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wrote:

:::Jerry:::: wrote:
wrote in message
ups.com...
Hi

I have a strut on a pram that has sheared. It's at a 'springy'
point
but isn't particularly load bearing. snip

So why has it fractured ?....

Because its flexed a lot? I can understand that it takes an amount of
force to flex it, but that isn't (to my mind) the same as it being
under a compressive or tensile load. I'm happy to be corrected on this
if I'm misunderstanding the cause of failure.

It's exactly the same.
To bend a tube, you've got to compress one side, and stretch the other.
Consider that when bending a tube, you are in effect shortening one side and
stretching the other.
This may be clearer if you think about how much force you'd usually need
to use to make the tube stretch/shrink by that much.

If the ends are sort-of held in place by the rest of the structure, then
you'll get a fracture at one point, that then opens and shuts, with the
crack moving along the diameter of the pipe till it's through.