Products: ABAQUS/Standard ABAQUS/CAE
The Oak Ridge National Laboratory (ORNL) constitutive model:
allows for use of the rules defined in the Nuclear Standard NEF 9–5T, “Guidelines and Procedures for Design of Class 1 Elevated Temperature Nuclear System Components,” in plasticity and creep calculations;
is intended for use in modeling types 304 and 316 stainless steel at relatively high temperatures;
can be used only with the metal plasticity models (linear kinematic hardening only) and/or the strain hardening form of the metal creep law; and
is described in detail in “ORNL constitutive theory,” Section 4.3.8 of the ABAQUS Theory Manual.
The ORNL constitutive model in ABAQUS/Standard is based on the March 1981 issue of the Nuclear Standard NEF 9–5T and on the October 1986 issue, which revises the constitutive model extensively. This model adds isotropic hardening of the plastic yield surface from a virgin material state to a fully cycled state. Initially the material is assumed to harden kinematically according to a bilinear representation of the virgin stress-strain curve. If a strain reversal takes place or if the creep strain reaches 0.2%, the yield surface expands isotropically to the user-defined tenth-cycle stress-strain curve. Further hardening occurs kinematically according to a bilinear representation of the tenth-cycle stress-strain curve.
You must specify the virgin yield stress and the hardening through a plasticity model definition and the elastic part of the response through a linear elasticity model definition. You specify the tenth-cycle yield stress and hardening values separately. The yield stress at each temperature should be defined by giving its value at zero plastic strain and at one additional nonzero plastic strain point, thus giving a constant hardening rate (linear work hardening).
| Input File Usage: | Use all of the following options in the same material data block: |
*PLASTIC *ORNL *CYCLED PLASTIC |
| ABAQUS/CAE Usage: | Property module: material editor: Mechanical |
ABAQUS/Standard also allows you to invoke the optional kinematic shift (
) reset procedure that is described in Section 4.3.5 of the Nuclear Standard. If you do not specify the reset procedure explicitly, it is not used.
| Input File Usage: | *ORNL, RESET |
| ABAQUS/CAE Usage: | Property module: material editor: Suboptions |
The ORNL constitutive model assumes that creep uses the strain hardening formulation. It introduces auxiliary hardening rules when strain reversals occur. An algorithm providing details is presented in “ORNL constitutive theory,” Section 4.3.8 of the ABAQUS Theory Manual. It can be used only when the creep behavior is defined by a strain-hardening power law.
| ABAQUS/CAE Usage: | Property module: material editor: Mechanical |
The ORNL formulation can also cause the center of the yield surface to translate during creep for use in subsequent plastic increments; this behavior is defined through two optional user-defined parameters.
You can specify A, the saturation rates for kinematic shift caused by creep strain as defined by Equation (15) of Section 4.3.3–3 of the Nuclear Standard. The default value is 0.3. Set A=0.0 to use the 1986 revision of the standard.
| Input File Usage: | *ORNL, A=A |
| ABAQUS/CAE Usage: | Property module: material editor: Suboptions |
You can specify H, the rate of kinematic shift with respect to creep strain (Equation (7) of Section 4.3.2–1 of the Nuclear Standard). Set H=0.0 to use the 1986 revision of the standard. If you do not specify a value for H, it is determined according to Section 4.3.3–3 of the 1981 revision of the standard.
| Input File Usage: | *ORNL, H=H |
| ABAQUS/CAE Usage: | Property module: material editor: Suboptions |
There are cases when we need to study the behavior of a material that has already been subjected to some work hardening. For such cases, initial equivalent plastic strain values can be provided to specify the yield stress corresponding to the work hardened state. See “Inelastic behavior,” Section 18.1.1, for additional details. Initial values can also be provided for the backstress tensor, 
, to include strain-induced anisotropy. See “Initial conditions,” Section 27.2.1, for more information.
| Input File Usage: | *INITIAL CONDITIONS, TYPE=HARDENING |
| ABAQUS/CAE Usage: | Initial hardening conditions are not supported in ABAQUS/CAE. |
For more complicated cases initial conditions can be defined through user subroutine HARDINI.
| Input File Usage: | *INITIAL CONDITIONS, TYPE=HARDENING, USER |
| ABAQUS/CAE Usage: | User subroutine HARDINI is not supported in ABAQUS/CAE. |
The ORNL constitutive model can be used with any elements in ABAQUS/Standard that include mechanical behavior (elements that have displacement degrees of freedom).
In addition to the standard output identifiers available in ABAQUS/Standard (“ABAQUS/Standard output variable identifiers,” Section 4.2.1), variables associated with creep (“Rate-dependent plasticity: creep and swelling,” Section 18.2.4) and the kinematic hardening plasticity models (“Models for metals subjected to cyclic loading,” Section 18.2.2) are available for the ORNL constitutive model.
