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Fatigue life and damage

Hi, i'm trying to do my first fatigue analysis, i've tried to setup a EN and a SN, and i'm not understanding some things:

1) why the number N (cycles) has to be lower than the NC1 number?

2)why for the SN aluminum curve there is a NC1 number? aluminum can't resist for an infinte number cycle

3)I've tried to put an event of 10^7 cycles and the results show me a life value of 10^24 (impossible for aluminum 6082 T6), how the life number could be over the cycle value?

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Adriano Koga_20259

Can you elaborate a little better your questions, please?

Are you using OptiStruct or HyperLife?

Did you get an error message for 1) in the software? If so, could you provide the error message?

Can you elaborate more on the statement for 2)? Why can't aluminum reach infinite life? The same for 3)

I was looking for some curves in google, and aluminum 6082T6 looks just like any other. Maybe i don't know it well enough.

https://www.researchgate.net/figure/Experimental-results-for-aluminum-alloy-EN-AW-6082-T6-under-constant-amplitudes_fig2_282696211

Joshua Pennington

Hi Nicolo,

1) Generally speaking the number of loading cycles should be lower than NC1 so you may review the number of cycles in the life results. If you have a constant amplitude loading, for example, you'd have 1 cycle in your loading setup and then review the total number cycles in the results. Not sure why you'd want to input N>NC1.

2) You are correct life for Al is not infinite. The curves in HL are general references. Ideally you'd put in your own material data from the manufacturer. If you want a more detailed SN curve you'd use the two slope method.

3) Not quite sure of this result, but as I mentioned in point 1 you should enter in a single cycle, or for cycle history data interpret the results as the number of repeats of that history data.

Nicolò Lodini_21627

Can you elaborate a little better your questions, please?

Are you using OptiStruct or HyperLife?

Did you get an error message for 1) in the software? If so, could you provide the error message?

Can you elaborate more on the statement for 2)? Why can't aluminum reach infinite life? The same for 3)

I was looking for some curves in google, and aluminum 6082T6 looks just like any other. Maybe i don't know it well enough.

https://www.researchgate.net/figure/Experimental-results-for-aluminum-alloy-EN-AW-6082-T6-under-constant-amplitudes_fig2_282696211

Hi, I'm using Optistruct.

I've set an event of 10^7 cycle and there aren't error messages.

Aluminum does not have a stress value below which it can resist for an infinite number of cycles, usually a very high conventional value is chosen for cycles to which it can resist, but it remains a finite number. Indeed if you look the second line of the graph that you have shared it isn't horizontal.

I don't understand why the software shows me a life's value higher than the cycle life, above the 10^7 load cycle that i've set, there's no load applied?

Thanks