|MadSci Network: Engineering|
Hi Mahboob! You've asked a good question, and if you can figure out an easy, economical solution, there will be a lot of businesses that will want to talk to you about it... Fatigue is when a metal fails due to fractures that were worsened by repeated ("Cyclic") loading and unloading. Most machinery parts fail by fatigue. (When you bend a paper clip back and forth until it breaks, that's fatigue fracture...) What actually happens is that when you apply a cyclic load, mic roscopic surface cracks grow a little bit each time you bend the metal. In addition to that, when you bend or deform a metal part, the material gets harder ("Work-Hardening") but also gets more brittle. After the part has been bent many many times, the crack is so big, and the remaining metal is so brittle, it cracks the rest of the way through. So to increase the fatigue strength of your leaf spring, you have to do one of the following: 1) Carefully polish the surfaces of the springs to remove any surface defects or machining marks that will be the location for crack initiation. 2) "Shot-Peen" the steel. Bombard the surfaces of the springs with steel shot, which cold works the steel, which hardens the surface and puts it into a state of compression, which impedes crack propagation. 3) "Temper" the steel. When initially forming the piece, cool the outer surface quickly to put the surface into a state of compression, which will also impede crack propagation. (There's another definition for "Tempering" steel, which is heating it to stabilize and de-stress the crystal structure.) 4) "Quench" the steel. When forming the piece, the faster you cool it down, the more steel gets locked into a Martensite phase, which has better fatigue properties. However, if you quench it too quickly, you develop thermal stresses and cracks, which will obviously weaken the steel. (Another method for increasing the Martensite percentage is to heat treat the steel before tempering it.) 5) During fabrication, prevent air pockets or contaminants ("non-metallic inclusions") from getting into the steel. These contaminants will provide starting points for cracks to form. 6) Increase the carbon content from .5% to about .95%. This will harden your steel and improve the Fatigue properties. 7) Finally, you could just decrease the applied load on your leaf spring. Steel has the nifty property of a "Fatigue Limit", which means that if your loads are less than a certain value, then you can cycle the part as many times as you want without ever seeing fatigue fractures. Most non- ferrous alloys such as aluminum or copper will not have a fatigue limit. I hope this helps! Let us know if you have any more questions... (By the way, for any people who have been wondering, "What is a leaf spring?", it's several pieces of flat, curved steel banded together. You normally find them under a truck bed, above the axle. They flex when you put weight into the truck bed, acting like shock absorbers. They can also be ripped out of the truck, and used to launch things into the air... Jeff Yap Mad Scientist References: "Physical Metallurgy Principles" by Reed-Hill & Abbaschian, PWS Publishing, Boston, MA Encyclopaedia Britannica Online WebElements The Learning Channel American Heritage Dictionary
Try the links in the MadSci Library for more information on Engineering.