Re: Dynamic modeling deinonychosaur claws

["Jaime A. Headden" <qilongia@yahoo.com>]

Phil Bigelow (bigelowp@juno.com) wrote:

<All well and good.  But note that my question was about the relative
*strength* of a curved blade compared to a straight blade with the same cutting
surface length.  In particular, the relative susceptibilities of each to
torsional stresses and the relative shear strengths of each.>

  Historically, long curved blades such as scimitars and katana, developed from
a need to increase the reach of a blade and thus the need to draw the blade,
which is eased with length as you increase the curvature of the blade. My
favorite curved blade, the samshir, is incredibly long with a stronly tapered
point and was deliberately designed for the ease of drawing a 4'+ blade from
the hip. Blades typically have their strongest point about 1/3 down from the
tip, while a curved blade allows you to maintain length of this point during a
drawing, slicing motion. Katana are designed for ONE thing, a single, fatal
slicing motion. They are NOT designed for catching other blades, despite
romantic _Highlander_ movies to the contrary. The long Celtic claymores are
also designed for a single cut, but in this case it's not a drawn slice but a
single chop: into, through, and out. The motion is akin to that used by axes.

  The development of a long decurved or incurved blade is to concentrate the
cut towards the base or "hilt" of the blade, not towards the tip. In sickles,
the motion is a sweep which is followed through with a drawn-in gathering of
the reeds and plants as you pull the tip of the blade towards you. Scycthes and
kama use similar motions, where motion follows the curve of the blade. Because
of this, such blades are tilted relative to the direction of cutting, so that
the follow-through motion results in the flat of the blade gathering the
materials. The sickles of dromaeosaurs are quite different, and likely followed
the point, not the edge. Thus, few blades are designed for piercing, or using
the tip, which is one of the weakest parts of a blade.

  In addition, I think I should point out that it is impossible to get the
ungual of *Deinonychus*, more strongly curved than in *Velociraptor*, to slice
into an object it has not already pierced. Using a slicing function when the
leg action may have a coincidental toe flexing action, will cause the ungual to
follow its curve into the flesh, as in gurkhas. If this mechanic was actually
employed, it would only support Manning et al.'s position, though only
initially. Manning et al. need to follow through this study with sharpened
edges, and various leg motions including toe flexure during piercing. I suspect
it will support tests done by H. Todd Wheeler on bison in simulating angle of
attack and effectiveness of various bite strategies.

<On straight blades, the tip is far from the "pivot point" (aka, the articular
surface).  On a strongly curved blade with the same cutting surface length, the
tip is closer to the pivot point.  This gives the curved blade less leverage,
but possibly greater strength.  For both dinosaur predators and knife users,
this may be a worthwhile tradeoff.>

  All a curved blade does is focus the stroke to the "power point" on the
blade, rather than provide any intrinsic strength to the blade itself.

<It may be useful to computer model and then test the relative shear strengths
and torsional strengths of the curved pedal unguals of various curved-claw
predatory dinosaurs.  It would be valuable to know the absolute maximum amount
of force that each species' unguals can take before they break.  Test for
maximum sagital force, maximum lateral force, and maximum twisting force.  Even
knowing the maximum compressive force may prove informative.>

  I agree. With the history of attention to sickle claws (over 75 years) little
study has been done on the actually _strength_ of the unguals or the
application of various forces to various points on the ungual. This would be a
useful study, and I think it's actually rather simple to do. If we can model
the various unguals in 3D, there are rather simple FEA tests that can be done
to provide analysis, including testing for a keratin covering of various
shapes.

  Cheers,

Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
__________________________________ 
Yahoo! FareChase: Search multiple travel sites in one click.
http://farechase.yahoo.com