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Understanding a Static vs Dynamic problem 4
- Thread starter Jlog50
- Start date
The decision to treat a problem as static or dynamic is one of engineering judgment. Some issues are clearly one or the other....some are not so clear. Sometimes they must be analyzed both ways to see the differences/effects.
There are numerous texts on how to analyze either, but the decision to analyze in a certain way is left to the engineer in responsible charge, and keeping with the standard of care of his practice area.
There are numerous texts on how to analyze either, but the decision to analyze in a certain way is left to the engineer in responsible charge, and keeping with the standard of care of his practice area.
Twoballcane
Mechanical
I think this is a broad question. What is your goal? If it vibs or shock it is a dynamic problem. If there is a constant load with no reversals, it is a static problem.
For electronic packaging, Dave Steinberg's Vibration Analysis for Electronic Equipment may be a good start.
Tobalcane
"If you avoid failure, you also avoid success."
For electronic packaging, Dave Steinberg's Vibration Analysis for Electronic Equipment may be a good start.
Tobalcane
"If you avoid failure, you also avoid success."
- Thread starter
- #4
Thanks for your input Ron and Twoballcane, I am looking at artificial hip joint in the body in terms of a structural problem under time dependant loading conditions. Its seems that not many in the field justify there determination for looking at the problems statically or dynamically, so I was trying to learn more about this.
Thanks again.
Thanks again.
Twoballcane
Mechanical
Time dependant loading, do you mean 5lb at 1sec, then 10lb at 2sec, then 20lb at 3sec in a linear rate or 5lb per sec vibration?
Tobalcane
"If you avoid failure, you also avoid success."
Tobalcane
"If you avoid failure, you also avoid success."
Twoballcane
Mechanical
maybe you are thinking of frequency to failure?
Tobalcane
"If you avoid failure, you also avoid success."
Tobalcane
"If you avoid failure, you also avoid success."
- Thread starter
- #7
Twoballacane thanks for you input, the time dependant loading profile (assumed linear between values) is:
0s 0.12s 0.32s 0.5s 0.62s 1s
300.0N 3000.0N 1500.0N 3000.0N 300.0N 300.0N
I don't see this as so much a vibration problem, but more of a shock impact.
0s 0.12s 0.32s 0.5s 0.62s 1s
300.0N 3000.0N 1500.0N 3000.0N 300.0N 300.0N
I don't see this as so much a vibration problem, but more of a shock impact.
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- #8
I would consider it a reversing fatigue problem. Which is kinda like a single statics problem with a bunch of factors to derate the situation over time. Then I would look at buckling. and lastly there are some (I'm trying to remember so the term might be wrong) energy to failure over time Those are all "static" problems and are well treated in machine design textbooks. Dynamics is usually all about the solution to position, work, acceleration, jerk problems.
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- #9
Wrenchbender
Mechanical
Chapter 1 of Tedesco - "Structural Dynamics" talks about this distinction. (Should be available used at a low price.) Other good references could be Meyers & Chawla "Mechanical Behavior of Materials"; or Hertzberg "Deformation and Fracture Mech of Engr Mat'l".
Strain rate can also be an influential factor. With polymers and bio materials visco-elastic responses should probably be looked at for either a static or a dynamic analysis.
Strain rate can also be an influential factor. With polymers and bio materials visco-elastic responses should probably be looked at for either a static or a dynamic analysis.
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- #10
"0s 0.12s 0.32s 0.5s 0.62s 1s
300.0N 3000.0N 1500.0N 3000.0N 300.0N 300.0N
From this type of force-time history you get a response from an elastic system which in simple terms depends on the natural frequencies of the structure.
In general, if the lowest structural natural frequency is significantly lower than the dominant spectral terms in the shock, then the problem can be treated as static, using the highest value of force in the shock as the input.Or, another way,if the width of the shock pulse is much less than the lowest natural period of the structure.
Otherwise you must treat it as a classical shock-vibration problem.
In that case , you get the spectral content of the shock and vectorially sum the response,to your dynamic system, a usually tedious process.Or use the numerical methods in the time domain ( e.g.Duhamel) to find the response.
Good references abound.I like the series, Shock and Vibration Handbook, Harris and Crede, McGraw Hill.
300.0N 3000.0N 1500.0N 3000.0N 300.0N 300.0N
From this type of force-time history you get a response from an elastic system which in simple terms depends on the natural frequencies of the structure.
In general, if the lowest structural natural frequency is significantly lower than the dominant spectral terms in the shock, then the problem can be treated as static, using the highest value of force in the shock as the input.Or, another way,if the width of the shock pulse is much less than the lowest natural period of the structure.
Otherwise you must treat it as a classical shock-vibration problem.
In that case , you get the spectral content of the shock and vectorially sum the response,to your dynamic system, a usually tedious process.Or use the numerical methods in the time domain ( e.g.Duhamel) to find the response.
Good references abound.I like the series, Shock and Vibration Handbook, Harris and Crede, McGraw Hill.
Disregard my previous post. I got my pulse widths and time periods reversed.It should read:
0s 0.12s 0.32s 0.5s 0.62s 1s
300.0N 3000.0N 1500.0N 3000.0N 300.0N 300.0N
From this type of force-time history you get a response from an elastic system which in simple terms depends on the natural frequencies of the structure.
In general, if the lowest structural natural frequency is significantly higher than the dominant spectral terms in the shock, then the problem can be treated as static, using the highest value of force in the shock as the input.Or, another way,if the width of the shock pulse is much greater than the lowest natural period of the structure.
Otherwise you must treat it as a classical shock-vibration problem.
In that case , you get the spectral content of the shock and vectorially sum the response,to your dynamic system, a usually tedious process.Or use the numerical methods in the time domain ( e.g.Duhamel) to find the response.
Good references abound.I like the series, Shock and
Vibration Handbook, Harris and Crede, McGraw Hill.
For your input it looks like the "shock" pulse is the order of about 1/2 second which means that for structures with natural frequencies >> 2pi/.5sec= about 12Hz , the static solution is valid.
0s 0.12s 0.32s 0.5s 0.62s 1s
300.0N 3000.0N 1500.0N 3000.0N 300.0N 300.0N
From this type of force-time history you get a response from an elastic system which in simple terms depends on the natural frequencies of the structure.
In general, if the lowest structural natural frequency is significantly higher than the dominant spectral terms in the shock, then the problem can be treated as static, using the highest value of force in the shock as the input.Or, another way,if the width of the shock pulse is much greater than the lowest natural period of the structure.
Otherwise you must treat it as a classical shock-vibration problem.
In that case , you get the spectral content of the shock and vectorially sum the response,to your dynamic system, a usually tedious process.Or use the numerical methods in the time domain ( e.g.Duhamel) to find the response.
Good references abound.I like the series, Shock and
Vibration Handbook, Harris and Crede, McGraw Hill.
For your input it looks like the "shock" pulse is the order of about 1/2 second which means that for structures with natural frequencies >> 2pi/.5sec= about 12Hz , the static solution is valid.
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- #14
Never completely rule out dynamics. In reality, every problem is that of dynamics. Many can be simplified to a simple static problem when something simpler is studies, such as deflection in a bridge. However, without proper modal analysis (dynamic) this bridge may certainly make for a good "pallatzzo".
My point is that we should never rule out dynamics just because we can or because dynamics is much more complicated.
btw, shock impact is generally treated as dynamic. But, zekeman's post is certainly is a very good reference for determining the coupling pt. between dynamic and static.
Although, for your application I think you should do a dynamic shock isolation analysis.
This way the shock that the body sees can be minimized while at the same time providing not too much compliance.
this would make for a good design instead of a "solid metal bar".
![[peace]](/data/assets/smilies/peace.gif "[peace] [peace]")
Fe
My point is that we should never rule out dynamics just because we can or because dynamics is much more complicated.
btw, shock impact is generally treated as dynamic. But, zekeman's post is certainly is a very good reference for determining the coupling pt. between dynamic and static.
Although, for your application I think you should do a dynamic shock isolation analysis.
This way the shock that the body sees can be minimized while at the same time providing not too much compliance.
this would make for a good design instead of a "solid metal bar".
Fe
- Thread starter
- #16
Thank-you all for you feedback, I have never done a dynamic shock isolation analysis before can I do this using FE methods or simple hand calcs? I think I do need to do some reading up on this.
Chicopee, the problem does involve movement and rotation.
Thanks again
Chicopee, the problem does involve movement and rotation.
Thanks again
- Thread starter
- #18
Is a dynamic shock isolation analysis the same as a modal analysis to determine the frequency of the system?
Also I understand that I need to base the analysis on a linear model for a modal analysis and also my component in my FE model are in contact. Will this cause any problem with the modal analysis?
Thanks in adavance
Also I understand that I need to base the analysis on a linear model for a modal analysis and also my component in my FE model are in contact. Will this cause any problem with the modal analysis?
Thanks in adavance
No it's not the same thing, but related in ways.
A good reference for shock isolation is this thesis:
I'm not sure what this means though "also my component in my FE model are in contact. Will this cause any problem with the modal analysis?"
Fe
A good reference for shock isolation is this thesis:
I'm not sure what this means though "also my component in my FE model are in contact. Will this cause any problem with the modal analysis?"
Fe
- Thread starter
- #20
Thanks for link, FeX32, what I meant by "also my component in my FE model are in contact. Will this cause any problem with the modal analysis?" was that I have a few parts in my FE model which are in contact and there is a comment on this in the abaqus manual saying..
The natural frequencies and modes shapes of a structure can be used to characterise its dynamic response to loads in the linear regime....
It then goes on to state:
A problem should have the following characteristics for it to be suitable for linear transient dynamic analysis:
The system should be linear: linear material behavior, no contact conditions, and no nonlinear geometric effects.
The response should be dominated by relatively few frequencies. As the frequency content of the response increases, such as is the case in shock and impact problems, the modal superposition technique becomes less effective.
The dominant loading frequencies should be in the range of the extracted frequencies to ensure that the loads can be described accurately.
The initial accelerations generated by any suddenly applied loads should be described accurately by the eigenmodes.
The system should not be heavily damped.
The natural frequencies and modes shapes of a structure can be used to characterise its dynamic response to loads in the linear regime....
It then goes on to state:
A problem should have the following characteristics for it to be suitable for linear transient dynamic analysis:
The system should be linear: linear material behavior, no contact conditions, and no nonlinear geometric effects.
The response should be dominated by relatively few frequencies. As the frequency content of the response increases, such as is the case in shock and impact problems, the modal superposition technique becomes less effective.
The dominant loading frequencies should be in the range of the extracted frequencies to ensure that the loads can be described accurately.
The initial accelerations generated by any suddenly applied loads should be described accurately by the eigenmodes.
The system should not be heavily damped.
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