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89student
Posts: 4
Registered: ‎11-04-2013

Influence of fiber length on fiber orientaion?

175 Views, 3 Replies
11-04-2013 09:46 AM

Hi to all,

 

I would like to ask in how far the the fiber length is taken into account during the prediction of the fiber oriantation in the fiber orientation models in Moldflow?

 

Thank you for any answers!

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Employee
Alex.Bakharev
Posts: 42
Registered: ‎04-10-2013

Re: Influence of fiber length on fiber orientation?

11-04-2013 12:54 PM in reply to: 89student

In the models used by Moldflow the fiber length by itself has no influence on fiber orientation. What is important is the "aspect ratio" AR=L/D where L is the fiber length and D is the fiber diameter.  If fiber aspect ratio is very large (infinite) then in shear-dominated flow the fiber once oriented in flow direction would stay in flow direction and any thickness-direction orientation will be caused only by fiber interactions (collisions between fibers). If the aspect ratio is finite then even oriented in flow direction the fiber would slowly rotate because the melt velocity on the opposite sides of the fiber is different. This rotation obviously affects fiber orientation. At AR>~50 movements of fiber due to fiber interactions completely dominate the rotation and the fiber behaves like having  an infinite aspect ratio. For typical commercial fiber filled plastics AR~20 and the aspect ratio affects fiber orientation only marginally. If AR=1 the "fiber" behave like a sphere and cannot be oriented at all. If AR<1 the "fiber" is like disk (e.g. mica scale). It orients very differently from normal fiber.

 

For most of the materials in standard Moldflow database the fiber length is not given, only the fiber aspect ratio. If the fiber length is given then Moldflow can predict fiber breakage. Fiber breakage reduces the average aspect ratio of fibers, so it affect fiber orientation.

 

Aspect ratio of fibers also affects predicted mechanical properties of composites and its shrinkage (and warpage) but this is another story.

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Contributor
MartinCavalry
Posts: 21
Registered: ‎03-01-2012

Re: Influence of fiber length on fiber orientation?

11-06-2013 05:39 AM in reply to: Alex.Bakharev
Hi All,

thanks for this thread and explanations.
Let me have a slightly different question please:
I wonder when you get the "Tensile modulus in 1st/2nd principal direction (fiber) " result using the dual-domain analysis type, does the modulus result regard to the fibre or to the part ?
Can you help me please in clarifying this.
sorry for the confusion, thanks for your help!
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Employee
Alex.Bakharev
Posts: 42
Registered: ‎04-10-2013

Re: Influence of fiber length on fiber orientation?

11-06-2013 01:28 PM in reply to: MartinCavalry

Yes, you are absolutely right. Predicted mechanical properties (tensile moduli, Poison ratios, CLTEe) are calculated from the predicted fiber orientation (and fiber breakage if it is done).

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