Load Limiting Shear Plate Material

[Shockdesigner331]

Hello All. I'm working on a project where I need a shear plate (think fusable link). Basically once a certain load is reached the "shear" disc fractures and the load is released. The disc will be grooved to set the load at which it fractures, but with the materials i'm used to the thickness is crazy small (.005 in"!) I'm looking for a material that's got about a 30 ksi or less shear strength, somewhat brittle (~Like 4140 is brittle, would like it to just shear cleanly), is machinable, somewhat corrosion resistant.

Any advice would be appreciated, I could go through all the books, or I can ask those who know...

-Mike

 

[EdStainless]

You can't do this with a pin? Wire makes great shear pins. It has precise diameter, know strength and it is available.

The biggest criteria for good repeatable shear is to use material with low work hardening. If you need corrosion resistance look at the ferritic or maternsitic stainless grades.

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[Shockdesigner331]

Thanks Ed, We don't want to use shear pins in this application. The shear feature has to be able to be replaced in the field. Our experience with pins is not that pleasant, removing the sheared pins has been a pain in the past.

We'd also like the shear to be such that there's a low percentage of penetration prior to shear. Is that related to low work hardening?

 

[EdStainless]

When I used to use shear pins I always made sure that the pin was 2-3 times the size needed for the load before we cut the groove in it. That way there would be no distortion of the pin. We did machine the grooves with diamond tools to control the shape. I liked medium strength martensitic stainless grades. They have enough ductility that fatigue was not a concern.
I always presumed that strain hardening was why some materials made it difficult to reproduce shear loads. They would be sensitive to strain rate.

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Rust never sleeps
Neither should your protection

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[Shockdesigner331]

OK, so now i'm thinking that I'm going to go with 416 stainless. I'm going with 416 (out of the martensite stainless) because of it's more "brittle" nature. Do you have any experience with shearing/punching fixtures? I'm wondering what sort of clearance that I would need. My setup is as such: 2 piece outer ring, and a 2 piece inner ring. The sheet to be sheared is sandwhiched inbetween (the respective rings are bolted together). I'm looking for a nice clean break, repeatablility is key. thanks

 

[NickE]

you could start with around 10% (of material thickness) for punch to die clearance.... (just make that whatever you are going to call your cutting components)

You also want to make sure that the parts that cut the plate are ~10pts greater HRc so that they dont wear each time the plate shears... unless they can be replaced each time also.

 

[EdStainless]

I would suggest that you go to 410. The free machining additions to 416 may make it difficult to calibrate in thin sections.
You need to have a groove or score line at the shear location. You know, like the thin lines stamped into blow out discs.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Shockdesigner331]

Ed, Do I really need the groove if I go with pretty tight clearances and the plate is loaded almost completely in shear? Adding the groove means that when the shear plate is replaced in the field, after failure, the groove needs to be aligned precisely. I suppose I could add a few dowel pins for precise alignment, but I was hoping just to use the bolt holes and bolt clamping force to locate and hold it in place.
To calibrate when it shears, I was going to change the length of the material being sheared (it doesn't need to be a complete ring)