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A study of the mechanical behaviour of a glass fibre reinforced polyamide 6,6: Experimental investigation
ARTICLE IN PRESS
POLYMER TESTING Polymer Testing 25 (2006) 544–552 www.elsevier.com/locate/polytest
Material Behaviour
A study of the mechanical behaviour of a glass fibre reinforced polyamide 6,6: Experimental investigation B. Mouhmida,, A. Imada, N. Benseddiqa, S. Benmedakhe`neb, A. Maazouzc a
Laboratoire de Me´canique de Lille, CNRS UMR 8107, Ecole Polytech’Lille, Universite´ de Lille1, Cite´ Scientifique, Avenue Paul Langevin, 59 655 Villeneuve d’Ascq cedex, France b AETech—Acoustic Emission Technology, 66, Av de Landshut, BP 50149, 60201 Compie`gne cedex, France c INSA de Lyon, Avenue Jean Capelle, 69621 Villeurbanne cedex, France Received 30 January 2006; accepted 12 March 2006
Abstract In this experimental work, we studied the mechanical behaviour of a short glass fibre reinforced polyamide frequently used in the automobile industry. In order to investigate the influence of glass fibre content, temperature and strain rate, we carried out a series of uniaxial tensile loadings on an unfilled polyamide and glass fibre reinforced polyamide with different weight fractions: 15, 30 and 50 wt%. Experimental results showed that the studied composite is a strain rate, temperature and fibre volume fraction dependant material. Both elastic modulus and tensile strength increase with strain rate and decrease with temperature. Glass fibre reinforced PA66 exhibits improvement in its mechanical strength. The evolution of the normalized modulus and tensile strength as functions of relative density can be described by a type of power function. The acoustic emission (AE) technique, which is recognized as an effective tool for non-destructive testing and material evaluation, has been used to determine the damage threshold and obtain information about fracture mechanisms in the studied composites. Scanning electron microscopy (SEM) analysis was made on the fracture surfaces to visualize the damage process: fibre fracture, matrix rupture and interface rupture. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyamide 66; Glass fibres; Mechanical behaviour; Damage; Acoustic emission
1. Introduction Glass fibre reinforced polyamides continue to be used with increased frequency in many applications, such as stressed functional automotive parts (fuel Corresponding author. Fax: +3 28 76 73 11.
E-mail address: [email protected] (B. Mouhmid). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.03.008
injection rails, steering column switches) and safety parts in sports and leisure (snowboard bindings). These materials are known for their stiffness, toughness and resistance to dynamic fatigue. Fibre reinforced thermoplastics compounds may be processed by conventional methods, such as injection moulding, and offer improvements in mechanical properties over unreinforced ones. These composites compete with metals in many
ARTICLE IN PRESS
2. Experimental procedure 2.1. Material and tensile test conditions Chopped E-glass and PA66 were used to produce moulded composites with 0, 15, 30 and 50% (wt) glass content. The glass bundles and pre-dried PA66 pellets were dry blended to the desired glass content and compounded on a single screw extruder. The compounds were moulded on an 80 ton moulding machine. Compounding and moulding temperatures were respectively 275 and 280 1C with a mould temperature of 80 1C. The mechanical properties testing were performed at a relative humidity of 50% using ISO 527 specimens (Fig. 1). The tensile tests were carried out in a 10 kN Instron machine equipped with a temperature controlled chamber. The temperatures
545
20
150 60
10
engineering applications because of their ease of fabrication, light weight and economy. However, there are problems concerning material defects such as voids or cracks that may be present or initiated in one of three regions: the matrix, the fibre or the fibre/matrix interface [1]. The mechanical properties of thermoplastic composites containing short fibres have been the subject of much attention. These properties result from a combination of the fibre and the matrix properties and the ability to transfer stresses across the fibre/matrix interface, but also depend on the injection conditions such as screw and barrel parameters, mould temperature and design [2–5]. Variables such as fibre ratio, diameter, length, orientation and the interfacial strength are of prime importance to the final properties of the thermoplastic composites according to the studies led by Thomasson [6] and Shao-Yun Fu [7]. In this study, we characterized mechanically the behaviour of glass fibre reinforced and unreinforced polyamide 66. The effects of glass fibre content, temperature and strain rate have been investigated. An AE monitoring technique was used to identify different types of failure in the investigated composites. In order to visualize the damage process, scanning electron microscopy (SEM) analysis was made on the fracture surface of the specimens. The stress–strain response of the PA66 depends strongly on the humidity conditions. PA66 is considered as dry when the water absorption is less than 0.2% and saturated at 7.2%. The tensile strength in glass fibre reinforced PA66 is 50% higher in the dry state than in the saturated one [22].
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B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552
Fig. 1. Tensile specimen dimensions (mm).
and strain rates chosen were, respectively, T ¼ 20, 50 and 80 1C and 1, 5 and 50 mm/min corresponding to e_ ¼ 1:1 103 ; 5:6 103 and 5:6 102 s1 . 2.2. Damage analysis technique The tensile test was made with acoustic emission monitoring by the use of a Vallen AMSY 5 system. Acoustic emission (AE) is elastic radiation generated by the rapid release of energy from sources within a material. These elastic waves are detected and converted to voltage signals by small piezoelectric sensors mounted on a convenient surface of the material. The result is that AE can be used to monitor a structure for active damage even when ambient noise levels are extremely high. Sources of acoustic emission include in this case all types of damage. 3. Results and discussions In this section, the influence of an intrinsic parameter (glass fibre content) and two extrinsic parameters (strain rate e_ and temperature T) on the three mechanical properties of tensile elastic modulus E, tensile strength sr and the failure strain er , has been investigated. 3.1. Glass fibre content effect Fig. 2 shows stress–strain curves obtained for the studied composites at T ¼ 20 1C and e_ ¼ 5:6 103 s1 . We can note the typical stress–strain curves of reinforced materials showing high strength levels and low deformation capabilities. Indeed, glass fibre reinforced PA66 presents brittle behaviour with a failure strain of about 5.5%, whereas unreinforced PA66 presents ductile behaviour with a failure strain of about 25%. This figure also shows
ARTICLE IN PRESS B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552
546
2.80
180.0 50% 160.0
2.60
140.0
2.40
30%
120.0
R2 = 0.99
2.20 15%
100.0
σr/σr0
σ (MPa)
y = 1x2.8
80.0 0%
60.0
2.00 1.80 1.60
40.0
1.40
20.0
1.20
0.0
1.00 5.0
10.0
15.0 ε (%)
20.0
25.0
30.0
Fig. 2. Stress–strain curves with different glass fibre ratios at 20 1C and 5:6 103 s1 .
clearly that the increase in fibre content leads to an increase in tensile strength and elastic modulus. In order to explain these evolutions, the elastic modulus and the tensile strength can be modelled using a simple rearrangement of the rule of mixture equations: E ¼ avf E f þ ð1 vf Þ E m and sr ¼ bvf srf þ ð1 vf Þ srm where E f ; srf are the modulus and the tensile strength of the fibre (E f ¼ 76 GPa and srf ¼ 3200 MPa) and E m ; srm are, respectively, the experimental values of the modulus and the tensile strength of the unreinforced PA66, vf is the fibre volume fraction and a and b are the fibre orientation factors [8]. In our case, the average values of a and b are: a ¼ 0:24 and b ¼ 0:072, these values depend on the fibre length and orientation. The evolution of these two mechanical parameters can also be modelled using a phenomenological description based upon the relative representations: normalized tensile strength sr =srm , normalized elastic modulus E=E m versus relative density r=r0 where r0 is the density of unreinforced PA66. Figs. 3 and 4 illustrate these evolutions and show that they are described by power relationships as follows: 2:8 sr r ¼ r0 srm
3:7 E r and ¼ , Em r0
with a good correlation coefficient: R2 ¼ 0:99 for both formulas. Imad [9] has suggested similar
1
1.1
1.2
1.3
1.4
1.5
ρ/ρ0 Fig. 3. Variation of normalized tensile strength as a function of relative density.
4.00 y = 1x3.7
3.50
R2 = 0.99 3.00 E /E0
0.0
2.50
2.00
1.50
1.00 1
1.1
1.2
1.3
1.4
1.5
ρ/ρ0 Fig. 4. Variation of normalized modulus as a function of relative density.
expressions in a study about the mechanical behaviour of expanded polystyrene. This observation is in good agreement with literature data [10,11] and can be explained by the contribution of the glass fibre as a brittle and tough material. The loss of ductility is confirmed by the
ARTICLE IN PRESS B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552
547
90.0
Stress MPa
70.0 50.0 30.0
0.0011 1/s 0.0056 1/s
10.0 -10.00.0
0.056 1/s 5.0
10.0
15.0 20.0 starin %
25.0
30.0
35.0
Fig. 6. Stress–strain curves of unreinforced PA66 for different strain rates.
function defined by E ¼ aE lnð_eÞ þ bE and sr ¼ asr lnð_eÞ þ bsr for the different studied materials [13,14]. Values of aE , bE , asr and bsr are given in Table 1. Fig. 5. (a) Ductile rupture. (b) Brittle fracture.
3.3. Temperature effect rupture specimen profile. In fact, the unreinforced PA66 clearly develops a striction zone. Contrariwise, the glass fibre reinforced PA66 shows a brittle fracture profile as shown in Figs. 5ab. 3.2. Strain rate effect Fig. 6 illustrates the stress–strain curves obtained from uniaxial tensile tests on unreinforced PA66 specimens at room temperature (20 1C) with the following strain rates: e_ ¼ 1:1 103 ; 5:6 103 and 5:6 102 s1 . These curves show that in unreinforced PA66 the tensile strength increases with the increase of strain rate, whereas the failure strain drops, which means the material gets less ductile. An explanation is that for high values of strain rate, molecular rearrangement does not have time to take place. This is in accordance with other results in the specialized literature [12]. The strain rate does not seem to have any significant effect on the modulus in the range considered. In glass fibre reinforced PA66, a typical stress– strain curve is shown in Fig. 7. Elastic modulus and tensile strength slightly increase as strain rate increases. Contrariwise, the failure strain is not affected by the strain rate in the range considered. The tensile modulus and the tensile strength are plotted against logarithms of strain rate in Figs. 8a and b and can be modelled using a simple regression
In order to evaluate the effect of temperature on the tensile properties of the glass fibre reinforced PA66, the stress–strain curves were plotted. Typical stress–strain curves at a constant strain rate of 5:6 103 s1 are shown in Fig. 9. It is noted that the tensile behaviour of the studied materials is strongly dependant on the temperature. It can be seen that there is an obvious region AC called the ‘‘toe region’’ as shown in Fig. 10. The ‘‘toe region’’ does not represent a material property but it is an artifact caused by the take up of slack as observed, e.g., by Shi et al. [15]. The slope between the origin B and point D on the elastic section in the stress–strain curve is taken as the Young’s modulus according to ASTM standard D882-97 [16]. The effect of temperature on the Young’s modulus is shown in Fig. 11. We can see that the normalized Young’s modulus decreases with the testing temperature, following a nonlinear relationship. The normalized tensile strength also decreases with increasing temperature as shown in Fig. 12. It can be noted that the failure strain increases as the temperature is increased up to 80 1C for the 15% (wt) short glass fibre reinforced PA66. Globally, the effect of temperature on the failure strain is much more accentuated in unreinforced PA66 than in short glass fibre reinforced PA66, as shown in Fig. 13.
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160.0 5mm/min 140.0
Stress (Mpa)
120.0
50mm/min 1mm/min
100.0 80.0 60.0 40.0 20.0 0.0 0.0
1.0
2.0
3.0 Strain (%)
4.0
5.0
6.0
Modulus (MPa)
Fig. 7. Stress–strain curves of 30% glass fibre reinforced PA66 for different strain rates.
6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 -7.00
PA66 0% PA66 15% PA66 30% PA66 50%
-6.00
-5.00
-4.00
-3.00
-2.00
Ln (strain rate)
(a)
200 180
140 120 100 80 60 PA66 0% PA66 15% PA66 30% PA66 50%
Tensile strength (MPa)
160
40 20 0
-7 (b)
-6
-5
-4
-3
-2
Ln (strain rate)
Fig. 8. (a) Variation of modulus as a function of ln ð_eÞ. (b) Variation of modulus as a function of ln ð_eÞ.
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Table 1 Values of aE and bE for the different studied materials
aE bE asr bsr
90
15% SGFR PA66
30% SGFR PA66
50% SGFR PA66
32.8 2079.1 1.71 71.17
88.6 3609.6 0.76 94.75
179.0 4791.7 2.94 142.57
170.7 6320.0 6.43 201.89
20°C 50°C 80°C
80 70
Relative modulus
60 50 40 30
E/Em
Stress (MPa)
0% SGFR PA66
20 10 0 0
2
6
4
8
10
Strain (%)
0% GFR PA66 15% GFR PA66 30% GFR PA66 50% GFR PA66
2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0
Fig. 9. Stress–strain curves of 15% glass fibre reinforced PA66 at strain rate of 5:6 103 s1 at different temperatures.
20
40 60 Temperature (°C)
80
100
Fig. 11. Normalized modulus versus temperature.
30 50°C
20 D σ/σm
Stress (MPa)
25
15
10
5
2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0
C 0 A 0
0% GFR PA66 15% GFR PA66 30% GFR PA66 50% GFR PA66
B 0.5
20
40 60 Temperature (°C)
80
100
Fig. 12. Normalized tensile strength versus temperature.
1 1.5 Strain (%)
2
1.5
Fig. 10. Stress–strain curves of 15% glass fibre reinforced PA66 at strain rate of 5:6 103 s1 at 50 1C.
4. Damage mechanisms 4.1. Acoustic emission The AE technique has been used to detect different damage stages in glass fibre reinforced
PA66 at room temperature. AE may have several physical causes: crack initiation and propagation, fibre/matrix interface and fiber failure. The number of AE events characterizes the global damage mechanism of the sample. It is possible to correlate each range of amplitude to a damage mechanism type. Many authors have already worked on this subject. Barre´ and Benzeggag [17,18], testing glass fibre reinforced polypropylene samples, reported that the acoustic signal varies with the corresponding damage mode: AE amplitude range from 40 to
ARTICLE IN PRESS B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552
550
70
Failure strain (%)
60 50 40
PA00 PA15 PA30 PA50
30 20 10 0 20
40
60 80 Temperature (T°C)
100
Fig. 13. Effect of temperature on the failure strain.
55 dB corresponds to matrix cracking, 60–65 dB to debonding, 65–85 dB to pull-out and 85–95 dB to fibre fracture. Figs. 14a and b illustrate AE results of 15% (wt) glass fibre reinforced PA66: force, counts and amplitude versus time. These curves highlight three main damage process stages of the composite:
Fig. 14. Acoustic emission results of a 15% glass fibre reinforced PA66: (a) number of events and (b) amplitudes.
(1) An elastic zone with an insignificant acoustic activity (zone A). (2) A second zone corresponding to the nonlinear behaviour controlled by matrix plasticity and microcracks (zone B: amplitude between 40 and 60 dB). The beginning of this zone corresponds to the yield stress. (3) A last zone, C, of important damage represented by consequent AE activity preceding rupture. These interpretations are consolidated by other results [19–21]. 4.2. Scanning electron microscopy For more comprehension of rupture mechanisms, fracture surfaces have been observed by SEM. Fig. 15 is a fracture surface photography of an unreinforced PA66 showing craters. Cavitations could have occurred and produced crazing. Crazing could also have occurred near initial voids or an existing defect that took place during moulding. However, the presence of spherolites, that are
Fig. 15. Unreinforced PA66 failure facies.
crystalline defects, is the most probable hypothesis. Craze propagation led then to cracks. This stage relates to the ductile behaviour of unreinforced PA66. Once the crack is created, it weakens the material and primes the brittle rupture of the remaining ligament.
ARTICLE IN PRESS B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552
Fig. 16. Fracture surface of a 50% (wt) short glass fibre reinforced PA66 showing a predominant orientation of the fibres.
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fibre percentage leads to higher values of tensile strength and modulus, and lower values of failure strain. Strain rate, in the studied range, has a less significant effect on glass fibre reinforced PA66 behaviour than on the unreinforced PA66. In unreinforced PA66, the strain rate increase results in a higher yield stress and a lower failure strain. Temperature plays a predominant role in the material behaviour, temperature increase leads to more ductility and less stiffness. AE testing is a powerful method for online detection and analysis of matrix, fibre and interface related active fracture processes in composite materials. A number of tools for identification and evaluation of damage stages and failure mechanisms exist. They are based on changes in AE activity or intensity features. Stress waves emitted by fibre breaks cause much higher AE amplitudes than other mechanisms, such as matrix cracking. SEM and AE analysis allowed working out the damage process and chronology. Unreinforced PA66 develops crazing followed by rupture of the remaining ligament. In short glass fibre reinforced PA66, fibre-matrix interface rupture is observed. In 30% (wt) and 50% (wt) glass fibre reinforced PA66, the damage is characterized by matrix plastication and microcracks, fibre pull out and fracture.
References Fig. 17. A 15% glass fibre reinforced PA66 failure surface.
Fig. 16 indicates predominant orientation of the fibres parallel to the flow direction in a random area of 50% (wt) short glass fibre reinforced PA66 specimen. Fig. 17 shows the fracture surface of the 15% glass fibre reinforced PA66. For this specimen, the fracture surface is rough and most of the fibres are oriented in the loading direction. This fracture surface is featured by some holes due to the pulled out fibres and the protuding fibres on the fracture surface. The same remarks can be made about reinforced PA66 with other short glass fibre contents. 5. Conclusions In this paper, the effects of strain rate, temperature and glass fibre content have been investigated. Glass fibre addition in PA66 results in loss of ductility and increased stiffness: increasing glass
[1] M. Akay, D.F. O’regan, Fracture behaviour of glass fibre reinforced polyamide mouldings, Polym. Test. 14 (1995) 149–162. [2] J.P. Tancrez, J. Pabiot, F. Rietsch, Damage and fracture mechanisms in thermoplastic-matrix composites in relation to processing and structural parameters, Compos. Sci. Technol. 56 (1996) 725–731. [3] G. Guerrica-Echevarria, J.I. Eguiazabal, J. Nazabal, Influence of the preparation method on the mechanical properties of a thermoplastic liquid crystalline copolyster, Polym. Test. 20 (2001) 403–408. [4] W.N. Ota, S.C. Amico, K.G. Satyanarayana, Studies on the combined effect of injection temperature and fiber content on the properties of the polypropylene–glass fiber composites, Compos. Sci. Technol. 65 (2005) 873–881. [5] A. Gu¨lu¨, A. O¨zdemir, E. O¨zdemir, Experimental investigation of the effect of glass fibres on the mechanical properties of polypropylene (PP) and polyamide 6 (PA6) plastics, Mater. Des. 27 (2006) 316–323. [6] J.L. Thomasson, The influence of fibre properties of the performance of glass-fibre-reinforced polyamide 66, Compos. Sci. Technol. 59 (1999) 2315–2328. [7] S.-Y. Fu, B. Lauke, A. San, Effect of fiber length and fiber orientation distributions on the tensile strength of short fiber
ARTICLE IN PRESS 552
[8]
[9]
[10]
[11]
[12]
[13]
[14]
B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552 reinforced polymers, Comopos. Sci. Technol. 56 (1996) 1179–1190. G.X. Sui, S.-C. Wong, R. Yang, C.Y. Yue, The effect of fiber inclusions in toughened plastics—Part II: determination of micromechanical parameters, Compos. Sci. Technol. 65 (2005) 221–229. A. Imad, A viscoelastic–plastic behaviour analysis of expanded polystyrene under compressive loading: experiments and modeling, International Journal of Cellular Polymers 20 (2001) 189–209. J.L. Thomason, Micromechanical parameters from macromechanical measurements on glass reinforced polyamide 6,6, Compos. Sci. Technol. 61 (2001) 2007–2016. S.Y. Fu, Tensile properties of short glass fiber and short carbon fiber reinforced polypropylene composites, Composites A 31 (2000) 1117–1125. Z. Wang, Y. Zhou, P.K. Mallick, Effect of temperature and strain rate on the tensile behavior of short fiber reinforced polyamide-6, Polym. Compos. 23 (2002) 858–871. Y. Zhou, P.K. Mallick, A non linear damage model for the tensile behavior of an injection molded short E-glass fiber reinforced polyamide 6,6, Mater. Sci. Eng. A 393 (2005) 303–309. M. Todo, K. Takahashi, P. Be´guelin, H. Kausch, Strain rate dependence of the tensile fracture behaviour of woven cloth reinforced polyamide composites, Compos. Sci. Technol. 60 (2000) 763–771.
[15] X.Q. Shi, Z.P. Wang, H.L.J. Pang, X.R. Zhang, Investigation of effect of temperature and strain rate on mechanical properties of underfill material by uses of microtensile specimens, Polym. Test. 21 (2002) 725–733. [16] ASTM (American Standards for Testing Methods) Standard, ASTM designation: D882-97 (1996) 159–167. [17] S. Barre´, M. Bezeggagh, On the use of acoustic emission to investigate damage mechanics in glass-fibre reinforced polypropylene, Compos. Sci. Technol. 52 (1994) 369–376. [18] M. Benzeggagh, M. Kennane, Measurement of mixed mode delamination fracture toughness of unidirectional glass–epoxy composites with mixed modebending apparatus, Compos. Sci. Technol. (1995) 439–449. [19] A. Laksimi, L. Bounouas, S. Benmedakhene, Z. Azari, A. Imad, Damage on polypropylene glass fibre composites: experimental analysis, C.R. Acad. Sci. Paris Se´rie II b (2000) 651–656. [20] S. Huguet, N. Godin, R. Gaertner, L. Salmon, D. Villard, Use of acoustic emission to identify damage modes in glass fibre polyester, Compos. Sci. Technol. 62 (2002) 1433–1444. [21] M. Benzeggagh, S. Benmedakhene, Residual strength of a glass/polypropylene composite material subjected to impact, Compos. Sci. Technol. 55 (1995) 1–11. [22] A. Bergeret, I. Pires, M.P. Foulc, B. Abadie, L. Ferry, A. Crespy, The hygrothermal behaviour of glass-fibre-reinforced thermoplastic composites: a prediction of the composite lifetime, Polym. Test. 20 (2001) 753–763.
POLYMER TESTING Polymer Testing 25 (2006) 544–552 www.elsevier.com/locate/polytest
Material Behaviour
A study of the mechanical behaviour of a glass fibre reinforced polyamide 6,6: Experimental investigation B. Mouhmida,, A. Imada, N. Benseddiqa, S. Benmedakhe`neb, A. Maazouzc a
Laboratoire de Me´canique de Lille, CNRS UMR 8107, Ecole Polytech’Lille, Universite´ de Lille1, Cite´ Scientifique, Avenue Paul Langevin, 59 655 Villeneuve d’Ascq cedex, France b AETech—Acoustic Emission Technology, 66, Av de Landshut, BP 50149, 60201 Compie`gne cedex, France c INSA de Lyon, Avenue Jean Capelle, 69621 Villeurbanne cedex, France Received 30 January 2006; accepted 12 March 2006
Abstract In this experimental work, we studied the mechanical behaviour of a short glass fibre reinforced polyamide frequently used in the automobile industry. In order to investigate the influence of glass fibre content, temperature and strain rate, we carried out a series of uniaxial tensile loadings on an unfilled polyamide and glass fibre reinforced polyamide with different weight fractions: 15, 30 and 50 wt%. Experimental results showed that the studied composite is a strain rate, temperature and fibre volume fraction dependant material. Both elastic modulus and tensile strength increase with strain rate and decrease with temperature. Glass fibre reinforced PA66 exhibits improvement in its mechanical strength. The evolution of the normalized modulus and tensile strength as functions of relative density can be described by a type of power function. The acoustic emission (AE) technique, which is recognized as an effective tool for non-destructive testing and material evaluation, has been used to determine the damage threshold and obtain information about fracture mechanisms in the studied composites. Scanning electron microscopy (SEM) analysis was made on the fracture surfaces to visualize the damage process: fibre fracture, matrix rupture and interface rupture. r 2006 Elsevier Ltd. All rights reserved. Keywords: Polyamide 66; Glass fibres; Mechanical behaviour; Damage; Acoustic emission
1. Introduction Glass fibre reinforced polyamides continue to be used with increased frequency in many applications, such as stressed functional automotive parts (fuel Corresponding author. Fax: +3 28 76 73 11.
E-mail address: [email protected] (B. Mouhmid). 0142-9418/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.03.008
injection rails, steering column switches) and safety parts in sports and leisure (snowboard bindings). These materials are known for their stiffness, toughness and resistance to dynamic fatigue. Fibre reinforced thermoplastics compounds may be processed by conventional methods, such as injection moulding, and offer improvements in mechanical properties over unreinforced ones. These composites compete with metals in many
ARTICLE IN PRESS
2. Experimental procedure 2.1. Material and tensile test conditions Chopped E-glass and PA66 were used to produce moulded composites with 0, 15, 30 and 50% (wt) glass content. The glass bundles and pre-dried PA66 pellets were dry blended to the desired glass content and compounded on a single screw extruder. The compounds were moulded on an 80 ton moulding machine. Compounding and moulding temperatures were respectively 275 and 280 1C with a mould temperature of 80 1C. The mechanical properties testing were performed at a relative humidity of 50% using ISO 527 specimens (Fig. 1). The tensile tests were carried out in a 10 kN Instron machine equipped with a temperature controlled chamber. The temperatures
545
20
150 60
10
engineering applications because of their ease of fabrication, light weight and economy. However, there are problems concerning material defects such as voids or cracks that may be present or initiated in one of three regions: the matrix, the fibre or the fibre/matrix interface [1]. The mechanical properties of thermoplastic composites containing short fibres have been the subject of much attention. These properties result from a combination of the fibre and the matrix properties and the ability to transfer stresses across the fibre/matrix interface, but also depend on the injection conditions such as screw and barrel parameters, mould temperature and design [2–5]. Variables such as fibre ratio, diameter, length, orientation and the interfacial strength are of prime importance to the final properties of the thermoplastic composites according to the studies led by Thomasson [6] and Shao-Yun Fu [7]. In this study, we characterized mechanically the behaviour of glass fibre reinforced and unreinforced polyamide 66. The effects of glass fibre content, temperature and strain rate have been investigated. An AE monitoring technique was used to identify different types of failure in the investigated composites. In order to visualize the damage process, scanning electron microscopy (SEM) analysis was made on the fracture surface of the specimens. The stress–strain response of the PA66 depends strongly on the humidity conditions. PA66 is considered as dry when the water absorption is less than 0.2% and saturated at 7.2%. The tensile strength in glass fibre reinforced PA66 is 50% higher in the dry state than in the saturated one [22].
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B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552
Fig. 1. Tensile specimen dimensions (mm).
and strain rates chosen were, respectively, T ¼ 20, 50 and 80 1C and 1, 5 and 50 mm/min corresponding to e_ ¼ 1:1 103 ; 5:6 103 and 5:6 102 s1 . 2.2. Damage analysis technique The tensile test was made with acoustic emission monitoring by the use of a Vallen AMSY 5 system. Acoustic emission (AE) is elastic radiation generated by the rapid release of energy from sources within a material. These elastic waves are detected and converted to voltage signals by small piezoelectric sensors mounted on a convenient surface of the material. The result is that AE can be used to monitor a structure for active damage even when ambient noise levels are extremely high. Sources of acoustic emission include in this case all types of damage. 3. Results and discussions In this section, the influence of an intrinsic parameter (glass fibre content) and two extrinsic parameters (strain rate e_ and temperature T) on the three mechanical properties of tensile elastic modulus E, tensile strength sr and the failure strain er , has been investigated. 3.1. Glass fibre content effect Fig. 2 shows stress–strain curves obtained for the studied composites at T ¼ 20 1C and e_ ¼ 5:6 103 s1 . We can note the typical stress–strain curves of reinforced materials showing high strength levels and low deformation capabilities. Indeed, glass fibre reinforced PA66 presents brittle behaviour with a failure strain of about 5.5%, whereas unreinforced PA66 presents ductile behaviour with a failure strain of about 25%. This figure also shows
ARTICLE IN PRESS B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552
546
2.80
180.0 50% 160.0
2.60
140.0
2.40
30%
120.0
R2 = 0.99
2.20 15%
100.0
σr/σr0
σ (MPa)
y = 1x2.8
80.0 0%
60.0
2.00 1.80 1.60
40.0
1.40
20.0
1.20
0.0
1.00 5.0
10.0
15.0 ε (%)
20.0
25.0
30.0
Fig. 2. Stress–strain curves with different glass fibre ratios at 20 1C and 5:6 103 s1 .
clearly that the increase in fibre content leads to an increase in tensile strength and elastic modulus. In order to explain these evolutions, the elastic modulus and the tensile strength can be modelled using a simple rearrangement of the rule of mixture equations: E ¼ avf E f þ ð1 vf Þ E m and sr ¼ bvf srf þ ð1 vf Þ srm where E f ; srf are the modulus and the tensile strength of the fibre (E f ¼ 76 GPa and srf ¼ 3200 MPa) and E m ; srm are, respectively, the experimental values of the modulus and the tensile strength of the unreinforced PA66, vf is the fibre volume fraction and a and b are the fibre orientation factors [8]. In our case, the average values of a and b are: a ¼ 0:24 and b ¼ 0:072, these values depend on the fibre length and orientation. The evolution of these two mechanical parameters can also be modelled using a phenomenological description based upon the relative representations: normalized tensile strength sr =srm , normalized elastic modulus E=E m versus relative density r=r0 where r0 is the density of unreinforced PA66. Figs. 3 and 4 illustrate these evolutions and show that they are described by power relationships as follows: 2:8 sr r ¼ r0 srm
3:7 E r and ¼ , Em r0
with a good correlation coefficient: R2 ¼ 0:99 for both formulas. Imad [9] has suggested similar
1
1.1
1.2
1.3
1.4
1.5
ρ/ρ0 Fig. 3. Variation of normalized tensile strength as a function of relative density.
4.00 y = 1x3.7
3.50
R2 = 0.99 3.00 E /E0
0.0
2.50
2.00
1.50
1.00 1
1.1
1.2
1.3
1.4
1.5
ρ/ρ0 Fig. 4. Variation of normalized modulus as a function of relative density.
expressions in a study about the mechanical behaviour of expanded polystyrene. This observation is in good agreement with literature data [10,11] and can be explained by the contribution of the glass fibre as a brittle and tough material. The loss of ductility is confirmed by the
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90.0
Stress MPa
70.0 50.0 30.0
0.0011 1/s 0.0056 1/s
10.0 -10.00.0
0.056 1/s 5.0
10.0
15.0 20.0 starin %
25.0
30.0
35.0
Fig. 6. Stress–strain curves of unreinforced PA66 for different strain rates.
function defined by E ¼ aE lnð_eÞ þ bE and sr ¼ asr lnð_eÞ þ bsr for the different studied materials [13,14]. Values of aE , bE , asr and bsr are given in Table 1. Fig. 5. (a) Ductile rupture. (b) Brittle fracture.
3.3. Temperature effect rupture specimen profile. In fact, the unreinforced PA66 clearly develops a striction zone. Contrariwise, the glass fibre reinforced PA66 shows a brittle fracture profile as shown in Figs. 5ab. 3.2. Strain rate effect Fig. 6 illustrates the stress–strain curves obtained from uniaxial tensile tests on unreinforced PA66 specimens at room temperature (20 1C) with the following strain rates: e_ ¼ 1:1 103 ; 5:6 103 and 5:6 102 s1 . These curves show that in unreinforced PA66 the tensile strength increases with the increase of strain rate, whereas the failure strain drops, which means the material gets less ductile. An explanation is that for high values of strain rate, molecular rearrangement does not have time to take place. This is in accordance with other results in the specialized literature [12]. The strain rate does not seem to have any significant effect on the modulus in the range considered. In glass fibre reinforced PA66, a typical stress– strain curve is shown in Fig. 7. Elastic modulus and tensile strength slightly increase as strain rate increases. Contrariwise, the failure strain is not affected by the strain rate in the range considered. The tensile modulus and the tensile strength are plotted against logarithms of strain rate in Figs. 8a and b and can be modelled using a simple regression
In order to evaluate the effect of temperature on the tensile properties of the glass fibre reinforced PA66, the stress–strain curves were plotted. Typical stress–strain curves at a constant strain rate of 5:6 103 s1 are shown in Fig. 9. It is noted that the tensile behaviour of the studied materials is strongly dependant on the temperature. It can be seen that there is an obvious region AC called the ‘‘toe region’’ as shown in Fig. 10. The ‘‘toe region’’ does not represent a material property but it is an artifact caused by the take up of slack as observed, e.g., by Shi et al. [15]. The slope between the origin B and point D on the elastic section in the stress–strain curve is taken as the Young’s modulus according to ASTM standard D882-97 [16]. The effect of temperature on the Young’s modulus is shown in Fig. 11. We can see that the normalized Young’s modulus decreases with the testing temperature, following a nonlinear relationship. The normalized tensile strength also decreases with increasing temperature as shown in Fig. 12. It can be noted that the failure strain increases as the temperature is increased up to 80 1C for the 15% (wt) short glass fibre reinforced PA66. Globally, the effect of temperature on the failure strain is much more accentuated in unreinforced PA66 than in short glass fibre reinforced PA66, as shown in Fig. 13.
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160.0 5mm/min 140.0
Stress (Mpa)
120.0
50mm/min 1mm/min
100.0 80.0 60.0 40.0 20.0 0.0 0.0
1.0
2.0
3.0 Strain (%)
4.0
5.0
6.0
Modulus (MPa)
Fig. 7. Stress–strain curves of 30% glass fibre reinforced PA66 for different strain rates.
6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 -7.00
PA66 0% PA66 15% PA66 30% PA66 50%
-6.00
-5.00
-4.00
-3.00
-2.00
Ln (strain rate)
(a)
200 180
140 120 100 80 60 PA66 0% PA66 15% PA66 30% PA66 50%
Tensile strength (MPa)
160
40 20 0
-7 (b)
-6
-5
-4
-3
-2
Ln (strain rate)
Fig. 8. (a) Variation of modulus as a function of ln ð_eÞ. (b) Variation of modulus as a function of ln ð_eÞ.
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Table 1 Values of aE and bE for the different studied materials
aE bE asr bsr
90
15% SGFR PA66
30% SGFR PA66
50% SGFR PA66
32.8 2079.1 1.71 71.17
88.6 3609.6 0.76 94.75
179.0 4791.7 2.94 142.57
170.7 6320.0 6.43 201.89
20°C 50°C 80°C
80 70
Relative modulus
60 50 40 30
E/Em
Stress (MPa)
0% SGFR PA66
20 10 0 0
2
6
4
8
10
Strain (%)
0% GFR PA66 15% GFR PA66 30% GFR PA66 50% GFR PA66
2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0
Fig. 9. Stress–strain curves of 15% glass fibre reinforced PA66 at strain rate of 5:6 103 s1 at different temperatures.
20
40 60 Temperature (°C)
80
100
Fig. 11. Normalized modulus versus temperature.
30 50°C
20 D σ/σm
Stress (MPa)
25
15
10
5
2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0
C 0 A 0
0% GFR PA66 15% GFR PA66 30% GFR PA66 50% GFR PA66
B 0.5
20
40 60 Temperature (°C)
80
100
Fig. 12. Normalized tensile strength versus temperature.
1 1.5 Strain (%)
2
1.5
Fig. 10. Stress–strain curves of 15% glass fibre reinforced PA66 at strain rate of 5:6 103 s1 at 50 1C.
4. Damage mechanisms 4.1. Acoustic emission The AE technique has been used to detect different damage stages in glass fibre reinforced
PA66 at room temperature. AE may have several physical causes: crack initiation and propagation, fibre/matrix interface and fiber failure. The number of AE events characterizes the global damage mechanism of the sample. It is possible to correlate each range of amplitude to a damage mechanism type. Many authors have already worked on this subject. Barre´ and Benzeggag [17,18], testing glass fibre reinforced polypropylene samples, reported that the acoustic signal varies with the corresponding damage mode: AE amplitude range from 40 to
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550
70
Failure strain (%)
60 50 40
PA00 PA15 PA30 PA50
30 20 10 0 20
40
60 80 Temperature (T°C)
100
Fig. 13. Effect of temperature on the failure strain.
55 dB corresponds to matrix cracking, 60–65 dB to debonding, 65–85 dB to pull-out and 85–95 dB to fibre fracture. Figs. 14a and b illustrate AE results of 15% (wt) glass fibre reinforced PA66: force, counts and amplitude versus time. These curves highlight three main damage process stages of the composite:
Fig. 14. Acoustic emission results of a 15% glass fibre reinforced PA66: (a) number of events and (b) amplitudes.
(1) An elastic zone with an insignificant acoustic activity (zone A). (2) A second zone corresponding to the nonlinear behaviour controlled by matrix plasticity and microcracks (zone B: amplitude between 40 and 60 dB). The beginning of this zone corresponds to the yield stress. (3) A last zone, C, of important damage represented by consequent AE activity preceding rupture. These interpretations are consolidated by other results [19–21]. 4.2. Scanning electron microscopy For more comprehension of rupture mechanisms, fracture surfaces have been observed by SEM. Fig. 15 is a fracture surface photography of an unreinforced PA66 showing craters. Cavitations could have occurred and produced crazing. Crazing could also have occurred near initial voids or an existing defect that took place during moulding. However, the presence of spherolites, that are
Fig. 15. Unreinforced PA66 failure facies.
crystalline defects, is the most probable hypothesis. Craze propagation led then to cracks. This stage relates to the ductile behaviour of unreinforced PA66. Once the crack is created, it weakens the material and primes the brittle rupture of the remaining ligament.
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Fig. 16. Fracture surface of a 50% (wt) short glass fibre reinforced PA66 showing a predominant orientation of the fibres.
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fibre percentage leads to higher values of tensile strength and modulus, and lower values of failure strain. Strain rate, in the studied range, has a less significant effect on glass fibre reinforced PA66 behaviour than on the unreinforced PA66. In unreinforced PA66, the strain rate increase results in a higher yield stress and a lower failure strain. Temperature plays a predominant role in the material behaviour, temperature increase leads to more ductility and less stiffness. AE testing is a powerful method for online detection and analysis of matrix, fibre and interface related active fracture processes in composite materials. A number of tools for identification and evaluation of damage stages and failure mechanisms exist. They are based on changes in AE activity or intensity features. Stress waves emitted by fibre breaks cause much higher AE amplitudes than other mechanisms, such as matrix cracking. SEM and AE analysis allowed working out the damage process and chronology. Unreinforced PA66 develops crazing followed by rupture of the remaining ligament. In short glass fibre reinforced PA66, fibre-matrix interface rupture is observed. In 30% (wt) and 50% (wt) glass fibre reinforced PA66, the damage is characterized by matrix plastication and microcracks, fibre pull out and fracture.
References Fig. 17. A 15% glass fibre reinforced PA66 failure surface.
Fig. 16 indicates predominant orientation of the fibres parallel to the flow direction in a random area of 50% (wt) short glass fibre reinforced PA66 specimen. Fig. 17 shows the fracture surface of the 15% glass fibre reinforced PA66. For this specimen, the fracture surface is rough and most of the fibres are oriented in the loading direction. This fracture surface is featured by some holes due to the pulled out fibres and the protuding fibres on the fracture surface. The same remarks can be made about reinforced PA66 with other short glass fibre contents. 5. Conclusions In this paper, the effects of strain rate, temperature and glass fibre content have been investigated. Glass fibre addition in PA66 results in loss of ductility and increased stiffness: increasing glass
[1] M. Akay, D.F. O’regan, Fracture behaviour of glass fibre reinforced polyamide mouldings, Polym. Test. 14 (1995) 149–162. [2] J.P. Tancrez, J. Pabiot, F. Rietsch, Damage and fracture mechanisms in thermoplastic-matrix composites in relation to processing and structural parameters, Compos. Sci. Technol. 56 (1996) 725–731. [3] G. Guerrica-Echevarria, J.I. Eguiazabal, J. Nazabal, Influence of the preparation method on the mechanical properties of a thermoplastic liquid crystalline copolyster, Polym. Test. 20 (2001) 403–408. [4] W.N. Ota, S.C. Amico, K.G. Satyanarayana, Studies on the combined effect of injection temperature and fiber content on the properties of the polypropylene–glass fiber composites, Compos. Sci. Technol. 65 (2005) 873–881. [5] A. Gu¨lu¨, A. O¨zdemir, E. O¨zdemir, Experimental investigation of the effect of glass fibres on the mechanical properties of polypropylene (PP) and polyamide 6 (PA6) plastics, Mater. Des. 27 (2006) 316–323. [6] J.L. Thomasson, The influence of fibre properties of the performance of glass-fibre-reinforced polyamide 66, Compos. Sci. Technol. 59 (1999) 2315–2328. [7] S.-Y. Fu, B. Lauke, A. San, Effect of fiber length and fiber orientation distributions on the tensile strength of short fiber
ARTICLE IN PRESS 552
[8]
[9]
[10]
[11]
[12]
[13]
[14]
B. Mouhmid et al. / Polymer Testing 25 (2006) 544–552 reinforced polymers, Comopos. Sci. Technol. 56 (1996) 1179–1190. G.X. Sui, S.-C. Wong, R. Yang, C.Y. Yue, The effect of fiber inclusions in toughened plastics—Part II: determination of micromechanical parameters, Compos. Sci. Technol. 65 (2005) 221–229. A. Imad, A viscoelastic–plastic behaviour analysis of expanded polystyrene under compressive loading: experiments and modeling, International Journal of Cellular Polymers 20 (2001) 189–209. J.L. Thomason, Micromechanical parameters from macromechanical measurements on glass reinforced polyamide 6,6, Compos. Sci. Technol. 61 (2001) 2007–2016. S.Y. Fu, Tensile properties of short glass fiber and short carbon fiber reinforced polypropylene composites, Composites A 31 (2000) 1117–1125. Z. Wang, Y. Zhou, P.K. Mallick, Effect of temperature and strain rate on the tensile behavior of short fiber reinforced polyamide-6, Polym. Compos. 23 (2002) 858–871. Y. Zhou, P.K. Mallick, A non linear damage model for the tensile behavior of an injection molded short E-glass fiber reinforced polyamide 6,6, Mater. Sci. Eng. A 393 (2005) 303–309. M. Todo, K. Takahashi, P. Be´guelin, H. Kausch, Strain rate dependence of the tensile fracture behaviour of woven cloth reinforced polyamide composites, Compos. Sci. Technol. 60 (2000) 763–771.
[15] X.Q. Shi, Z.P. Wang, H.L.J. Pang, X.R. Zhang, Investigation of effect of temperature and strain rate on mechanical properties of underfill material by uses of microtensile specimens, Polym. Test. 21 (2002) 725–733. [16] ASTM (American Standards for Testing Methods) Standard, ASTM designation: D882-97 (1996) 159–167. [17] S. Barre´, M. Bezeggagh, On the use of acoustic emission to investigate damage mechanics in glass-fibre reinforced polypropylene, Compos. Sci. Technol. 52 (1994) 369–376. [18] M. Benzeggagh, M. Kennane, Measurement of mixed mode delamination fracture toughness of unidirectional glass–epoxy composites with mixed modebending apparatus, Compos. Sci. Technol. (1995) 439–449. [19] A. Laksimi, L. Bounouas, S. Benmedakhene, Z. Azari, A. Imad, Damage on polypropylene glass fibre composites: experimental analysis, C.R. Acad. Sci. Paris Se´rie II b (2000) 651–656. [20] S. Huguet, N. Godin, R. Gaertner, L. Salmon, D. Villard, Use of acoustic emission to identify damage modes in glass fibre polyester, Compos. Sci. Technol. 62 (2002) 1433–1444. [21] M. Benzeggagh, S. Benmedakhene, Residual strength of a glass/polypropylene composite material subjected to impact, Compos. Sci. Technol. 55 (1995) 1–11. [22] A. Bergeret, I. Pires, M.P. Foulc, B. Abadie, L. Ferry, A. Crespy, The hygrothermal behaviour of glass-fibre-reinforced thermoplastic composites: a prediction of the composite lifetime, Polym. Test. 20 (2001) 753–763.