Dinamic Stiffnes Improvement

[Caronte]

Hello all,

I have to improve an automotive part according to dinamic stiffnes criteria and I need to know wich parameters affect to the sistem.

The sistem is loaded with a force of 1 N from 0 Hz to 100 Hz and the measured result is the acceleration at 1 point. This acceleration has to be under a limit curve in all the spectrum.

Adding material or making the sistem more rigid (in static sense) does not seem to help to reduce accelerations (even it make it worst!!!).

Any advice will be very helpfull.

Thanks all.

 

[electricpete]

I don't have any practical experience with automobile stuff, but some comments from simple theory.

Do you have one dominant peak in your spectrum indicating resonance? If near the top of your range maybe you can stiffen to move it up above your range.

For simple sdof damped mass spring system, the effect of adding masss is to move resonance frequency down. If measured in velocity the height of the peak is constant. If measured in acceleration the height of the peak should decrease in proportion to resonant frequency.

Have you considered adding damping. That should reduce the height of any significant peaks. Maybe it can a washer made of damping material inserted at the joint where the part is bolted down.

 

[brad]

This can be a difficult problem to solve using only test methodology. I have dealt with this problem in the past, using a combination of test and FEA. Typically, when the response target is violated, one wants to evaluate which structural modes dominate the response and address these individually.

Stiffening the part often increases the mass, and as you've observed can result in even worse performance. For this reason, it is important to selectively stiffen the part.

Using an FEA code, the best way to address these problematic modes is to identify the areas of peak strain energy and stiffen them locally. Therefore, even if mass is added, if done selectively the stiffness improvement is greater than the degradation due to increased mass. Lacking an FEA evaluation, this can be done by using engineering judgment, but this can often be difficult to evaluate (at least in complex parts).

As a simple example, consider a cantilever beam in a first mode bending. The best way to dynamically stiffen this is not to increase the thickness through the entirety, but rather thicken it near the constrained end (as this will have positive stiffness implications, without as negative of a mass implication). Conversely, adding mass at the free end will degrade the performance (even though the static stiffness increases).

Hope this helps.
Brad

 

[GregLocock]

Caronte, this was my bread and butter for 10 years.

It sounds as though you may be testing the component free-free. If it is a small component then this is usually unrealistic, and will make it very difficult to optimise the driving point impedance. The results will be pretty meaningless as well.

If it is large, such as an engine block or a car body, then it is appropriate to test it free-free, in that frequency range.

Are you physically testing the part? you may find that SMURF (structural modification using response functions, I kid you not) will lead you to a better structure.

If this is an FEA test then try using an optimiser.









Cheers

Greg Locock

 

[Caronte]

First, thank you all for your fast reply.

At this step of design I am analyzing the sistem in FEA code (using Nastran). As first issue I had a first peak of resonance near 84 Hz, but I solved this adding more stiffness to one of the components of the assembly (where the part was clamped).

The damping used is a design constraint. A customer constant value stablished for calculation and I don't have possibilitie to add any damping component in the fixing interface.

The part I am analizing is big and with welded subcomponents. The analisys I am doing considers the assembly clamped at the fixing points

As you propose me I will try to add more stiffness near the fixing points and evaluate the effect. Also modify some thickness of many parts.

Thanks again for your help and I will keep you informed.

 

[GregLocock]

Good, this is much easier in FEA!


Typically to improve the point impedance below the first flexural resonance you will have to improve the global stiffness of the system. Increases in mass should not have too much effect, and damping won't either.

I don't understand why increasing the rigidity of the part did not work - perhaps you did not improve the global rigidity.

Above the first resonance you may get some benefit by local stiffening actions.





Cheers

Greg Locock

 

[brad]

Greg and I are essentially saying the same. I've found the best way to locally stiffen the part is to evaluate peak strain energy density and focus on those areas. Stiffening near the constraints presumes that this is the critical area. Check this assumption against your strain energy plots.

Brad

 

[Tmoose]

"The part I am analizing is big and with welded subcomponents. The analisys I am doing considers the assembly clamped at the fixing points.
As you propose me I will try to add more stiffness near the fixing points and evaluate the effect. Also modify some thickness of many parts."

I'd look at "Element strain" in my last model to pick which of the "many parts" need stiffening. If the "fixing points" are bolted flanges then moving the fasteners closer to the web or thickening the flanges often increases stiffness 10 X real quick.

System stiffness and avoiding dangerous stresses levels from over-restrained welds are sometimes at odds with each other.

 

[blackeye]

GregLocock

I haven't seen a reference to SMURF for years. You're not really using Modal Plus are you?

 

[GregLocock]

No, it's just an acronym that I always remember! We use LMS, I doubt that we'll be dropping that in the near term.

We /do/ still have a Genrad 2515, but it is only used for one particular rig.

Cheers

Greg Locock