Need help with modelling viscoelastic material in ANSYS

[Waqas Khan]

Hi,
I am a student working on a project entitled "finite element of a viscoelastic damper". I am taking natural rubber as the viscoelastic material. In order to define a viscoelastic material in ANSYS, we have to define some parameters of the prony series:

- the relative shear modulus and the relaxation time in shear.
- the relative volumetric modulus and the bulk relaxation time.
- the WLF equation constants (can be found out from literature).

My problem is that I don't have to deal with experimentation (b/c of the lack of equipment). So I cannot perform any tests. So can u give me any previously measured prony series parameters,

a(alpha) and t(relaxation time)

for natural rubber and a few more materials. If they are dependent on specimen size and shape, can i also have the sizes and shapes used in the actual experiment. Plz, i am really in need of these parameters and would really appreciate the help..

[Jorgen]

I virtually never use the linear viscoelasticity model so unfortunately I do not have the material parameters that you are looking for. You might find them in the Plastic Design Library books discussed in this thread (http://polymerfem.com/forums/showthread.php?t=354).

- Jorgen

[Jorgen]

I don't use Ansys often enough to remember the Ansys structure. But in case you are interested, Abaqus has a CREEP subroutine that would do what you are interested in...

-Jorgen

[Jorgen]

I wrote one exemplar VUMAT that you can download from this thread (http://polymerfem.com/forums/showthread.php?t=42).

Note that writing a VUMAT is not an easy assignment.
Best of luck,
Jorgen

[Frank]

Hello,

here is the code for a sinh-creep law in Abaqus.
Good luck,

Frank

-------------------

SUBROUTINE CREEP(DECRA,DESWA,STATEV,SERD,EC,ESW,P,QTILD,
1 TEMP,DTEMP,PREDEF,DPRED,TIME,DTIME,CMNAME,LEXIMP,L END,
2 COORDS,NSTATV,NOEL,NPT,LAYER,KSPT,KSTEP,KINC)
C
INCLUDE 'ABA_PARAM.INC'
C
CHARACTER*80 CMNAME
C
DIMENSION DECRA(5),DESWA(5),STATEV(*),PREDEF(*),DPRED(*),
1 TIME(2),COORDS(*),EC(2),ESW(2)
C
C DEFINE CONSTANTS
C
A=
SIG0=
AN=
C
T1=EXP(QTILD/SIG0)
T2=EXP(-QTILD/SIG0)
DECRA(1) = A*(.5*(T1-T2))**AN*DTIME
IF(LEXIMP.EQ.1) THEN
DECRA(5) = AN*A*(.5*(T1-T2))**(AN-1.)*DTIME/
1 SIG0*.5*(T1+T2)
END IF
C
RETURN
END

[manfred]

Ready made measurements of dynamical mechanical data from NR compounds may be available by Dr. Michael Herdy the founder of the ViscoData software (determining PRONY series). Dr. Herdy can also deliver experimental data sets for various different compounds (EPDM,FKM,NBR . . . ) Usually he applied those data to demonstrate the utility of above mentioned software!



Hi,
I am a student working on a project entitled "finite element of a viscoelastic damper". I am taking natural rubber as the viscoelastic material. In order to define a viscoelastic material in ANSYS, we have to define some parameters of the prony series:

- the relative shear modulus and the relaxation time in shear.
- the relative volumetric modulus and the bulk relaxation time.
- the WLF equation constants (can be found out from literature).

My problem is that I don't have to deal with experimentation (b/c of the lack of equipment). So I cannot perform any tests. So can u give me any previously measured prony series parameters,

a(alpha) and t(relaxation time)

for natural rubber and a few more materials. If they are dependent on specimen size and shape, can i also have the sizes and shapes used in the actual experiment. Plz, i am really in need of these parameters and would really appreciate the help..

[Jorgen]

Hello manfred,

I am curious, what is your relationship to the ViscoData software? Are you working for that company? Do you have the contact information for Dr. Michael Herdy?

- Jorgen

[manfred]

dear Jorgen ,

no, I do not working for this company, I'm just a frequently user and satiesfied customer of this software package and many years ago I was a kind of consultant in polymer physics when Dr. Herdy starded to develope this software. Dr. Herdy is a specialist in evolutionary computation and genetic algorithm. All of the fitting procedures applied in that package is based on his expertise in that field. That is why for example the determination of the prony series goes so smart in ViscoData; one never needs for instance to set initial values in order to start the fitting process as is usually required when using gradient methods.

Manfred