Hi Toymaker, I see a tooling mark along the tube which I suppose comes from bending process.
Could be stress from bending tool and wall thinning (in that area seem to be very high contact forces and material push outwards) the reason for failure happening in that neutral fiber (or close to) of tube?
But other than that, bulging looks quite uniform.
Good catch NapierDeltic. The die marks you see on the tube are created by the rolling process and I believe are due to the groove in the die being over-size for the tube. I machined all the dies myself and sized them for use with 0.625" (5/8) after I decided to use 5/8 in my boiler, but before I actually had any of the tube in hand. In retrospect, I should have waited. Tubing sold in Thailand as 5/8" (16mm) is neither and actually measures between 0.59X to 0.60X resulting in my 0.625" dies not providing adequate side support to the smaller-than-expected tube. I didn't even notice the issue until after I had already rolled the first 15 meter pancake coil. Given the work that goes into making these boiler coils, I decided at that point to continue.
The dies for the 8mm tube shown below bending an aluminum tube do a splendid job, because the 8mm tube is actually 8mm. You can't see the 5/8" dies very well in the photo below,...but they are indeed the problem, or the smaller-than-should-be tube is the problem,...depends on your perspective.
Back to your question: I measured a few tube scraps I have laying around and determined the ruptured area actually occurred at a wall thickening area; the wall is 0.003" thicker than in most other areas around the tube. The tool mark area is the thickest section, measuring 0.006" thicker than other parts.
The first sample tube ruptured on the outermost bend where presumably the wall is at it's thinnest, so no surprise there. The wall thickness on the outermost bend on the sample tube we're discussing measures thinner than wall areas directly adjacent to the rupture. This is a bit of a head-scratcher for me,...lots of maybes and could-have-been's, but nothing definite.
Last edited: