If there were a crack on an ejector shroud but repair facilities are not available, would it be more advisable to (1) stop drill the crack extremities or (2) leave the crack as it is and fly? For 1), while we can be quite sure the cracks will not propagate, is there any danger of having hot gases seeping through the stop drilled hole and affecting the airframe?
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stop-drilling on ejector shrouds
- Thread starter packard
- Start date
packard...
You have not mentioned wether the duct is thin or thick skinned. However, I will try to give You some rules-of-thumb learned by both testing and field experience.
If this is a hot gas duct, leakage of hot gas will occur thru the crack... and obviously thru a stop-drill hole. If You can't tolerate this, perhaps removing the part from service, or welding [as per appropriate methods, off-acft], or PATCHING would be viable alternates.
If crack is not leaking substantially, and/or it doesn't matter, then STOP-DRILLING the crack tip(s) is a viable alternative. Use the largest diameter stop-drill possible, MINIMUM diameter, of 0.187"D maximum 0.375"D, and DEBURR very CAREFULLY!!!! Then PLUG this hole with a fastener to match the environment, IE: aluminum duct use an aluminum driven rivet; or CRES/Inconel duct use a monel or A286 driven rivet. Install an NAS1097 [flush-shear hd] or equiv in a very thick-walled duct; or, if thin-walled [or cracking-prone], install an MS20426-style [brazier hd] fastener with head inside duct, tail outside... then grind-down head to approx 50%--60% of the original height. The fastener will swell in the SD hole and provide a clamping affect on the hole lips... extending re-crack time significantly beyond a simple deburred stop-drilled hole. I have also had success using a MONEL cherry-max style rivet and tight-fitting tension-rated Hi-Loks [thick head and cress collar] that are tightly fitted to the hole [flat surface required]... DO NOT GRIND DOWN THE HEADS of thes fasteners!!!
One-other option, superior to stop-drilling/plugging the crack-tip is to stop-drilled hole, COLD-WORK [CX] the hole and then plug it. See the FTI website noted below for the SsCx process. If you want a lot-more life out of Your SD/plugged holes this will do it!
NOTE for weld-repairing a part: stop-drill the crack-tip with a small diameter bit [than can easily be plugged by a welder], then slott-out [or grind-out] the crack to eliminate the contaminated crack-faces. Use 100% surrounding/dry inert gas-purge [all sides], appropriate TIG and weld-rod; then post-weld stress-relieve [ferrous and HRA materials]. I found that the stop-drill minimizes thermal stresses/strains on the weld, minimizing re-cracking and distortion during welding . Regards, Wil Taylor
You have not mentioned wether the duct is thin or thick skinned. However, I will try to give You some rules-of-thumb learned by both testing and field experience.
If this is a hot gas duct, leakage of hot gas will occur thru the crack... and obviously thru a stop-drill hole. If You can't tolerate this, perhaps removing the part from service, or welding [as per appropriate methods, off-acft], or PATCHING would be viable alternates.
If crack is not leaking substantially, and/or it doesn't matter, then STOP-DRILLING the crack tip(s) is a viable alternative. Use the largest diameter stop-drill possible, MINIMUM diameter, of 0.187"D maximum 0.375"D, and DEBURR very CAREFULLY!!!! Then PLUG this hole with a fastener to match the environment, IE: aluminum duct use an aluminum driven rivet; or CRES/Inconel duct use a monel or A286 driven rivet. Install an NAS1097 [flush-shear hd] or equiv in a very thick-walled duct; or, if thin-walled [or cracking-prone], install an MS20426-style [brazier hd] fastener with head inside duct, tail outside... then grind-down head to approx 50%--60% of the original height. The fastener will swell in the SD hole and provide a clamping affect on the hole lips... extending re-crack time significantly beyond a simple deburred stop-drilled hole. I have also had success using a MONEL cherry-max style rivet and tight-fitting tension-rated Hi-Loks [thick head and cress collar] that are tightly fitted to the hole [flat surface required]... DO NOT GRIND DOWN THE HEADS of thes fasteners!!!
One-other option, superior to stop-drilling/plugging the crack-tip is to stop-drilled hole, COLD-WORK [CX] the hole and then plug it. See the FTI website noted below for the SsCx process. If you want a lot-more life out of Your SD/plugged holes this will do it!
NOTE for weld-repairing a part: stop-drill the crack-tip with a small diameter bit [than can easily be plugged by a welder], then slott-out [or grind-out] the crack to eliminate the contaminated crack-faces. Use 100% surrounding/dry inert gas-purge [all sides], appropriate TIG and weld-rod; then post-weld stress-relieve [ferrous and HRA materials]. I found that the stop-drill minimizes thermal stresses/strains on the weld, minimizing re-cracking and distortion during welding . Regards, Wil Taylor
- Thread starter
- #3
Wil, shroud is thin skinned 0.021" and is surrounded by firewalls attached to the airframe. Going by 3x thickness rule of thumb, stop-drill size is relatively small, but I cannot quantify gas leakage. Shroud encloses the passage for hot exhaust gas ejected from aircraft rear. My personal opinion is that leaked gas will still be protected by the firewall and should not pose a problem to the airframe.
Blinds have to be used because of access, but plugging stop-drilled holes is not the norm. Perhaps you can educate us on the circumstances which will lead to a need for plugging up of stop-drilled holes?
- Thread starter
- #4
Wil, shroud is thin skinned 0.021" and is surrounded by firewalls attached to the airframe. Going by 3x thickness rule of thumb, stop-drill size is relatively small, but I cannot quantify gas leakage. Shroud encloses the passage for hot exhaust gas ejected from aircraft rear. My personal opinion is that leaked gas will still be protected by the firewall and should not pose a problem to the airframe.
Blinds have to be used because of access, but plugging stop-drilled holes is not the norm. Perhaps you can educate us on the circumstances which will lead to a need for plugging up of stop-drilled holes?
Packard... A few precautions...
IF this is an exhaust duct, or other duct seeing direct impingment of hot gasses, then stop-drilling is Your only option. A fastener head protruding into the hot & High-velocity airflow would be bad news [erosion, heat, etc].
If no critical systems are in cavity between duct and fire-wall(S), and/or cavity is well vented away from systems, probably no problems with hot gas leakage thru a tiny SD hole.
If cracking has occured, then SD hole MUST be large-enough, and placed properly, to not only capture the crack-tip 100%** but also "clean-out" the strain-hardened zone at the tip of the crack. Drilling with a SHARP drill-bit to minimize localized strain hardening and heat-build, combined with deburing is essential to retard re-cracking. Stop-drill-holes at the crack-tip essentially change the crack from being an extremely long and very narrow sharp-tipped cut into equivalent of a "radiused slot". The bigger the SD hole Dia, the wider the "slot" [aspect-ratio W/L] becomes, reducing stresses proportionaly. A hole is a special-case slot where Width = Length = 2X Radius.
**CAUTION: some cracks are deceiving and may crack thru to each surface at a severe angle... such that it LOOKS like the crack-tip was captured... but on opp side the crack tip extends past the SD hole!!!! This has happened to me!!!!
Plugging a hole with a fastener....
Every open hole distorts [strains] under loading. Cyclic distortion is the mechanism for fatigue-crack initiation and growth. A tightly installed plug [fastener] in a hole inhibits strain deformation of the hole... during both compression and tension loading [strain] cycles. Minimize or eliminate hole deformation and cracks have a hard time initiating and growing! Plugging a hole can reduce the effective stress concentration from Kt = 3, down to Kt roughly =~1... resulting in a huge rise in fatigue "life" [time for crack initiation and time to grow beyond affects of the SD/Plugged hole]. This is Why ALL "open holes" ABSOLUTELY should be plugged. Holes that cannot be plugged [systems-pass-thru, lightening, etc] usually have-to-be reinforced [flanged, "padded", etc] to minimize strain-induced cracking! Plugged crack-tip SD holes get a huge rise in retardation for the same reasons as "open holes".... but can never be equivalent to an un-damaged plugged hole. Regards, Wil Taylor
IF this is an exhaust duct, or other duct seeing direct impingment of hot gasses, then stop-drilling is Your only option. A fastener head protruding into the hot & High-velocity airflow would be bad news [erosion, heat, etc].
If no critical systems are in cavity between duct and fire-wall(S), and/or cavity is well vented away from systems, probably no problems with hot gas leakage thru a tiny SD hole.
If cracking has occured, then SD hole MUST be large-enough, and placed properly, to not only capture the crack-tip 100%** but also "clean-out" the strain-hardened zone at the tip of the crack. Drilling with a SHARP drill-bit to minimize localized strain hardening and heat-build, combined with deburing is essential to retard re-cracking. Stop-drill-holes at the crack-tip essentially change the crack from being an extremely long and very narrow sharp-tipped cut into equivalent of a "radiused slot". The bigger the SD hole Dia, the wider the "slot" [aspect-ratio W/L] becomes, reducing stresses proportionaly. A hole is a special-case slot where Width = Length = 2X Radius.
**CAUTION: some cracks are deceiving and may crack thru to each surface at a severe angle... such that it LOOKS like the crack-tip was captured... but on opp side the crack tip extends past the SD hole!!!! This has happened to me!!!!
Plugging a hole with a fastener....
Every open hole distorts [strains] under loading. Cyclic distortion is the mechanism for fatigue-crack initiation and growth. A tightly installed plug [fastener] in a hole inhibits strain deformation of the hole... during both compression and tension loading [strain] cycles. Minimize or eliminate hole deformation and cracks have a hard time initiating and growing! Plugging a hole can reduce the effective stress concentration from Kt = 3, down to Kt roughly =~1... resulting in a huge rise in fatigue "life" [time for crack initiation and time to grow beyond affects of the SD/Plugged hole]. This is Why ALL "open holes" ABSOLUTELY should be plugged. Holes that cannot be plugged [systems-pass-thru, lightening, etc] usually have-to-be reinforced [flanged, "padded", etc] to minimize strain-induced cracking! Plugged crack-tip SD holes get a huge rise in retardation for the same reasons as "open holes".... but can never be equivalent to an un-damaged plugged hole. Regards, Wil Taylor
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