SAE AS81914 - Convoluted Tubing Tensile Stress

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bbarbee

I'm not sure if this is the correct sub forum to put this in, but I'm sure it will find its way to the right place.

I'm a quality engineer at a plant that makes tubing. We're qualifying some convoluted plastic tubing per SAE AS81914. There's a requirement to measure the stress at 10% strain, which I guess it to give some indication as to the springiness of the convolutions. The problem is the way the spec is written (4.7.2 "Full sections of tubing ... shall be extended to 10% strain in an ASTM D 638 tensile apparatus, or equivalent").

The problem I have is that to calculate the stress, you have to know the cross-sectional area. Convoluted tubing has a helical profile so you have to pick a diameter somewhere between the OD and the ID and assume that's the effective tubing diameter. This is confounded by the fact that the convolutions are irregular. The peaks are wider than the troughs, so you can't say the diameter is right in the middle. The peak to trough ratio is variable from one part to the next, so you can't just come up with one formula. I'd say to use a dog-bone sample like with other materials, but SAE wants that full section of tubing.

Any insights?
 
B

bbarbee

I found the answer in an old standard and figured I'd post it in case people need it in the future.

There are a couple of variables used here.

"T" is the (minimal) wall Thickness.
"M" is the Mean diameter which is (OD+ID)/2
"B" is the convoluted OD which is M+T
"C" is the convoluted ID which is M-T

A is the (minimal) cross sectional area which is 3.1415*(B^2-C^2)/4
 
howste

howste

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bbarbee said:
I found the answer in an old standard and figured I'd post it in case people need it in the future.

There are a couple of variables used here.

"T" is the (minimal) wall Thickness.
"M" is the Mean diameter which is (OD+ID)/2
"B" is the convoluted OD which is M+T
"C" is the convoluted ID which is M-T

A is the (minimal) cross sectional area which is 3.1415*(B^2-C^2)/4
Click to expand...

What was the old standard you found the information in?
 
B

bbarbee

It's a PDF I have that was scanned from an old notebook. The bottom of the page says "Mcdonnell Douglas" and the rest of the footer is illegible.
 
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