Monday, June 15, 2015

Stresses and Strains When Sewing

In this weeks' tutorial on the Betties blog, I'm going with a pretty nerdy theme - stresses and strains when sewing.

The effect of stresses and strains on materials have been a consistent theme for me for the past 15 years - during my degree in Materials Engineering, my PhD in Materials Science, and my time at an engineering software company.

So, it isn't surprising that I would spend a lot of time considering how stresses and strains affect my sewing.

What are stresses and strains?

From a strictly theoretical point of view, stress is pressure, or force per unit area.  Strain, on the other hand, is how the object stretches due to stress.  All materials experience some strain when put under stress - steel, titanium, carbon fiber, or diamond.

You'll never see stress without some strain, but for the sake of this tutorial, we'll consider just the stresses.

Where do the stresses come from when I'm sewing?

Consider what is happening to the fabric when sewing:




The feed dogs are pushing the fabric back.  The foot is pressing downward.  You may be twisting or pulling as well.


This creates shear stresses, like when you rub your hands against each other when it's cold outside.  The shear stresses are particularly strong in the top layer of fabric.



 What is happening to the fabric?

Let's consider how a fabric could react to stress.  Look at a single thread, pulled along its axis, called tensile stress:


It will stretch a bit while under stress, but after, it will return to its original shape.  That means no permanent strain (or stretching) has occurred.

Alternatively, let's consider a string under shear stress:



In this case, the string shifts with the stress, and remains in that shape afterwards.  So, there has been some permanent (plastic) strain.

Now let's extend this to woven fabric.  Consider sewing along the grainline, which means the stresses are pulling along the threads any stresses will stretch the threads parallel to the seam line, but they will return to their original shape afterwards.

 
No consider the bias of the fabric, diagonal to the grainline.  In this case, when we apply a stress or create a seam, it acts like when we applied a shear stress to the thread.  The string bends to accommodate, and doesn't return to its original shape afterwards.


 
Hopefully, it is now obvious why we have to be so careful when we sew along the bias.  We are applying shear stresses to the fabric, which bends the threads.

How do we fix this?

Obviously, the easiest solution is to always sew on the grainline.  And I am sure that some people look great in potato sacks.  Meanwhile, for the rest of us, I have 4 categories of solutions (I like categories).

1. Put the stress in something else

Interfacing or stay tape are designed to absorb stresses the way the grain absorbs it - elastically, with a full recovery afterwards.


2. Stiffen the threads

Consider shear stresses on a fiber which is much stiffer than thread, such as carbon fiber:

This time, it goes back to its original shape after the stress is removed.  I don't think carbon fiber would be particularly comfortable (or cost-effective) for clothes.  However, we can chemically stiffen the threads in our fabric using laundry starch.

Before I start sewing, I usually soak the fabric in a starch solution, air-dry it, and then iron the starch to make the fabric as stiff as possible.

When dealing with a seam where the fabric may slip and slide, I'll do a spot-spray with my spray starch.

Yes, I have to wash the starch out before I wear the clothes, but I think that is a small price to pay for good seams!

3. Mechanical intervention

Overlockers /sergers or a walking foot on a regular machine will pull the fabric along the top and bottom, eliminating the problem altogether.  I always use my walking foot when doing topstitching.

4. Roll with it

You can also make the stresses work for you. If one side of a seam is a little bit longer than the other, then put the longer side on the bottom, so that the top stretches to fit it. If sewing a convex curve to a concave (such as a princess bust), put the concave (shorter seam allowance) side on top.

I hope that understanding where these stresses are coming from, as well as knowing some coping mechanisms, helps take the stress out of your sewing!

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