Which engineering equation describing the yield strength of a structure includes time as a variable?

 

A better question might be, what assumptions are made in those equations, and are they only useful for static loads.

Dann

 

The best clause I have seen in this is from Savington... "if you want to".. in this statement: "Keep the timing soft and you can probably make 220wtq at 3000rpm on a stock motor if you want to."

Because as I have thought about it more, there are few times, if any, where torque at such a low RPM has value. Even at the drag strip, I would plan to launch at higher RPM; and on the street, I cannot hook that much torque in first gear, and all other gears are then above 4-5 K if running up through them.

And in first gear, there is not enough time or load to spool the turbo before the engine is well past 3K.

 

A thought experiment:

Take a 3 foot long steel roundbar, 1/2" in diameter.

You stand it on its end and apply increasing mass to the top of the steel rod until you find the force required to fold the rod onto itself.

If you watched the rod fold on high speed you would see that the rod is being initially bent through its elastic deformation stage and quite a few milliseconds would pass before the rod reaches its plastic deformation stage.

Therefor if the load is applied for a lesser period of time than it takes to reach plastic deformation the rod will not permanently deform.

Assuming 8,000 rpm in a 4 stroke, and 20 crankshaft degrees of peak cylinder pressure the load is applied for less than 1 millisecond.


What do you all think? I have run this past a mechanical engineering phd (a friend) and a physics phd (my auntie) who agree.

Dann

 

you might feel differently if you had low end torque Power everywhere, regardless of gear or rpm, is pretty nice on the street.

 

I ran 200-210 at 2500 on a stock 1.6 bottom end for 6 months without failure.

 

210tq at 2500 rpm? what turbo setup can achieve this on a 1.6L?

a gt2554r cant even do 210tq at 2500 rpm on a 1.8L vvt motor.

 

Gt2871 churbo + Autorotor OA2076 compound twin

 

impressive.

sup with the neg cats? am i expected to know each MT.net members wild setups??

chill out people, it was just a question.

 

Strain-rate dependent (i.e. time dependent) behavior is a pretty big research field in solid mechanics. I'd imagine the time scales you see in engine combustion are small enough to make this relevant, but I'm definitely not informed enough to say for certain,

 

This is what I have been referring to, thankyou for sharing your knowledge and giving me some terms to google.

Dann

 

The most literal form of "breaking as a function of time at applied load" is creep.

Creep describes plastic deformation below yield stress as a function of time. So the more general question of "what stress will this part meaningfully deform at" can include time as a variable, even if it's not strictly called the yield stress. But I agree that this is not relevant to this discussion, since a con rod run at 3000rpm for 1 minute and a con rod run at 7000rpm for 1 minute spend the exact same amount of time in compression.

 

FWIW, I ran 240-ish rwhp (dynojet) for 50K+ miles on a totally stock 99 motor, and when I took it apart to build it (because I wanted to make more), the rods were straight as an arrow. That motor never ran boost with less than 95 octane though.

--Ian

 

But each time force is applied it's applied for less time the higher the revs are, and therein lies the root of the question.