Strain Measurements on an Old Aircraft

[AliBaig1991]

I have recently started working on fatigue life estimation of very old aircraft which have already accumulated more than 2500 flying hours. Our parent organization installed strain gauges on an aircraft and recorded strain values during a simulated sortie. I am confused about the significance of strain values. Since the aircraft is old and it will have accumulated lot of damage during its service, will the accumulated damage in anyway affect strain readings? Put in another way, consider two aircraft, one is brand new and has zero damage and one is very old and both of them are flown for one hour in exactly same fashion. Will the strains readings be same for both or different? I will really appreciate if you guys share any good reference material for understanding this?

Thank you so much.

 

[Sparweb]

Is the accumulated damage significant enough to redistribute stresses through the fuselage?
Probably not, assuming the plane doesn't have any gaping holes in it.
Gut feel: The amount of damage needed to affect the distribution of strain in strain gauges would have to be so significant that it could be seen visually.

The damage you are looking for and addressing in an airframe life extension is "small amounts of cracking less than an inch long". That's a flagrant oversimplification and I'm sure I'll be lambasted for writing it on the internet, but as far as a conceptual framework is concerned, I am using it to attach your expectations to a specific visual image of a typical skin crack or beam web crack that is:
a) big enough to just detect during a detailed inspection
b) not yet big enough to compromise the airframe
c) will soon grow to a size that CAN compromise the airframe.

An inch-long crack on the crown skin will have a stress concentration around it in an area at most 6 inches around it. Farther away, any strain gauge wouldn't see a change.

So there could be spots in high-stress areas (wing roots, stabilizer roots, landing bear beams, etc.) where a fairly small crack could possibly cause a redistribution of stress (and hence, strain) and your strain gauges could tell you lies, if placed nearby. Otherwise, they're basically telling you the stress in the structure they're attached to.

Just generally about life extensions... In a modern aircraft designed with the DTA philosophy (unlike your old bird) the time to get to this state of cracking should be about 100,000 hours. Older planes without any fracture mechanics in their design don't benefit from that intent, and are overdesigned as a result. At least they should be... Since it can't be proven, you are now faced with RETROACTIVELY applying fracture mechanics to the old plane's structure and working it out.

Bear in mind there are plenty of OTHER ways that strain gauges can lie to you or fool you. If you're posting this question because you have the strain-gauge data and you can't make sense of it, well that's a whole new direction of inquiry.

 

[SWComposites]

Presumably aluminum airframe?
2500 hrs doesn’t sound like much at all. What kind of aircraft?
Does the airframe have any corrosion?

 

[drawoh]

Put in another way, consider two aircraft, one is brand new and has zero damage and one is very old and both of them are flown for one hour in exactly same fashion. Will the strains readings be same for both or different?

They will be the same, unless one airframe is damaged to the point that the loads pass through in a different direction. If the strain gauges had been installed when the aircraft were new, you should see plastic strain due to creep.

Do airframes creep?

--
JHG

 

[rb1957]

getting good data from strain gauges is very difficult.

to answer your question, like the other replies, there should be no difference between a "virgin" airframe and one that has done some time. There are of course caveats like very specialised aircraft or structures and corrosion.

but when you installed the s/gauges, how did you ensure that the gauge was unloaded (when you installed the s/gauge) ?

Did you s/gauge the wing of the fuselage ?

Are we talking about a highly concentrated loadpath (like a strut) ? something you could take off the plane and calibrate the s/guage ?

More sensible answers will require more data ... like airplane type, why you're extending the fatigue life (2500 hrs doesn't sound like much ?) ? how much you're extending it, do you have original data ? do you know the current fatigue life ? do you know the conditions that that life was derived from ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

Also, another consideration...

OLD general aviation airframes are FAR MORE susceptible to corrosion and wear-damage due to materials-used, poor/minimal protective finishes, rough/loose assembly practices, poor maintenance/inspections, etc.

AND YES, 2500-hrs is very low-time for a GA aircraft. I was the USAF engineer for Cessna T-41A [modified C-172] trainer aircraft with well-over 25,000-hrs/Acft. We accomplished the random partial disassembly [NOT structural de-fastening]... and detailed visual inspections of... 10% of the fleet and found 'normal' wear and corrosion damage... and relatively minor cracks in secondary structure and 'rare' substantial damage in flight critical primary structure [primarily wear-fretting].

CAUTION. The USAF T-41A aircraft were assembled with maximum corrosion protective finishes and under the eyes of USAF contract inspectors... and included nominal sheet-metal deburring, etc. Many 'high volume production' GA aircraft in the 1960s and 1970s were often built with minimal corrosion protective finishes... unless specifically ordered as an 'option' [paid-for] by the customer to include alodine and primer on the sheet metal. In many cases, these low-cost/fast assembly practices did NOT include deburring sheet metal parts. MANY of these aircraft had short/hard lives and fared-poorly in corrosive climates... and are 'mostly retired' [scrapped].

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[WKTaylor]

NOTE1.
Often older-design/certified [waaaaay-out-of-production] airframes** are randomly selected for formal/detailed destructive tear-down. This procedure often involves the structure, mechanical and electrical/electronic systems. First the major assemblies are separated... interiors, doors/panels, cowling, engines/mounts, control surfaces, wings, stabilizers, tanks, landing gear etc. Second the 'systems components' are carefully removed and cataloged for each 'section'... brackets, bearings, pulleys, cables/rods, common hardware, electrical wires/harnesses, clamps, etc etc... and their condition is cataloged for 'condition and serviceability'... superficially or to any higher definition... depending on the task-at-hand. Finally the structural elements are carefully de-fastened/separated by experienced technicians, piece-by-piece... and their condition is cataloged for 'condition and serviceability'. In many cases, unanticipated deterioration is often uncovered and cataloged... and provides significant 'ah-ha' moments regarding these designs.

**For multiple years of manufacturing... and/or various/significant 'model' configurations... this usually involves several aircraft, simply because design/model variations, manufacturing practices and 'other changes' naturally occur/evolve over time.

The resulting 'catalog' of visual and nondestructive inspection findings are used to build a 'picture' of the aging/damage condition unique to that aircraft and it's details. Together, the findings from several aircraft form a 'picture of the overall aircraft trends'... structural, mechanical, electrical, corrosion, etc.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[crackman]

AliBaig1991

The 2500 hours you quoted in many cases is not atypical of past failures. In fact, many GA aircraft in the past have had wing failures. Here are a few for reference all of which occurred at less than 10,000 hours:

Piper J-3 Cub - 1947 - 566 flight hours
Twin Pioneer - 1957 - 563 flight hours
DHC2 Beaver - 1964 - 1158 flight hours
Beech Model 18 - 1972 - 8227 flight hours
Piper PA-28 - 1987 - 7488 flight hours
Commander 680 - 1990 - 8346 flight hours
North American T-6 - 2005 - 7742 flight hours
PZL M18A - 2013 - 7013 flight hours

Anyways, the usage of the aircraft is paramount in understanding any failures at low time.

Best of luck