What’s new in Abaqus 2021 non linear functionalities
The greatest value of Abaqus lies in its non-linear functionalities so it is obvious that new developments would revolve around these features. The 2021x has gone through a very exhaustive feature upgrade process. It may not be possible to cover all of it in a blog article but I have tried my best to include major enhancements.
- Element Technology enhancement:
All three dimensional continuum solid elements are now supported with composite solid section in Abaqus standard. These elements are available from 2021xFD04 release. The partial list includes C3D6, C3D15, C3D15V and its hybrid counterparts.
The section definition further supports spatially varying thickness of each ply layer using distribution. In case sum of ply thicknesses do not match the element thickness, a normalization scheme is used. The variable ply thickness has been introduced in 2020x FD02 for standard. It’s worth to mention that ply thickness variation using analytical fields has also been introduced in Abaqus explicit from 2020x FD04
CSS8 enhancement CSS8 is a continuum solid shell element. Its behavior has been modified for a user defined orientation. In prior releases, orientations are projected on the element mid-surface in initial configuration such that local material 3-direction is normal to element mid-surface. Beginning 2020x FD02, if environment variable CSS8_ORI_PROJECTION is set to OFF, orientations will not be projected on element mid surface.
Beam cross section profiles in library
The standard cross section profiles supported for beams has already been outstanding. Starting 2021xFD03, two new profiles have been added. These are called CHANNEL and HAT. They are supported in both standard and explicit and further in 2D as well as 3D beams.
Meshed composites cross section
Predefined or even user defined general cross sections for beams do not work in case of very complex geometry or multi material make up of the cross section. However the dimensionality of model may still call for the beam element approach. In such a case a meshed cross section can be used to model the beam cross section and beam section properties can be generated using *BEAM SECTION GENERATE functionality introduced in 2021xFD03. A new family of warping elements has been introduced to model meshed composite beam cross sections. These are WARPF2D3, WARPF2D4 and their quadratic counterparts. They can capture in plane and out of plane warping for composites.
- Material Enhancements:
LaRC05 damage criterion
It is a damage initiation criterion for polymer matrix fiber reinforced composites that earlier existed as user subroutine but now integrated in abaqus standard. It has four different damage mechanisms with respective output variable. Matrix Cracking: LARCMCCRT Fiber Kinging: LARCFKCRT, Fiber splitting: LARCFSCRT, Fiber Tension: LARCFTCRT. 3D Solid, plane stress, shell and membranes are supported. XFEM is supported as well.
Hosford Coulomb damage initiation criterion:
This criteria is a special case of ductile, with the equivalent plastic strain at the onset of damage assumed to be of following form:
The damage criterion output field variable is HCCRT. Available in both standard and explicit and can be used with XFEM.
Ply fabric damage initiation criterion:
Yet another failure criteria has been introduced. This one is for bi-laminar woven fabrics that follows orthotropic elasticity behavior. The criteria has been introduced in 2021xFD04 and supports damage evolution.
Low density foam model enhancement:
The foam model has been there since long but it works only for compressible response. A deviatoric component has been added to the model to include near incompressibility. Abaqus will automatically include this component if poison ratio is greater than 0.475. It has been introduced in 2021xFD04
Hyperelasticity subroutine modification:
The UHYPER_STRETCH has been introduced in 2021xFD04 to include hyperelasticity definition using principal stretches. The conventional HYPER routine is limited to strain variants I1, I2 and J.
- Fracture Mechanics:
XFEM element enhancements:
There are quite a few enhancements on XFEM side. First one is inclusion of thermal and fluid effects. Coupled temp-displacements are now available to support heat transfer due to conductance and heat generation. Small sliding contact or finite sliding general contact are
supported. A prominent application would be damage evaluation due to frictional heat in disc brakes. Coupled pore pressure-temperature-displacement elements have also been introduced in XFEM to support hydraulic fracture. Both of these element types have been introduced in 2020xFD02.
XFEM robustness and accuracy:
Two enhancements have been made in 2020xFD04 to improve the robustness of 3D non planar crack propagation. These are non-local stress averaging and smoothing algorithms.
*DAMAGE INITIATION, POSITION=NONLOCAL
Use of least square procedure to smooth out the normal of individual crack facets in elements along the crack front that satisfy damage initiation.
In the past only one crack was allowed to initiate in single enrichment zone. Additional cracks would not nucleate until all pre-existing cracks have moved through boundary of enrichment zone. This limitation has been removed. More than one cracks can now propagate per enrichment zone. User specifies a relative radius around the crack tip where elements are excluded from crack nucleation. This is small region called crack initiation exclusion zone. Multiple cracks can initiate outside this zone.
In the past, crack propagation around stress free boundaries used to show unphysical behavior at times. This has been fixed by using weighted average value of previous and current element crack direction.
*DAMAGE INITIATION, POSITION=NONLOCAL
*FRACTURE CRITERION, POSITION=NONLOCAL
The linear elastic fatigue crack growth (*FATIGUE), which is a way to estimate fatigue in abaqus now supports specification of a non-zero damage based tolerance for the least number of cycles to damage an interface element. Previously, such an analysis required specification of nonzero cycle increment based tolerance. Damage based tolerance results in smoothing of crack front in a 3D fatigue crack growth analysis with a skewed mesh.
Algorithm has also been improved to handle unstable crack growth. An additional data entry has been introduced in *CONTROL to avoid the overshoot of crack prior to the activation of unstable growth tolerance in an increment. This not only applicable to VCCT but to damage evolution as well.