Hi All,

The trouble with the term ductile is it means many things to many  
people. Plastic also has its problems too because it can just mean the  
non recoverable strain. I too think the Kohstedt et al. (1996) paper  
is great and they are very careful to describe what they mean and you  
can see in the abstract they do not use the term brittle-ductile  
transition but it is in the figures. My favourite term for this  
transition would be cataclastic to thermally activated creep  
transition but that is a little bit of a mouthful.

Pedants unite

Julian

Quoting Mark Brandon <[log in to unmask]>:

> This continues to be an interesting discussion. Julian and Michael  
> seem to be in agreements with points raised in comments by Ernie  
> Rutter and myself.
> Julian and Michael also raise some other interesting issues. So I am  
> venturing to add some additional thoughts....
>
> First, I think it is useful to remember that all of this terminology  
> comes from literature in engineering and material sciences.
> There is no need to special definitions for geology...
>
> Second, pressure does suppress localization in granular materials,  
> but in the general case,  it is not the sole factor to control the  
> transition from
> brittle to ductile behavior (see Julian's second comment). The  
> reason is that there are many factors that can induce localization,  
> and there is
> generally no single variable, such as pressure or temperature, that  
> will inhibit it.
>
> Third, the term plastic is not a good substitute for ductile. The  
> formal definition of a plastic is a material with a finite yield  
> strength. Frictional
> materials are often called Coulomb plastics because they have a  
> pressure-sensitive yield strength. Plastic materials and viscous  
> materials
> can deform both ductily and brittlely. Viscous materials have no  
> yield strength, and will flow given any deviatoric stress. Power-law  
> viscous
> behavior, which applies to the dislocation mechanism in crystalline  
> materials, has the appearance of a yield strength (given the large  
> change in
> strain rate with increasing deviatoric stress). Individual crystals  
> have a plastic-like yield stress given that dislocations motion  
> requires a finite
> resolved shear stress on the glide plane. These two factors account  
> for why engineers view crystalline materials, such as metals, as  
> plastic
> materials, because on human time scales they appear to have a finite  
> strength.  That said, the viscous behavior of crystalline solids is  
> clearly
> apparent on geologic time scales.
>
> Best,
> Mark Brandon
>
>
> On 7/30/10 9:53 AM, Michael Stipp wrote:
>> Hi Julian,
>>
>> great, many thanks! I thought about making the same comment on the wrong
>> use of the term "ductile". But one word in favor of Dave Kohlstedt and
>> co-workers and their great 1995-review paper: They clearly explain it in
>> their text, brittle-ductile denotes the change in deformation mode and
>> brittle-plastic the change in dominant deformation mechanism. And they
>> refer to Ernie's paper from 1986. So, the use of "plastic" and "ductile"
>> does not seem to be so difficult. Nevertheless, many people seem to
>> ignore it, even when it is corrected in a review.
>>
>> I always use the term plastic shear zone as synonym for mylonite zone,
>> but this will probably raise the next discussion round on terminology.
>>
>> Best wishes
>> Michael
>>
>>
>>
>> Julian Mecklenburgh schrieb:
>>
>>> Dear All,
>>>
>>> Prompted by the recent emails about Shear Zones. I am always surprised
>>> about how much incorrect use words such as ductile is about in the
>>> literature.
>>>
>>> <rant>
>>>
>>> Ductile is a non-localised distributed deformation it says nothing
>>> about the deformation mechanism
>>>
>>> Brittle to ductile transition is the transition from localised to
>>> distributed deformation with an increase in pressure.
>>>
>>> The transition from Cataclatic deformation to thermally activated
>>> deformation with depth in the Earth is not the brittle to ductile
>>> transition. But cataclatic to thermally activated transition is a bit
>>> of a mouthful so what about other options. The suggestion of the
>>> seismogenic transition is ok what about brittle-plastic transition or
>>> brittle-viscous transition. There is no consensus on this apart from
>>> that most people incorrectly use the brittle-ductile transition. Even
>>> Kohstedt et al. (1995) get it wrong (see fig1).
>>>
>>> Ernie published a paper on this in the 80's but everybody seems to
>>> ignore it. See Rutter (1986).
>>>
>>> I do not like the term ductile shear zone as it is an oxymoron ductile
>>> implies distributed deformation but shear zone implies localised. A
>>> much better term would be plastic shear zones.
>>>
>>> </rant>
>>>
>>> All the best
>>> Julian
>>>
>>> KOHLSTEDT, D. L., EVANS, B.&  MACKWELL, S. J. 1995. Strength of the
>>> lithosphere: constraints imposed by laboratory experiments. Journal of
>>> Geophysical Research 100(B9), 17,587-17,602.
>>>
>>> RUTTER, E. H. 1986. On the Nomenclature of Mode of Failure Transitions
>>> in Rocks. Tectonophysics 122(3-4), 381-387.
>>>
>



-- 

I've done the 3 peaks yacht race for Mothers of Africa
Please donate at: http://www.justgiving.com/teamkishinoor