Fatigue failure of components made from Delrin (Ertacetal) 1

[rattler]

Hi,

I have a component made from Delrin (Ertacetal) which has failed; I suspect the failure is due to fatigue driven by vibration. I've looked at the failed surface under a low magnification microscope and there appears to be what looks like "beach marks"; as the material is white plastic these look a bit like flakes of cod! They are quite large and visible to the naked eye with a pitch of approximately 1 mm. Does fatigue failure of this type of material exhibit "beach marks" similar to metal?

Thanks

 

[1gibson]

What was the manufacturing process? If injection molded, could these be weld lines?

 

[rattler]

Hi,

The part is axisymmetric, turned from bar material - it is essentially a tube with a flange on the end. The failure has occurred between the flange and tube.

 

[TVP]

Yes, plastic parts show somewhat similar marks on fracture surfaces. Take a look at this article with excellent images of both metal and plastic parts:

http://www.imrtest.com/cms/repository/media/ArticleFractography.pdf

 

[Compositepro]

Plastic bars are extruded, so the molecular orientation and strength is in the length direction. A sharp radius at the transition from tube to flange is a stress concentrator and is poor design practice for plastic parts or even metal parts.

 

[mrfailure]

Beachmarks (in metals and plastics) indicate crack propagation and are most often associated with fatigue but be careful - they can also indicate another crack propagation mechanism. Does vibrationally-induced fatigue make logical sense? Delrim (POM) is susceptibel to chemical degradation and stress corrosion cracking in the right environments. Is it being exposed to a chemical environment? You may need additional lab analysis to really tell why this failed.

 

[rattler]

Hi - thanks very much for your replies; the component is used in the sea at a depth of approximately 50 metres and is attached to a device that is likely to generate vibrations.