[peppercorn (OP)]

My thinking is that frozen wood is more brittle & the the ice formed would potentially fracture the cell walls and other areas.

Clearly the concussive forces of the flail would start to heat up the wood, but assuming a constant low temp was maintained would there be a significant saving in mechanical energy needed?

[LeeE]

I think it would depend upon exactly how cold the wood is.  Many trees seem to survive ok after being frozen in winter, so unless it's super-cooled i.e. quite a long way below 0C, I would expect it to require more energy as you're not only having to break up the wood but also the frozen water.  Mind you, and not being a tree scientist, I think it's possible that living trees may contain little free water as it'll be bound up in the sap, which probably has a relatively low freezing point.

Even then though, wooden structures that are periodically frozen don't seem to suffer over the short term from the freezing (they probably will over the long term, but then structural wood will eventually decay over the long term anyway).

[peppercorn (OP)]

No idea about the freezing point of sap, but I think quite alot of water is in the cell walls (or something!) and sap is only just under the bark isn't it.

I imagine living trees are going to take lower temperatures before ice forms due to the transmission of water up the tree. Cut wood full of ice crystals is definitely more brittle than normal, just trying to get an idea of energy needs.

[chris]

Interesting discussion, which I just stumbled into and it got me thinking...

One of the demos we do on the Naked Scientists stage shows is to immerse flowers and rubber gloves in liquid nitrogen to demonstrate embrittlement. The tissue freezes and also the polymers become unyielding so they fall apart much more readily rather than deform. Cold tissue will therefore definitely chip more readily rather than just deform but may need more energy to do so...

Because, as alluded to above, wood contains water in the xylem tubes, which are in the woody core of a trunk, and also sap in the phloem tubes, which sit beneath the bark. If you freeze wood then the water in both of these places will also freeze; my gut instinct is that this is likely to be harder to smash up than the non-frozen equivalent, so the chipper will consume more energy, not less.