Lusas Composite Balsa Data Values??? 1

[adam0bmx0]

Hi chaps,

Basically i need to add a new material in Lusas Comosite, i want to add a typical 'End Grain Balsa'. Ive already checked in the avialible materials supplied in the software and there is no Balsa.

I've attached 2 pictures, one shows the inputs in the Lusas software i need to input, and the other is the information im using for Balsa.

Im stuck on what to put in the Lusas software.

Any help is much appreciated.

For reference im trying to model a 3-point bending test on a sandiwch composite, carbon skins with a balsa core.

Cheers,

Adam

 

[adam0bmx0]

And here is the Lusas options window i need to fill with Balsa values.

 

[adam0bmx0]

Also is Balsa Orthotropic or Isotropic?

 

[btrueblood]

The values from the balsa vendor are strictly for the direction parallel to the grain of the wood (i.e. axis of the tree it came from). They have not given you properties for the direction(s) perpendicular to the grain of the wood. Those stiffnesses may or mayn't be important to you.

For a first go, you could plug the average of the tensile and compressive modulus in for the (for example) z-axis Young's Modulus. Set the x- and y- Young's modulus to, say 1/10th of the z-axis value. The x-z and y-z shear modulus correspond to the shear modulus from the vendor. You have no data for xy shear modulus, so again guess at 1/10th of the value for shear along the grain direction. You have no data for Poisson's ratio, so guess at 0.3 for all values.

Yes, wood of all kinds is a classic example of an orthotropic material (properties depend on orientation). Strength and modulus of woods depend on whether the stresses are applied across the grain or in the direction of the grain, with the latter being stronger, and across-the-grain being typically 1/10th the value of with-the-grain. You will need to make sure that the direction of the elements matches the direction of the grain as it is used in your panels (presumably the grain is perpendicular to the sheets, so make sure your element's z-axis is aligned that way too. Unless you pick the x-axis to be your "grain" axis, in which case everything rotates.)

Run some FEA test cases, hopefully modelling sample sandwich panels that you have tested and have load/deflection/strain data for. Compare. Try running values of your guesses adjusted upwards/downwards by a factor of .3 or so, see if it has any affect on the sandwich properties. Try varying the reported values by 10 to 20% as well (wood is a natural product, hopefully the vendor is reporting typical or averaged values, and not "best" values).

...or go back to your vendor and ask for the numbers perpendicular to the grain, and averaged values, and use those.

 

[adam0bmx0]

The main problem is i cant ask the vendor for the values as the balsa used was un-marked and just an avialible sample from the classroom.

Im trying to create 2 FEA modles, the 1st;

Surface features meshed with thick shell composite elements

and then;

Volume features meshed with solid composite elements

Basically the 2 examples from the Lusas manual;

8. Linear Analysis of a Composite Strip

Ok, below is a picture showing my element orientation.

And at current the Balsa Values;

And my Carbon skin values

Which 2 Young's Modulus oreintations need to be reduced by 1/10th based on my element direction?

Thanks you for your help so far, this is a steep learning curve for me so sorry for all the questions!

 

[GBor]

What unit system are you in? Your carbon prepreg numbers look very low. And if you are using Baltek balsa, which one? Can you tell?

Get a density on the balsa. This should be easy enough...measure it to get the volume, weigh it to get the mass. Baltek can tell you just about anything you need from the density.

I belive btrueblood was stating that your shear values in line with the fiber should be 1/10 of the shear value of the plane crossing the fibers. Is this a uni-directional sample? If not, the shear values will be higher.

Garland E. Borowski, PE
Engineering Manager
Star Aviation

 

[adam0bmx0]

Hi GBor.

The units used are N, mm, t, s, C throughout.

As mentioned, unfortunatly i have no idea what make the balsa is, so am using the normal grade Baltak Balsa data for calculations.

Also, due to being back home and the lab shut over holidays, i have no access to the balsa in order to determine its density.

Yes my carbon is all UD.

I just need some figures to plug into the Lusas material boxes so i can then start my anaylsis and compare it to my tested specimens.

from these results i can then tweak the lusas values to a certain degree.

Adam