that would have the capacity to detect and characterise such anisotropy is under development.
What are the requirements, in terms of material and size, for a sample to be suitable for indentation plastometry?
Firstly, the test is designed for metals and is not suitable for brittle materials such as ceramics or glasses. It's also not suitable for polymers or other soft or visco-elastic materials. The sample dimensions can vary over a fairly wide range (up to about 10 cm in lateral dimensions and about 5 cm in thickness). They can also be much smaller than this, although an indent cannot normally be made very close (within about 2 mm) to a free edge and the sample thickness should also ideally be at least about 2 mm. (Thinner samples than this can be processed, but some small changes are needed to the operation, which will be explained.)
What are the requirements, in terms of sample preparation?
The surface to be indented must be reasonably flat and horizontal. However, a high polish is not required and indeed polishing as such can be avoided. A ground surface, with a surface roughness of no more than about 5 microns, is fine. Furthermore, while it should be approximately horizontal - ie normal to the direction of indenter motion, inclinations of up to a few degrees, which would actually be quite noticeable, are not a problem. Furthermore, samples can be mounted or unmounted. (For thin sheet, it is in general preferable for them to be unmounted.)
What is the output from a standard indentation plastometry test?
The underlying outcome is a set of parameter values in a constitutive law for the true stress v. true strain relationship exhibited by the sample. This can be converted (via integrated software for FEM simulation of a tensile test) to a nominal stress v. nominal strain plot that would be produced in such a test (with specified sample dimensions). There are options for users to specify information about their own test outcomes, such as the (nominal) stress and strain at the fracture point, which can be converted to a critical fracture strain for the material. Other outcomes include information about the stress and strain fields during the indentation test that was carried out, plus the load-displacement plot and the residual indent profile (and comparisons with corresponding best fit model predictions).
What range of strains is created during indentation?
This depends on several variables, including the plasticity characteristics, with a strong work hardening rate tending to inhibit the creation of large strains. In general, however, the strain level might typically range up to about 50-60%. (Such levels are rarely produced in a controlled way during tensile testing, so that indentation plastometry has a superior capability for study of the plasticity characteristics at very high strains). The average strain during an indentation test (weighted by the plastic work done in different strain ranges) is typically about 10-20%.
What volume of material is deformed in a typical test and what is the potential spatial resolution for mapping over a surface?
The standard indenter radius is 1 mm and a typical depth of penetration is about 200 microns, giving an indent diameter of around 1 mm. The (grossly) deformed volume is therefore of the order of a tenth of a cubic mm, or about 10^8 cubic microns. For a typical grain size of 100 microns, there are thus a few hundred grains in this volume. It is important that the deformed volume should contain enough grains for it to have a (plasticity) response that is representative of the bulk, but that is likely to be the case even for relatively coarse-grained samples (with grain sizes of several hundred microns). In terms of studying variations over a surface, indents should not be closer to each other than about 3 mm, but this still gives a relatively fine scale mapping capability.