Seam Strengths

[Gerry]

I found this study fascinating:

https://apps.dtic.mil/sti/tr/pdf/ADA137383.pdf

The Canadian military commissioned the report to find alternatives to the double lapped seams used for their uniforms. It gives a list of seams in terms of their strength, and from the diagram I identified the following, from weakest to strongest:

Plain seam
Plain seam with reinforcement stitch line close to first
French seam
Lapped seam with single, central stitch line (not sure what this is called, anyone?)
Overlapped seam
Flat felled seam
Double lapped seam
Double lapped seam with wider spacing
Overlapped seam with double stitch.

Interestingly, chain stitch gives a slightly stronger double-lapped seam. People commonly mistake stitch strength for seam strength, but they're different things. Lockstitch is a stronger stitch than chain stitch, but due to its relative inflexibility the seam is more prone to ripping.

So, ironically, those wide, double-lapped, chain-stitched shirt seams on ready-to-wear that we all love to criticise are actually quite strong. Obviously if the thread breaks the whole thing unravels, but that's less likely to happen due to the flexibility of the stitch. And if polyester thread is used then longevity should be reasonable.

Personally, I've never had a seam burst on any ready-to-wear shirt that I've owned. The cloth tends to develop holes first, due to quality issues. And no, I won't be jumping ship and buying a chain-stitch machine. Interesting though.

[Schneiderfrei]

Great Sleuthing!

[Gerry]

I was surprised to see overlapped seams coming out on top, but then I've seen sailmakers on youtube using this very seam, so it makes sense.

All of this came about from a shirt I was partially dissecting yesterday. I'm used to seeing chain-stitching for the sleeve seams and often those of the sides, but this company had used the stitch to secure a sew-on, raised, front placket. Clearly for reasons of economy in this case, but it got me thinking about why ready to wear shirting companies still use these machines. There had to be something more than penny-pinching involved. If the seams were truly as weak as many think, the returns would be crippling ... and I've never had seams on jeans or shirts burst when they've been chain-stitched - the cloth usually wears thin and/or develops holes first.

[Gerry]

The merits of constructing seams with a double chain stitch have been known about for a long time. This conclusion from a 1917 report:

Tests made with a variety of fabrics, thread sizes, and types of test specimens have shown that seams made with the double-locked [chain] stitch are stronger and are less weakened by thread breaks than seams made with the shuttle [lock] stitch.

Hence their continued use today.

https://archive.org/details/comparativetest191796lewi/page/n1/mode/2up

[TSjursen]

Another factor is that the chain stitch does not require a bobbin. The savings in a factory setting are quite significant.

[Schneiderfrei]

TSjursen, yes stopping to replace empty bobbins becomes very significant.

[Gerry]

When working with 100% cotton thread, sew slow is the conclusion of this report (which opens as a download); especially with finer coths:

https://jtatm.textiles.ncsu.edu/index.php/JTATM/article/view/7575/4088

It was concluded that increase in Fabric GSM [weight] and sewing speed had exerted a detrimental impact on cotton sewing threads due to the increased abrasion of the sewing threads ... but it was observed that the reduction of all tensile properties was greater for finer counts than coarser counts; due to their lower parent thread strength and also lesser surface area to be offered to the machine abrasion.

It seems that super fast, industrial machines can cause damage when working with cotton thread. Abrasion from the needle and fabric weakens it.

[Greger]

It is better to have the threads bust than the cloth tear.

[Gerry]

It is better to have the threads bust than the cloth tear.

That's not really what the study meant. Generally speaking, cotton thread is weaker compared to polyester, so it's already more prone to breaks when stressed (as you say, not necessarily a bad thing). What they're talking about is when the thread is degraded to the point that it becomes weak per se. Speed of sewing, cloth density and weight all accelerate that process (apparently).

[Greger]

I slipped this in because it is related.
Something about hand sewing. Shorter threads are stronger. And waxed helps.
A sewing machine handles thread different.
Any comparison between sewing machines and overcasters (serger)?

[Bifurcator]

Thanks for Sharing!  This is really informative and an interesting deep dive.  As mentioned some of the findings seem counterintuitive, but that seems to be a common theme with garment making

From experience I do think the chainstitch tends to break and fall apart over (a long) time.  I've seen it in more than 1 vintage military pant I've owned.  I'm guessing partly due to it sitting on top of the fabric rather than sinking in like a lockstitch.  Though I don't know if the thread was cotton or poly, which obviously would be a big factor.

[Gerry]

From experience I do think the chainstitch tends to break and fall apart over (a long) time.  I've seen it in more than 1 vintage military pant I've owned.  I'm guessing partly due to it sitting on top of the fabric rather than sinking in like a lockstitch.  Though I don't know if the thread was cotton or poly, which obviously would be a big factor.

Yes, that was mentioned in one of the studies that I read. As you say, the chain stitch isn't embedded in the cloth so it's more prone to abrasion. And a lot of the time the chain is on the inside of the garment, so the problem is made worse due to constant contact with the skin/body. Obviously polyester would be less prone to this problem. I think it was in the conclusion of the Canadian study where the writer said that all this research needs to be updated in light of developments in thread technology (polyester etc).

I can see why the military pants would show stitch failure. The cloth used in those garments is meant to last, so the stitching would probably be the first thing to fail. As I mentioned, with a lot of ready to wear the fabric thins and develops holes long before the seams have a chance to degrade. Such is the poor quality of a lot of the cloth used in manufacturing (built in obsolescence).

Sorry Greger, couldn't find much about overlocker seams. Only a study showing their strength as a fail-safe/temporary hold should the main seam rip. Not that useful (or interesting frankly).