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Crack propagation

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Simon Spooner, Feb 2, 2007.


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    All,

    Something for the weekend...

    From the discussion with Paul B here:http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=3445 in which paul claimed he could associate a specific injury with a specific direction of force.

    Question: what factors determine the direction of propagation of a crack through a material?
     
  2. Everyone knows that answer, Simon........the image of the the obese plumber bending over to fix the drain pipe.......or a red eyed dog that needs a fix :confused: :eek: CRACK KILLS!!!! :eek: :p
     
  3. That picture is perhaps the most disgusting thing I've ever seen. I'm honestly interested in this topic as my best friends PhD was on fracture mechanics of leather. When we were writing up our respective works we spent many nights in the local pub splitting atoms with axes and discussing such things as crack propagation. As I recall, at one point he was thinking about lyapunov exponents and chaos mathematics. He is running his first half marathon this weekend, if he survives I'll give him a call to get his angle on it.
     


  4. Simon,

    I couldn't resist the photo.....BTW, the chihuahua was previously a punk rocker in his youth. :p
     
    Last edited: Feb 4, 2007
  5. David Smith

    David Smith Well-Known Member

    Hello Simon

    Only just came across your thread on crack propagation.
    This is a major consideration in structural engineering and especially in steel jackets in offshore installations. Crack detection, inspection and reair were a major part of the work when I was a sub sea inspection engineer in the oil industry. Methods used for detection include, visual / video, metalic partical inspection, ultrasonic detection and radiography. This comes under the heading of NDT non destructive testing (BTW Simon have you ever tried testing a crack to destruction, it's hard work and very messy but someone's got to do it eh! :D )

    I'll have to see if I can dig out my old NDT books but of the top of my head-

    Anyway as far as I remember cracks propagate at right angles to the lines of stress providing the material is of uniform stiffness and grain.
    Cracks in steel often start where there are inclusions / contamination or gas bubbles in the steel left during production or included in a weld during construction. Also they start from pitting errosion due to oxidisation and galvanic corrosion or damage from mechanical errosion. These areas are weak and the crack will propagate from them. The crack itself acts as a stress raiser IE like a little lever for the internal forces to act on within the material.
    These cracks once detected and providing they were small could be made safe by grinding or drilling a small hole to below the depth of the crack this has the effect of spreading, and therefore reducing the local stress concentration. If the cracks were large and particularly if they were full thickness then measures to increase the local stiffness of the member were undertaken EG fitting of a grout filled clamp that envelopes the node. Node are generally where crack start since they are areas of construction welding and the junction of 2 or more caissons or bracings and therefore also act as levers for external forces to act on and induce high internal forces.

    Therefore one may see that cracks propagate wher the material is weakest or the internal forces are highest or a combination of the two. To stop propagation several things can be done.
    One - reduce local internal stress eg negate the lever effect of the crack, Two - increase local node or bracing stiffness, Three- reduce external applied forces, Four - increase internal resisting forces, Five do not allow inclusions in the steel, Six - do not allow corrosion and errosion.

    I'm sure one could apply similar principles to bones or orthotic materials.

    All the best Dave Smith
     
  6. As a matter of fact I have. Ignoring the obvious double entendre, I spent many a Saturday morning "helping" my good friend Dr David O'Leary in his lab with his instrom and leather experiments. Really, I was watching Withnail and I, making up the health and safety numbers and tossing in a thought or two when asked.

    Problem is I don't think the materials in question meet these criteria. Moreover, their fibrous nature means we also get things like fibre pull-out, which we don't see in metallic materials.
     
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