Bypass Transient Analysis?

[Talos]

Hi,

Is it possible to 'simulate' a classical shock pulse by applying an acceleration in a 2:1 ratio and is this advisable

 

[GBor]

To quote a great saying, "What?" Are you asking if you can apply a static 'g' loading of 2-g's to simulate a dynamic shock event, the answer is a resounding "maybe", but this would be a VERY small shock pulse.

I see you are a military engineer, so let me say that "classical shock pulse" depends on what branch you are dealing with. For the navy...not a chance. You probably need to run a DDAM (Dynamic Design Analysis Method) or an equivalent transient (Time dependent loading) analysis. For the Army, it depends on what scenario you are designing towards (the "shock" of a gun against your shoulder is very different than a mine exploding under your vehicle). Air Force...completely different beast...the list goes on.

In short, though, the answer is "no"...it is not advisable. You have design safety factors, combat loads, etc. to consider and your threat.

If you are in a much less threatening situation, lifting and handling of heavy equipment overhead has different design safety factors than standard ground operation. There should be a code or a spec governing those factors.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
Magnitude The Finite Element Analysis Magazine for the Engineering Community

 

[Talos]

Thanks Gbor,

Firstly Military no, pressed wrong button on forum set up should have been mechanical(don't know hoe to change it).

The reason I am asking this question is because I have some previous analysis where the requirement was to look at equivalent stress when a half sine 4g 11ms pulse is applied. As this could not be achieved in the FEA package, a linear acceleration was applied in a 2:1 ratio (ie 8g) to simulate

I was asking whether this was a sensible assumption

 

[GBor]

Guess I have to ask why it wasn't possible in the FEA package, but the linear acceleration equivalent depends somewhat on the structure that is being analyzed. A flexible structure has a low "Dynamic Load Factor" where are stiff structure may have a pretty high DLF. 2 is probably conservative...that suggests an infinitely stiff structure, if I recall correctly. Usually 1.8 is pretty reasonable for strong, steel structures. You may use 1.2 to 1.4 for flexible composites.

An 11ms pulse is pretty short. There probably wouldn't be a great deal of energy imparted into the system over such a short time period...gut instinct says you're probably OK...just my opinion, though.

 

[Talos]

Thanks GBor,

The structures in question are relatively simple steel and aluminium brackets securing loads to walls. The FEA package used at the time could not perform any model transient analysis.

I had looked into dynamic load factors for various classical pulses and derived a figure of 1.8 (more like 2 for pulse waveforms)

I just need a second opinion from somebody more knowledgable in FEA than I

Thanks again