Je voudrais savoir si il existe une valeur GRMS par défaut à prendre dans le cas où nous n′avons aucun moyen de la calculer.
I would like to know if we can consider a GRMS value by default when we can not estimate it.
Thank you for your answer.
Si votre produit est un COTS et que vous cherchez un profil de vie standard par défaut, utilisez celui du guide FIDES 2009 (page 51). Sinon, pour quel type de produit cherchez vous un niveau de GRMS ?
Considéreriez vous qu′il y a une température par défaut si vous n′avez aucun moyen de la calculer ? La même réponse s′applique aux vibrations !
If your product is a COTS and you look for a standard by default life profile, use the one given in FIDES Guide 2009 (page 51). Otherwise, for what kind of product do you search a GRMS level?
Would you consider there is a default temperature if you have no mean to calculate it? The same answer applies to vibrations!
A very good reading about this topic:
Dear M. Lambda, let me first express –again – my compliments with the 2009 FIDES guide. I is a great help for reliability calculations and the guide is written in a very clear way. Perhaps that is the reason that my question is more or less about one of the side topics. For a manufacturer of galley equipment I am preparing an accelerated life test. Just as for the FIDES reliability calculations I need a mission profile for this. I asked the manufacturer for the relevant vibration levels in a galley during standstill, taxiing, and flight. However, information on this matter seems to be scarce and he was not able to deliver specific values. In fact, the most relevant profile I could find is the one of the “Medium haul civil aircraft, computer in avionics bay” presented as an example by FIDES in the 2009 guide, issue A (page 75). Now, I realize that the profiles given are mainly meant as examples, but I still hope that you could help me with the following: is there a “typical” relation between vibration levels in the galley as compared to those in the avionics bay? I am assuming here that one can use the spectrum from FIDES for the avionics bay when no specific values are available. And if I look at the phases, taxiing is only one single phase thus having only one single vibration level. I suppose this is a well-considered choice, but at the same time it seems to me that vibration levels highly vary during taxiing, due to the different speeds. Can one recalculate vibration levels based on a mechanical model, and the value given (5 Grms): is this a maximum or an average value? Sorry to be bothering you about these details, but I am rather stuck; any comment would be appreciated, a good reference as well. Many thanks in advance!
Yes, you can use the life profile given in the FIDES as a basis to build your own life profile. Of course you will have to tune the levels and most probably add some phases to be representative of your products' life. We have done our best to put good mission profiles in the FIDES Guide but we have to struggle hard against the trend to reuse them without understanding, a behaviour that always leads to wrong result and poor reliability engineering.
A usefull note is thate the energy of vibrations is more or less additive (for moslty random vibrations).
The taxiing is indeed a phase during which vibrations may change a lot. It is reduced to a single phase because it would be very complex to improve it and it wouldn't improve a lot the accuracy of the reliability prediction. It is a common difficulty to assess the right level (of whatever vibration or temperature or others) for the phases where the real level is not stabilized. This tuning may be key for the accuracy of a reliability prediction.
Every time there is an uncertainty on the life profile, I recommend to do a sensitivity analysis to understand and be aware of the consequences of possible errors.
Once again, all of this is applicable for reliability predictions, not for characterization trials or environment qualification tests.