NDE Fasteners 1

[Jimbo2010]

I have some zinc plated fasteners that have seen approximately 5 years of service and the usual policy is to exchange new for old. However timescales are preventing the procurement of new ones and I am looking to perform either DPI or MPI to the fasteners to see if they have developed any surface cracks. Originally, at manufacture the studs were fully volumetrically inspected. Has anyone any advice/experience in performing surface inspections through zinc coated items? And the feasibility of getting meaningful results?

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[stanweld]

If you are removing the fasteners for examination, wet fluorescent MT using a bench tester has proven effective at detecting cracking in ferritic steel fasteners whether zinc plated or not. UT has been effectively used to detect cracking in fasteners left in place. I always required blind tests of my UT Examiners.

 

[CoryPad]

I agree with stanweld, magnetic penetrant examination is suited for this work. What size are the fasteners?

 

[Jimbo2010]

The thread is 1.5/8" so relatively big fastener. How do you determine the difference between a crack in the Zinc coating and in the steel of the fastener??

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[ndeguy]

MPI is flux-leakage based. Zinc is non-magnetic and so no flux will be induced in the plating, thus if there is a discontinuity in it but the magnetic steel substrate is sound there will be no magnetic particle indication. Only if there is a discontinuity in the steel substrate will the particles be held by the leakage flux around its location. Essentially you are testing the integrity of the steel fastener and not the integrity of the zinc plating.

 

[CoryPad]

Great answer by ndeguy. The fluid can puddle in certain areas due to surface roughness and scratches, but you won't see a distinct, bright indication like you would for a crack.

 

[Jimbo2010]

Thank you for your help guys. I understand how this would be applicable in the shank of the fasteners that is straight but will this technique still work in the threaded region?

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[stanweld]

Yes.

 

[CoryPad]

Agreed, definitely suitable.