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error: Failed to Evaluate Expression
Posted Jan 31, 2012, 12:33 p.m. EST Battery Design Version 5.1 5 Replies
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Hi everyone,
I am a new COMSOL user so I apologize for any stupid questions.
I am working on soil mechanics [very simple verification footing problem for now] and I try to implement a material with stiffness-stress dependency, in the form of:
[stiffness]=a*[stress]^b.
I understand that I need some initial values for stress, so I also included them under the material model.
However, when I try to run the analysis, I get the message below as an error:
Failed to evaluate variable.
- Variable: p
- Geometry: 1
- Domain: 1
Failed to evaluate variable.
- Variable: mod1.solid.E
- Defined as: (unit_Pa_cf*mod1.mat1.def.Ecross((p/unit_Pa_cf)))
Failed to evaluate variable.
- Variable: mod1.solid.D33
- Defined as: (((1-mod1.solid.nu)*mod1.solid.E)/((mod1.solid.nu+1)*(1-(2*mod1.solid.nu))))
Failed to evaluate variable.
- Variable: mod1.solid.SlZ
- Defined as: (mod1.solid.siZ+((mod1.solid.eelZ*mod1.solid.D33)+((mod1.solid.D13*mod1.solid.eelR)+(mod1.solid.D23*mod1.solid.eelPHI))))
Failed to evaluate variable.
- Variable: mod1.solid.SZ
- Defined as: mod1.solid.SlZ
Failed to evaluate expression.
- Expression: 1.5*((mod1.solid.SR-(mod1.solid.SR+mod1.solid.SPHI+mod1.solid.SZ)/3)^2+2*mod1.solid.SRPHI^2+2*mod1.solid.SRZ^2+(mod1.solid.SPHI-(mod1.solid.SR+mod1.solid.SPHI+mod1.solid.SZ)/3)^2+2*mod1.solid.SPHIZ^2+(mod1.solid.SZ-(mod1.solid.SR+mod1.solid.SPHI+mod1.solid.SZ)/3)^2)
Does anyone happen to know what is going wrong?
Thanks everyone for your time!
Thymios
I am a new COMSOL user so I apologize for any stupid questions.
I am working on soil mechanics [very simple verification footing problem for now] and I try to implement a material with stiffness-stress dependency, in the form of:
[stiffness]=a*[stress]^b.
I understand that I need some initial values for stress, so I also included them under the material model.
However, when I try to run the analysis, I get the message below as an error:
Failed to evaluate variable.
- Variable: p
- Geometry: 1
- Domain: 1
Failed to evaluate variable.
- Variable: mod1.solid.E
- Defined as: (unit_Pa_cf*mod1.mat1.def.Ecross((p/unit_Pa_cf)))
Failed to evaluate variable.
- Variable: mod1.solid.D33
- Defined as: (((1-mod1.solid.nu)*mod1.solid.E)/((mod1.solid.nu+1)*(1-(2*mod1.solid.nu))))
Failed to evaluate variable.
- Variable: mod1.solid.SlZ
- Defined as: (mod1.solid.siZ+((mod1.solid.eelZ*mod1.solid.D33)+((mod1.solid.D13*mod1.solid.eelR)+(mod1.solid.D23*mod1.solid.eelPHI))))
Failed to evaluate variable.
- Variable: mod1.solid.SZ
- Defined as: mod1.solid.SlZ
Failed to evaluate expression.
- Expression: 1.5*((mod1.solid.SR-(mod1.solid.SR+mod1.solid.SPHI+mod1.solid.SZ)/3)^2+2*mod1.solid.SRPHI^2+2*mod1.solid.SRZ^2+(mod1.solid.SPHI-(mod1.solid.SR+mod1.solid.SPHI+mod1.solid.SZ)/3)^2+2*mod1.solid.SPHIZ^2+(mod1.solid.SZ-(mod1.solid.SR+mod1.solid.SPHI+mod1.solid.SZ)/3)^2)
Does anyone happen to know what is going wrong?
Thanks everyone for your time!
Thymios
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5 Replies Last Post Feb 2, 2012, 12:35 p.m. EST
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Posted:
1 decade ago
Hi
its rather simple,:you define your Young modulus E w.r.t. a variable "p" but "p" is not defined in your model, hence a series of variables alld epending on E are unknown for COMSOL
You ave defined an analytical equation, but you could also define units for the argument and the results, then you do not need to use the [1/Pa] units trick to make your arguments without units while calling your function
Also the body load, normally one writes "-solid.rho*g_const" along the axis direction. In this way COMSOL looks after the gravity of Earth (g_const = 9.81...m/s^2, except if it was the step on the Moon pls correct accordingly, but I do not believe COMSOL as the gravity constant of the Moon stored by default ...) Like that you do not need to change all values, if you change the material
Also for the pressure parametric load, its easier to use a "solver continuation" sweep, rather than a "external" Parametric Sweep, the former launches one load, then continues to solve with the next etc, by using last parameter value as starting point for the next analysis. While the latter "external" Parametric sweep, runs through the geometry, and the mesh to update these, then starts each new solving process by applying the initial (mostly 0,0) conditions. The differences are subtile, and in many cases makes you gain quite some processing time.
--
Good luck
Ivar
its rather simple,:you define your Young modulus E w.r.t. a variable "p" but "p" is not defined in your model, hence a series of variables alld epending on E are unknown for COMSOL
You ave defined an analytical equation, but you could also define units for the argument and the results, then you do not need to use the [1/Pa] units trick to make your arguments without units while calling your function
Also the body load, normally one writes "-solid.rho*g_const" along the axis direction. In this way COMSOL looks after the gravity of Earth (g_const = 9.81...m/s^2, except if it was the step on the Moon pls correct accordingly, but I do not believe COMSOL as the gravity constant of the Moon stored by default ...) Like that you do not need to change all values, if you change the material
Also for the pressure parametric load, its easier to use a "solver continuation" sweep, rather than a "external" Parametric Sweep, the former launches one load, then continues to solve with the next etc, by using last parameter value as starting point for the next analysis. While the latter "external" Parametric sweep, runs through the geometry, and the mesh to update these, then starts each new solving process by applying the initial (mostly 0,0) conditions. The differences are subtile, and in many cases makes you gain quite some processing time.
--
Good luck
Ivar
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Posted:
1 decade ago
Hello Ivar and thank you for your flash reply,
I tried most of the things that you mentioned. They were quite helpful and the analysis ran. More specifically I related E to mean stress, as I initially wanted to.
However, I was not sure I was getting the correct results, so I included a parameter on the model and I ran a parameter sweep to check if the model takes into consideration the stress dependent stiffness.
As you can see from the model, for all parameters (namely initial stiffness of 10-100MPa), the results are the same, which means that the model take into consideration only the constant part of the equation. I double checked that by also running a parameter sweep for that value, and the results proved the same.
Could you possibly know why is that happening?
Thank you!
Thymios
ps: I cannot upload my new model for some reason. I get a message it's too large.
I tried most of the things that you mentioned. They were quite helpful and the analysis ran. More specifically I related E to mean stress, as I initially wanted to.
However, I was not sure I was getting the correct results, so I included a parameter on the model and I ran a parameter sweep to check if the model takes into consideration the stress dependent stiffness.
As you can see from the model, for all parameters (namely initial stiffness of 10-100MPa), the results are the same, which means that the model take into consideration only the constant part of the equation. I double checked that by also running a parameter sweep for that value, and the results proved the same.
Could you possibly know why is that happening?
Thank you!
Thymios
ps: I cannot upload my new model for some reason. I get a message it's too large.
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Posted:
1 decade ago
I managed to include it, but without the solution..
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Posted:
1 decade ago
Hi
you are right the stress does not change, BUT the displacement does ;)
add a Plot More Max/Min Line to your strss plot, or add a separate surface defomation plot and you will notice it
--
Good luck
Ivar
you are right the stress does not change, BUT the displacement does ;)
add a Plot More Max/Min Line to your strss plot, or add a separate surface defomation plot and you will notice it
--
Good luck
Ivar
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Posted:
1 decade ago
Hi again!
Well, If you notice, I changes only for change in the "constant" part of the stiffness k0.
If you do the same parametric study for the stress dependency factor k1, you will notice that all the displacements coincide.
I cannot understand how this works, since I get no error during the analysis, but still, half the equation is not taken into account :(.
Thank you again for your time!
Thymios
Well, If you notice, I changes only for change in the "constant" part of the stiffness k0.
If you do the same parametric study for the stress dependency factor k1, you will notice that all the displacements coincide.
I cannot understand how this works, since I get no error during the analysis, but still, half the equation is not taken into account :(.
Thank you again for your time!
Thymios