Anisotropic Material Behaviour & the influence of different Failure Criteria - Hook
- Anne Duechting-Rotsch (Unlicensed)
- Lennart Krueger
- SHK
Goal of this tutorial
Learn how different material properties & failure criteria influence the design
Usage of isotropic, transversely isotropic and orthotropic material behaviour
Usage of von Mises, FFF Thumb Rule and directional dependent Tsai Wu failure criteria
Set up different optimisations to exploit the full potential of Generative Design
Training:
Relevant data for this tutorial:
Step 1: Start MSC Apex Generative Design
The program starts and you can directly create your optimisation model
Step 2: Model generation
You can either create the geometry directly in MSC Apex Generative Design or import already existing files. You can import for example .xb, .xt, .step, and .sldprt files into the program.
Import/create the Design Space including the Non-Design Spaces in MSC Apex Generative Design as one solid. For this Hook the already prepared Design Space was imported.
Open the Optimization Tools to select the imported Geometry as the Design Space
Material Assignment
Create the material in the Materials editor and assign it to the Design Space
Assuming the part should be printed with a Manufacturing Method like FFF, a Transversely Isotropic material is applied.
The specific values needed are the Young's Modulus in build direction (1700 MPa) and in in-plane direction (1900 MPa), the Shear Modulus (730 MPa) and the Poisson ratio (0.3) for xy and (0.39) for yz. The density is set to 0.9e-6 kg/mm3.
As the last input the material limits should be entered. Which of these are required depends on the optimisation intention and the chosen Failure Criterion. In this case we want to take advantage of the directional dependent material limits as well as different limits for tension and compression (directional dependent Tsai-Wu).
These values are optional, if a different Failure Criterion is selected, less material input is required
| Axis | In-Plane |
|---|---|---|
Tension Strength | 48 MPa | 62 MPa |
Compression Strength | 96 MPa | 124 MPa |
Shear Strength | 50 MPa | |
After assigning the material to the solid, a material coordinate system has to be placed which indicates the build direction in x-direction.
The top surface can be selected so that the material coordinate system is aligned with the global coordinate system
Step 3: Definition of boundary conditions
Go to the Loads & Boundary Condition Tool to enter the loads and fixations. Displacements, Forces, Moments, Gravity and Pressure Loads can be applied using different selection options.
One direct load is created (Force - Moment 1) on the shown surface with the given value in the table.
Name | Force/Moment/Pressure/Gravity | Direction (depending on local coordinate system) | Value in N/Nmm |
|---|---|---|---|
Force - Moment 1 | Force on faces | y | -500 |
One Constraint on the mounting holes inner surface is created:
Name | Direction |
|---|---|
Constraint 1 | x, y, z (=0) |
Therefore, the Loads & Boundary Condition Tool is needed.
Under Displacement Constraints a “clamped” constraint can be chosen, which locks translations in all three directions. On the left side of the Tool the relevant geometry choice can be selected. In this case the inner surface of the hole is selected to attach the constraint.
Step 4: Interface Creation
Interfaces have to be created for every functional surface - so every surface where a boundary condition is applied to. With this Tool an offset to the inside with the input “Non-Design Space Thickness” and an offset to the outside with the input “Machining Allowance” is created. The Offset Distance is expanding the Interface to the set value to create material on front faces.
All surfaces on which a boundary condition is applied can be selected directly as an interface. Therefore, the “Select Faces from Loads and Boundary Conditions” button can be used. The Boundary Condition surfaces will be highlighted. With “Apply” the Non-Design Space Thickness and the Machining Allowance values will be applied to these surfaces.
In this case an Non-Design Space Thickness of 3 mm and a Machining Allowance of 1 mm is entered. Because not only the inner faces touching the screw but also the front and back face are supposed to contain material and have sharp, functional faces, an Offset Distance of 1 mm is entered.
Note: Sometimes the Interface Offset (usually displayed in red) is not visualized due to a limitation. The correct value will be considered in the optimisation.
Step 5: Definition of load cases
The next steps are defined in the Studies area.
All boundary conditions must be assigned to the specific load cases, which are defined as Events. The number of Events can be changed by adding/deleting Events to the GD Scenario. The assignment of the boundary conditions to the Events can be made in the Loads & Constrains Window. The already created loads and constraints that concern the Design Space are listed in this window and can be activated for each Event individually.
Event1: Force-Moment 1, Constraint 1
Step 5: Definition of optimisation parameters and Generative Design Settings
The optimisation parameters are selected in the Studies Area as well.
Select the Manufacturing Method: FFF
Selected the Failure Criterion: Directional Dependency (Tsai Hill)
Leave the Build Direction to the X Axis*
Enter the Safety Factor: 4
Select the Strut Density: Medium
Select the Shape Quality: Balanced
Set the Complexity Setting: 10
*Hint: The build direction can be modified in the studies tab. This is the same coordinate system as for the material, thus the material properties will change/rotate accordingly.
Tip: Take a look at the Safety Factor calculation:
The material limits are scaled with the Safety Factor! The values in brackets are the goal of the optimisation.
You can check the status of the optimisation in the GD Status and get more information on Warning and Error messages. This can be done directly in the Post-Processing as well as in the Studies tab for an optimisation that has already been executed.
Step 6: Starting the optimisation and visualize the results
If all data is correct, the optimisation can be started and tracked in the Post Processing. The Analysis Readiness function checks if all information is provided and the optimisation can start.
All result iterations are displayed as soon as they are available. Furthermore, you are able to stop the optimisation in this selection area. However, a Restart is not directly possible.
The optimisation is finished after 64 iterations (Shape Quality: Balanced).
Step 7: Visualization of Failure Criteria, Displacements etc.
Inside the Post Processing the Failure Criteria, the displacements, the optimisation achievement index and the volume/mass are visible for all iterations
The Scale can be influenced individually
Mass: 45.6 g – This is a weight saving of 15 % compared to the default calculation with isotropic material and von Mises Stress as a Failure Criterion by taking advantage of the different directional dependent material limits!
Influence of Material Properties & Failure Criteria
The chosen material properties and Failure Criteria influence the resulting design. The Failure Criterion with the chosen material limits for all directions uniformly (von Mises), directional dependent (FFF-Thumb Rule or Tsai-Hill) or taking into account different tension and compression strength besides directional dependency (Tsai-Wu) has the biggest impact on the Design.
Transversely Isotropic Material Behaviour - Failure Criterion: Directional Dependency (Tsai-Wu)
In this case the Failure Criterion is set to directional dependent (Tsai-Wu) instead of Tsai-Hill. With the Tsai-Wu failure criterion the compression strength besides the tensile strength can be taken into account.
Mass: 42.2 g – This is a weight saving of 21 % compared to the default calculation with isotropic material and von Mises Stress as a Failure Criterion by taking advantage of the different tension and compression strength besides the directional dependent material limits!
Isotropic Material Behaviour - Failure Criterion: FFF Thumb Rule
The stiffness of the material is in all directions uniformly. As the Failure Criterion the FFF Thumb Rule is calculated. With the FFF Thumb Rule the maximum allowable Stress (Stress Goal) in build direction can be scaled. This is especially useful for a Manufacturing Method like FFF. The result is shown in the following picture with the same optimisation set up except the material properties and Failure Criterion.
Mass: 47.5 g – This is a 12 % weight saving compared with the von Mises Stress Failure Criterion by taking advantage of the higher allowable stress for the in-plane directions!
Isotropic Material Behaviour - Failure Criterion: von Mises Stress
The stiffness of the material is in all directions uniformly. As the Failure Criterion the von Mises Stress is calculated. The result is shown in the following picture with the same optimisation set up except the material properties and Failure Criterion.
Mass: 53.7 g – This is a weight increase of 21 % compared with the transversely isotropic material by taking advantage of higher allowable stresses for the in-plane directions and compression (Directional dependent Tsai-Wu Failure Criterion)!
You can go back to the model setup by clicking the Exit button in the right bottom corner.
The whole MSC Apex Generative Design project with all results can be downloaded here:
In case of anisotropic material stiffness and the directional dependent Tsai-Wu transversely isotropic Failure Criterion with huge material property differences for the in-plane directions and the build direction, as well for the maximum allowable tensile and compression strength, the optimisation may not give stable results.
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