why is compression beter for fatique loading? 1

[321GO]

During testing it is suposedly observed that fatigue crack's "always" start in tensile loaded area's, "never" in equally loaded compression area's?

If so, then why are compression cycling loads much less damaging?

p.s. the material is a spring steel(heat treatmented)


Gracias

 

[rb1957]

'cause metals don't (significantly) fatigue in compression cycling ...
read up on it in fatigue texts ... i suspect it'll be something to do with dislocation mobility (or immobility).
tension-compression cycling (-ve R) is a different thing, so too compression overloads (occassional high compression loads)

 

[geesamand]

The basic philosophy of fatigue is that any tensile stress of a significant magnitude will add microscopic damage to the material. The general description of this is that each tensile stress cycle pulls on cracks within the material and makes them grow. At the end of the day, the repeated tensile stress cycles extend the cracks to the macroscopic level and lead to failure.

Compressive stresses of similar magnitude do not expand the cracks and do not cause fatigue type failures.

That's enough of a layman's description to be useful but not good enough to answer a test question. Just in case that's a factor here.

 

[Tmoose]

Hi 32160,

If this was not the era of GD&T and Solidworks I'd say grab a Paper Mate Pink Pearl and use a sharp knife to make a ~ 3/16" deep cut across the short dimension of the large face. Probe the depth of the cut with the corner of an index card and record the depth. Then bend the ends together toward the cut face Like the letter "C" and hold it there for 60 seconds. Measure and record the depth of the cut. Then bend P. Pearl the other way, so the cut is on the outside of the C, while watching the cut closely. Hold for 60 seconds and count the number of erasers on the floor.

 

[btrueblood]

What Tmoose and geesa said. Cracks don't open under compression loading, and thus don't grow. Cracks do open under tension loading, and do grow.

 

[MiketheEngineer]

You can tear paper but when you crumple it - it is still all there!!

 

[IceStationZebra]

Along the same line of reasoning you can largely discount the surface roughness. ISZ

 

[SafetyFactorOf1]

http://blogs.mhsl.uab.edu/engineering/wp-content/uploads/2010/03/464042a-f22.jpg

Look at figure 1. In the simplest of explanations, tensile load is pulling the crack apart, increasing its size, which decrease cross sectional area, thus increasing effective stress, and leading to our failure. Compression is pushing it back together, theoretically never increasing the crack size. This is generally referred to as "type 1". If you were to have a crack in the middle of a plate (like the paper example mentioned above) it would effectively be the same thing, with propagation likely to occur on both ends. Note that many formulas you will use for crack growth, the "width" is given is "2a" or "2b" or something like that, where the two is accounting for two crack tips. Don't miss that on the exam

For a more technical reference:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0685377

Jim