- Apr 2, 2020
- Reaction score
- Sunderland , UK.
K2, do you have a reference for this data? I've never heard of a metal having compressive strength that is 20% of the tensile strength. Metals (especially ductile metals like copper) have nearly equal tensile and compressive strengths, in my experience. I'd like to learn more about this phenomenon.
Cylinders collapsing when they "shouldn't", on the other hand, I have lots of experience with. That happens because they don't fail in yield; they fail under elastic buckling. And elastic buckling is very sensitive to geometric imperfections.
Anyway, if you can point me in the direction of that strength data for copper, I'd be grateful.
I'll try and show some stuff...
Copper reduces strength with temperature increase... e.g. at 200C compared to 20C. :
But this data table quotes Compressive strength and Tensile strength ranges...
In this table, the compressive strength of copper is defined as a minimum of 45MPa. And a max of 330MPa. These figures are for everything from fully annealed to hardest condition at room temperature. (20C).
The Tensile strength is quoted as 210MPa to 380MPa - under the same conditions:
Therefore I deduced that the "worst case" for Annealed copper is the compressive strength of 45MPa in the same condition as tensile strength of 210MPa. : I.E. Compressive strength of 21.4% of the Tensile strength.
This correlated with the University graph I had previously studied where the Compressive strength versus Tensile strength at 200C followed a similar value.
ASME Code Table 1B of Section 2, part D, requires the "limiting stress" in tension at 400deg.F. to be 3000psi.
Considering the "Compressive strength" (worst case at 21.4% of Tensile strength) I therefore use the limiting factor of stress for tubes in compression to be 898psi.
Of course, you will note that I have not considered any variation of tube wall thickness, deformation, ovality or other factors that are major contributors to failure of tubes in compression (Uniform external pressure). But I do consider the ASME Stress Concentration Factor "standard value" of 3.3 whenever there are any penetrations of the tube - such as a firing hole, hole for cross-tubes in a flue or fire-tube, etc. as every discontinuity of stress (such as a hole = weakening, or reinforcement = strengthening) becomes a focal point where compression failure is likely to be initiated.
I agree that "once initiated" the tube fails in Buckling. It happened to me on one Hydraulic test, where I was going to 4 x NWP, and the tube collapsed at about 3.8 x NWP. My pre-test calculations had used the "tensile strength limit" for hoop stress, so I should have been OK for more than 8 x the NWP, but I suspect a deformity of the tube initiated the failure. However, that is when I found the "Min Compressive strength" to be so much lower than the tensile strength... and believe this to be significant. (Is it? - I really am not sure if I am right or wrong?).
But a "better educated Engineer" than I can explain if I am right or wrong? (I have a Physics degree, which is NOT the same as an Engineering degree!). I am glad of your expertise and input. (I sometimes confuse "Apples and Bananas" on subjects where I am not an expert!).