reveal zones plastic deformation steel structure

Is "polish and coat with a hard brittle varnish" the answer to this question?
I've made a "square-cornered U" of RHS100x50by8thk (4"x2"by5/16"thk.).
45degree mitres full-pen'd for the two corners. About 150mm (6") between the parallel Rectangular Hollow Sections (RHS) heading towards the open end of the "U". Lying flat, presents the 100mm (4") faces of the RHS (obviously(?!)). It's about 490mm long (about 19") overall.
This was for practicing various skills and getting used to the facilities & equipment - plus the following intention...
I'd like to put a hydraulic cylinder / hydraulic jack toward the open end of the parallel sections forming the "U", and see what happens when it's increasingly loaded.
Precautionary note - one assumes it would be a good idea to lash the hydraulic cylinder with rope to constrain it if it were elastically ejected from its original position...
If things go well and forces pass those necessary to cause yielding - how would one reveal where that plastic yielding is happening? I've heard of polish surfaces smooth and apply a hard brittle lacquer... If so, exactly what type? Economical preferred - this is very rough-and-ready and my own interest. Nail varnish sounds alright - but might not be the cheapest.
I've finite-element'ed the thing and get a prediction of about 8 tonnes force near the outer "open" end to reach general yielding. More accurately - at 250mm from the closed end, the thing should caliper as having grown by 3.9mm across the overall width at the moment at which it starts to yield.
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"Richard Smith" wrote in message
Is "polish and coat with a hard brittle varnish" the answer to this question?
I've made a "square-cornered U" of RHS100x50by8thk (4"x2"by5/16"thk.). 45degree mitres full-pen'd for the two corners. About 150mm (6") between the parallel Rectangular Hollow Sections (RHS) heading towards the open end of the "U". Lying flat, presents the 100mm (4") faces of the RHS (obviously(?!)). It's about 490mm long (about 19") overall.
This was for practicing various skills and getting used to the facilities & equipment - plus the following intention...
I'd like to put a hydraulic cylinder / hydraulic jack toward the open end of the parallel sections forming the "U", and see what happens when it's increasingly loaded.
Precautionary note - one assumes it would be a good idea to lash the hydraulic cylinder with rope to constrain it if it were elastically ejected from its original position...
If things go well and forces pass those necessary to cause yielding - how would one reveal where that plastic yielding is happening? I've heard of polish surfaces smooth and apply a hard brittle lacquer... If so, exactly what type? Economical preferred - this is very rough-and-ready and my own interest. Nail varnish sounds alright - but might not be the cheapest.
I've finite-element'ed the thing and get a prediction of about 8 tonnes force near the outer "open" end to reach general yielding. More accurately - at 250mm from the closed end, the thing should caliper as having grown by 3.9mm across the overall width at the moment at which it starts to yield.
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You would have to test the nail polish to find the elongation that cracks it. In my experience, using it to mark wires, electrical connectors, USB flash drives etc, it's not brittle enough.
Try to buy it from a cute young clerk with a sense of humor. They don't initially know what to make of a dignified and respectable-looking man asking for cheap nail polish in all the rainbow (resistor code) colors.
I'd consider lightly scribing a grid on the metal and measuring it after stretching the frame beyond its elastic limit, where the hydraulic pump handle stops becoming progressively harder to pump. I you have a set of machinists' parallels you could leapfrog them down the frame to keep the line spacing accurate.
I added a tee and pressure gauge to my hydraulic ram kit, and calibrate the rams with a second-hand Tuffaloy spot-welding force gauge. That gauge is presently displaying blade tension on my bandsaw sawmill. I couldn't do much mechanical experimentation without tension and compression gauges that measure up to 1000Kg or more.
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On Sunday, November 15, 2020 at 2:14:47 PM UTC-8, Richard Smith wrote:

...

I'd think, in testing-machine fashion, you'd want to step the pressure (good hydraulic gage required for that) while watching a machinist's dial gage on the deflection. You know what direction it moves... no need for a two-D grid, and buckling would require 3-D before-and-afters. Do multiple pressure trials, relieving pressure and looking for permanent deflection.
I'm not sure I could get full penetration on those internal corners, but maybe cut-a-slot/insert-a-wedge in two or three places could make a U with continuous inner metal, all the welds being under some amount of compression. You'd want to hot-work the bends, of course.
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Thanks for responses. Things moved on a long way quickly.
This is the write up about what I was then in the early stages of... http://www.weldsmith.co.uk/tech/struct/201113_u_rhs/201120_U_RHS_make_analyse_test.html
This one gets testing conditions directed at the test weld http://www.weldsmith.co.uk/tech/struct/201123_wssb_test/201123_weld_RHS_beam_test.html The Lu..ders Bands spall the mill scale - "witnesses" the deformation penetrated around the weld. Found what I wanted... Rich S
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