Fatigue life in concrete under compressive forces with Miner's rule

[andresdeantonio]

I have found a fatigue S-N curve for concrete in a Danish code in order to evaluate a concrete wind tower under fatigue cycles. Most of the literature I have read for fatigue refers to metals, S-N curves have an alternating stress in the vertical axis and the cycles number in the horizontal axis. There are also some formulas (Goodman) that relate the mean stress to the alternating stress for brittle materials.

I have run some cases where I am not sure if the alternating stress is the stress to employ (particularly for concrete), and I will cite it with an example:

The concrete has a f'c = 45 MPa.

The S-N fatigue curve was done with the following values (theoretically in compression):

stress = 30.00 MPa, allowable cycles = 1
stress = 27.86 MPa, allowable cycles = 10
stress = 25.71 MPa, allowable cycles = 100
stress = 23.57 MPa, allowable cycles = 1,000
stress = 21.43 MPa, allowable cycles = 10,000
stress = 19.29 MPa, allowable cycles = 100,000
stress = 17.14 MPa, allowable cycles = 1,000,000
stress = 15.00 MPa, allowable cycles = 10,000,000
stress = 14.85 MPa, allowable cycles = 100,000,000 (endurance limit)

Now, let's suppose I get a maximum stress of 20 MPa and a minimum stress of 0 MPa. Therefore my alternating stress and mean stress will both be 10 MPa (according to the curve I am inside the endurance limit).

As a second example, let's suppose I get a maximum stress of 40 MPa and a minimum stress of 20 MPa, my mean stress will be 30 MPa but my alternating stress will continue being 10 MPa.

And to conclude, let's suppose I get a maximum stress of 50 MPa (I already exceeded my concrete capacity) and a minimum stress of 30 MPa, my alternating stress will continue being 10 MPa but it's clear that my compressive forces are much higher and could even break the concrete! But following the alternating stress criteria I am still in a safe zone!

Any input on how these stresses are employed for concrete, or S-N curves for compressive forces? Thanks so much.

 

[rb1957]

i'd question your interpretation of the data. the data listed is "stress = X MPa" and i think you're interpreting that as alternative stress. but that'd put the concrete in tension ???

i think (without anything else to go by) that the data is cycle max stress and cycle min stress = 0.

just my 2c ...

 

[andresdeantonio]

Thanks for your response, I thought the same, but I also feel that this information could apply for a "regular metal S-N curve". What if by employing a metal curve I exceded the yield strength in compression, then I shouldn't employ the alternating stress? Do you have any thoughts on this?