The fracture surface characteristics of off-axis high-modulus graphite-fiber/epoxy composite specimens were studied, using a scanning electron microscope (SEM). The specimens were subjected to tensile loading at various angles (0 deg - 90 deg) to the fiber direction. SEM photomicrographs of the fractured surfaces revealed three different load angle regions with distinct fracture characteristics. Based on these revelations, criteria were established which can be used to characterize fracture surfaces with respect to a predominant single stress fracture mode

Chamis, C. C.

Sinclair, J. H.

English

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	 FRACTURE SURFACE N78-13137
CHARACTERISTICS OF OFF-AXIS COMPOSITES
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FRACTURE SURFACE CHARACTERISTICS OF OFF-AXIS COMPOSITES
by J. H. Sinclair and C. C, Chamis
( Lewis Research Center
Cleveland, Ohio 44135 1-
F
° TECHNICAL PAPER to be presented at the
Fourteenth Annual Meeting cfthe Society of Engineering Science, Inc.
Beth,	 em, Pennsylvania, November 14-16, 1977
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FRACTURE SURFACE CHARACTERISTICS OF OFF-AXIS COMPOSITES
J. H. Sinclair and C. C. Chamis
National Aeronautics and Opace Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
C11
	
The fracture surface characteristics of off-axis high-modulus
o	
graphite-fiber/epoxy composite specimens were studied using a scan-
o^	 ning electron microscope (SEM). The specimens were subjected to
w	 tensile loading at various angles (0° - 90°) to the fiber direction.
SEM photomicrographs of the fractured surfaces revealed thrse differ-
ent load angle regions with distinct fracture characteristics. Based
on these revelations, criteria were established which can be used to
characterize fracture surfaces with respect to a predominant "single-
stress" fracture mode.
INTRODUCTION
Off-axis tensile data for unidirectional composites are of con-
siderable interest to the fiber composite community for several im-
portant reasons. Some of these are: (1) determination of the variation
of elastic properties and fracture stress (strain) as a function of
load angle (angle between fiber and load directions), (2) verCOication
of composite macromechanics theories for elastic properties ,:._d for
combined-stress fracture, and (3) generation of fundamental information
for assessing angleplied laminate mechanical behavior.
An investigation of the first two aspects dealing with boron-fiber/
epoxy composites is reported in (1). A brief review of previous work
is also covered in this reference. However, the third aspect has not
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been examined with respect to fracture modes, morphology of fracture
surfaces, and criteria for identifying these fracture modes.
The objective of this investigation was to study the surface mor-
phology of fractured specimens made from unidirectional high-modulus
graphite-fiber composites and subjected to off-axis tensile lead. The
focus of the investigation was on identifying distinct features of
fracture surfaces using the scanning electric microscope and on estab-
lishing criteria to characterize these fracture surfaces. A broader
scope of the investigation from which this was a part is described in
a forthcoming NASA IN publication now being prepared.
SPECIMEN PREPARATION, TESTING, AND EXAMINATION
The laminate consisted of eight unidirectional plies of Modmor-I
graphite fibers in a matrix of ERLA-4617 epoxy resin cured with meta-
phenylene di-amine (MPDA). Tensile specimens were cut from the lami-
nate plate at the desired load angles and dressed down to the required
0.500 inch width by diamond wheels. Specimen ends were reinforced with
adhesively bonded fiber glass tabs. The tensile specimens were then
instrumented with strain gages. Figure 1 shows a completed specimen;
a schematic of specimen geometry and strain gage arrangement is shown
in figure 2. The test specimens were placed in the grips pictured in
figure 3 and loaded to fracture using a hydraulically actuated univer-
sal testing machine. Loading was incremental to facilitate periodic
recording of strain gage data.
Fractured surfaces from each tensile specimen shown in figure 4 were
observed by scanning electron microscope, and typical photomicrographs
were made to illustrate fracture modes.
Segments of tested laminates containing the fracture surfaces of
interest were cut (while carefully preserving fracture surfaces) from
each specimen and cemented (on edge with fracture surface up) to alumi-
num mounts. In order to facilitate observation using the SEM, the
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specimens were made electrically conductive by coating them with a
gold-palladium film approximately 200 angstron units (2x10- 6
 cm) thick
which was applied by vapor deposition in a vacuum evaporator. They
were then studied and photographed with a JUL-JSM-2 scanning electron
microscope.
RESULTS
The results of tensile testing and of the SEM studies are discussed
in this section
TensI.Lv'est Results
A summary of some of the tensile properties determined during
this study is presented in table T. The specimens tested along the
fiber direction broke at 81.7 ksi and the fracture strengths of the
specimens decreased gradually with increasing load angle; the transverse
specimen (90° off-axis) broke at 4 ksi. The moduli decreased in a
similar manner with increasing load angle and lie between almost 35 mil-
lion psi for the longitudinal specimen (0° off-axis) and just over
1 million psi for the 90° off-axis specimen. Stress strain curves for
the specimens tested at the various angles are presented in figure 5.
Note that they are all linear to fracture.
S.E.M. Study Results
The fractured surfaces of the specimens were examined using a
scanning electron microscope (SEM) for the purpose of identifying re-
gions of distinct or predominant fracture modes associated with the
different load angles. SEM photomicrographs of all the specimens tested
are presented in figures 6 to 14 at three different magnifications.
The fracture surface of the 0° specimen is shown in figure 6.
As-can be observed, in figure 6(a) the fracture surface is irregular.
An area replete with pulled-out fibers that broke off at varying lengths
can be observed in the left foreground of figure 6(a). This type of
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fracture suggests poor bonding between matrix and fibers. The remain-
ing areas of figure 6(a) show irregularly shaped tiered or stepped sur-
faces that are typical of the fracture surface of specimens tested in
longit„ .dinal tension. At some regions, fibers over relatively large
areas fractured at approximately the same level, and they show little
fiber pull-out. This is the type of fracture surface shown in fig-
ures 6 (b) and (c). There was strong bonding between the fibers and
matrix in these areas. The fracture mode of the resin was cleavage
(fig. 6 (c)). Lateral fiber surfaces of these stepped zones show some
evidence of sheared resin lying between the fibers as seen in the upper
left of figure 6(c).;
The fracture surface in figure 6 (c) reveals several distinct
features. These features and some significant implications following
therefrom are as follows:
1. The fiber fracture surface is irregular but appears to be
symmetric from the perimeter towards the center. This type of fiber
fracture surface morphology will be considered to be typical of tensile
fracture. i
2. The matrix fracture surface between fibers at the same level
is smooth indicating tensile brittle fracture by cleavage.
3. The matrix fracture surface between fibers at different
levels is lacerated indicating fracture by tear which will be considered
to "fracture by intralaminar shear.”
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4. The surface of fibers that pulled-out appears to be clear of
matrix residue, in general. This indicates weak interfacial bonding
which may be construed to imply that " the interfacial bond is predomi-
nantly mechanical."
5. The transverse split in the left foreground is probably
caused by the elastic energy release after fracture "back-lash."
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5The fracture surface of the 5° off-axis tensile specimen, fig-
ure 7, reveals the following features:
1. Surface tiering (fig. 7(b)) and matrix cleavage (fig. 7(c))
both of which are indicative of tensile fracture.
2. Extensive matrix surface lacerations indicating substantial
fracture by intralaminar shear.
3. Fiber pull-out and surfaces free of matrix residue indicating
weak interfacial bonding, Recall that all of the above features were
observed in the fracture surface of the 0 0 tensile specimen.
i
The fracture of the 10 0 off-axis tensile specimen of figure 8
reveals: (1) matrix surface lacerations and (2) fiber surfaces free of
matrix residue. Both of these features indicate that the intralaminar
shear fracture mode became dominant between the 5° and 10 0 off-axis load-
ings. We will consider this angle range to fracture by intralaminar shear
stress. This observation as well as certain theoretical considerations
not covered herein led to the recommendation that: "The 10 0 off-axis
tensile specimen is suitable for intralaminar shear characterization of
unidirectional fiber composites [21."
The fracture surfaces of the 15 0 and 30° off-axis tensile spect-
mens figures 9 and 10 reveal similar features to those of the 10° ort-
axis tensile specimen. These surfaces, too, indicate that fracture was
induced mainly by intralaminar shear stress.
The fracture surface of the 45° off-axis, figure 11, tensile
specimen reveals the following features:
1. Matrix cleavage with irregular boundary incicating some
transverse tensile fracture.
2. Matrix debris resulting probably from tensile matrix fracture.
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3. Some matrix lacerations indicating some intralaminar shear
stress fracture interaction.
The above surface fracture features show that another fracture mode be-
comes active somewhere between a load angle of 30° and 45°. This mode
will be construed to be induced by the transverse tensile stress.
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The fracture surfaces of the 60° off-axis tensile specimen, fig-
ure 12, and the 75°, figure 13, reveal mainly matrix cleavage and some
fiber surface free of matrix residue; both of these features indicate
that fracture was induced by transverse tensile stress,
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The fracture surface of the 90° tensile specimen, figure 14,
reveals the following features:
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1. Matrix cleavage indicative of tensile fracture of brittle
materials
2. Fiber surfaces free of matrix residue indicative of weak in-
terfacial bond.as was already mentioned.
The above , features indicate that transverse tensile stress produces
fracture surfaces characterized by: (1) matrix cleavage and (2) fiber
surfaces clear of matrix residue.
The above discussion leads to the following criteria for study-
ing and classifying fracture and fracture surfaces in MOD I - graphite
fiber/resin matrix composites.
1. Fiber tensile fracture - The surface morphology is tiered
	
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and is characterized by the following: j
a. irregular fiber surface with some symmetry from the
perimeter inward
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b. matrix cleavage surface 	
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c. fiber pull-out with fiber surfaces clear of matrix
residue
d. some matrix lacerations on inter-fiber surfaces con-
necting two different tier levels
	
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e, fiber fracture appears to occur at load angles near 0°
and possibly up to 50.
2. Intralaminar shear stress fracture - the fracture surface
produced by this stress is mainly level or plane and is characterized,
by the following:
a. extensive matrix lacerations
b. fiber surfaces free of matrix residue
c. intralaminar shear stress appears to induce fracture at
the load angle (0) range 5° < 6 < 30° and contributes significantly to
fracture in the load angle (0) range somewhere between 15° and 30° to
45°.
3. Transverse tensile stress fracture - The fracture surface
T•.oduced by this stress is level and is characterized by the following:
a. matrix cleavage
b. fiber surfaces free of matrix residue
c. some matrix debris
d. transverse tensile stress appears to induce fracture-in
the.load.angle (0) range 45° < 0 < 90' and contributes significantly to
fracture in the load angle (B) range somewhere between 15° and 30° to
45°.
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4. Mixed mode fracture - The fracture surface produced by mixed
mode is regular and is characterized by a mixture of lacerated and
cleavaged surfaces which are produced by combinations of intralaminar
shear and transverse tensile fracture. This mode is prevalent in the
load angle range (0) between 15° and 30 0 to 450.
The four criteria stated above and the physical characteristics
associated with them have not been reported previously to the authors'
knowledge. These should provide researchers with a set of definite
guidelines for identifying and classifying fracture in graphite -fiber/
resin-matrix composites. And, since the matrix appears to dominate
fracture in the load angle (9) range 5 0 < 0 < 90 0 via either intralami-
Par shear or transverse tensile, the above criteria should be appli-
cable to all fiber/resin-matrix composites.
SUMMARY OF RESULTS AND CONCLUSIONS
The major results and conclusions of an investigation to study the
fracture surface characteristics of fibercomposites subjected to off-
axis tensile loadings by examination with a scanning electron micro-
scope are:
1. Fracture surfaces of off-axis tensile specimens exhibit distinct
morphological characteristics:
a. Irregular with fiber pull-out near load angle of 0° to the
fiber direction.
b. Extensive amount of matrix lacerations in the load angle
range 5° < B < 30°.
c. Matrix cleavage in the load angle range 45° < B < 90°.
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d. Mixed mode lacerations and cleavage between 15° to 30° and
45°.
2. Criteria were established for characterizing fracture surfaces
and identifying the associated loading angle in off-axis tensile speci-
mens;
	
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a. Longitudinal tensile stress fracture - irregular fracture
surface with fibar pull-out, 0° < 0 < V.
b. Intralaminar shear stress fracture - regular fracture sur-
face with extensive matrix lacerations, 5° < 6 < 30°.
c. Transverse tensile stress fracture - regular fracture sur-
face with extensive matrix cleavage, 45 0
 < 6 < 900.
d. Mixed mode fracture - regular surface with lacerations and
cleavage between 15° and 30° to 45 0 .
3. The results of this investigation should provide a good founda-
tion for identifying, characterizing and quantifying fracture modes
in off-axis and angleplied laminates.
REFERENCES
1. R. S. Sandu, "Nonlinear Behavior of Unidirectional and ?agle Ply
Laminates," J. Aircraft, 13, 104-111 (1976).
2. C. C. Chanis and J. H. Sinclair, "loo
 Off-Axis Tensile Test for
Intralaminar Shear Characterization of Fiber Composites," NASA
TN D-8215 (1976).
10
TABLE I. - SUMMARY OF MEASURED TENSILE DATA (MODMOR I/EPDXY SPECIMENS)
Specimen Load Specimen Fracture Fracture Fracture Modulus,
angle, thickness, load, stress, strain, psi
deg in. lb ksi Cxxp
percent
A-0 0 0.053 '2152 81,7 0.231 34..9x106
A-5 5 .056 1540 55.2 188 29.3
A-10 10 .056 11.30 49.8 286 17.8
A-15 15 .056 -198 28.7 .284 10.4
A-30 30 .057 357 12.7 .365 3.49
A-45 45 .056 210 7.5 .390 1.97
A-60 60 .056 160 5.7 .413 1.31
A-75 75 .057 128 4.5 .385 1.20
A-90 90 .052 1	 104 1	 4.0 1	 .364 1.12
allominal spec- width - 0.500 inch.
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Figure 4, - Fractured tensile specimens,
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Figure 5. - Stress-strain curves for specimens subjected
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Figure 6. - Scanning electron photomicrographs of fractured surface of MOD Apoxy composite Dlg tested parallel to the fiber direction.
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Figure 7. - . Scanning electron photomicrographs of fractured surface of MCD UEpoxy composite (01 8 tested at 5 degrees to the fiber direction,
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Aqure 8. - Scanning electron photomil-rographs of fractured surface of MOD IlEpoxy composite [0}8 tested at 10 degrees 10 the fiber direction.
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Figure 9. -Scanning eIectrun photomicrographs of fractured surface of MOD lEpoxy composite (Q g tested at 15 degrees to the fiber direction,
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Figure 14. • Scanning electron photomicrographs of fractured surface of MOD IlEpoxy composite [0]8 tested transverse to the fiber direction.
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Fracture surface characteristics of off-axis composites

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