Structural Fiber Glass Question (ISO 12215-5)

Good afternoon I am working on analyzing the strength of a fiberglass layout using ISO 12215-5. Even if you aren’t familiar with the standard I would welcome your input. In the ISO standard the flexural strength is calculated using the mass content of the fibers in the laminate (w_f or psi). (psi as in the greek letter NOT pressure) ISO provides a spreadsheet that, among other things, calculates the flexural strength of a layout. The ISO paper gives a table(C.2) which gives the actual psi values. Depending on the direction and weave of each layer some layers use an "effective psi" which is lower than the actual psi provided in the table. Even when using the effective psi the strengths in the ISO spreadsheet don’t match the chart in the paper.

Here is my question:

Why would you use an effective psi (as opposed to the actual) and how is it calculated?

Is anyone familiar with the standard who can explain the difference between the spreadsheet and table(Table C.4 b) values?

Thanks Guys!
-Will

Additional Information:
If anyone is curious the psi is used to calculate minimum hull thickness.

The ISO spreadsheet is available here:

http://standards.iso.org/iso/12215/-5/Module 1 GRP Motor/

Mass content of the fibers in the laminate can be calculated with this equation:

psi= 2.56/(3.072 *t/w + 1.36)

t= plate thickness
w= glass content (kg/m^2)


Example:

For CSM
t=0.7 mm
w= 0.3 kg/m^2
psi= 2.56/(3.072 *0.7/0.3 + 1.36)= 0.3
Using ISO 12215-5 Table C.4 b):
Flexural strength(sigma_uf) =152 MPa

Output from ISO Spreadsheet:
Flexural strength =122 MPa

 

What I believe is that the value of psi is not the result of applying a formula (C.1) but, on the contrary, psi is a known data, for each layer, with which the mechanical properties of the layer are calculated (Table C.4).
Furthermore, to calculate the final thickness is essential to consider the evaluation level (EL-a, EL-b or EL-c) for mechanical properties (Table C.1).

 

Thanks for the response,

My problem did have to do with evaluation lvl(mine is EL-c). I figured out my problem was I was multiplying my psi value by 0.8 instead of my flexural strength value.

Also here is a little extra info in hopes that I can save someone else a little time.

If you are trying to evaluate multilayer laminates with one being sprayed chop strand mat you must use the equation in example C.3.5. As far as I can tell it does not appear with the other equations in Table C.4a.

Thanks Again!
-Will

 

This only refers to the calculation of Sigma-uf. The formula is the one in C.4-a) table but, for this case, the coefficient 502 is replaced by another coefficient "C", whose value is:

C = 502-202 * Rp,​

where : Rp = mass of sprayed chop / total mass of glass.

I hope I have helped.