Standard Parts are not DDPs (fatigue)? 1

[banag]

Why are the standard parts (actuator rod, bearing, bush or fastener) not considered as critical DDPs?
How does the inspection routines are derived for those standard parts?

The Manufacturer would have conducted the fatigue test. Unless those have super fatigue life, why aren't those parts considered in deriving inspection routine of the load paths (which includes those standard parts)?

 

[der8110]

Actually they are (or should have been!), keep looking through Chapter 4/5 for examples. And if not, look at another aircraft manual. In most cases these types of parts have "super fatigue life" as you call it, just as a matter of good design practice. Either intentional or by luck. And there have been/are/will always be those horses that get out of the barn, which will keep the AD process going until there is perfection in the universe.

On the military side, these components might fall under the MECHSIP program.

Under the FAA, many of these types of parts will fall under the "not required if the applicant establishes that their application for particular structure is impractical", which means linear elastic fracture mechanics does not predict well, and not that showing compliance is impractical for cost or schedule reasons (still happens in the 21st century). In these cases, the part is removed and replaced with new. "Safe life".

Some airframes have manuals that are very telling if you know what to look for.

 

[rb1957]

DDP ?

and I agree with der ... the designer using hardware should establish that it is adequate for the job ... statically, fatigue, thermally, ....

another day in paradise, or is paradise one day closer ?

 

[banag]

Thanks der8110 and rb1957.

I meant the 'Detail Desing Points' (potential for fatigue crack initiation) which are considered for deriving inspection routine for SSI (Structurally Significant Item) as DDPs.

I am still looking for airframe manuals. Meanwhile, few more thoughts on this.

Bolts bigger in size (relatively) are considered as DDPs. In other words, critical load carrying bolts should be analysed and exclusive inspection routine should be quoted. Mostly inspected by Magnetic particle technique.
And bolts smaller in size (relatively) are not considered as DDPs, even when the smaller bolts failure scenarios are studied to derive the inspection intervals for a fail-safe PSE (Primary Structural Element - equivalent to SSI).

I too agree, it is all in the hands of the designer to put the right parts in the right place. As a analyst (beginner), I am trying to get the feel of the fatigue life for the standard parts which seems kind of clouded (Again I am a beginner).

 

[rb1957]

I am currently working on a project where they do DTA on bolts (common hardware) and develop inspection procedures. What IMHO nonsense! Any operator will happily replace hardware (cheaper than inspecting). Now special hardware is a different question. Standard hardware should have a threshold based on DTA (using fatigue would create a safelife and unnecessary (and expensive) part tracking) so maintenance program is threshold, TBD procedure, no repeat, and part replacement.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Baskybang

I am sorta confused about Your question.

MOST aerospace bolt specs [at least NASxxxx bolts] refer to a very detailed procurement spec that manufacturers are obliged to adhere to. This spec details every aspect of fastener manufacturing and performance testing that cannot be detailed in a simple 'base spec'. Embedded within these specs are a host of secondary specs related to layers of details that mere mortal [dumb structure] engineer never-even dream of [nightmare of fastener manufacturing]. NOT to mention that the FAA has intervened with the fastener quality act that mandates 'traceability' for airworthiness.

Suggest looking at the following procurement specs [typical 160-KSI bolts]...

NAS4002 FASTENER, ALLOY STEEL, EXTERNALLY THREADED, 160 KSI Ftu, 95 KSI Fsu, 450 ºF

NAS4003 FASTENER, A286 CORROSION RESISTANT ALLOY, EXTERNALLY THREADED, 160 KSI Ftu, 95 KSI Fsu, 1000 °F

NAS4004 FASTENER, 6AL-4V TITANIUM ALLOY, EXTERNALLY THREADED, 160 KSI Ftu, 95 KSI Fsu, 450 °F

NAS4007 TESTING, METALLURGICAL, FASTENERS, EXTERNAL THREADS

NAS4008 FASTENERS, CORROSION RESISTANT NICKEL ALLOY 718, EXTERNAL THREADED, 220 KSI Ftu, 125 KSI Fsu, 900 °F

The noted procurement specification [examples only] provide clear requirements/layers of requirements that lead to high reliability parts.

Corporate fastener standards [BAC, DAC, MDC, Airbus, etc] also have very detailed procurement standards for fabrication, inspection and performance testing [etc]... and are sometimes more rigorous than their look-alike NAS counterparts... even though most are made by the same fastener companies on the same equipment.




Regards, Wil Taylor

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