Neil:
maybe rambling-- as you say-- but some good, solid, engineering points.
However, I think you mean the S-N curve is exponential, not "logarithmic";
as stress increases, the number of cycles to failure becomes exponentially
less (curve becomes more-and-more vertical). Yes? No?
Russ, #1226B
-----Original Message-----
From: owner-land-speed@autox.team.net
[mailto:owner-land-speed@autox.team.net]On Behalf Of Albaugh, Neil
Sent: Friday, June 06, 2003 11:02 AM
To: 'ardunbill@webtv.net'; 'land-speed@autox.team.net'
Subject: RE: to buy or not to buy??????
Bill;
You are absolutely correct. The "S-N" diagram "number of cycles" scale is
logarithmic and at low stresses, the curve flattens out so the fatigue life
is essentially indefinite.
When we're talking about high-performance engines built for endurance
applications-- boats, aircraft, 500 mile NASCAR events, Indy, Le Mans or
Sebring, we're solidly into the area of concern about fatigue life. Short
events like drag racing or , perhaps, Bonneville require a focus more on
component strength so that they can survive very high stresses for a much
shorter time.
Fatigue life is usually secondary for short events but there can be
something called "low-cycle fatigue" that can occur. This is when a part is
stressed almost to the breaking point (into the yield region), it will fail
after only a few cycles.
Try bending a coat hanger back and forth severly by hand. First the place
where the wire is bending back and forth gets hot-- this is because it is
absorbing energy by yielding (like a roll cage in a crash), then it breaks.
This in a good example of "low-cycle fatigue".
I'm rambling on too much on this subject so I'll shut up and let others take
it from here.
Regards, Neil Tucson, AZ
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