Hi Lok Installation and cold working
Hi Lok Installation and cold working
(OP)
I have recently been in a discussion with one of my esteemed colleagues concerning the value of installing hi-loks in interference fit holes. According to what I can ascertain from the hi-shear site and our own process specifications, hi-loks are supposed to be installed in transition fit or clearance fit holes. It is my understanding that hi-shear developed the hi-tigue fastener with it's rounded transition from shank to thread that allows installation in interference fit holes therefore burnishing and cold working the hole upon installation. It is my contention that driving hi-loks into interference holes does not cold work the surrounding area (at least in a positive way as to increase fatigue life) but rather damages the hole and decreases fatigue life. Simply put,...does a hi lok "cold work" a the material when driven in to an interference fit hole and increase fatigue life of the material. Your feedback is greatly appreciated





RE: Hi Lok Installation and cold working
take a look into thread31-121526: Fatigue life enhancement with Hi-lite..
wk-taylor peovided quite an extensive explanation there..
Basically I have the same doubt as you are.
To answer your question, the best thing is for you to perform coupon test with different interference level. But you have to be carefull, which type of specimen to be used, a low load or a high load transfer with high secondary bending? And variation of specimen thicknesses and installation procedure (automatic or manual) will complicate you results even more..
RE: Hi Lok Installation and cold working
RE: Hi Lok Installation and cold working
A hi-lok body does not expand to fill a hole like a rivet does.
Cold working of holes is done with a "coining" process, prior to fastener installation.
RE: Hi Lok Installation and cold working
They open up the holes through the steel/titanium parts only with larger hole diameters in an attempt to match the spring rates of the base material for strain if there is an Aluminum and Steel/Titanium stackup.
RE: Hi Lok Installation and cold working
What is not in doubt is that fatigue life improves with an interference fit fastener. This is seen in the SSF analysis method used by Niu. The fit factors for an 0.001" interference pin exceed those for a driven rivet. See his Section 7.7 of his Airframe Structural Design. There's a small paragraph that states this explicitly, and his beta factors in Fig. 7.7.29 indicate that the best among the listed fasteners is a taper-lok (which is a high interference fit).
Coining or split sleeve cold working also improve fatigue life, but that is because of the way they prepare the hole, introducing a band of compressive stresses around the edge. FTI (the company that produces the split sleeve stuff) specifically shows in their test data that adding an interference fit fastener on top of the cold working produces a further increase in fatigue life (though whether it is worth the effort of doing both is debatable). Frequently, the interference fit and cold working are used only when you need that last extra bit of fatigue life to show the part life good.
Hope this helps.
RE: Hi Lok Installation and cold working
Thanks,
Skypunk
RE: Hi Lok Installation and cold working