joints welded, study of tensions of hot spots

[mambo1988]

Good morning my name is matteo student of mechanical engineering, and I am making a group project concerning the realization of a machinery, the problem we have is the fatigue study of welded joints. I read the cnr 10011 regulations and the eurocode 1993 where we talk about different methods of verification or method of admissible tensions and the hot-spot method. Sincerely I found the eurocode not so clear about the procedure to be adopted for the hot spot method, do you have any material about it?? I tried to shape the welding cord through ansys and imposing all the boundary conditions (forces, flender moments, touches and constraints present in the element) I compared the tension near the welding foot with the notch case reported on the norm (although there is no case where all the stress similar to my case are present)....what do you think?thanks

 

[meccanicamg]

reading en 1993-1-9:2005 shows in the annex b that the hot-spot method is the geometric method for calculating fatigue resistance. in taboo b1 there are some categories of typical welds with their indicated characteristics.

in Taboo 8.1 of the en 1993-1-9 there are the categories of welding under the form of typical joints.

According to my opinion they are basically the same thing, only that in one you evaluate the type of joint in the specific while in the other case you have generic categories of welding but with more refined characteristics of execution.

verification methods for en 1993-1-9:2005 still remain the same:
- calculation by directional method (Static)
- calculation by simplified method (Static)
- calculation by miner method attached 5 "cycle to failure" (fatigue) where the evaluation of the evaluation of the amplitudes according to the fact that it is damage tolerant or safe life (change coefficients but the method of calculation is the same)
- in the case of joint actions and therefore combined actions (fatigue) must be evaluated according to the specific formulation of the equivalent delta.

I don't understand where your problem is. there is only to make calculations with the good excel sheet or to handle with paper and pen. the fem simulation is something that is not explicitly foreseen by the norm, so there opens an undefined world.

 

[mambo1988]

Thank you for your answer.

the machine is an air platform to "sphile" that allows to work a worker at altitude, composed of several beams that rise one compared to the other, where there is a beam with a c section welded on the frame that supports the entire column .
I dimensioned welding in the static case, also helping me with the standard uni en 280 (person load, wind etc.).
My problem with fatigue study is that I don't know the load spectrum and so I can't know the lady delta.
In fact I tried to find a delta sigma thinking that there are variable forces (operative position, wind, manual load, etc.) that urge the welding cord differently, but for the prof this speech was not good, so it is not like facing the problem.

 

[meccanicamg]

Thank you for your answer.

the machine is an air platform to "sphile" that allows to work a worker at altitude, composed of several beams that rise one compared to the other, where there is a beam with a c section welded on the frame that supports the entire column .
I dimensioned welding in the static case, also helping me with the standard uni en 280 (person load, wind etc.).
My problem with fatigue study is that I don't know the load spectrum and so I can't know the lady delta.
In fact I tried to find a delta sigma thinking that there are variable forces (operative position, wind, manual load, etc.) that urge the welding cord differently, but for the prof this speech was not good, so it is not like facing the problem.

I believe that your prof will lead you to make the following reasoning:

- statically uni en 280 to know what load to consider
- always for the fatigue testing you have that the uni en 280 establishes fatigue verification for a number of cycles equal to * 40mila for light service
* 100mila for heavy dutythe aerial platform operates in various configurations and during a load cycle the stresses of the various construction elements are subject to several working cycles. the correct verification of each element should be carried out through multiple load cycles, evaluating for each stress excursion the number of cycles (reservation method).

to keep in mind that it is not possible in the design know the load cycles as you do not know a priori (not even in statistical terms) the number and working methods of the platform.

it can be assumed in favor of safety, that the platform during a working cycle, can be carried out in the most heavy work configuration so that for each structural element in absolute the maximum value of its stress.

Unfortunately I do not have the norm under hand but if you have it you should also find the indication on the load to be used for fatigue analysis.

 

[mambo1988]

you are just like you say, you have to study the work cycle according to the steps:
-Residence machine (sigma min)
- development machine (where I consider the load situation in the worst situation leading to max sigma)
Knowing the delta sigma now I can study the fatigue knowing from the norm the number of cycles.

Thank you for the comprehensive answers, I will update you at work.

 

[meccanicamg]

therefore it should not be difficult to make calculation to fatigue. I look forward to developments