What is quai static analysis?

[kammieorama]

Hello ,
I have done my engineering project in which i have applied some load on differet parts. Its a static analysis of a vehicle and we are trying to see if it exceed the yield stress.

One person said this is called as Quasi static analysis. I would like to know three things:-

1. What quasi static analysis means?

2. Is it a substitute to dunamic loads?

3. What are the fields of its application? I mean in which all industries and for what all applications it is used?

Thank you in advance for your reply

 

[IRstuff]

Is this for school? Student posting is not allowed.

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[kammieorama]

no this is not for school.

I had done this project in college. But now i have to present it to my colleagues in justification for development of a new methodolgy.
so i need to convince them and hence i need to understand what it is all about.

 

[IRstuff]

You probably need to find a better search engine:

http://www.chalice-engineering.com/Analysis_Basics/Introduction_to_FEA1.htm#Quasi

"Quasi" means "sort of," and therefore represents those analyses that ought to be dynamic, but for a variety of reasons, can be treated as static. In general, they're related to whether any modes are excited or excitable.

I would recommend that you stay away from using terms that you are not intimately familiar with; you run the risk of looking like a fool, or worse yet, develop a reputation of simply spouting buzzwords. Your credibility to your peers is more important than to look good or smart; if you don't know what you're talking about, you will be found out. Do not fear saying, "I don't know, but I'll go and find out." Presumably, this is early in your career, and you will get some passes for not knowing certain things.

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[GregLocock]

Here's an example: driving through a pothole generate a series of forces in the front suspension. The peak longitudinal force at the contact patch is say 40 kN, although it only reaches that value instantaneously.

rather than try and dynamically model this car crash of an event, the designers might just take that 40 kN load and treat it as if it is static and then work out the loads in the rest of the system.

That's a typical quasistatic approach, extremely rough and ready. In that particular case it would result in massive overdesign of the suspension arms - not much good for production, but very handy if you are designing a car to collect actual data during that event.

I suppose you could argue that using D'Alembert forces you reduce a dynamic analysis to quasistatic, but I wouldn't say that.



Cheers

Greg Locock


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[IRstuff]

Note that in Greg's example, the shock event duration must be smaller than the transient response of the system, so as to not excite the closest mode.

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[kammieorama]

thanks a lot greglock and IRstuff.

 

[IRstuff]

Oh, actually I wrote that incorrectly; my bad. The quasi-static regime requires the shock event's frequency content to be lower than the natural frequency.

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[amanuensis]

Actually, for understanding what is the quasi-static regime, you have to visualyze two classical curves:

1. The FRF (Frequency Response Function) of a SDOF (Single Degree Of Freedom) System or a MDOF (Multiple Degree Of Freedom).
This curve have to be considered if the excitation is a vibration (sinus or random). All the frequency range between 0Hz and the first resonance frequency corresponds to the quasi-static regime. It means that your dynamical system can be approximated by just one equivalent spring. No waves travel or stand in the dynamical system.

2. The SRS (Shock Response Spectrum) of a half-sine.
This curve have to be considered if the excitation is a shock. The shape of this curve exibits a constant level or a step just after a bump. This part of the curve also indicates a quasi-static regime. It means that the "wavelenght" of the shock is much bigger than the one of the sytem.

 

[spongebob007]

Answers to your questions:

1.Generally if your lowest mode of vibration is at least one octave above your forcing frequency you can treat the problem as quasi-static. Your load is applied as a static force acting through the center of gravity of your structure.

2.Yes it can be-see #1 above. In fact in many cases even if resonance effects are important you can still perform a quasi-static analysis if you have an FRF or SRS curve plotted as a ratio of frequencies. You simply take the appropriate ratio and apply it as a multiplier to your load.

3.My guess is that it is a technique that is widely used. I use it frequently in my work.


Since it sounds like you really don't know what you are talking about and you need to convince others of the validity of this method, I suggest you get studying and calculating. Make sure you are familiar with terms like "octave rule", FRF, and SRS. You should also do some very simple problems both ways in order to convince yourself and others that it works out. When I first started using the method I made some FEA models of very simple structures and ran both dynamic and quasi-static analysis and lo-and-behold the results were the same. I will stress one more time, please do some research and understand what the method is and when you can and when you can't use it.

 

[amanuensis]

Sorry spongebob, but I absolutely don't agree with you to define the quasi- static from the octave rule. Octave rule is just a rule of thumb to say that two modes are uncoupled if resonances frequencies are at least one octave apart.

I have worked for the french company MBDA (missile manufacturer), so SRS, MRS (Maximum Response Spectrum)and FDS (Fatigue Damage Spectrum) are some very familiar concepts for me.

A FRF have to be thought as a modes superposition.
Each mode is equivalent to a SDOF system or a mass-spring-damper system.
The part of the FRF curve below the resonance frequency only corresponds to the response a the spring.
The part of the FRF curve above the resonance frequency only corresponds to the response a the mass.
The part of the FRF curve around the resonance frequency only corresponds to the response a the damper.

So, the quasi-static regime corresponds to the frequency range below the first resonce frequency where you can add all the stiffnesses (and only the stiffnesses) of all the springs of each modes. You then obtain a equivalent stiffness.

 

[spongebob007]

For a single DOF system subject to a harmonic force plot the transmissibility curve. The x axis is the ratio of the natural frequency to the driving force. For a ratio of .5 (equivalent to the natural frequency of the structure being one octave above the forcing frequency) the transimissibilty ratio is somewhere between 1.1 and 1.3 depending on damping. For many sturcutres of practical interest there is little error introduced by just assuming the transmissibility is 1.

Also I am not defining quasi-static analysis in terms of the octave rule. I am simply stating a general rule of thumb. Quasi-static analysis is using a body load in a static analysis in lieu of performing a dynamic analysis. That's a definition.