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Poisson’s Ratio and Shear Stresses

February 22, 2012
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Warning: if you’re not a Mechanical, Matieral, or Civil engineer this might make absolutely no sense because I’m going to talk as if you have prior knowledge of the two concepts in the post title. Sorry. I had an epiphany.

So, I’m a Civil engineering grad student and one class I’m taking this semester is on composite materials, specifically fibrous materials (like fiber glass and such). Lately we’ve been studying single ply laminas (only one layer, with fibers running in only one direction) and how applying stresses at different angles creates different amounts of strain. Just like BIAS FABRIC.

Bam. Molly’s mind = Blown.

Fabric cut on the bias or sewn on the bias is reacting to a shear stress (pulling) applied to it, causing it to strain (or stretch) and due to Poisson’s ratio at a 45 degree angle the fabric will contract in the perpendicular off-axis direction (or perpendicular to the true bias)!

You have no idea how excited I was when I thought of this. And again, I’m sorry if you didn’t understand a word I just said, I just really had to get it out of my system because none of the guys at school would have known what bias cut fabric is… It’s like I live in two completely different worlds.

~Molly

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7 Comments leave one →
  1. Pedro permalink
    February 22, 2012 7:54 am

    Interesting, interesting… Yes.

    • February 22, 2012 8:56 am

      Amazing isn’t it? I knew you’d understand

  2. scooter permalink
    February 22, 2012 9:24 am

    I’m not an engineer (though I certainly am surrounded by them), but I have wondered: if stretched at a non-true bias (say, a 30 degree angle to the grain), does the contraction still occur exactly perpendicular to the direction of the stretch, or is it off-perpendicular because of the grain direction?

    What does Poisson’s Ratio (which I have never heard of!) have to say?

    • February 22, 2012 10:10 am

      I’m not sure it would be perpendicular if it wasn’t on the true bias, I think that’s why it’s so important to get your fabric perfectly aligned on the bias otherwise the stretch will be all wonky.
      As for Poisson’s ratio, it’s basically the ratio of the amount of contraction (or expansion in the case of styrofoam) in the perpendicular direction over the amount of expansion in the direction you’re pulling. That’s probably as best I can explain it.

  3. February 22, 2012 11:03 am

    I don’t know squat about engineering, and even the engineers I know are all computer/software ones, but I love that you thought about bias-cut fabric as a direct application! As a science girl, I can appreciate that kind of geekiness. And yes, sometimes I feel like I’m in two different worlds. One time my students came in after school and instead of cleaning up their AP Chem lab, I was drafting a bodice sloper…

  4. February 22, 2012 6:20 pm

    Oh it makes sense, its just not that interesting without pretty pictures of 1920s bias dresses *nudge nudge* 😄

  5. February 26, 2012 12:48 am

    Geekiness is very cool, especially when applied to fabric. Love that your brain was blown – just hope it returned to normal, um, orientation, shortly thereafter. With stupid grin on face of course.

    PS amazingly it does make sense to me so well explained :o)

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