airframe wear concern

[qmechanics]

Just had an incident where a actuator motor housing had been rubbing against part of an welded (4130 chromoly) frame. I've since moved the motor but it left a wear mark about 1/8 inch wide and 1/4 inch long and .006 to .010 deep. the chromoly had an OD of .765 and is .047 thick. I'm wondering if this has substancially weakened the frame that the point of wear? There's about a 60 pound shear load on the frame at that point but it's intermittant.
I can't find any information on this. Any help would be appreciated.

 

[qmechanics]

Jetmaker,

The depth:length ratio is 1:31.5 where would that put the stress concentration? Last night I went over it very carefully....it's does look like someone had been trying to blend the spot in. It's very shallow only about .006 or less. the spot is elliptical major axis about 3/16 of an inch...maybe a bit longer. It's very difficult to measure because of the location. OD is .764 and thickness .047 inches.
No info from OEM available. Sure looks like someone was determined to fill it in and paint over it. The sanded spot came off in one piece....left rough edges of paint around the wear mark. I'm thinking this happened at the factory during initial installation. There's no evidence the frame has ever been repainted...Hmmm. Interesting. Thanks guys.

 

[jetmaker]

Check Peterson for the stress concentration. I think you will find it works out to 1.0. This is a typical repair for airframe skins and in landing gear components. Often to save time, we would show that the reworked dimension was still within mfg tolerance of the part and then just sign it off.

jetmaker

 

[qmechanics]

I filed and sanded the area nice and smooth. There's no evidence of a wear mark left. It was small and the area I smoothed blended very nicely. You can hardly tell it's there at all. The blending did not change the depth and there are no edges. Just a nice smooth blend. I don't think there's any concern about stress risers. The depth is still .006 with a blended area about the size of a dime. I have Petersons but cannot find what you're referring to Jetmaker? Also read that the variation in thickness from the manufacture can be as much as 12% which works out to .00564 inches. Pretty close.

 

[jetmaker]

ok, pretty close does not fly tho. However, if you calculate the properties based on minimum drawing dimensions and can show worse than those for the blended condition, then you are good.

As for Peterson, it is in the notched bar section. It is a chart which shows that the variation in angle of the notch (v-notch vs U-notch). Unfortunately, I do not have a copy of Peterson available at this time. If you can not find it, I'll try and get the book.

jetmaker

 

[bf109g]

I think this is the chart jetmaker is talking about. As the notch angle approaches 180, the stress concentration approaches 1.

 

[jetmaker]

star for you bf109g...

 

[qmechanics]

Thanks guys. This is very helpful.
I computed the angle to be 178.645...
which corresponds to a stress concentration factor
of 1.05

 

[qmechanics]

dye pen revealed NO cracks, pits or edges. nice and clean.
Can't get the manufactures specs jetmaker.
Did all the steps b109g suggested in an earlier post. The manufacture is not forthcoming with data.
Not much more I can do short of replacement. I have to add that if I do that it's likely to do more damage. If the attachment member is removed and a new one welded in place the new part will have to be exactly where the original one was. Off by as little as a few mils and the tailcone will not go back on. It's that tight. That would require a whole new tailcone or the replacement of the last bay then re-rigging the drive shafts etc. Could take months.

 

[rb1957]

i think you've done a thorough job on this ... you've got a very benign blend, possibly from the OEM build; you've looked into a bunch of aspects and considered the damage form a whole bunch of directions.

i don't think you need to install a s/gauge and collect real data.

you think the damage won't get worse with continued ops ('cause you think it is original); that'd be my main concern.

 

[qmechanics]

That's what we think rb1957. Looks like it came from the factory like this. We will never know the truth but by looking at the geometry there's just no explanation for how this
wear mark could have been made. There's nothing there to interfere...not on this helicopter. I think this frame may have come from a different ship that was at the factory being overhauled. Who knows? This isn't the first time something strange like this has happened. They had a crew grinding the shot peen off grips because the crew thought it was a defect. Thanks again for all your help!