Date: Sat, 9 Dec 2000 01:38:52 -0600
To: hetaken <kenott@uclink.berkeley.edu>
From: xxxxx
Subject: Re: ApplePBG3 Strategy

Hi Ken,

[...]

   I double-checked one of my textbooks, and I overstated the case for steel (which can also fail by cyclic fatigue), and probably understated the case for aluminum. The big idea is that there is a stress limit for a given material, and if you exceed that stress limit, the part will fail under cyclic loading. The more one exceeds the limit stress, the shorter the life. If the stress is below the stress limit, then it probably won't fail by cyclic fatigue. With this in mind, the designer is supposed to make the part big enough (stress = force/area, and generally, larger part = lower stress) and/or shaped smooth enough (gradual flowing curves, not sharp angular features) to minimize the stress it is exposed to, and thus prolong its life under cyclic loading. To be real wordy, a good example would be a paper clip. Under the minor deflections it normally experiences by holding 5 - 10 papers together, it has a long life and shouldn't fail by cyclic fatigue. However, if you bend one of the arms back and forth a 180°, it is exposed to much larger stresses, and it will fail after several back and forth bendings. (By the way, its not uncommon to see important parts designed to have life-span of a million cycles.)

   Unfortunately, it appears that the hinges in our powerbooks obviously were not designed effectively, or else not manufactured correctly (e.g. bad mold/machining that leaves burs or sharp angular features on the hinge). Thus they are failing prematurely.

   Thanks for the reply, and good luck. By the way, I saw the posting to your site on Mike Breeden's site. I really enjoy XLR8yourmac a lot. Take care.

xxxxx