Learning Forum

[sachin kumar]

Hi,

Why harmonic analysis stress results are differ from transient structural (sinusoidal load) analysis (Non-linear large deflection on) even I have run transient analysis long enough to get steady state response.

details of load and settings are given below.

Harmonic analysis: acceleration in x- direction = 5g, damping ratio = 0.03 first i have run modal and got frequency range 1000 to 4000Hz than i have run harmonic and evaluate von-mises stress at 1000Hz result shows 1.51Mpa max value.

In next step, I ran transient structural with Non-linear mat properties with large deflection ON/OFF both.

load: acceleration =49050*sin(2*pi*1000*time)). damping control Beta coeff 9.53e-06 calculated basis 0.03 damping ratio

min. time step cal: 1/(20*1000) = 0.00005sec. in graph control No of points = 20*1000*time.

I ran for 1 sec and got periodic type results of von mises stress and max value comes out to be = 0.73Mpa which is lower than harmonic.

my question is why there is a difference in harmonic and transient eventhough i got steady state periodic value which we can say harmonic.

as per my understanding non-linear mat properites should not impact the result becaouse stress value is well below the plastic region.

can anyone explain why there is a difference in both harmonic and transient structural????

[danielshaw]

Harmonic analyses are linear.  It appears that you ran a non-linear transient.

[sachin kumar]

yes I ran non-linear transient, but how it will affect the results even the stress values are still in elastic region

[dlooman]

Large deflection wouldn't be included in the harmonic.  You could have numerical damping in the transient.  What's is the gamma value on the TINTP command?

[sachin kumar]

I have tried both

large deflection OFF: 1.13Mpa

Large deflection ON: 0.73Mpa

Gamma = 0.1

[dlooman]

So, you should be comparing results with large deflection off to get an apple to apple comparison.  Gamma = 0.1 is like a log decrement of 0.1.  Damping ratio is about equal to log decrement divided by 2pi.  So in the transient you have more damping.  How different are the two stresses?  What's the maximum harmonic stress?

[sachin kumar]

maximim harmonic stress = 1.58Mpa@5g load and 3.17Mpa@10g load

Max transient structural stress = 0.78Mpa @5g load and 2.32Mpa @ 10g load

there is a constant difference of about 25%

so as per your understanding the large deflection is off and what about gamma = ??

[sachin kumar]

i tried with gamma = 0.005 as well but the results are still same

[dlooman]

Since the transient is linear, shouldn't the 10g result be 2X the 5 g result?  Is it possible there is another nonlinearity, such as nonlinear contact?

[sachin kumar]

oh sorry that was typo mistake it is 1.16MPa instead of 0.78MPa

we have bonded contact in both transient and harmonic

[dlooman]

That's a good verification that the transient is linear then.  Just to confirm, the excitation is at 1000 Hz and you run the transient for 1 second with a time step of 0.00005 secs.  Even with auto-time stepping the solution should probably maintain this time step since the response frequency should be close to 1000 hz.  Is that the case?  The time step doesn't open up?  A possible source of confusion on the harmonic side is that in recent years we have switched from damping ratio (dmprat) to constant structural damping coefficient (dmpstr) for a full harmonic.  If you are actually using constant structural damping coefficient, it produces only half the amount of damping that damping ratio of the same value does.

[sachin kumar]

Thanks Dave for prompt reply.

Initial time step = 0.001

min time step = 0.000001

max = 0.1

time step is auto control and it is varying with time

I got your point i think we should keep the time equal to 0.00005 for complete iteration.

as we have sinusoidal loading if time step will be more than above then there might be slip in magnitude of load.

am i right??

[dlooman]

I just wanted to make sure we didn't have a large integration error.  A time step of 0.00005 would insure that.

[sachin kumar]

Now I am getting 3.13Mpa max stress which is very close to harmonic results 3.17Mpa

 

[dlooman]

So, we can declare victory?

[sachin kumar]

????