12 replies to this topic

[MatsNorway]

I am making a tool at work that i am thinking i should design to break in order to save the axle the tool is working on.

 

I will make operator recommendations stating lower values than the tool can take. but we all know those have a tendency to be ignored from time to time.

 

Does anyone have any experience on making designs made to break?

Edited by MatsNorway, 19 September 2015 - 18:12.

[Greg Locock]

Yes, but I suspect it won't help much. The #2 cross member (often known incorrectly as the front subframe) in a car tends to short circuit the crash structure, as it needs to be stiff and strong, the crash structure needs to be neither. So, typically the rear bolted joints that hold the x member to the main longitudinals are designed to break away. This is rather a tricky balancing act, we don't want them to break away if you drive through a pothole and over a kerb, we do want them to break away if you drive into another car. The bolts don't break as such, the joint does, it slides out under the head of the bolt via a shaped piercing in a plate. I know the LS DYNA boys went through a few iterations of shape for the slot, but not as many as I'd have expected, and obviously once we had a validated design we've never touched it since.

[bigleagueslider]

It can be extremely difficult to get a metal component to fail in shear at a consistent/precise load point, if that is what you are looking for. A better approach would be to use some type of mechanical overload clutch or mechanism that can be adjusted to release at the exact load point you require.

[MatsNorway]

It can be extremely difficult to get a metal component to fail in shear at a consistent/precise load point, if that is what you are looking for

Not in tearing no. A clean pull only. It does not sound too tricky in my mind but what do i really know.. I guess it gets tricky once you add cycles to the equation.

 

Complex designs is out of the question. Then we would rather only inform the user.

[GreenMachine]

It can be extremely difficult to get a metal component to fail in shear at a consistent/precise load point, ...

 

A bit o/t, but ...

 

Back in the day, when I had a professional interest in the subject, I was shown over a hypersonic wind tunnel.  They used automotive steel in the diaphragms, because they found it was the material which had the most predictable and consistent fail mode - an important, if not critical, element in the value of the tunnel's experimental results.

[gruntguru]

Mats, perhaps post some details of the tool and what it does. You might get some interesting ideas thrown around.

 

GreenMachine - UQ?

[MatsNorway]

 

Blue part in question on getting a thin section.

Edited by MatsNorway, 21 September 2015 - 06:55.

[Canuck]

When making a joint that needed to fracture between a specific load value (above tool string load, below $$$$ wireline breaking load), we made numerous weak-point samples for testing at a 3rd-party lab using most of the assembly and went from there. 

[GreenMachine]

UQ?

Yes

[gruntguru]

Blue part in question on getting a thin section.

What does it do?

Where are the forces/torques applied?

[MatsNorway]

The bolt going through transfers the pull from the other part.  the black bolts holds back and thats how the blue gets its stretch.

[bigleagueslider]

Will elongation/yielding of the blue part before fracture pose a problem? Also what if just enough force is applied to yield but not fracture the blue part?

[gruntguru]

So the blue part is in pure tension left to right?

 

Perhaps make the vertical bolt a sacrificial part? Notch it to fail in shear. Use brittle material to avoid the "yeild without fracture" scenario.