MadSci Network: Engineering

Re: How might the fatigue strength of a leaf spring be increased?

Date: Mon Apr 23 17:57:36 2001
Posted By: Jeff Yap, Materials Engineer
Area of science: Engineering
ID: 987083893.Eg

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

"Physical Metallurgy Principles" by Reed-Hill & Abbaschian, PWS 
Publishing, Boston, MA
Encyclopaedia Britannica Online
The Learning Channel
American Heritage Dictionary

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