×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Composites, Strain Gauges and FE Models

Composites, Strain Gauges and FE Models

Composites, Strain Gauges and FE Models

(OP)
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?

RE: Composites, Strain Gauges and FE Models

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 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.

RE: Composites, Strain Gauges and FE Models

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.

RE: Composites, Strain Gauges and FE Models

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.

RE: Composites, Strain Gauges and FE Models

(OP)
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.


RE: Composites, Strain Gauges and FE Models

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?

RE: Composites, Strain Gauges and FE Models

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources