Hi, As per the document "Fatigue

If you apply the individual spectra you miss some fatigue damage.

Adding the GAG cycle is a simple fix for this. defining GAG as flight max to ground min is another easy way to define it ... more precisely it is flight mean to ground mean.

The better fix is to run a rainflow analysis of the composite spectrum.

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

GAG cycle is defined as one cycle of load by combining max load from flight and minimum load from ground
Be careful - what exact load conditions are you using for the max flight and min ground loads?
The GAG cycle is typically defined from the normal 1g flight and ground load conditions.

To add on to what RB said, the way "damage" accumulates due to cyclic loading is due to both major and minor load cycles in a spectrum. You may have a bunch of smaller exceedances in some order that come from maneuver, gust, etc. But if you simply sum the damage from all of those smaller excursions, you will under-predict the actual percentage of life you use up per mission. There is damage that comes from major load cycles comprised of smaller fluctuations as well. The GAG cycle may be defined as the absolute min to absolute max in the mission.

Again, as RB mentioned, the way to analyze for the major cycles is to do a damage accumulation on a spectrum which has been cycle-counted, preferably using a technique which accounts for the order of loads.

Then your GAGDR becomes the ratio of GAG cycle damage to total damage (including the GAG cycle).

Usually for aircraft structures we will analytically define a single-cycle equivalent stress that causes the same amount of fatigue damage as the total spectrum. This makes F&DT a bit simpler because you can apply a munch simpler spectrum in a rogue flaw analysis.

Keep em' Flying
//Fight Corrosion!

Hi Jnnal
There are many papers on spectrum development in the public domain and in particular on the gag cycle, see the ASTMs. Just be very careful about trying to use one equivalent gag cycle to represent the same damage as a full spectrum, This usually does not work. You need to first know the damage from a full flight by flight spectrum to the figure out an equivalent gag spectrum and usually it is not as simple as a single cycle and takes several cycles if at all possible. There have been several short courses presented on this at both the ASIP and AA&S conferences in recent years, seek those out.
Good luck.

GAG stress cycle is the by far largest cycle in a transport spectrum, possibly 90% of the damage. In a fuselage most of the damage is due to pressure.

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

Where is the OP? (seems to be an all too common problem)

Hi

Hate to disagree with anyone here but gag damage being the most significant is just not true. It actually depends on the mission profiles and utilization as well as where on the airframe. For example,actual data shows large transport aircraft like 747, 767, 777, 787, A330, A340, etc have GAG damage of only 20% to 30% in the center fuselage. These are aircraft with average flight lengths of 10 to 14 hours where the gag cycle produces very little damage. Most damage is from gust since flights are long and exposed to turbulence for long periods of time. Now small narrowbodies flying short 1 hour hops like Aloha are driven by gag damage. I have as well as others presented much of this data at several structures conference in the past few years, all public domain.

I've made the same mistaken assumption as RB has. My work experience primarily centers around regional carriers (jets and turboprops) and smaller, meaning my fatigue philosophy is going to be swayed by simplified methodologies. There are some still in use today (the Chicago ACO's paper and the Seattle ACO's guidelines from the 1990's) which are meant to provide a "reverse-engineering" method to make modifications of these aircraft easier and safer. So I use this method, when appropriate, because it has the FAA's blessing, and that's as close to the truth as I need to get for a "hockey-puck antenna".

But those simplified methods are not meant to be the truth.

Hi Sparweb

Yes, lots of folks have made the same mistake and there are some issues with the simplified method too. Along with a friend of mine we teach a 40 hour class twice a year which goes thru the full flight by flight spectrum development process as well as the simple once per flight gag and detail the differences. We even present actual crack growth testing of the two methods. Anyways, unfortunately folks think DTA is all about fracture mechanics. But, if your fatigue loads and spectra are off 10 to 20%, your life will be off by orders of magnitude. Also, I have seen people use overly conservative methods and end up with very harsh inspections which end up unnecessarily damaging structure by frequent access. Over the years I have been tasked by customers to redo many of these DTAs with more representative flight by flight spectra, keeps me busy I guess

I'm sure many here might have seen OEM plots of GAG contours, specifically I'm thinking of those contained in Boeing's xx, colloquially referred to as "Book 2". I've not worked for Boeing but I have seen this data in a few contexts. The key parameter that will characterize what we're discussing is the GAGDR - that is, the ratio of GAG cycle damage to total damage.

As crackman pointed out it can be all over the place. For example for a 747, which is a widebody and makes long haul flights, there is more time for the gusts to have an effect on the accumulated damage. In the crown skin of the fuselage over the wing box, the GAGDR can be as low as 0.25. Whereas in the forward fuselage it can be over 0.90.

What I mentioned earlier with the concept of "single-cycle equivalent stress" will take the GAGDR into account. This can be worked into a "spectrum factor". As crackman has indicated (and as with any method) I think it's important to understand what's being done and how it works in order to know if it's a valid way of characterizing damage.

There is pretty brief description of the basic idea in Appendix D of AFRL-VA-WP-TR-2000-3030 (Enhancements to Repair Assessment Procedure and Integrated Design (RAPID). But there are a lot of other more detailed references.

Keep em' Flying
//Fight Corrosion!