Extrusion vs milling

[dan100]

Hi!
We have a problem obtaining a 113 deg angle for a helicopter repair. We figure if we mill it out of a solid bar it will lack some fatigue strength due to surface differences, but we cannot find any literature on it. Is bead blasting a good idea? Does anybody have experience with these things?
Dan100

 

[Somber]

don't have time at the moment to get into details, but I'll give you my quick answer from my experience... machined parts are always a concern for long term applications in which fatigue cycling is a major factor (few environments more severe than helicopter structure)... cold working the surface (normally a peening operation) can help regain some of the fatigue life by closing off the grain boundaries opened by machining and inducing pre-stress into the surface...

basically, without knowing more about the specific application you are working toward, I don't want to go too far with recommendations... what I can say is that we use machined parts, even in extremely critical fatigue environments of fighter A/C... if you make a good material choice, take care to control the macro concerns (generous radii, avoid abrupt cross section changes, good surface finish), and use some technique to cold work the surface - I feel you can use a machined part in most applications...

all that being said, despite very careful process controls, etc - we still lose machined parts due to fatigue related failures, but in several cases they have performed better overall than the composite components they replaced...

if you do go with a machined repair, I recommend adding an appropriate inspection interval for the repair... you are obviously repairing some damage currently - which means you had some sort of noticeable failure without the loss of the helicopter - hopefully that means any future problems in this area will evidence themselves before something catastrophic...

 

[dan100]

Thank you, you sort of confimed my thoughts on this.
Dan100

 

[JSF]

Dan,
You have to check what is the origanl part extrusion. If the extrusion is 7075T6511 you need to check what is the dimension of the extrusion. If you machine the part out of 7075T73 Plate you need to check the Ftu, Fty property to make sure your orignal applied load and material allowable has adequate margin of safety. If you machine out of a very thick plate your material allowable decreases with thicker block. Since you change the material property you need FAA/DER approval on your 337.

Rich. Chwa

 

[dan100]

Thank you, JSF.

 

[jetmaker]

dan,

Just an additional note on material form substitution. If you are machining from a plate, one of the 'legs' of the extrustion will be oriented along the SHORT-TRANSVERSE direction of the plate. Consequently, you will see reduced properties in that leg. Make sure your plate material has adequate strength in that direction for both fatigue and fastener bearing loads. As a suggestion, you might do better to hog the part out of extruded bar as it does not really have a SHORT-TRANSVERSE direction due to the extruding process.

Later,

TSH

 

[dan100]

Thank you, Jetmaker. (cool name!)
Dan100

 

[debarra]

Hi Dan100
Perhaps it was a bit later but anyway
you should visit this site

http://www.tiernay.com

deba