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Effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposite
Materials Science and Engineering A 527 (2010) 2826–2830
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposite K.R. Rajeesh a , R. Gnanamoorthy b,∗ , R. Velmurugan c a b c
Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India Indian Institute of Information Technology, Design and Manufacturing (IIITD&M) Kancheepuram, IIT Madras Campus, Chennai 600036, India Department of Aerospace Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 9 June 2009 Received in revised form 17 December 2009 Accepted 22 January 2010
Keywords: Polymers Nanocomposites Indentation hardness Flexural fatigue
a b s t r a c t Humidity affects the mechanical properties of polymers and their composites. Understanding the influence of humidity on the strength, stiffness and fatigue characteristics will aid in better product design. The effect of relative humidity (RH) on indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposites is reported. Indentation hardness and indentation modulus of the material reduces up to ∼50% in the samples conditioned in water due to the plasticization and associated increased polymer chain mobility. Cantilever bending fatigue tests conducted at different relative humidity levels at constant displacement amplitude revealed increased fatigue life for polyamide 6 nanocomposites at high humidity. Hysteresis heating and molecular reorientation lowers the modulus during fatigue process and causes a reduction in the force amplitude at high humidity levels. The failure mechanisms at different humidity levels are discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer nanocomposites are promising for structural applications due to their high stiffness-to-weight ratio compared to pristine polymer. Exfoliated layered silicate polymer nanocomposites exhibit remarkable improvement in mechanical properties such as flame retardancy, gas barrier properties, ionic conductivity, thermal stability, tensile properties and tribological behavior compared to pristine polymer even at very low nanoclay content [1–4]. In many structural applications, components have to sustain not only various types of loads but also different types of environmental conditions. Temperature and humidity severely affect the mechanical properties of polymers and their composites [5–7]. Naval applications are examples where the components are exposed to extreme humidity levels, which will affect the chemical structure and there by the mechanical behavior of polymer and their composites [8,9]. It is essential to understand the longterm behavior of components made of these materials under severe operating conditions for better product design. The fatigue crack propagation behavior of nylon 66 containing different water content was reported by Bretz et al. [10]. At a constant stress intensity factor range, the fatigue crack propagation decreased as the water content was increased to 3%. The fatigue crack propagation rates at the moisture saturation level (8%) were
higher than those observed in dry specimen. An increase in water content in nylon causes an increase in the slope of crack length versus number of cycles [11]. Fatigue properties of two engineering rubbers in air and water have been studied by Selden [12]. In polychloroprene rubber, a factor of two to three times lower crack growth rates was observed in water compared to that in air. Water was found to act as a plasticizer in polyhydroxy ester ether and lowers the room temperature tensile strength and modulus [13]. The strain at failure increased with increasing water content and a change in the mode of failure was reported. Brittle failure occurred when the moisture content was low. As the absorbed water content increased, the samples necked and exhibited extensive plastic deformation. Scaffaro et al. [14] reported the effect of humidity, temperature and ultraviolet on creep behavior of polyamide 6. The flexural fatigue life of the polyamide 6 is enhanced by the addition of nanoclay [15]. This paper reports the effect of relative humidity (RH) on the flexural fatigue behavior of dry polyamide 6 nanocomposite samples. The effects of humidity on mechanical properties of polyamide 6 nanocomposite were studied using the microindentation technique. The failure mechanisms of polyamide 6 nanocomposite at different humidities are discussed. 2. Materials and experimental methods 2.1. Test materials and characterization
∗ Corresponding author. Tel.: +91 44 2257 4691; fax: +91 44 2257 4691. E-mail address: [email protected] (R. Gnanamoorthy). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.01.070
The nanocomposite used in the present study was prepared using melt compounding of commercial grade polyamide 6 gran-
K.R. Rajeesh et al. / Materials Science and Engineering A 527 (2010) 2826–2830
2827
Fig. 1. Injection molded flexural fatigue specimen used.
ules and hectorite clay (organically modified with a hydrogenated tallow quaternary amine complex) in a counter rotating twin-screw extruder. Prior to extrusion, the polyamide 6 pellets were dried at 333 K for 24 h. The required quantity of polyamide granules and nanoclay (5 wt.%) was fed into the extruder. The extrudate was cooled and then shredded into pellets. Prior to injection molding, all the pellets were dried in an oven at 333 K for 4 h. ASTM-D 671-71 Type-A specimens with a specimen thickness of 4 mm were used (Fig. 1). The geometry permits a uniform stress distribution. Molded specimens were conditioned in a vacuum chamber at 333 K for 24 h. X-ray diffraction studies were made to infer the exfoliation of clay in the polymer matrix [16]. The moisture content present in the dry sample was measured using a moisture analyzer by heating the sample for 15 min at 378 K. The dried samples contain approximately 0.1% of water. The differential scanning calorimetry (DSC) tests were performed at a heating rate of 10 K/min between 303 and 573 K to understand the effect of absorbed water. The tests were conducted on the dry sample as well as on the sample containing approximately 8% of water. Microindentation tests were carried out to understand the effect of humidity on the properties of polyamide 6 nanocomposite. Tests were conducted in a dynamic ultra-microhardness instrument using a triangular indenter with 115◦ tip angle. The schematic diagram of the indentation test is shown in Fig. 2a. The typical loading–unloading curve obtained from an instrumented indentation is explained in the Fig. 2b. The indentation hardness is estimated from the maximum depth reached by the indenter under specified force. The modulus can be obtained from the slope of the initial part of unloading curve using the inbuilt software. The dry polyamide 6 nanocomposite samples were kept in a chamber maintained at 35% RH, 60% RH, and 90% RH at 300 K, a few samples were immersed in water for 24 h before indentation testing. Indentation tests were conducted on the conditioned samples at room temperature. Five indentations per sample were performed and the average value is reported.
Fig. 2. (a) Schematic diagram showing the indentation parameters and (b) typical loading–unloading curve obtained from an instrumented indentation.
stant temperature of 300 K. At each test condition, at least three specimens were tested. The displacement is precisely controlled using a programmable logical controller. Every specimen was conditioned inside the chamber for 4 h at the test relative humidity before subjected to flexural testing. Specimens were subjected to alternate bending (R = −1) and all tests were conducted at a constant displacement amplitude of ±20 mm and a frequency of 2 Hz. The number of cycles which causes complete failure of the specimen was considered as the fatigue life of the sample. The force necessary to inflict the bending displacement with constant amplitude of ±20 mm was measured using a precision load cell. Load and displacement as a function of time were recorded using a data acquisition system and a personal computer.
2.2. Flexural fatigue test details Fatigue tests were conducted in a cantilever bending fatigue test rig developed in-house [17]. Fig. 3 shows the schematic of flexural fatigue testing. An environmental chamber which encloses the specimen unit is attached to the fatigue testing machine. A microprocessor connected with the environmental chamber precisely maintains the required temperature and humidity. The temperature and relative humidity conditions of the environmental chamber can be varied from 278 to 353 K and from 35% to 95% with an accuracy of ±1 K and ±3%, respectively. Flexural fatigue tests were conducted at different relative humidities and at a con-
Fig. 3. Schematic diagram of flexural fatigue test.
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K.R. Rajeesh et al. / Materials Science and Engineering A 527 (2010) 2826–2830 Table 1 Hardness and modulus of the test materials evaluated using indentation tests. Sample condition
Indentation hardness (N/mm2 )
Dry 35% RH 60% RH 90% RH Wet
76 69 53 44 38
± ± ± ± ±
3 2 2 1 2
Indentation modulus (N/mm2 ) 2461 2193 1415 1198 1182
± ± ± ± ±
80 100 50 22 30
Fig. 4. DSC data of dry and wet polyamide 6 nanocomposite.
3. Results and discussion 3.1. Material characterization It is well known that in many polymers the water absorption affects their properties [13,14,18]. The differential scanning calorimetry data of dry and wet polyamide 6 nanocomposite is shown in Fig. 4. The broad endothermic peak observed around 350 K in the case of wet samples is due to the evaporation of moisture absorbed by the sample. The melting temperature for the both dry and wet samples is nearly same. The initial part of the DSC data indicates that the water has been absorbed by polyamide 6 nanocomposite and during the heating process the water is evaporated. This indicates that the absorption of the water is a reversible process. When specimen deforms, the absorbed water can act as lubricant in between the molecular chains. Fig. 5 shows the typical single indentation loading–unloading behavior of the polyamide 6 nanocomposite sample conditioned at different humidity levels. The maximum depth reached by the indenter in order to attain a load of 500 mN is larger in samples conditioned at high relative humidity. High indentation depth implies low hardness of the material. The modulus of the samples was estimated from the slope of the unloading curve. The indentation modulus and indentation hardness of polyamide 6 nanocomposite are given in Table 1. Hardness and modulus of the material reduces with an increase in humidity indicating the softening due to water molecules that had entered into the samples. The hydrophilic sili-
Fig. 5. Loading–unloading curves obtained from microindentation experiments.
Fig. 6. Influence of relative humidity on the flexural fatigue life under constant displacement amplitude conditions.
cate layers are surface modified to make the clay organophilic using quaternary ammonium. Hence, the possibility for deterioration of the clay with humidity is less. In fiber-reinforced polymers, water molecules reduce the interfacial bonding between fiber and matrix [5,19,20]. There can be a similar effect on the interface adhesion between the polymer chain and the organoclay layers. 3.2. Flexural fatigue behavior Flexural fatigue lives of polyamide 6 nanocomposite at different relative humidities (35%, 60% and 90%) are shown in Fig. 6. Under displacement controlled flexural fatigue conditions, polyamide 6 nanocomposites exhibit higher fatigue life at high humidity levels. Fig. 7 shows the force amplitude variation experienced by the
Fig. 7. Force–amplitude variation plotted against number of cycles for polyamide 6 nanocomposite tested at different humidities at constant displacement amplitude.
K.R. Rajeesh et al. / Materials Science and Engineering A 527 (2010) 2826–2830
polyamide 6 nanocomposite samples with respect to the number of cycles measured during the fatigue tests, which were conducted at different relative humidities. At all humidity levels, the force–amplitude curve shows a slope that decreases from initial value and reaches a stable value. The resistance of the sample decreases due to the reduction in the modulus. The reduction in modulus may occur either due to the molecular reorientation of the frozen molecules by the external stresses or from the hysteresis heating associated with the fatigue process [16]. The fatigue cracks initiated propagate and result in further drastic reduction of the force amplitude in the final stage. Humidity affects the force amplitude experienced by the samples. Although the samples were dried before the tests, it is observed that the force amplitude experienced during the initial stages decreased with increasing relative humidity of the surroundings. The surface of the sample absorbs moisture from the surrounding environment and reduces the surface hardness and modulus. Under flexural condition, the maximum resistance to deformation is provided by the outer portion, so that the force required to impose the same displacement varies in the initial stages itself with surrounding humidity level. The number of cycles experienced by the sample to reach a stable force amplitude increases with humidity. This can be attributed to the variation in hysteresis heating due to moisture. The internal frictional heating is reduced due to the presence of water particles between the polymer chains. At high humidity levels, the amorphous region in the polyamide 6 nanocomposite absorbs more water from the surrounding environment and plasticizes the molecular chains [14,21]. Plasticization leads to increased chain mobility in the amorphous region of the polyamide 6 nanocomposite and reduces the stiffness. The force required to impose the same displacement is less in the case of the plasticized polyamide 6 nanocomposite compared to a dry specimen. High force amplitude indicates high stress levels experienced by the specimen and reduces the fatigue life. In the present study, the life quantified is the total life, which includes the crack initiation component and the crack propagation component. Moisture content in the material can delay the crack initiation, and also causes blunting of the crack tip. The crack tip blunting slows down the crack propagation speed. 3.3. Failure mechanism Observation of failure surfaces revealed the significant effect of humidity on the fracture surface morphology (Fig. 8). Cracks were initiated at both edges of the sample, which propagated towards
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Fig. 8. Macroscopic flexural fatigue fracture surface of the samples tested at different moisture levels. (a) 35% RH, (b) 60% RH and (c) 90% RH (area inside the box shows the dominant fast fracture zone and outside the dotted line shows the controlled crack growth region).
central region because the maximum stress level occurs on the outer layer in a bending specimen. It is well known that the slow crack propagation area have smooth surface and fast crack propagation area have relatively rough surface [22]. The fracture surface of the sample tested at 35% relative humidity revealed a dominant fast fracture region covering ∼60% of the overall fracture surface. The fast fracture region shows a rugged topology with ripples. However, in the case of samples tested at 60% relative humidity the fast propagation region is less (∼40% of total area) compared to the sample tested at 35% relative humidity. A dominant flat and smooth fractured surface is observed on the samples tested at 90% rela-
Fig. 9. Schematic representation of crack propagation showing the molecular orientation occurring at (a) low and (b) high humidities.
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ular chain mobility of polyamide 6 nanocomposite is due to the plasticization effect of water and/or reduced interfacial strength between the nanoparticles and polymer chain. The central region reveals the fast fracture because the absorbed water at the surface might not have reached the inner core and it is insufficient to improve the chain mobility. The failure mechanism in polymeric material at different humidities is shown schematically in Fig. 9. Fig. 10 shows the scanning electron micrograph of the fatigue fracture surfaces near the crack initiation region in samples tested at different relative humidities. The sample tested at 35% RH (Fig. 10a) shows ripples perpendicular to the direction of crack propagation. Discontinuous growth bands and less stress whitening regions were observed in the samples tested at 60% RH (Fig. 10b). In the samples tested at 90% RH (Fig. 10c) prominent stress whitening and a few discontinuous growth bands were observed, due to increased plastic deformation. The occurrence of whiter region at higher humidity level reflects significant plastic deformation in these samples [21]. Humidity increases the chain mobility in polymers and thereby causes severe plastic deformation at the crack tip. 4. Conclusion The effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 clay nanocomposites is investigated. The hardness and modulus of the polyamide 6 nanocomposite is reduced in humid atmospheres. Under displacement controlled condition, flexural fatigue life of polyamide 6 nanocomposite is increased with humidity. The increase in the surrounding humidity and absorption of water reduce the hardness and modulus, and improve the flexural fatigue life. Acknowledgement The authors would like to thank the Naval Research Board for financial support provided for this research project. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 10. Scanning electron micrographs of fracture surfaces near the crack initiation site in the sample tested at (a) 35% RH, (b) fatigue striations and plastic deformation found at 60% RH and (c) severe plastic deformation and stress whitening observed at 90% RH (arrow indicates the direction of crack propagation).
tive humidity. When the surrounding humidity level increases, the area of the smooth region also increases, indicating that at higher humidity levels the material absorbs more moisture from the atmosphere. When the crack propagates, the molecular chains in front of the crack tip deform and blunt the crack tip as the absorbed water particle increases the chain mobility. The increase in the molec-
[16] [17]
[18] [19] [20] [21] [22]
D.R. Paul, L.M. Robeson, Polymer 49 (2008) 3187–3204. G. Srinath, R. Gnanamoorthy, J. Mater. Sci. 40 (2005) 2897–2901. G. Srinath, R. Gnanamoorthy, Mater. Sci. Eng. A 435–436 (2006) 181–186. G. Srinath, R. Gnanamoorthy, J. Mater. Sci. 42 (2007) 8326–8333. J.R.M. d’Almeida, R.C. de Almeida, W.R. de Lima, Compos. Struct. 83 (2008) 221–225. D.P.N. Vlasveld, S.G. Vaidya, H.E.N. Bersee, S.J. Picken, Polymer 46 (2005) 3452–3461. U.S. Ishiaku, H. Hamada, M. Mizoguchi, W.S. Chow, Z.A. Mohd Ishak, Polym. Compos. (2005) 52–59. G. Srinath, R. Gnanamoorthy, Compos. Sci. Technol. 67 (2007) 399–405. J.L. Willett, W.M. Doane, Polymer 43 (2002) 4413–4420. P.E. Bretz, R.W. Hertzberg, J.A. Manson, J. Mater. Sci. 14 (1979) 2482–2492. H.A. El-Hakeem, L.E. Culver, Int. J. Fatigue 1 (3) (1979) 133–140. R. Selden, J Appl. Polym. Sci. 69 (1998) 941–946. S. St Lawrence, J.L. Willett, C.J. Carriere, Polymer 42 (2001) 5643–5650. R. Scaffaro, N. Tzankova Dintcheva, F.P. La Mantia, Polym. Test. 27 (2008) 49– 54. K.R. Rajeesh, R. Gnanamoorthy, R. Velmurugan, Proceedings of the 2nd International Symposium on Advanced Materials and Polymers for Aerospace and Defense Applications (SAMPADA), Pune, India, December, 2008, Paper ID: PSS21-OR-1. A. Ramkumar, R. Gnanamoorthy, Compos. Sci. Technol. 68 (2008) 3401– 3405. A. Ramkumar, R. Gnanamoorthy, High temperature flexural fatigue behavior of polyamide 6 nanocomposites, in: IISc Centenary—International Conference on Advances in Mechanical Engineering (IC-ICAME), IISc Bangalore, July, 2008. W. Gottfried, Ehrenstein, Polymeric Materials, Carl Hanser Verlag, Munich, 2001. A. Hodzic, J.K. Kim, A.E. Lowe, Z.H. Stachurski, Compos. Sci. Technol. 64 (2004) 2185–2195. R. Selzer, K. Friedrich, Compos. A 28A (1997) 595–604. E. Philip Bretz, W. Richard, Hertzberg, A. John, Manson, J. Mater. Sci. 16 (1981) 2070–2078. C.C. Chen, J. Shent, J.A. Sauer, Polymer 163 (1985) 89–96.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposite K.R. Rajeesh a , R. Gnanamoorthy b,∗ , R. Velmurugan c a b c
Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India Indian Institute of Information Technology, Design and Manufacturing (IIITD&M) Kancheepuram, IIT Madras Campus, Chennai 600036, India Department of Aerospace Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 9 June 2009 Received in revised form 17 December 2009 Accepted 22 January 2010
Keywords: Polymers Nanocomposites Indentation hardness Flexural fatigue
a b s t r a c t Humidity affects the mechanical properties of polymers and their composites. Understanding the influence of humidity on the strength, stiffness and fatigue characteristics will aid in better product design. The effect of relative humidity (RH) on indentation hardness and flexural fatigue behavior of polyamide 6 nanocomposites is reported. Indentation hardness and indentation modulus of the material reduces up to ∼50% in the samples conditioned in water due to the plasticization and associated increased polymer chain mobility. Cantilever bending fatigue tests conducted at different relative humidity levels at constant displacement amplitude revealed increased fatigue life for polyamide 6 nanocomposites at high humidity. Hysteresis heating and molecular reorientation lowers the modulus during fatigue process and causes a reduction in the force amplitude at high humidity levels. The failure mechanisms at different humidity levels are discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polymer nanocomposites are promising for structural applications due to their high stiffness-to-weight ratio compared to pristine polymer. Exfoliated layered silicate polymer nanocomposites exhibit remarkable improvement in mechanical properties such as flame retardancy, gas barrier properties, ionic conductivity, thermal stability, tensile properties and tribological behavior compared to pristine polymer even at very low nanoclay content [1–4]. In many structural applications, components have to sustain not only various types of loads but also different types of environmental conditions. Temperature and humidity severely affect the mechanical properties of polymers and their composites [5–7]. Naval applications are examples where the components are exposed to extreme humidity levels, which will affect the chemical structure and there by the mechanical behavior of polymer and their composites [8,9]. It is essential to understand the longterm behavior of components made of these materials under severe operating conditions for better product design. The fatigue crack propagation behavior of nylon 66 containing different water content was reported by Bretz et al. [10]. At a constant stress intensity factor range, the fatigue crack propagation decreased as the water content was increased to 3%. The fatigue crack propagation rates at the moisture saturation level (8%) were
higher than those observed in dry specimen. An increase in water content in nylon causes an increase in the slope of crack length versus number of cycles [11]. Fatigue properties of two engineering rubbers in air and water have been studied by Selden [12]. In polychloroprene rubber, a factor of two to three times lower crack growth rates was observed in water compared to that in air. Water was found to act as a plasticizer in polyhydroxy ester ether and lowers the room temperature tensile strength and modulus [13]. The strain at failure increased with increasing water content and a change in the mode of failure was reported. Brittle failure occurred when the moisture content was low. As the absorbed water content increased, the samples necked and exhibited extensive plastic deformation. Scaffaro et al. [14] reported the effect of humidity, temperature and ultraviolet on creep behavior of polyamide 6. The flexural fatigue life of the polyamide 6 is enhanced by the addition of nanoclay [15]. This paper reports the effect of relative humidity (RH) on the flexural fatigue behavior of dry polyamide 6 nanocomposite samples. The effects of humidity on mechanical properties of polyamide 6 nanocomposite were studied using the microindentation technique. The failure mechanisms of polyamide 6 nanocomposite at different humidities are discussed. 2. Materials and experimental methods 2.1. Test materials and characterization
∗ Corresponding author. Tel.: +91 44 2257 4691; fax: +91 44 2257 4691. E-mail address: [email protected] (R. Gnanamoorthy). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.01.070
The nanocomposite used in the present study was prepared using melt compounding of commercial grade polyamide 6 gran-
K.R. Rajeesh et al. / Materials Science and Engineering A 527 (2010) 2826–2830
2827
Fig. 1. Injection molded flexural fatigue specimen used.
ules and hectorite clay (organically modified with a hydrogenated tallow quaternary amine complex) in a counter rotating twin-screw extruder. Prior to extrusion, the polyamide 6 pellets were dried at 333 K for 24 h. The required quantity of polyamide granules and nanoclay (5 wt.%) was fed into the extruder. The extrudate was cooled and then shredded into pellets. Prior to injection molding, all the pellets were dried in an oven at 333 K for 4 h. ASTM-D 671-71 Type-A specimens with a specimen thickness of 4 mm were used (Fig. 1). The geometry permits a uniform stress distribution. Molded specimens were conditioned in a vacuum chamber at 333 K for 24 h. X-ray diffraction studies were made to infer the exfoliation of clay in the polymer matrix [16]. The moisture content present in the dry sample was measured using a moisture analyzer by heating the sample for 15 min at 378 K. The dried samples contain approximately 0.1% of water. The differential scanning calorimetry (DSC) tests were performed at a heating rate of 10 K/min between 303 and 573 K to understand the effect of absorbed water. The tests were conducted on the dry sample as well as on the sample containing approximately 8% of water. Microindentation tests were carried out to understand the effect of humidity on the properties of polyamide 6 nanocomposite. Tests were conducted in a dynamic ultra-microhardness instrument using a triangular indenter with 115◦ tip angle. The schematic diagram of the indentation test is shown in Fig. 2a. The typical loading–unloading curve obtained from an instrumented indentation is explained in the Fig. 2b. The indentation hardness is estimated from the maximum depth reached by the indenter under specified force. The modulus can be obtained from the slope of the initial part of unloading curve using the inbuilt software. The dry polyamide 6 nanocomposite samples were kept in a chamber maintained at 35% RH, 60% RH, and 90% RH at 300 K, a few samples were immersed in water for 24 h before indentation testing. Indentation tests were conducted on the conditioned samples at room temperature. Five indentations per sample were performed and the average value is reported.
Fig. 2. (a) Schematic diagram showing the indentation parameters and (b) typical loading–unloading curve obtained from an instrumented indentation.
stant temperature of 300 K. At each test condition, at least three specimens were tested. The displacement is precisely controlled using a programmable logical controller. Every specimen was conditioned inside the chamber for 4 h at the test relative humidity before subjected to flexural testing. Specimens were subjected to alternate bending (R = −1) and all tests were conducted at a constant displacement amplitude of ±20 mm and a frequency of 2 Hz. The number of cycles which causes complete failure of the specimen was considered as the fatigue life of the sample. The force necessary to inflict the bending displacement with constant amplitude of ±20 mm was measured using a precision load cell. Load and displacement as a function of time were recorded using a data acquisition system and a personal computer.
2.2. Flexural fatigue test details Fatigue tests were conducted in a cantilever bending fatigue test rig developed in-house [17]. Fig. 3 shows the schematic of flexural fatigue testing. An environmental chamber which encloses the specimen unit is attached to the fatigue testing machine. A microprocessor connected with the environmental chamber precisely maintains the required temperature and humidity. The temperature and relative humidity conditions of the environmental chamber can be varied from 278 to 353 K and from 35% to 95% with an accuracy of ±1 K and ±3%, respectively. Flexural fatigue tests were conducted at different relative humidities and at a con-
Fig. 3. Schematic diagram of flexural fatigue test.
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K.R. Rajeesh et al. / Materials Science and Engineering A 527 (2010) 2826–2830 Table 1 Hardness and modulus of the test materials evaluated using indentation tests. Sample condition
Indentation hardness (N/mm2 )
Dry 35% RH 60% RH 90% RH Wet
76 69 53 44 38
± ± ± ± ±
3 2 2 1 2
Indentation modulus (N/mm2 ) 2461 2193 1415 1198 1182
± ± ± ± ±
80 100 50 22 30
Fig. 4. DSC data of dry and wet polyamide 6 nanocomposite.
3. Results and discussion 3.1. Material characterization It is well known that in many polymers the water absorption affects their properties [13,14,18]. The differential scanning calorimetry data of dry and wet polyamide 6 nanocomposite is shown in Fig. 4. The broad endothermic peak observed around 350 K in the case of wet samples is due to the evaporation of moisture absorbed by the sample. The melting temperature for the both dry and wet samples is nearly same. The initial part of the DSC data indicates that the water has been absorbed by polyamide 6 nanocomposite and during the heating process the water is evaporated. This indicates that the absorption of the water is a reversible process. When specimen deforms, the absorbed water can act as lubricant in between the molecular chains. Fig. 5 shows the typical single indentation loading–unloading behavior of the polyamide 6 nanocomposite sample conditioned at different humidity levels. The maximum depth reached by the indenter in order to attain a load of 500 mN is larger in samples conditioned at high relative humidity. High indentation depth implies low hardness of the material. The modulus of the samples was estimated from the slope of the unloading curve. The indentation modulus and indentation hardness of polyamide 6 nanocomposite are given in Table 1. Hardness and modulus of the material reduces with an increase in humidity indicating the softening due to water molecules that had entered into the samples. The hydrophilic sili-
Fig. 5. Loading–unloading curves obtained from microindentation experiments.
Fig. 6. Influence of relative humidity on the flexural fatigue life under constant displacement amplitude conditions.
cate layers are surface modified to make the clay organophilic using quaternary ammonium. Hence, the possibility for deterioration of the clay with humidity is less. In fiber-reinforced polymers, water molecules reduce the interfacial bonding between fiber and matrix [5,19,20]. There can be a similar effect on the interface adhesion between the polymer chain and the organoclay layers. 3.2. Flexural fatigue behavior Flexural fatigue lives of polyamide 6 nanocomposite at different relative humidities (35%, 60% and 90%) are shown in Fig. 6. Under displacement controlled flexural fatigue conditions, polyamide 6 nanocomposites exhibit higher fatigue life at high humidity levels. Fig. 7 shows the force amplitude variation experienced by the
Fig. 7. Force–amplitude variation plotted against number of cycles for polyamide 6 nanocomposite tested at different humidities at constant displacement amplitude.
K.R. Rajeesh et al. / Materials Science and Engineering A 527 (2010) 2826–2830
polyamide 6 nanocomposite samples with respect to the number of cycles measured during the fatigue tests, which were conducted at different relative humidities. At all humidity levels, the force–amplitude curve shows a slope that decreases from initial value and reaches a stable value. The resistance of the sample decreases due to the reduction in the modulus. The reduction in modulus may occur either due to the molecular reorientation of the frozen molecules by the external stresses or from the hysteresis heating associated with the fatigue process [16]. The fatigue cracks initiated propagate and result in further drastic reduction of the force amplitude in the final stage. Humidity affects the force amplitude experienced by the samples. Although the samples were dried before the tests, it is observed that the force amplitude experienced during the initial stages decreased with increasing relative humidity of the surroundings. The surface of the sample absorbs moisture from the surrounding environment and reduces the surface hardness and modulus. Under flexural condition, the maximum resistance to deformation is provided by the outer portion, so that the force required to impose the same displacement varies in the initial stages itself with surrounding humidity level. The number of cycles experienced by the sample to reach a stable force amplitude increases with humidity. This can be attributed to the variation in hysteresis heating due to moisture. The internal frictional heating is reduced due to the presence of water particles between the polymer chains. At high humidity levels, the amorphous region in the polyamide 6 nanocomposite absorbs more water from the surrounding environment and plasticizes the molecular chains [14,21]. Plasticization leads to increased chain mobility in the amorphous region of the polyamide 6 nanocomposite and reduces the stiffness. The force required to impose the same displacement is less in the case of the plasticized polyamide 6 nanocomposite compared to a dry specimen. High force amplitude indicates high stress levels experienced by the specimen and reduces the fatigue life. In the present study, the life quantified is the total life, which includes the crack initiation component and the crack propagation component. Moisture content in the material can delay the crack initiation, and also causes blunting of the crack tip. The crack tip blunting slows down the crack propagation speed. 3.3. Failure mechanism Observation of failure surfaces revealed the significant effect of humidity on the fracture surface morphology (Fig. 8). Cracks were initiated at both edges of the sample, which propagated towards
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Fig. 8. Macroscopic flexural fatigue fracture surface of the samples tested at different moisture levels. (a) 35% RH, (b) 60% RH and (c) 90% RH (area inside the box shows the dominant fast fracture zone and outside the dotted line shows the controlled crack growth region).
central region because the maximum stress level occurs on the outer layer in a bending specimen. It is well known that the slow crack propagation area have smooth surface and fast crack propagation area have relatively rough surface [22]. The fracture surface of the sample tested at 35% relative humidity revealed a dominant fast fracture region covering ∼60% of the overall fracture surface. The fast fracture region shows a rugged topology with ripples. However, in the case of samples tested at 60% relative humidity the fast propagation region is less (∼40% of total area) compared to the sample tested at 35% relative humidity. A dominant flat and smooth fractured surface is observed on the samples tested at 90% rela-
Fig. 9. Schematic representation of crack propagation showing the molecular orientation occurring at (a) low and (b) high humidities.
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ular chain mobility of polyamide 6 nanocomposite is due to the plasticization effect of water and/or reduced interfacial strength between the nanoparticles and polymer chain. The central region reveals the fast fracture because the absorbed water at the surface might not have reached the inner core and it is insufficient to improve the chain mobility. The failure mechanism in polymeric material at different humidities is shown schematically in Fig. 9. Fig. 10 shows the scanning electron micrograph of the fatigue fracture surfaces near the crack initiation region in samples tested at different relative humidities. The sample tested at 35% RH (Fig. 10a) shows ripples perpendicular to the direction of crack propagation. Discontinuous growth bands and less stress whitening regions were observed in the samples tested at 60% RH (Fig. 10b). In the samples tested at 90% RH (Fig. 10c) prominent stress whitening and a few discontinuous growth bands were observed, due to increased plastic deformation. The occurrence of whiter region at higher humidity level reflects significant plastic deformation in these samples [21]. Humidity increases the chain mobility in polymers and thereby causes severe plastic deformation at the crack tip. 4. Conclusion The effect of humidity on the indentation hardness and flexural fatigue behavior of polyamide 6 clay nanocomposites is investigated. The hardness and modulus of the polyamide 6 nanocomposite is reduced in humid atmospheres. Under displacement controlled condition, flexural fatigue life of polyamide 6 nanocomposite is increased with humidity. The increase in the surrounding humidity and absorption of water reduce the hardness and modulus, and improve the flexural fatigue life. Acknowledgement The authors would like to thank the Naval Research Board for financial support provided for this research project. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 10. Scanning electron micrographs of fracture surfaces near the crack initiation site in the sample tested at (a) 35% RH, (b) fatigue striations and plastic deformation found at 60% RH and (c) severe plastic deformation and stress whitening observed at 90% RH (arrow indicates the direction of crack propagation).
tive humidity. When the surrounding humidity level increases, the area of the smooth region also increases, indicating that at higher humidity levels the material absorbs more moisture from the atmosphere. When the crack propagates, the molecular chains in front of the crack tip deform and blunt the crack tip as the absorbed water particle increases the chain mobility. The increase in the molec-
[16] [17]
[18] [19] [20] [21] [22]
D.R. Paul, L.M. Robeson, Polymer 49 (2008) 3187–3204. G. Srinath, R. Gnanamoorthy, J. Mater. Sci. 40 (2005) 2897–2901. G. Srinath, R. Gnanamoorthy, Mater. Sci. Eng. A 435–436 (2006) 181–186. G. Srinath, R. Gnanamoorthy, J. Mater. Sci. 42 (2007) 8326–8333. J.R.M. d’Almeida, R.C. de Almeida, W.R. de Lima, Compos. Struct. 83 (2008) 221–225. D.P.N. Vlasveld, S.G. Vaidya, H.E.N. Bersee, S.J. Picken, Polymer 46 (2005) 3452–3461. U.S. Ishiaku, H. Hamada, M. Mizoguchi, W.S. Chow, Z.A. Mohd Ishak, Polym. Compos. (2005) 52–59. G. Srinath, R. Gnanamoorthy, Compos. Sci. Technol. 67 (2007) 399–405. J.L. Willett, W.M. Doane, Polymer 43 (2002) 4413–4420. P.E. Bretz, R.W. Hertzberg, J.A. Manson, J. Mater. Sci. 14 (1979) 2482–2492. H.A. El-Hakeem, L.E. Culver, Int. J. Fatigue 1 (3) (1979) 133–140. R. Selden, J Appl. Polym. Sci. 69 (1998) 941–946. S. St Lawrence, J.L. Willett, C.J. Carriere, Polymer 42 (2001) 5643–5650. R. Scaffaro, N. Tzankova Dintcheva, F.P. La Mantia, Polym. Test. 27 (2008) 49– 54. K.R. Rajeesh, R. Gnanamoorthy, R. Velmurugan, Proceedings of the 2nd International Symposium on Advanced Materials and Polymers for Aerospace and Defense Applications (SAMPADA), Pune, India, December, 2008, Paper ID: PSS21-OR-1. A. Ramkumar, R. Gnanamoorthy, Compos. Sci. Technol. 68 (2008) 3401– 3405. A. Ramkumar, R. Gnanamoorthy, High temperature flexural fatigue behavior of polyamide 6 nanocomposites, in: IISc Centenary—International Conference on Advances in Mechanical Engineering (IC-ICAME), IISc Bangalore, July, 2008. W. Gottfried, Ehrenstein, Polymeric Materials, Carl Hanser Verlag, Munich, 2001. A. Hodzic, J.K. Kim, A.E. Lowe, Z.H. Stachurski, Compos. Sci. Technol. 64 (2004) 2185–2195. R. Selzer, K. Friedrich, Compos. A 28A (1997) 595–604. E. Philip Bretz, W. Richard, Hertzberg, A. John, Manson, J. Mater. Sci. 16 (1981) 2070–2078. C.C. Chen, J. Shent, J.A. Sauer, Polymer 163 (1985) 89–96.