Controlled Yield Material for Shear Failure 1

[adchampion]

I am in search of a material to add a controlled failure piece to a current valve design. I am looking for a material that has a tensile strength capable of taking the operating load of the valve (approx. 35000 psi tensile yield), but that shears at a relatively low and predicable value.

My rough calculations are based on Shear Yield = 2/3*Tensile Yield, but we are now exploring the use of brass for this part and it seems that this assumption is invalid for brass. If there is some relation that is constant then the geometry can be adjusted to take that into account.

Other than the estimate of shear strength as some fraction of tensile yield, there is another issue. If there is a constant relationship to yield then we need to specify a relatively narrow band of tensile yield in order to get the corresponding narrow band in the shear yield that will allow us to predict the failure of the threads within the required window.

When we order material, can we just specify this narrow band of tensile yield (5000 or 10000 psi)? How will this affect the cost of the material (rough percentage)? Further, if anybody has any suggestions for a material, they would be welcome. We have looked some at ceramics, but keep coming back to the brass.

 

[swall]

What kind of material and what operating evironment is involved here?

 

[adchampion]

Thanks for the quick response.

Existing materials for the connecting pieces are 17-4 and 4130. Material for the piece to fail is to be determined.

The worst it shoud see is salt air. This took aluminum of our list.

~Ben

 

[metengr]

What kind of valve design or operating function is intended with this part -(I am just trying to understand conceptually what your intentions are from your original post)?

 

[swall]

You might try Carpenter Project 70 182-FM, which is a free machining ferritic stainless. The Carpenter data sheet shows that this material work hardens at a low rate, which suggests that once it started to shear, it would go pretty quickly at fairly constant load. Stated typ properties are 48ksi yield and and 70ksi tensile. If having stainless in this system would be a problem due to galvanic couplint, you might try a free machining carbon steel.

 

[adchampion]

The application is a MOS choke valve. The piece that I am working on is one that goes beteen the stem and the drive screw to fail if the pass through ports in the stem are clogged and stem suddenly sees pressure across the whole diameter. The piece that shears is basically a plug that threads into the stem and the threads into the drive screw. The plug threads that go in the drive screw are the ones designed to shear.

One of my main concerns is getting a material that will shear at a predictable value. The data sheets taht I have seen specify a minimum value for tensile yield, but no maximum value, so it could come in at anything. Is it possible to just specify a band of acceptable yield strength (either tensile or shear)?

~Ben

 

[swall]

Unless you are buying directly from the mill (i.e. large quantities), it is going to be difficult to specify a band for mechanical properties. This may force you into adding an annealing operation to the part and testing every batch for mech properties.

 

[metengr]

I would suggest using a standard shear pin.

 

[adchampion]

This is what I was afraid was going to have to happen, but I was hoping to avoid testing every batch. Any thoughts on using a ceramic of some type. I have no real experience with ceramics, but it seems like it could be an option. Do they have a more narrow band of yield strength (read: predicatable)?

 

[adchampion]

The pin idea was suggested at the beginning of the project. However, there are more complicated issues with the use of a pin. There are different configurations and different pressures that would result in the use of different pins (or a different number of pins). Even if we did use a pin, there would still have to be a narrowly controlled shear strength.

Further, we are trying avoid the situation where the end user can just grab a commercial piece and shove it in there with no control over whether or not it will work.

~Ben

 

[metman]

If you order certified material, it will have the heat lot number and you should get consistant properties within a heat.

You could set up a simple torsion test to determine shear strength vs hardness and from this establish your hardness range limits. I cannot tell you the extra cost but you can get some suppliers to hardness test a heat lot to fullfill yuor requirements.