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responses to 'Help, my indentor keeps sliding !!'

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  • responses to 'Help, my indentor keeps sliding !!'

    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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