failure of plastic part when bolted

[cameprak]

Hello to all,

I have a plastic(20% talc filled pp) and aluminium sheet.Both are assembled using 6mm bolt by automatic operation(applied torque is constant). plastic part(2.8 mm thick) fails during this process.

Now, I want to find out the following results.

1. Why the part fails

2. What is the maximum stress between bolt and plastic sheet due to preload

3. How the contact changes between plastic part and bolt when temperature increases by 50 C.

My qeustion is, how to approach this problem. If i do contact analyis, will it be right. Any input about amount of preload, friction between plastic and bolt(static and slip).

Hope, I explained clearly. Any further info needed, please let me know. Thanks in advance for your help

cameprak

 

[GregLocock]

The good news is that you don't need FEA to solve this. The bad news is that you need to understand what is going on.

What are the possible failure mechanisms of the plastic?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SWComposites]

Please explain in detail the failure mode/location. What does the temperature increase have to do with installing the fastener?

 

[cameprak]

Hi,

SW composites,

1. When assemble, plastic part cracks in the particular region with in the span of 10 degree. We did a moldflow analysis to find out weld line formation in that area. As suspected, weld line is formed exactly there. But as per anylsis, it is a strong weld line, so dont have to worry.(material joins at proper melting temperature, not much temperature reduction when it joins). But, what happens in reality is remains to be checked since it is manufactured in other location.

2. initially we approached in this way, because not every component fails. 40 to 50% only. Hope fully, once we follow a proper molding process, problem will be solved.

3. other requirements I mentioned are part of a development process. We would like to answer those questions. Then, temperature increment has nothing to do with fastener operation. But, what happens to the bolt connection when temperature increases or decreases by 50 C. Lack of FEA knowledge leads to this question. Looking for your suggestions and way to do the analysis.

Greg,

could you pleasegive some more tips about failure methods to look for.

Thanks for ur replies.

Cameprak

 

[GregLocock]

No, sorry. Your original question sounded rather naive, now that you have expanded it, it is not so easy. But, your major error is still going to be predicting the strength across the shut line, at a rough guess. No amount of FEA will really help.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.