Today's KNOWLEDGE Share :Fatigue Failure

Today's KNOWLEDGE Share

Fatigue is a complicated aspect of plastic parts failure.

While most are aware of the need to keep frequency low in a fatigue test to avoid local heating of the sample, many may not be aware of the following extremely interesting tests to be done.

Let us say you test your sample (at some T and a given Stress Ratio, like 0.1) at 1 Hz and also 2 Hz.

Would you expect your part to fail after :

- the same number of cycles ? or

- the same testing time ?

Well, in a nutshell, if you fail at the same total time (twice the number of cycles) that indicates that your failure is Plasticity Controlled. A DUCTILE failure.

If you fail at the same number of cycles (i.e. half the testing time) your failure is Crack Growth controlled. A BRITTLE failure.

Of course things may be somewhat intermediate, but such simple tests will really inform you about the failure mechanism and help consider better materials for the application.

The test is particularly interesting for GF or CF filled polymers because the lack of visible deformation is often wrongly interpreted as BRITTLE fail !

source:Vito leo