Re: [CR]Re: Frame Fatigue


Example: Framebuilders:Rene Herse

Date: Tue, 10 Jun 2003 08:14:15 -0700 (PDT)
From: tarik saleh <tsaleh@rocketmail.com>
Subject: Re: [CR]Re: Frame Fatigue
To: BobHoveyGa@aol.com, classicrendezvous@bikelist.org
In-Reply-To: <144.1345ddde.2c1748ae@aol.com>


Jan writes:
> (I am sure
> >somebody will explain that frames get soft with use
> and "reset" after
> >a rest.)

BobHoveyGa@aol.com wrote:
> Bicycles are the same way...
> after a ride, they must have a rest period for the
> frames to "reset".
> Therefore it is essential for any serious rider to
> have a stable of bikes to prevent
> "frame fatigue"... at least that is what I have told
> my wife.

Interestingly, cyclic softening and hardening are real phenomena, as is strain rate sensitivity.

Cyclic softening is when, with (for example) a constant load, each fatigue cycle results in more deflection. Cyclic hardening is the opposite. I am pretty sure the reason I get beat in sprints is that my bike is exhibiting cyclic softening, while my competitors is just the opposite allowing him or her to beat me by ensuring more of their power goes to beating me and less to making their bike softer with age. Damn them.

Additionally, I think strain rate sensitivity would be a good description, if very loosely and inaccurately applied, to the interesting observations of Jan and Bob above. Correctly, one sees strain rate sensitivity when puling a piece of silly putty. Pull it real slow and it exhibits super plastic behavior, extending many times its original length before it breaks. Pull it quickly and it snaps at very small strain.

Quasi-tangentially, I think the idea of given frames a rest between rides can be loosely described as letting the fatigue damage anneal itself out with time (as the atoms migrate about) at room temp. Aluminum is even better for this as many alloys will undergo room temp aging phenomena, but of course this is OT.

So, in conclusion, when asked "why I need so many bicyles " I will tell people the following:

Due to concerns about cyclic softening and strain rate sensitivity, I need to age my bikes, reversing fatigue damage, so I rotate each bike in and out of service to ensure that each bike is at its very freshest each time I ride, anything else would be uncivilized.

(phenomena are real, application of said phenomena was done EXCEPTIONALLY poorly for humorous effects)

Tarik, materials magician

===== Tarik Saleh PO box 1326, Knoxville, TN 37901 tsaleh at rocketmail dot com Bicycles, bicycles, bicycles: http://www.engr.utk.edu/~tsaleh/bike.html