Fatigue Testing

[dansoarr]

Hi I'm after some advice

If I have a component that must flex 50 times through its life (2 months), I have an automated test fixture that will flext the component and check for a failure, (true return position).

If I wanted to prove that the component woudl not fail in normal operation say to 1:10,000 at 95%, I belive I'd have to test 30,000 parts to 50 flex's.

Now I only have 10 parts available, although completly representative of the final part.

So can I test each of the 10 parts to 3,000 actuations, and deduce the same fact. i.e

Is it true or fair to say, To test to a failure rate 1:10,000 @ 95%, I could either :-
Test 30,000 components, once each Or
Test 10,000 components, 3 times each Or
Test 1,000 components, 30 times each Or
Test 100 components, 300 times each Or
Test 10 components, 3,000 times each Or
Test 1 component, 30,000 times each ?

Some help would be great.

Thanks

Arron

 

[IRstuff]

Clearly, the last case cannot possibly yield a plausible answer. You could survive 50 car crashes, but you'd be unlikely to survive 30,000 car crashes.

Moreover, since there are a number of long-term and thermal-related issues with fatigue failures that going past 500 actuationson a single unit wouldn't make much sense. Some materials tend to work-harden, so exercising beyond a certain point fundamentally changes the parameters of the material itself, thereby nullifying any potential conclusions. Likewise, mechanical working of a material that can fatigue induces heat, which can accelerate or otherwise alter material behavior as well.


On the other hand, if all pieces can handle 3,000 actuations, then there isn't a problem beyond whether the pieces you have really encompass ALL possible process and material variations.

TTFN

Eng-Tips Policies FAQ731-376

 

[IceStationZebra]

What do you mean by "1:10,000 at 95%"? Are you saying you want a 95% confidence that less than 0.001% of the components will fail?

What is the material? If metal, what is the loading vs. yield strength?

ISZ

 

[melone]

Sounds like you are looking for Mean Time To Failure (MTTF)....

 

[WGJ]

Using the true installation loadings, run about ten samples until failure and then do a Weibull analysis and follow it up by applying a 95% confidence limit. From the 95% conf curve, read off the B5 (the point where 5% of the population has failed, with 95% confidence)

Use some Weibull software, it takes away the headaches.


Bill