Beginner needs help.

[trtkbk]

Good day.

I'm a total beginner with elmer and I'm struggling to get the results I should get.

I have a cylinder which I compress by means of an axial force of 10kN.

I've calculated the stress generated by the force as well as the resulting displacement.

My problem is that if I apply the load as a boundary condition set as force the results I get do not correspond to the calculated values, however, when setting the boundary condition to the calculated displacement, elmer provides the correct stress.

Could anyone please have a look at my sif file and let me know what I'm doing wrong?

Thanks in advance.

Header
CHECK KEYWORDS Warn
Mesh DB "." "."
Include Path ""
Results Directory ""
End

Simulation
Max Output Level = 5
Coordinate System = Cartesian
Coordinate Mapping(3) = 1 2 3
Simulation Type = Steady state
Steady State Max Iterations = 1
Output Intervals = 1
Timestepping Method = BDF
BDF Order = 1
Solver Input File = case.sif
Post File = case.ep
Coordinate Scaling = 0.001
End

Constants
Gravity(4) = 0 -1 0 9.82
Stefan Boltzmann = 5.67e-08
Permittivity of Vacuum = 8.8542e-12
Boltzmann Constant = 1.3807e-23
Unit Charge = 1.602e-19
End

Body 1
Target Bodies(1) = 1
Name = "Body 1"
Equation = 1
Material = 1
Initial condition = 1
End

Solver 1
Equation = Linear elasticity
Procedure = "StressSolve" "StressSolver"
Variable = -dofs 3 Displacement
Exec Solver = Always
Stabilize = True
Bubbles = False
Lumped Mass Matrix = False
Optimize Bandwidth = True
Steady State Convergence Tolerance = 1.0e-5
Nonlinear System Convergence Tolerance = 1.0e-7
Nonlinear System Max Iterations = 1
Nonlinear System Newton After Iterations = 3
Nonlinear System Newton After Tolerance = 1.0e-3
Nonlinear System Relaxation Factor = 1
Linear System Solver = Iterative
Linear System Iterative Method = BiCGStab
Linear System Max Iterations = 500
Linear System Convergence Tolerance = 1.0e-10
Linear System Preconditioning = ILU0
Linear System ILUT Tolerance = 1.0e-3
Linear System Abort Not Converged = False
Linear System Residual Output = 1
Linear System Precondition Recompute = 1
End

Equation 1
Name = "Axial Compression"
Calculate Stresses = True
Active Solvers(1) = 1
End

Material 1
Name = "Steel (carbon - generic)"
Heat expansion Coefficient = 13.8e-6
Heat Conductivity = 44.8
Sound speed = 5100.0
Heat Capacity = 1265.0
Mesh Poisson ratio = 0.285
Density = 7850.0
Poisson ratio = 0.285
Youngs modulus = 200.0e9
End

Initial Condition 1
Name = "InitialCondition 1"
Displacement 2 = 0
Displacement Velocity 3 = 0
Displacement 1 = 0
Displacement 3 = 0
Displacement Velocity 2 = 0
Displacement Velocity 1 = 0
End

Boundary Condition 1
Target Boundaries(1) = 3
Name = "Force"
Force 3 = -10000
End

Boundary Condition 2
Target Boundaries(1) = 4
Name = "Floor (Constrained in the Z direction)"
Displacement 3 = 0
End

[mika]

Hi,

Unfortunately the nomenclature used in Elmer in this connection may not be perfect. The keyword Force is actually used to prescribe a surface force per unit area which is usually referred to as the surface traction. In your setting the total force amounts to F = s * A where s is the prescribed surface traction as defined in the sif file and A is the total area of the corresponding boundary surface.

- Mika

[trtkbk]

Hi Mika.

Thanks for the reply.

So if I understand you correctly the value, in this instance, that i have to put into the boundary condition called "Force 3", when using the GUI, should in actual fact be the pressure?

For simple geometry such as this, I can easily calculate the pressure, however what should i do when more complex geometry is used?

Is there a work around allowing me to specify the force and it automatically calculates the area, or a method that will allow me to apply a force to nodes an a specific surface?

[Takala]

Hi,

could you just calculate the area in CAD? For example if you are using "Salome", there is a tool called "Basic Properties" under "Measures" menu.
If not, for example your surface shape is changing over time due to some forces: You could make a user function that calculates the boundary function
where you can access all the details of your model, see chapter 12: http://www.nic.funet.fi/pub/sci/physics ... Manual.pdf

I'm not terribly familiar with "StressSolver" module. Maybe there is an easier way.

Cheers,

Eelis

[mika]

Hi again,

If the force per unit area is cumbersome to calculate, I believe that pointwise loads can also be specified by modifying the sif file as

Boundary Condition k
Displacement 3 Load = -10000
Target Coordinates(1,3) = 1.0 2.0 3.0
End

Here the string before 'Load' ('Displacement 3') specifies the variable component corresponding to the direction of the point load and the value of the Target Coordinates keyword defines the location of the point load.

- Mika