sandwich panel

[shearlag]

Dear all,
Can any of you kindly help me in successfully executing a Nastran program for sandwich panel?
Since the problem is of sandwich wing skin, the upper and lower wing skin is represented using CQUAD4 using SHELL topology. The top facing and the bottom facing ( 15 layup of 0.13 mm each) is assumed as 2D orthotropic material with UD carbon tapes and the core is NOMEX (HRH-10) of 10 mm thick. Now what material property should the core be assumed? If I want to use 3D orthotropic for core,
the stiffness properties are as follows,
E11=blank or 0.1,E22= blank or 0.1, E33=160 Mpa, G12=25 Mpa, G23=25Mpa, G31=55 Mpa, u12= u23= u31=0.3
And strength properties are as follows,
sig+11=sig+22=sig+33=blank and sig-11=sig-22=sig-33=blank and Tow12=0.5 Mpa, Tow23=0.5 Mpa and Tow13=0.86Mpa
Why I want to use 3D orthotropic material properties for core is to have the effectiveness of E33=160 Mpa. Else I would be loosing this stiffness, which is important for core local compression. I was able to run successfully with core as 3D orthotropic and indeed got the results. My concern is about the credibility of the results. But when contacted MSC technical support, I was told 3D orthotropic do not support SHELL elements with SOL 101. It was shocking.I was suggested to use 3D orthotropic with SOL 600 with linear analysis option for which PATRAN writes MAT8 card.
Would it be ok to use 2D orthotropic properties for core, wherein PATRAN has only 2D options as
E11= blank or 0.1, E22=blank or 0.1, G12=25 Mpa, G23=25Mpa, u12= u23= 0.3
sig+11=sig+22=blank and sig-11=-22=blank and Tow12=0.5 Mpa,Tow23=0.5 Mpa .
And finally what does negative failure indices means ?
Thanks in advance.
shearlag

 

[JJ22mW]

I've always used 2D ortho for core. I don't think you'd be capturing any transverse compressive forces or stiffnesses with the CQUAD4's, considering they don't have two nodes in element Z direction that could move closer to eachother. And I wouldn't think that the deformations resulting from local core compression would have a large impact on the overall stiffness of your structure.

Now I do think that local core compression can have an impact on the compression strength of the face sheet (from bending moments, or inplane loads) since it puts an eccentricity into the face sheet and it generally fails in local buckling/wrinkling.

 

[rb1957]

-ve failure indices means it's failed, no?

you're using a CQUAD as the element and PCOMP as the property, yes?

 

[shearlag]

JJ22mW,
yes my concern was from your last line that "local core compression".If i take freebody forces at any section to do a local check for core compression, i should'nt end up with increasing the core thickness or going for a high density core to cater for those forces which are results from 2D orthotropic. Today I tried the same panel with 2D orthotropic and again with 3D orthotropic, surprisingly, both gave same failure index and stresses.PAtran gives a message while assinging 3D orthotropic material properties to core modelled with CQUAD4 elements saying that " 3D orthotropic properties would be converted into 2D orthotropic as it is modelled with 2D elements". This message I had not noticed earlier.

rb1957, truly i did,nt get your first remarks,
yes ihave used CQUAD4 elements for modelling sandwich panel and PCOMP as property.

thanks for both of your response