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Thread: question for Darryl

  1. #1
    opti-tipster harry a saake's Avatar
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    question for Darryl

    Darryl, at a seminar tonight mike disanto, showed a study done by the us air force on lense that showed, when scratched on the front a 20 percent chance they would break, when scratched on the back of the lens an 80 percent chance they would break, my question of course why the disparity, BTW, they were talking cr-39 and poly

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    I ain't der Miester but:

    I can see why it would make a difference on glass. (You remember it makes a difference on whether you put the scored side down when cutting same). Not too sure why it would matter on plastic. However with CR-39 having very little "flexure" it might matter a bunch more on this than poly or Trivex.
    Might be interesting to have someone credible like Colts to do some drop ball or drop point tests on these materials with and without scratches.

    Chip

  3. #3
    ABOM Wes's Avatar
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    Try this at home.

    Take just about any solid homogenous material. Score it on one side. Filp the scored side away from you, and smack it with something. It'll break far easier than it would if the scored side was facing you. This'll work on glass, plastic, etc. Same as on a lens. Luckily, most major scratches end up on the front of lenses.
    In fact, I'm going to try this at work monday with some dummy lenses, a screwdriver and a hammer. (And some eye protection!)
    Wes
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    ATO Member HarryChiling's Avatar
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    When an object hits a suface the force applied is compression and this force spreads throughout the material with most of the force being directed in the downward direction or the direction of travel of the object that striked the surface, so if the scratch is on the front surface the lens deals with a force perpendicular to the scractch. If the scratch is on the back surface the force spreads through the material and radiates from it's source so some of that force applied to the back surface is tension which pulls the scratched area or crack apart. I made examples, I hope that illustartaes it better that I said it.
    Attached Thumbnails Attached Thumbnails frontforce.jpg   backforce.jpg  
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  5. #5
    Master OptiBoarder Darryl Meister's Avatar
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    Harry is on the right track.

    The difference is due mainly to the the fact that materials, in general, have a much weaker resistance to "tension" forces (or being pulled apart) than to "compression" forces (or being crushed). When a lens is struck, the front surface is put under a state of compression while the back surface is put under a state of tension.

    Further, any point of damage to the material serves as a point of potential propagation of a fracture or tear, just as it is easier to tear materials that already have a small "rip" of some sort. Consequently, damage points on the back surface of the lens, which is placed under tension, serve as more critical points of "fracture origin" when the lens absorbs an impact.
    Darryl J. Meister, ABOM

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