Bolt (hexagonal) too tight on crank/chain case

[RichardNL]

The bolt on top of the crank case, connecting to the Crosspiece bar, is too tight. This means I cannot open up the case for repairs/maintenance

Even when I apply great strength, it is useless: does not move a bit. To apply this much force only results in breaking the hexagonal 'hole' in the bolt, thus losing grip with the hexagonal wrench.
For a "second opinion", I've taken the A-bike to a 'regular' bicycle shop. They tried it with both a hexagonal wrench and a regular wrench/spanner, but with no result.
Now I don't know what to do... Any ideas?

Update: finally I managed to get it to turn, with a 'Mole' wrench (Locking pliers) and now rather sore hands
But the nut/ring rotates with it. And it still won't come out. So there's yet another challenge for me to overcome...

[rongood]

Try soaking it in acetone, or just apply a few drops and let it stand. You can find it in hardware stores or just buy a small container of nail polish remover, which is basically just acetone. You can also try WD-40 or similar products. The purpose of the acetone is to dissolve the "lock nut" compound the factory used to make sure the bolts stay tight.

[RichardNL]

Thankyou Ronald, for this tip! Actually I had bought a WD-40 "pen" some time ago (for squeaky hinges on doors). I believe it worked - as I got that bolt out, finally!
The bolt itself looks heavily 'abused' by now (because of the brute force applied ), so I'll get a replacement later on.
But I can open the crank case, to fix my A-bike.