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viscoelastic material simulation in radioss

Hi everyone, I need an urgent help from all of you. I am doing a viscoelastic material simulation in radioss using kelvin maxwell law. i have the material data in storage modulus vs frequency so i changed my data into g(t) vs t. now when i go for curve fitting the value of prony term gi is coming greater than one.. As i have read in abaqus manual that value of gi should be less than 1 ,, so my question is this theory applied with radioss also or I can use my prony term coefficient which are coming greater than one.Please clear my concepts regarding this or also as per radios can i take the no of prny term more than 5 during curve fitting,, clear my concpts regarding this

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Altair Forum User

i mannu,

There is no mention of this condition for the Kelvin Maxwell Law in RADIOSS

Is there a reason specified why this has to be less than 1 elsewhere?

See in help - Altair RADIOSS > Reference Guide > Starter Input > Starter Keywords > /MAT:
/MAT/LAW40 (KELVINMAX)

Altair Forum User

The linear viscoelastic model in Abaqus is defined in the shear relaxation modulus as:

G(t) = Go{ 1 - Sum series (i=1, I=n) gi exp(-t/Taui)}

As you know Go is the instantaneous shear modulus which is the highest modulus value of the prony series at time = 0.0. After that all moduli should be reduced less numbers than G0 till they go to G-Infiniti and this occurs at t goes to infiniti. Therefore,
G-infiniti is called the long term modulus at t = inifinit defined by

G_infinit = Go { 1 - Sum series (gi)}

There is no way in above equation that gi will exceed 1

Altair Forum User

@Prakash Pagadala I have also some questions regarding this MATLAW.

I have a constitutive model for a polymer material that I want to test its compression in radios but I do not know how to put my data into radios.

can you help me to do so?

thanks

Altair Forum User

Hi @SamuelIshak

Sorry for a late reply.

You can use LAW65 and if you are using solid elements try LAW 33