A strain invariant failure theory (SIFT) has been developed to predict resin failure in damaged and pristine composite structures The finite element (FE) analysis in this work uses shell elements consistent with common practice in the aeronautical industry. The new SIFT is similar in nature to a characteristic length method that requires a matched finite element mesh. It samples the strains in brick elements lofted between two layers of shell elements, each representing half of the damaged laminate in the failure critical zone. Experimental tests involving three laminate materials have been carried out to validate t he modified SIFT approach for notched laminates, including single and multiple level delamination tests, and stiffened panel tests under shear or compression load. These results are summarised in a table. Square and rectangle single delamination tests are presented in more detail and indicate that failure location and load predicted by the modified SIFT approach correlates well with the experimental results

Kelly, D.

Li, R.

Mikulik, Z.

University of Queensland eSpace

5th Australasian Congress on Applied Mechanics, ACAM 2007  
10-12 December 2007, Brisbane, Australia 
A SIFT approach for analysing failure by delamination and disbonding in 
composite structures 
 
Li R.1, Kelly D.1, and Mikulik Z.2 
 
1 School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, 
NSW, 2052, Australia 
2 Cooperative Research Centre for Advanced Composite Structures Limited, 361 Milperra Road, 
Bankstown, NSW, 2200, Australia 
 
Abstract: A strain invariant failure theory (SIFT) has been developed to predict resin failure in 
damaged and pristine composite structures The finite element (FE) analysis in this work uses shell 
elements consistent with common practice in the aeronautical industry. The new SIFT is similar in 
nature to a characteristic length method that requires a matched finite element mesh. It samples the 
strains in brick elements lofted between two layers of shell elements, each representing half of the 
damaged laminate in the failure critical zone. Experimental tests involving three laminate materials 
have been carried out to validate t he modified SIFT approach for notched laminates, including single 
and multiple level delamination tests, and stiffened panel tests under shear or compression load. 
These results are summarised in a table. Square and rectangle single delamination tests are 
presented in more detail and indicate that failure location and load predicted by the modified SIFT 
approach correlates well with the experimental results. 
Keywords: strain invariant failure theory, composite structure, delamination. 
1 Introduction 
A strain invariant failure theory (SIFT) has been developed at UNSW to predict resin failure in pristine 
and delaminated composite structures [1,2], undertaken as part of the research program of the 
Cooperative Research Centre for Advanced Composite Structures Ltd. The intra-fibre resin within a 
lamina such as epoxy is a typical isotropic material, and thus its failure can be predicted by two strain 
invariant failure criteria for dilatation and distortion. These criteria were initially proposed by Ford and 
Alexander [3] for failures of isotropic materials in general. Previous studies have shown that they are 
applicable when the intra-fibre resin is modelled in a finite element analysis separately from the fibre 
as multi-layers of brick elements within a single lamina [2], or when the strain state of the intra-fibre 
resin is derived from laminate models using matrices of magnification factors extracted from 
micromechanical modelling of the resin-fiber system, initially proposed by Gosse and Christensen [4].  
The focus on development of a new failure criterion for fibre reinforced composites is due to the 
rapidly widening use of these materials in high performance structures and the recognition that the 
reliable prediction of failure requires criteria that consider separately failure of the fibres and failure of 
the resin. New generation commercial airliners such as the Airbus A380 and the Boeing 787 are using 
carbon fibre reinforced polymer composites because of their weight saving (fuel efficiency) and fatigue 
tolerance (long service life). An Airbus A380 uses approximately 25% composite by weight, including 
an all-composite centre wing-box, empennage and tailcone. Carbon fibre reinforced polymer 
composites account for about 50% of structural weight of a Boeing 787, particularly in the wing, 
empennage and fuselage. 
The increased use of carbon fibre composites in aircraft structures is also posing a challenge for 
analysing composite laminates damaged in service, such as impact damage due to bird strikes, 
especially to the lower portion of a composite fuselage, or by hard object indentation when being 
stationed at an airport. Analysing damage tolerance of carbon fibre composite has therefore become 
one of the main safety issues and a focus in aircraft design and certification processes. Consequently, 
the SIFT approach developed at UNSW and the CRC-ACS for resin failure in pristine composite 
structures has been further modified to analyse notched composite structures that contain 
delaminations or disbonds caused by artificial inclusions in the laminate. 
  
2 Modified SIFT approach 
In the strain invariant failure theory failure is predicted to occur if the first or second strain invariant 
exceeds a limiting value. In order to extract SIFT volumetric J1 and deviatoric J2 critical values in an 
equivalent manner to that employed when predicting failure in real structures, The analysis is 
implemented in MSC.NASTRAN and MSC.PATRAN. The programming interface in Patran was used 
to automatically generate failure index (FI) plots for both coupon tests and structure test models. In the 
coupon test models, the critical values are captured when the FEA models are loaded to the 
measured deformation at test failure loads, given SIFT is strain based. In structure test models, the 
SIFT J1 and J2 critical values are applied to predict failure mode, location and load. The finite element 
analysis (FEA) developed in this paper uses beam and shell elements consistent with common 
practice in the aeronautical industry. The new SIFT approach, as shown in Figure 1, is similar in 
nature to a characteristic length method that requires a matched finite element mesh. Large structure 
or sub-component models need mesh refinement in the critical regions. 
 
SIFT FEA
Magnification 
factors & 
thermal 
residual strain 
Coupon 
tests
Fibre, resin 
properties; Vf, 
Cure temperature; 
fibre preform 
Micromechanical FEA
Linear analysis 
using Nastran
Redraw fibre -
resin boundary 
according to v f
Modify 
material 
properties if 
fibre preform 
is fabric
Predefined 3D 
Nastran model for 
vf =0.5
Geometry, 
Load/BCs, 
failure type 
and load
Two sets of SIFT J 1 + 
J2 critical values for 
pristine and notched 
laminates
Create SIFT J 1 + J2 
failure index 
subcases and results
Non linear 
analysis using 
Nastran /Marc
Mesh 
refinement in 
critical area
Lofting brick 
element 
stacks at 
crack fronts
Pristine or notched 
shell element 
model
Structure 
tests
Geometry, 
Load/BCs
Prediction of  
failure mode, 
location and 
load
 
Figure 1 Flowchart of the modified SIFT approach. 
  
2.1 Shell element models for the strain invariant failure theory 
SIFT samples the strains in brick elements lofted between two layers of shell elements, each 
representing half of the damaged laminate in the failure critical zone. To reduce the impact on the 
shell element model, brick elements have only two to three rows followed by rigid links. The strain 
state of the intra-fibre resin is then determined from the lamina strain of the brick elements, including 
the thermal residual strain from the cure process and strain and magnification factors from micro-
mechanical modelling of the fibre-resin system. Mode I and Mode II failure indices are calculated from 
the SIFT critical values extracted from coupon tests of the same resin-fibre system, e.g. a dual 
cantilever beam (DCB) test for the J1 critical value and an end notched flexure (ENF) test for the J2  
critical value. 
   
Figure 2 SIFT model with deformation, Notched DCB (Nastran). 
FE models of these damaged laminate tests were created and analysed using MSC.Nastran/Marc. 
Tensile failure of the specimens away from the delamination front were post-examined by ultrasonic 
C-scan, and correlated with the FE predictions using both fracture mechanics (FM) and SIFT. Failure 
location and the failure load from test and analysis were compared and showed good correlation.  
This SIFT approach works for both pristine and damaged composite structures using the same 
meshing strategy, the same FE tool, but only with two sets of critical values. This SIFT approach could 
be not only a good alternative to a fracture mechanics approach, but also able to analyse pristine 
structures. It is a 3-D interactive failure criterion, and therefore is more sensitive to complex load cases 
than the fracture mechanics approach. 
2.2 Thermal residual strain in modified SIFT 
The SIFT procedure presented in [2] has been modified to conform with the established practice in 
industry to analyse through-thickness fai lure of stiffened panels. Thermal residual stress analysis 
applying the post-cure temperature change to the global models of structures has been removed from 
the third step of the SIFT procedure described in the previous paper due to the following reasons. 
1. General practice in the aviation industry is to model skin and stringer in stiffened panel 
structures using shell elements. Shell elements are not able to model the through-thickness 
thermal shrinkage, the main component of thermal residual strain for composite laminates. 
2. General practice in industry is to  predict failure in stiffened panel structures without thermal 
analysis of the global models. 
3. There is no widely accepted formulation in industry to calculate thermal shrinkage of 
composite structures.  
The estimated thermal residual strain as described in the previous paper is generally small (about 1%) 
compared to the strain induced by mechanical load when modelling structures using shell elements.  
2.3 Micromechanical modelling of the resin-fibre system 
Thermal residual strains induced in the resin by shrinkage relative to the fibre are included in the SIFT 
analysis. Strain magnification factors are determined by finite element analysis. Only a square array 
micromechanical model is considered in this modified SIFT approach as it produces the worst strain 
  
concentration and so the highest magnification factors compared to the other fibre arrays such as an 
hexagonal array. A Patran Command Language (PCL) function has been compiled to automatically 
execute micromechanical modelling of the resin-fibre system for the modified SIFT procedure. It 
modifies an existing micromechanical model of fibre-resin system according to fibre volume fraction 
(limits are from 0.39 to 0.62), fibre preform type (unidirectional tape or bi-directional fabric), cure 
temperature and material properties of resin and fibre. The first invariant thermal residual strain used 
in SIFT.  
In this PCL code, the SIFT transverse dilatation magnification factor is calculated by dividing the 
maximum value of the X strain component with the strain caused by a uniform unit displacement 
applied to the model (0.001 in all subcases), as indicated in the left image in Figure 3. Transverse 
shear magnification factor is calculated by dividing maximum value of the XY Engr strain component 
with uniform strain from unit shear displacement (0.001), as shown in the right image in Figure 3. The 
thermal residual strain is calculated by subtracting the free-body terms (3*alpha*DeltaT) for the resin 
from the minimum first invariant strain. 
   
Figure 3 Magnification of X direct and XY shear strain components in micromechanical model. 
2.4 FEA of structure in SIFT global model. 
The next step in the SIFT procedure is the FEA of the structure. The only requirement for a global 
structure model run for the SIFT approach is the presence of brick elements in the critical region. The 
PCL codes have been compiled to place the brick elements as required and assign them 3D 
orthotropic properties. 
3 Validation of modified SIFT approach for notched laminates 
Experimental tests involving three laminate materials have been carried out to validate the modified 
SIFT approach for notched laminates, including single and multiple level delamination tests, and 
stiffened panel tests under shear or compression load. The specimen sizes vary from about 100 to 
500 mm. Some results are summarised in Table 1.  
Table 1: Experimental results and SIFT prediction of notched laminate tests 
Experimental test SIFT predicted 
failure location 
Deviation of SIFT predicted 
failure load from test 
Square single delamination Correct 10% 
Rectangle single delamination  Correct -13% 
Circular single delamination  Correct -2.5% 
Square multiple delamination  Correct -3.5% 
L-stringer compression panel Correct -12 
L-stringer shear panel Correct -8.2% 
Two-bay box Correct -2.7% 
 
The delamination was established by embedding ETFE release film in the layup with desired 
delamination sizes, shapes and levels. Failure of the specimens from delamination front were post-
  
examined by ultrasonic C scan. , and correlated to the FEA predictions using SIFT. Failure location 
and the failure load from test and analysis were compared and showed good correlation. 
 3.1 Application to Square and Rectangular Delaminated Coupons. 
The delaminated specimens were modelled with two layers of shell elements. The plies connection at 
the delamination edge were modelled with solid elements (as shown in light blue squares in the figure 
below, and ten elements through the thickness). The prepreg properties were applied to the solid 
elements. Away from the delamination area rigid bars were used to join all six degrees of freedom of 
nodes, shown as purple circles. Within the delamination area there was no connection between the 
elements representing the remaining laminate above and below the delamination.  
As shown in the following picture, the square delamination model consisted of 8640 nodes, 7520 
elements and 936 multi-point constraints (MPCs) defined using the rigid bar elements. The 
dimensions of the 2400 solid elements were 2.5 x 2.5 x 0.22 mm3 . The rectangular delamination 
model consisted of 11146 nodes, 9618 elements and 1727 MPCs. The dimensions of the 3200 solid 
elements were 1.43 x 1.93 x 0.22 mm3. 
 
Load
Upper sub -laminate
(Loading plies)
Lower sub -laminate
(Resting plies)
MPC
Solid elements
 
Figure 4: Square delamination model. 
3.2 Testing 
The single delamination test was performed on the INSTRON 8504 machine. Tests were conducted 
under Room Temperature Dry (RTD) conditions, and provided the following information: 
· The load-displacement curve of load cell recorded in the INSTRON test machine. 
· The out-of-plane displacement of the specimen, recorded by two LVDTs attached to the loading 
fixture holding the specimen in the test. 
· Acoustic emissions (failure load and crack growth). 
Test specimens were visually inspected. They were also NDI inspected before and after the test.  
3.3 Experimental and SIFT correlation 
The FEA load-displacement curve of the square delamination mapped in the middle of the 8 test 
curves. The failure indices predicted using SIFT indicated a failure load of 0.919 kN at 80% 
displacement (2.40 mm), 10% higher than the actual test (0.837±0.071kN). SIFT predicted that the 
delamination growth happened at the delamination edge the closest to the loading rod (bolt hole), 
consistent with C-scan images of the tested specimens. 
  
The FEA load-displacement curve for the rectangular delamination also mapped in the middle of the 8 
test curves. The failure indices predicted using SIFT a failure load of 0.418 kN at 36% displacement 
(1.08 mm), 13% lower than the actual test (0.481±0.080 kN). It predicts that the delamination growth 
occurs at the delamination edge closest to the loading rod (bolt hole), consistent with the C-scan 
images of the tested specimens. 
 
 
 
Figure 5   Crack growth at the rectangular delamination, top: SIFT prediction; bottom C-scan of the 8 specimens. 
4 Conclusions  
A modified SIFT approach has been developed to predict resin failure in damaged as well as pristine 
composite structures by adopting two different sets of SIFT critical values. The finite element (FE) 
analysis developed in this paper uses beam and shell elements consistent with common practice in 
the aeronautical industry. The new SIFT is then similar in nature to a characteristic length method that 
requires a matched finite element mesh when extracting the critical values and when applying the 
analysis to predict failure.  
Experimental tests have been carried out to validate the modified SIFT approach. In this paper square 
and rectangular single delamination tests are presented in detail. They indicates that failure location 
and load predicted by the modified SIFT approach correlate well with the experimental results. 
5 Acknowledgement 
The authors would like to acknowledge helpful discussion with A. Rispler and L. Ramirez of the 
Hawker de Havilland Ltd. Thanks are also given to Hawker de Havilland Ltd. for providing us the 
information on materials and tests. 
References 
[1]  Li, R., Kelly, D. and Crosky, A. (ICCS, 2001). An evaluation of failure criteria for matrix induced failure in 
composite materials, Composite Structures, 57: 385-391. 
[2]  Li R., Kelly D and Ness R. Application of a first invariant strain criterion for matrix failure in composite 
materials. .  Journal of Composite Materials, Vol 37 No 22 pp 1977-2000 (2003) 
[3] Ford, H. and Alexander, J. Advanced Mechanics of Materials, 2nd Ed, Ellis Horwood Ltd, Chichester, UK. 
(1977). P402. 
[4] Gosse, J.H. and Christensen, S. (2001). Strain invariant failure criteria for polymers in composite materials. 
AIAA-2001-1184. 


A SIFT approach for analysing failure by delamination and disbonding in composite structures

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A SIFT approach for analysing failure by delamination and disbonding in composite structures

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