Should you worry about bending your wires? (A response to Bare Essentials)

For those who are unfamiliar,"Bare Essentials: Bras" is a book by Jennifer Lynne Fairbanks, which is most known for its unique bra drafting techniques which allow you to make a custom bra draft based on your measurements. I won't be talking about those in this post - the book is expansive and also covers bra construction, pattern alterations, and information for developing your custom pattern into a few different styles. 

When I was writing Part 3 of the Bra Physics series (which is primarily about the physics of underwires), I recalled a passage from the Bare Essentials book which discussed underwires, stress and strain and fatigue, and upon rereading I found it draws some conclusions that I don't agree with from a materials/engineering point of view. I think these issues stem from a minor (but common) misunderstanding of stress and strain, so I hope this post helps clear up these issues for some of you as well, as well as to potentially allay any fears you may have about damaging your bras.

As before with my response to Monica O'Rourke Bravo's physics lesson, I do not mean any disrespect to Fairbanks or her knowledge of lingerie design, I just think that in this particular chapter she misses the mark slightly and I'm very passionate about science and engineering education and communication.

The excerpts in this post are from "Adjusting Patterns for Wire Spring", page 78-80 (Third Edition).

What is a Stress-Strain Curve?

Stress-strain curves are diagrams that engineers (among other people) use to compare the stress and strain properties of materials. 

To recap, stress is the force transmitted through an object divided by its area

strain is the amount of deformation that an object has undergone

 Diagram demonstrating how forces on an item cause stress and strain
Now, for a given material the amount it strains, given a certain stress, is specific and replicable, since stress and strain are independent of a material's shape and size. What this means is that, if we take enough measurements, we can plot a graph and say "for a certain type of steel, it will strain by 0.01 for every 100N/m^2 we put on it". This graph we've made has stress on the y axis and strain on the x axis. Fairbanks includes a nice example of one in her book, which I have added below.


Stress-Strain Diagram from Bare Essentials: Bras
Fairbanks' demonstration of stress and strain - ignore the dotted line for now. From Bare Essentials: Bras - Third Edition p.78

In this graph, we can see that for a certain amount of stress, up to that first bar, the curve is labelled "elasticity". This is referring to the deformation of the wire being elastic while under that low amount of stress. The second part is labellled "plasticity", meaning the deformation is plastic - i.e. with a high amount of strain the wire will permanently deform and become a new shape.  

One thing that's missing from stress-strain graphs is time. I think many people who are new to the idea of stress and strain will kind of imagine there is some element of time to it, because we can perhaps imagine a machine applying more and more stress to a material before it breaks. I think this assumption led to this passage in Bare Essentials:

"When a wire is sprung, it is considered to be in a state of elasticity. There is a point in a wire’s life, when the strain of a wire becomes greater than the stress from wear. This is when a wire reaches a point called plasticity."

I believe this passage is implying that, given a constant stress that the bra puts on the wire  when worn, over time the strain in the wire will increase until it reaches a certain threshold and stops elastically deforming and starts plastically deforming, but this isn't really how stress strain curves work.

A wire may go through its entire lifetime without ever undergoing plastic deformation, because it keeps being bent and unbent within its elasticity range. Now once you bring fatigue into it, this doesn't really apply as much and it can deform over time, but stress-strain curves do not consider fatigue. Fatigue definitely has an impact on wear and is the cause of wires breaking in almost all cases - more on this in a minute.

Furthermore, I think this phrasing is a little confusing and implies that stress and strain are antagonistic properties instead of a set relationship for any given material. Yes, past the yield strength (ie the stress required to reach the end of the elastic region) the material strains more with additional stress than before, but this is just a material property rather than the strain overcoming the stress. Due to stress-strain curves, for a given stress, you know exactly what the strain is going to be.

Does bending your wires cause them to break easily?

Many people like to bend their underwires outwards at the gore, curve them around their ribcage, or sometimes wider or narrower to better suit their breast roots. People are often scared to do this because they're worried about damaging their underwires - but should you worry?

On this subject, Fairbanks says

"In the elasticity phase of a carbon alloy wire, the wire will always return to its original resting position. Once a wire is bent past its elasticity point, the wire reaches the plasticity phase. It is at this phase that the shape of the wire changes.
Dependent on the wire’s composition, the new shape becomes either its new permanent shape with limited elasticity or the wire becomes pliable and brittle. If reshaping a wire, I do not recommend building in any wire spring, as once the wire passes into its plasticity phase, the wire can fail with very little force and will snap.
Chart A demonstrates the new elasticity of a wire that has passed into the plasticity phase.This is displayed with the dotted line. The new shape has a very limited ability to spring and not fail."
Stress-Strain Diagram from Bare Essentials: Bras
From Bare Essentials: Bras - Third Edition p.78

 Let's break this argument down to analyse it.

The first paragraph is correct: with additional stress past the yield point (what Fairbanks calls the elasticity point) the wire's shape changes. The thing that is slightly problematic to me here is that the use of the word "phase", which brings up the mental image that your material is now locked into this point on the stress-strain curve and is now destined to only plastically deform from now on. 

What actually happens is displayed on the chart but is not explained in the text: when you plastically deform a bit of steel (until it breaks, anyway), it still maintains its elasticity. If you bend a bit of steel to the end of the dotted line on the diagram, when you let go it will spring back - not to zero but to a new point on the x axis (where the dotted line returns to). This is the permanent strain that "plasticity" refers to. You can try it for yourself with a paperclip: even if you bend the paperclip enough to deform it, it will still spring back a little. 

Fairbanks then claims that the material composition of an underwire will determine if a wire becomes either "the new shape with limited elasticity" or "pliable and brittle". Either way, she says, the wires can fail with very little force.

Pliable and brittle are essentially antonyms so I'm not exactly sure what she means here - pliability implies a material which is easily bent out of shape, whereas brittleness describes a material that does not bend out of shape and instead breaks under force. I believe that she means it will not readily maintain its new shape, instead deforming easily with little force but with a little more force it will break. While it is true that steel's composition affects its behaviour, this isn't going to be how it behaves.

Work Hardening and Wire Snapping

Full Stress-Strain diagram from wikipedia
A full stress-strain curve for a material like steel. Credit: Nicoguaro, CC BY 4.0, via Wikimedia Commons

There's a little extra to the stress-strain curve that Fairbanks cut off, which I've shown here. This downwards curve at the far end is called the "necking" phase and refers to the point where the wire has deformed so much that its cross-sectional area becomes smaller and it becomes weaker - it cannot withstand even the force that got it to that stage, so it breaks very easily. If you bend your wire to this point then you have weakened the wire and it is liable to break (sorry), but this is a pretty rare thing to do unintentionally, and requires substantial deformation.

But what's that upwards curve? That's called the "strain hardening" region. Strain hardening is also known as work hardening - if you do hobbyist metalwork you have probably come across this before. A permanently bent underwire will have an initial deformation that puts it in this strain hardening region. The interesting thing about strain hardening is that it does not have limited elasticity but actually increased elasticity. This is because the new straight "elasticity" line on the stress-strain curve reaches a bit higher up than the original one, meaning it can withstand more stress before deforming plastically. 

 

Altered stress-strain diagram showing the effect of strain hardening
The purple, thicker line indicates the stress-strain of a wire that has been bent. You can see that it has a greater potential to elastically deform than its unbent counterpart, due to the strain hardening. Credit: Nicoguaro, CC BY 4.0, via Wikimedia Commons. Edited by HugsforYourJugs

Metal that is strain hardened is more resilient because it is more elastic than metal that has not been strain hardened, but at the expense of a reduced ability to plastically deform further.

What this means in the grand scheme of things is that it’s totally fine to spring a bent wire, it is not more liable to break under regular spring. A good rule of thumb for springing a bent wire is that as long as you're springing it to within the original strain you put it under to get it to bend in the first place, you're not pushing the wire further along the stress-strain curve.

Fatigue and Creep, the enemies

The big however after this, though, is the role that fatigue plays. Fatigue is the idea that, when you are applying forces to a piece of metal, it will accumulate damage over time and eventually will break. Imagine if I handed you a paperclip and asked you to snap it for me. You would probably bend it back and forth until it broke due to fatigue. Going back to bras, the more you spring a wire, the more fatigue it will undergo, and the fewer wears it will be able to withstand before bending out of shape or breaking. 

Fatigue is caused by microscopic cracks forming when you strain the wire - it stands to reason that the process of plastically deforming the wire to bend it could kickstart the crack creation process. However, there may also be a protective effect from the increase in elasticity, as well as lower elastic deformation during wear. It's not clear cut to me whether bending your wires will make them fatigue faster, and I couldn't find much relevant literature on the topic. 

One thing to be mindful of when bending your wire is not creating any kinks. Kinks will concentrate stress in your wire and will make them fatigue much more quickly. Say yes to kinkshaming!

If you're wondering why old bras often have deformed wires, even if you have only sprung them to within the elastic region, this is because of a different type of fatigue known as creep. Creep causes that slow deformation over time, even if you are careful to keep your spring within the elastic range. Creep is unavoidable and happens in all materials, essentially, but reducing the strain your bras are undergoing by choosing thicker wires will help to reduce its effects - more on this in a bit.

Why Do Wires Break, Then?

In my experience, if you are frequently breaking your wires, there are three main causes: 

  1. Improper care
  2. Poor fit
  3. Unsuitable wire gauge

The first is easily remedied - avoid the tumble dryer, as this can bend your wires in unpredictable ways, causing the dreaded kinks mentioned earlier. It also will prematurely age your bra, contributing to poor fit.

"Poor fit" is a pretty nebulous term, I'll grant you that, but there are so many issues that I couldn't cover them all here in much detail. Suffice it to say: if your wire is sitting on breast tissue, it will bounce around with movement and fatigue a lot faster. If your wire is oversprung from the band being too tight, it will fatigue a lot faster. If your wire is sliding down over the course of the day, it can fatigue faster. If your gore is floating, again the wire is going to be shifting a lot more with wear. The main takeaway is that your wire should be as stable and as unmoving over the course of the day as is comfortable for you.

Which leads me on to point three. In hobbyist bra making, I've noticed that the vast majority of wires are much softer than typical UK/Polish DD+ brands use. If you are a large busted person who hasn't experienced a well fitting ready-to-wear bra, you may be surprised at just how different they are. The heaviest gauge hobbyist underwires I know of are made by Porcelynne, these wires remind me of Panache in terms of their strength. If your wire is too light a gauge, it will undergo a lot of elastic deformation and fatigue faster, as well as being more likely to float at the gore.

Conclusion

So while, in my view, you shouldn't worry about your wires breaking imminently after bending them, if you're frequently breaking them it's more likely to be one of the three reasons listed above. So make sure you're wearing an appropriately strong wire, get the fit right, care for your bras properly and wire breaking shouldn't be a problem for you.

In the extended edition of this blog post, I go on to discuss some further points relating to the 3D nature of wire bending and how it affects the issues of stress, strain and fatigue when it comes to a wire's lifetime. If you would like to, you can read it - and all other extended editions of my posts - for just £2 on my Ko-fi.

Comments

  1. Thanks for this detailed analysis. I'd wondered about this section in Jennifer's book. My DH is a boilermaker, and bends major structural components of heavy duty machinery for a living. In order to control the effect on the performance of the bar or rod he is bending to change its shape, he also heats it, and cools it under controlled conditions for specific results in terms of the properties this creates. In his work, bending without doing this is mostly undesirable. Does any of this apply to changing the shape of a bra wire. And can I safely consult him (or better, get him to do it) if I need to alter my wire ðŸĪŠðŸ˜ƒ

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    Replies
    1. Hi!
      The process of heating and cooling it is called annealing and it essentially resets the crystalline structure of the metal. There's definitely literature to support that when working under extreme loading, especially with things like steam, work hardening reduces the lifespan. Additionally, when making things like pipes you're bending the metal a lot more than, say, wrapping a wire around yourself (which is what happens during wear anyway).
      Like I said in the post there is a potential theoretical protective effect to wire bending, but I cannot say for sure whether this really exists or not. Annealing your wires would remove any impact on fatigue that comes from the bending process, protective or deleterious, but would also reduce the elastic deformation of the wires during wear (presuming you're bending them to have that effect), increasing lifespan.
      If you have access to it, then it sounds like it might be worth a try, but really your wires should be lasting much longer than the rest of the components of your bra, so it probably won't be worth it - unless you want to reuse your wires for as long as possible.

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