In this hot summer, Brett Davis' remark about the unlikelihood of pure shear
in nature may trigger some discussion on our geotectonics list. Let us not
forget that this discussion goes back to J.G. Ramsay's mindful suggestion
that STRAIN COMPATIBILITY EFFECTS ("How on Earth do you terminate pure
shear zones?") would tend to preclude pure shear deformation. My own
favorite thought: if God does not throw dice in physics then why would we
settle for just pure shear in geoscience? Does not the required presence of
delightful deformation gradients mean we actually have all kinds of 3-D
strains out there? So why this persistent interest for just one type of
strain out of the endless spectrum? The only true scientific justification I
can think of is that "pure shear" provides a useful soundbite that may lure
students into thinking about strain in the first place.
I don't know what serious purpose these online blurps may serve, but if you
are interested in the topic, Falk Koenemann's suggestion that simple shear
would require 30% less energy made me think of my own detailed work on
strain paths. Figure 9 in J. Struct. Geol. 1991, vol. 13, no. 9, p. 1073
quantifies how simple shear actually requires more time to establish than
any other type of plane strain (assuming all physical quantities being
equal - viscosity, strain rate, stress - you choose). This conclusion holds
for isotropic viscous materials, where pure shear is clearly the most energy
effective. However, this conclusion reverses when an anisotropy fabric
develops in the shear direction (Figs. 7 in J. Struct. Geol. 1993, vol. 14,
no. 6, p. 731). The anisotropy fabric slows down pure shear and hastens or
favors simple shear flow. My analytical work shows that you can't talk about
30% or any other fixed percentage disparity between the two, because the
difference in energy required actually increases with the amount of finite
strain (a runaway effect) and, of course, the intensity of the anisotropy
factor.
>From later work I have seen published, I am afraid that the graphs mentioned
have been poorly understood, because they represent a level of quantitative
accuracy which has not been matched yet in other work, as far as I know.
Take a fresh look at these graphs to salt your opinion on this summer's hot
topic!
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-----Oorspronkelijk bericht-----
Van: Falk H. Koenemann <[log in to unmask]>
Aan: [log in to unmask] <[log in to unmask]>
Datum: woensdag 4 augustus 1999 23:30
Onderwerp: Re: Your very best students with great research potential
Brett Davis schrieb:
> Does pure shear exist anywhere in the real world? I have a running battle
> with one of the geos here, where I argue that it is extremely, extremely
> rare.
>
Experiments have shown that plastic simple shear requires 30% less energy
than pure shear. So why would you expect pure shear to be common?
_______________________________________________________________
Falk H. Koenemann Aachen, Germany [log in to unmask]
http://home.t-online.de/home/peregrine/hp-fkoe.htm
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