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, 

Usually the +- on a cloth like that is because the fibres are orientated in 2 directions, eg a typical woven cloth rotated or a specific biaxial cloth. So a biaxial cloth often comes as +- 45 where as often people will rotate a plain or twill weave cloth 45 to get that orientation.

Yes ideally you want a symmetrical layup - the thicker the layup the more likely there is likely to be warping - especially with a core material where the spacing of the core on the skins means an imbalanced layup is more likely to result in a warp on the panel.

On complex shapes it is less of an issue as often the shape adds stiffness to the part in itself which can help reduce the likelyhood of warping.

Yes the 2 layers are orientated in the same direction so are balanced in that sense.



Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thank you Warren for your answer! I got some new questions.

What will the effect be if you have a twill and rotate it -45 ?

when thinking about a layup and warping do you know the effect of having different cloth weight. For example if you are doing a layup with 210g 0deg with a 400g in 45deg. Will the 400g count as two 210g in the 45deg orientation?

do you have any sources e.g books or webpages to read up on this ?

thank you 

Generally rotating layers isn't done to avoid a warp but to get more balanced mechanical properties by become a quasi-isotropic layup. Specifically on most flat panels, adding sheets at +- 45 improves torsional stiffness.  

A 400g is approximately 2 x 200g in general use.

There aren't any specific books I can recommend as a lot of this gets covered in various composites modules at colleges etc but there are a few books out there that may help.  Also watch videos on our site as some explain fabric basics etc. Composites Learning Area

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

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