- Email: [email protected]
Development of SENB toughness measurement for thermoset resins
ARTICLE IN PRESS
POLYMER TESTING Polymer Testing 26 (2007) 445–450 www.elsevier.com/locate/polytest
Test Method
Development of SENB toughness measurement for thermoset resins Jun Maa,, Qing Qia, Jessica Bayleya, Xu-Sheng Dua, Mao-Song Moa, Li-Qun Zhangb, a
Center for Advanced Materials Technology, School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia b Key Laboratory for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China Received 31 October 2006; accepted 10 December 2006
Abstract Conventional single-edge notched bending (SENB) fracture toughness measurement is ideal for plastic materials, but it is intrinsically difficult to initiate a sufficiently sharp crack for brittle thermoset resins. The problem was addressed in this research by an instantly propagated crack with a geometry-modified SENB specimen. Because the crack sharpness was natural, sufficient and repeatable, the obtained toughness value was accurate and reproducible, which was verified by double cantilever beam and compact tension measurements. The methodology developed here will provide a guide for brittle thermoset toughness testing. r 2007 Published by Elsevier Ltd. Keywords: Fracture toughness; Single-edge notched bending; Thermoset
1. Introduction Plane-strain fracture toughness (K1c)/critical strain energy release rate (G1c) is the most important material property for brittle thermoset resins; it is the worst-case scenario toughness test widely used in materials science research and structural engineering design. ISO 13586 and ASTM D5045-99 provide two methods for toughness measurement: singleedge notched bending (SENB) and compact tension (CT). SENB is much more popular, as evidenced by publication numbers [1–27], owing to convenient Corresponding author. Also corresponding author.
E-mail addresses: [email protected] (J. Ma), [email protected] (L.-Q. Zhang). 0142-9418/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.polymertesting.2006.12.011
processing with less material used. However, both standards are insufficient as regards information on how to initiate a sufficiently sharp crack, which is crucial for a brittle thermoset resin toughness test. As a result, the details of crack preparation are absent in a number of publications using SENB. Even from the reported crack preparation [19–27], it is still not clear if and how a sufficiently sharp crack can be made for SENB. A typical procedure is to chill a razor blade in liquid nitrogen and then tap it on the specimen, but without specifying (a) the blade temperature at which a crack was initiated, as the crack sharpness depends on the temperature, and (b) if the crack is reproducible with sufficient sharpness. Both standards advise that a sufficiently sharp crack is a prerequisite for toughness measurement.
ARTICLE IN PRESS 446
J. Ma et al. / Polymer Testing 26 (2007) 445–450
Fig. 1. Specimen geometries of (a) single edge notched bending (SENB), (b) modified single edge notched bending (m-SENB), (c) compact tension (CT), and (d) double cantilever beam (DCB).
Tapping, pressing and sliding a fresh razor blade are the common procedures used to initiate cracks. The crack sharpness made by pressing or sliding a blade certainly depends on the force as well as the blade sharpness. Xiao [28] found a razor blade tapping method produced a sufficiently sharp crack for CT specimens. However, there are two questions yet to answer: whether a crack by tapping is necessarily sufficiently sharp and how to initiate such a crack for a SENB specimen of thermoset resin. Both standards advise machining a notch on the SENB specimen followed by tapping across the notch root. In Fig. 1(a), the SENB geometry commonly used is 70–100 mm 8–12 mm (4–6) mm. As a notch takes off at least 3 mm, the remaining width 5–9 mm is not enough to accommodate a sharp instantly propagated crack (see below). Hence, it is intrinsically difficult to initiate a sufficiently sharp crack for the conventional SENB specimen. Tapping a razor blade into a thermoset specimen initiates two types of crack: non-propagated or instantly propagated. In this paper, we will investigate the influence of these two crack types on the fracture toughness of thermoset resin. The instantly propagated crack will be initiated for geometry-modified SENB specimens; and its measurements on neat epoxy, epoxy/ liquid rubber composite and epoxy/core-shell rubber composite will be verified with measurements by CT and double cantilever beam (DCB). 2. Experimental section 2.1. Materials Epoxy resin Diglycidyl Ether of Bisphenal A (DGEBA, Araldite-F) with epoxide equivalent weight
182–196 g/eq was supplied by Ciba-Geigy, Australia. Piperidine hardener was ordered from Aldrich. The liquid rubber was amine-terminated butadiene acrylonitrile copolymer (1300 35ATBN), ordered from Hycar-reactive liquid polymers. The core-shell rubber was kindly provided by Kaneka Corp. 2.2. Preparation of materials Epoxy resin was mixed with a given amount of liquid rubber or core–shell rubber at 120 1C for 10 min, and then piperidiene was added at a weight ratio of 100/5 (epoxy/hardener). After mixing at 120 1C for 5 min, the mixture was degassed, poured into an aluminum mould with a target sample size of 250 mm 80 mm 5 mm followed by curing at 120 1C for 17.5 h. It is noteworthy that a SENB specimen with 5–6 mm thickness is suitable for tapping, because with a thicker specimen it is more difficult to instantly propagate a crack and often breaks a razor blade. 2.3. Preparation of modified SENB specimen As analyzed in the Introduction, it is intrinsically difficult to initiate a sufficiently sharp crack for the conventional SENB thermoset. Hence, we modified the geometry by omitting the notch, as shown in Fig. 1(b). The modified SENB is denoted as m-SENB. The cured epoxy sample was marked and cut by a band saw to obtain specimens of 70 mm 8–12 mm 5 mm. Each side of the specimens was polished with emery paper until all visible marks disappeared. As pointed out by both standards, the crack length a should preferably be in the range given by 0.45pa/wp0.55. Since the
ARTICLE IN PRESS J. Ma et al. / Polymer Testing 26 (2007) 445–450
crack length actually depended on resin brittleness, tapping force and blade sharpness, the specimen width was estimated by tapping several specimens before cutting the samples off. Samples for CT and DCB specimens were both machined by a workshop because of the complicated geometry. 2.4. Razor blade tapping Initiating a crack by tapping is a tricky procedure. Hence, ISO 13586 suggests that it is essential to practice this since, in brittle test specimens, although a natural crack can be generated by this process, some skill is required in avoiding too long a crack or local damage. When tapping was made by hand, the razor blade did not always cut perpendicularly to the specimen length. In this situation, the razor blade must be taken out, which left a tiny cut, and it was then mounted again on a new region of the same specimen but with a new blade edge. Being much blunter and shorter than instantly propagated cracks, the small cut had little influence on the toughness. Tapping a razor blade into brittle thermoset resins experiences three stages, as shown in Fig. 2. At the first stage, the blade gradually cut into the specimen with the blade edge always adjacent to the crack tip, and the blade advanced with each tap. After the blade cut into the resin for 0.5–0.7 mm, the second stage started. At this stage, the blade appeared not to move despite repeated taps, and the crack did not grow. The cracks initiated through these two stages are termed non-propagated cracks. Obviously, the sharpness of this type of crack depends on the tapping force and the blade
447
sharpness. At the final stage, a crack instantly propagated with a single tap. As shown in Fig. 2, this crack was able to propagate 3–10 mm, depending on the tapping force and the resin brittleness. This crack sharpness is reproducible, because this instantly propagated crack is naturally sharp and repeatable. The instantly propagated cracks made in stage three are much sharper and longer than the non-propagated cracks initiated in stages 1 and 2. The procedure we used was as follows. A m-SENB specimen was placed horizontally and a new razor blade was perpendicularly mounted over it. A 272-g hammer was used for tapping. An instantly propagated crack appeared after the blade was tapped into the specimen 0.5 mm for neat epoxy and 0.7 mm for toughened samples. Since a controllable crack was desired, we tapped the specimen as slightly as possible. However, even the slightest tap instantly propagated a 3–6 mm long crack for each type of material. The following requirements were used for choosing specimens for calculation: (a) the instantly propagated crack should be perpendicular to the specimen length with similar length on both sides; (b) the ratio of the crack length with the width should be in the range of 0.3oa/wo0.7; and (c) the fracture surface should be flat. At least 10 specimens should be measured. 2.5. Similar cracks were made for CT and DCB specimens Due to the larger geometries of CT and DCB specimens, as shown in Fig. 1, instantly propagated cracks are easier to make. Six specimens were tested for each set of data. 2.5.1. Calculation The K1c and G1c values of m-SENB and CT specimens were calculated according to ISO13586. The calculation for DCB specimens was conducted according to Refs. [26,29]. 3. Results and discussion 3.1. Development of SENB measurement with two types of cracks
Fig. 2. Schematic of an instantly propagated crack by tapping.
As described earlier, tapping can initiate two types of cracks: non-propagated cracks and instantly propagated cracks. We investigated the
ARTICLE IN PRESS J. Ma et al. / Polymer Testing 26 (2007) 445–450
448
a
b 4.0 3.5
1.0
K1c, MPa·m1/2
K1c, MPa·m1/2
1.2
0.8 0.6 0.4
3.0 2.5 2.0 1.5 1.0
0.2
0.5
0.0 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80
0.0 0.35 0.40 0.45 0.50 0.55 0.60 0.65
Crack length/width
Crack length/width
d 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
3.5 3.0
K1c, MPa·m1/2
K1c
c
2.5 2.0 1.5 1.0 0.5
0.2
0.3
0.4
0.5
0.6
0.0 0.40
0.7
0.45
Crack Leangth ratio
0.50
0.55
0.60
0.65
Crack length/width
f
e
3.5
2.5
3.0
K1c, MPa·m1/2
K1c,MPa·m1/2
4.0 3.0
2.0 1.5 1.0
2.5 2.0 1.5 1.0
0.5
0.5
0.0 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70
0.0
Crack length/width
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Crack length/width
Fig. 3. Fracture toughness measurement for neat epoxy, epoxy/liquid rubber and epoxy/core-shell rubber composite with the two types of cracks. (a) Neat epoxy toughness measured with an instantly propagated crack, (b) neat epoxy toughness measured with a non-propagated crack, (c) epoxy/liquid rubber toughness measured with an instantly propagated crack, (d) epoxy/liquid rubber toughness measured with a non-propagated crack, (e) epoxy/core-shell rubber toughness measured with an instantly propagated crack, and (f) epoxy/core-shell rubber toughness measured with a non-propagated crack.
influence of the two crack types on fracture toughness of thermoset resins as below. Figs. 3(a) and (b) show plane-strain fracture toughness (K1c) with the ratio of crack length over width for neat epoxy resin. The specimens with instantly propagated cracks shows K1c 0.73 MPa m1/2, much lower than the value 2.62 MPa m1/2 for the specimens with non-propagated cracks. This indicates that only an instantly propagated crack is sufficiently sharp for the fracture toughness test. The test for Fig. 3(a) was separately repeated twice
by different researchers, and similar results were obtained, demonstrating credible reproducibility of the instantly propagated cracks. As liquid rubber and core–shell rubber are important tougheners for brittle thermoset resins, we measured the toughness of epoxy toughened by both materials. In Figs. 3(c) and (d), there is a significant toughness difference between these two types of cracks for epoxy/liquid rubber composite, demonstrating instantly propagated cracks are much sharper than non-propagated ones for brittle
ARTICLE IN PRESS J. Ma et al. / Polymer Testing 26 (2007) 445–450
449
Table 1 Comparison of critical strain energy release rate (G1c, kJ/m2) of neat epoxy, epoxy/liquid rubber composite and epoxy/core-shell rubber composite using various measurements Materials
Measured by the modified single-edge notched bending method (m-SENB)
Measured by compact tension method (CT)
Measured by double cantilever beam method (DCB)
Neat epoxy Epoxy/liquid rubber composite Epoxy/core-shell rubber composite
0.1870.03 0.4270.06 1.4370.13
0.1570.01 0.4170.01 1.4070.02
0.1770.02 0.4470.02 1.5170.15
materials. In Figs. 3(e) and (f), toughness difference is not significant for toughened epoxy, but it still indicates that instantly propagated cracks are sharper than non-propagated ones. This is because the toughened resin is less sensitive to crack sharpness than the brittle neat resin. 3.2. Evaluation of m-SENB measurement by CT and DCB We developed the instantly propagated crack for the toughness measurement of thermoset m-SENB specimens. This will be verified below. There are three methods for toughness measurement: SENB, CT and DCB, of which DCB is reliable and favored in fracture research, as a number of sufficiently sharp cracks are naturally initiated during the test. These measurements should achieve similar toughness values, because true fracture toughness is independent of the testing method so long as reproducible and sufficiently sharp cracks are made. In Table 1 listed the fracture toughness values of neat epoxy, epoxy/liquid rubber and epoxy/coreshell rubber composite by CT and DCB methods. G1c values of all samples measured by m-SENB, are similar to the values by DCB and CT. This means that this measurement using the instantly propagated crack on a m-SENB specimen is reliable. However, it is worthy to note that there are some discrepancies in previous literature. Rosso [30] reported G1c 0.25 kJ m 2 for neat epoxy resin using the same raw materials and preparation, higher than ours 0.15 kJ m 2. In our opinion, this was due to the specimen thickness 12 mm, which was harder to instantly propagate a crack and, thus, the obtained crack was less sharp than our thinner specimen. Wang [26] reported DCB values much lower than SENB. In the authors’ view, this was caused by the
insufficient crack sharpness of the SENB specimens made by pressing instead of tapping. 4. Conclusion A geometry-m-SENB fracture toughness measurement was developed with an instantly propagated crack: 1. Instantly propagated crack by tapping: This type of crack is much sharper than a non-propagated crack and leads to a minimum of fracture toughness. 2. Sample geometry: A thinner sample is easier for tapping the crack, but it might be more difficult to process. The thickness range of 5–6 mm is ideal for tapping and processing. Specimen width is determined by the length of an instantly propagated crack. A machined notch may be necessary. 3. Correctness verification: The toughness values measured by the m-SENB were verified by DCB and CT methods. 4. Comparison: The CT and DCB specimens have to be submitted to a workshop for processing; which may take couples of weeks, depending on the availability of technicians. By contrast, the m-SENB specimen can be easily processed within hours, independent of a workshop. Therefore, the m-SENB is the more simple and efficient method for fracture toughness tests. Acknowledgments This project is supported by the Australian Research Council (ARC). JM thanks the Australian Research Council (ARC) for the award of an Australian Postdoctoral Fellowship, tenable at the University of Sydney. The authors thank Prof. Gordon J Williams and Prof. Yiu-Wing Mai for useful suggestions.
ARTICLE IN PRESS 450
J. Ma et al. / Polymer Testing 26 (2007) 445–450
References [1] J. Y Choi, A. F Yee, R.M. Laine, Macromolecules 37 (2004) 3267. [2] J. Frohlich, R. Thomann, R. Mulhaupt, Macromolecules 36 (2003) 7205. [3] A. Zubeldia, M. Larran˜aga, P. Remiro, I. Mondragon, J.Polym. Sci.B: Polym. Phys. 42 (2004) 3920. [4] H. Harani, S. Fellahi, J.Appl. Polym. Sci. 71 (1999) 29. [5] M. Imanaka, Y. Nakamura, A. Nishimura, Compos. Sci. Technol. 63 (2003) 41. [6] L. Be´cu-Longuet, A. Bonnet, C. Pichot, H. Sautereau, A. Maazouz, J. Appl. Polym. Sci. 72 (1999) 849. [7] J.J. La Scala, J.A. Orlicki, C. Winston, E.J. Robinette, J.M. Sands, G.R. Palmese, Polymer 46 (2005) 2908. [8] L. Valette, Macromol. Mater. Eng. 288 (2003) 867. [9] S.J. Park, F.L. Jin, J.R. Lee, Mater. Sci. Eng. A 374 (2004) 109. [10] M. Preghenella, A. Pegoretti, C. Migliaresi, Polymer 46 (2005) 12065. [11] B. Qi, Q.X. Zhang, M. Bannister, Compos. Struct. 75 (2006) 514. [12] Q. Wu, J.J. Xu, Y.Z. Yu, J. Appl. Polym. Sci. 80 (2001) 1237. [13] E.J. Robinette, S. Ziaee, G.R. Palmese, Polymer 45 (2004) 6143. [14] S. Fellahi, N. Chikhi, M. Bakar, J. Appl. Polym. Sci. 82 (2001) 861.
[15] S. Bhuniya, B. Adihikari, J. Appl. Polym. Sci. 90 (2003) 1497. [16] S.J. Park, H.C. Kim, J. Polym. Sci. B: Polym. Phys. 39 (2001) 121. [17] S.C. Chen, S.T. Chiu, C.P. Ye, J. Appl. Polym. Sci. 86 (2002) 3740. [18] T. Iijima, K.I. Fujimoto, M. Tomoi, J. Appl. Polym. Sci. 84 (2002) 388. [19] R. Bagheri, R.A. Pearson, Polymer 37 (1996) 4529. [20] J. Brunner, A. Necola, M. Rees, P. Gasser, X. Kornmann, R. Thomann, R.M. Barbezat, Eng. Fract. Mech. 73 (2006) 2336. [21] C. Kanchanomai, S. Rattananon, Polym. Test. 24 (2005) 886. [22] C. Kaynak, Polym. Test. 25 (2006) 296. [23] R. Bagheri, Polym. Eng. Sci. 37 (1997) 245. [24] M. Rutnakornpituk, Eur. Polym. J. 41 (2005) 1043. [25] T. H Yoon, J. E Mcgrath, J. Appl. Polym. Sci. 80 (2001) 1504. [26] K. Wang, J. Wu, M.L. Toh, C. He, A.F. Yee, Macromolecules 38 (2005) 788. [27] Z. Zhang, Y.Y. Zhao, J. Appl. Polym. Sci. 84 (2002) 1223. [28] K.Q. Xiao, L. Ye, Y.S. Kwok, J. Mater. Sci. 33 (1998) 2831. [29] J. Du, M.D. Thouless, A.F. Yee, Int. J. Fract. 92 (1998) 271. [30] P. Rosso, L. Ye, K. Friedrich, S. Sprenger, J. Appl. Polym. Sci. 100 (2006) 1849.
POLYMER TESTING Polymer Testing 26 (2007) 445–450 www.elsevier.com/locate/polytest
Test Method
Development of SENB toughness measurement for thermoset resins Jun Maa,, Qing Qia, Jessica Bayleya, Xu-Sheng Dua, Mao-Song Moa, Li-Qun Zhangb, a
Center for Advanced Materials Technology, School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia b Key Laboratory for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China Received 31 October 2006; accepted 10 December 2006
Abstract Conventional single-edge notched bending (SENB) fracture toughness measurement is ideal for plastic materials, but it is intrinsically difficult to initiate a sufficiently sharp crack for brittle thermoset resins. The problem was addressed in this research by an instantly propagated crack with a geometry-modified SENB specimen. Because the crack sharpness was natural, sufficient and repeatable, the obtained toughness value was accurate and reproducible, which was verified by double cantilever beam and compact tension measurements. The methodology developed here will provide a guide for brittle thermoset toughness testing. r 2007 Published by Elsevier Ltd. Keywords: Fracture toughness; Single-edge notched bending; Thermoset
1. Introduction Plane-strain fracture toughness (K1c)/critical strain energy release rate (G1c) is the most important material property for brittle thermoset resins; it is the worst-case scenario toughness test widely used in materials science research and structural engineering design. ISO 13586 and ASTM D5045-99 provide two methods for toughness measurement: singleedge notched bending (SENB) and compact tension (CT). SENB is much more popular, as evidenced by publication numbers [1–27], owing to convenient Corresponding author. Also corresponding author.
E-mail addresses: [email protected] (J. Ma), [email protected] (L.-Q. Zhang). 0142-9418/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.polymertesting.2006.12.011
processing with less material used. However, both standards are insufficient as regards information on how to initiate a sufficiently sharp crack, which is crucial for a brittle thermoset resin toughness test. As a result, the details of crack preparation are absent in a number of publications using SENB. Even from the reported crack preparation [19–27], it is still not clear if and how a sufficiently sharp crack can be made for SENB. A typical procedure is to chill a razor blade in liquid nitrogen and then tap it on the specimen, but without specifying (a) the blade temperature at which a crack was initiated, as the crack sharpness depends on the temperature, and (b) if the crack is reproducible with sufficient sharpness. Both standards advise that a sufficiently sharp crack is a prerequisite for toughness measurement.
ARTICLE IN PRESS 446
J. Ma et al. / Polymer Testing 26 (2007) 445–450
Fig. 1. Specimen geometries of (a) single edge notched bending (SENB), (b) modified single edge notched bending (m-SENB), (c) compact tension (CT), and (d) double cantilever beam (DCB).
Tapping, pressing and sliding a fresh razor blade are the common procedures used to initiate cracks. The crack sharpness made by pressing or sliding a blade certainly depends on the force as well as the blade sharpness. Xiao [28] found a razor blade tapping method produced a sufficiently sharp crack for CT specimens. However, there are two questions yet to answer: whether a crack by tapping is necessarily sufficiently sharp and how to initiate such a crack for a SENB specimen of thermoset resin. Both standards advise machining a notch on the SENB specimen followed by tapping across the notch root. In Fig. 1(a), the SENB geometry commonly used is 70–100 mm 8–12 mm (4–6) mm. As a notch takes off at least 3 mm, the remaining width 5–9 mm is not enough to accommodate a sharp instantly propagated crack (see below). Hence, it is intrinsically difficult to initiate a sufficiently sharp crack for the conventional SENB specimen. Tapping a razor blade into a thermoset specimen initiates two types of crack: non-propagated or instantly propagated. In this paper, we will investigate the influence of these two crack types on the fracture toughness of thermoset resin. The instantly propagated crack will be initiated for geometry-modified SENB specimens; and its measurements on neat epoxy, epoxy/ liquid rubber composite and epoxy/core-shell rubber composite will be verified with measurements by CT and double cantilever beam (DCB). 2. Experimental section 2.1. Materials Epoxy resin Diglycidyl Ether of Bisphenal A (DGEBA, Araldite-F) with epoxide equivalent weight
182–196 g/eq was supplied by Ciba-Geigy, Australia. Piperidine hardener was ordered from Aldrich. The liquid rubber was amine-terminated butadiene acrylonitrile copolymer (1300 35ATBN), ordered from Hycar-reactive liquid polymers. The core-shell rubber was kindly provided by Kaneka Corp. 2.2. Preparation of materials Epoxy resin was mixed with a given amount of liquid rubber or core–shell rubber at 120 1C for 10 min, and then piperidiene was added at a weight ratio of 100/5 (epoxy/hardener). After mixing at 120 1C for 5 min, the mixture was degassed, poured into an aluminum mould with a target sample size of 250 mm 80 mm 5 mm followed by curing at 120 1C for 17.5 h. It is noteworthy that a SENB specimen with 5–6 mm thickness is suitable for tapping, because with a thicker specimen it is more difficult to instantly propagate a crack and often breaks a razor blade. 2.3. Preparation of modified SENB specimen As analyzed in the Introduction, it is intrinsically difficult to initiate a sufficiently sharp crack for the conventional SENB thermoset. Hence, we modified the geometry by omitting the notch, as shown in Fig. 1(b). The modified SENB is denoted as m-SENB. The cured epoxy sample was marked and cut by a band saw to obtain specimens of 70 mm 8–12 mm 5 mm. Each side of the specimens was polished with emery paper until all visible marks disappeared. As pointed out by both standards, the crack length a should preferably be in the range given by 0.45pa/wp0.55. Since the
ARTICLE IN PRESS J. Ma et al. / Polymer Testing 26 (2007) 445–450
crack length actually depended on resin brittleness, tapping force and blade sharpness, the specimen width was estimated by tapping several specimens before cutting the samples off. Samples for CT and DCB specimens were both machined by a workshop because of the complicated geometry. 2.4. Razor blade tapping Initiating a crack by tapping is a tricky procedure. Hence, ISO 13586 suggests that it is essential to practice this since, in brittle test specimens, although a natural crack can be generated by this process, some skill is required in avoiding too long a crack or local damage. When tapping was made by hand, the razor blade did not always cut perpendicularly to the specimen length. In this situation, the razor blade must be taken out, which left a tiny cut, and it was then mounted again on a new region of the same specimen but with a new blade edge. Being much blunter and shorter than instantly propagated cracks, the small cut had little influence on the toughness. Tapping a razor blade into brittle thermoset resins experiences three stages, as shown in Fig. 2. At the first stage, the blade gradually cut into the specimen with the blade edge always adjacent to the crack tip, and the blade advanced with each tap. After the blade cut into the resin for 0.5–0.7 mm, the second stage started. At this stage, the blade appeared not to move despite repeated taps, and the crack did not grow. The cracks initiated through these two stages are termed non-propagated cracks. Obviously, the sharpness of this type of crack depends on the tapping force and the blade
447
sharpness. At the final stage, a crack instantly propagated with a single tap. As shown in Fig. 2, this crack was able to propagate 3–10 mm, depending on the tapping force and the resin brittleness. This crack sharpness is reproducible, because this instantly propagated crack is naturally sharp and repeatable. The instantly propagated cracks made in stage three are much sharper and longer than the non-propagated cracks initiated in stages 1 and 2. The procedure we used was as follows. A m-SENB specimen was placed horizontally and a new razor blade was perpendicularly mounted over it. A 272-g hammer was used for tapping. An instantly propagated crack appeared after the blade was tapped into the specimen 0.5 mm for neat epoxy and 0.7 mm for toughened samples. Since a controllable crack was desired, we tapped the specimen as slightly as possible. However, even the slightest tap instantly propagated a 3–6 mm long crack for each type of material. The following requirements were used for choosing specimens for calculation: (a) the instantly propagated crack should be perpendicular to the specimen length with similar length on both sides; (b) the ratio of the crack length with the width should be in the range of 0.3oa/wo0.7; and (c) the fracture surface should be flat. At least 10 specimens should be measured. 2.5. Similar cracks were made for CT and DCB specimens Due to the larger geometries of CT and DCB specimens, as shown in Fig. 1, instantly propagated cracks are easier to make. Six specimens were tested for each set of data. 2.5.1. Calculation The K1c and G1c values of m-SENB and CT specimens were calculated according to ISO13586. The calculation for DCB specimens was conducted according to Refs. [26,29]. 3. Results and discussion 3.1. Development of SENB measurement with two types of cracks
Fig. 2. Schematic of an instantly propagated crack by tapping.
As described earlier, tapping can initiate two types of cracks: non-propagated cracks and instantly propagated cracks. We investigated the
ARTICLE IN PRESS J. Ma et al. / Polymer Testing 26 (2007) 445–450
448
a
b 4.0 3.5
1.0
K1c, MPa·m1/2
K1c, MPa·m1/2
1.2
0.8 0.6 0.4
3.0 2.5 2.0 1.5 1.0
0.2
0.5
0.0 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80
0.0 0.35 0.40 0.45 0.50 0.55 0.60 0.65
Crack length/width
Crack length/width
d 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
3.5 3.0
K1c, MPa·m1/2
K1c
c
2.5 2.0 1.5 1.0 0.5
0.2
0.3
0.4
0.5
0.6
0.0 0.40
0.7
0.45
Crack Leangth ratio
0.50
0.55
0.60
0.65
Crack length/width
f
e
3.5
2.5
3.0
K1c, MPa·m1/2
K1c,MPa·m1/2
4.0 3.0
2.0 1.5 1.0
2.5 2.0 1.5 1.0
0.5
0.5
0.0 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70
0.0
Crack length/width
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Crack length/width
Fig. 3. Fracture toughness measurement for neat epoxy, epoxy/liquid rubber and epoxy/core-shell rubber composite with the two types of cracks. (a) Neat epoxy toughness measured with an instantly propagated crack, (b) neat epoxy toughness measured with a non-propagated crack, (c) epoxy/liquid rubber toughness measured with an instantly propagated crack, (d) epoxy/liquid rubber toughness measured with a non-propagated crack, (e) epoxy/core-shell rubber toughness measured with an instantly propagated crack, and (f) epoxy/core-shell rubber toughness measured with a non-propagated crack.
influence of the two crack types on fracture toughness of thermoset resins as below. Figs. 3(a) and (b) show plane-strain fracture toughness (K1c) with the ratio of crack length over width for neat epoxy resin. The specimens with instantly propagated cracks shows K1c 0.73 MPa m1/2, much lower than the value 2.62 MPa m1/2 for the specimens with non-propagated cracks. This indicates that only an instantly propagated crack is sufficiently sharp for the fracture toughness test. The test for Fig. 3(a) was separately repeated twice
by different researchers, and similar results were obtained, demonstrating credible reproducibility of the instantly propagated cracks. As liquid rubber and core–shell rubber are important tougheners for brittle thermoset resins, we measured the toughness of epoxy toughened by both materials. In Figs. 3(c) and (d), there is a significant toughness difference between these two types of cracks for epoxy/liquid rubber composite, demonstrating instantly propagated cracks are much sharper than non-propagated ones for brittle
ARTICLE IN PRESS J. Ma et al. / Polymer Testing 26 (2007) 445–450
449
Table 1 Comparison of critical strain energy release rate (G1c, kJ/m2) of neat epoxy, epoxy/liquid rubber composite and epoxy/core-shell rubber composite using various measurements Materials
Measured by the modified single-edge notched bending method (m-SENB)
Measured by compact tension method (CT)
Measured by double cantilever beam method (DCB)
Neat epoxy Epoxy/liquid rubber composite Epoxy/core-shell rubber composite
0.1870.03 0.4270.06 1.4370.13
0.1570.01 0.4170.01 1.4070.02
0.1770.02 0.4470.02 1.5170.15
materials. In Figs. 3(e) and (f), toughness difference is not significant for toughened epoxy, but it still indicates that instantly propagated cracks are sharper than non-propagated ones. This is because the toughened resin is less sensitive to crack sharpness than the brittle neat resin. 3.2. Evaluation of m-SENB measurement by CT and DCB We developed the instantly propagated crack for the toughness measurement of thermoset m-SENB specimens. This will be verified below. There are three methods for toughness measurement: SENB, CT and DCB, of which DCB is reliable and favored in fracture research, as a number of sufficiently sharp cracks are naturally initiated during the test. These measurements should achieve similar toughness values, because true fracture toughness is independent of the testing method so long as reproducible and sufficiently sharp cracks are made. In Table 1 listed the fracture toughness values of neat epoxy, epoxy/liquid rubber and epoxy/coreshell rubber composite by CT and DCB methods. G1c values of all samples measured by m-SENB, are similar to the values by DCB and CT. This means that this measurement using the instantly propagated crack on a m-SENB specimen is reliable. However, it is worthy to note that there are some discrepancies in previous literature. Rosso [30] reported G1c 0.25 kJ m 2 for neat epoxy resin using the same raw materials and preparation, higher than ours 0.15 kJ m 2. In our opinion, this was due to the specimen thickness 12 mm, which was harder to instantly propagate a crack and, thus, the obtained crack was less sharp than our thinner specimen. Wang [26] reported DCB values much lower than SENB. In the authors’ view, this was caused by the
insufficient crack sharpness of the SENB specimens made by pressing instead of tapping. 4. Conclusion A geometry-m-SENB fracture toughness measurement was developed with an instantly propagated crack: 1. Instantly propagated crack by tapping: This type of crack is much sharper than a non-propagated crack and leads to a minimum of fracture toughness. 2. Sample geometry: A thinner sample is easier for tapping the crack, but it might be more difficult to process. The thickness range of 5–6 mm is ideal for tapping and processing. Specimen width is determined by the length of an instantly propagated crack. A machined notch may be necessary. 3. Correctness verification: The toughness values measured by the m-SENB were verified by DCB and CT methods. 4. Comparison: The CT and DCB specimens have to be submitted to a workshop for processing; which may take couples of weeks, depending on the availability of technicians. By contrast, the m-SENB specimen can be easily processed within hours, independent of a workshop. Therefore, the m-SENB is the more simple and efficient method for fracture toughness tests. Acknowledgments This project is supported by the Australian Research Council (ARC). JM thanks the Australian Research Council (ARC) for the award of an Australian Postdoctoral Fellowship, tenable at the University of Sydney. The authors thank Prof. Gordon J Williams and Prof. Yiu-Wing Mai for useful suggestions.
ARTICLE IN PRESS 450
J. Ma et al. / Polymer Testing 26 (2007) 445–450
References [1] J. Y Choi, A. F Yee, R.M. Laine, Macromolecules 37 (2004) 3267. [2] J. Frohlich, R. Thomann, R. Mulhaupt, Macromolecules 36 (2003) 7205. [3] A. Zubeldia, M. Larran˜aga, P. Remiro, I. Mondragon, J.Polym. Sci.B: Polym. Phys. 42 (2004) 3920. [4] H. Harani, S. Fellahi, J.Appl. Polym. Sci. 71 (1999) 29. [5] M. Imanaka, Y. Nakamura, A. Nishimura, Compos. Sci. Technol. 63 (2003) 41. [6] L. Be´cu-Longuet, A. Bonnet, C. Pichot, H. Sautereau, A. Maazouz, J. Appl. Polym. Sci. 72 (1999) 849. [7] J.J. La Scala, J.A. Orlicki, C. Winston, E.J. Robinette, J.M. Sands, G.R. Palmese, Polymer 46 (2005) 2908. [8] L. Valette, Macromol. Mater. Eng. 288 (2003) 867. [9] S.J. Park, F.L. Jin, J.R. Lee, Mater. Sci. Eng. A 374 (2004) 109. [10] M. Preghenella, A. Pegoretti, C. Migliaresi, Polymer 46 (2005) 12065. [11] B. Qi, Q.X. Zhang, M. Bannister, Compos. Struct. 75 (2006) 514. [12] Q. Wu, J.J. Xu, Y.Z. Yu, J. Appl. Polym. Sci. 80 (2001) 1237. [13] E.J. Robinette, S. Ziaee, G.R. Palmese, Polymer 45 (2004) 6143. [14] S. Fellahi, N. Chikhi, M. Bakar, J. Appl. Polym. Sci. 82 (2001) 861.
[15] S. Bhuniya, B. Adihikari, J. Appl. Polym. Sci. 90 (2003) 1497. [16] S.J. Park, H.C. Kim, J. Polym. Sci. B: Polym. Phys. 39 (2001) 121. [17] S.C. Chen, S.T. Chiu, C.P. Ye, J. Appl. Polym. Sci. 86 (2002) 3740. [18] T. Iijima, K.I. Fujimoto, M. Tomoi, J. Appl. Polym. Sci. 84 (2002) 388. [19] R. Bagheri, R.A. Pearson, Polymer 37 (1996) 4529. [20] J. Brunner, A. Necola, M. Rees, P. Gasser, X. Kornmann, R. Thomann, R.M. Barbezat, Eng. Fract. Mech. 73 (2006) 2336. [21] C. Kanchanomai, S. Rattananon, Polym. Test. 24 (2005) 886. [22] C. Kaynak, Polym. Test. 25 (2006) 296. [23] R. Bagheri, Polym. Eng. Sci. 37 (1997) 245. [24] M. Rutnakornpituk, Eur. Polym. J. 41 (2005) 1043. [25] T. H Yoon, J. E Mcgrath, J. Appl. Polym. Sci. 80 (2001) 1504. [26] K. Wang, J. Wu, M.L. Toh, C. He, A.F. Yee, Macromolecules 38 (2005) 788. [27] Z. Zhang, Y.Y. Zhao, J. Appl. Polym. Sci. 84 (2002) 1223. [28] K.Q. Xiao, L. Ye, Y.S. Kwok, J. Mater. Sci. 33 (1998) 2831. [29] J. Du, M.D. Thouless, A.F. Yee, Int. J. Fract. 92 (1998) 271. [30] P. Rosso, L. Ye, K. Friedrich, S. Sprenger, J. Appl. Polym. Sci. 100 (2006) 1849.