asymptote- man I must really use my brain on a Sunday. lol
I read your link "fatigue limit" and the wiki article "fatigue (material) after googling S-N curve. One thing I did see in the second article was the process of shot peening. Shot peening is utilized in spring making. I had forgotten about this.
Surface quality. Surface roughness cause microscopic stress concentrations that lower the fatigue strength. Compressive residual stresses can be introduced in the surface by e.g.
shot peening to increase fatigue life. Such techniques for producing surface stress are often referred to as
peening, whatever the mechanism used to produce the stress.
Low plasticity burnishing,
laser peening, and
ultrasonic impact treatment can also produce this surface compressive stress and can increase the fatigue life of the component. This improvement is normally observed only for high-cycle fatigue.
http://en.wikipedia.org/wiki/Fatigue_(material)
True enough, if the material is kept below it's fatigue limit, it can theoretically last forever. Add mother nature into the equation and she will see to it that it deterioration will set in and the material will fail. But for the intended purpose of the material, given that it is correct for the application, designed properly and utilized within its parameters, it most certainly could "last" forever.
Now on to the other point I ran across:
[h=3]
Material change[/h]Changes in the materials used in parts can also improve fatigue life. For example, parts can be made from better fatigue rated metals. Complete replacement and redesign of parts can also reduce if not eliminate fatigue problems. Thus
helicopter rotor blades and
propellers in metal are being replaced by
composite equivalents. They are not only lighter, but also much more resistant to fatigue. They are more expensive, but the extra cost is amply repaid by their greater integrity, since loss of a rotor blade usually leads to total loss of the aircraft. A similar argument has been made for replacement of metal fuselages, wings and tails of aircraft.[SUP]
[16][/SUP]
With that said, composites can be utilized in the place of alloys to further elevate the fatigue level.
Now, my point. Auto manufacturers tend to stick by what they know because it is familiar and an acceptable level of cost has been figured. Aluminum alloys, carbon fibers and exotic plastics are utilized, to a degree, in auto making. You know this. Aluminum, as you may no, requires a greater mass to steel in order for it to exhibit similar properties to steel. This greater mass is cost prohibitive when compared to the steel. Manufacturing of aluminum, or the shaping, forming, stamping, etc. is also different in relation to steel. Basically, steel is what we know and steel is cost effective, easy and produces an acceptable desired outcome. Lets also not forget what happens when we combine steel with other alloys such as aluminum. Galvanic corrosion will/can occur and certain measures have to be used in order for this to not happen... driving up said cost and the real possibility of premature failure happening, especially at the hands of end users who don't know any better. Remember this the next time you thread a steel bolt into your aluminum calipers.
Carbon fiber and other exotic materials are also cost prohibitive. And as you may be aware, their applications can be limited.
Lotus is big user of aluminum. Ferrari, Lamborghini and Corvette, among others, also use aluminum and other exotic metals and composites. Their prices all show it.
In the end of this. I believe the spring is not representing it's fatigue limit when it "settles" and more investigation is needed... obviously.
However, this raises the point that some manufacturers may be using substandard steel and poor manufacturing processes in order to keep cost low and profit high. Theoretically, a spring of good quality materials and design should out live the life of a car, in relation to fatigue.
I'll pick some brains, along with my own refreshing and researching, tomorrow and this week and see what comes up.
We are having a good time...mmmmkay?
