1. 1 ABAQUS USER SUB-ROUTINE DEVELOPMENT A. KAMSANAHALLY, S. VAN DER VEEN, K. GADHAMSETTY
Development of Abaqus User Sub-Routine to Predict
Orthotropic Crack Initiation
ARAVIND KAMSANAHALLY SRIRAMAMURTHY (MSc AMA 2016)
SJOERD VAN DER VEEN (Responsable de Stage – Airbus Toulouse)
KIRAN KUMAR GADHAMSETTY (Responsable de Stage – Airbus India)
September 12, 2016
1 Introduction
The applications of sheet metal in the manufacturing of the
aircraft control surfaces are immense. The components that
are fabricated undergo fatigue cycles during the lifetime of
the aircraft and this proves to be a critical aspect for sheet
metal as the rolling operation changes the material's
properties from isotropic to orthotropic behaviour. This
change in the behaviour increases the significance of material
plane orientations that are imbibed in the sheet metal the
moment they are rolled. The need to simulate and analyse the
crack initiations in orthotropic sheet metal is done by
MATFEM’s Modular material models which is a concept
implemented in their MatCard for the simulations.
Figure 1: Schematic of the material orientation and
conventions of the test specimens. [3]
The provision given by ABAQUS in the utilization of User-
defined field outputs is exploited by the MatCard in
generating solution-dependent parameters that are used to
predict the crack initiation. The objective of this work is to
recreate the material model definition present with the
MatCard by creating User subroutine to redefine field
variables at a material point. The subroutine is built in
FORTRAN by independently developing the algorithm
through the usage of material failure models present in the
MatCard. The developed code is used as a part of ABAQUS
USDFLD to generate the User-defined field outputs similar
to the solution-dependent parameters of MatCard. The
eventual development, testing and validation of the
subroutine is expected to provide clarity on the functioning of
the MatCard. The goal of the subroutine development is to
implement the concept of 'Linear Tensorial Damage
Accumulation' where the orthotropic behaviour is
implemented into the isotropic material by combining with a
coefficient that will define the influence of weak material
orientation planes in the initiation and propagations of cracks.
The simulations are done on a predefined Double edged
notch bending specimen and a compatibility validation of the
subroutine with ABAQUS Explicit through a Single element
specimen. The primary validation on the single element
model resulted in the debugging of the Sub-routine and the
resulting modifications were incorporated into the code that
used on the DENB model.
2 Results
The significant field output is the Shear Fracture Risk that is
calculated through MATFEM and through the Sub-Routine.
A comparison of this parameter for the various scenarios
provides the characteristic behavior of the orthotropic
material for different loading conditions and different
material properties. However, other field outputs can also be
used to validate the functions of the Sub-routine in
comparison to the values of similar parameters being
generated by MATFEM and ABAQUS. The initial tests of
the sub-routine were done on the Single-Element model to
observe if the post-processing through LTDA was
comparable to the one shown by MATFEM. The ABAQUS
input file containing the geometry details, boundary
conditions, loading definitions and solver parameters for the
Single-Element model was used for both MATFEM and Sub-
routine.
Figure 2: Smooth variation of Single element SFR after the
debugging.

2. 2 ABAQUS USER SUB-ROUTINE DEVELOPMENT A. KAMSANAHALLY, S. VAN DER VEEN, K. GADHAMSETTY
The preliminary validation involved the development of the
Input file to create the Single-element on ABAQUS (GUI not
used) with the geometry, boundary conditions, loading
conditions and solver definitions. Various tests were
conducted on the single element to counter the erratic
behaviour of the SFR plot. The preliminary validation of
Psi_shear being more than SDV2 by a constant ratio paved
way for the secondary validation that involved testing the
Sub-routine on the DENB model by varying various
parameters in sync with MATFEM’s parameters.
Figure 3: Contour plot of LTDA SFR at Critical Time step.
3 Conclusions
The results obtained through the Sub-routine were not
directly conclusive of the perfect representation of
MATFEM’s modular material definition concept but the
LTDA being a critical component of the orthotropic
definition was a key improvement in taking the sub-routine
program closer to the MatCard’s definitions. The errors in the
validation process gave an insight into the key parameters at
the computation and the execution level needed to make the
material behavior robust. The troubleshooting of the
problems encountered during the validation process of the
single element led to the optimization of the code and the
eventual modification in the behavior of the SFR of the sub-
routine. It was seen that a small difference of a definite ratio
exists between the SFR of sub-routine and MATFEM. The
reason for the difference can be deduced based on the
observations obtained by subjecting the single element model
for other tests. Pure compressive and pure shear loading of
the single element might lead to a conclusive evidence for the
difference in the value of the SFR and it may also provide
solutions optimization of the existing sub-routine algorithm.
This mode of approach to the current situation can be the
connecting link to next phase of development of the sub-
routine. The results can be improvised by going a step ahead
of MATFEM and their material models by considering better
formulations like Modified Mohr-Coulomb model and Shear-
Modified Gurson Model that accurately describe the crack
initiation and the interaction phenomenon with lesser
parameters.
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