Sai Phanindra

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Posts posted by Sai Phanindra


Hi,
I have checked and compared the .out files for the model with and without global rigid body. The model with global rigid body does not account for the added inertia.

Hi,
I have added mass and Inertia at the CG of the system, which I want to retain with the global rigid body. My analysis involves impact of a body on hard ground and the objective is to predict the subsequent dynamics. For the same, I want to use a global rigid body to reduce the time taken during the bouncing phase.

Thanks for the suggestion. That works.

Hi Gopal,
Thanks for the info. It would be great if you could elaborate the following:
1. Suppose I add some external inertia to one of the rigid bodies which are deactivated during freefly, What happens to the inertia? Is it also deactivated.
2. If that is the case, can I add external inertia to the global rigid body to compensate for the deactivated Inertia?
Thanks,
Sai

Hi,
I am trying to run a dynamic impact analysis problem with bouncing involved. In my model, I have added inertia specified in global skew frame to replicate the actual case. I want to reduce the computation time when the body is off the ground by introducing a rigid group consisting of all the nodes in the model. For the same, I am using a sensor of TYPE: interface. According to the RBODY definition, is SENS_ID is nonzero, the added inertia is ignored. Please confirm if this is true? If so, how do I ensure that the added inertia is incorporated in the model when I make SENS_ID nonzero.
Thanks and Regards,
Sai Phanindra

Hi,
I am using INTER/TYPE7 for contact definition of a piston sliding inside a cylinder. The contact parameters are:
Istf=4, Igap=1, FRIC=0.1, gapmin=0.01, INACTI=6, Ifric=0, Iform = 2.
Please find attached the Resultant normal force and the Resultant Tangential force plot (created by TH/INTER output block). As it can be observed from the plot, the peak normal force is ~12 KN for which the expected tangential force should be 1.2 KN (for mu=0.1). However, the observed resultant tangential force peak is upto ~3 KN.
Please resolve this problem. Thanks a ton.
Regards,
Sai Phanindra

Hi Prakash,
I know the value of thermal expansion coefficient (alpha). Can you guide me how it converts to the temperature exponent in LAW2?
Thanks,
Sai

Hi,
I want to do a thermal analysis using RADIOSS, as it involves impact as well. In optistruct, I have done the thermal analysis using MAT1 wherein I specified the thermal expansion coefficient (A). What material should I use in RADIOSS and how can I specify the thermal expansion coefficient.
Thanks and Regards,
Sai Phanindra

Hi Prakash,
I am trying to model soil in RADIOSS and I have used the equations as given above to find the yield coefficients A0, A1 and A2. The results I am getting are different from the actual properties. Can you give more insight on how J2 coefficients are calculated. I am unsure of the equation you have given for J2 because the curve given below (J2 vs P curve) can never occur for equation given by J2 = 1/3*(alpha^2*P^2 2alpha^2*t0*P/tan(alpha) + alpha^2*t0^2/tan^2(alpha), since the P^2 coefficient is positive. Also, Is J2' the same as J2 in that text?
Also, Can you kindly guide what property to use for modeling soil in RADIOSS. I am currently using P14_SOLID with Isolid=24 and ismstr = 10. Please let me know if there is any better property. Further, is it advisable to have coarse mesh or fine mesh for modeling soil?


Thanks Prakash.

Hi,
I am using RADIOSS Block 120 solver. I have defined a point mass (elem type M). How can I define moment of Inertia for the point mass. I couldn't find any options to do so in CARD EDIT. Thanks.

Hi,
I am facing a similar error. My model was working fine until I deleted the mesh for a component and remeshed it. I am now getting he following error:
ERROR ID: 78
** ERROR : UNDEFINED NODE NUMBER
DESCRIPTION:
IN NODE GROUP DEFINITION DEFINITION :
NODE ID = 2096 DOES NOT EXIST
I have deleted the empty contact surfaces and sets. I am unsure what I can do now to resolve the error.

Hi Prakash,
Thanks for your suggestion. I checked the material model. The problem was with the definition of stressstrain curve. I had previously defined Stress vs. total strain (elastic + plastic) but now I have changed it to Stress vs. Plastic strain and the model is now working fine. Thanks again for your help... You are awesome man. Keep rocking .
Rahul R likes this 
Hi Prakash,
I have referred to the RADIOSS FAQ and have followed the recommendations: I have used ISMSTR=1 (Small strain from t=0) to avoid excessive compression. However, the zero volume error in my model is not due to excessive compression as I have not defined any compressive load. According to the defined load, the foam should attain the same velocity as leg guide and move along with the guide with very little compression. I think a bunch of elements are failing as soon as the guide comes in contact with the foam without any compression whatsoever. So, I believe there is some instability is the foam itself which is coming into play upon contact. Please let me know in case you need any more details.

Hi,
I have tried using IStiff = 4 as suggested but still I am getting the same error. I believe the problem is with the formulation of foam elements itself and not with the contact definition as the foam elements are distorting without any contact penetration. Can you please check the foam formulation I used and suggest any correct approach?

Hi,
I am trying to model a problem involving crushable foam modeled using LAW33 and coarse brick elements with property as ISOLID=14, ISMSTR=1, ICPRE=2. I have defined a relative velocity between foam and a metal part, so that the metal part comes in contact with the foam. As soon as the metal part comes in contact with the foam, the foam elements encounter negative internal energy leading to "zero or negative volume" error. Please help me resolve the instability. I have attached the model for reference.

I think this error results if you are using mass scaling in your model (DT/NODA/CST option) in your model and there is any factor causing the time step to decrease such as element distortion/penetration . Because the time step tends to decrease to very low values, too much mass is added to maintain the minimum time step,which results in this error. I suggest you to check for any penetrations through the interface or any element distortion happening in the model.
Rahul Ponginan likes this 
Hi,
I want to plot the stressstrain/ forcedisplacement curve along a section cut for uniaxial compression analysis using Hyperview/Hypergraph. I am using Hypermesh/RADIOSS to perform the analysis. Please let me know in detail how it needs to be done.
Thanks and Regards,
Sai Phanindra

Hi Prakash,
I have used the TYPE 7 interface for contact between an Aluminum shell and foam. Since the foam is a soft material, I have defined two interfaces  one with foam=master, Al=slave and another with foam=slave and Al=master. The parameters for the foam are as given below:
For foam=master, Al = Slave:
Stfac = 382
Igap = 0
gapmin=1.0
For foam = slave, Al = master:
Stfac: Default value
Igap=0
gapmin = 1.0
I am getting the ZERO OR NEGATIVE SUBVOLUME : DELETE 3DELEMENT error when I used LAW 36 property for the foam. For compression, should I define the plastic strain as negative in the stressstrain input curve for LAW36? Also, I am using a crushable foam with zero poisson's ratio, therefore true stress = engineering stress. I have defined the engineering stressplastic strain as the input curve. Is it correct?
Please provide your views on the same.
Thanks and Regards,
Sai

Hi Prakash,
Thanks for your suggestion. The density of foam I am using is 239 Kg/m3, which is high compared to Polyurethane foams (Density: ~30 Kg/m3) but is much less compared to metals. Are you suggesting that LAW33 is useful only for very low densities like PU materials?
Also, I am not sure if I can use LAW36 to model the foam due to the following differences:
1. The stressstrain behavior in compression (Please find attached) for foam is very different from the mentioned behavior for LAW36.
2. No strain rate effect exhibited by the foam.
I would be grateful if you can suggest the appropriate material model for Al foam, the properties of which are given below:
 Isotropic, open cell foam
 Poisson's ratio = 0
 No strain rate effect
 Density of core material (Al 6061) = 2700 Kg/m3
 Relative density = 8.8%. Therefore, Density of foam = 239 Kg/m3

Hi,
I am trying to model Aluminium crushable foam (porosity 8%) in impact analysis problem using RADIOSS block120. I am using LAW 33 viscoelastoplastic material model and P14_Solid property brick elements (ISOLID =12, ISMSTR = 2, ICPR=2).
I am however getting severe element distortion of foam elements, leading to reduced time step and ultimately resulting in "Zero or negative volume error". How can I resolve this. Am I using the correct material model and property? Please help me. This is very very critical for my analysis.
Also, I need help in the interface definition between the soft foam material and rigid aluminum solid part touching it. What parameters to use for defining such a contact?
Thanks.
Introducing Inertia with Rigid Groups
in Altair RADIOSS
Posted · Report reply
Hi,
I tried giving the additional mass and inertia to the global rigid body. However, during the RIGID BODY INITIALIZATION of the global rigid body (in _0000.out), the added mass and inertia are ignored and only the slave node mass is considered. Is there any workaround where I can add mass and inertia externally?
Also, I am facing another problem with global rigid body:
The model is initially off the ground, so the global rigid body is activated. Once it reaches the ground after free fall phase, the global rigid body is turned off and the computation is carried out as expected (with a lower time step and all elements active) for 30 ms. Beyond that, the error (which is 5% at this point) starts increasing and reaches +99% at which point the dynamics become nonphysical. i.e. the kinetic energy of the system increases to 10,000 J even though the initial total energy of the system is 460 J. Is there any reason for this sort of instability?