B
Bret Cahill
Guest
A fatigue limit just means the material can be stressed an infinite
number of times below that limit, i. e. a steel in a watch spring.
If a material doesn't have a fatigue limit, i. e., an Al in an
aircraft wing, then, no matter how low the strains, with enough
cycling, it is 100% guaranteed to crack.
At first it sounds crazy to intentionally design a plane with wings
that are 100% guaranteed to, sooner or later, fall off, but the stress
profile over the life of a wing is hard to predict, microbursts, rough
landings, etc., so using a material with a fatigue limit doesn't help
a designer much in predicting reliability. Al structures can easily
last longer than steel that occasionally cycles above its fatigue
limit.
In actual practice aircraft designers just keep the strains so low
that 99.999% of the time the wing will last ten thousand years. Do a
cost/benefit risk analysis to optimize your overall odds at survival
and wing fatigue failure will be at the bottom of the list.
In fact, lot of lives probably could be saved _overall_ by shifting
toward fuel efficiency and away from overly robust wings. The big
picture must be spread sheeted.
Alloys seem to have lower conductivities than their pure element
parent metals. The conductivity of iron might be higher than a steel
but it's too brittle.
What about carbon fiber?
I once rewound a fan motor with copper of the same gauge. I couldn't
get the coils tight and the fan never quite had the power of the
original. I'm guessing it drew even more current than the original
because there was less of an inductive effect to restrict current.
Basically it was a short. The number of turns was less than the
original but this would also be true for a tight Al coil using the
same motor frame.
The effect on the field should be similar for the tight Al as the
loose Cu using the same motor frame.
Maybe this could be mitigated somewhat if a motor could be
specifically designed for a lower conductivity wire, but there still
is no way to avoid at least some loss in efficiency.
.. . .
No one mentions it because it was so gradual but the reliability of
electric motors has gone way up. It's hard to find a vehicle without
power windows and the windows always seem to work.
The relative cost of electric motors for power has always been low.
That's why efficiency won't be easily sacrificed for cost. The cost
of the electricity passing through a stationary motor running 24/7 is
easily an order of magnitude more than the motor itself.
In contrast a rechargeable battery costs at least twice as much as the
electricity it will ever hold in its lifetime.
To be sure an EV doesn't run 24/7 but if an EV lasted 100 years the
cost of supplying batteries to an EV might cost 20X more than the
electric motors.
The place to sacrifice efficiency to reduce costs is the battery.
Either the initial cost must be very low or it must cycle tens of
thousands of times.
Bret Cahill