Some questions about ISO 12215-5: 2019

I have seen before that ISO has an open line to answer queries from users. The only downside is that they answer only the most relevant question.

 

No, they say: 'address your questions to national bodies'...

 

Finally we did that... Set of spreadsheets for composite craft consits of 6 pages. Plus there are separate spreadsheets for design loads of monohulls and catamarans.

 

Alik- The layup appears it is a single plate with stiffeners. What formula did you use for the bending moment? I could not read the unit on the Mb even I enlarge the attachment. Image is fuzzy.

 

We use fixed ends condition formula from 2019. Same it is was used in 2008/2014 standard.
In fact, first we programmed the old 2008 standard, and then we modified it to 2019. Otherwise is impossible to understand what is required in 2019.

 

I think there are some "small" differences 2008/2019. As for the bending moment (BM), the 2018 norm has its formulas but the 2019 one specifies the values of the panels BM in the short direction and in the long direction and the most unfavorable value is chosen, of course.
Another example of the many differences that exist and that have forced me, not to adapt, but to reprogram my application: the properties of the fibers are now calculated using different formulas. This leads to the contradiction that the same material now has different mechanical properties than 2 years ago. My material database has had to be totally changed.
Another small difference that I would like to know your opinion about. In the 2008 standard, it was understood that for FRP hulls it was mandatory to perform layers analysis of the laminate. The 2019 standard allows the use of "Method 1 (Simplified)" which does not require layers analysis. So are we going backwards in quality requirements?. If one wanted to optimize the structure as much as possible, far beyond what Method 1 allows, one would go directly to the FEA. You would not need "Method 2 (Enhanced)". Or the other way around, if you want a little extra quality you would never use Method 1. Perhaps I have misunderstood it.

 

Our approach was to simplify the 2019, otherwise not usable. Our laminates for panels are always balanced, so we can exclude some excessive cases from calcs.

Regarding FEA: we tried that for local strength, and we found that a) the results are highly subjective to meshing and boundary conditions b) require huge number of manhours c) any changes in stiffeners and panels would required re-modelling - start from scratch. With the reduction in safety factors (via structural properties) the standard offers, not worth unless boats with small number of stiffening members and largely curved panels. It is all about stress concenrators and how one models them. Say, if plate is fixed at all edges, it will pass. If it is part of structure and loaded by unit load from 2019 standard - likely not.

Structural properties of materials - yes they are very different, we compared them to 2008, and are probably targeted for FEA use.
BUT these properties are not usable for FEA anyway: modelling of BX (45/-45) in FEA would require LT (0/90) input with 45 deg rotation. Otherwise, FEA will used degraded properties.

 

In other words, the new ISO 12215-5: 2019 only has advantages for the user, am I right? But we must use it until someone reasonable comes up with a better solution. Well, "better" no, "more useful".
By the way, your spreadsheet, at first glance, looks great.

 

I see no advantages excerpt ego satisfaction of the writers, who are not practicing boat designers

Complicated calc method works only if the results of calculations can be verified. In this 2019 standard, the results can't be verified - even the developers can't produce correct calcs in the samples they show

 

I spoke with the "father of the child" to explain that it was impossible for me to validate my calculations according to the Excel tables presented in the standard. He replied that he was not happy with those tables, that I did not use them to validate anything. Very professional answer, right?
The funny thing is that he is very proud of the work he has done. In what world does this gentleman live? But the standard has been approved and we are obliged to use it, unless we have more sophisticated means and are great experts in structural calculation. These types of standards, in my opinion, are created for those "not so expert" designers and should, therefore, respond to their circumstances. The rest are nothing but theoretical, shallow, and very impractical speculations.

 

Few years ago we met, I tried to explain him that complicated standards are not workable in small craft industry, where most of the clients have limited budgets are not ready to pay excessive costs for engineering. Well, major brands can still do it, even if the boat does not comply.

My manhousrs to design a recreational boat have increased four times since 2008. And saying that, we have quite large office with 14 staff, where one person is specialized on weight and stability calcs only. So we get benefit from experience of doing same type of work. Take typical boat design office or 'one-man-show' boat designer, they can't do it at all.

Meanwhile, I will try to validate our spreadsheets with IMCI. They have been quiet for last few months on this matter.

 

It is really impossible. The ISO and LR method of "predicting" minimum properties are just that. It is a formula to "fit a curve" with a known minimum properties of the material. The engineering Constants used to predict properties and is based on 4 basic properties which has a very narrow range. It is accurate only only to a point where the different matrix and fibers "conform" to the curve. The matrix method has been around a long time and works only on the predicted behavior of material properties based on metals, not composites. It is supposed to be linear.

In short, material properties can only be verified by coupon test, compared against the manufacturers published result. The formula is still useful fit a factor is used to "adjust" the result. Like Leo L. said, All formulas are wrong, but some are useful.

 

Rx, I am not talking about the theoretical part of the standards because I am not prepared to discuss this topic in English and that part of composite materials is not my "specialty", as you already know. What I'm saying is that if you use the formulas and input data from ISO 12215-5: 2019, you don't get the same results as those given in the EXAMPLE tables of the standard. One might think, and it is legitimate to do so, that these examples could be used to validate the results obtained by oneself but, as I gather from what the author said, those tables are not reliable.

 

Tell us, please, your experiences with them and if they have given any answer, on these topics, that is useful for all of us.

 

That's how we do it...have done so for years too.
Reliance on 'pure numbers' is a race to the bottom, not a fit-for-purpose structure.

'tis true of any FEA, regardless of the materials used.
FEA is just a tool - it must be viewed as a "good approximation" (assuming certain conditions in model have been met) , not an absolute..