Upload
k-senthil-kumar
View
218
Download
2
Embed Size (px)
Citation preview
Mechanical Properties of Injection Molded LongFiber Polypropylene Composites, Part 2:Impact and Fracture Toughness
K. Senthil Kumar, Naresh Bhatnagar, Anup K. GhoshDepartment of Mechanical Engineering, Centre for Polymer Science and Engineering,Indian Institute of Technology, New Delhi, India
This study describes the effect of fiber length andcompatibilizer content on notched izod impact andfracture toughness properties. Long fiber polypropyl-ene (LFPP) pellets of different sizes were prepared byextrusion process using a new radial impregnation die,and subsequently, pellets were injection molded asdescribed in previous publication [1]. The content ofglass fiber reinforcement was maintained same for allcompositions. Maleic-anhydride grafted polypropylene(MA-g-PP) was chosen as a compatibilizer to increasethe adhesion between glass fiber and PP matrix and itscontent was maintained at 2 wt%. Notched izod impactproperty was studied for LFPP composites preparedwith and without compatibilizer for different pelletsizes. Failure mechanism due to sudden impact wasanalyzed with scanning electron micrographs and wascorrelated with impact property of LFPP composites.Fracture and failure behavior of injection molded LFPPcomposite were studied and relationship between frac-ture toughness and microstructure of LFPP compositewas analyzed. The microstructure of the compositeswas characterized by the dimensionless reinforcingeffectiveness parameter, which accounts for the influ-ence of fiber layer structure, fiber alignment, fiber vol-ume fraction, fiber length distribution, and aspect ratio.Matrix stress condition factor and energy absorptionratio were determined for LFPP composites preparedwith and without compatibilizer. Failure mechanismof both the matrix and fiber, revealed with SEMimages, were discussed. POLYM. COMPOS., 29:525–533, 2008.ª 2008 Society of Plastics Engineers
INTRODUCTION
The Part 1 of the present work [1] dealt with the de-
velopment of long fiber pellets and long fiber composites,
optimization of compatibilizer content, and determination
of fiber length distribution in the injection molded sam-
ples. LFPP pellets have the fiber length equal to the pellet
size (the aspect ratio is generally maintained over 100).
Since these pellets are fed into the injection molding
machine for developing long fiber-reinforced composites,
these pellets can also be referred as initial feedstock fiber.
Although the final fiber length after injection molding
decides the mechanical properties of composite, correla-
tions here are done with initial feedstock fiber length.
This is to find out the optimum pellet size, to be consid-
ered for injection molding.
Long fiber-reinforced thermoplastic composites used in
structural applications are subjected to impact and/or
other loading conditions requiring high fracture toughness.
Therefore, it is important to assess their material perform-
ance well in advance. This paper is emphasized on the
analysis of impact and fracture toughness of long fiber-re-
inforced polypropylene composites prepared with and
without compatibilizer. The effect of fiber length on
notched impact properties of long fiber thermoplastic
composite materials at low temperature was investigated
by Marvin [2]. A detailed analysis of impact failure
mechanism is carried out by scanning electron microscopy
(SEM) images to identify the factors affecting the impact
strength of composite [3, 4]. Thomason et al. [5] dis-
cussed the influence of fiber length and concentration on
the impact property of injection molded long fiber poly-
propylene composites. The author also correlated the ex-
perimental property with fiber pullout model and fiber
strain energy model. Since glass fiber reinforced thermo-
plastics are principally related to stiffness and toughness,
it becomes necessary to evaluate the fracture mechanics
data, which provides material constants. Fracture tough-
ness variations of injection molded long fiber-reinforced
thermoplastics were studied [6, 7]. Since the mode of test-
ing for both the fracture toughness and tensile strength is
same, a relationship between them was attempted. A lin-
ear relation between them indicates that the material vari-
ables that determine the tensile strength also determine
the fracture toughness [8, 9]. Fiber properties and matrix
properties are the two major contributions for failure of
Correspondence to: Anup K. Ghosh; e-mail: [email protected]
DOI 10.1002/pc.20369
Published online in Wiley InterScience (www.interscience.wiley.com).
VVC 2008 Society of Plastics Engineers
POLYMER COMPOSITES—-2008
composites. To find out the failure mechanism of fiber
and matrix in detail, SEM analysis has been done and the
microstructure of composite was correlated with the rein-
forcement efficiency factor (R). The various energy
absorbing mechanism such as fiber debonding and pullout
for fibers and crazing and shear yielding on polypropylene
matrix were observed by SEM images [10–12].
The present work aims at the impact and fracture
toughness analysis of injection molded long fiber-rein-
forced polypropylene composites. Figure 1 shows the
block diagram of the work interested in this part of the
manuscript. The purpose of this study is to examine the
effect of long glass fiber reinforcements on these mechan-
ical properties and to correlate this relationship with
microstructural properties such as fiber length, fiber orien-
tation, and fiber content. The description of the results is
supported by extensive studies of the failure mechanisms
using SEM.
EXPERIMENTAL
The materials and processing of injection molded long
fiber-reinforced polypropylene composites are explained
in Part 1 [1]. Test specimens for the evaluation of impact
properties and fracture toughness were prepared by injec-
tion molding long fiber pellets (Table 1).
Specimen Preparation
Notched izod impact strength was determined by test-
ing injection molded specimens of materials in accord-
ance with the ASTM D 256. Impact testing was carried
out by Izod Impact Testing machine. In this test, a 12.7-
mm wide rectangular bar with a 2.5-mm milled notch was
clamped in a sample vise with the notch facing the direc-
tion of the impact. A pendulum is raised to a fixed height
and then released to break the sample. The notch serves
to concentrate the stress of impact and direct the fracture
to that area. The energy required to break the specimen is
calculated from the difference in pendulum height at the
beginning and end of the flow through swing. Five speci-
mens were tested and their standard deviation values are
presented.
Rectangular strips of dimensions 63.5 � 12.7 � 3
mm3 were injection molded. Each specimen was then
notched to produce a single edge notched test (SENT)
specimen. Edge notches were inserted midway along the
length of the specimens in two steps: first a saw cut was
made, which was then sharpened with a razor blade. The
notches are in transverse direction to the melt flow direc-
tion as shown in the Fig. 2. Fracture toughness measure-
ments, Kc, were carried out on these specimens with a
gauge length of 45 mm, in an Instron Universal Testing
Machine at a crosshead speed of 5 mm/min. All tests
were performed at room temperature.
Calculation of Fracture Toughness, Kc
Griffith introduced an energy approach to crack propa-
gation in which, a flaw is unstable when the strain energy
FIG. 1. Block diagram showing the work aimed in this manuscript.
TABLE 1. Injection molding parameters for LFPP.
Injection
pressure
(MPa)
Holding
pressure
(MPa)
Back
pressure
(MPa)
Screw
speed
(rpm)
Temperature
profile
(8C)
88 88 2.0 30–35 190–230
526 POLYMER COMPOSITES—-2008 DOI 10.1002/pc
change that emerges due to an increment of a crack
growth is greater than the surface tension of the material.
Linear elastic fracture mechanics (LEFM) relates the
energy concept referred to as G, to the stress intensity
factor K. The critical value of the stress intensity factor
Kc at which the crack grows unstable is called as fracture
toughness. Kc can be calculated as
Kc ¼ scyffiffiffia
p(1)
where, sc the apparent maximum stress, a the crack
length, and y is the geometry factor of the testing speci-
men that includes the ratio of the crack length to the
width (w) of the specimen. Harris [13] obtained an
expression for the geometry factor y, in which the single
edge notch test (SENT) specimen ends are clamped and
the distance between the clamped ends is three times the
specimen width (w).
y ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
25pða=wÞ20� 13ða=wÞ � 7ða=wÞ2
s(2)
The y function given by Eq. 2 was used for calculating
the fracture toughness Kc in Eq. 1.
RESULTS AND DISCUSSION
Impact Strength
Figure 3 shows the results of notched izod impact
strength of injection molded LFPP composites with and
without compatibilizer at 238C. It can be seen from the
Fig. 3 that the impact strength increases with respect to
increase in feedstock fiber length up-to 9 mm and
decreases for 12 mm pellets sized LFPP composite. The
trend is same for both LFPP composite with and without
compatibilizer. As mentioned earlier, the average fiber
length after injection molding decides the mechanical
properties of composites. The average fiber length
increases up-to a feedstock fiber length of 9 mm and it
remains almost same for 12 mm pellet size. There is no
increase in average fiber length for 12 mm feedstock fiber
length. Therefore, the mechanical properties of injection
molded LFPP composite for a feedstock fiber length of 12
mm has shown no improvement.
The impact strength of LFPP composite with 40 wt%
fiber increases by �50% when compared with that of the
polypropylene. This is because of the addition of fibers
which restricts the failure behavior of composites. Simi-
larly, for LFPP composite with compatibilizer, the impact
strength increases by 105%. This proves that the interfa-
cial adhesion between glass fiber and polypropylene ma-
trix is improved by addition of optimum content of com-
patibilizer. Among LFPP composite, there is an increase
of 15%, on an average, in impact strength with increase
in feedstock fiber length, whereas for composite with
compatibilizer, the increase is about 11%. Impact strength
of LFPP composite with 12-mm feedstock fiber length
decreases, due to the less retention of final fiber length af-
ter injection molding.
The major energy absorbing mechanisms when a dis-
continuous fiber reinforced composite fractures from an
existing notch are fiber pullout, debonding, and fiber frac-
ture. When a sudden impact load is applied, the matrix
gets deformed and fracture takes place at the crack tip.
The matrix transfers the stress by shear to the fiber
through the fiber–matrix interface. If the stress transfer is
more than the interfacial shear strength, the composite
fails by fiber debonding. Transfer of stress may still be
possible to a debonded fiber through frictional forces
along the interface. The composite may fail by fiber frac-
ture if the fiber stress level exceeds the local fiber
strength. Fibers that have fractured away from the crack
interface will be pulled out of the matrix, which may also
FIG. 3. Effect of feedstock fiber length on impact strength of composites.
FIG. 2. Fracture toughness specimen dimensions (in mm).
DOI 10.1002/pc POLYMER COMPOSITES—-2008 527
involve energy dissipation. Fiber pullout occurs due to the
poor adhesion between fiber and matrix.
Scanning electron micrographs of impact tested sam-
ples for LFPP composite with and without compatibilizer
were given in the Fig. 4. The figure explains that the
LFPP composite fails in a brittle fashion, since the load-
ing rate is very high in this testing. The high and low
value impact strength resulted samples were chosen for
scanning the morphology in order to analyze their failure
behavior. Figure 4 also reveals that the fiber fails due to
debonding, fracture, and pullout behavior. For LFPP com-
posite without compatibilizer (Fig. 4a and b), failure by
fiber pullout behavior is dominant near the notch. This
shows that less energy has been required for the fiber to
pullout due to poor adhesion between glass fiber and
polypropylene matrix. For compatibilized composite fiber,
fracture behavior is observed more. When compatibilizer
is added (Fig. 4c and d), the interfacial shear strength is
high, which could transfer the stress from matrix to fiber
and more energy is absorbed by the fiber before it fails.
This is observed only in the presence of compatibilizer
content. The improved adhesion between glass fiber and
polypropylene matrix due to the presence of compatibil-
izer requires more stress for the fiber to fail, and hence
improved impact strength in case of composite with com-
patibilizer. But, increase in feedstock fiber length does
not have any significant effect on the failure behavior of
composite. This is because there is no significant increase
in final fiber length after injection molding. Therefore, it
is very difficult to find out the change in failure behavior
FIG. 4. Scanning electron micrographs of impact tested samples: (a) A-3, (b) A-9, (c) B-3, and (d) B-9.
TABLE 2. Fracture toughness values of different LFPP composites.
Injection molded samples
Fracture
stress (MPa)
Fracture
toughness
(MPaHm)
Tensile
strength
(Mpa)
PP 14.46 2.11 32.65
LFPP composite without
compatibilizer
3 18.60 2.72 34.16
6 17.80 2.60 34.74
9 17.34 2.53 33.27
12 17.37 2.54 33.72
LFPP composite with
compatibilizer
3 18.34 2.68 42.13
6 18.22 2.66 44.50
9 20.21 2.95 45.45
12 19.14 2.80 45.60
528 POLYMER COMPOSITES—-2008 DOI 10.1002/pc
of LFPP composite unless there is significant increase in
average fiber length after injection molding.
Fracture Toughness
A fracture toughness value of LFPP composite with and
without compatibilizer was tabulated in Table 2 along with
fracture stress and their tensile strength. The tensile
strength of the composite is explained in Part 1 of the pub-
lished work [1]. Crack length is maintained as 6.35 mm for
all samples. Geometry factor y is calculated using the Eq. 2.
Table 2 shows that the fracture toughness value of LFPP
composite with and without compatibilizer. There is no sig-
nificant increase in fracture toughness value with increase
in initial feedstock fiber length. The increase in fracture
toughness for LFPP composite with compatibilizer is mar-
ginal when compared with that of the LFPP composite
without compatibilizer. Since the mode of testing for frac-
ture toughness and tensile strength of composite is same, a
linear relationship between fracture toughness and tensile
strength of composite was observed and shown in Fig. 5.
This explains that the material variables, such as the type of
FIG. 5. Fracture toughness versus tensile strength of LFPP composites (a) without compatibilizer and (b)
with compatibilizer.
DOI 10.1002/pc POLYMER COMPOSITES—-2008 529
fiber, fiber length, fiber content, fiber orientation, matrix na-
ture, fiber–matrix adhesion, that determine the tensile
strength and fracture toughness are same. The trend line for
LFPP composite without compatibilizer fits to an agree-
ment of 66% (Fig. 5a) and LFPP composite with compati-
bilizer confirms to an agreement of 92% (Fig. 5b). The vari-
ation in the slope of the straight line and the intercept value
determines the processing variables and due to some exper-
imental error. On comparison the linear relationship
between fracture toughness and tensile strength for LFPP
composite with compatibilizer fits well.
Structure–Toughness Correlation
The microstructure across the thickness of the injection
molded fiber reinforced composites was strongly inhomo-
geneous. SEM analysis of LFPP composite showed a three-
layered structure as shown in Fig. 6. The two outer shell
layers (S) show a fiber orientation predominantly parallel to
the flow direction and the intermediate core layer (C) con-tains fiber oriented in transverse direction to the melt flow
direction. This kind of layered fiber orientation is formed
due to the difference in the injection velocity of the flow
front. The velocity of fluid flow in the core region is higher
than the velocity at the wall. Therefore, the fiber orientation
in the core region will be different from the fiber orientation
in the shell region. Moreover, there exists two kinds of
interaction i.e. fiber–fiber and fiber–mold interactions when
a fluid flows and fills the mold. In the shell layer, the fiber–
fiber interaction will be less and fiber–mold interaction will
be more and there exists a high shear between mold and
fiber. Because of this, the fibers are aligned towards the
melt flow direction. In the core region, shear force between
the fiber and mold is zero, and the fibers are aligned in
transverse direction to the melt flow direction due to the
higher fiber–fiber interaction. T is the total thickness of the
sample. The thickness of the shell and core layer is in the
ratio of 1:2. Incorporation of long fibers of an aspect ratio
greater than 100 increases C/T ratio [14].
The fracture toughness Kc of a thermoplastic composite
can increase or decrease with increasing amounts of stiff
and strong fibers in a polymer matrix. The tendency and
degree of variation in toughness are mainly a function of
the initial fracture toughness of the base polymer and sev-
eral microstructural effects related to the fibers and fiber/
matrix interface.
According to microstructural efficiency concept devel-
oped for injection molded thermoplastic composites, the
relative fracture toughness (the ratio of fracture toughness
of composite to the fracture toughness of the matrix, Kc/
Km) is linearly related to the microstructural efficiency
factor, M.
Kc
Km
¼ M ¼ aþ nR (3)
where a is the matrix stress condition factor, n is the
energy absorption ratio, and R is the reinforcing effective-
ness parameter. R can be calculated from
R ¼ 2S
Tfp;effðSÞ þ
C
Tfp;effðCÞ
� �Vf
l
d
� �max
lnlw
(4)
where, fp,eff values correspond to the fiber orientation fac-
tor, fp, in a way that both expresses the hindrance effect
FIG. 6. SEM images of LFPP composite (�70).
530 POLYMER COMPOSITES—-2008 DOI 10.1002/pc
of fibers due to their local orientation in different layers
C/T and 2S/T with respect to the actual crack direction.
Using the Friedrich’s nonlinear correlation [8] between fpand fpeff, the value of fp,eff (for core 0.05 and for shell
0.95) was found considering fp as 0.4. The value of R was
calculated by maintaining constant Vf as 0.167 and the
values are tabulated in Table 3 with lmax and ln/lw ratio.
Detailed analysis of fiber length distribution is explained
in Part 1 of the published work [1].
Figures 7 and 8 show the plot between microstructure
efficiency factor and reinforcing effectiveness parameter
for LFPP composite without and with compatibilizer,
respectively. LFPP composite without compatibilizer has
y intercept of 1.4 and a negative slope of 0.01 (Fig. 7).
This intercept value indicates that the measured value of
Km value is small when compared with the real stress
state conditions in the matrix of the composite. Negative
slope means that the loss of energy dissipation by the ma-
trix is larger than the increase of energy dissipation by
the fibers during breakdown of composite. This happens
for very ductile matrices and it holds good for PP. But,
for LFPP composite with compatibilizer, the y intercept
has a value of 0.9 and a positive slope of 0.02 (Fig. 8).
This explains that the fracture toughness value of matrix
is high when compared with that of the real stress state
conditions in the matrix of the composite and/or the fiber
hinders the matrix deformation. Positive slope of 0.02
gives the idea that the loss of energy dissipation by the
matrix is smaller than the increase of energy dissipation
by the fibers during breakdown of composite. This shows
that the composite has become brittle by adding compati-
bilizer and further increase in interfacial shear strength
between the glass fiber and polypropylene matrix favors
positive energy absorption ratio.
Figure 9 explains the relative change in both micro-
structure efficiency factor (M) and relative modulus (E*¼ Ec/Em) of LFPP composites with Vf ¼ 16.7%. Without
compatibilizer the trend is following a negative slope and
with compatibilizer the trend is of positive slope. There-
fore, the addition of compatibilizer enhances the fracture
toughness marginally.
Fracture Toughness––Failure Mechanism
Microscopic failure mechanisms occurring in discontin-
uous long fiber reinforced polypropylene are shown sche-
matically in the Figure 10. They can be grouped into PP
matrix related and glass fiber related failure mechanisms.
Failure initiates first at the fiber ends and stress concentra-
tion in these damage zones gives rise to crazing phenom-
ena in PP matrix. For LFPP composite without compati-
bilizer, fiber debonding and fiber pullout behavior are tak-
ing place more common, because of poor interfacial bond
strength between ductile PP and glass fibers. From the fig-
FIG. 8. Effect of reinforcement on microstructure efficiency for LFPP
composite with compatibilizer.
FIG. 9. Microstructure efficiency factor versus relative modulus of
LFPP composites with Vf ¼ 16.7%.
TABLE 3. Reinforcing effectiveness parameter values for different
feedstock fiber length.
Feedstock fiber
length (mm) lmax (mm) ln/lw R
3 2.4 0.873 14.50
6 2.6 0.817 14.78
9 3.2 0.876 19.50
12 2.9 0.879 17.70
FIG. 7. Effect of reinforcement on microstructure efficiency for LFPP
composite without compatibilizer.
DOI 10.1002/pc POLYMER COMPOSITES—-2008 531
ure, it is evident that the mode of failure in these compo-
sites is ductile. For LFPP composite with compatibilizer
fiber fracture occurs more along with matrix shear yield-
ing mechanism. In these compatibilized composite, since
most of the applied load is transferred to the fibers by the
matrix through the interface, contribution of fiber is more.
FIG. 10. SEM images of (a) PP, (b) LFPP composite without compatibilizer, and (c) LFPP composite with
compatibilizer (�1,500).
532 POLYMER COMPOSITES—-2008 DOI 10.1002/pc
These fibers fail in a brittle manner and hence fiber frac-
ture is more than fiber pullout and debonding. At the time
of fiber fracture, the matrix shear yields and the compos-
ite fails. The final crack path emerges by connection of
the different craze planes due to fiber debonding, pullout,
and fracture events with concomitant matrix deformation
(shear yielding, plastic deformation). With respect to
feedstock fiber length, there is no significant change in
failure mechanism or fracture toughness values, since the
fiber length after injection molding is marginal. But with
little addition of compatibilizer, it is possible to have
good interaction between fibers and matrix and in turn the
failure behavior changes.
Table 4 shows that the coefficient of variation for
impact strength and fracture toughness on LFPP compo-
sites with and without compatibilizer. The coefficient of
variation signifies the repeatability of the experiment and
thereby the homogeneous microstructure of the injection
molded plaques. It is defined as the ratio of standard devi-
ation and average value and expressed in percentage. The
value of coefficient of variation is, on an average, not
larger than 10%.
CONCLUSION
The variations of impact strength and fracture tough-
ness were examined for LFPP composite with and without
compatibilizer, with different initial feedstock fiber length.
In conclusion, it can be stated that the addition of compa-
tibilizer does improve the impact strength values and mar-
ginally on fracture toughness values. The effect of glass
fiber length on impact strength is highly significant and
the effect of glass fiber length on fracture toughness is
quite marginal. Scanning electron micrographs also sup-
ported the major energy absorbing mechanism such as
fiber fracture and fiber pullout occurring on LFPP compo-
sites with and without compatibilizer. A linear relation-
ship between fracture toughness and tensile strength of
composites explains that the material variables that deter-
mine the fracture toughness and tensile strength are same.
The ratio between shell and core to part thickness was
also found out using SEM images. Structure–toughness
correlation was analyzed for LFPP composite with and
without compatibilizer. For LFPP composite without com-
patibilizer, the loss of energy dissipation by the matrix is
more than the increase of energy dissipation of fibers dur-
ing breakdown of composite. Matrix is very ductile in
this composition and poorly adhered with glass fiber. Ma-
trix fails by crazing phenomena. For LFPP composite
with compatibilizer, strong adhesion with glass fiber
increases the energy dissipation by fibers than that of the
matrix during breakdown of composite. The final break-
down emerges by different craze planes due to fiber
debonding, pullout, and fracture with concomitant matrix
deformation (shear yielding, plastic deformation).
REFERENCES
1. K. Senthil Kumar, N. Bhatnagar, and A.K. Ghosh, Polym.Compos., 28(2), 259 (2007).
2. M.J. Voelker, Polym. Compos., 12(4), 119 (1991).
3. A. Hassan, R. Yahya, A.H. Yahaya, and A.R.M. Tahir,
J. Reinforced Plast. Compos., 23(9), 969 (2004).
4. T.P. Skourlis, S.R. Mehta, C. Chassapis, and S. Manoo-
chehri, Polym. Eng. Sci., 38(1), 79 (1998).
5. J.L. Thomason and M.A. Vlug, Compos. A, 28, 277 (1997).
6. S. Hashemi and M. Koohgilani, Polym. Eng. Sci., 35(13),1124 (1995).
7. F. Gonzalez and C. Chassapis, J. Reinforced Plast. Compos.,20(10), 810 (2001).
8. J.K. Kocsis, Compos. Sci. Technol., 48, 273 (1993).
9. J.K. Kocsis, T. Harmia, and T. Czigany, Compos. Sci.Technol., 54, 287 (1995).
10. K. Friedrich, D.E. Spahr, J.M. Schultz, and R.S. Bailey,
J. Mater. Sci., 25, 4427 (1990).
11. K. Friedrich and J.K. Kocsis, Compos. Sci. Technol., 32,
293 (1988).
12. K. Friedrich, Compos. Sci. Technol., 22, 43 (1985).
13. D. Harris, J. Basic Eng., 3, 49 (1967).
14. J.K. Kocsis, Polypropylene Structure, Blends and Compo-
sites, Chapman and Hall, London, 166 (1995).
TABLE 4. Coefficient of variation (%) of the impact strength and
fracture toughness on LFPP composites.
Injection molded samples
Coefficient of variation (%)
Impact
strength
Fracture
toughness
PP 14.8 2.7
LFPP composite without compatibilizer
3 8.7 5.3
6 7.0 10.6
9 7.8 10.3
12 9.1 5.8
LFPP composite with compatibilizer
3 2.2 4.6
6 7.6 3.2
9 8.9 3.3
12 3.9 7.0
DOI 10.1002/pc POLYMER COMPOSITES—-2008 533