Composites, Strain Gauges and FE Models

[gwolf]

I have a small carbon/epoxy structure upon which multiple strain gauges have been placed. The locations and orientations of the gauges were designed using FE (not by me) to detect and identify a large number of natural frequencies up to a few KHz. There is a single gauge rather than a rosette at each location and the gauges do not line up with the fibres on the surface. It will be necessary to accurately assess strain amplitudes.

How should I relate the strain gauge results to fibre axial strains and the FE model?

 

[GBor]

gwolf,

There are some transformation equations that you can use. Most, if not all, composites textbooks will have these within the first couple of chapters. You may also be able to find them at

http://www.materials-sciences.com

This site also has some online calculators that may transform the strains for you. The author of these calculators is Gerry Flannagan...a VERY capable composites analyst. Matrials Sciences used to be owned by Walt Rosen, if that name means anything to you.

 

[GregLocock]

One option might be to add dummy strain gauges to your FEA model, with practically zero stiffness and mass. We do a similar thing in a similar situation.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[johnhors]

The original intention of the single strip gauges was to measure frequencies, which they are very capable of doing.

However for strain (and thus stress) measurements, unless you are absolutely sure that your single strip gauges are in a uniaxially stress field and inline with the stress, then only rosette gauges will give you a true reading of strain. Using single strip gauges in a non-uniaxial stress field is meaningless.

 

[gwolf]

Thanks guys, I'll look up the equations.

Johnhors - I agree that using a single gauge it is impossible to know the full shear state of the model. However, I have an FE model so if I can match up the one gauge to test I can use the FE model to fill in the blanks.

 

[crisb]

Gwolf... following on from your comment above but not knowing anything about your model!....would an option be to compare FE strain with the (strain gauge) strain in the direction of the strain gauge, then use this ratio to adjust the (strain gauge) strain in the direction of the fibres?

 

[GregLocock]

Um, if you align your dummy strain gage so that it measures the same strain as your real strain gage you don't need a rosette.

Remenmber your first priority is to get correlation, not measure the maximum strain at the strain gage's location.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.