DAMAGE TOLERANCE AND NOTCH SENSITIVITY TEST METHOD DEVELOPMENT
FOR SANDWICH COMPOSITES
Daniel O. Adams, Bradley Kuramoto, and Marcus Stanfield
Department of Mechanical Engineering
University of Utah
Salt Lake City, UT 84112
ABSTRACT
Although the development of coupon-level test methods for notch sensitivity and
damage tolerance of monolithic composites has reached a high level of maturity, less
attention has been directed towards the development of standardized test methods for
sandwich composites. For monolithic composites, these test methods are commonly
performed as part of composite materials characterization programs to provide design
allowables associated with notch sensitivity and damage tolerance. Additionally, results
from such tests have been used to validate numerical modeling approaches for
simulating damage progression in composite laminates.
The objective of this research is to develop and evaluate candidate notch
sensitivity and damage tolerance test methodologies for sandwich composites and to
propose specific methodologies and configurations for standardization. Two test
methodologies have been developed for damage tolerance testing of sandwich
composites. The first utilizes an end-loaded Compression After Impact (CAI)
configuration. The second methodology consists of a four-point flexure test
configuration in which the damaged facesheet of the composite sandwich panel may be
loaded in compression or tension. For both test methodologies, research has focused
on their limits of applicability, the required sandwich specimen dimensions, and their
recommended testing procedures. For notch sensitivity, initial research has focused on
the development of test methods for sandwich composites under in-plane loading, with
an expected progression to more complex out-of-plane loading cases.
Additionally, finite element analysis with progressive damage modeling is being
assessed for use in predicting strength reductions of sandwich composites with impact
damage and notches (open holes). Previously developed analysis methodologies for
monolithic composite laminates are being extended to sandwich composites using
ABAQUS with add-on software NDBILIN. Experimental validation is being aided through
the use of Digital Image Correlation (DIC) to provide full-field deformations and strain
fields associated with damage progression.
INTRODUCTION
Damage tolerance test method development for monolithic composite laminates
has reached a high level of maturity relative to sandwich composites. There currently
exists a standardized damage resistance test method (ASTM D7136 [1]), as well as a
damage tolerance standard (ASTM D7137 [2]) for monolithic composites. For sandwich
composites, relatively little effort has been placed towards arriving at one or more
standardized damage tolerance test methods for sandwich composites until recently.
However, the need for such a standardized test recently has been identified by the
aviation industry in general, and ASTM Committee D30 on Composites specifically. In
October 2009, a joint SAMPE/ASTM D30 Panel Session on “Damage Resistance and
Damage Tolerance of Sandwich Structures” was held in Wichita, KS.
This panel,
organized by the PI of this research project, focused on the current status and future
needs of damage resistance and damage tolerance test methods for sandwich
composites. Considerable interest was identified for standardized damage resistance
and damage tolerance test methods for sandwich composites, and work commenced
towards a damage resistance test method. In 2011, a damage resistance standard
practice for sandwich composites (ASTM D7766 [3]) was published by ASTM
Committee D30.
Later in 2011, a second joint SAMPE/ASTM D30 Panel on “Damage
Resistance of Composite Sandwich Structures” was held in Ft. Worth, TX.
At this
panel, interest in a subsequent damage tolerance test method for sandwich composites
was reaffirmed. At a subsequent ASTM D30 meeting, the University of Utah (Dr. Dan
Adams) was identified as the lead institution to lead the effort towards the development
of a standardized test method in damage tolerance.
One objective of this research is to identify and develop one or more test
methodologies for assessing the damage tolerance of sandwich composites. The
possibility of having multiple test methods or practices exists in order to accommodate
the largest number of sandwich configurations and loading conditions as well as
intended usages of sandwich composites.
Additionally, efforts have focused on
investigating possible methodologies to scale test results from small-scale sandwich
panel tests to larger sandwich structures. Following an initial assessment of candidate
test methodologies, two have been selected for further development: edgewise
compression after impact and four-point flexure after impact.
Testing has been
conducted to evaluate these methods, and finite element analysis performed using
ABAQUS/NDBILIN to investigate specimen configurations and predict the residual
strength of the specimens.
Another objective of this research is to investigate the notch sensitivity of
sandwich composites. Although the development and numerical modeling of couponlevel test methods for notch sensitivity of monolithic composite laminates has reached a
high level of maturity, little research has been performed towards the development of
such test methods and numerical modeling methodologies for sandwich composites.
Two standardized tests currently exist for notched monolithic composites under in-plane
loading: open-hole tension (ASTM D 5766 [4]) and open-hole compression (ASTM D
6484 [5]). However, currently no standardized test methods exist for assessing notch
sensitivity of sandwich composites. A survey of the literature has been conducted to
determine candidate notched sandwich composite test methods. Based on this review,
the initial focus of this research has been on an open-hole edgewise compression test.
Emphasis has been placed on both suitable specimen and hole sizing as well as the
numerical simulation of the test specimen to predict the notched strength. Digital Image
Correlation (DIC) is used during testing to provide full-field deformations and strain
fields.
LITERATURE REVIEW
Research began with a literature review to identify candidate damage tolerance
test methodologies for sandwich composites. Emphasis was focused on both papers
and reports in the open literature as well as common practices in the aerospace industry
and from commercial test labs. From the candidate test configurations identified, two
were selected for investigation and further development. The first methodology utilizes
an end-loaded compression after impact (CAI) test configuration. Second, a four-point
flexure test methodology was identified for evaluating post-impact performance where
the damaged facesheet can be loaded in compression or tension depending on
specime
en setup. Further de
evelopmentt and evalu
uation of th
he two ide
entified dam
mage
toleranc
ce test meth
hodologies is currently
y underway..
Additionally,
A
h identifie
ed several potential n
notch sensiitivity
a literaturre review has
tests tha
at could be investigate
ed for sand
dwich comp
posites. A b
brief descrip
ption follow
ws for
each ca
andidate tes
st method identified. The first ccandidate ttest is an in
n-plane loa
aded,
open-ho
ole sandwic
ch compression test. This
T
candida
ate test wo
ould incorpo
orate aspeccts of
two existing stan
ndardized tests: the edgewise
e compresssion test for sand
dwich
composites (ASTM
M C364 [6
6]) and th
he open-ho
ole compre
ession testt of mono
olithic
M D6484 [5
5]). This ca
andidate te st would fe
eature a ce
entered cirrcular
composites (ASTM
hole, eitther through
h the entire
e sandwich specimen or through
h one facessheet. Thiss test
methodo
ology, also used in a previous research
r
invvestigation by Tombliin et al. [7]] has
been selected as th
he initial foc
cus of this research
r
in vestigation
n.
Another
A
can
ndidate test is an in-p
plane loade
ed, open-hole sandwich tensile test.
While a standardiz
zed open-h
hole tension
n test existts for monolithic com
mposites (A
ASTM
D5766 [4]),
[
no sta
andardized in-plane te
ensile tests exist for ssandwich ccomposites.. To
introduc
ce sufficientt tensile loa
ad into the sandwich
s
sspecimen, rrelatively co
omplex fixtu
uring
is expec
cted to be required, as
a evidence
ed by the open-hole tension tesst configuration
used pre
eviously by Tomblin ett al. [7].
The
T
out-of-plane flexu
ural streng
gth of notcched sandw
wich comp
posites can
n be
determin
ned using a four-pointt flexure test configurration as sh
hown in Fig
gure 1. Su
uch a
test wo
ould be pe
erformed similarly
s
to the stand
dardized fo
our-point fflexure test for
monolith
hic compos
sites (ASTM
M D7249 [8
8]), but witth modificattions to acccommodatte an
open-ho
ole composiite sandwic
ch specimen
n.
Figure 1. Four-point
F
t flexure te
est for notc
ched sandw
wich comp
posites.
A three-poin
nt flexure test
t
configuration hass been used to indu
uce an in-p
plane
bending moment in
n a sandwic
ch composite specime
en by placin
ng the sand
dwich on its side
otch was used
u
in th
he compos ite sandwicch specimen, which was
[9]. An edge V-no
ed to both static and fatigue testing. Thiss three poin
nt flexure cconfiguratio
on is
subjecte
limited to
o a notch lo
ocated on the
t tension side of the
e sandwich. However, if the test w
were
modified
d to four-po
oint flexure as shown in Figure 2
2, the edge
e notch cou
uld be place
ed at
the tension or compression sid
de or a dou
uble edge n
notch could be used.
Fig
gure 2. Fo
our-point in
n-plane flex
xure for no
otched san
ndwich com
mposites.
Parmigiani
P
et
e al. at Orregon State
e Universityy [10] has d
developed an out-of-p
plane
shear (M
Mode III te
earing) test method to
o investigatte the notcch sensitivitty in mono
olithic
composites. As sho
own in Figu
ure 3, this test
t
uses a keyhole no
otch in the specimen. This
test methodology will need to
t be inves
stigated further for its possible applicability to
sandwic
ch composittes.
Figure
F
3. Out-of-plan
O
ne shear te
est for notc
ched sandw
wich comp
posites.
RESEAR
RCH FOCU
US: SANDWICH COM
MPOSITE D
DAMAGE T
TOLERANC
CE
The
T primary
y focus of th
he sandwich damage ttolerance e
effort is to e
evaluate the
e two
general test metho
odologies, edgewise
e
compressio
c
n and four--point flexu
ure, for posssible
standard
dization.
However, a secondary focus is to pro
ovide a residual stre
ength
comparison of sand
dwich pane
els with idea
alized dam
mage as well as impact damage u
using
both of the
t propose
ed test methodologies. Additiona lly, digital im
mage corre
elation and finite
elementt analyses have bee
en performed to esta
ablish prop
per specim
men sizing and
predictin
ng failure sttrengths of damaged sandwich
s
pa
anels.
osite Edgew
wise Comp
pression A
After Impac
ct
Sandwich Compo
One
O
of the most com
mmonly us
sed method
ds to asse
ess damag
ge tolerancce of
sandwic
ch compos
sites is edgewise
e
compression testing
g. Typicallly, end-loa
aded
Compression After Impact (CAI) testing on sandwich structures has been performed
using ASTM D7137 [2], the standardized method for CAI testing of monolithic composite
plates, as a reference. This test method requires that the loading ends of the specimen
are machined to be parallel within tight tolerances, and then loaded edgewise until
failure. The specimen sides are typically supported along their edges to prevent
buckling failure of the sandwich panel. This test methodology has been used previously
on Nomex honeycomb panels as well as stitched and unstitched foam panels with
success [11-14]. On undamaged or barely damaged high-strength specimens, however,
edgewise loading using this test configuration can lead to undesired failure modes such
as end crushing before failure in the gage section [12].
Tomblin et al. [11-12] and Butterfield and Adams [13-14] have used edgewise
compression to assess the damage tolerance of sandwich composites. The sandwich
panels tested in both laboratories were 216 mm (8.5 in.) wide by 267 mm (10.5 in.) in
height. Impactor diameters of 25 mm and 76 mm (1.0 in. and 3.0 in.) were used.
Whereas the Wichita State University researchers [11-12] used honeycomb core
sandwich panels, the subsequent University of Utah investigation [13-14] focused on
unstitched and stitched foam sandwich composites. In both investigations, sandwich
composites with 9.5 mm and 19 mm (0.375 in. and 0.75 in.) thick cores were tested.
Composite sandwich specimens were clamped on the upper and lower loading edges,
and knife-edge supports were used on the specimen sides. The test fixture used for the
University of Utah investigation is shown in Figure 4. In this investigation, strain gages
were placed 25 mm (1.0 in.) from the lower corners in order to align the panels prior to
testing. A hemispherical alignment stage, placed under the test fixture, was used in
conjunction with the corner strain gages and a small preload to provide proper panel
alignment.
Figure 4. Comp
pression After
A
Impac
ct (CAI) test fixture fo
or composiite sandwich
panells used at the
t Univerrsity of Uta
ah.
Based
B
on th
he review of
o the litera
ature as w
well as from
m past experiences of the
authors, several aspects
a
of sandwich CAI
C testing
g have bee
en identified that requ
uired
further investigatio
on prior to
o test stan
ndardizatio n. For ma
any core materials, it is
necessa
ary to reinfforce the core
c
at the
e specimen
n ends to allow prop
per end loa
ading
without premature end failure
es due to eiither facesh
heet broom
ming or buckling. Sincce an
y imposed on the san
ndwich pan
nel at the e
ends,
external clamp-type restraint is typically
o the entirre end of th
he sandwic
ch specime
en is not de
esirable. R
Rather, the core
potting of
material may be filled
f
or re
eplaced witth a highe
er strength potting orr filler material.
Addition
nally the choice of side
e supports must be cconsidered. Such sup
pports may take
the form
m of knife edge supp
ports or cla
amp-type ssupports, d
depending on the typ
pe of
boundarry condition
n desired; knife edge
e supports are used tto replicate
e a pinned joint
whereas
s clamped supports are
a used to
o minimize
e panel rottation at th
he edges.
The
selection
n of suppo
ort condition
ns are imp
portant whe
en considerring scaling
g the resullts of
CAI san
ndwich pan
nel testing to larger size structtures.
The
e best-suitted size off the
sandwich panel as well as the boundary conditions may both be selected to represent
the composite structure of interest. Tomblin et al. [11-12] investigated the effects of
scaling the planar dimensions of sandwich specimen both experimentally and using
finite element analysis. For sandwich configurations with thin facesheets, the residual
strength increased with panel width.
However, this increase was not observed for
sandwich configurations with thicker facesheets. Results suggest that CAI-level tests
may be useful for scaling if the width-to-damage ratio is sufficient to avoid edge effects.
This topic is currently under investigation as part of the standardized test method
development.
Testing conducted to evaluate the sandwich edgewise CAI test method used
composite
sandwich
panels
fabricated
from
1.5
mm
(0.06
in.)
thick
G11
fiberglass/epoxy and 0.38 mm (0.015 in.) thick carbon/epoxy facesheets bonded to 13
mm (0.5 in.) thick Nomex honeycomb core using FM300 film adhesive. The sandwich
specimens were machined to 216 mm (8.5 in.) in width by 267 mm (10.5 in.) in height,
the same dimensions as used in previous studies [11-14]. To prevent the loaded ends
from crushing prior to failure in the gage section, they were potted using a room
temperature cure epoxy potting compound. In order to minimize the variability in initial
test results due to variations in damage states from impact testing, the use of idealized
damage in the form of circular holes machined in one facesheet was employed rather
than actual drop-weight impacting. By testing these sandwich configurations with and
without the idealized damage, these tests provided an initial comparison of residual
strength between different damage sizes.
Following these initial idealized damage
states, actual impact damage was created in composite sandwich panels.
Testing was conducted using the same fixture as used in previous testing at the
University of Utah (Figure 4). The specimens were preloaded and subsequently aligned
using the hemispherical stage to adjust the strain readings from strain gages bonded
near the bottom corners of both facesheets. Two different idealized damage sizes, 25
mm (1 in.) and 76 mm (3 in.) through holes on the damaged facesheet were compared
to undamaged and impact damaged specimens. The impact damaged specimens were
impacted at an energy level of 3.4J (30 in-lbf) in accordance with ASTM D7766 [3]. As
shown in Figure 5, the damage state had a significant impact on the residual strength,
especially for the sandwich composites with the relatively thin carbon/epoxy facesheets.
The strength reduction of the impacted specimens with carbon/epoxy facesheets was
between that measured in specimens with a 25 mm (1 in.) and 76 mm (3 in.) hole. The
sandwich composites with the thicker G11 facesheets were not affected as significantly,
Normalized Compression Strength with less strength reduction due to all forms of damage investigated.
1.0
Glass/Epoxy
0.9
Carbon/Epoxy
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Undamaged
1" Hole
3" Hole
Impacted
Figure 5. Normalized results of edgewise compression after impact testing.
Sandwich Four-Point Flexure After Impact
The second test method selected for investigating damage tolerance of sandwich
composites is the four-point flexure after impact test. As shown in Figure 6, this test
configuration allows for a constant bending moment and zero shear stress in the central
gage section of the specimen. The four-point flexure configuration allows for the
damaged facesheet to be placed under either compression or tension loading
depending on how the specimen is placed into the fixture (damaged facesheet placed
either upward or downward). In a majority of studies performed to date, the damaged
facesheet has been loaded in compression. The University of Utah test set-up for fourpoint flexure testing of sandwich composites is shown in Figure 7.
Fig
gure 6. She
ear and be
ending mom
ment diagrram for fou
ur point fle
exure test.
Figure
e 7. Four point
p
flexurre test fixtu
ure used fo
or assessin
ng damage
e tolerance
e of
sandwich composite
es at the University o
of Utah.
Based on a review of the literature and past experiences of the authors, a
number of aspects of sandwich four-point flexure testing are under investigation in
anticipation of test standardization. The central test section, or region of the sandwich
panel between the inner supports, must be of sufficient length and width such that the
damage present in the panel is not influenced by the panel edges or the presence of the
inner loading rollers. Similarly the outer spans of the panel must be of sufficient length
to generate an adequate bending moment in the central test section prior to shear
failure in the outer spans or compression failure at the load application points. Shear
failure of the core in the outer spans can be avoided through the use of a higher density
core material, or the filling of the cells of a honeycomb core material.
Similarly,
precautions must be taken to avoid crushing failure of the facesheet and/or core at the
loading points. Thus, the sandwich panel and span lengths of the test fixture must be
properly designed to prevent undesirable failure modes from occurring under four-point
flexure loading.
An initial round of testing was conducted to evaluate four-point flexure loading for
assessing damage tolerance of sandwich composites. The sandwich specimens were
sized in accordance with the ASTM D7249 [8] sizing guidelines. The sandwich panels
were fabricated using 0.5 mm (0.02 in.) carbon/epoxy facesheets bonded to 13 mm (0.5
in.) thick Nomex honeycomb core using FM300 film adhesive. The sandwich specimens
were machined to 216 mm (8.5 in.) in width by 813 mm (32 in.) in length, the same
dimensions as used in previous studies [13-14]. Initial tests were performed using a
760 mm (30 in.) outer support span and 229 mm (9 in.) inner loading span. Due to the
relatively low shear strength of the Nomex honeycomb, a higher density honeycomb
core material was spliced in the outer section to prevent core shear failures. Testing
was performed using idealized damage in the form of 25 mm (1 in.) and 76 mm (3 in.)
circular holes machined in one facesheet.
Additionally, sandwich panels without
damage were tested to determine strength reductions due to the idealized damage.
As shown in Figure 8 the two idealized damage states had a significant effect on
the residual strength of the sandwich composites. The 25 mm (1 in.) facesheet hole
produced a 42% reduction in strength and the 76 mm (3 in.) hole produced a 60%
reduction in strength.
When discussing these test results at a recent ASTM Committee D30 on
Composites meeting, it was suggested that to simplify the specimen preparation
procedures (avoiding the splicing of higher density core in the outer sections), a greater
core thickness could be used in the test specimen, even if this thickness was greater
than for the intended application. The rationale was that the damage tolerance of a
sandwich composite under flexure is sensitive to facesheet damage, but insensitive to
the core thickness. Thus, the core thickness can be used as a design variable towards
preventing undesirable failure modes from occurring. However, the core thickness must
be sufficient to prevent damage from occurring in the opposite facesheet during the
impact event.
Normalized Flexural Strength 1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
No Hole
1" Hole
3" Hole
Figure 8. Normalized four-point flexure after impact strength of idealized damage
specimens.
RESEARCH FOCUS: SANDWICH COMPOSITE NOTCH SENSITIVITY
The initial focus of the sandwich notch sensitivity research has focused on the
measurement and prediction of the open-hole compression strength of composite
sandwich specimens. The selected specimen size was 114.3 mm (4.5 in.) in width and
152.4 mm (6.0 in.) in height. The diameter of the through-hole in the center of the
specimen was 19 mm (0.75 in.), producing the same 6:1 specimen width-to-hole
diameter ratio used with monolithic open-hole composite specimens [4-5]. The test
fixture was the same as used in ASTM D7137 [2] for compression-after-impact testing
of monolithic composite specimens Fixed end conditions were used on the top and
bottom ends of the specimen and knife edge supports were used on specimen edges.
The composite sandwich panels were fabricated using 0.5 mm (0.02 in.)
carbon/epoxy facesheets bonded to 13 mm (0.5 in.) Nomex honeycomb core using
FM300 film adhesive. The specimens were machined to their final measurements and
the core region at the specimen ends were potted using a room-temperature cure epoxy
to prevent end crushing during loading. The central through-hole was milled using a
UKAM Universal Metal Bond Core Drill.
Four strain gages were bonded 25 mm (1 in.) from the bottom and side edges for
use in aligning the fixture at a given preload. The front face was painted white and
speckled using black ink for Digital Image Correlation (DIC) measurements. The load
and crosshead displacements were also measured during the test. Initial test results
from the notched sandwich composite specimen show a 32% reduction in compression
strength compared to the unnotched specimen. Axial strains measured using DIC and
bonded strain gages were found to be in good agreement as shown in Figure 9.
5.00E‐03
4.50E‐03
4.00E‐03
Strain (in/in)
3.50E‐03
3.00E‐03
DIC Left
2.50E‐03
Left Gage
2.00E‐03
DIC Right
1.50E‐03
Right Gage
1.00E‐03
5.00E‐04
0.00E+00
0
20
40
60
80
100
120
DIC Index
Figure 9. Axial strain gage and DIC comparison from open-hole compression
testing of composite sandwich specimens.
FINITE ELEMENT MODELING
The failure analysis of notched sandwich specimens was modeled using the
commercial finite element code ABAQUS. NDBILIN, a user-defined nonlinear material
model (UMAT) developed by Materials Sciences Corporation [15, 16] was used for
stiffness degradation based progressive damage modeling. Using this UMAT, stiffness
degradation is prescribed at the lamina level within the composite facesheets and is
shaped by inputting the failure criteria (ex: max stress/strain, Hashin, phase averaged
constituent), the stiffness degradation factors, and stiffness transition factor. A graphical
representation of the material model is shown in Figure 10. NDBILIN allows for two
different material inputs for fiber stiffness degradation and matrix stiffness degradation.
When matrix damage occurs, the stiffness reduction is applied to all matrix modes (22,
33, 12, 13, and 23).
Figure
F
10. NDBILIN stiffness
s
de
egradation
n model.
In
nitial progrressive dam
mage anallysis using
g ABAQUS
S/NDBILIN focused o
on a
monolith
hic compos
site open-h
hole tension specime
en.
The m
modeled sp
pecimen was a
quasi-iso
otropic lam
minate of IM
M7/8552 carbon/epoxyy, and pred
dictions were compare
ed to
experimental resu
ults from three
t
spec
cimens tessted.
The
e predicted load ve
ersus
displace
ement resu
ults, shown in Figu
ure 11, were in go
ood agreem
ment with the
experimental resultts.
Figure 11. ABAQUS
S/NDBILIN open-hole
o
tension re
esults and model.
Following
F
th
he initial modeling
m
off the mono
olithic com
mposite ope
en-hole ten
nsion
specime
en, the AB
BAQUS/NDB
BILIN mod
deling meth
hodology w
was applied
d to open--hole
compres
ssion testin
ng of the co
omposite sa
andwich spe
ecimens. F
Full-field strains meassured
during mechanical
m
testing usin
ng DIC werre in good a
agreement with prediccted strain ffields
from AB
BAQUS/ND
DBILIN analysis as sh
hown in Fig
gure 12.
These initial results w
were
useful in
n determinin
ng minimum
m specimen
n sizing req
quirements to produce
e far-field strains
at the specimen boundaries and at the location o f the strain
n gages. R
Results sug
ggest
4.5 in.) wide and 152 mm (6.0 in
n.) tall speccimen with a 19 mm ((0.75
that the 114 mm (4
in.), center hole is suitable
s
forr investigating notch se
ensitivity un
nder in-plan
ne compresssion
loading in composite sandwich specimen
ns.
Figu
ure 12. Comparison of Digital Image Corrrelation (le
eft) and finite elemen
nt
analy
ysis results
s.
Although
A
the
e predicted
d strain field
ds in the viicinity of the open holle were in g
good
agreeme
ent with ex
xperimental results, th
he initial an
nalysis did not accura
ately predicct the
failure load.
As shown
s
in Figure 13, the finite element ssimulation predicts fu
urther
s reduction and a con
nsiderably higher
h
com
mpression ffailure load than obse
erved
stiffness
experimentally. These differrences sug
ggest that a
additional ffailure mod
des need to be
included
d into the finite elem
ment simula
ations to p roperly mo
odel the fa
ailure sequence
occurring in sandw
wich open-hole compression tesst. As a rresult, additional modeling
nding as w
well as face
esheet bucckling
capabilitties to simulate both facesheet//core debon
are currrently underr investigattion. The use
u of cohe
esive eleme
ents is und
der investigation
for face
esheet/core debonding
g, whereas
s the posssibility of in
ncorporating an insta
ability
analysis
s is being ex
xplored.
3500
3000
Nyy (lbf/in)
2500
2000
1500
Experimental
1000
ABAQUS/NDBILIN
500
0
0
0.002
0.004
Strain (in/in)
0.006
0.008
Figure 13. Open-hole compression load versus strain comparison.
SUMMARY
The two candidate test configurations under development for damage tolerance
of sandwich composites, edgewise compression after impact and four-point flexure after
impact, appear to be well suited for future ASTM standardization. The current focus of
the sandwich damage tolerance research is the evaluation of progressive damage
modeling approaches for predicting strength reductions in composite sandwich
structures. Additionally, a combined experimental/numerical methodology for scaling of
damage tolerance results from smaller test specimens to larger sandwich structures is
under investigation.
Research into the notch sensitivity of sandwich composites has focused initially
on in-plane compression loading of sandwich composites with centrally located open
holes. Progressive-damage finite element analysis has been performed using ABAQUS
with a user-defined nonlinear material model, NDBILIN. This modeling approach, used
previously for monolithic composite laminates, appears to be a promising approach for
sandwich composites. Experimental validation is being aided through the use of Digital
Image Correlation (DIC) to provide full-field deformations and strain fields associated
with damage progression.
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