... this may need a little more thought.
I made some sample test pieces and a test bed, and the first thing that became apparent is that the original design is all a bit unstable for the sorts of loads that are required to load the test pieces to destruction - basically, the whole assembly will probably topple over long before you can get the full load applied, if you use my original part designs.
So I modified the designs to make it all a bit more stable, by adding some broader feet to the test bed, and making the supports on the test pieces a bit wider. I also added the option of a removable Load Platen, to allow the test load to be applied more easily to the load pad on the top flange of the test piece, if desired:
and here are the parts I made:
and here's the complete assembly (using the lighter, weaker I-Beam test piece, rather than the solid rectangular beam) sitting on my kitchen scale (so I can do some quick load testing):
and here it is with my thumb applying a 5 kg load:
You can see that the test piece is bending, but it has only deflected about 3 mm with 5 kg load, and it is nowhere near structural failure. At a guess, it is going to be able to sustain more than 10 kg of point load.
So, what's the problem? Well, 10 kg is going to be quite a pile of weights, and I am not sure how to apply that much weight to such a small load pad in a stable fashion , and still have room to get my digital calipers in to accurately measure the deflection for the various load increments. I'm going to have to work out a simpler loading mechanism which can apply 10 or 20 kg of load, while still being able to measure both applied load and the resulting deflections.
Does anyone have any suggestions?
The easiest approach would be place the test pieces across a hole or slot in the table and hang a bucket at the load point. Then you can pour sand or add discrete weights to the bucket. The center of gravity will always be below the hanger point, which should keep things more stable than trying to balance weights on top.
ReplyDeleteIf you add a spring scale strain gauge to the bucket hanger, you could even read off the weight directly as you fill.
Pure tensile samples?
ReplyDeletee.g. http://classes.mst.edu/ide120/lessons/tension/fractures/index.html
The load weight could be sat on some scales and be steadily lifted off. You might need to print a weight hanger to connect the weights to the sample.
my first thought was to put the entire assembly on a scale at the edge of your workbench, and then use a giant c-clamp to apply increasing pressure until failure.
ReplyDeletethe bucket idea isn't bad either. I'd definitely use sand, not water.