On Aug 10, 5:42 pm, William M Connolley <[EMAIL PROTECTED]> wrote:
> On Fri, 10 Aug 2007, Eric Swanson wrote:
> >> This is unlikely. Sea ice has essentially no strength in tension anyway.
> > I think their point about the permanence of landfast ice is important,
> > since we know it exists and isn't easily dislodged. This suggests
> > that the tensile strength of sea-ice away from the coast may also be
> > important.
>
> The common model used in GCMs is viscous plastic (or more primitive). Or the
> more modern elastic-viscous-plastic, which is supposedly scientifically
> equivalent (look up Hunke). Elastic-plastic is rather different and certainly
> not used in GCMs.
>
> On the resistance to motion, is doesnt matter a great deal how thick it is,
> in the sense that acceleration can usually be neglected. But clearly it
> affects
> its resistance to compression or shear (but the pure tensile strength remains
> negligible).
Having been educated as a mechanical engineer, I learned a bit about
structures and stresses. I've also done a small bit of work with
fatigue failure. That said, I think it's obvious that ice does
exhibit tensile strength, all thought how that applies to sea-ice, I
can't yet say.
An obvious example is the fact that vehicles can and are driven over
frozen lakes. There are even people that hold races on the frozen
lakes. The weight of the vehicles produces a tensile load on the
lower surface of the ice. If, as you claim, "tensile strength remains
negligible", then the ice would fail and the vehicles would go to the
bottom of the lake. Furthermore, the ability of the ice to support a
load is a direct function of it's thickness.
>From my limited knowledge, I would expect that the sea-ice would be
anything but homogeneous. The thickness would be highly variable,
judging from some photographs I've seen taken from below the ice.
Under such conditions, the ice would fail at the weakest point, i.e.,
where it is thinnest. Using an average thickness would give a
confusing picture of what would actually be happening in a failure. I
would guess that tension failure might be similar to a fatigue failure
in a metal, which begins with small crack which slowly increases in
size, which, in effect, reduces the cross section of the material
subject to loading. In a fatigue failure, the apparent load based on
the initial dimensions is much below that which would have caused
failure before crack formation when a part is new. That does not
imply that there is no tensile strength.
If one has been in a lab and seen the failure of a material placed
under a controlled loading, one quickly notes the speed with which the
final failure occurs. The stress builds up as strain and when the
failure happens, there's a sudden release of the load. Such failures
can be very spectacular, such as the bridge failure in Minnesota last
week. I viewed a video of that collapse the other day. The camera
showed things looking quite normal, then, the center section dropped
into the water in a few seconds. Earthquakes are another example of
the effects of a stress buildup followed by failure, with spectacular
consequences.
Sea-ice dynamics may be analogous to that of earthquakes, in that the
stresses are spread over a large area, but the failures are local.
The appearance of leads and polynyas is said to be rather sudden. The
breakup of river ice and lake ice in spring can also be very sudden.
My point is that we are likely to see such a breakup of Arctic sea-ice
in some sudden fashion, as the extent and thickness continue to
decline. That's not to say the sea-ice would then instantly disappear
afterwards. The sea-ice could re-freeze into another continuous
sheet, if the temperatures were low enough. Or, the shattered sea-ice
could become more mobile, as apparently happened last year with the
large polynya to the north of Alaska.
I'm sure that these ideas is not new and I will need to do some more
study to catch up with the present understanding. I do think that the
models could benefit from some representation of a threshold failure
mode based on thickness between elastic and viscous behavior. Whether
this sort of approach is presently included, I can't say. Time for
more reading...
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