Some questions about ISO 12215-5: 2019

This is what I could not wrap my head around it. Given that ISO uses the formula MD = 83.33 x kc2 x 2k2 x p x b2 x 10-6. compared to the standard formula of Ma = q L2 /12 and M1 = qL2/24. See attach.
LR uses k pb2/12 for single skin panel which is (1/12) x pb2. If you take a closer look, that ISO 83.33 is really 1/12 x 10^3. This 83.33 or 1/12 changes depending on the zone where you are looking at in analyzing
the laminate. The one near the base of the stiffener or the one at the center.

Using natural sign convection method on fixed end beam (Engineering Toolbox), the Mb is positive (downwards) when it is at the center , negative when at the edge. Conversely, when the center (outer laminate) is in tension the inner laminate region at the base of the stiffener is also in tension due to the reaction of the stiffener (fixed ends).

Thus when looking at the result of the ply analysis, I look at the result of the tabulation. when using 1/12 constant, is the result negative (in compresion). Am I looking at the result of the analysis of the laminate near the inner laminate at the base? Is the Mb negative or positive?

Plate with stiffeners is a different story as shown in the attached. But this is LR and I am still validating it using the moment diagram by parts method.

 

Unfortunately such tests do not fit in budget of recreational small craft design. Say, the 10-12m craft - what is the cost of the boat, and what is the design budget?
Note that such craft will use 5-10 different layups for different areas, and one should test all of them - full program will be quite expensive.
Unfortunately, the recreational boat manufacturer has limited funds and he would rather spend them for marketing, not for R&D - reality of today's world.

Thus, most of societies and standards provide approximation formulas for material properties, for composites. We compared results obtained from such formulas from LR, BV, ISO, GL, etc. with sample test results - test points are always higher than predicted using formulas. Thus, formulas are the way to go for small craft below 24m...

 

I also noted the issue with signs in 2019. However, as I said, we used formulas from 2008 standard, and then modified the spreadsheet to 2019. It seems to work this way.

 

The ISO formulas is almost the same as LR. It is used to predict what is supposed to be the minimum properties that you will use in calculations using the tabulated method. That means, use this value as the minimum, don't go below it. ISO is limited in the resin (polyester) that was used and is concerned only with common materials.

If you start using more sophisticated materials like twill weave fabric and epoxy resin, your material properties may go up a little but that is not ISO concern.

 

It is the same formula, just written differently.

Take the constant 1/12 = 0.08333. When converting into cm^3 = 83.33

This is also used in ABS rules.

 

Yes I have seen that. That is for a plate. What I am studying right now is why it changes when it is a plate with primary or secondary stiffener. My structural design reference book seems to have a different answer from LR or ISO.

 

Hi All,

Has anyone reviewed the sandwich laminate stack analysis presented in table H.4 of 12215-5:2019?

Immediately I notice anomalies surrounding the design moment calculation. Has anyone else encountered this?

What do people think about the design moment calculation in table A.4?

Thanks,

 

Table A.4 is not correct. It should say Mdb = - (1/6) * K2b * P ^ 2 * b * 10 ^ -3

Regarding Table H4., I cannot check it now but I do not remember having detected any anomaly. It must be said that it is very difficult to know what it does in these example tables and that it is impossible to obtain the same results.

 

Thanks for your response TANSL,

I did see this resolution noted in the publication from RG Souppez. If we apply P^2, then equate the units of the expression we are left with: (N^2)/(mm) which does not represent the units of a bending moment.

I have reviewed 12215-5:2008 which presents calculations with P * b^2 for moments in both b & l direction. These equations also yield values within the same order of magnitude, whereas the 2019 calculation (with P^2) provides a bending moment in b direction that is much smaller.

Any comments on this observation would be most appreciated.

 

In a thin panel, subjected to forces perpendicular to its plane, it may happen that the bending moment in one direction is less than the perpendicular direction.
In some formulas it must be taken into account that what is being calculated is a 1 cm wide slice.

 

What does K2b represent?

 

K2b : Aspect ratio factor for transverse bending moment.
and also
K2l : Aspect ratio factor for longitudinal bending moment.

 

Thanks. But isn't the pressure already squared as a unit. That is lb/in2 or N/mm2? if squared it becomes lb/in4.

 

You are surely right. The only thing I can say, and I already know that it is not a scientific or very professional answer, that applying the square of the pressure gives the same results for the Mdb (design bending moment in the b direction) that the standard gives in its examples . And since it is not difficult to find errors in the norm, everyone interprets things in a different way. Is K2b a dimensionless coefficient? I don't know, I couldn't say.

 

Forgive me asking the obvious question.
But do you not offer ISO rules as software you have developed?
If so, surely you should know what these rules mean, otherwise, what are you offer in your to-be-paid software?