Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Force given by blades on cloth

  1. Feb 13, 2009 #1
    I have been wracking my brains about how to work out how much force I am calculating when doing some drop testing on armour.
    I was wondering if someone woudl be able to walk me through a good way to show this as simply as possible. My physics knowledge is limited but I learn fast so say what you think and I'll ask if I don't get it.

    Right my problem is I am dropping various blades onto fabrics from two different heights (1.2 and 0.7m) the drop test machine has minimal friction and just drops straight downwards.

    I am using different amounts of cloth to see where it starts to become effective against certain weapons strikes. However the points of blades are obviously microscopic and there is my dilema. I am having difficulty saying how many kilos per m squared force I am giving or how many joules I am releasing within a time frame.

    I have some more data if someone is willing to help.

    Thanks in advance

  2. jcsd
  3. Feb 13, 2009 #2
    Try having a read through this. It's not the most thrilling document in the world, but it is a standard describing current testing methodologies and reporting for knife protection. The approach is to drop a blade of specific dimensions attached to a carriage of known mass. Specific threat levels are described by the kinetic energy of the striker upon impact. The depth of penetration into the backing material is measured, and if it's over a certain distance then the sample is not safe. A material that prevents the

    The problem lies with associating force with the tests. For instance, try to appreciate the difference between a knife impact on steel, and a knife impact on butter. The steel will arrest the knife's motion over a very short distance (very small degree of penetration into the steel) but at a very high force. The butter will arrest the motion over a very long distance but at a very low force. Now ask yourself which protects you better from knife penetration.

    It gets even more complicated when you consider that a knife or spike going through Kevlar weave often doesn't interact a huge amount with the material itself, instead finding space between fibres and forcing them apart and through. That's why fibre armour manufacturers often supply a stab resistant layer that roughens the blade, making it less able to slip between fibres.
  4. Feb 14, 2009 #3
    That article is perfect cant believce I hadn't found that yet. The machine and blades I have used are the ones int he pictures, it is actually the MET police drop test equipment!


  5. Feb 16, 2009 #4
    No worries, it's probably one of the better standards I've had to work to.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook