TEST CONDITIONS EFFECT ON THE FRACTURE TOUGHNESS OF
HOLLOW GLASS MICRO-SPHERES FILLED COMPOSITES
C. Capela*, J.D. Costa**, J.A.M. Ferreira**
* Mechanical Engineering Department
Polytechnic Institute of Leiria
Morro do Lena - Alto Vieiro, 2400-901 Leiria, Portugal; Email: [email protected]
** Mechanical Engineering Department, ICEMS
University of Coimbra
Polo II da Univ. de Coimbra, Pinhal de Marrocos, 3030, Coimbra, Portugal
ABSTRACT
Low-density sheet moulding compounds based on hollow glass micro-spheres are usually classified as syntactic foams if the
filler content is relatively high. Syntactic foams are potential good materials for applications where impact loads occur since
they are able to reduce impact force. The addition of hollow micro-spheres trends to increase the specific values in terms of
impact force and marginally in flexural modulus for high volume fractions of micro-spheres. In current work they were studied
the effects of load rate and of immersion of the specimens in water up to sixty seven days on the flexural mechanical
properties and particularly on the fracture toughness KIC. Hollow micro-spheres Verre ScotchitTM-K20 with epoxy and
polyester polymer binging were used. Fracture toughness KIC, flexural stiffness modulus and ultimate strength were obtained
as function of the load rate and the immersion time. The increase of the load rate trends to increase stiffness modulus, but only
marginally effects on KIC were observed. Ultimate strength increases significantly with the increasing of load rate for epoxy
based composites, but for the case of the polyester based foams a negligible effect was observed. The increase of the
immersion time in water trends to reduce stiffness modulus. KIC decreases slightly after 15 days for the polyester based
composites and after 67 days for epoxy based foams, and only negligible effects on ultimate strength were observed.
Introduction
Low-density sheet moulding compounds based in hollow glass micro-spheres are being increasingly used, namely in
automotive industry, boats and core materials, where it can present advantages compared with traditional metal, such as:
lower weight, less expensive for low volume production in consequence of lower tool costs, no corrosion effects, more design
freedom, etc. These materials are usually classified as syntactic foams when the filler content is relatively high.
Numerous studies are reported on the mechanical properties of the filled composites, such as: the fracture toughness in epoxy
resin modified with rubber particles [1-3], thermo-plastic particles [4,5] and hard particles [2,6,7]. The hard particles can
contribute to enhancement in fracture toughness. Hollow glass micro-spheres can offer this possible toughening effect in
addition with the beneficial lower density and consequent weight reduction.
Syntactic foams are potential good materials for applications where impact loads occur since they are able to reduce impact
force [8,9] which can be an important parameter in design or materials selection. The mechanical properties of syntactic foams
used in automobile industry had been studied by Oldenbo [10] and Gregl [11]. The last reference presents also some studies
concerning the surface quality aspect.
Hollow glass spheres are commercially provided in many different densities, strengths and diameter sizes. The grade chosen
depends on the application and overall weight reduction. Micro-spheres can be formulated for thermoplastic or thermoset
resins and also some grades tough enough for reaction injection moulding and resin transfer moulding.
As reported by Kim [9] and Oldenbo [10] the addition of hollow micro-spheres trends to reduce the Young modulus and
ultimate strength and even the specific values are only increased in terms of impact force and, marginally, in flexural modulus
for high volume fractions of micro-spheres [9]. Oldenbo [10] did verify that flexural Young modulus can be increased by using
materials with different charged formulation layers: low density in the centre and standard sheet moulding compounds in the
outer layers. Also the thickness and particle size of micro-spheres can produce important changes on the mechanical
behaviour. Wouterson [12] concludes that the effect of the micro-spheres volume fraction on the specific tensile and flexural
strength and stiffness depends on the micro-sphere density and thickness to radius ratio.
Materials and experimental tests
TM
In the present study it was used a batch of hollow micro-spheres Verre Scotchit -K20 manufactured by 3M was used. The
average value particles size obtained by microscopy analysis was 33.5 µm with a standard deviation of 17.8 µm. Two different
resins were used for binding micro-spheres: epoxy 520 with hardener 523 and polyester Hetron 92 FR supplied by Ashland
Chemical Hispania. Resins and hardener were mixed in a mixing pot. A predetermined amount of resin/hardener and microspheres was calculated as function of intended volume fraction of micro-spheres. Resins and hardener were mixed in a mixing
pot and afterwards the micro-spheres were added while stirring. Composite sheets were manufactured by using an aluminium
mould with a rectangular parallelepiped cavity of 400x200x6 mm. The mould was cleaned by using acetone and treated with a
fluid green release agent, MCP. The volume fraction of the foams and densities obtained according to Archimedes principle is
presented in Table 1.
Table 1: Volume fraction of micro-spheres and corresponding foam densities.
Base polymer
Epoxy
Polyester
Volume fraction of micro-spheres
26.3
30.1
3
Foam density [g/cm ]
0.92
0.83
Rectangular parallelepiped specimens were cut from the moulded plates and machined for the dimensions of 65x12x6 mm for
flexural and mode I fracture toughness tests. Mode I fracture tests were performed for three-point bending loading with a span
of 48 mm as is shown in Figure 1. A pre-crack was produced by tapping and razor blade at the crack tip of the mechanical
notch on each specimen. Crack length was measured after the test by using a microscopy mounted in a X-Y base.
6
P
12
65
6
x
L=48
12
65
Figure 1: Schematic view of specimen and loading for mode I fracture tests.
The tests were performed by using an Instron universal testing machine, equipped with computer controller and recorder
system according to ASTM D790-98, 1998 [13] and ASTM D5045-96, 1996 [14], for flexural properties and mode I fracture
toughness, respectively. For the flexural properties analysis the load versus displacement curves were obtained directly and
the stresses were calculated by using the relationships for linear bending beams.
According with ASTM D5045-96, 1996 standard [14] the stress intensity factor for mode I fracture toughness in three-point
bending loading is calculated by the equation (1)
K IC =
[
]
 6 c 1.99 − c(1 − c)(2.15 − 3.93c + 2.7c 2 ) 



B W 
(1 + 2c)(1 − c)3 / 2
PQ
(1)
where: PQ is the maximum force in this case of brittle behaviour of the materials, c=a/W, a is the crack length, B is the
specimen thickness and W the specimen depth.
Strain energy release rate was calculated using the equation (2)
G c = U /(BWΦ )
(2)
where: U is the integrated area measured in load-displacement curve up PQ loading and Φ is a calibration factor given, in
function of a/W, according ASTM D5045-96, 1996 [14].
Two batches of tests were performed to study the influence of the load rate and the exposure water, respectively. For the first
objective the specimens were stored at dry ambient and tested for load rates from 0:05 to 500 mm/min. The specimens for the
second objective were immersed in water at 20 ºC during periods until 67 days. Before the test the specimens were removed
from the water, dried and tested at a load rate of 1 mm/min. For each test condition they five specimens were tested. Then, the
medium values and standard deviation of the current mechanical property was calculated for each test condition.
Results and discussion
Flexural modulus [MPa] .
The results of the effect of load rate on the stiffness modulus, ultimate flexural strength and fracture toughness KIC, are
summarized in the Figures 2,3 and 4, respectively, in which the medium values and the standard deviation are plotted against
the load rate. These results show that the values of all the three properties are significantly lower for polyester based foams
than for the epoxy based materials, particularly the ultimate strength and the fracture toughness which indicates a strong
influence of the base material and a poor interface micro-sphere/polymer adhesion and efficiency obtained with polyester base
material.
2000
1800
1600
1400
1200
1000
Epoxy
Poliester
800
0.01
0.1
1
10
100
1000
Load rate [mm/min]
Figure 2. Flexural modulus versus the testing load rate
Flexural strength [MPa] .
80
70
Epoxy
Poliester
60
50
40
30
20
0,01
0,1
1
10
100
1000
Load rate [mm/min]
Figure 3. Flexural ultimate strength versus the testing load rate
A dissimilar effect of load rate in the different properties was observed. As was expected, flexural modulus increases
significantly with load rate for both base polymers. In opposite, the ultimate strength increases significantly with the increasing
of load rate for epoxy based composites, but in opposite for the case of the polyester based foams a negligible tendency to
decrease was observed. On the other hand the analysis of Figure 4 shows only marginally effects of the load rate on KIC.
K1c [MPa.m
0.5
]
2
1,5
1
0,5
Epoxy
Poliester
0
0,01
0,1
1
10
100
1000
Load rate [mm/min]
Figure 4. Fracture toughness versus the testing load rate
Figures 5, 6 and 7 summarize the results of the effect of immersion time of the specimens in water at 20 ºC on the stiffness
modulus, ultimate flexural strength and fracture toughness KIC, respectively. These results were complemented with a study of
water absorption that did indicate that the absorption process follows the Fick` s Law in agreement with Gupta [15] and the
water saturation was reached quickly. Also, a scanning electronic microscopic observation of fracture surface was performed
to understand the failure process. In spite of the quick water diffusion process the analysis of the results shows a negligible
and not well defined effect of the water on the ultimate strength. Fracture toughness shows some tendency to decrease
slightly, but the time for this decreasing depends on the base polymer being earlier for polyester than for epoxy based material
since the diffusion process is quickly in the first base polymer foam. The decrease occurs after 15 days for the polyester based
composites and after 55 days for epoxy based foams. These results agree with surface analysis (Figure 8) where no
significant effect of water immersion on failure aspect was observed. By other side a significant effect of the immersion time on
the stiffness modulus was obtained (Figure 4) for both base polymers which can probably be associated with an easier
dislocation in micro-spheres/polymer interfaces caused by the water lubrication.
Flexural modulus [MPa] .
2200
2000
1800
1600
1400
1200
1000
Epoxy
800
Polyester
600
0
20
40
60
80
Water immersion time [days]
Figure 5. Flexural modulus versus the immersion time
80
Flexural strength [MPa] .
Epoxy
Polyester
70
60
50
40
30
20
0
20
40
60
80
Water immersion time [days]
Figure 6. Flexural ultimate strength versus the immersion time
K1c [MPa.m
0.5
]
2
1,5
1
0,5
Epoxy
Poliester
0
0
20
40
60
80
Water immersion time [days]
Figure 7. Fracture toughness versus the immersion time
The fracture surfaces were observed in scanning electronic microscopy to better understand the failure mechanisms
associated with the parameters studied here. Figure 8 shows SEM images of the morphological features of mode I fracture
surfaces for specimens manufactured with epoxy resin and Vf=26.3 %. The figure reports the failure of three specimens
exposed for a long time in dry air and one specimen immersed in water during 67 days and tested at the loading rates of 0.05
to 500 mm/min. The pictures show low presence of micro-porosities indicating a correct procedure followed in manufacturing
cur process. The analysis of these pictures also shows a brittle fracture with low deformation of the matrix. In agreement with
failure mechanisms reported by Kishore [16] significant of pull outs of micro-spheres was observed together with some fracture
of the micro-particles. On the other hand according to these observations, no significant effect of the load rate and water
immersion on failure mechanisms was observed.
a) Specimen stored in dry air and loaded at 0.05 mm/min
b) Specimen stored in dry air and loaded at 0.05 mm/min
c) Specimen stored in dry air and loaded at 1 mm/min
d) Specimen stored 67 days in water, loaded at 1 mm/min
Figure 8. SEM observations of fracture surfaces
Conclusions
In the current work it was studied the effects of load rate and of the immersion time of the specimens in water up to sixty seven
days on the flexural mechanical properties and particularly on the fracture toughness KIC. Fracture toughness KIC, flexural
stiffness modulus and ultimate strength were obtained as function of the load rate and the immersed time. The increase of the
load rate tends to increase stiffness modulus, but only marginally effects on KIC were observed. Ultimate strength increases
significantly with the increasing of load rate for epoxy based composites, but for the case of the polyester based foams a
negligible effect was observed. The increase of the immersion time in water trends to reduce stiffness modulus. KIC decreases
slightly after 15 days for the polyester based composites and after 67 days for epoxy based foams, and only negligible effects
on ultimate strength were observed.
Acknowledgments
The authors would like to acknowledge Project nº PTDC/EME-PME/66549/2006, for funding the work reported.
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