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The essential work of fracture of polyamide 66 filled with TiO2 nanoparticles
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 65 (2005) 2374–2379 www.elsevier.com/locate/compscitech
The essential work of fracture of polyamide 66 filled with TiO2 nanoparticles Jing-Lei Yang a, Zhong Zhang a
a,b,* ,
Hui Zhang
a
Institute for Composite Materials, University of Kaiserslautern, 67663 Kaiserslautern, Germany b National Center for Nanoscience and Technology of China, 100080 Beijing, China Received 3 June 2005; accepted 7 June 2005 Available online 8 August 2005
Abstract The fracture property of 21-nm TiO2-nanoparticle filled polyamide 66 was studied based on the essential work of fracture method. An energy-partitioned work of fracture approach was introduced, in which the resistance to crack initiation, wini, and the resistance to crack propagation, wprop, were applied. Double-edge-notched-tension specimens with different original ligament lengths were tested at a constant cross-head speed. The results showed that the essential work term of composites filled with low nanoparticle concentration notably increased, while the plastic work fairly decreased compared to that of neat matrix at room temperature. Fractography analysis suggests a three-stage evolution of crack initiation. The individual nanoparticles acted as stress concentration points, which promoted cavitations and thus induced relatively large local deformation. Thereafter, the tiny cavitations coalesced into sub-micro ones and rapidly grow into micro-voids and crack initiation due to the high-level stress concentration. The plastic work of composites was decreased with increasing nanoparticle fractions, which was due to unavoidably aggregated nanoparticles leading to high level stress concentration that favouring the crack propagation. 2005 Elsevier Ltd. All rights reserved. Keywords: Essential work of fracture; Thermoplastics; Spherical nanoparticles; Nanocomposites; Toughening mechanisms
1. Introduction Polyamide 66 (PA66), i.e. Nylon 66, is widely used in many engineering applications due to its excellent mechanical performances and other advantages. Studies of introducing additional phases, e.g., inorganic fillers, have been successfully developed and thus extended its applications in the past decades [1]. Effects of fillers on the mechanical performances of composites strongly depend on their property, shape, dimension, size and aggregate degree, surface characteristics and concentration. Studies have shown that mechanical properties of * Corresponding author. Tel.: +49 631 2017213; fax: +49 631 2017196. E-mail addresses: [email protected], zhong.zhang@ nanoctr.cn (Z. Zhang).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.06.008
polymer composites filled with smaller particles are superior to those with larger ones at micron level, which were deeply investigated in rubber particle toughened PA [2,3]. In recent years, great efforts devoted to the development of nanoparticles filled polymer composites have made it possible to investigate the effect on the mechanical properties of these nanocomposites [4–6]. A major specific feature of nanocomposite materials is their huge interfacial area between filler and matrix, which can dramatically reach up to 1000 m2/g filler and is considered as a key factor to be able to intensely improve mechanical properties of a polymer matrix. Due to this reason, layered clays, with fairly large aspect ratios, have been widely researched in recent years [7]. Although many attractive enhancements were obtained in physical and mechanical performances of polymer/ clay nanocomposites, a major disadvantage of the
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379
reduction of fracture toughness has greatly restricted their engineering applications. Consequently, various kinds of spherical nanoparticles, such as silica, titanium oxide and calcium carbonate, have been attempted for preparing nanocomposites [8–10]. However, study on fracture toughness of polymer nanocomposites was scarcely found in the literatures [10–12]. The method of essential work of fracture was originally proposed by Broberg [13] and thereafter successfully developed toward a standard method by some other researchers [14–16] in order to study the fracture behaviour of a wide range of polymers and composites. Double edge notched tension (DENT) specimens were used for the tensile test, as schematically shown in Fig. 1. Typical load vs. elongation curve was showed in Fig. 1 as well. At the maximum load in tensile test, the material was fully yielded and crack initiated, and then crack propagated with advanced material necking and tearing until the specimen totally fractured. Consequently, Karger-Kocsis [17], Hashemi and Williams [18] introduced a method of energy-partitioned work of fracture, which divided the total energy dissipated into contributions from crack initiation (yielding) and crack propagation (advanced necking/tearing and specimen separating). As schematically depicted in Fig. 1, the specific work of resistance to crack initiation (wini) and to crack propagation (wprop) can be expressed as wini ¼ we;ini þ bini wp;ini l; wprop ¼ we;prop þ bprop wp;prop l;
ð1Þ ð2Þ
where we, ini and we, prop are the specific essential work of resistance to crack initiation and propagation, respectively. wp, ini and wp, prop are the volumetric energy dissipated during yielding/crack-initiating and crack
Fig. 1. Schematic diagram of energy partitioned EWF method. (a) Dimensions of DENT specimen, in this study W = 39 mm, Zt = 80 mm, t = 2 mm, and u = 30 mm; (b) typical load–elongation curve; and (c) fracture work of resistance to crack initiation (wini) as a function of ligament length (l).
2375
propagating, respectively. bini and bprop are geometry factors related to the shape of the plastic zone during crack initiating and propagating stages, respectively. In the present study, the fracture property of PA66 and its nanocomposites filled with 1 and 3 vol% 21-nm TiO2 nanoparticles was studied by using the method of energy partitioned work of fracture with DENT specimens produced by injection moulding. The validity of essential work of fracture (EWF) method was discussed. Fractography analysis of fractured specimens was provided. The effect of nanoparticle on the fracture behaviour and the toughening mechanisms in nanocomposites were discussed.
2. Experimental 2.1. Materials preparation A commercial Polyamide 66 (DuPont, Zytel 101) was considered as the matrix material in this study. TiO2 nanoparticles (Degussa P25) were applied as nano-fillers with a density of 4 g/cm3 and a mean diameter of 21 nm. The volume contents were in the range of 1–3%, respectively. Before mixing, PA66 and nanoparticles were dried in an oven at 70 C for about 72 h. Compounding of matrix and nanoparticles was carried out in a corotating twin-screw-extruder (Berstoff ZE 25A Æ 44D-UTS). The barrel temperatures were set at 55/260/270/280/285/ 285/285/285/285 (in C), respectively, and the screw speed 150 rpm. During melt extrusion, ventilation was kept on to remove trapped air in blends. After mixing, the blend granules were injection-molded into 2-mm thick plates for tensile tests by using an injection-molding machine (Alburg Allrounder 320S). For all blends, the injection parameters were maintained constant. The barrel temperature ranged from 280 to 295 C and the mold temperature was kept constantly at 70 C. The injection pressure and speed were 500 bar and 80.0 cm/s, respectively. Specimens were then prepared according to the ESIS protocol [19]. Three series of samples, namely PA66 or PA, 1TPA (1 vol% TiO2/PA66) and 3TPA (3 vol% TiO2/PA66), were cut as 80 · 40 · 2 mm3 with different initial ligaments by using a band saw. Specimens were dried in a vacuum oven at 75 C for 3.5 h before testing, and then placed in a drying chamber under 25% relative humidity and room temperature. Before testing, a fresh razor blade was carefully tapped into ligament of specimen to introduce two aligned sharp pre-cracks. The free ligament length was left in the range from 6 to 20 mm and measured with a light microscope. 2.2. Mechanical testing All mechanical tests reported here were performed at room temperature under a constant crosshead speed of
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379
1 mm/min using a Zwick 1485 universal testing machine. The elongation was measured through an extensometer with a gauge length of 30 mm. The specific work of fracture was obtained by dividing the integrated area of the load–elongation curve with the initial ligament area l · t with l – ligament length and t – specimen thickness. 2.3. Fracture behaviour observation Fracture behaviour analysis of the specimens was processed by means of microscope. A light microscope (LM) was used to observe the difference of the macro plastic zone of the fractured specimen with the same original ligament length. A scanning electron microscope (SEM, Jeol 5400) was applied to study the topography of fracture surface, which was sputtered with gold before observation. The fractographies at the pre-notched tips and the centres of the fractured specimens were presented, corresponding to crack initiation and propagation, respectively.
3. Results and discussion 3.1. Validity check of EWF methodology During the maximum tensile load, the ligament was fully yielded with some kind of plastification before crack propagation, which fulfilled the condition that crack should initiate after plastification of the ligament [19,20]. The load vs. elongation curves of PA66 were illuminated in Fig. 2. The curves of all tested specimens exhibited the self-similarity with no pronounced drop at maximum load, which showed that there was no interaction of the plastic zone with the specimen boundary behind the opposite crack or no tough-to-brittle transition [20]. Therefore, the specimens with a maximum ligament longer than W/3 were still available. To ensure the EWF data were obtained under plane stress and to remove data where fracture occurred prior to full ligament yielding, the graphs of the net
a
1.1σm
70
σm
60 Net section stress, σmax [MPa]
2376
61
0.9σm
50
b
60 50
1.1σm
40
0.9σm
σm
80
45
c
1.1σm
σm
60
0.9
63 m
40 4
8
12 16 Ligament length [mm]
20
Fig. 3. Stress criteria of EWF data: (a) neat PA66; (b) 1TPA (1 vol% TiO2/PA66); and (c) 3TPA (3 vol% TiO2/PA66). The gray points are invalid data outside stress limits.
section stress (rmax = Fmax/lt, where Fmax is the maximum load in the load–elongation curve) vs. ligament length for the tested materials were provided in Fig. 3. The stress criterion in protocol [19] was considered in following steps: (1) for all data, determine a mean value for the net section stress rmax denoted by rm; (2) then apply the criterion that any essential work data, for which rmax < 0.9 rm or rmax > 1.1 rm be rejected from the determination of EWF; (3) after that the independence of net section stress on ligament length was confirmed to ensure the existence of predominant plane stress state. However, because the overlapped fracture process zone (FPZ) in crack propagating at room temperature would lead to a minus specific essential work of resistance to crack propagation, the fracture work of resistance to crack propagation, wprop, was not considered here. The specific work of resistance to crack initiation, wini, could also represent toughness before crack propagating. It will be further discussed in the following text. 3.2. Effects of nanoparticle volume content
0
th (m m) leng
18.2 12.2
0
11.7
8.4
8.1
7.3
2
Liga men t
14.3
1
men t len gth
20.6 17
Liga
Load [kN]
2
c
3TPA (mm )
b
1TPA
(mm )
a
Liga men t len gth
3 PA
19.1 16 9 8.7
4 6 0 2 4 6 0 2 Elongation from extensometer [mm]
4
6
Fig. 2. Influence of different ligament length on the form of the load– elongation curves of: (a) neat PA66; (b) 1TPA (1 vol% TiO2 /PA66); and (c) 3TPA (3 vol% TiO2/PA66).
As described before, the influence of different ligament length on the shape of the load–displacement curves was presented in Fig. 2. The load increased almost linearly until to the peak point where the ligament is fully yielded and crack initiation occurred. In the case of PA and 1TPA, after the maximum point, stable crack propagation proceeded with nearly constant crack growth speed. In the last stage, the crack propagated unstably and finally break occurred. For 3TPA, there was stable crack growth, however, with high propagating speed. Some brittle part in the fracture surface could be seen by naked eyes.
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379 Table 1 The parameters of the specific work of resistance to crack initiation and the ratio of the maximum load Fmax to the displacement at break smax as a function of nanoparticle volume content Specimen
we, ini (kJ/m2)
bini wp, ini (MJ/m3)
R2
Fmax/smax (N/mm)
PA66 1TPA (1 vol% TiO2) 3TPA (3 vol% TiO2)
6.89 11.66 19.49
1.76 0.82 0.28
0.96 0.92 0.96
525 ± 56 586 ± 95 972 ± 82
As a result of decreasing ductility, the ratio of maximum load Fmax to displacement of extensometer (gauge length 30 mm, in this study) at break smax increased as well, as listed in Table 1. Compared to the ratio of neat PA, it was increased slightly for 1TPA, but substantially for 3TPA, which was in accordance with the tensile results [21]. Furthermore, these changes were represented in the fracture mechanics values we, ini and bini wp, ini. The linear regressions of wini vs. l of neat PA66, 1TPA and 3TPA are presented in Fig. 4. Nanocomposites filled with 1 and 3 vol% particles showed higher essential works than neat PA by 69% and 183%, respectively. Nevertheless the plastic work, bini wp, ini, decreased by 53% for 1TPA and considerably reduced by 84% for 3TPA compared to that of neat PA, respectively, which agreed with the above analysis of ratio Fmax/smax that material became brittle with increasing nanoparticle content. All results are also summarized in Table 1, as demonstrated in which, it is interesting to note that the increase in the term of we, ini associated with increasing particle volume fraction is always accompanied by a decrease in the term of bini wp, ini. 3.3. Toughening mechanisms In order to deeply understand the toughening mechanisms of nanoparticles filled PA66, the profile of the
Specific work of fracture 2 wf [kJ/m ]
50
wini,PA wini,1vol% TiO2/PA wini,3vol% TiO2/PA
40 30 20 10 0 0
4
8 12 Ligament L [mm]
16
20
Fig. 4. Specific work of resistance to crack initiation (wini) vs. ligament length (l) for the DENT specimens.
2377
outer plastic zone and the fractography of the fractured specimens with the same original ligament length were observed using light microscope (LM) and scanning electron microscope (SEM, Joel 5400), respectively. It is noteworthy to point out that due to the slight change of crystallinity in 1TPA and 3TPA [22], its minor influence on wini was not taken into account in this text. From the macro-photographs (shown in Fig. 5(a), (c), and (e)), a clear trend was easily seen there existed different levels of plastic deformation, illustrating in dark parts at the pre-crack tips of the three specimens and the area of the total plastic zone diminished with increasing particle volume content. It could be almost found that there were tiny fibrils only in neat matrix, indicated by arrow in Fig. 5(a). The plastic area of neat PA was around two and four times larger than that of 1TPA and 3TPA, respectively. Correspondingly, the necking section areas of PA and 1TPA were nearly 1/3 and 1/7 of original ones, respectively, as shown in Fig. 5(b) and (d). However, there was no obvious necking in 3TPA (reference Fig. 5(f)), indicating very low plastic deformation. It could be obviously drawn that the plastic work of fracture term, biniwp, ini, decreased with increasing nanoparticle volume content, which was in accordance with the measured results. Subsequently, SEM fractographies were taken near the pre-notched tip, where crack initiated, and in the middle part of the fractured specimen, where crack propagation completed. As illustrated in Fig. 6, the photos presented the topographic form of the fractured specimen near the pre-notched tips and in the middle parts of the samples. Near the pre-notched tips of neat matrix and composites, there were many dimples, whose density was much higher and the size was much smaller in nanocomposites than those in neat PA66 (reference Fig. 6(a), (c), and (e)), which resulted from the amount of fillers. This naturally illuminated that energy consumed to initiate crack in nanocomposites was much more than that in neat matrix. Moreover, in Fig. 6(b) there was clear and full fibrillation in the middle part of fractured neat PA, indicating much work dissipated for mass plastic deformation. However, there were no such sights instead of deep fringe dimples in 1TPA and even delaminating brittle layers observed in 3TPA due to the addition of nanoparticles, as provided in Fig. 6(d) and (f). Another photo with higher magnification in Fig. 6(d) clearly revealed a three-stage-evolution of the cavitation and crack initiation. First, individual particles acted as stress concentration points, which resulted in numerous tiny cavitations with relatively large local deformation (Stage I), as indicated in Fig. 7 with arrows named 1. With increasing tensile loading, those cavitations coalesced into sub-micro ones (Stage II), where high-level stress concentration rapidly led to micro voids (Stage III), and thus crack
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J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379
Fig. 5. Light microscope pictures of the fractured specimens with equal original ligament length. Side views: (a) PA; (c) 1TPA (1 vol% TiO2/PA66); and (e) 3TPA (3 vol% TiO2/PA66); and sectional views: (b) PA; (d) 1TPA; and (f) 3TPA.
Fig. 6. SEM microphotographs taken within the white rectangular parts in Fig. 5 of the tested specimens showing different fracture behaviors. Crack initiating regions with: (a) PA; (c) 1TPA (1 vol% TiO2/PA66); and (e) 3TPA (3 vol% TiO2/PA66); Crack propagating regions with: (b) PA; (d) 1TPA; and (f) 3TPA. Arrows indicated crack processing direction.
initiated with large dimples, as depicted arrows named 2 and 3, respectively, in Fig. 7. The amount of cavitations in stages I and II was subjected to the filler concentration. In the observed range, more particles resulted in more cavitations, where consequently much energy was participated during crack initiation.
Here, it is natural to believe that a homogenous dispersion of nanoparticles in matrix will bring best results. A transmission electron microscopy (TEM) picture of a 2 vol% 21-nm TiO2/PA66 specimen after injection molding was shown in Fig. 1 of [21]. A relatively satisfactory dispersion of nanoparticles, with large number of single
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379
2379
to unavoidable aggregated fillers which lead to high stress concentration favouring the crack propagation. Conclusively, the performance of 1TPA is satisfactory with enhanced EWF and acceptably decreased plasticity.
Acknowledgements
Fig. 7. Dimples showing a three-stage-evolution of the cavitations and crack initiations. Arrows 1 indicated cavitations formed around individual nanoparticle; Arrows 2 indicated cavitations formed around small aggregates; and Arrows 3 indicated cavitations formed around large aggregates.
particles, can be observed. However, there were still aggregating particles with non-ignore amount. It is reasonable to believe that the filler dispersion will be better in 1TPA with relatively substantive single particles, but worse in 3TPA with large amount of aggregates. It was easily to be estimated from the resulting Fig. 6(c) and (e) that the size of aggregates was less than 100 nm in 1TPA but several hundred nanometers in 3TPA. Thus, much more dimples resulted from macro voids occurred with increasing nanoparticle content, shown in Fig. 6(e), which favoured the crack propagation and impaired the plasticity of material.
4. Conclusions The fracture behaviour of PA66 filled with various fractions of TiO2 nanoparticle (21 nm) was studied using an energy-partitioned work of fracture method. The essential work term of composites filled with lower concentration, well-distributed nanoparticles, was enhanced significantly. A three-stage evolution of crack initiation was proposed. The individual nanoparticles act as stress concentration points, which promoted cavitations and thus induced relatively large local deformation; the tiny cavitations coalesce into sub-micro ones and rapidly grow into micro voids and crack initiation due to high-level stress concentration. The plastic work of composites was decreased with increasing nanoparticle fractions, which was due
Z. Zhang is grateful to the Alexander von Humboldt Foundation for his Sofja Kovalevskaja Award, financed by the German Federal Ministry of Education and Research (BMBF) within the German Governments program for investment in the future. The authors appreciate Professors K. Friedrich and J. Karger-Kocsis, IVW, for their valuable discussions during the course of this work and the preparation of this paper.
References [1] Kohan MI. Nylon plastics handbook. Hanser Gardner Publications; 1995. [2] Wu S-H. J Polym Sci 1983;21:699–716. [3] Wu S-H. J Appl Polym Sci 1988;35:549–61. [4] Ajayan PM, Schadler LS, Braun PV. Nanocomposite science and technology. Weinheim: Wiley-VCH Verlag GmbH Co. KgaA; 2003. [5] Klabunde KJ. Nanoscale materials in chemistry. New York: Wiley; 2001. [6] Karger-Kocsis J, Zhang Z. In: Balta Calleja JF, Michler G, editors. Mechanical properties of polymers based on nanostructures and morphology. New York: CRC Press; 2005. p. 547–96. [7] Pinnavaia TJ, Beall GW. Polymer clay nanocomposites. Chichester: Wiley; 2000. [8] Ng CB, Schadler LS, Siegel RW. Nanostruct Mater 1999;12:507–10. [9] Liu J, Gao Y, et al. J Mater Sci 2002;37:3085–8. [10] Chan CM, Wu J-S, et al. Polymer 2002;43:2981–92. [11] Chen L, Wong S-C, Pischarath S. J Appl Polym Sci 2003;88:3298–305. [12] Bureau MN, Perrin-Sarazin F, Ton-That M-T. Polym Eng Sci 2004;44:1142–51. [13] Broberg KB. J Mech Phys Solids 1975;23:215–37. [14] Mai Y-W, Cotterell B. Int J Fract 1986;32:105–25. [15] Hashemi S. Plast Rub Compos Process Appl 1993;20:229–37. [16] Karger-Kocsis J. J Macromol Sci B 1999;38:635–46. [17] Karger-Kocsis J, Cziga´ny T, Moskala EJ. Polymer 1997;38:4587–93. [18] Hashemi S, Williams JG. Plast Rub Compos 2000;29:294–302. [19] Test protocol for essential work of fracture, vol. 28. ESIS Publication; 2001. [20] Lach R et al. Eur Polym J 2005;41:383–92. [21] Zhang Z, Yang J-L, Friedrich K. Polymer 2004;45:3481–5. [22] Zhang H, Zhang Z, Yang J-L. Temperature dependence of crack initiation fracture toughness for polyamide nanocomposites [submitted].
The essential work of fracture of polyamide 66 filled with TiO2 nanoparticles Jing-Lei Yang a, Zhong Zhang a
a,b,* ,
Hui Zhang
a
Institute for Composite Materials, University of Kaiserslautern, 67663 Kaiserslautern, Germany b National Center for Nanoscience and Technology of China, 100080 Beijing, China Received 3 June 2005; accepted 7 June 2005 Available online 8 August 2005
Abstract The fracture property of 21-nm TiO2-nanoparticle filled polyamide 66 was studied based on the essential work of fracture method. An energy-partitioned work of fracture approach was introduced, in which the resistance to crack initiation, wini, and the resistance to crack propagation, wprop, were applied. Double-edge-notched-tension specimens with different original ligament lengths were tested at a constant cross-head speed. The results showed that the essential work term of composites filled with low nanoparticle concentration notably increased, while the plastic work fairly decreased compared to that of neat matrix at room temperature. Fractography analysis suggests a three-stage evolution of crack initiation. The individual nanoparticles acted as stress concentration points, which promoted cavitations and thus induced relatively large local deformation. Thereafter, the tiny cavitations coalesced into sub-micro ones and rapidly grow into micro-voids and crack initiation due to the high-level stress concentration. The plastic work of composites was decreased with increasing nanoparticle fractions, which was due to unavoidably aggregated nanoparticles leading to high level stress concentration that favouring the crack propagation. 2005 Elsevier Ltd. All rights reserved. Keywords: Essential work of fracture; Thermoplastics; Spherical nanoparticles; Nanocomposites; Toughening mechanisms
1. Introduction Polyamide 66 (PA66), i.e. Nylon 66, is widely used in many engineering applications due to its excellent mechanical performances and other advantages. Studies of introducing additional phases, e.g., inorganic fillers, have been successfully developed and thus extended its applications in the past decades [1]. Effects of fillers on the mechanical performances of composites strongly depend on their property, shape, dimension, size and aggregate degree, surface characteristics and concentration. Studies have shown that mechanical properties of * Corresponding author. Tel.: +49 631 2017213; fax: +49 631 2017196. E-mail addresses: [email protected], zhong.zhang@ nanoctr.cn (Z. Zhang).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.06.008
polymer composites filled with smaller particles are superior to those with larger ones at micron level, which were deeply investigated in rubber particle toughened PA [2,3]. In recent years, great efforts devoted to the development of nanoparticles filled polymer composites have made it possible to investigate the effect on the mechanical properties of these nanocomposites [4–6]. A major specific feature of nanocomposite materials is their huge interfacial area between filler and matrix, which can dramatically reach up to 1000 m2/g filler and is considered as a key factor to be able to intensely improve mechanical properties of a polymer matrix. Due to this reason, layered clays, with fairly large aspect ratios, have been widely researched in recent years [7]. Although many attractive enhancements were obtained in physical and mechanical performances of polymer/ clay nanocomposites, a major disadvantage of the
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379
reduction of fracture toughness has greatly restricted their engineering applications. Consequently, various kinds of spherical nanoparticles, such as silica, titanium oxide and calcium carbonate, have been attempted for preparing nanocomposites [8–10]. However, study on fracture toughness of polymer nanocomposites was scarcely found in the literatures [10–12]. The method of essential work of fracture was originally proposed by Broberg [13] and thereafter successfully developed toward a standard method by some other researchers [14–16] in order to study the fracture behaviour of a wide range of polymers and composites. Double edge notched tension (DENT) specimens were used for the tensile test, as schematically shown in Fig. 1. Typical load vs. elongation curve was showed in Fig. 1 as well. At the maximum load in tensile test, the material was fully yielded and crack initiated, and then crack propagated with advanced material necking and tearing until the specimen totally fractured. Consequently, Karger-Kocsis [17], Hashemi and Williams [18] introduced a method of energy-partitioned work of fracture, which divided the total energy dissipated into contributions from crack initiation (yielding) and crack propagation (advanced necking/tearing and specimen separating). As schematically depicted in Fig. 1, the specific work of resistance to crack initiation (wini) and to crack propagation (wprop) can be expressed as wini ¼ we;ini þ bini wp;ini l; wprop ¼ we;prop þ bprop wp;prop l;
ð1Þ ð2Þ
where we, ini and we, prop are the specific essential work of resistance to crack initiation and propagation, respectively. wp, ini and wp, prop are the volumetric energy dissipated during yielding/crack-initiating and crack
Fig. 1. Schematic diagram of energy partitioned EWF method. (a) Dimensions of DENT specimen, in this study W = 39 mm, Zt = 80 mm, t = 2 mm, and u = 30 mm; (b) typical load–elongation curve; and (c) fracture work of resistance to crack initiation (wini) as a function of ligament length (l).
2375
propagating, respectively. bini and bprop are geometry factors related to the shape of the plastic zone during crack initiating and propagating stages, respectively. In the present study, the fracture property of PA66 and its nanocomposites filled with 1 and 3 vol% 21-nm TiO2 nanoparticles was studied by using the method of energy partitioned work of fracture with DENT specimens produced by injection moulding. The validity of essential work of fracture (EWF) method was discussed. Fractography analysis of fractured specimens was provided. The effect of nanoparticle on the fracture behaviour and the toughening mechanisms in nanocomposites were discussed.
2. Experimental 2.1. Materials preparation A commercial Polyamide 66 (DuPont, Zytel 101) was considered as the matrix material in this study. TiO2 nanoparticles (Degussa P25) were applied as nano-fillers with a density of 4 g/cm3 and a mean diameter of 21 nm. The volume contents were in the range of 1–3%, respectively. Before mixing, PA66 and nanoparticles were dried in an oven at 70 C for about 72 h. Compounding of matrix and nanoparticles was carried out in a corotating twin-screw-extruder (Berstoff ZE 25A Æ 44D-UTS). The barrel temperatures were set at 55/260/270/280/285/ 285/285/285/285 (in C), respectively, and the screw speed 150 rpm. During melt extrusion, ventilation was kept on to remove trapped air in blends. After mixing, the blend granules were injection-molded into 2-mm thick plates for tensile tests by using an injection-molding machine (Alburg Allrounder 320S). For all blends, the injection parameters were maintained constant. The barrel temperature ranged from 280 to 295 C and the mold temperature was kept constantly at 70 C. The injection pressure and speed were 500 bar and 80.0 cm/s, respectively. Specimens were then prepared according to the ESIS protocol [19]. Three series of samples, namely PA66 or PA, 1TPA (1 vol% TiO2/PA66) and 3TPA (3 vol% TiO2/PA66), were cut as 80 · 40 · 2 mm3 with different initial ligaments by using a band saw. Specimens were dried in a vacuum oven at 75 C for 3.5 h before testing, and then placed in a drying chamber under 25% relative humidity and room temperature. Before testing, a fresh razor blade was carefully tapped into ligament of specimen to introduce two aligned sharp pre-cracks. The free ligament length was left in the range from 6 to 20 mm and measured with a light microscope. 2.2. Mechanical testing All mechanical tests reported here were performed at room temperature under a constant crosshead speed of
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379
1 mm/min using a Zwick 1485 universal testing machine. The elongation was measured through an extensometer with a gauge length of 30 mm. The specific work of fracture was obtained by dividing the integrated area of the load–elongation curve with the initial ligament area l · t with l – ligament length and t – specimen thickness. 2.3. Fracture behaviour observation Fracture behaviour analysis of the specimens was processed by means of microscope. A light microscope (LM) was used to observe the difference of the macro plastic zone of the fractured specimen with the same original ligament length. A scanning electron microscope (SEM, Jeol 5400) was applied to study the topography of fracture surface, which was sputtered with gold before observation. The fractographies at the pre-notched tips and the centres of the fractured specimens were presented, corresponding to crack initiation and propagation, respectively.
3. Results and discussion 3.1. Validity check of EWF methodology During the maximum tensile load, the ligament was fully yielded with some kind of plastification before crack propagation, which fulfilled the condition that crack should initiate after plastification of the ligament [19,20]. The load vs. elongation curves of PA66 were illuminated in Fig. 2. The curves of all tested specimens exhibited the self-similarity with no pronounced drop at maximum load, which showed that there was no interaction of the plastic zone with the specimen boundary behind the opposite crack or no tough-to-brittle transition [20]. Therefore, the specimens with a maximum ligament longer than W/3 were still available. To ensure the EWF data were obtained under plane stress and to remove data where fracture occurred prior to full ligament yielding, the graphs of the net
a
1.1σm
70
σm
60 Net section stress, σmax [MPa]
2376
61
0.9σm
50
b
60 50
1.1σm
40
0.9σm
σm
80
45
c
1.1σm
σm
60
0.9
63 m
40 4
8
12 16 Ligament length [mm]
20
Fig. 3. Stress criteria of EWF data: (a) neat PA66; (b) 1TPA (1 vol% TiO2/PA66); and (c) 3TPA (3 vol% TiO2/PA66). The gray points are invalid data outside stress limits.
section stress (rmax = Fmax/lt, where Fmax is the maximum load in the load–elongation curve) vs. ligament length for the tested materials were provided in Fig. 3. The stress criterion in protocol [19] was considered in following steps: (1) for all data, determine a mean value for the net section stress rmax denoted by rm; (2) then apply the criterion that any essential work data, for which rmax < 0.9 rm or rmax > 1.1 rm be rejected from the determination of EWF; (3) after that the independence of net section stress on ligament length was confirmed to ensure the existence of predominant plane stress state. However, because the overlapped fracture process zone (FPZ) in crack propagating at room temperature would lead to a minus specific essential work of resistance to crack propagation, the fracture work of resistance to crack propagation, wprop, was not considered here. The specific work of resistance to crack initiation, wini, could also represent toughness before crack propagating. It will be further discussed in the following text. 3.2. Effects of nanoparticle volume content
0
th (m m) leng
18.2 12.2
0
11.7
8.4
8.1
7.3
2
Liga men t
14.3
1
men t len gth
20.6 17
Liga
Load [kN]
2
c
3TPA (mm )
b
1TPA
(mm )
a
Liga men t len gth
3 PA
19.1 16 9 8.7
4 6 0 2 4 6 0 2 Elongation from extensometer [mm]
4
6
Fig. 2. Influence of different ligament length on the form of the load– elongation curves of: (a) neat PA66; (b) 1TPA (1 vol% TiO2 /PA66); and (c) 3TPA (3 vol% TiO2/PA66).
As described before, the influence of different ligament length on the shape of the load–displacement curves was presented in Fig. 2. The load increased almost linearly until to the peak point where the ligament is fully yielded and crack initiation occurred. In the case of PA and 1TPA, after the maximum point, stable crack propagation proceeded with nearly constant crack growth speed. In the last stage, the crack propagated unstably and finally break occurred. For 3TPA, there was stable crack growth, however, with high propagating speed. Some brittle part in the fracture surface could be seen by naked eyes.
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379 Table 1 The parameters of the specific work of resistance to crack initiation and the ratio of the maximum load Fmax to the displacement at break smax as a function of nanoparticle volume content Specimen
we, ini (kJ/m2)
bini wp, ini (MJ/m3)
R2
Fmax/smax (N/mm)
PA66 1TPA (1 vol% TiO2) 3TPA (3 vol% TiO2)
6.89 11.66 19.49
1.76 0.82 0.28
0.96 0.92 0.96
525 ± 56 586 ± 95 972 ± 82
As a result of decreasing ductility, the ratio of maximum load Fmax to displacement of extensometer (gauge length 30 mm, in this study) at break smax increased as well, as listed in Table 1. Compared to the ratio of neat PA, it was increased slightly for 1TPA, but substantially for 3TPA, which was in accordance with the tensile results [21]. Furthermore, these changes were represented in the fracture mechanics values we, ini and bini wp, ini. The linear regressions of wini vs. l of neat PA66, 1TPA and 3TPA are presented in Fig. 4. Nanocomposites filled with 1 and 3 vol% particles showed higher essential works than neat PA by 69% and 183%, respectively. Nevertheless the plastic work, bini wp, ini, decreased by 53% for 1TPA and considerably reduced by 84% for 3TPA compared to that of neat PA, respectively, which agreed with the above analysis of ratio Fmax/smax that material became brittle with increasing nanoparticle content. All results are also summarized in Table 1, as demonstrated in which, it is interesting to note that the increase in the term of we, ini associated with increasing particle volume fraction is always accompanied by a decrease in the term of bini wp, ini. 3.3. Toughening mechanisms In order to deeply understand the toughening mechanisms of nanoparticles filled PA66, the profile of the
Specific work of fracture 2 wf [kJ/m ]
50
wini,PA wini,1vol% TiO2/PA wini,3vol% TiO2/PA
40 30 20 10 0 0
4
8 12 Ligament L [mm]
16
20
Fig. 4. Specific work of resistance to crack initiation (wini) vs. ligament length (l) for the DENT specimens.
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outer plastic zone and the fractography of the fractured specimens with the same original ligament length were observed using light microscope (LM) and scanning electron microscope (SEM, Joel 5400), respectively. It is noteworthy to point out that due to the slight change of crystallinity in 1TPA and 3TPA [22], its minor influence on wini was not taken into account in this text. From the macro-photographs (shown in Fig. 5(a), (c), and (e)), a clear trend was easily seen there existed different levels of plastic deformation, illustrating in dark parts at the pre-crack tips of the three specimens and the area of the total plastic zone diminished with increasing particle volume content. It could be almost found that there were tiny fibrils only in neat matrix, indicated by arrow in Fig. 5(a). The plastic area of neat PA was around two and four times larger than that of 1TPA and 3TPA, respectively. Correspondingly, the necking section areas of PA and 1TPA were nearly 1/3 and 1/7 of original ones, respectively, as shown in Fig. 5(b) and (d). However, there was no obvious necking in 3TPA (reference Fig. 5(f)), indicating very low plastic deformation. It could be obviously drawn that the plastic work of fracture term, biniwp, ini, decreased with increasing nanoparticle volume content, which was in accordance with the measured results. Subsequently, SEM fractographies were taken near the pre-notched tip, where crack initiated, and in the middle part of the fractured specimen, where crack propagation completed. As illustrated in Fig. 6, the photos presented the topographic form of the fractured specimen near the pre-notched tips and in the middle parts of the samples. Near the pre-notched tips of neat matrix and composites, there were many dimples, whose density was much higher and the size was much smaller in nanocomposites than those in neat PA66 (reference Fig. 6(a), (c), and (e)), which resulted from the amount of fillers. This naturally illuminated that energy consumed to initiate crack in nanocomposites was much more than that in neat matrix. Moreover, in Fig. 6(b) there was clear and full fibrillation in the middle part of fractured neat PA, indicating much work dissipated for mass plastic deformation. However, there were no such sights instead of deep fringe dimples in 1TPA and even delaminating brittle layers observed in 3TPA due to the addition of nanoparticles, as provided in Fig. 6(d) and (f). Another photo with higher magnification in Fig. 6(d) clearly revealed a three-stage-evolution of the cavitation and crack initiation. First, individual particles acted as stress concentration points, which resulted in numerous tiny cavitations with relatively large local deformation (Stage I), as indicated in Fig. 7 with arrows named 1. With increasing tensile loading, those cavitations coalesced into sub-micro ones (Stage II), where high-level stress concentration rapidly led to micro voids (Stage III), and thus crack
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Fig. 5. Light microscope pictures of the fractured specimens with equal original ligament length. Side views: (a) PA; (c) 1TPA (1 vol% TiO2/PA66); and (e) 3TPA (3 vol% TiO2/PA66); and sectional views: (b) PA; (d) 1TPA; and (f) 3TPA.
Fig. 6. SEM microphotographs taken within the white rectangular parts in Fig. 5 of the tested specimens showing different fracture behaviors. Crack initiating regions with: (a) PA; (c) 1TPA (1 vol% TiO2/PA66); and (e) 3TPA (3 vol% TiO2/PA66); Crack propagating regions with: (b) PA; (d) 1TPA; and (f) 3TPA. Arrows indicated crack processing direction.
initiated with large dimples, as depicted arrows named 2 and 3, respectively, in Fig. 7. The amount of cavitations in stages I and II was subjected to the filler concentration. In the observed range, more particles resulted in more cavitations, where consequently much energy was participated during crack initiation.
Here, it is natural to believe that a homogenous dispersion of nanoparticles in matrix will bring best results. A transmission electron microscopy (TEM) picture of a 2 vol% 21-nm TiO2/PA66 specimen after injection molding was shown in Fig. 1 of [21]. A relatively satisfactory dispersion of nanoparticles, with large number of single
J.-L. Yang et al. / Composites Science and Technology 65 (2005) 2374–2379
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to unavoidable aggregated fillers which lead to high stress concentration favouring the crack propagation. Conclusively, the performance of 1TPA is satisfactory with enhanced EWF and acceptably decreased plasticity.
Acknowledgements
Fig. 7. Dimples showing a three-stage-evolution of the cavitations and crack initiations. Arrows 1 indicated cavitations formed around individual nanoparticle; Arrows 2 indicated cavitations formed around small aggregates; and Arrows 3 indicated cavitations formed around large aggregates.
particles, can be observed. However, there were still aggregating particles with non-ignore amount. It is reasonable to believe that the filler dispersion will be better in 1TPA with relatively substantive single particles, but worse in 3TPA with large amount of aggregates. It was easily to be estimated from the resulting Fig. 6(c) and (e) that the size of aggregates was less than 100 nm in 1TPA but several hundred nanometers in 3TPA. Thus, much more dimples resulted from macro voids occurred with increasing nanoparticle content, shown in Fig. 6(e), which favoured the crack propagation and impaired the plasticity of material.
4. Conclusions The fracture behaviour of PA66 filled with various fractions of TiO2 nanoparticle (21 nm) was studied using an energy-partitioned work of fracture method. The essential work term of composites filled with lower concentration, well-distributed nanoparticles, was enhanced significantly. A three-stage evolution of crack initiation was proposed. The individual nanoparticles act as stress concentration points, which promoted cavitations and thus induced relatively large local deformation; the tiny cavitations coalesce into sub-micro ones and rapidly grow into micro voids and crack initiation due to high-level stress concentration. The plastic work of composites was decreased with increasing nanoparticle fractions, which was due
Z. Zhang is grateful to the Alexander von Humboldt Foundation for his Sofja Kovalevskaja Award, financed by the German Federal Ministry of Education and Research (BMBF) within the German Governments program for investment in the future. The authors appreciate Professors K. Friedrich and J. Karger-Kocsis, IVW, for their valuable discussions during the course of this work and the preparation of this paper.
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