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4 point bending and ASTM 790 - concerns

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  • 4 point bending and ASTM 790 - concerns

    Dear All

    I am a little unhappy with the oft quoted Standard ASTM 790 in the use of 4
    point bending in bone.

    Can I get peoples' reaction to my concerns?

    ASTM 790 "Standard test methods for flexural properties of unreinfroced
    plastics and electrical insulating materials" is quite typical of the
    literature on four point bending.

    It considers only two configurations for the two inner contacts: the span
    of the sample or 1/3 of the span. There is no reason for this fractions
    except there are equations for these.

    The supports or 'noses' are large - 9.5mm diameter - which is appropriate
    for the huge range of hardness found in plastics

    The equation used to calculate surface strains requires measurement of
    displacement at the mid-point -not the displacement of the contacts which is
    measured by the test machine - this Standard requires an extra channel of
    information measured independently using an extensometer at the mid-point -
    yet is does NOT make that clear.

    My concerns are:
    1. It makes more sense bringing the inner contacts as close as possible
    (so increasing the applied moment for a given force) while allowing for a
    'butterfly' fragment - I would suggest 9mm separation of the contacts for a
    5mm deep sample
    2. It makes sense to have the contact diameter as small as possible -
    otherwise the sample will roll around the contact and shorten the span - I
    am tempted to use 3mm diameter contacts - though these would have to be
    solid and so not applicable to fatigue testing because of the incumbent wear
    (Griffin et al 1997)
    3. The calculation of strain should be from the applied displacement of
    the contacts i.e. from the known crosshead position (allowing for the system
    stiffness) - other channels of information are helpful but redundant

    Am I wrong to be concerned?

    Many thanks

    Dr Edward Draper
    Orthopaedic Research Fellow / Divisional Finance Officer
    Dept of Musculoskeletal Surgery, Imperial College Faculty of Medicine
    Basic Sciences Unit, Royal Veterinary College
    Dept of Orthopaedic Science, Institute of Orthopaedics, RNOH

    Tel: 020 8746 8312
    Mob: 07785 785369
    Fax: 020 8746 8787

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