how to find crack initiation in gear tooth root

[LMGK]

Hi all,
I am doing gear tooth failure analysis for that i could know the following in fracture mechanics:
1. How to find crack initiation and crack propagation ?. (i found some articles about crack propagation but still i dunno about crack initiation, i mean to find a, where it initiates and what r the parameters associated to crack initiate)

I hope someone have a solution or ideas. thanks in advance.

regards,
lmgk

 

[metengr]

My suggestion is that you send the gear to a reputable metallurgical lab for analysis. Visit the lab and discuss with their metallurgist exactly what you need. Most metallurgical labs like to have customers stop in and review their results.

 

[LMGK]

I havent started practical test yet before that i could analysis theortically how the crack initiates in tooth root by using fracture mechanics. thanks

 

[EricZhao]

You need FEA software to do this kind of analysis. For the complex gear tooth profile, there is no theoretical stress solution. You need fatigue analysis first. In the fatigue life analysis, you find the various stress amplitudes for different loads of your gear tooth. Then you use Miner’s law to calculate the cumulative fatigue damage: \sigma n/N=1. The life (cycle) you get at this point is the fatigue life.

It is assumed that after \sigma n/N>1, cracks will be initiated somewhere on the gear surface. Mostly it would be at the root. Use the FEA to calculated stress intensity factor at the crack tip, there are relationships between this stress intensity factor and crack propagation speed dx/dN, then you set up the crack depth criteria. Lets say you set 1mm depth for the crack inspection criteria. A gear with crack depth more than 1mm will fail the inspection. From the dx/dN value you can get the life (cycles) from 0mm crack to 1mm crack.

The total gear life is the fatigue life + 0mm to 1mm crack life.

Most of the time, people will just replace the part after it reached the fatigue life limit, unless the cost to replace the part is so high.

As I said, I don’t think there is a crack initiation theory in crack analysis. Usually it is assumed the crack is initiated by a small imperfection on the surface. You have to assumed the worst case - crack at where the max stress intensity factor is. Or if you got some gear already failed, examine the crack location of your gear.

 

[LMGK]

Thanks, At the moment we dont have license for MSC.Fatigue. is it possible to do fatigue analysis in MSC.Nastran ?. or any other FEA softwares which are able to fatigue life analysis ?.

 

[EricZhao]

I am not sure about the fatigue capability of MSC/NASTRAN. I am sure ANSYS structural can deal with fatigue analysis and stress intensity factor for crack analysis. Why not consult your manual or call your MSC support?

 

[TVP]

LMGK,

Take a look through the publications at the following NASA website:

http://www.grc.nasa.gov/

http://WWW/5900/5950/Publicat.htm

There are several that discuss gears, crack detection, finite element simulation, etc.

 

[LMGK]

thanks tvp

 

[Carburize]

In order to get some information on the possible crack initiation locations and potential growth paths I would suggest that you review the following publications

AGMA 1010 "Appearance of Gear Teeth - Terminology of Wear and Failure"

AGMA 912 "Mechanisms of Gear Tooth Failure"

CETIM Publication "Aspect des dentures d'engrenages apres functionnement"