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Model Forum / General / Rockets / July 2005Physics 101 was a long time back...
I'm trying to determine the force required to break a piece of cord. Setup: A fixed boom at the top, with tensionless attachment suspends a piece of cord of the type being tested. A weight of 8 pounds is secured to the lower end, and the weight dropped from successively greater height until the cord breaks. A new piece of cord is used for each drop to avoid cumulative damage to the sample being tested. Now, my (probably erroneous) thinking says that the velocity of the weight (derived from v=at) in feet/second * mass of the weight (8 pounds) yields a force in foot/pounds. Example: The 8 pound weight does not break the cord when dropped 18". A drop of 22" breaks the cord. My method suggests a breaking strength of about 80 foot/pounds, which was significantly higher than I expected. The rated strength of the line is 22 lbs, so my numbers are not out of the ballpark, but are perhaps way out in left field... May be apples and oranges as well, i.e. dynamic vs static loads. This is all about engineering parachute shroud lines, and managing the dynamic loads of deacceleration at opening. TIA for setting me straight on the proper method of figuring the forces involved. (yeah, yeah, I know... the Knacke manual) Kevin OClassen In a word, No. You need to hook a load cell or other force transducer to the string so you can measure the force in real time. your calculations are 8way* too simplistic, and do not take into consideration things like the accelleration and jerk that are accutaly applied to the string. The forces are not instantanious due to the stretch of the string. A simple real-time data logging system is inexpensive and easy to put together, even if you boild it yourself. > In a word, No. You need to hook a load cell or other force transducer to > the string so you can measure the force in real time. your calculations [quoted text clipped - 4 lines] > even > if you boild it yourself. Agreed. Am I closer to say that what I described essentially calculates the total force, which is then dissipated over a (relatively short) period of time? KO > Agreed. Am I closer to say that what I described essentially > calculates the total force, which is then dissipated over a > (relatively short) period of time? Well, if you're looking for a simple numeric formula to do something with, I'd recomend using a formula that puts more emphasis on the velocity - mv^2 than a simple linear formula. > I'm trying to determine the force required to break a piece of cord. > [quoted text clipped - 21 lines] > > Kevin OClassen Your calculations and units are off. Velocity times mass is momentum, not a force. In English units, a measured pound weight in not a mass, it is a force. If you convert to SI units, you can eliminate the units issue, since most SI scales are calibrated in mass units (kg), not weight (Newtons). A pure calculation of dynamic forces will be difficult without knowing precise times and distances (cord stretch), or making some assumptions about them. (And I assume you do not have tech info on the cord you are testing.) I would imagine a static pull would tell you the ultimate yield strength of the cord to suit your purposes. You could try a spring scale or tie a bottle on it and add weight (water?) till the cord breaks. If you can get cord stretch info (zero to max weight), you could estimate the dynamic performance of the cord.  SignatureGary Bolles summum jus, summa injuria est To contact me; bollesg at comcast dot net http://home.comcast.net/~bollesg/rockets/rockets.html Help make r.m.r. a No Flame Zone >I'm trying to determine the force required to break a piece of cord. >This is all about engineering parachute shroud lines, and managing the >dynamic loads of deacceleration at opening. TIA for setting me straight on >the proper method of figuring the forces involved. (yeah, yeah, I know... >the Knacke manual) Here's the simple way: Send a sample of your cord to "Doc" Damerau, and he'll run tests on honest-to-goodness materials testing equipment, and post the results online at http://www.rocketmaterials.org for the rest of us to use. It's what I've done, and what lots of others of us have done, too. Check out the site -- lots of great info there! - Rick "Pay it forward" Dickinson SignatureRick Dickinson - rtd@notesguy.com National Association of Rocketry (NAR) # 73975 Level 2 Tripoli Rocketry Association (TRA) #10498 Level 2 http://www.crayonrocketry.com > >I'm trying to determine the force required to break a piece of cord. > [quoted text clipped - 6 lines] > > Send a sample of your cord 3+ samples. > to "Doc" Damerau, and he'll run tests on > honest-to-goodness materials testing equipment, and post the results [quoted text clipped - 4 lines] > > - Rick "Pay it forward" Dickinson SignatureJerry Irvine, Box 1242, Claremont, California 91711 USA Opinion, the whole thing. <mail to:01rocket@gte.net> Please bring common sense back to rocketry administration. (too late) Produce then publish. http://www.usrockets.com Ebay. http://tinyurl.com/6wlp8 >> Send a sample of your cord > >3+ samples. Well, one really long sample is what I sent, and Doc seemed to do ok with that.... - Rick "How long is a cord?" Dickinson SignatureRick Dickinson - rtd@notesguy.com National Association of Rocketry (NAR) # 73975 Level 2 Tripoli Rocketry Association (TRA) #10498 Level 2 http://www.crayonrocketry.com >>>Send a sample of your cord >> [quoted text clipped - 4 lines] > > - Rick "How long is a cord?" Dickinson I'm pretty sure Doc has some scissors. >>>>Send a sample of your cord >>> [quoted text clipped - 6 lines] > > I'm pretty sure Doc has some scissors. Actually... I don't have a pair at work. I use a blueprint shear. :-) Doc SignatureDrake "Doc" Damerau President, NEPRA NAR Section 614 L3CC member TRA 9934 www.nepra.com www.rocketmaterials.org http://home.sprynet.com/~monel/home.htm > - Rick "How long is a cord?" Dickinson > > I believe it's 4 ft. by 4 ft. by 8 ft... OOOHHHH.... You weren't talking about wood... David Erbas-White > Here's the simple way: > [quoted text clipped - 7 lines] > - Rick "Pay it forward" Dickinson > -- Thanks for the link. That is one useful site. Kevin OClassen >> Here's the simple way: >> [quoted text clipped - 11 lines] > > Kevin OClassen Send me samples and I'll be happy to test them for you. I have a back log of rocket materials to test. But I'm going to be doing some Kevlar cord in a week or so, so I can test it then. The Doctor is IN! Doc SignatureDrake "Doc" Damerau President, NEPRA NAR Section 614 L3CC member TRA 9934 www.nepra.com www.rocketmaterials.org http://home.sprynet.com/~monel/home.htm |
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