Minimum Allowable Cycles in Cumulative Damage Report

Hi,

In Cumulative Damage Report in CEASAR, the top two lines show "Highest Usage Ratio" and "Minimum Allowable Cycles"

What is meant by "Minimum Allowable Cycles" and how it is calculated?

Thanks

MW

Any body?

Usage Ratio is actually the Accumulated Damage Ratio. It is calculated for EACH NODE.

The APPLIED NO. OF CYCLES is IDENTICAL FOR THE WHOLE MODEL, so that, for each NODE, the Applied No. of Cycles is the same.

Consequently, the HIGHEST "Usage Ratio", representing the Applied No. of Cycles vs. Allowable No. of Cycles, yields to the MINIMUM Allowable No. of Cycles.

NOTE.

If ONLY ONE CYCLIC LOAD or STRESS RANGE is considered in analysis, then the Highest Usage Ratio and Minimum Allowable No. of Cycles correspond to the HIGHEST CYCLIC STRESS RANGE RATIO found by CII analysis. The Minimum Allowable No, of Cycle may be (actually it is!) taken from the applicable Fatigue Curve, based on that Highest Stress Range found by the stress analysis.

Otherwise, if 2 or more cyclic loads or stress ranges are to be considered, the Accumulated Damage Ratio (or "Usage Ratio" as named by CII developers) needs to be calculated for each section/node. The actual/minimum allowable no. of cycles is established from the Highest "Usage Ratio" value.

Thanks Dorin for the reply. As usual you have been very helpful.
In my case I have three cases of fatigue as shown in the image attached. The minimum alloable cyles reported by CAESAR are 238698.
I have few queries.
What is the significance of "Minimum Allowable Cycles" and why it is termed as "Minimum"?

Since there are three cases, I could not understand which case minimum allowable cycles are for.

Another question is how to calculate "Minimum allowable cycles" from cumulative damage ratio which in this case is 0.23.

Thanks for your help

MW

For multiple cyclic actions, like your analysis case (thermal + wave induced movements), the single relevant result, with obvious significance, is the so-called "Usage Ratio", meaning the Accumulated Damage ratio, which is:

[ (Applied Cycles No)/(Allowable No of Cycles) ]_Load#1 +
[ (Applied Cycles No)/(Allowable No of Cycles) ]_Load#2 +
[ (Applied Cycles No)/(Allowable No of Cycles) ]_Load#3 =

(N_applied/N_Allow)1 + (N_applied/N_Allow)2 + (N_applied/N_Allow)3 < 1.00,

and which needs not to exceed 1.00.

Personally, I am always interested in that resultant/overall "Usage Ratio", which is the relevant fatigue analysis result.

I am not aware about that "Minimum Allowable Cycles" number. Have a look in caesar II documentation, but it's unlikely you'll find the clarification there...
Maybe somebody from COADE/Intergraph/Hexagon... might clarify this issue, which is anyway, in my opinion, less relevant for a fatigue analysis result interpretation.

Thanks Dorin for your help.

Perhaps Intergraph people will explain this.

Cheers and stay safe

MW

The "Minimum Allowed Cycles" is the smallest allowed cycles, on any node, on any element, anywhere in the report. To see this, send the Cumulative Usage Report to either Word or Text, then search for that (minimum) value.

Dear Mr. Ay,

Thank you for your response.

However, could you please be more specific, and clarify how is established that "smallest allowed cycles", probably on the highest loaded/stressed node/element?

For one cyclic load and one stress range accounted, the matter is simple indeed, and no explanations are needed.

But, for two or more cyclic loads accounted, that "smallest allowed cycles" number is not so clear anymore, at least from my point of view.

For Mr. Waheed case, there are 3 cyclic stress ranges accounted: S1,S2,S3. If, from the applicable fatigue curve, the corresponding maximum Allowed No. of Cycles are Na1, Na2 and Na3 respectively, and the effective applied No. of ccycles are N1, N2 and N3 respectively, then the "Usage Ratio" = Accumulated Damage Ratio is:

D = N1/Na1 + N2/Na2 + N3/Na3 (that should be < 1.00).

Such assessment is performed for each Node/Element, and the highest "D" value is stated, corresponding to the highest "fatigue loaded/damaged" section of the piping system.

However, is still unclear how that "smallest allowed cycles" number is established in such case.

In general, N1, N2, N3 applied cycles are different, the S1, S2, S3 stresses vary from one node/element to another, which means that Na1, Na2 and Na3 also vary from one node to another, and, it may happen the highest "stressed" Node/Element (e.g. where the MAX value from S1, S2, S3 is found) NOT TO COINCIDE with the Node/Element where the highest "D" usage ratio is found.
In such situation, which is the "smallest allowed cycles" concept significance, and what approach is employed to evaluate it?

In my opinion, an "Allowed No of Cycles" should be related to a Stress Range Value and to an applicable Fatigue Curve. In the situation described above, there are 3 Cyclic Stress Range values, with 3 different applied numbers of load-unload cycles.
Therefore, the "smallest allowed cycles" concept is not anymore so clear.

Thank you for your time,

Kind regards,

The report shows the maximum allowed cycles for the conditions analyzed and summarized. Of all of those maximum values, one of them is the minimum. This is what is reported as the "Minimum Allowed Cycles".

For example, lets say your model has two elements, and the maximum allowed cycles as computed is:

Element 1: 50,000
Element 2: 48,500

The minimum of these two maximums is 48,500. The report would list 48,500 as the "minimum allowed cycles".