Physical testing optimises material properties

Many industries are currently re-evaluating the materials used in products, generally to improve upon cost, weight and performance. However, with this wave of change comes a need for thorough analysis. And while this has seen design engineers make intensive use of simulation software, the need for physical test data to correlate virtual predictions against has become another vital tool in the development process.

"With simulation it's easy to miss subtle changes in a material," says Dan Bailey, Digital Image Correlation (DIC) project manager at Instron. "With so many materials coming to the fore there might be a slight change in the additives, pigments or fibres, and that all makes a difference. You need to calibrate between what you think is happening, and what is actually happening."

This demand has seen test equipment manufacturer Instron produce software to make its machines more straightforward to use, but also yield more effective test data for analysis. Its DIC is a software driven optical technique to allow virtual extensometers to be placed on a test sample.

The data is initially captured using a video camera and test piece that is sprayed with a 'speckled pattern'. Once the test has been carried out, the DIC can be used to produce full field strain and displacement maps that resemble FEA-style images, but using the data from the physical test specimen.

Like any extensometer, DIC tracks changes between two points. The difference, however, is its ability to change parameters and position of the virtual extensometers on the test piece during 'replay'. So the physical test of one sample can be used to find axial strain and displacement, transverse strain and displacement, shear strain, Poisson's ratio, as well as maximum and minimum normal strains.

"Engineers get quite excited by this as it's easy to prove if FEA simulations are right and prove [or disprove] all that information they've predicted," says Bailey. "You can draw extensometers between any two points. You can put one in the normal central position, have one above it and one below it; and up to five at a time. You're able to generate load against displacement curves between any two point and you can replay that, with multiple gauges in different places and then export that data to analyse further."

So, virtual extensometers can be placed near the point of failure to assess localised strain concentration or placed away from the point of failure around another point of interest such as a hole in the sample. It gives scope to analyse how the material behaves away from its point of failure, or next to it, to make material properties and mechanical behaviour easier to clearly define.

While DIC?is unlikely to replace traditional extensometers – as these are often required by test standards – it will be increasingly useful for R&D departments that need to carry out analysis or investigation on different materials, to more fully understand a materials behaviour, and identify any potential issues prior to production.

Justin Cunningham

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