Talk Composites - The Forum for Advanced Composites

Layup symmetry

http://www.talkcomposites.com/Topic40499.aspx

By Hansson43 - 8/19/2022 4:02:41 PM

Hi, I have been reading some about symmetry and I have a couple of questions that I hope you can answer.

1: when a layup is written +45/-45 why do you go about having minus and plus. Is that only applicable when using unidirectional fibers ? Or is it applicable when using a twill 2x2 as well (If yes, then why)? 

2. I have read that if you are doing a flat panel then symmetry can be important to mitigate warping. How are you thinking about symmetry of your layup ? For exempel when doing complex shapes ? I are you always striving for it in any layup ?

3. When doing flat parts with core I have read that it might be good to have more layers on the side of the core that will bend. If doing that won’t you disrupt the symmetry when the is core is counted as midplane ?

4. Does a two layer layup of 0/45 count as symetrical when there is no mid plane?

Thank you, 
By SleepingAwake - 8/23/2022 8:22:07 PM

 
Hansson43 - 8/23/2022 7:29:27 PM
Thank you Warren, 

so if you do à layup with twill and have +45, should you also have a layer of -45 as well, to reach quasi tropic ? It seems like it would just be the same since it is a twill 


0/90 _ +/-45 _ 0/90 with twill fabric will be pretty stable in terms of warping, as it is symmetric. However, it will not be quasi-isotropic as there are more fibres running at 0/90 th diagonally, which means it will be stronger in this direction. This may or may not be a problem for your specific application.
Heck, quasi-isotropic layups are rarely ever the best layup, but rather an attempted to cover all bases for missing information on specific load cases, lack of knowledge on how to engineer it, or cost saving measures or lack of time.
But to get back to the warping, for thin layups even having a more resin rich side towards the mould compared to the bag side which can confirm to the weave can depending on the cure cycle be enough to warp a plate.
And one more thing: i disagree with warren for once. In my opinion/experience is a sandwich panel much more stable in terms of warping
Hope that makes sense!