10V45 Exterior Threaded Fastener Failure

[roberthathaway]

We have run into an interesting failure with this thread that is at the end of a small hydraulic cylinder. 90% of the failure is brittle cleavage, however 10% at thread root initiation is dimple rupture. We believe that the cyclinder experience impact loading, but why would it initiate through a ductile mode, why not brittle fracture across then entire cross section. The fastener is at 23 HRC but is not heat treated and the threads are cut. Any ideas as fas as why a ductile failure would progress into brittle would be appreciated.

 

[1gibson]

Crack propogation due to cyclic loading can appear to be ductile failure at first glance. Check "ductile" surface for signs of longer term exposure to atmosphere?

 

[metengr]

Did you examine the adjacent thread roots for similar cracks using NDT or a low power hand held magnifier? Second question - did you observe any signs of deformation (bending or stretch) in the entire threaded section?

It is possible that the threaded section could have been exposed to either higher than design tensile forces (or less severe impact loading) that induced either single or multiple thread root cracks.

After this event occurred you now have a pre-existing crack which can result in a one-time cleavage failure given notch senstive material and impact loading conditions.

 

[Tmoose]

can you post pictures of the fracture surfaces, and the cylinder installation?