Ultem 1000 failure criterion 1

[elinah34]

I started calculating the stress in a 6-32 female thread in ultem 1000 plate. I suddenly realized that I am using VON MISES equivalent stress that we usually use when dealing with metals that aren't brittle. What is the failure criterion in Ultem 1000? I know how to calculate the shear stress in the teeth, but how do I go on from there to finding the tightening force in which the ultem 1000 female thread will fail?

 

[Tmoose]

80% tensile elongation at break ?

https://www.boedeker.com/Product/ULTEM-1000-Natural

https://www.sabic.com/en/products/specialties/ultem-resin-family-of-high-heat-solutions/ultem-resin

 

[3DDave]

Note that the usual load sharing limits are because the screw elongation in the first few threads limits the ability for contact farther down the fastener to matter.

However, in this case, the modulus of elasticity of the Ultem will be so much lower that essentially all engaged threads will be carrying the load. Sure, there may be a limit, but it might be at 30 turns of thread. This will be quite different if the material is filled with glass fiber or other material.

 

[Pud]

I would be concerned with creep - the decay of stress for a given strain.
All plastics creep, amorphous the poorer of flavours.
Some companies have isochronous stress/strain curves which will give an indication of an allowable initial condition.

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[centondollar]

Creep is the increase of deformation (and strain) under constant stress. Decay of stress for a given strain is relaxation.

 

[Sparweb]

Creep is the increase of deformation (and strain) under constant stress. Decay of stress for a given strain is relaxation.

So can a fastener tightened in Ultem be trusted to remain torqued? No. This is poor design.

In any fastening situation, where you find the joint strength is critical enough to warrant an analysis, then DON'T thread the fastener directly into the plastic part.
In aerospace, we even found a long time ago that Aluminum is too soft for steel fasteners to be directly tapped into.
Instead use a pressed/bonded/mechanically locked insert that is threaded to engage with the threads of your screw.

Helicoils, Keenserts, PEMs easily come to mind. There are 1001 different products to do this.

Therefore, elinah34, the reason it's not clear where your analysis is flawed is because your analysis shouldn't exist.

 

[Pud]

Decay of stress for a given strain is called creep in the world of plastics.
They all creep (thermoplastics that is).

And as Sparweb noted.

Screws designed for plastics are better than tapping etc. Commonly called PT screws.


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[elinah34]

Hello everyone and thanks for replying.
I am going to reply to differnt things that came up in the discussion:
1. It was mentioned that in plastics more teeth (than usually taken in metals - 3 teeth) bear the load. Is there any reliable source to evaluating the load distribution in such cases?
2. I was told to consider the strain instead of the stress. But what would be the right comparison in case of shear and not pure tension?
3. My design includes Tangless Helicoil, but in my report I am trying to demonstrate the improvement of strength as a result of using Helicoil, and that's the reason I am starting by calculating the strength in the simplest case of directly threading the Ultem 1000.
4. You mentioned creep, and you are right.
We plan to take into account the creep (that depends on the stress level, which is a result of the initial tightening force), and to ensure that even after tightening force reduction, we are still fine.
4. Do you know if using Helicoil decreases the creep?

 

[Sparweb]

3. My design includes Tangless Helicoil, but in my report I am trying to demonstrate the improvement of strength as a result of using Helicoil, and that's the reason I am starting by calculating the strength in the simplest case of directly threading the Ultem 1000.

That's better. Well one way to show the advantage of the Helicoil is to divide the pitch diameter of the HC by the pitch diameter of the 6-32 screw. You could collect all the data and do all the math, but the bottom line is: if you wrote it out as one equation, a whole lot of terms cancel, and you are left with just the ratio of the thread diameters.