http://polymerfem.com/ The latest news about constitutive modeling and simulation of polymers. en Fri, 18 May 2012 03:15:00 GMT vBulletin 60 http://polymerfem.com/images/misc/rss.png http://polymerfem.com/ http://polymerfem.com/showthread.php?2643-Polycrystal-Shape-Memory-Alloys&goto=newpost Wed, 16 May 2012 13:56:37 GMT Hi all, I am modelling polycrystal shape memory alloys upon uniaxial loading using Abaqus UMAT. I modelled single crystal which are unrotated or... Hi all,

I am modelling polycrystal shape memory alloys upon uniaxial loading using Abaqus UMAT. I modelled single crystal which are unrotated or rotated successfully. But when I model polycrystal shape memory alloys I get always this error in Abaqus message file: "***NOTE: THE SOLUTION APPEARS TO BE DIVERGING. CONVERGENCE IS JUDGED UNLIKELY. ***ERROR: TOO MANY ATTEMPTS MADE FOR THIS INCREMENT" . I know that this problem occurs because of the material Jacobian. I calculated my Jacobian numerically using finite difference.

I used this algorithm which Matsgd wrote for me in another thread:

1. Calculate the stress at time t+dt based on the strain at t+dt (and the state variables at time t).
2. Update STRESS and STATEV in the UMAT. Do not modify these when calculating DDSDDE.

3.

Define SIX strain increment vectors:
de1 = alpha*[1 0 0 0 0 0]
de2 = alpha*[0 1 0 0 0 0]
de3 = alpha*[0 0 1 0 0 0]
de4 = 2*alpha*[0 0 0 1 0 0]
de5 = 2*alpha*[0 0 0 0 1 0]
de6 = 2*alpha*[0 0 0 0 0 1]

If your constitutive model uses engineering shear strains, ignore the factor "2" above.

For each of the strain increment vectors above, calculate a stress vector, using the known strain epsilon at time t+dt, and the strain increment vectors above. In other words, calculate stresses just the same way you would do normally, only this time based on the SIX strain vectors:

(epsilon + de1) -> sigma1
(epsilon + de2) -> sigma2
(epsilon + de3) -> sigma3
(epsilon + de4) -> sigma4
(epsilon + de5) -> sigma5
(epsilon + de6) -> sigma6

The first column of C is then given by
(sigma1-sigma)/alpha.
The second is given by
(sigma2-sigma)/alpha





This algorithm is useful for single crystal problems. However, when I use polycrystal model, my solution does not converge. What can I do? Can it be an another problem different from Jacobian calculation? or Can I try different things in calculation of Jacobian?

Thank you.
Nazim Babacan ]]> Constitutive Models NazBaba http://polymerfem.com/showthread.php?2643-Polycrystal-Shape-Memory-Alloys http://polymerfem.com/showthread.php?2642-creating-with-equation&goto=newpost Tue, 15 May 2012 03:27:00 GMT can i create a part with equation... example: create a circle with y = x+2 thanks can i create a part with equation...

example: create a circle with y = x+2

thanks ]]> Finite Element Modeling Ng Theng Pin http://polymerfem.com/showthread.php?2642-creating-with-equation http://polymerfem.com/showthread.php?2641-Add-new-variable-in-subroutine-DLOAD&goto=newpost Fri, 11 May 2012 02:06:33 GMT Hi

I'm doing something related to Subroutine in Abaqus. I got stuck in problem:" I try to add more variables in subroutine: example: I want to have a variable 'A' in DLOAD and After an increment, 'A' variable will change Value."
When I try to do it. I pass this case:

in DLOAD, I add some code :
IF (KINC.lt.102) THEN
ALLOCATE( CV(100), STAT = AllocateStatus)
IF (AllocateStatus /= 0) STOP "*** Not enough memory ***"
END IF
CV(1)=10+KINC
CV(2)=99
The result of CV(1) change, I think that CV changes depend on KINC (KINC will change after an increment). After do that, I try to change CV but I dont want to use KINC, Someone have some ideas about my problem can Help me !!!!

Thank you so much ]]> Finite Element Modeling luonganh89 http://polymerfem.com/showthread.php?2641-Add-new-variable-in-subroutine-DLOAD http://polymerfem.com/showthread.php?2640-Umat&goto=newpost Thu, 10 May 2012 03:49:07 GMT ist possible UMAT subroutine make the element to active or deactive the element? what the coding should input? thanx.. ist possible UMAT subroutine make the element to active or deactive the element? what the coding should input?
thanx.. ]]> Finite Element Modeling Ng Theng Pin http://polymerfem.com/showthread.php?2640-Umat http://polymerfem.com/showthread.php?2639-Help-about-Allocating-Variables-in-Subroutine-Abaqus&goto=newpost Tue, 08 May 2012 01:10:42 GMT I try to allocate some variables in fortran Subroutine abaqus. but i get some error. Anyone has met this problem can share me some experience ? I try to allocate some variables in fortran Subroutine abaqus. but i get some error. Anyone has met this problem can share me some experience ? ]]> Finite Element Modeling luonganh89 http://polymerfem.com/showthread.php?2639-Help-about-Allocating-Variables-in-Subroutine-Abaqus http://polymerfem.com/showthread.php?2638-GHM(Golla-Hughes-Mac-Tavish-model)-for-viscoelastic-materials&goto=newpost Mon, 07 May 2012 15:02:16 GMT Bonjour;
J'ai une structure sandwich à coeur viscoélastique que je veux la modéliser sous Ansys.
La loi du comportement du coeur viscoélastique suit le modèle GHM suivant
G(s)=G0(1+sum(alpha(i)*(s^2+2*omega*xsi*s/s^2+2*omega*xsi*s+omega^2))
avec:
G0:le module statique de cisaillement
alpha,xsi,omega:sont les paramètres du minioscillateurs correspondant(vu que ce module est équivalemnt à la somme de termes des mini-oscillateurs).
Sous Ansys,j'ai constaté qu'il ya soit le modèle réhologique de Maxwell, soit les séries de Prony pour définir cette loi.
Je veux modéliser cette loi sous ansys,mais je n'ai trouvé aucun tutorial qui parle sur les structures viscoélastiques(non-linéarité matérielles).

Est ce que vous pouvez me rensigner et guider pour ce faire ce genre de trucs.
Merci bcp ]]> Viscoelasticity SOSO http://polymerfem.com/showthread.php?2638-GHM(Golla-Hughes-Mac-Tavish-model)-for-viscoelastic-materials http://polymerfem.com/showthread.php?2637-Layered-Shell-Modeling&goto=newpost Sun, 06 May 2012 14:03:38 GMT Hi, I am confused regarding displacements calculated through layered shell model in Ansys using Shell 181. The model is composed of 5 layered... Hi,

I am confused regarding displacements calculated through layered shell model in Ansys using Shell 181.

The model is composed of 5 layered sections of different materials (with thicknesses) assigned to an area. The whole laminate is cooled from a reference (zero-strain) (150 C) temperature to a uniform temperature (-40 C).

I need to calculate the difference of displacements between two nodes in the meshed area, for a specific location in layer no. 3. The issue is that I only have nodes in one plane and the results (i think) are interpolated along the thickness. My question is that, is there any method to find displacements at a specific layer in the shell model [as we can find all other results (stress, strains etc.) for different layers]

I have also developed a 3D model (solid 185) for the same problem and the displacements calculated verifies with the literature but has a difference of the order of 10 with the shell model. I need to use the shell model in order to decrease the problem size.

Any help will be appreciated. Thanks! ]]> Finite Element Modeling osmias http://polymerfem.com/showthread.php?2637-Layered-Shell-Modeling http://polymerfem.com/showthread.php?2636-can-i-adding-element-in-abaqus-by-calling-the-subroutine&goto=newpost Fri, 04 May 2012 07:05:48 GMT may i know can i adding element in abaqus by calling the subroutine? and what subroutine should i use?

example: when the stress at the element "A" exceed the certain and i want to call to add the element so that can reduce the stress at the part? ]]> Finite Element Modeling Ng Theng Pin http://polymerfem.com/showthread.php?2636-can-i-adding-element-in-abaqus-by-calling-the-subroutine http://polymerfem.com/showthread.php?2635-analysis-step-problem&goto=newpost Thu, 03 May 2012 15:25:51 GMT i had set my step with the some value of load.... and may i know why the analysis of the stress and strain will be the same with the different time... i had set my step with the some value of load....
and may i know why the analysis of the stress and strain will be the same with the different time period set?

is't means the time period with 100 with load, A kN = 100 cycle with load, A kN?

thank you..... ]]> Finite Element Modeling Ng Theng Pin http://polymerfem.com/showthread.php?2635-analysis-step-problem http://polymerfem.com/showthread.php?2634-adding-element&goto=newpost Thu, 03 May 2012 06:49:35 GMT may i know can i adding the element during the analysis is running? and how i can do it\.... example: when the stress value at element AA... may i know can i adding the element during the analysis is running? and how i can do it\....

example:
when the stress value at element AA greater the threshold, i would like to add the element at the element AA so that can reduce the the stress value...

thank you... ]]> Computer Software Ng Theng Pin http://polymerfem.com/showthread.php?2634-adding-element http://polymerfem.com/showthread.php?2633-Depvar&goto=newpost Thu, 03 May 2012 04:22:30 GMT For the abaqus documentation. depvar is used to allocate space at each integration point for solution-dependent state variables.. im not very... For the abaqus documentation. depvar is used to allocate space at each integration point for solution-dependent state variables.. im not very understand on that... can somebody give an simple example of using depvar.... thank you.. ]]> Finite Element Modeling Ng Theng Pin http://polymerfem.com/showthread.php?2633-Depvar http://polymerfem.com/showthread.php?2632-Applying-initial-stress-field-in-Ansys-using-INISTATE&goto=newpost Wed, 02 May 2012 22:39:34 GMT I am having some difficulty using the INISTATE feature in Ansys 13.0. Let's say I have a square sheet modelled using shell elements. I am using a non-linear hyperelastic law. The edge of the sheet is fixed in all DOF. I apply an anisotropic initial stress field to the sheet where Sx=0.01 and Sy-0.02 using:

INISTATE,SET,DTYP,STRE,CYS,-2
INISTATE,DEFINE,,,,,0.01,0.02

I check that the stress field is what I want it to be by typing:

INISTATE,LIST

I run two analyses where I displace the centre node of the domain and calculate the reaction force on the node. In the first analysis, I displace the node in the positive X direction. In the second analysis, I displace the same node in the positive Y direction. As the material is non-linear, I expect the stiffness of the sheet in the Y direction to be greater than in the X direction due to the initial stress state. Therefore, the reaction force in the second analysis should be higher for a given displacement than the reaction force for the same displacement in the first analysis. However, this is not what I get. The reaction force is higher when the node is displaced in the X direction.

Instead of using INISTATE, if I apply the initial stress by applying appropriate forces along the edge of the sheet in a loadstep prior to displacing the nodes, the reaction force results are what I expect. I have run equivalent ABAQUS analyses in the past using the *INITIAL CONDITIONS, TYPE=STRESS feature and the results were what I expected also.

Am I misunderstanding/misusing the INISTATE feature in Ansys? Or is my logic wrong?

Thanks for any input.

Cormac ]]> Computer Software CF http://polymerfem.com/showthread.php?2632-Applying-initial-stress-field-in-Ansys-using-INISTATE http://polymerfem.com/showthread.php?2631-Max-temperature-in-Dynamic-Mechanical-Testing&goto=newpost Wed, 02 May 2012 19:25:00 GMT Hi, I am trying to evaluate viscoelastic properties of a rubber material using Dynamic Mechanical Analyser. The frequency range of my interest is... Hi,

I am trying to evaluate viscoelastic properties of a rubber material using Dynamic Mechanical Analyser. The frequency range of my interest is up to 8 KHz. I read in literature that using Time Temperature Superposition technique (or frequency shifting) we can convert data at extended temperatures to higher / lower frequencies.

Can someone suggest if there is a formula or thumb rule as to the minimum temperature down to which I should test the sample for getting master curve up to 8 KHz ?

Thank you.

Kind regards
Kiran ]]> Viscoelasticity KiranK http://polymerfem.com/showthread.php?2631-Max-temperature-in-Dynamic-Mechanical-Testing http://polymerfem.com/showthread.php?2630-COMSOL-Setting-up-simple-optimization-problem&goto=newpost Wed, 02 May 2012 11:03:07 GMT Hi all, i have some doubts about setting the COMSOL optimization module. My aim is quite simple, i should find the optimal value of a variable to... Hi all, i have some doubts about setting the COMSOL optimization module.

My aim is quite simple, i should find the optimal value of a variable to minimize the objective equation.
In particular i should set up the parameter C in this equation(which represents the outlet pressure of a laminar flow model):

S=Q*C+po

in order to minimize the following equation:

abs(p-15000)

Where p is the pressure coming out from linear flow model (spf) and 15000 is a reference pressure, both applied to domain.
The algorithm is quite simple, comsol should:

1)assign a value to C
2)estimate the value of domain pressure( coming out from laminar flow model)
3)verify if optimality condition is satisfied, otherwise start again from 1

My doubt is about implementation: which kind of objective should i use (integral objective, probe objective...)? Should i use inequality constraints too?

Best regards ]]> Finite Element Modeling tdh89 http://polymerfem.com/showthread.php?2630-COMSOL-Setting-up-simple-optimization-problem http://polymerfem.com/showthread.php?2629-How-to-plot-pole-figure-after-CPFEM-modeling-by-using-UMAT&goto=newpost Wed, 02 May 2012 06:02:19 GMT Thanks for sharing your knowledge. Thanks for sharing your knowledge. ]]> Constitutive Models ilovebluesky http://polymerfem.com/showthread.php?2629-How-to-plot-pole-figure-after-CPFEM-modeling-by-using-UMAT