Fatigue Doubt

I understand in the aerospace industry that fatigue is number of life cycles to crack initiation and damage tolerance is the remaining life. In other industry like consumer or automotive (correct me if I am wrong), only fatigue is used to define the failure of the part.

My question is if fatigue is defined as number of life cycles to crack initiation, how would fatigue testing to stop when crack initiation happens and record the number of cycles for S-N curve?


There are a lot of misnomers and confusion that gets perpetuated regarding this terminology.

In the United States at least, this is in part due to how the continued airworthiness requirements per the FAA have evolved over time. Particularly FAR 25.571. Back in the day, when things were based only on safe-life criteria, there was no account taken for potentially rogue flaws and everything was just referred to as “fatigue”. The remnant of this still exists in 25.571 ©.

However, we have evolved from a safe-life approach to a fail-safe approach, and finally to the current standard of “damage tolerant design” This basically just means we consider the potential for macroscopic initial cracks and use fracture mechanics principles to account for widespread fatigue damage (WFD) and multi-element damage (MED). We still use safe life approach for things where inspection methods or time between detectable and critical could be very small, like landing gear.

Now first misnomer - DTA is not synonymous with crack growth. Notice the language of 25.571(b), the DTA requirement includes determinations for damage due to “fatigue, corrosion, and accidental damage”.

The bottom line:

  1. DTA is a damage tolerance assessment.

  2. Fatigue is defined as the accumulation of damage due to cyclic or repeated load. Period. What do you think is happening when “they” do a rotating beam test on a specimen? You have microscopic inclusions, grain boundary defects, etc which are growing. This is crack growth. All damage that results from cyclic load is crack growth. There are flaws that propagate in the structure and lead to failure. All damage that results from cyclic loading is fatigue, by definition. They are synonymous. This is why you usually see the terms “fatigue crack initiation” for the early stage and “fatigue crack growth”. These in my opinion, are more correct terms.

  3. The real distinction is how we deal with analysis to develop our continued airworthiness inspection programs. On one hand there is a requirement to check our inspection thresholds based on statistical fatigue data. That is, S-N curve data which has been statistically generated via test. The REAL distinction is that these specimen had no initially induced macroscopic flaw. That is, the life is from “pristine” condition, to fracture. [fracture is simply defined as the separation of a piece of material into parts]. So yes, you are correct they do not stop the test when they can detect a crack. The point is, if there is no initial “rogue” flaw, the total life is dominated by the time spent growing a grain-level flaw to the point where is is large enough to grow quickly. When people say fatigue is the number of cycles to get to the point of crack initiation, this is a bit of a misnomer as well. Remember, a crack is a crack, whether it is microscopic or not.
    In reality, the period you refer to is the time taken for flaws in a structure to reach Region II of the da/dN vs. deltaK curve (and the flaw may or may not be detectable at that point. At this point, the growth happens much more quickly. “Fatigue” is not at all defined as this life. Nor is “damage tolerance” defined as the remaining life. The total statistical fatigue life, with a scatter factor applied, is one criteria for the inspection threshold.

  4. Another criteria for the threshold, and the way we develop a repeat interval, is by using linear elastic fracture mechanics to determine the fatigue crack growth life of a piece of structure with an assumed initial macroscopic rogue flaw. So we more or less skip region I of the da/dN vs. deltaK curve (but no scatter factor here). We take to total fatigue life of this assumed initial configuration and apply factors to develop another potential threshold. Then we look at the detectable crack length we have and figure out the amount of time between detectable and critical. [Side note - critical is not always fracture, it might be NSY or some other criteria].

So in short, people like to say “fatigue” when referring to S-N data, and “crack growth” or “damage tolerance” when referring to LEFM. But in reality, they are the same principles, but different approaches. One approach is based on statistical data with no induced flaws. One is based on statistical data with an assumed flaw.

I think the confusion stems from the fact that the FAR used to only require a “fatigue” analysis, and this language was retained when another paragraph was added to include what we call damage tolerant design, which includes the account of rogue flaws.

In cyclic load ie fatigue, structures (especially assembled structures) which are normally ductile, can eventually behave in a brittle manner. We want to encapsulate this and there are multiple approaches. One way is to simply use test data & statistics. Another way to is assume we have a flaw and treat it with the principles of FM.

Above is a snippet.

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