LIST OF ILLUSTRATIONS. Figure. 1. ... the shear strength. [I], but ASTM. C273 is recommended. The ASTM standard calls for a ..... Hexel International. "'Bonded.
-//W -_7 i
NASA Technical mPaper - 3108
i-
A Novel Method of Testing the Shear Strength of Thick Honeycomb Composites
,,_±
i
IL
i
A. J. Hodge and
......
i
A. T. Nettles
NASA Technical Paper 3108 1991
A Novel Method of Testing the Shear Strength of Thick Honeycomb Composites
A. J. Hodge and
National Aeronautics and Space Administration Office of Management Scientific and Technical Information Division
A. T. Nettles
George
C. Marshall
Marshall
Space
Space
Flight
Flight
Center,
Center Alabama
TABLE
OF CONTENTS
Page I. II.
INTRODUCTION SPECIMEN i.
DESCRIPTION
Materials
2. Specimen Ill.
TEST l.
IV.
V.
Double
TEST
.........................................................................
1 1
Fabrication
3
METHODS
Impact
1
............................................................................................
Testing
2. Four-Point 3.
.....................................................................................
.............................................................................
.........................................................
4
......................................................................................
Bend
Testing
4
..........................................................................
Lap Shear Testing
RESULTS
............................
4
........................................................................
5
.......................................................................................
7
I. Impact Testing ...................................................................................... 2. Four-Point Bend Test ..............................................................................
7 7
3. Double
7
Lap Shear Test ............................................................................
CONCLUSION
APPENDIX-Photographs REFERENCES
........................................................................................ of Testing
Techniques
8
..........................................................
9
..................................................................................................
10
iii
I_AGEjLj_
INTENTIONALLY'_AI_
PRECED;NG
PAGE
BLANK
NOT
FILMED
LIST
OF ILLUSTRATIONS
Title
Figure 1.
Lap shear
2.
Fabrication
3.
Dimensions
4.
Method
5.
DLS test fixture
6.
Shear
Page
test ......................................................................................... of lap shear of four-point
of bonding
stress
specimen
.................................................................
bend tests ..............................................................
specimens
for DLS tests
....................................................
......................................................................................
versus
impact
energy
.................................................................
2 3 4 5 6 7
iv h
TECHNICAL
A NOVEL
PAPER
METHOD
OF TESTING
OF THICK
HONEYCOMB
THE
SHEAR
STRENGTH
COMPOSITES
I. INTRODUCTION
Determination
of the
shear
strength
of a honeycomb
core
and carbon-fiber
facing
composite
is
normally performed utilizing the American Society for Testing Materials (ASTM) standard ASTM C273, "'Shear Test in Flat Sandwich Constructions or Sandwich Cores." ASTM C393, "Flexure Test of Flat Sandwich
Constructions,"
recommended. thickness
The
and
specimens
ASTM
a width
may
can
also
be used
standard
calls
not less than twice
be too large
to fabricate
to determine
the shear
for a specimen
length
the specimen's
thickness.
and fit into laboratory
strength
[I],
not less than Thus,
scale
testing
but
ASTM
12 times
for thick
C273
the specimen's
honeycomb
machines.
is
The
cores,
honeycomb
core evaluated in this study, which was being tested for the Advanced Launch System (ALS) program, was 3.6-cm (1.4-in) thick and required a specimen length of at least 42.7 cm (16.8 in). Materials available
limited
the specimen
Previous there
studies
can still be damage
age in the form
of dents
length [I,2]
to 30.5
have
shown
to the core.
Cross
or delaminations
cm (12 that
in).
at low-impact
sections
of these
on the face
energies low-impact
sheets.
However,
with
no visible
energy
surface
specimens
a slight
ductile
damage
show buckle
no damoccurs
in
both types of core under the point of impact. Therefore, it is possible that an impact which causes no visible damage to the composite may damage the core sufficiently to reduce the composite's shear stress. The residual
shear
strength
of impacted
honeycomb
composites
with carbon
fiber
facesheets
and
aluminum and glass-phenolic cores was to be determined by two methods: four-point bend and lap shear tests. Since ASTM C273 could not be used to specifications, lap shear testing was attempted with smaller lengths.
When
this was performed,
(fig. 1). An alternate was devised.
method
the facesheets
of lap shear
testing
II. SPECIMEN
of the composite was
needed.
Thus,
tended
to peel
the "double
away
from
lap shear"
the core
(DLS)
test
DESCRIPTION
1. Materials Only tured tured
one carbon
fiber prepreg
system,
T300/934,
was used in the tests.
by Amoco and preimpregnated with Fiberite 934 resin. by Hexel were compared: glass/phenolic and aluminum.
(2 lb/ft3),
a cell size of 4.76
mm (3/16
in), and a thickness
The T300
was manufac-
Two types of honeycomb cores manufacBoth cores have a density of 314.3 N/m 3
of 35 mm [2]. The crush
strengths
ously determined to be 1,158 kPa for the aluminum and 896 kPa for the glass/phenolic [I]. film adhesive used to bond the honeycomb cores and the facesheets was EA9684 produced
were previThe epoxy by Hysol.
Lap Shear Test Configuration Honeycomb, ASTM C273
Face sheet peels
F
Figure
I. Lap
shear
test.
for
2. Specimen
Fabrication
The T300/934 figurations. A layer bond the face sheets programmable
platen
180 °C (355
°F),
When in) in length These
were release
figure
making
was made
into eight-ply
The composite cooled
lay-ups
to be
was heated
to room
the specimens
and 7.6 cm (3.0
The specimens--made the sarne dimensions. center,
press.
and then
fully cured,
beams
specimens,
prepreg
with (0,45,
- 45,90),
quasi-isotropic
con-
of epoxy adhesive film was placed between the facings and honeycomb in order to to the core [4]. The composite was vacuum bagged and pressurized at 80 kPa from a
film was placed
The beam's
bend between
tested
bend length
tested
was in the "L" direction
following
the honeycomb
were cut into beams
ASTM
C393
12.7 cm (5.0 in) apart.
for 2 hours
at
The outer
honeycomb
29.2
cm ( I 1.5
of the honeycomb.
guidelines.
core and the faceplate
with one eight-ply facing and one (0,90)_ four-ply Two incisions were then made on opposite facings
the incisions
cured
at 2.8 °C (5 °F) per minute.
to be four-point
in) in width. four-point
at 1.7 °C (3 °F) per minute,
temperature
For
as shown
the
DLS
in figure
2.
facing--were cut into beams of each 6.35 cm (2.5 in) from the was removed
as can be seen
in
2.
Release
Film
/
Incision Side
up to face
sheet
view
\ _..,,Incision 8.9 cm
-'_"-i -
up to face sheet 12.7 cm
c -!11111k1111 One Half of Lap Shear Specimen
Figure
2.
Fabrication
of lap shear
specimen.
III. TEST
1. Impact
Testing
Samples a Dynatup system.
were impacted
8200
drop
The impacting
pneumatically
2.
Four-Point
at various
weight tup's
impactor,
in place
Bend
Testing were tested
of Flat Sandwich Constructions." and the outer contact points were in) from
energy
levels
ranging
and data were
obtained
mass was t,2t
clamped
The specimens
(2.5
METHODS
the center--halfway
by two aluminum
for residual
plates
shear
stress
using
the Dynatup
of I. 27 cm (0.50
with
holes
utilizing
of 7.62
ASTM
between
the center of contact
and outer
force
to help distribute
730 data
acquisition
in). The specimens cm (3.0
standard
contact
were
in) in diameter.
C393,
3.8cm 25.8 cm
3.
Dimensions
of four-point
bend
points.
the force
.f
__
Figure
was done with
"'Flexure
Test
The inner force contact points were 3.8 cm (1.5 in) from the center, 12.7 cm (5.0 in) from the center. The impact points are located 6.35 cm
3.8 crn (1.5 in), were placed on the area crushing would not occur (fig. 3).
Impact Damage
kg with a diameter
from 0 to 8.6 J. hnpacting
tests.
Impact
Aluminum
so that
Damage
localized
tabs, core
3.
Double
Lap
Shear
Lap shear
testing
Testing of honeycomb
ever, due to the thickness Because the shear strength ing. The during
combination
the
testing
composites
and thick
core
was thus devised
in the test fixture the strength The specimen
component
of stress
men and the weaker the actual
strength
caused
to test the residual
nique was termed the DLS test. After two specimens eight-ply facing, they were bonded with the four-ply to find
ASTM
standard
C273.
a peeling
of figure
5 and pulled
of the individual should
be under
involved.
between
the facings
and core
shear
However, using
strength
of the composite.
were impacted at the same energy levels on the facesheets together (fig. 4). The sample was then
in an Instron
1125. The strength
obtained
was divided
stress
completely,
since two halves fails,
while
were
the average
C273
actually
has a tensile
bonded
observed
strength
one DLS speci-
should
be lower
C273.
damage
Four ply face
sheets
|
V
1
bonded together Two halves
_Impact
Figure
4.
by 2
or compressive
to make
[
ply face
This tech-
sample.
half of the DLS specimen if tested
shear
Impact
Eight
How-
I).
A new technique
placed
done using
of the honeycomb, the specimens could not be made to the proper length. after impact was needed, the original thin facings were used during the test-
of the thin facings (fig.
is normally
Method
of bonding
specimens
I
damage
for DLS
tests.
sheets
than
"Double Lap Shear"TestSetup All dimensions in inches. J
4.0
0.50\
I
3.0 0.50
±I
Dia 0.50
l 1
4.75
0 0
.25
0 0
:0
0
5.0
m
0
0
t 2.5
0 - -__t_ -
-
0.75
m
--..o _._
0.50
Figure
5.
DLS
test
fixture.
0
_t
4.5
IV.
1.
Impact
of impacted
Small
dents
could
occur
until
1.90
cores
samples rather
2.
be felt but not seen
energy
impacted
at that energy
failing
through
Bend
As expected, failed
glass/phenolic
3.
Damage
all failed
at 810 kPa and steadily specimen,
Lap
Four
sustained
however,
impact
strength
decreased
energy
J--failed
Shear
undamaged
increased
J, while
was
levels,
penetration
first
visible
damage
did not
with glass/
first occurring
than the glass/phenolic.
to the extent core crushed
facing.
damage
felt in samples
with visible
less damage
of the
at
At 5.2 J, for
that a hole the diameter of the beneath. The aluminum core
less face sheet and core damage,
with the core buckling
fracture.
the specimens
damage--5.2
Double
level
brittle
to actual
Test
of 1 to 2 J of impact
greatest
energy
received
visible
as low as 0.66
cores.
impact
the aluminum
none
levels
aluminum
the same
levels
from
at energy
with
at approximately
Four-Point
range
ranged
the glass/phenolic core samples had fiber breakage tup was punctured into the facing with the brittle
than
samples
specimens
J in the samples
1.95 J. At higher example, impacting
RESULTS
Testing
Damage
phenolic
TEST
damage,
due to core
decreased
failed
in shear
failure.
strength
The
undamaged
aluminum
core
to 750 kPa at 8.6 J. The undamaged
A sharp
decrease
(fig. 6), with a drop in strength
of over
at 530
at 700 kPa.
shear
in strength 100 kPa.
was
noticed
The specimen
in the with the
kPa.
Test DLS
aluminum
the strength
core
specimens
steadily
decreased
to 590 kPa at 4.2 J. Three
undamaged
failed
at an average
as exhibited
value
of 720 kPa.
in the four-point
glass/phenolic
specimens
bend
were tested
The
and failed
at an average value of 640 kPa, and again a sharp decrease in strength was observed at an impact of about I to 2 J. The composites's strength gradually decreased to 450 kPa at 5.4 J.
900
With
tests.
energy
o ooo
o
o
800 o
700
o
[] [] •
I I
600
[]
m 13
•
500
[]
•
O •
AI 4-pt bd GI/Ph DLS
•
AI DLS
• • •
I
2
0
Figure
GI/Ph 4-pt bd
[]
•
400
n
6.
• I
"
Shear
stress
!
I
4 6 Impact Energy versus
8
10
impact
energy.
(J)
7
V.
Although
much
scatter
observed.
The honeycomb
the impact
energy
a rapid decrease phenolic
core The
stress
in the data obtained
composites
increased.
with aluminum
The composites
in strength specimens
existed
between continued
DLS tests gave
lower
in the DLS tests is probably
CONCLUSIONS
cores
tended
with glass/phenolic
i and 2 J of impact to gradually values
for the shear
in shear
stress
tests,
to gradually
cores,
energy.
decrease
strength
With
a general
decrease
on the other hand, higher
impact
trend was
in strength tended
energies,
as
to have the glass/
in strength. than the four-point
due to the fact that two samples
bend
are bonded
tests.
together
The
lower
shear
and the weaker
of
the two fails. The DLS specimens also have less strength added by shear stress in the face sheets than the four-point bend specimens. The curves for impact energy versus shear stress, however, followed the same general
trend for both tests. Thus,
the modes
of failure
were similar,
and the tests are comparable.
APPENDIX PHOTOGRAPHS
OF TESTING
Four-Point
DLS
TECHNIQUES
Bend
USED
Test
Test 9
ORIGINAL BLACK
AND
WHITE
PAGE PHOTOGRAPH
REFERENCES
I
,
,
Nettles, Phenolic
A.T., Lance, D.G., and Hodge, and Aluminum Honeycomb Core
Nettles,
A.T.,
and
Graphite/Epoxy .
,
Face
Hexel International California, 1989. Loken, Seminar,
10
Hodge,
H.Y.:
"DuPont
SAMPE,
A.J.:
"Impact
Sheets.'"
35th
"'Bonded
Honeycomb
Aramids
Midwest
Testing
International
for Advanced
Chapter,
A.J.: "An Composite
March
Examination of Impact Damage in Glass/ Panels." NASA TP 3042, August 1990. of Glass/Phenolic
SAMPE
Sandwich
Construction."
Composites." 9,
1982.
Symposium,
Presented
Honeycomb April Hexel
Panels
2-5,
1990.
TSB
124,
at the Industrial
With
Dublin,
Composites
Report Documentation
National Aeronautics and Space Administration 1.
4.
Report
No.
2.
NASA
TP-3108
Title
Subtitle
and
A Novel Thick
7.
Method
of Testing
Honeycomb
Government
the Shear
Accession
Strength
Page
No.
Recipient's
5.
Report
Composites
Performing
12.
Sponsoring
Space
Agency
National
Performing
Organization
Code
8.
Performing
Organization
Report
and
No.
M-658
Flight
Center,
11.
Contract
13.
Type
or Grant
Alabama
35812 of Report
and
Technical
and Space
No.
Center
Address
by Materials
strength
in two ways; (DLS)
Administration
and Processes
composites before
14.
Laboratory,
the use of common
yielded
similar
Words
(Suggested
Composite Epoxy
Science
Sponsoring
and Engineering
Period
Covered
Paper Agency
Code
Directorate.
to the supplier's
values
The assessment
of sandwich
beams
was developed scale
fabrication
slightly for shear
by Author(s))
lower
core were
of shear
strength
and by a novel
tested
for
was performed
"double
specimens
and testing
possible.
The two techniques
values
of the two methods
shear
strength
could
lap shear"
so smaller
be used
thus
strength.
18.
Distribution
Statement
Unclassified--Unlimited
Tolerance
Classif.
(of this
Subject
Impact report)
Unclassified 1626
laboratory
data. The DLS test gave
damage:
honeycomb
Materials
Instrumented
FORM
technique
and glass/phenolic
Resins
Damage
Security
bend testing
testing
making
Key
of aluminum
and after impact
by four-point
test. This novel
but were closer
NASA
Unit
Abstract
shear
19,
Work
DC 20546
Sandwich
17.
No.
Notes
Prepared
16.
Space
Aeronautics
Supplementary
1991
and Address
Flight
Name
Washington, 15.
Name
C. Marshall
Marshall
Date
and A.T. Nettles
Organization
George
No,
6,
10.
9.
Catalog
March
of
Author(s)
A.J. Hodge
3.
Category:
24
Testing 20.
Security
Classif.
(of this
Unclassified
page)
21.
No.
of pages
16
22.
Price
A03
OCT 86
For sale by the National Technical
Information Service, Springfield, VA 22161-2171 NASA-L(mgley,
1991
