Cycle scaling for submarines models

1. static and cyclic pressure testing of model is carried out before actual design for underwater vessels. How cycle scale is carried out for loading cycles. Actual structure might experience many thousands of pressure cycles.
2. When we carry out fatigue testing for the model inside a tank, the tank is itself subjected to fatigue loading, how fatigue damange to the testing tank is prevented.

 

Here in Europe we use the ISO standards 13445 (series) for pressure vessels/ tanks and 13480 (series) for piping.

If you state that max no of cycles are less than 500, or not to exceed this, fatique as a consideration can be ignored. Simplifyes things a bit.

If you wants it to be possible to certify the product for more than 500 full cycles, calculations for fatique must be documented also. (unless the max working pressure not exceeds a certain amount (%) of the max yield for the pressure vessel material, I'm not quite sure about the last one here, but it was at least like that earlier....).

Another thing to be really concerned about, is uniform thickness and roundness, for pressure vessels subjected to vacuum or overpressure from the outside. Internal ppressure, well it only kepps the vessel pretty round, even at yield pressure, external pressure, well then that area may cave in... "Pfloppp" ("hey, who the f... calculated this....!).

 

Completely forgot, the pressure equipment directive (PED)...
(Just the looks, no brain...).

If I understand you correctly; the regulations you will have to compy with are here:

http://ec.europa.eu/enterprise/pressure_equipment/ped/index_en.html

Click around in there, you'll find the kategories, (volume, pressure media), your equipment will have to be categorized in. They state that equipment are to be designed in accordance to the "hormonized" standards. If the equipment are kategorized lowlow, you may get away with designing the pressure vessels according to para 3.3 in the regulations, witch states "sound enginering practice"/SEP (also referrred to as "somebody else's problem, the national authorities then often like the "harmonized standards"..) But you will not be allowed to apply the CE mark to this equipment.

That was the regulations...

The rules for calculations again as mentioned above; are the ISO standards 13445, 13480.
A couple of thousand pages... The 3 part is for the calculations, use material, with certificates that comply to one Europeean standard, 3.1b certificates are they referred to (materials with dual certificates is often ok).

Good luck..

 

Hi, thanx for ur reply.I feel that i was unable to explain my question properly. I wanted to ask that I want to carry out tests on models of submarine and if i am knowing the actual cyclic pressure on the submarine, then how can i scale it down to apply it on my submarine model for carrying out tests on submarine structure, as everything is scaled down for the model tests?hope i m able to make u understand abt my question now?
thanx a lot in advance

 

? Seriously ? Do not take any offence to that.. for full scale design...

I thought you were considering tanks to be for instance put on the outside of a sub..

Ok, The physics are are like that;

A sphere, perfect round, Diameter "D", exposed to pressure (internal or external) will for instance survive a given pressure it is exposed for by a shell thickness of 10 mm.

A cylinder, perfect cylindric in shape, Diameter "D", will for the same pressure collapse, To have the same stress in the material, the thickness will have to be 20 mm.

(Figures; Hot dogs in water always burst in the length, even the skin is the same thickness all over (I've checked..) I always tell the kids that I make them for the Ketchup, and that its not all people who can do that trick).

The rules:
Therfore the longitudinal welds in a pressure wessel is of more importance to the design, and also more interesting to check/ control. Also it's normal to reduce the max allowable yield stress by some factors (varies for each Class society, more control (X-ray, MPI) will normally allow you to use more of the strength in the material.

So; Everything can be scaled down, the thickness, the shell thickness. To get that approved by a class society, you would have to ask. I would also expect that they for the full size design would require a over pressure launch (unmanned) of let's say 30% overload, and with stress gauges (in the well known language Norwegian we call it "strekklapper") fitted/ glued to the hull.

So, if the calculated shell thickness is 30 mm, working dept is 100 m, a model scaled 1:10, with a shell thickness of 3 mm, and a test pressure equivalent that of 10 m would be pretty close.

(If I now have understood correctly...)

 

yeah, this is what i want to know.but what is the procedure for scaling down the pressure cycles. how the actual pressure cycles which are quite high in value can be scaled down to be applied on the model in the testing tank.

 

Ok then, fatique is not my field....

But; I do not think that the requiered no. of cycles can be scaled down, unless you also increase the load, But then; I have no clue of how to determine the reduced no of cycles against how much more stress to put on the design..

I have a thought at the back of my head...
If the stress amplitude is kept within certain limits of the yield in the material, there's a saying that fatique will not occur. I do not recall the boundries to those limits, but I'll try to look around during the weekend. Can't guarantee that I find anything though. If I remember correctly, Its pretty conservative....