Fatigue Loads

[lLouie]

Hi

I have written about fatigue here many times. I found a source that I thought might be helpful, and I have a question.
To find the stresses in each spectrum (taxi, cruise), should I consider the fuel mass used by the aircraft in that spectrum? For example, should I include the fuel mass spent during 100 flights to calculate 1g stress in cruise? Or should I take the critical mass?

You can find the source in attachment.

 

[lLouie]

Damage = (Load)^4 I understood this. The analysis program (nCode DesignLife) calculates the damage. I don't think I need to go through this formula again. The main purpose is, after each flight of my plane, there will be damage to part X. There will be damage again on the next flight. My purpose is to add up these damages and find out what percentage of my plane's life has decreased.

The analysis program calculates these;
Damage of fligth 1 = 0.00244
Damage of flight 2 = 0.03432
Damage of flight n = 0.0542

Total damage = 0.09
1- 0.09 = 0.91

We think same, however mathematic is different

 

[rb1957]

ok, all is very unclear ! I don't get the last calc ?

ok, ncode has calculated the damage of three different flights

so how many flight 1 can you have ? ... 1/0.00244
how many of flight 1 and flight 2 ? ... 1/0.03676
how many of flight 1 + flight 2 + flight "n" ? ... 1/0.09096

can I have 10 flight "n" + 10 flight 2 + 100 flight 1 = 10*0.0542 +10*0.0342 +100*0.00244 = 0.542+0.3432+0.244 = 1.1292 ... so no (total damage must be <1)

clear as mud ??

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

 

[lLouie]

Thank you so much. I am grateful to you

 

[rb1957]

ok, now I've taken the time to re-read your previous post, what you're doing is fine (why you're doing it I'm not so sure).

Yes, your 3 flights have a total damage of 0.091, so yes you have 0.91 damage left.

Mind you, this is ignoring safe life factors and such.

What it does tell you is flight "n" is 20x more damaging than flight 1 (so you might look for a reason for this).

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

 

[lLouie]

Actually, I like learning so, I try to improve myself. Thanks to you, I have an idea for aircraft's fatigue method. The analysis program can be learned, but the theory of fatigue is the most important.

 

[lLouie]

Do you also calculate how much life is left after each flight after giving a certain life to the aircraft? Or how accurate is it to calculate this situation after each flight?

 

[rb1957]

that's not the usual way of doing the calcs. "how much life remains" depends on what type of flights you want to use. you can assume a flight mix to start with ... all flight 1, equal numbers of flight 1, 2, n and figure out what you can do. as you go along you can sum the damage accumulated, calc the remaining "damage", calculate the number of flight 1 that would equal this. The most significant problem with this is you are not accounting for a safe life factor, which you could by using sum damage = 1/SLF = 0.25 or 0.20 depending on how conservative you want to be.

Out of interest, how have you defined your flight types ? what loads ?

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

 

[lLouie]

As in the file I shared before, I am doing 5 analyses (taxi - climb - cruise etc.) for its flight profile. First, my loads will be in unit load(1g), then the fatigue analysis program will calculate these values ​​according to the flight profile and the life will be obtained.
After the stress values ​​in the unit load are determined, I will write a code that will calculate the remaining life of the aircraft in hours according to the flight profile in Matlab after each flight without making any analysis.