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Biaxial Tensile-Compressive Progressive Damage
Behavior of Notched and Unnotched Carbon-Epoxy
Crossply Laminates
T.E. Tay*, U. Kureemun & M. Ridha
Department of Mechanical Engineering
National University of Singapore
COMPTEST 2015
7th International Conference on Composites
Testing and Model Identification
IMDEA, 2015
Dept of Mechanical Engineering:
~ 68 Academic Staff
~ 1,400 Undergraduate Students
~ 300 Graduate Students
National University of Singapore
1. Introduction.
2. Behavior of unnotched and open-hole
crossply laminates under biaxial tensioncompression.
3. Modeling and analysis of progressive
damage and failure under biaxial loading.
4. Conclusion and Outlook.
Objectives
•
Study behaviour of progressive damage in
laminates under biaxial tension-compression.
Various mechanisms of damage progression.
•
Model and analyze progressive damage and
failure with a micromechanics-based failure
criterion (Tsai-Ha).
•
Investigate role of mesh design on accuracy
of prediction.
Experimental Setup
• A custom-designed fixture,
mounted on a uniaxial
loading machine.
• Provides in-plane biaxial
tension-compression.
• This fixture has been used in
previous biaxial tests on
glass epoxy composites [1].
• Operates under a
mechanism that allows the
horizontal arms to displace
vertically as loading
progresses, in order to avoid
in-plane bending of
specimen arms.
[1]
Sun X.S., Haris A., Tan V.B.C., Tay T.E., Narasimalu S., Della C.N.,
‘A multi-axial fatigue model for fibre reinforced composite laminates based
on Puck’s criterion’, Jour. of Composite Materials, 46 (4), 2012, 449-469,
(doi: 10.1177/0021998311418701).
Experimental Setup
•
Biaxialty ratio is dependent on the
angle the fixture’s slanted arms
make with the horizontal.
•
Hence, the biaxialty ratio deviates
from its initial value in tests
involving specimens that undergo
large displacements prior to failure.
•
For α=64.30, the fixture yields unity
biaxialty ratio
•
This configuration has been used in
all tests conducted in this study.
Illustration of fixture’s operating
mechanism
Specimen Geometry
Specimen Fabrication
Unnotched
specimen
• Laminates with a stacking sequence of [0/90]2S were autoclave cured
from RS-36/T700 unidirectional CFRP prepreg following the
manufacturer’s recommendations.
• Woven glass epoxy tabs: VARTM and cured at room temperature and
pressure.
• Tabs were bonded to the CFRP test specimens, forming a sandwich
structure, which was water-jet cut according to the geometry shown.
Failure Criteria (Tsai-Ha)
Micromechanics-based, amplification of stresses at matrix, fiber, and
fiber-matrix interface
(Christensen’s formula)
Modeling strategy
• Regions of the cruciform arms gripped by the biaxial fixture are not included in the mesh.
• Each ply is modeled using a single layer consisting of Abaqus quadrilateral continuum shell
element SC8R in the thickness direction.
1 mm
• Cohesive layers of
COH3D8 elements
are introduced
between the carbon
epoxy test
plies to model the
cohesive interaction
between plies.
1
mm
• Nodes of ply
structural elements
contacting with
corresponding
nodes of cohesive
layer elements
were tied.
1 mm
1 mm
Aligned Mesh (along fiber directions) in unnotched cruciforms.
Strains are generally uniform in the
centre of the specimen where strain
measurements are used for strength
determination
DIC (left) and FEA (right) strain fields εxx (A.I, A.II) and εyy (B.I,
B.II) in unnotched [0/90]2S laminates at 0.15 mm applied arm
displacement.
DIC (left) and FEA (right) strain fields εxx (A.I, A.II) and
εyy (B.I, B.II) in unnotched [15/-75]2S laminates at 0.15
mm applied arm displacement.
0.69 mm
0.61 mm
Failure progression
Failure progression
Off-axis cases
• The deformed shape of the
laminates illustrate shearing of
elements along the -750
direction predominantly,
•
responsible for change of fiber
orientations along these crack
lines in the 150 plies,
respectively,
•
trigger fiber micro-buckling.
Failure progression
•
Failure initiation is marked by transverse tensile matrix
failure in the 150 plies followed by shear matrix failure in the
-750 plies.
0.265 mm
0.225 mm
Failure progression
300 off-axis
Failure progression
•
Similar phenomenon occurs in
the 300 off-axis case.
Predicted biaxial strengths are generally in good agreement wit
h experimental values. unlike the on-axis case where material b
ehavior is linear prior to initial ply fiber failure, some non-linearit
y is displayed prior to failure when laminates are loaded at 150
and 300 off-axis
Aligned Mesh (along fiber directions) of open-hole cruciforms.
DIC Strain fields
The introduction of an open-hole
disturbs the uniform strain fields
significantly, creating regions of
localized high strain
concentrations around the notch.
Failure Progression
00 on-axis
•
•
•
Transverse matrix
cracking initiates in 00
plies around the hole.
Transverse
compressive matrix
failure in 900.
Fiber compressive
failure in 00 followed
by fiber tensile failure
in 900 plies.
Failure Progression
150 off-axis
•
•
•
Similar to on-axis case
Transverse tensile
matrix cracking and
transverse
compressive matrix
failure in 150 and -750
plies, respectively,
Fiber failure (tensile
and compressive) in
150 and -750 plies,
respectively.
Failure Progression
300 off-axis
•
Matrix shear failure
nucleate from hole
propagates along 300
and -600 directions.
•
Extensive matrix
damage predicted.
•
In experiments, fibers
in 300 plies fail
prematurely, due to
change in fiber
orientation post matrix
failure triggering local
fiber buckling.
Effects of open-hole on biaxial strength
•
Predicted strengths of open-hole laminates are consistent with experimentally determined
values.
• The 300 off-axis case exhibits significant non-linear behavior due to the large off-axis
loading angle.
•
Experimental and predicted strains at
failure, however, are not more than 6.33%
apart in these cases
Failure Progression
•
FE overall
prediction is
satisfactory until
initial fiber
failure, beyond
which loading is
no longer purely
biaxial.
Aligned vs Non-Aligned Mesh
Although stresses at
failure are not very
different, crack
patterns are not
similar.
Concluding Remarks
•
Cruciform specimens are a viable way to investigate both
notched and unnotched composites under biaxial loading.
•
Biaxiality increases the complexity of failure modes interplay.
•
Mesh design influences progressive damage patterns, at least
for cross-plies, material property degradation or smeared crack
models.
Publication
Biaxial tensile-compressive loading of
unnotched and open-hole carbon epoxy
crossply laminates.
U Kureemun, M Ridha, TE Tay.
Journal of Composite Materials, 2015.
Thank You