[quote]diesel25 wrote:
Dumbells are commonly constructed of Cast Iron.
Dumbells are most likely to fail in SHEAR. Cast Iron’s Shear Ultimate Strength is 240 MPa - or 35,000 psi.
But let’s add a BIG factor of safety to account for potential corrosion and fatigue: 0.5. This’ll make the shear Strength 17,500 psi.
We’ll take the dumbell handle width to be 1 inch ~ 25 mm
After ALL the factors of safety and VERY conservative estimations, your dumbell will not fail before pi^2*(25/1000)/4 x 17500 = 1,080 lbs
Its strength is likely significantly higher than that.
Load on dumbell when you drop it:
Newton’s 2nd law: F = m*a = (W/g)*a, where a = accleration as you push the dumbell (10 ft/s - you wish), g = gravitational acceleration (32.2), W = 500 lbs/dumbell (yeah right).
Then, F = (500/32.2)*10 = 155 lbs.
The weight of the dumbel is spread into two disks at the ends, each roughly half its nominal weight. Each end of the handle will be subjected to HALF the value of F in shear - 155/2 = 77.5 lbs
Factor of safety involved in the exercise = 1080/77.5 = 14.
Using 500 lb dumbells and pushing them up at 10 ft/s - both HIGHLY questionable) you would be 14x under the dumbells capacity.
This is AFTER our factors of safety accounting for potential corrosion and metal fatigue.
So, WHAT did you break again?[/quote]
The stress is a combination of shear and moment
A 500 LB dumbell with a 1" bar would see a max static stress of about 15,000 psi
When dropped from a height of 3 feet the impact stress would be multplied by 3.6, from 6 ft multiply by 4.7
repetetive impacts could cause crack propogation and eventual failure on larger dumbells, especialy the 3 piece dumbells where the weights are fastened to the bar with a bolt.