Cad-plate fatigue life
Cad-plate fatigue life
(OP)
As standard practise, we specify the cad-plate of all steel parts we design to QQ-P-416F. However, it has come to our attention that this may dramatically reduce the fatigue life.
On page 287 of "Mechanical Engineering Design" by Shigley and Mischke it states "Metallic coatings, such as... cadmium plating, reduce the endurance limit by as much as 50 percent."
As a note, AN bolts are CAD plated.
Any thoughts, comments or, hopefully, an explanation?
On page 287 of "Mechanical Engineering Design" by Shigley and Mischke it states "Metallic coatings, such as... cadmium plating, reduce the endurance limit by as much as 50 percent."
As a note, AN bolts are CAD plated.
Any thoughts, comments or, hopefully, an explanation?





RE: Cad-plate fatigue life
Cadmium plating provides excellent corrosion protection, that's why it was widely used. Cadmium can also be toxic, so that's why it is not being used for new designs. The typical material now used for medium strength fasteners in the aircraft world is A286 cres (usually with silver plating on the nut to prevent galling).
I've never seen where the fatigue endurance limit was adversely affected by a properely applied plating. However, plating ferrous materials with tensile strengths above about 150ksi requires a post plating embrittlement relief. This consists of baking the part in an oven, for a specific time and temperature immediately after plating, to drive off any hydrogen in the surface of the part.
Here's a link to a typical embrittlement relief spec:
http://www.raytheonaircraft.com/scm/supserv/contract/specs/bs_pdfs/BS25526%20Specification%20for%20Stress%20and%20Embrittlement%20Relief%20Requirements%20for%20Chemical%20Processes%20Revision%20-.pdf
Hope that answers your question.
Terry
RE: Cad-plate fatigue life
Providing post-plate embrittlement relief and pre-plate stress relief has been carried out properly, I can see no reason for the assertion stated in your 'Mechanical Engineering Design' Is this a recent publication ? Furthermore, how can cad. plate be deleterious to fatigue resistance when the very fact that it is there to prevent corrosion means it must afford some protection against corrosion-induced fatigue initiation.
The best thing is to get away from cad. plate on steel bolts altogether and use A286 as tbuelna suggests, or precipitation-hardening steels (15-5PH, 17-4PH, 17-7PH)if even higher strength is required.
RE: Cad-plate fatigue life
The precipitation hardening CRES alloys 15-5 or 17-4 are never used for fasteners, due to stress corrosion problems under sustained tension loads. A286 (or sometimes even 718 Inconel) is the material of choice. I have ocassionally seen 13-8Mo used, since it seems to exhibit fairly low notch sensitivity.
As a side note, most aircraft structural fasteners (Huck bolts, Hilocks, etc) are now titanium alloys. The cost break point on a typical commercial aircraft, like a 737, is about $300 per pound of aircraft weight. Titanium materials will cost about $50 per pound, so their use becomes quite cost effective.
regards,
Terry
RE: Cad-plate fatigue life
Just a little note about silver plated nuts.
They were by Textron for the assembly of the BAe146 wing skins to the spars and the result was a huge problem of multiple site corrosion damage inside the wings on the spar flanges. It has resulted in the scrapping of badly affected aircraft and mayor repairs on others and just about finished off the BAe 146 as a design.
Use silver plated nuts in high temperature applications (like on engines ), but NEVER use them on aluminium, especially in fuel tanks (just look at where silver sits in the galvanic series ).
Cad., Cat-Ti, or IVD aluminium is definitly the way to go for fasteners, although Airbus has recently been having notable success with a kind of ferrous coating on clip nuts (I don't recall all the detail but it was covered in an article in their company magazine a couple of years ago ).
If you seek more info about the BAe146 problems talk to National Jet in Adelaide, Australia.
Regards,
RE: Cad-plate fatigue life
RE: Cad-plate fatigue life
If Cad plating is followed by proper post plating treatment, its use will have minimal affect on the fatigue life. Look out for other platings, such as chrome, though, as chrome plating is full of small cracks that can propogate into the metal. They will reduce fatigue life just by being there
RE: Cad-plate fatigue life
Do not use Cad plating in parts that are subject to extreme temperatures as this can result in cad diffusion into the base material and result in cad-embrittlement.
RE: Cad-plate fatigue life
RE: Cad-plate fatigue life
www.eaa1000.av.org/technicl/corrosion/ticad.htm
RE: Cad-plate fatigue life
RE: Cad-plate fatigue life
Hi all
When I last worked @ Boeing we were using the Hi KOTE FASTENERS ON THE C-17. Though Lightening strike were still being tested for commerical use aircrafts. Here is a link you may find intresting.
http://www.hi-shear.com/fastener_hk_hk1.htm
Hi-Kote 1 is used as a protective coating on Titanium, Nickel based (Inco 718) and Cres alloy (A-286, 300 series stainless) products, most of which are externally threaded fasteners. Hi-Kote 1 facilitates the installation of fasteners as it's durable surface prevents galling during push in. It has also been successfully used on aluminum rivets to provide primary sealing around the fastener and eliminate the need for wet sealant.
Hi-Kote 1 is widely accepted as the leader in protective aluminum coatings for aircraft fasteners, and conforms to the requirements of Hi-Shear Specification 294.
RE: Cad-plate fatigue life
I have also seen fastener service failures of cadmium plated fasteners that initiated at localized plating "arc burns".
RE: Cad-plate fatigue life
RE: Cad-plate fatigue life
3.2.1 Stress relief treatment: Unless otherwise specified (see 6.2), parts which have been machined,
ground, cold formed, or cold straightened after heat treatment, shall receive a stress relief
treatment in accordance with Table I or Table IA, as applicable, prior to shot peening, cleaning, and
plating for the relief of damaging residual tensile stresses. For fastener hardware, cold working of
the head-to-shank fillet radius and thread rolling after heat treat produce compressive stresses and
a stress relief treatment is not required on parts thus affected.
Regards, Wil Taylor