Define a Helius PFA Cohesive Material

Use three keyword statements to define the user-defined cohesive material.

A Helius PFA cohesive material is defined similar to a composite material using the three keyword statements *MATERIAL, *DEPVAR, and *USER MATERIAL. Consider the following lines from an Abaqus input file that completely specify a Helius PFA user-defined cohesive material.

*MATERIAL, name=cohesive
*DEPVAR
9
*USER MATERIAL, constants=11
23, 1.0E+10, 1.0E+10, 1.0E+10, 1.0E+6, 1.0E+6, 1.0E+6, 100
200, 200, 1.25

For any given Helius PFA cohesive material, the number of user material constants must be between 8 and 11. Appendix B provides a detailed description of each user material constant, including the range of allowable values for each constant and the impact that each constant has on the constitutive relations used to represent the material. Each of the user material constants typically defined in an analysis incorporating a Helius PFA cohesive material are listed below along with a brief description. For a more detailed description of any particular user material constant, refer to the appropriate section of Appendix B.

1. Damage Criteria - The first user material constant selects the damage initiation and damage evolution criteria. It is a two digit integer where the tens place holds the damage initiation criterion selection and the ones place holds the damage evolution type selection. The damage initiation flag can be 1 for maximum traction or 2 for a quadratic based criterion. The damage evolution flag can be 1 for displacement based softening, 2 for energy based, or 3 for energy based using a mixed mode power law. For example, if the first user material constant is 12, the maximum traction damage initiation criterion will be used with the energy based softening law.
2. Stiffnesses - User material constants 2-4 specify the material stiffness in the normal, first shear, and second shear directions respectively.
3. Strengths - User material constants 5-7 specify the maximum tractions the material can sustain before damage initiates in the normal, first shear, and second shear directions respectively.
4. Displacement Based Damage Evolution - The following user material constant must be defined if the displacement based damage evolution is chosen.
   • Effective Displacement at Failure - User material constant 8 is a positive number which defines the difference in effective displacement at complete failure and at damage initiation.
5. Energy Based Damage Evolution - The following user material constant must be defined if the energy based damage evolution is chosen.
   • Total Fracture Energy - User material constant 8 is a positive number which defines the total energy dissipated due to a failure. In mathematical terms, this is the area under the traction - separation curve.
6. Energy Based Damage Evolution (Mixed Mode Power Law) - The following user material constants must be defined if energy based damage evolution with a mixed mode power law is chosen.
   • Normal Mode Fracture Energy - User material constant 8 is a positive number which defines the total energy dissipated due to a pure normal mode failure.
   • First Shear Mode Fracture Energy - User material constant 9 is a positive number which defines the total energy dissipated due to a pure first shear mode failure.
   • Second Shear Mode Fracture Energy - User material constant 10 is a positive number which defines the total energy dissipated due to a pure second shear mode failure.
   • Power Law Exponent (Alpha) - User material constant 11 is a positive exponent used in the mixed mode power law function used to determine the rate of softening in the damaged cohesive material.