Safety
Use the Safety subnode to set up variables which can be used to check the risk of failure according to various criteria. It can be used in combination with Linear Elastic Material and Nonlinear Elastic Material. Four different variables describing the failure risk will be defined, as described in Table 4-2.
You can add any number of Safety nodes to a single material model. The contents of this feature will not affect the analysis results as such, so you can add Safety nodes after having performed an analysis and just do an Update Solution in order to access to the new variables for result evaluation.
Table 4-2: Variables for safety factor evaluation
Variable
Description
Criterion Fulfilled
Criterion violated
Failure index, FI
For a linear criterion, this is the ratio between the computed value and the given limit.
FI<1
FI>1
Damage index, DI
A binary value, indicating whether failure is predicted or not. DI is based on the value of FI.
DI=0
DI=1
Safety factor, SF
For a linear criterion, this is 1/FI.
SF>1
SF<1
Margin of safety, MoS
SF-1
MoS>0
MoS<0
For orthotropic and anisotropic failure criteria, the directions are given by the coordinate system selection in the parent node.
The Safety node is only available with some COMSOL products (see http://www.comsol.com/products/specifications/)
Failure Model
Select a Failure Criterion. The available choices depend on the physics interface, as indicated in Table 4-3.
Table 4-3: Available failure criteria by physics interface
Criterion
Solid Mechanics
Shell, Plate
Membrane
Beam
Truss
von Mises
X
X
X
X
X
Tresca
X
X
X
X
X
Rankine
X
X
X
X
X
St. Venant
X
X
X
X
X
Mohr-Coulomb
X
-
-
-
-
Drucker-Prager
X
-
-
-
-
Jenkins
X
X
X
-
-
Waddoups
X
X
X
-
-
Azzi-Tsai-Hill
Plane stress
X
-
-
-
Norris
Plane stress
X
-
-
-
Modified Tsai-Hill
Plane stress
X
-
-
-
Tsai-Hill
X
X
X
-
-
Hoffman
X
X
-
-
-
Tsai-Wu Orthotropic
X
X
-
-
-
Tsai-Wu Anisotropic
X
X
-
-
-
User defined
X
X
X
-
-
•
When Failure Criterion is von Mises Isotropic, enter Tensile strength σts.
•
When Failure Criterion is Tresca Isotropic, enter Tensile strength σts.
•
When Failure Criterion is Rankine Isotropic, enter Tensile strength σts and Compressive strength σcs.
•
When Failure Criterion is St. Venant Isotropic, enter Ultimate tensile strain εts.and Ultimate compressive strain εcs.
•
When Failure Criterion is Mohr-Coulomb Isotropic, select Material parameters — Cohesion and angle of friction or Tensile and compressive strengths to determine the type of input data.
When Cohesion and angle of friction is used, enter Cohesion c and Angle of internal friction φ.
When Tensile and compressive strengths is used, enter Tensile strength σts and Compressive strength σcs.
In either case, you can select Include elliptic cap to limit the allowed compressive stress. When selected, enter the Elliptic cap parameters pa and pb.
•
When Failure Criterion is Drucker-Prager Isotropic, select Material parameters — Drucker-Prager parameters, Tensile and compressive strengths, or Mohr-Coulomb parameters to determine the type of input data.
When Drucker-Prager parameters is used, enter Drucker-Prager alpha coefficient α and Drucker-Prager k coefficient k.
When Tensile and compressive strengths is used, enter Tensile strength σts and Compressive strength σcs.
When Mohr-Coulomb parameters is used, enter Cohesion c and Angle of internal friction φ.
In either case, you can select Include elliptic cap to limit the allowed compressive stress. When selected, enter the Elliptic cap parameters pa and pb.
•
When Failure Criterion is Jenkins Orthotropic, enter Tensile strengths σts, Compressive strengths σcs, and Shear strengths σss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Waddoups Orthotropic, enter Ultimate tensile strains εts, Ultimate compressive strains εcs, and Ultimate shear strains γss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Azzi-Tsai-Hill Orthotropic, enter Tensile strengths σts, Compressive strengths σcs, and Shear strengths σss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Norris Orthotropic, enter Tensile strengths σts, Compressive strengths σcs, and Shear strengths σss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Modified Tsai-Hill Orthotropic, enter Tensile strengths σts, Compressive strengths σcs, and Shear strengths σss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Tsai-Hill Orthotropic, enter Tensile strengths σts, Compressive strengths σcs, and Shear strengths σss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Hoffman Orthotropic, enter Tensile strengths σts, Compressive strengths σcs, and Shear strengths σss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Tsai-Wu Orthotropic, enter Tensile strengths σts, Compressive strengths σcs, and Shear strengths σss. All entries have three components, related to the principal axes of orthotropy.
•
When Failure Criterion is Tsai-Wu Anisotropic, enter Second rank tensor, Voigt notation f, and Fourth rank tensor F. Enter the components of the tensors with respect to the directions of the coordinate system in the parent node.
•
When Failure Criterion is User defined, you enter two expressions describing the Failure criterion g(S), used in the failure index, and the Safety factor sf(S) respectively. As an example, if you would like to replicate the von Mises Isotropic criterion with tensile strength 350 MPa, you could enter g(S) as solid.mises/350[MPa]-1 and sf(S) as 350[MPa]/(solid.mises+eps).
For all input fields, the default is to take the value From material. Change to User defined to enter other values or expressions.
For a detailed description of the various criteria, see Safety Factor Evaluation in the Structural Mechanics Theory chapter.
Location in User Interface
Context Menus
 Solid Mechanics>Linear Elastic Material>Safety
 Solid Mechanics>Nonlinear Elastic Material>Safety
Membrane>Linear Elastic Material>Safety
 Membrane>Nonlinear Elastic Material>Safety
Shell>Linear Elastic Material>Safety
Plate>Linear Elastic Material>Safety
Beam>Linear Elastic Material>Safety
Truss>Linear Elastic Material>Safety
Ribbon
Physics tab with Linear Elastic Material or Nonlinear Elastic Material node selected in the model builder tree:
Attributes>External Strain