Compression of UHMW sheet in bend

[markbannister]

We are bending a 12"x5"x.125" UHMW extruded sheet around a 4.5" radius. When we do so, the sheet seems to compress and the overall length decreases by 1.8%.
Is that really what is happening or is there something obvious we are missing?
If it is compressing, over time will the material creep and grow any significant amount?

Details:
Once bent the 12" dimension forms almost a half circle.
We restrain it firmly to shape as it is quite stiff.
We see the same compression even when we have heat formed the material to near net shape prior to fitting (low heat on the outside face of the sheet).

 

[rickfischer51]

How are you bending it? Cold mechanical bending can be done in a slip roll, between a couple of dies, or perhaps by twisting the ends. Adding heat would primarily reduce load required to form and reduce springback. when a beam is bent, the inside is in compression, but the outside is in tension, and the center is at zero stress, save for a small shear in thin sections. In the theory, the center plane, called the neutral axis, does not change length. Could this be due to friction with your forming tool? Try lubrication the dies. You may not want to do this in production for contamination reasons, but try it as a diagnostic exercise to see if it is the problem. Also, if you heat from the outside while bending over a cold mandrel, the mandrel will act like a heat sink, and the part will never be heated to a uniform temp. Try preheating your tooling, then heat the blank from both sides. Again, maybe not practical in production, but perhaps useful in diagnosing your problem

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[rickfischer51]

Also, try annealing the stock before you cut the 12" length.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[markbannister]

When we bent it just used heat on one side and let it curl from the stress.
But the part is smaller whether we pre bend or not.
To install we just strap it to the curve.