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 savings. 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 tends to reduce the Young modulus and ultimate
strength. Even the specific values are only increased in terms of impact force and, marginally, in terms of 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 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. 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. A predetermined amount of resin/hardener and micro-spheres was calculated as function of the intended volume
fraction to be attained. Resin and hardener were mixed in a pot and afterwards the micro-spheres were added while stirring.
Composite sheets were manufactured by injection 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 in three-point bending loading with a span
of 48 mm as 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 sliding 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 [13] and ASTM D5045-96 [14] standards, for flexural and mode I fracture toughness tests,
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 ASTM D5045-96 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 where a brittle behaviour of the materials was observed, c=a/W, a is the crack
length, B is the specimen thickness and W the specimen width.
Strain energy release rate was calculated using the equation (2)
G c  U /( BW )
(2)
where: U is the integrated area of the load-displacement curve up to PQ load and Φ is a calibration factor given as a function of
a/W, according to ASTM D5045-96 [14].
Two batches of tests were performed to study the influence of the load rate and the time of exposure to 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. For the
second objective the specimens were immersed in water at 20 ºC during increasing 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 five specimens
were tested. Then, the average and standard deviation values of the current mechanical property were calculated for each test
condition.
Results and discussion
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. 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.
Flexural modulus [MPa] .
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 that the load rate only marginally influences KIC
results.
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 which indicates that the water saturation was reached quickly following the absorption process of Fick` s Law
in agreement with the work of Gupta [15] and the. 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 trend to decrease
slightly with the time of exposure 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 loading rates of 0.05, 1
and 500 mm/min. The pictures show a low presence of micro-porosities indicating that the procedure followed in
manufacturing cur process was correct. 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 pull outs of micro-spheres was observed
together with some fracture of the micro-spheres. 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 500 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 a function of the load rate and the water immersion time. The
increase of the load rate tends to increase stiffness modulus, but only marginal 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; 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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