Pipe elements For pipe elements the stretch tensor and the deformation gradient tensor are not available. The axial strain, strainInc(*,1), and the shear strain, strainInc(*,4), associated with twist are provided along with the hoop stress, stressNew(*,2). The hoop stress is predefined based on your pipe internal and external pressure load definitions (PE, PI, HPE, HPI, PENU, and PINU), and it should not be modified here. The thickness stress, stressNew(*,3), is assumed to be zero and any value you assign is ignored. You must define the axial stress, stressNew(*,1), and the shear stress, stressNew(*,4). You must also define hoop strain, strainInc(*,2), and the pipe thickness strain, strainInc(*,3).
Component 1 2 3 4 5 6 7 8 9
2-D Case 3-D Case
Initial calculations and checks
In the data check phase of the analysis Abaqus/Explicit calls user subroutine VUMAT with a set of fictitious strains and a totalTime and stepTime both equal to 0.0. This is done as a check on your constitutive relation and to calculate the equivalent initial material properties, based upon which the initial elastic wave speeds are computed.
1.2.17–1
VUMAT
Component 1 2 3 4 5 6
2-D Case 3-D Case
The shear strain components in user subroutine VUMAT are stored as tensor components and not as engineering components; this is different from user subroutine UMAT in Abaqus/Standard, which uses engineering components.
Beam elements For beam elements the stretch tensor and the deformation gradient tensor are not available. For beams in space you must define the thickness strains, strainInc(*,2) and strainInc(*,3). strainInc(*,4) is the shear strain associated with twist. Thickness stresses, stressNew(*,2) and stressNew(*,3), are assumed to be zero, and any values you assign are ignored.
1.2.17
VUMAT: User subroutine to define material behavior.
VUMAT
Product: Abaqus/Explicit
WARNING: The use of this user subroutine generally requires considerable expertise. You are cautioned that the implementation of any realistic constitutive model requires extensive development and testing. Initial testing on a single-element model with prescribed traction loading is strongly recommended.
Shell elements When VUMAT is used to define the material response of shell elements, Abaqus/Explicit cannot calculate a default value for the transverse shear stiffness of the element. Hence, you must define the element’s transverse shear stiffness. See “Shell section behavior,” Section 26.6.4 of the Abaqus Analysis User’s Manual, for guidelines on choosing this stiffness.
The component ordering of the symmetric and nonsymmetric tensors for the three-dimensional case using C3D8R elements is different from the ordering specified in “Three-dimensional solid element library,” Section 25.1.4 of the Abaqus Analysis User’s Manual, and the ordering used in Abaqus/Standard.
Nonsymmetric tensors
For nonsymmetric tensors there are ndir+2*nshr components, and the component order is given as a natural permutation of the indicesቤተ መጻሕፍቲ ባይዱof the tensor. The direct components are first and then the indirect components, beginning with the 12-component. For example, the deformation gradient is passed as
The component ordering depends upon whether the tensor is symmetric or nonsymmetric.
Symmetric tensors For symmetric tensors such as the stress and strain tensors, there are ndir+nshr components, and the component order is given as a natural permutation of the indices of the tensor. The direct components are first and then the indirect components, beginning with the 12-component. For example, a stress tensor contains ndir direct stress components and nshr shear stress components, which are passed in as
• is used to define the mechanical constitutive behavior of a material; • will be called for blocks of material calculation points for which the material is defined in a user
Special considerations for various element types
The use of user subroutine VUMAT requires special consideration for various element types.
Shell and plane stress elements You must define the stresses and internal state variables. In the case of shell or plane stress elements, NDIR=3 and NSHR=1; you must define strainInc(*,3), the thickness strain increment. The internal energies can be defined if desired. If they are not defined, the energy balance provided by Abaqus/Explicit will not be meaningful.