View Full Version : responses to 'Help, my indentor keeps sliding !!'

Robert Whiteside
12-17-1999, 12:16 AM
Thank-you for the responses!

I as asked to post the results by a few, so here they are...

The original posting was...

Hello all,

I am in the pilot work phase of a study that involves placing an indentor on
a specific patch of cartilage and measuring the resulting stress when a
constant strain rate is applied. Although the indentor is perpendicular to
the surface, it slides relative to the cartilage (actually, it is probably
strain in the tissue sample causing the misalignment).

If anyone has come across this problem and has any suggestions that may
help, it would be greatly appreciated. I have thought of a indentor cap
with a small pin in the centre to hold them still relative to each other but
I'm not sure what effect this will have on the stress ditribution to the
underlying bone.


Robert Whiteside
M.Sc. (Biomechanical Engineering) candidate
Clinical Mechanics Group
Department of Mechanical Engineering
Queen's University
Kingston, Ontario
Canada. K7L 3N6

and the responses...

You may gain a lot of insight into this problem by reviewing Ian
Sneddon's work on the punch problem on the infinite half-space. See the
Dover edition of his book on transform theory. I also published some
work on this problem with a layered medium with Yoav Serig in the 70's.

I don't have any particular advice for you, but I did want to say Hi and
let you know that other Canadians are working in the area.

I assume that you are using a flat indentor tip... Where is the motion
coming from? Could you reduce the play in the specimen clamp or the
indentor mechanism to reduce the motion?

Please let me know if you receive any tremendous responses...

I did basically the same thing at the CMG about 2 yrs ago. You should be
able to find my thesis at the CMG or with Dr. Small. I remember my pilot
testing with pig shoulder cartilage giving me the same problems. You have
to make sure that your indentor is centered and perpendicular on the tissue
and that you use a small enough indentor. You should also make sure that
your strain rate is fairly quick.

Have you considered doing a confined compression test whereby the
cartilage is constrained along the sides and the indentor is approximately
the radius of the constraining device?

Thanks to all who responded.

Cheers (and happy holidays)


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