an internal damping in epoxy composite...

An internal damping in epoxy composite systems

D. Kroisová

Technical University of Liberec, Department of Material Science,

Studentská 2, 461 17, Liberec, Czech Republic

email: [email protected]

Abstract This work is focused on an internal damping in epoxy composite materials. Composite systems designed

for experiments were created by two-component epoxy resin and selected types of fillers (lead particles

and chippings, alloy hollow tubes, ferrous-ferric oxide particles, titanium dioxide particles, cork particles,

carbon chopped fibres) differ in their composition, particle size and shape and the weight fraction in the

polymer matrix. The damping tests of casted and cured epoxy composite systems were performed at a

temperature of approximately 22oC, an atmospheric pressure and a low frequency at a range of 50 Hz to

100 Hz as a common condition of a dynamic stress of structures. The photo-electric equipment was used

to measure the deflection of samples the lost coefficient tan δ was calculated by the common method. A

scanning electron microscopy was used to observe filler shape, size and surface as well as fracture

surfaces of composites and the evaluation of their interfaces.

1 Introduction

An internal damping of composite materials is a very important parameter related to the dynamic

behaviour of materials and can be used as a source of interesting knowledge about their structure, state and

behaviour. A lot of composites are built up from polymer materials as a matrix and a different type of

reinforcement. The viscoelasticity of polymers is the main reason of their using as materials for vibration

damping. The damping mechanisms of composite systems include not only behaviour of viscoelastic

polymer but intrinsic damping of fillers, the boundary sliding of fillers, the interfacial sliding between

matrix and fillers or the damage of composite systems too [1, 2, 3]. The particle type, the size, the shape,

the surface and the volume fraction of filler can contribute to damping capacity of composite systems.

Material parameters as a hardness or as a stiffness can influence an internal damping of systems too. In

addition, the testing conditions including the temperature, the frequency and the oscillation amplitude can

affect the damping behaviour of composite systems as well [4]. The aim of this work is to study the effect

of selected types of fillers and their weight fraction in epoxy matrix composite samples during common

working conditions.

2 Experimental

2.1 Materials

Two-component epoxy resin (ChS-Epoxy 371, diethylentrimine hardener P11 - Spolek pro chemickou a

hutní výrobu, a. s. Czech Republic) was used as the matrix having a good adhesion to many substrates,

chemical, moisture and heat resistance and, therefore having wide application. Lead particles, lead

chippings, Pb-Sn hollow tubes (Spolek pro chemickou a hutní výrobu, a. s. Czech Republic), ferrous-ferric

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oxide particles (Sigma-Aldrich, spol. s r. o., Czech Republic ), cork particles (different particle size

fractions were prepared by grinding and sieving of common cork compact material), titanium dioxide

particles (Precheza, a. s. Přerov, Czech Republic), carbon fibres (Havel Composites CZ s. r. o., Czech

Republic) were used as fillers of epoxy composite systems. Their basic parameters are presented in table

1.

Designation of Fillers Components Specific Gravity

[g/cm3]

Shape and Size

of Fillers

[μm]

Lead spherical

particles 99.9% Pb

11.3

Particles

Diameter of particles

100 μm

Lead chippings 99.9% Pb

11.3

Chippings

Diameter of chippings

1 – 1.5 mm

Length of chippings

3 – 7 mm

Pb-Sn hollow tubes 60% Pb

40% Sn 8.9

Tubes

Diameter of tubes

500 – 600 μm

Length of tubes

5 – 6 mm

Tubes wall thickness

50 μm

Cork tiny particles

45% suberin

27% lignin

12% cellulose

0.25 – 0.5

Particles


20 – 50 μm

Cork particles

45% suberin

27% lignin

12% cellulose

0.25 – 0.5

Particles


300 – 500 μm

Ferrous-ferric oxide

particles

99.9% Fe3O4

4.9 – 5.1

Aggregates from spherical particles

Diameter of aggregates

20 – 100 μm


< 1 μm

Ferrous-ferric oxide

particles in own

polymer phase

99.9% Fe3O4 4.9 – 5.1



10 – 20 μm


< 1 μm

Titanium dioxide

particles

92% TiO2

4% Al2O3

3% SiO2

1% ZrO2

4.2



20 – 100 μm


< 1 μm

Carbon fibres 99.9% C 1.6

Fibres

Diameter of fibres 8 μm

Length of chopped fibres

2 – 5 mm

Table 1: Basic parameters of used fillers

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2.2 Composite materials preparation

The desired amount (weight fraction phr – part per hundred of resin – g per 100 g epoxy resin) of all types

of filler was added to fluid epoxy resin and mixed together. The mixtures were cured in a three-part

aluminium mould at a temperature 22oC for 24 hours under pressure 0.8 MPa to get homogeneous

materials. To standardize the samples, they were tested after 21 days because of slow post-curing of the

epoxy resin at laboratory temperature - figure 1.

Figure 1: The curve of tan δ dependence on post-curing time of pure epoxy resin

2.3 Characterization

A damping capacity of epoxy composite samples was determined by the photoelectric equipment at a

laboratory temperature (22 ± 2)oC and a low frequency at a range of 50 Hz to 100 Hz. These parameters

were selected as common conditions of dynamic stress of structures although a lot of scientists are

interested in a damping capacity at the glass transition temperature of viscoelastic polymer matrix. The

composite samples (length 100 mm, width 10 mm, thickness 2 mm) were used as cantilever beams. The

standard initial deflection of the cantilever beam from the equilibrium state was specified by the pinpoint

of the measuring apparatus. The photoelectric equipment was used to measure the damped oscillation of

samples. The numerical values of these oscillations were read from a memory oscilloscope. The damping

ratio was found using the logarithmic decrement method. The loss factor tan δ was determined from the

vibration curve decay.

Scanning electron microscope (VEGA/TESCAN, Czech Republic) was used at an accelerating voltage of

30 kV to evaluate the character of sample interfacial surface and the homogeneity of the systems. The

fracture surfaces were coated with a thin layer of gold – palladium alloy.

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3 Results and discussion

3.1 The effect of material type and volume fraction on internal damping

The addition of fillers to viscoelastic polymer matrix can make a contribution to increase the internal

damping of composites. The contribution to the internal damping can come from the intrinsic damping of

fillers, the boundary sliding of fillers, the interfacial sliding between matrix and fillers or from the damage

of composite systems [4, 5]. Part of vibration energy is dissipated due to shear forces that are formed at

the interfaces of loaded system if the Young modulus of used compound is different [6]. Another part of

the vibration energy is dissipated due to viscoelastic behaviour of polymer matrix [7].

As we can see from figure 2, the highest contribution to the internal damping comes from the Pb-Sn

hollow tubes. The part of the vibration energy can be dissipated by the deformation of alloy tubes due to

their low stiffness. An adhesion between tubes and an epoxy resin matrix is low – figure 3c). The

difference in the contribution to the internal damping between lead chippings and lead particles can be

explained by the shape of fillers. Lead chippings with low Young modulus can be deformed during the

loading whilst spherical particles cannot. An adhesion in these both examples is low too – figure 3a), 3b).

Figure 2: The curves of tan δ fraction dependence of the selected particles


a) b) c)

Figure 3: The fracture surfaces of the selected composite systems

a) lead particles – epoxy resin matrix

b) lead chippings – epoxy resin matrix

c) alloy (Pb-Sn) hollow tubes – epoxy resin matrix

As we can see from figure 4, higher contribution to the internal damping comes from the bigger cork

particles. The part of the vibration energy can be dissipated by the deformation of cork particles consisting

from hollow cells. An adhesion between cork particles and an epoxy resin matrix is good – figure 5d). The

difference in the contribution to the internal damping between small and big cork particles can be

explained by the different size of the filler – figure 5a), b), c).

Figure 4: The curves of tan δ fraction dependence of the cork particles

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a) b)

c) d)

Figure 5: The shape and the size of two different size cork particles

a) tiny cork particles

b) bigger cork particles

c) bigger cork particles – hollow cells

d) cork particles in epoxy resin matrix

As we can see from figure 6, the higher contribution to the internal damping comes from ferrous-ferric

oxide particles deposited in the own polymer phase. The part of the vibration energy can be dissipated by

the movement of the tiny particles in the own polymer phase – figure 7a). Ferrous-ferric oxide particles

without polymer phase – figure 7b) do not contribute to the internal damping. An adhesion between both

types of fillers and the epoxy resin matrix is good – figure 7c), d). The difference in the contribution to the

internal damping between two types of ferrous-ferric oxide particles can be explained by the presence of

the secondary viscoelastic polymer phase and the possibility of tiny ferrous-ferric oxide particles to move

in this polymer phase during loading.


Figure 6: The curves of tan δ fraction dependence of two different ferrous-ferric oxide particles

a) b)

DAMPING 1213

c) d)

Figure 7: The character of two different ferrous-ferric oxide particles

a) ferrous-ferric oxide particles in the own polymer phase

b) ferrous-ferric oxide particles without own polymer phase

c) the fracture surface of ferrous-ferric oxide particles in the own polymer phase – epoxy resin

matrix

d) the fracture surface of ferrous-ferric oxide particles without polymer phase – epoxy resin matrix

As we can see from figure 8, particles of titanium dioxide do not contribute to the internal damping of

composite system. An adhesion between this type of filler and the epoxy resin matrix is good – figure 9b).

There is no intrinsic damping of filler particles and no interfacial sliding between matrix and particles of

filler.

Figure 8: The curves of tan δ fraction dependence of titanium dioxide particles


a) b)

Figure 9: Titanium dioxide particles

a) aggregates of titanium dioxide particles

b) the fracture surface of titanium dioxide particles – epoxy resin matrix

As we can see from figure 10, the chopped carbon fibres do not contribute to the internal damping of this

composite system. An adhesion between used carbon fibres and the epoxy resin matrix is quite good –

figure 11b). The energy cannot be dissipated by the deformation of the high-modulus carbon fibres.

Figure 10: The curves of tan δ fraction dependence of chopped carbon fibres

DAMPING 1215

a) b)

Figure 11: Carbon fibres

a) a surface of carbon fibres

b) the fracture surface of chopped carbon fibres - epoxy resin matrix

4 Conclusion

The fillers type, the fraction of the filler particles in the matrix, their size and shape and basic mechanical

parameters can influence the internal damping of composite systems.

Low modulus particles of filler with a nonspherical shape have an influence on the internal damping as a

result of their deformation during the vibration of the matrix.

Spherical and high modulus particles and fibres practically do not deform themselves and do not increase

the internal damping of composite systems.

The samples were tested at the low frequency at a range of 50 Hz to 100 Hz and the temperature 22oC.

Data dispersion is to 2 %.

Acknowledgements

The author gratefully acknowledges MSM number 4674788501, Czech Republic for the financial support.

References

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an internal damping in epoxy composite...

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