Different results between NASTRAN and ABAQUS
Different results between NASTRAN and ABAQUS
(OP)
Hello everyone,
I made the same analysis in NASTRAN and ABAQUS in order to compare their performance and results.
It's a modal dynamic analysis (half sine shock), the shock is 0.011 s long and my analysis time is 0.22 s in order to see the behavior of the structure after the shock.
In ABAQUS I used the following command lines:
*STEP, INC=200
*MODAL DYNAMIC, CONTINUE=NO
0.0011 , 0.22
*MODAL DAMPING,MODAL=DIRECT, DEFINITION=FREQUENCY RANGE
0.0,0.03
400.0,0.03
*SELECT EIGENMODES, DEFINITION=FREQUENCY RANGE
0.0,400.0
*BASE MOTION,DOF=1,AMPLITUDE=CHOC_11ms,SCALE=9.81,TYPE=ACCELERATION
Where CHOC is an input file where my half sine coordinate are set.
For NASTRAN I used the following commands:
SOL 112
DLOAD = 101
TSTEP 100 200 0.0011 1
TLOAD2 101 101 ACCE 0.0 1.1E-2 45.45455270.0
TABDMP1 4 Q
+ 0.0 16.7 400. 16.7 ENDT
SPCD 101 174482 1 9.81
ENDDATA
SOL 112 is based on a cosine so I used a phase of 270 in order to obtain a sine.
The thing is that if I plotted the acceleration OUTPUT of the same node from both analysis I have a similar behavior but I observe differences (see attached picture).
I do not know where the differences come from, do you have an idea?
The blue curve is NASTRAN and the orange one is ABAQUS
Thks
I made the same analysis in NASTRAN and ABAQUS in order to compare their performance and results.
It's a modal dynamic analysis (half sine shock), the shock is 0.011 s long and my analysis time is 0.22 s in order to see the behavior of the structure after the shock.
In ABAQUS I used the following command lines:
*STEP, INC=200
*MODAL DYNAMIC, CONTINUE=NO
0.0011 , 0.22
*MODAL DAMPING,MODAL=DIRECT, DEFINITION=FREQUENCY RANGE
0.0,0.03
400.0,0.03
*SELECT EIGENMODES, DEFINITION=FREQUENCY RANGE
0.0,400.0
*BASE MOTION,DOF=1,AMPLITUDE=CHOC_11ms,SCALE=9.81,TYPE=ACCELERATION
Where CHOC is an input file where my half sine coordinate are set.
For NASTRAN I used the following commands:
SOL 112
DLOAD = 101
TSTEP 100 200 0.0011 1
TLOAD2 101 101 ACCE 0.0 1.1E-2 45.45455270.0
TABDMP1 4 Q
+ 0.0 16.7 400. 16.7 ENDT
SPCD 101 174482 1 9.81
ENDDATA
SOL 112 is based on a cosine so I used a phase of 270 in order to obtain a sine.
The thing is that if I plotted the acceleration OUTPUT of the same node from both analysis I have a similar behavior but I observe differences (see attached picture).
I do not know where the differences come from, do you have an idea?
The blue curve is NASTRAN and the orange one is ABAQUS
Thks





RE: Different results between NASTRAN and ABAQUS
RE: Different results between NASTRAN and ABAQUS
I made a previous modal analysis and I obtained the same frequencies, modes and effective masses with both codes.
Yes this is MSC NASTRAN.
I first thought that it was problem of a difference between the sine defined by ABAQUS and NASTRAN but I plotted the curves of total acceleration at the excitation nodes to see if both codes used the same input excitation. They are the same, so I think maybe the problem comes from the damping coefficient.
n NASTRAN I have the following values for the damping (I used CRIT type): Q=30, G/2=1/2Q so G=0.03333 and CRIT=0.01667
In ABAQUS I put G as the modal damping values (don't know if it's the right approach)
RE: Different results between NASTRAN and ABAQUS
RE: Different results between NASTRAN and ABAQUS
Well in fact I am not sure it is the source of my problem here. I just run a sine sweep frequency on both codes and I obtained the same curve for the acceleration vs frequency (following the direction of the sine sweep). So I suppose my damping coeff are good here.
So I join you on your first impression it's maybe the way the diff eqt are solved.
RE: Different results between NASTRAN and ABAQUS
another day in paradise, or is paradise one day closer ?
RE: Different results between NASTRAN and ABAQUS
I agree with you, I first put 90 deg but my input signal was negative and I had to apply a -1 factor to my shock in order to have the same input than the one in ABAQUS. I then tried 270 deg and I have a positive signal without adding my -1 factor.
By the way in both case (phase=90 and -1 factor and phase=270 without -1 factor) give the same results (the curve in my first post)
RE: Different results between NASTRAN and ABAQUS
sin(x) = cos(x+270)
another day in paradise, or is paradise one day closer ?
RE: Different results between NASTRAN and ABAQUS
So adding a phase of 270 is right we are agree?
My half sine shock is 0.011s long so the max is at 0.0055s, so a period of 0.022s.
RE: Different results between NASTRAN and ABAQUS
another day in paradise, or is paradise one day closer ?
RE: Different results between NASTRAN and ABAQUS
The critical damping you calculate is 0.01667, but that not what you have entered in your nastran SOL112??
RE: Different results between NASTRAN and ABAQUS
You use a 0.03 (or 3%) modal damping in ABAQUS analysis whereas for NASTRAN you use a Q damping of 16.7. Which equates to a critical damping ratio of 1/(2*16.7) = 3%, so your modal damping becomes =2*3% = 6%. This doesn't explain the phase shift that you see but maybe you can redo your analysis with consistent inputs for both solvers.