Forces around a bolt hole in a plastic part.

[superptrucker]

I'm trying to understand why I am seeing cracks around bolt holes in a polycarbonate part that is made out of 1/8" thick sheet. I understand that PC is subject to environmental stress cracking, which involves chemicals accelerating small micro cracks under tension. What I'm struggling with is whether a clamping force of a fastener will result in hoop stress around the bolt hole. I thought this was a simple phenomenon, but I can't find any information on it so I'm starting to doubt my intuition. Does a clamping or compression force of a fastener going through the hole impart a resulting hoop stress around that hole? Thanks in advance.

 

[Compositepro]

Polycarbonate is notorious for cracking under clamping loads. I once used split washers under screw heads and every hole cracked.

 

[MikeHalloran]

Yes, there will be hoop stress, induced by the compression forces from the bolt.

Not widely understood; PC is subject to Environmental Stress Cracking, even from normally innocuous fluids used to cool drills.

Much more widely not understood; the cracking doesn't always occur immediately after contact with the fluid. ... and still occurs even if the fluid exposure is brief, and the fluid is immediately removed, and the PC washed with a compatible soap and water. ... and still occurs even if the cleaned part is stored in a protected environment for substantial time, pretty much immediately when the stress is applied. I.e., the environmental exposure and the stress application can be displaced in time, and cracking still occurs.







Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

Resistant materials with similar properties exists. Transparent ABS e.g. Terlux or similar is one example if you need transparency and impact resistance with added chemical resistance.

Chris DeArmitt PhD FRSC
President

Plastic materials consultant to the Fortune 100

http://www.phantomplastics.com

 

[superptrucker]

Thank you guys for the information. To follow up, I did a quick simulation on a simple washer, and when I use a compression load the displacement shows the OD getting bigger, and the ID getting smaller. If there was hoop stress around the hole, wouldn't we expect the ID to get bigger? Maybe the hoop stress is experienced near the OD of the fastener? Unfortunately, I'm using the express version of the simulation in SolidWorks, which doesn't allow me to properly constrain the part.

 

[MikeHalloran]

Try using a washer with an OD of about ten bolt head diameters.

... and make the axial load large enough to shrink the ID into contact with the bolt.
... and keep going with more axial load.



Mike Halloran
Pembroke Pines, FL, USA