Interference mountings 2

[shingouz]

Hi,

I am intersted in interference fitted screws and found in literature that compressive stresses introduced in plates reduces maximum stress of a stress cycle and also stress amplitude, I was wondering how this effect is achieved?
Thanks

 

[debodine]

My background is electrical so this is not my field and I cannot answer your question, but i would like to ask a related question of the experts who do respond. Is this phenomenon described in the OP the process known as "cold working" a hole? I have heard that term and for my own education I wanted to know if I was correctly defining that term in my own mind.

Thanks in advance for the enhancement of my education!

 

[rb1957]

google Fatigue Technologies Inc, ForceMate, etc ...

basically, the pin is bigger than the hole (causing the interference). pin is installed either mechanically using a mandle to force it into the hole, or freezing the fasteners (with liquid N2).

 

[berkshire]

debodine,
Also google work hardening, or strain hardening.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[YoungTurk]

Basically, the stress field from the cyclic stress is superimposed over the compressive stress around the hole perimeter. So the net tension stress is reduced due to the pre-existing compressive stress.

Interference fit may cold work the hole to some extent, though the amount depends on the particulars of the stackup. The compressive stress around the hole is a result of the combination of the residual stress due to yield around the fastener hole (the cold work) and the elastic stress due to the outward force held by the fastener shank. A hole may be cold worked with a split mandrel which is then removed, leaving an open hole with the cold work improvement but no elastic compressive stress.

 

[debodine]

berkshire:

This is how the Wikipedia entry for Work Hardening begins: "Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation." Engineers Edge makes the same connection between the three terms. Thank you for the suggestion as that definitely helps me to relate these terms. I found several good articles that talk about the forming of tiny dislocations that eventually prevent future dislocations, thus as the metal is under strain it becomes less ductile and more brittle.

YoungTurk:

If I am understanding you correctly, the cyclic stress is the continual application and removal of the load. So the stress introduced would normally also deform the metal around the fastener if the metal around the fastener were not already work hardened. Instead, because the hole is already work hardened, the load transfers completely (in a perfect world at least) through the metal and fastener and into the load path until disspated as intended, rather than some of the energy being dissapated in plastic deformation of the metal around the fastener. Thus the work hardening means that the design uses controlled conditions (work hardening per a deliberate process with a reliable result) rather than uncontrolled conditions where the amount of plastic deformation introduced by the cyclic load could make unintended and undesired changes in the metal properties. Do I correctly understand your point?

Many thanks to both of you for taking the time to improve my knowledge!

 

[debodine]

Man I took two stabs at spelling "dissipated" and blew it both times!

 

[YoungTurk]

Not quite. The point is that the compressive stress around the hole due to the interference fit counteracts the peak tensile stress concentration around the hole.

The "cyclic" stress is just a term for stresses which occur under normal operating conditions, as opposed to the stress when it is sitting in the hangar. The term "cyclic" is usually used when talking about fatigue, which is typically the primary driver for specifying interference fit fasteners. This also explains why the compressive stress around the hole is beneficial, since it reduces tensile stress, which is the cause of most fatigue failures (cracks beginning at holes).

My point about cold working was that the compressive stress has two components, the elastic component, which would go away if the fastener or tool is removed, and the plastic component (due to "cold working") which remains even if the fastener or tool is removed. The plastically deformed circumference of the hole is held in compression because it has "pushed out" the material around the hole, which therefore has a force pushing back in toward the hole circumference. This means you can get some of this benefit by cold working the hole with a tool even is the hole is left open (no fastener).

The strain hardening is not, in my experience, a significant factor in the specification or analysis of the interference fit fastener joint. The strain hardening is not required to "control" the load path as you suggest. Maybe berkshire was making a point with the strain hardening that is beyond me.

 

[debodine]

YoungTurk:

Thank you for the follow up, and I think I better understand the scenario. This sentence really helped me:

"The plastically deformed circumference of the hole is held in compression because it has "pushed out" the material around the hole, which therefore has a force pushing back in toward the hole circumference. This means you can get some of this benefit by cold working the hole with a tool even is the hole is left open (no fastener)."

What you were trying to teach me is that the compression from the cold working and the tensile stress concentration around the hole balance, in the absence of any other forces influencing them. I believe that the term used by persons in your field is "equilibrium". So from a static standpoint (in the hanger, no other acting forces except for gravity) the forces around the hole are in equilibrium.

I appreciate your feedback.

 

[rb1957]

"the compression from the cold working and the tensile stress concentration around the hole balance" not quite, the way i see it.

say you have a plain hole in a plate ... stress concentration at the edge of the hole is 3, right?, so when the plate is in tension the local stress is 3 times the applied.

now cold work the hole. expand the perimeter of the hole. this puts the edge material in compression. this compression is balanced by tension stresses in the far field of the plate. now there is a new stress at the edge of the hole (was zero, now is the compression). now when you apply tension, the hole stress concentration still applies, so the stress increases but it's increasing from some compression stress (not zero). so the fatigue stress cycle, the stress range (the difference between minimum and maximum stress) is the same but the mean stress of the cycle is lower (since the minimum stress is compression), the fatigue cycle R (= min/max) has reduced from 0 to something -ve.

... that's the way i see it

 

[berkshire]

Young Turk,
I was giving the search pointer on strain hardening, Because although plastic working of a hole is really compression hardening.
The methods are lumped together under work hardening or strain hardening to differentiate the method from heat treating.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[shingouz]

rb1957,
I agree with your analysis, mean stress is reduced due to the presence of compressive stresses but stress amplitude is the same. Now suppose that compressive stresses are elastic, Is the same effect occuring?
The original question I asked was about compressive stresses introduced when the screw diameter is larger (some micrometers) than the hole diameter so it introduces elastic compressive stresses near the hole edge

 

[rb1957]

"now suppose that compressive stresses are elastic" ... ? i think you mean "inelastic" as in "above yield" ... yes?

if you mean cycling (tension) stresses applied to a compression yielded material then this is obviously a more complicated/specialised problem. The people at Fatigue Technologies Inc (for example) make their livelihood out of this problem. As a result of their testing of their products (eg ForceMate) i'm sure they have some knowledge base, they might even have analysis tools to predict the life of their products; they might just say "it's much longer than it was" and leave it up to the customer to look into their own situation. I would expect that the interference effect would reduce over time (can you have creep in compression ?)