how to find crack initiation in gear tooth root
how to find crack initiation in gear tooth root
(OP)
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
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





RE: how to find crack initiation in gear tooth root
RE: how to find crack initiation in gear tooth root
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
www.borowskiengineering.com
RE: how to find crack initiation in gear tooth root
lmgk
RE: how to find crack initiation in gear tooth root
If it is due to an overtorque, you need to know the post yield behaviour up to ultimate strength and rupture. This would require a pretty good knowledge of the entire stress-strain curve, including the strain-hardening portion.
This is really a pretty complex problem...more complicated than anyone should try to fully explain on a forum. You may want to find a metallurgist in your local area and have a long talk with him.
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
www.borowskiengineering.com
RE: how to find crack initiation in gear tooth root
1) "Safe life" in which you use an S-N curve, appropriately factored from test to design the gears to survive a certain number of load cycles.
2) "Damage Tolerant" in which you assume that there is an initial crack (corner quarter or semi-circular). For the initial crack size you choose the largest crack which would NOT be detectable by the inspection methods used for the component. You then use fracture mechanics to see how many cycles it takes to grow the crack to the critical crack length.
In either case you need to know the stresses at the tooth root and this is difficult. You can either use an FE model or resort to empirical gear tooth design software. I personally prefer the empirical approach in this case because it is based on a lot more real world data than the FE. With FE you run into problems in determining the stresses around tooth-tooth contact points.
You will also have difficulty in determining the real loading on the tooth because gearboxes flex (well, the lightweight aerospace ones do) and will often bias the load and contact towards one end of the tooth. This is a significant effect if the gearbox/gear ensemble has not been designed to generate even contact at the most commonly used load condition.
You are into specialist analysis here - consult experts better than me locally as GBor suggests.
RE: how to find crack initiation in gear tooth root
There are two important theories for fatigue studies:
1) Stress Life Theory (S-N) approach estimates total life without distinguishing crack initiation from crack propagation.
2) Strain Life Theory (E-N) approach is called as the crack initiation method.
AAY
RE: how to find crack initiation in gear tooth root
I have tooth root stress values which are taken from MSC Nastran linear static analysis. I like to know at what torque and speed (normally load distribution along the face width may be constant or variable) , the crack initiates in the root and minize it by profile correction which is my ultimate aim as i said before.
I hope the theories and ideas which u people suggested could help me. Let me go thro it and see what is says. thank u guys.
We dont have msc.fatigue module which is easier to find the fatigue life. Instead of that i am looking alternative method to do ie. MSC Nastran/Ansys/or ..(i dunno)
lmgk
RE: how to find crack initiation in gear tooth root
i'd be surprised if the crack growth portion of your damage is significant, compared with the crack initiation life. if this is what you're interested in, (the crack growth portion of the crack life), then your DT would start at a non-detectable flaw size (something like 0.05" is tyipcal in aerospace to represent manufacturing (or in-service) damage, or 0.005" for typical material imperfections). I'd think that you could do your fatigue life as a hand calc, you have a simple spectrum (i think, 1 stress cycle per gear revolution). maybe something like ncode could analyze your entire model ...
anyways, i'd also be surprised if linear FEM can give you an accurate stress concentration stress, i'd expect plasticity at the root of the gear tooth.
modifying the gear tooth profile is a way to increase fatigue life, remember sometimes taking material away can improve the fatigue life (ah, the magic of fatigue !).
consider too, surface treatments (maybe you can shot-peen a non-contact part of the tooth), surface hardening, cold working (again in a non-contact area) are also ways to improve fatigue life. also, maybe you can modify how the gear is used; possibly changing the time-profile of how the gear is loaded can help (if the load is rapidly applied, maybe this'll increase the stresses on the tooth).
good luck