Advances in Extensometry

Things have come a long way since the original extensometer by J. A. Ewing (c.1890) which was screwed onto the specimen and manually read from a Vernier scale; now we have the ability to continuously and automatically measure strain without touching the specimen, but achieving better than micron resolution at up to 0.5m/s!

The latest video extensometer is capable of providing live strain measurement with only a few milliseconds delay, meaning that it can be used for highly dynamic tests, including direct strain control on the specimen.

Strain Measurement & Control for Fatigue

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Using the latest non-contact strain measurement provides new potential for fatigue testing of composite materials, by making continuous measurements of global strains throughout every cycle. These can include:

• tracking strain without specimen damage
• measuring transverse strain dynamically
• active control of test waveform by gauge length strain

Any or all of these capabilities can be used to provide more insightful data and more repeatable tests – key challenges for generating useful studies in composites fatigue.

Strain Rate Effects

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It is widely accepted that strain rate affects the behaviour of all materials, but that polymer matrix composites tend to be especially sensitive.
Taking the example of a carbon fibre reinforced polyamide compound for injection moulding, video extensometry allows observation of the changes in stress-strain curve, which would be challenging to measure with a clip-on extensometer at anything above the slowest speed.
Clearly quasi-static test data is a poor point of reference for most practical fatigue tests, where even small stress levels and moderate frequencies the strain rate can be greater than 5%/s.

 

This article is a summary of a paper published by Dr. Peter Bailey and Maeve Higham.

Read the full paper provided by PROSTR (Procedia Structural Integrity).

doi:10.1016/j.prostr.2016.06.017