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Dynamic fracture toughness of polyurethane foam
Polymer Testing 27 (2008) 941–944
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Short Communication: Material Properties
Dynamic fracture toughness of polyurethane foam Liviu Marsavina a, b, Tomasz Sadowski a, * a b
Lublin University of Technology, Faculty of Civil and Sanitary Engineering, 20-618 Lublin, Nadbystrzycka 40 street, Poland POLITEHNICA University of Timisoara, Department Strength of Materials Blvd. M. Viteazu, Nr.1, Timisoara 300222, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2008 Accepted 12 August 2008
This paper is a first attempt to determine the dynamic fracture toughness of polyurethane foam and to study the effect of impregnation on the fracture toughness. Instrumented impact tests were performed using notched specimens. In order to study the effect of impregnation on the impact properties two different resins were used. The obtained results show that the impregnation increases the dynamic fracture toughness by 27%. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Polyurethane foam Impregnation layer Dynamic fracture toughness Instrumented impact test
1. Introduction Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing and cushioning. They are made of interconnected networks of solid struts and cell walls incorporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity [1]. Particularly for the design of high performance sandwich composites, a good knowledge of the behaviour of different grades of foam is necessary. Of particular interest is the fracture toughness of such foams because foam cracking weakens the structure’s capacity for carrying loads. Many efforts have been made in recent years to determine the fracture toughness of different types of foam in static and dynamic loading conditions. Micromechanical models and experimental investigations were used for estimating the fracture toughness. The first correlation between fracture toughness of PUR foams and density (<200 kg/m3) was proposed by McIntyre and Anderson [2] in a linear form. The same behaviour was observed by Danielsson [3] on PVC Divinycell foams and Viana and Carlsson on Diab H foams [4]. A correlation * Corresponding author. E-mail address: [email protected] (T. Sadowski). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.08.006
between the static fracture toughness and relative pffiffiffiffiffi density
r/rs was proposed in [1] in the form: KIc ¼ C sfs plðr=rs Þm , where sfs is the modulus of rupture of the cell wall material in bending, l is the cell dimension, C represents a constant of proportionality and m an exponent (equal with 3/2 for open cell). In Ref. [4], a value m ¼ 2 for closed cell foam is used. Kabir et al. [5] investigated the dynamic fracture toughness and found a maximum value of 2.74 MPa m0.5 for PVC foam with 260 kg/m3 density which is 3.75 times higher than the static fracture toughness of the same foam. They observed that fracture is brittle without yielding and is produced in Mode I. Mills and Kang [6] used a falling mass with a compact tension specimen in order to determine the dynamic fracture toughness, and Mills [7] also proposed a correlation between the dynamic fracture toughness and foam density. However, the dynamic fracture toughness for PUR foams has not been reported in the literature. On the other hand, the effect of impregnation on mechanical properties of foams was pointed out by Apostol et al. [8] but is less studied compared with the effect of density, strain rate or temperature. For the determination of the dynamic fracture toughness of polyurethane foam an instrumented impact test was considered. A 200 kg/m3 density rigid foam was used in the experimental program. The foam was impregnated
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L. Marsavina, T. Sadowski / Polymer Testing 27 (2008) 941–944
Table 1 Comparison of static characteristics for polyurethane foams Foam type
Tested
Divinycell
Properties
Units
Un-impreg. Polyester Epoxy H200 impreg. impreg. [11]
Density Compressive strength Compressive modulus Tensile strength Tensile modulus Flexural strength Flexural modulus
kg/m3 MPa MPa MPa MPa MPa MPa
200 5.07 113 5.39 169.3 6.84 146
200 4.97 131.3 5.12 191.4 6.66 211.8
200 4.95 132.7 4.05 259.4 6.91 294.5
200 4.5 150 4.8 210 n.a. n.a.
with polyester and epoxy resins and a study of impregnation on the impact energy and fracture toughness was performed. Fig. 1. The microstructure of PUR foam.
2. Test methodology The investigated material is a 200 kg/m3 closed cell polyurethane (PUR) rigid foam. The foam faces were impregnated with epoxy (layer of 170 mm) and polyester (layer of 100 mm) resin in order to increase durability of the foam used for lightweight boats. The foam was manufactured and supplied in the form of flat panels of 12 mm thickness. The microscopic structure of the foam is presented in Fig. 1, and the characteristic dimensions are cell – wall size 200–500 mm and wall thickness 3–4 mm. The main mechanical characteristics of the investigated foam were determined experimentally [9,10] and are summarised in Table 1 side by side with the same characteristics of commercial Divinycell foam of the same density [11]. From Table 1 it can be observed that the impregnation layer has no effect on the tensile and flexural strength but has important influence on the tensile and flexural modulus [9]. The results of the impact test on this type of foam on unnotched specimens show that the impregnation layer decreases the energy absorbed to fracture [10]. Three point bend specimens (12 12 60 mm) were adopted with a notch of 1.5 mm (cut with a razor blade), impregnated with epoxy (0.170 mm layer thickness) and polyester (0.100 mm layer thickness) resin. A span of 40 mm was used for the test and the impact load was applied in the transverse direction. The principle of impact and instrumented impact tests of plastic materials are given in the EN ISO 179 [12,13] and, for example, by Kalthoff [14]. A KB Pruftechnik pendulum (Germany) was used for the instrumented impact tests, with the following main characteristics: pendulum mass 2.04 kg, pendulum length 0.386 m, drop height 0.742 m, drop angle 157.32 , pendulum energy 7.5 J, impact velocity 3.815 m/s. The tup has a built-in electronic sensor which allows recording the load with 1 MHz frequency. A four-channel data acquisition
A/D card (AdLink NuDAQ PCI-9812) was used for recording the load in time. Tests were performed at room temperature. The load history F(t) was recorded by using strain gages bonded near the striker edge. The displacement s(t) of the specimen during the test was calculated according to [13,14]:
sðtÞ ¼ v0 t
Lp g MH
Z
t
Z
0
t1
FðtÞdt dt1
½m
(1)
0
where t is the time after impact in which the deflection is calculated [s], Lp is the pendulum length [m], MH is the horizontal moment of the pendulum [Nm], F(t) is the force measured at time t after impact [N], g is the gravitational acceleration [m/s2]. The energy is given by:
W ¼
Z
s
FðsÞds ½ J
(2)
0
where s is the deflection [m] and F is the force in [N]. The Charpy impact strength for notched specimen acU is given by:
acU ¼
wf 1000 BHN
kJ=m2
(3)
where B is the width of the specimen and HN is the width remaining at the base of the notch in the specimen [mm]. The dynamic fracture toughness was calculated according to [15]:
KId ¼
Fmax S f ða=HÞ BH3=2
MPa m0:5
(4)
where Fmax is the maximum force, S is the span, and B and H are specimen dimensions. The function f(a/H) is given by:
h i 2 pffiffiffiffiffiffiffiffiffi1:99 ða=HÞð1 a=HÞ 2:15 3:93ða=HÞ þ 2:7ða=HÞ f ða=HÞ ¼ 1:5 a=H : ð1 þ 2a=HÞð1 a=HÞ2
(5)
L. Marsavina, T. Sadowski / Polymer Testing 27 (2008) 941–944
180
Table 2 Instrumented impact test results
un-impregnated
160
epoxy impreg.
140
943
Specimen Maximum force [N]
polyester impreg.
Impact energy [J]
Force [N]
120 100 80 60 40 20 0 0
0.5
1
1.5
2
2.5
Displacement [mm] Fig. 2. Comparison of force–displacement curves for notched specimens (40 mm span).
Un-impregnated 1 129.2 127.1a 2 129.2 3 121.0 4 129.2 Polyester impregnated p1 164.1 165.1a p2 174.3 p3 160.0 p4 162.0 Epoxy impregnated e1 172.2 167.1a e2 164.1 e3 162.0 e4 170.2 a
3. Experimental results A comparison of the load–displacement curves obtained during instrumented impact testing for the notched, unimpregnated and impregnated foam specimens is shown in Fig. 2. The first peak in the force–time diagram is caused by the inertia of that part of the specimen which is accelerated after the initial contact with the striker. The peak force and duration of the inertial peak depends on the contact stiffness. This peak occurs after approximately 0.2 ms. After this peak, the maximum peak corresponds to maximum force need for breaking the specimen, and occurs after 0.3 ms. This force was used for determination of dynamic fracture toughness in Eq. (4). After this peak, the force decreases to zero. On some impregnated specimens, another peak appears which could be related to the breaking of the impregnation layer. The impact energy (calculated as the difference in the potential energy between the initial and final hammer positions) and the energy absorbed to fracture (calculated with Eq. (2) up to the fracture point) are approximately equal for un-impregnated and impregnated specimens. However, an increase in force of approximately 30% was
120
un-impregnated
Impact strength [kJ/m2]
0.774a
Dynamic fracture toughness [MPa m0.5]
0.102 0.091 0.092 0.102
0.097a 0.802 0.718 0.742 0.832
0.205 0.202a 0.206 0.193 0.205
0.109 0.106 0.112 0.117
0.111a
0.850 0.861a 0.820 0.864 0.909
0.256 0.270 0.248 0.253
0.108 0.121 0.101 0.112
0.110a
0.838 0.857a 0.939 0.783 0.868
0.266 0.256a 0.253 0.248 0.257
0.257a
Mean values.
obtained for impregnated specimens, Fig. 2, but the displacements to fracture were smaller, resulting in the same energy. The energy versus displacement for notched unimpregnated and impregnated specimens is shown in Fig. 3. Similar behaviour can be observed for impregnated and un-impregnated specimens, and the same amount of energy corresponds to the same displacement. Table 2 presents the experimental results for the maximum force, impact energy, impact strength and dynamic fracture toughness obtained in the instrumented impact tests. Similar values of maximum force, impact energy, impact strength and dynamic fracture toughness were obtained for both impregnations, and these values are higher compared with the un-impregnated foams by 30% for maximum force, 11% for impact strength and by 27% for dynamic fracture toughness. Comparing the dynamic fracture toughness with the static fracture toughness of rigid PUR (0.3–0.4 MPa m0.5, [1]) it can be observed that for the investigated foam with relative density 0.17 the dynamic fracture toughness is decreased to only 50–66%. The fractured specimens after instrumented impact tests are shown in Fig. 4. The fracture is fairly brittle and occurs in opening mode (Mode I).
epoxy impreg.
100
polyester impreg.
4. Validation of experimental data
W [J]
80 60 40 20 0 0
0.5
1
1.5
2
Displacement [mm] Fig. 3. Comparison of energy versus displacement curves.
2.5
A check for loses due to friction was performed prior to testing and it was found that the frictional loss was 0.059 J which represents 0.4% of the nominal energy of the pendulum 14.847 J. According to the standard [13] the energy loss due to friction should be less than 1%. Taking into account that the maximum ratio between the energy to fracture to nominal energy of the pendulum for foam specimens was approximately 0.013, the second term in Eq. (1) is practically negligible. From the displacement–time curves, the impact velocity was determined by the slope of the linear interpolated data and is in the limits v0 ¼ 3.8 0.2 m/s prescribed by Ref. [13]. The coefficient of
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L. Marsavina, T. Sadowski / Polymer Testing 27 (2008) 941–944
Further studies on the effect of foam density, loading rate and anisotropy in rise and transverse directions are necessary for fully characterising the behaviour of PUR foams under impact loading. Acknowledgments The authors acknowledge the support of Marie Curie Transfer of Knowledge project MTKD-CT-2004-014058, Research for Excellence Grant 202/2006 from Romanian National Authority for Scientific Research and Polish Ministry of Science and Higher Education – grant No. 65/ 6PR UE/2005–2008/7. References
Fig. 4. Fractured foam specimens after instrumented impact test.
determination R2 for the linear interpolation has a value of unity, which confirms that the second term in Eq. (1) has no influence. The load versus time record shows oscillatory behaviour. The frequency of the specimen fs can be evaluated measuring the period from the load–time record. This period is approximately 0.5–0.7 ms, which gives a frequency fs between 1.43 and 2 kHz. Taking into account that the natural frequency of the force-measurement system fn ¼ 1 MHz, the validity criteria: fn 3 fs is fulfilled. 5. Conclusions An instrumented impact test was used for determination of dynamic fracture toughness of PUR rigid foam. Even although the Charpy impact tests are considered not particularly suitable for testing cellular materials [12], the experimental results allow drawing some important conclusions about the effect of impregnation on the capacity to absorb energy of such materials. The mean values of the dynamic fracture toughness for un-impregnated specimens was 0.202 MPa m0.5 and 26% higher for the impregnated specimens.
[1] M.F. Gibson, L.J. Ashby, Cellular Solids, second ed. Cambridge University Press, 1997. [2] A. McIntyre, G.E. Anderson, Fracture properties of a rigid PUR foam over a range of densities, Polymer 20 (1979) 247–253. [3] M. Danielsson, Toughened rigid foam core material for use in sandwich construction, Cell. Polym. 15 (1996) 417–435. [4] G.M. Viana, L.A. Carlsson, Mechanical properties and fracture characterisation of cross-linked PVC foams, J. Sandw. Struct. Mater. 4 (2002) 99–113. [5] M.D. Kabir, M.C. Saha, S. Jeelani, Tnsile and fracture behavior of polymer foams, Mater. Sci. Eng. A 429 (2006) 225–235. [6] N.J. Mills, P. Kang, The effect of water immersion on the fracture toughness of polystirene bead foams, J. Cell. Plast. 30 (1994) 196–222. [7] N.J. Mills, Polymer Foams Handbook, Butterworth-Heinemann, 2007. [8] D. Apostol, M. Miron, D.M. Constantinescu, Experimental evaluation of the mechanical properties of foams used in sandwich composites, in: Proceedings of the 24th Danubia – Adria Symposium, Sibiu, 2007, pp. 35–36. [9] L. Marsavina, T. Sadowski, D.M. Constantinescu, R. Negru, Failure of polyurethane foams under different loading conditions, Key Eng. Mater. 385–387 (2008) 205–208. [10] L. Marsavina, T. Sadowski, R. Negru, M. Knec, Non-linear behaviour of foams under static and dynamic loading, in: Proceedings of the Euromech Colloquium 498, Kazimierz Dolny, 2008, pp. 35–36. [11] Divinycell H grade, technical manual, DIAB International AB, Laholm, 1999. [12] EN ISO 179–1:1999, Plastics – Determination of Charpy impact properties. Part 1: Non-instrumented impact test. [13] EN ISO 179–2:2000, Plastics – Determination of Charpy impact properties. Part 2: Instrumented impact test. [14] J.F. Kalthoff, Characterization of the dynamic failure behaviour of a glass-fiber/vinyl-seter at different temperatures by means of instrumented charpy impact testing, Comp. Part B 35 (2004) 657–663. [15] Y. Murakami, Stress Intensity Factors Handbook, vol. 2, Pergamon Press, Oxford, 1988, p. 1447.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Short Communication: Material Properties
Dynamic fracture toughness of polyurethane foam Liviu Marsavina a, b, Tomasz Sadowski a, * a b
Lublin University of Technology, Faculty of Civil and Sanitary Engineering, 20-618 Lublin, Nadbystrzycka 40 street, Poland POLITEHNICA University of Timisoara, Department Strength of Materials Blvd. M. Viteazu, Nr.1, Timisoara 300222, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2008 Accepted 12 August 2008
This paper is a first attempt to determine the dynamic fracture toughness of polyurethane foam and to study the effect of impregnation on the fracture toughness. Instrumented impact tests were performed using notched specimens. In order to study the effect of impregnation on the impact properties two different resins were used. The obtained results show that the impregnation increases the dynamic fracture toughness by 27%. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Polyurethane foam Impregnation layer Dynamic fracture toughness Instrumented impact test
1. Introduction Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing and cushioning. They are made of interconnected networks of solid struts and cell walls incorporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity [1]. Particularly for the design of high performance sandwich composites, a good knowledge of the behaviour of different grades of foam is necessary. Of particular interest is the fracture toughness of such foams because foam cracking weakens the structure’s capacity for carrying loads. Many efforts have been made in recent years to determine the fracture toughness of different types of foam in static and dynamic loading conditions. Micromechanical models and experimental investigations were used for estimating the fracture toughness. The first correlation between fracture toughness of PUR foams and density (<200 kg/m3) was proposed by McIntyre and Anderson [2] in a linear form. The same behaviour was observed by Danielsson [3] on PVC Divinycell foams and Viana and Carlsson on Diab H foams [4]. A correlation * Corresponding author. E-mail address: [email protected] (T. Sadowski). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.08.006
between the static fracture toughness and relative pffiffiffiffiffi density
r/rs was proposed in [1] in the form: KIc ¼ C sfs plðr=rs Þm , where sfs is the modulus of rupture of the cell wall material in bending, l is the cell dimension, C represents a constant of proportionality and m an exponent (equal with 3/2 for open cell). In Ref. [4], a value m ¼ 2 for closed cell foam is used. Kabir et al. [5] investigated the dynamic fracture toughness and found a maximum value of 2.74 MPa m0.5 for PVC foam with 260 kg/m3 density which is 3.75 times higher than the static fracture toughness of the same foam. They observed that fracture is brittle without yielding and is produced in Mode I. Mills and Kang [6] used a falling mass with a compact tension specimen in order to determine the dynamic fracture toughness, and Mills [7] also proposed a correlation between the dynamic fracture toughness and foam density. However, the dynamic fracture toughness for PUR foams has not been reported in the literature. On the other hand, the effect of impregnation on mechanical properties of foams was pointed out by Apostol et al. [8] but is less studied compared with the effect of density, strain rate or temperature. For the determination of the dynamic fracture toughness of polyurethane foam an instrumented impact test was considered. A 200 kg/m3 density rigid foam was used in the experimental program. The foam was impregnated
942
L. Marsavina, T. Sadowski / Polymer Testing 27 (2008) 941–944
Table 1 Comparison of static characteristics for polyurethane foams Foam type
Tested
Divinycell
Properties
Units
Un-impreg. Polyester Epoxy H200 impreg. impreg. [11]
Density Compressive strength Compressive modulus Tensile strength Tensile modulus Flexural strength Flexural modulus
kg/m3 MPa MPa MPa MPa MPa MPa
200 5.07 113 5.39 169.3 6.84 146
200 4.97 131.3 5.12 191.4 6.66 211.8
200 4.95 132.7 4.05 259.4 6.91 294.5
200 4.5 150 4.8 210 n.a. n.a.
with polyester and epoxy resins and a study of impregnation on the impact energy and fracture toughness was performed. Fig. 1. The microstructure of PUR foam.
2. Test methodology The investigated material is a 200 kg/m3 closed cell polyurethane (PUR) rigid foam. The foam faces were impregnated with epoxy (layer of 170 mm) and polyester (layer of 100 mm) resin in order to increase durability of the foam used for lightweight boats. The foam was manufactured and supplied in the form of flat panels of 12 mm thickness. The microscopic structure of the foam is presented in Fig. 1, and the characteristic dimensions are cell – wall size 200–500 mm and wall thickness 3–4 mm. The main mechanical characteristics of the investigated foam were determined experimentally [9,10] and are summarised in Table 1 side by side with the same characteristics of commercial Divinycell foam of the same density [11]. From Table 1 it can be observed that the impregnation layer has no effect on the tensile and flexural strength but has important influence on the tensile and flexural modulus [9]. The results of the impact test on this type of foam on unnotched specimens show that the impregnation layer decreases the energy absorbed to fracture [10]. Three point bend specimens (12 12 60 mm) were adopted with a notch of 1.5 mm (cut with a razor blade), impregnated with epoxy (0.170 mm layer thickness) and polyester (0.100 mm layer thickness) resin. A span of 40 mm was used for the test and the impact load was applied in the transverse direction. The principle of impact and instrumented impact tests of plastic materials are given in the EN ISO 179 [12,13] and, for example, by Kalthoff [14]. A KB Pruftechnik pendulum (Germany) was used for the instrumented impact tests, with the following main characteristics: pendulum mass 2.04 kg, pendulum length 0.386 m, drop height 0.742 m, drop angle 157.32 , pendulum energy 7.5 J, impact velocity 3.815 m/s. The tup has a built-in electronic sensor which allows recording the load with 1 MHz frequency. A four-channel data acquisition
A/D card (AdLink NuDAQ PCI-9812) was used for recording the load in time. Tests were performed at room temperature. The load history F(t) was recorded by using strain gages bonded near the striker edge. The displacement s(t) of the specimen during the test was calculated according to [13,14]:
sðtÞ ¼ v0 t
Lp g MH
Z
t
Z
0
t1
FðtÞdt dt1
½m
(1)
0
where t is the time after impact in which the deflection is calculated [s], Lp is the pendulum length [m], MH is the horizontal moment of the pendulum [Nm], F(t) is the force measured at time t after impact [N], g is the gravitational acceleration [m/s2]. The energy is given by:
W ¼
Z
s
FðsÞds ½ J
(2)
0
where s is the deflection [m] and F is the force in [N]. The Charpy impact strength for notched specimen acU is given by:
acU ¼
wf 1000 BHN
kJ=m2
(3)
where B is the width of the specimen and HN is the width remaining at the base of the notch in the specimen [mm]. The dynamic fracture toughness was calculated according to [15]:
KId ¼
Fmax S f ða=HÞ BH3=2
MPa m0:5
(4)
where Fmax is the maximum force, S is the span, and B and H are specimen dimensions. The function f(a/H) is given by:
h i 2 pffiffiffiffiffiffiffiffiffi1:99 ða=HÞð1 a=HÞ 2:15 3:93ða=HÞ þ 2:7ða=HÞ f ða=HÞ ¼ 1:5 a=H : ð1 þ 2a=HÞð1 a=HÞ2
(5)
L. Marsavina, T. Sadowski / Polymer Testing 27 (2008) 941–944
180
Table 2 Instrumented impact test results
un-impregnated
160
epoxy impreg.
140
943
Specimen Maximum force [N]
polyester impreg.
Impact energy [J]
Force [N]
120 100 80 60 40 20 0 0
0.5
1
1.5
2
2.5
Displacement [mm] Fig. 2. Comparison of force–displacement curves for notched specimens (40 mm span).
Un-impregnated 1 129.2 127.1a 2 129.2 3 121.0 4 129.2 Polyester impregnated p1 164.1 165.1a p2 174.3 p3 160.0 p4 162.0 Epoxy impregnated e1 172.2 167.1a e2 164.1 e3 162.0 e4 170.2 a
3. Experimental results A comparison of the load–displacement curves obtained during instrumented impact testing for the notched, unimpregnated and impregnated foam specimens is shown in Fig. 2. The first peak in the force–time diagram is caused by the inertia of that part of the specimen which is accelerated after the initial contact with the striker. The peak force and duration of the inertial peak depends on the contact stiffness. This peak occurs after approximately 0.2 ms. After this peak, the maximum peak corresponds to maximum force need for breaking the specimen, and occurs after 0.3 ms. This force was used for determination of dynamic fracture toughness in Eq. (4). After this peak, the force decreases to zero. On some impregnated specimens, another peak appears which could be related to the breaking of the impregnation layer. The impact energy (calculated as the difference in the potential energy between the initial and final hammer positions) and the energy absorbed to fracture (calculated with Eq. (2) up to the fracture point) are approximately equal for un-impregnated and impregnated specimens. However, an increase in force of approximately 30% was
120
un-impregnated
Impact strength [kJ/m2]
0.774a
Dynamic fracture toughness [MPa m0.5]
0.102 0.091 0.092 0.102
0.097a 0.802 0.718 0.742 0.832
0.205 0.202a 0.206 0.193 0.205
0.109 0.106 0.112 0.117
0.111a
0.850 0.861a 0.820 0.864 0.909
0.256 0.270 0.248 0.253
0.108 0.121 0.101 0.112
0.110a
0.838 0.857a 0.939 0.783 0.868
0.266 0.256a 0.253 0.248 0.257
0.257a
Mean values.
obtained for impregnated specimens, Fig. 2, but the displacements to fracture were smaller, resulting in the same energy. The energy versus displacement for notched unimpregnated and impregnated specimens is shown in Fig. 3. Similar behaviour can be observed for impregnated and un-impregnated specimens, and the same amount of energy corresponds to the same displacement. Table 2 presents the experimental results for the maximum force, impact energy, impact strength and dynamic fracture toughness obtained in the instrumented impact tests. Similar values of maximum force, impact energy, impact strength and dynamic fracture toughness were obtained for both impregnations, and these values are higher compared with the un-impregnated foams by 30% for maximum force, 11% for impact strength and by 27% for dynamic fracture toughness. Comparing the dynamic fracture toughness with the static fracture toughness of rigid PUR (0.3–0.4 MPa m0.5, [1]) it can be observed that for the investigated foam with relative density 0.17 the dynamic fracture toughness is decreased to only 50–66%. The fractured specimens after instrumented impact tests are shown in Fig. 4. The fracture is fairly brittle and occurs in opening mode (Mode I).
epoxy impreg.
100
polyester impreg.
4. Validation of experimental data
W [J]
80 60 40 20 0 0
0.5
1
1.5
2
Displacement [mm] Fig. 3. Comparison of energy versus displacement curves.
2.5
A check for loses due to friction was performed prior to testing and it was found that the frictional loss was 0.059 J which represents 0.4% of the nominal energy of the pendulum 14.847 J. According to the standard [13] the energy loss due to friction should be less than 1%. Taking into account that the maximum ratio between the energy to fracture to nominal energy of the pendulum for foam specimens was approximately 0.013, the second term in Eq. (1) is practically negligible. From the displacement–time curves, the impact velocity was determined by the slope of the linear interpolated data and is in the limits v0 ¼ 3.8 0.2 m/s prescribed by Ref. [13]. The coefficient of
944
L. Marsavina, T. Sadowski / Polymer Testing 27 (2008) 941–944
Further studies on the effect of foam density, loading rate and anisotropy in rise and transverse directions are necessary for fully characterising the behaviour of PUR foams under impact loading. Acknowledgments The authors acknowledge the support of Marie Curie Transfer of Knowledge project MTKD-CT-2004-014058, Research for Excellence Grant 202/2006 from Romanian National Authority for Scientific Research and Polish Ministry of Science and Higher Education – grant No. 65/ 6PR UE/2005–2008/7. References
Fig. 4. Fractured foam specimens after instrumented impact test.
determination R2 for the linear interpolation has a value of unity, which confirms that the second term in Eq. (1) has no influence. The load versus time record shows oscillatory behaviour. The frequency of the specimen fs can be evaluated measuring the period from the load–time record. This period is approximately 0.5–0.7 ms, which gives a frequency fs between 1.43 and 2 kHz. Taking into account that the natural frequency of the force-measurement system fn ¼ 1 MHz, the validity criteria: fn 3 fs is fulfilled. 5. Conclusions An instrumented impact test was used for determination of dynamic fracture toughness of PUR rigid foam. Even although the Charpy impact tests are considered not particularly suitable for testing cellular materials [12], the experimental results allow drawing some important conclusions about the effect of impregnation on the capacity to absorb energy of such materials. The mean values of the dynamic fracture toughness for un-impregnated specimens was 0.202 MPa m0.5 and 26% higher for the impregnated specimens.
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