Failure of a bonded joint that has a thread 3

[elinah34]

Hello,
I have a question about a thread which exists in a bonded joint.
I am adding a simplified picture of such a joint, that we have some issues with:

Please pay attention that the thread goes a little bit over the second lower bonded Ultem part.
After manufacturing this joint (Including inserting the Tangles Helicoil, a special Helicoil for Plastics which doesn't require exerting any force for inserting it to its place, but only threading it to its place) everything was fine, and there was no evidence of separation in the interface of both Ultem parts while examining it visually and ultrasonically.
After tightening the bolt exactly as described in the picture above, exerting 10 Lb*Inch, we found out that it caused a separation between the 2 Ultem parts (the Adhesive didn't succeed holding them together).
We can't understand the root cause of this failure because the tightening force should attach the parts to each other.
We thought about a potential failure mechanism, but we aren't sure about it – the external load is never uniformly distributed along the thread's teeth (see an example below), and maybe this may cause a relative movement?

 

[elinah34]

Hey again Brian Malone (Industrial)
I thing there is enough place beneath the bolt for chips accumulation.

 

[3DDave]

We are all guessing due to the lack of information and photos showing the sort of failure you have.

The screw thread is acting as a wedge. The wedge causes spread.

 

[Orange_kun]

Is the thermistor removed and reinstalled often? What kind of loads is that bracket subject to? I don't see a pressing need for the helicoil. Just tap and bolt directly to the plastic. Use a coarser thread, and possibly a nylon screw.

How certain are you that:
1. The tapped threads are deep enough
2. The helicoil isn't being installed too deep.
3. The joint was prepared and bonded properly.

I'd lose the helicoil, unless you've already seen issues with just plastic threads.

 

[elinah34]

I am adding several pictures.

.

 

[elinah34]

Hey Orange_kun,
The thermistor is installed only once and isn't removed. This specific joint in the plate bwlow was aubjected only to compression due to the initial preload of 10 lb-in.

 

[Compositepro]

Item is a plastic with much lower strength and modulus than metals. When you tighten the screw it pulls the top sheet of plastic upwards and the adhesive is all that is holding it down. This is also a peel loading, which to be avoided in any adhesive joint. The Ultem between the helicoil and the thermistor is under compression, and will creep.