deleted_user wrote:Do you recycle the plastic bubble or get a new one each time?
I get a new straw and plastic bubble each time. Your inquiry does beg the question of how long can a full “unit” last. I predict the straw would fail first.
deleted_user wrote:Do you recycle the plastic bubble or get a new one each time?
evilcheadar wrote:deleted_user wrote:Do you recycle the plastic bubble or get a new one each time?
I get a new straw and plastic bubble each time. Your inquiry does beg the question of how long can a full “unit” last. I predict the straw would fail first.
momuuu wrote:But what pool cleaner should I use? Let's stop discussing these idle subjects and get to the real matter!
Jam wrote:Brain freeze tally.
kami_ryu wrote:do you not pay for refills?
deleted_user wrote:kami_ryu wrote:do you not pay for refills?
Likely discounted.
momuuu wrote:But what pool cleaner should I use? Let's stop discussing these idle subjects and get to the real matter!
evilcheadar wrote:Fresh refill, cup shows some wear.
deleted_user wrote:Whoa boy there are a lot of little dings in that sucker. Here's something for consideration: the critical Griffith crack length.
Griffith studied fracture mechanics in the early 1900s when the only mechanics of materials concepts around were simply stress and strain. When a little scratch happens like that on your cup, or in the metal of a ship hull, the material has "yielded," it has surpassed its tensile strength, surpassed its yield stress and theoretically it can hold no load and the structure is in danger of collapse. But of course, this is not really how things work because metal gets dinged up all the time, the golden gate bridge has scratches on it, and still structures survive.
Griffith explained why this is by approaching the problem in terms of energy rather than just stress and strain. There is a certain amount of energy that a crack needs to automatically propagate. A stress concentration (crack) is just a means of converting strain energy to fracture energy, like how a can-opener converts muscle energy into mechanical energy -- if there is no continuous energy put into the opener it fails to do anything. So while stress concentrations exist in a crack, the material can release some of its internal strain energy to keep the net energy in the system less than what needs to propagate the crack, for a certain length of crack, that is. The critical Griffith crack length is explained graphically here:
Essentially, in a closed energy system, when stresses exist (when your cup is full) and a crack forms, there is a critical, acceptable length until it propagates until acted upon. What is the critical Griffith crack length of your cup?
Consider the formula:
Modern foam cups are polystyrene. Polystyrene Properties:
E = 0.4 x10^6 psi
W = 5.5 ft-lbf/in^2 = 66 in-lbf/in^2
S = 6200 psi
Then,
Lg = 0.45 inches (approximately)
Sources:
1) http://www.arvindguptatoys.com/arvindgu ... gordon.pdf
2) http://www.dielectriccorp.com/downloads ... tyrene.pdf
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