Design bracket for failure at a certain load 10

[ajdb]

Hello all,

As the subject line says, I am curious about designing a bracket that will fail at a specific load.

At the moment my question is where should I start. When searching around on google I haven't been able to find much on the subject.

 

[WARose]

Would it not be possible to test until failure to verify that it will fail around the desired loading?

Sure....but for the situation I was describing above.....you have the predictable variables i was describing above. You'd just find out what you already know.

 

[Edu09]

There's probably many more sensible examples and when you mention bracket I guess you weren't thinking of reinforced concrete but what about an over reinforced beam. It's usually something we look to avoid but if designed correctly(!) then a sudden brittle failure at a certain load is guaranteed!

https://www.youtube.com/watch?v=3xw9_33uNJA

 

[Tmoose]

"Is there a general term for components that are designed to fail at ..... Y torsion ...? "

https://www.regalpts.com/products/c.../torque-overload-couplings/Pages/default.aspx

https://www.rw-america.com/products/industrial-couplings/safety-couplings/st1/

 

[rb1957]

Structural fuzes are common enough, to protect more expensive equipment.

The problem is how precise do you Need to be. You can design for a standard bolt and have it exceed it's allowable shear at some loading. The issue here is that most stock of standard bolts exceed the spec minimum strength, so actual in-service failure load is going to exceed the load based on spec minimums. If you Need to fail at a load, the common approach is to machine a groove in the bolt, creating a high Kt and creating a more predictable failure load. There should be a bunch of testing (and maybe batch testing too) to validate the failure load.

another day in paradise, or is paradise one day closer ?

 

[JStephen]

One problem you have is that in a lot of design, all the equations and methods are set up to maintain a minimum design strength, but nobody cares how much over that design strength the item actually is.

Without knowing the specifics- another possibility is something that gives way, not by breakage/shear, but by mechanical action. This has the potential to be a lot more accurate, and has the potential to be resettable.

 

[ajdb]

Just for anyone who may be interested or for future people who may see this post.

In December I did a project where we looked at the end connection for an anchor chain on a ship. These are designed to withstand 15% of the ultimate strength of the anchor chain, and fail before 30% of the ultimate strength.

http://thrmarine.com/friksbeheer/wp-content/uploads/2017/11/Tech-Spec-Anchor-end-connection-THR.pdf

The one here is designed based on a german norm, which is easy because it describes exactly the hand calculation to use, and what variables you need to know.

In our project the design was based on the norm but had a few changes which required some extra checks in FEM.

 

[BridgeSmith]

CalTrans has done some interesting work with controlled-failure concrete shear blocks for seismic applications for bridges. It employs a concrete block that is cast isolated from the abutment (w/ 2 layers of building paper/felt) except for a calibrated amount of reinforcement crossing the joint that fails at a fairly predictable shear force.

 

[btrueblood]

I was working with a vendor in California on an unrelated project, but they showed me one of their other product lines. It's a burst disk used on the pneumatic conveying systems that unload grain cars. Except that the burst disk is activated by a snap-through buckling of a Belleville washer (loaded by a diaphragm) that drives a spike through the center of the burst disk to rupture it. The design gives a very repeatable rupture of the disk (and release of pressure) at very repeatable pressure differentials, much more repeatable than could be expected from the burst disk material itself (though that is the secondary, backup release mode). The idea being, you can build/fabricate the trigger mechanism and test it for function non-destructively, modify it if needed and re-test, then add it to the structural element that is to be used.