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Deformation behavior of human enamel under diametral compression
Materials Letters 136 (2014) 130–132
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Deformation behavior of human enamel under diametral compression Dmitry Zaytsev n, Peter Panfilov Ural Federal University, Ekaterinburg, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 3 April 2014 Accepted 26 July 2014 Available online 14 August 2014
Deformation behavior of human enamel under diametral compression is examined. Enamel is a brittle substance under tensile stress: the deformation curve can be approximated by the straight line and the sample fails as soon as the maximal stress is reached. The diametral tensile strength of human enamel is 40 MPa, the elastic modulus is 3.5 GPa and the total deformation is 1.3%. The trajectory of crack is macroscopically straight and it lies between the contact points of the sample with the compression plates of testing machine. There is extensive cracking prior separation of the sample in its central part. Many satellite cracks are situated ahead the main crack tip. Therefore, fracture behavior of human enamel corresponds to a ductile solid. & 2014 Elsevier B.V. All rights reserved.
Keywords: Enamel Deformation Fracture Tension Diametral compression
1. Introduction Diametral compression test is the alternative method to the conventional tensile testing [1,2]. Compression load applies to the opposite ends of the tablet shape sample that induces the pure tensile stress in the diametral plane (Fig. 1). Such method allows estimating the tensile strength of both small-size samples and brittle materials. This technique was successfully applied to the estimation of the tensile properties of human dentin [3,4]. In addition, diametral compression testing is widely used for examination of the mechanical properties of the dental restorative materials [5,6]. The data on tensile properties of human enamel at static conventional testing is absent in a literature, because the volume of enamel in a human tooth is limited and, as a result, samples for the conventional tensile test cannot be manufactured. Therefore, diametral compression testing is the suitable technique for examination of deformation and fracture behavior of human enamel in the field of tensile stress. The diametral tensile strength of enamel was reported in the sole work (33–35 MPa); however, no any description of its deformation behavior was given [7]. Examination of fracture behavior of human enamel under fatigue test has shown that there are several mechanisms of fracture toughening in enamel, such as crack bridging, crack deflection and crack bifurcation [8]. Information on tensile properties of human enamel will be useful for manufacturers of dental materials, insomuch as the mechanical properties of restorative materials
n Correspondence to: Department of Physics, Institute of Natural Sciences, Ural Federal University, Lenin Avenue, 51, 620083 Ekaterinburg, Russia. Tel.: þ 7 343 261 5343; Mobile: þ 7 922 222 9455; fax: þ7 343 261 5978. E-mail address: [email protected] (D. Zaytsev).
http://dx.doi.org/10.1016/j.matlet.2014.07.189 0167-577X/& 2014 Elsevier B.V. All rights reserved.
should be close to the properties of tooth hard tissues. The aim of this work is examination of the deformation behavior of human enamel under diametral compression.
2. Experimental procedure Fifteen human molars and premolars without any visible damages were used in this work. They were extracted from the mature male and female subjects according to the medical diagnosis and the Ethic Protocol of the Urals State Medical University at Ekaterinburg. Fifteen samples for mechanical testing have been cut by means of the diamond saw with water irrigation from the lateral part of dental enamel. The shape of the samples was cylindrical/tablet with the diameter of 2.5 mm and 1.25 mm in height. The samples consist of the intermediate enamel. Enamel rods lie mainly along the height of tablet sample due to the choice of the geometry of cutting and the arrangement of the rods in enamel. Samples with other orientations of the enamel rods cannot be manufactured because of small thickness of the enamel in other directions. Therefore, such orientation of enamel rods is the sole one for the samples having such size. Shimadzu AGX50 kN testing machine was used for the mechanical testing (traverse rate was 0.1 mm/min). Processing of the results including the statistical analysis was carried out by the Trapezium—X package, which is the standard software of Shimadzu facilities. The diametral tensile stress and the diametral strain were calculated by σ ¼ 2F=π Dh and ε ¼ Δx=D, respectively, where F—applied force, D—diameter of samples, h—height of sample and Δx— movement of the traverse. Elastic modulus was calculated from the slope of the linear / elastic part of deformation curve. Back
D. Zaytsev, P. Panfilov / Materials Letters 136 (2014) 130–132
surface of the samples were examined with a help of the optical microscope in reflected light.
3. Results and discussion Under testing, the samples separate in the direction of applied loading on two equal parts as soon as the maximal stress is reached (Fig. 2). The deformation curve of enamel sample under
Fig. 1. Scheme of diametral compression.
131
diametral compression that is more close to the average curve is given in Fig. 3. The curve may be divided on two parts. The first part is nonlinear: it begins from the origin of coordinates and it finishes at 5 MPa and 0.2%. Further, the straight line can approximate the trend of the deformation curve. Nonlinear character of the deformation curve on the first part is caused by the imperfection of the geometry of samples. Therefore, intrinsic response of enamel on the tensile stress is linear in all range of loading. It may be concluded that deformation behavior of human enamel is brittle, insomuch as the deformation curve is the straight line and the sample fails when the maximal stress is reached. The diametral tensile strength is 39.97 4.7 MPa, the elastic modulus is 3.3370.24 GPa and the total deformation is 1.3 70.1%. The diametral tensile strength obtained in this work is higher than in Ref. [7] (33–35 MPa). This difference may be caused by the distinction of the orientation of enamel rods in the samples, because fracture behavior of enamel strongly depends on the arrangement of enamel rods in the sample [8]. Unfortunately, no orientation of the enamel rods in samples was given in Ref. [7]. In addition, the diametral tensile strength of enamel samples is close
Fig. 3. Deformation curve of human enamel under diametral compression.
Fig. 2. Back surfaces of the samples of human enamel for the diametral compression: a—prior the test; b—after the test.
132
D. Zaytsev, P. Panfilov / Materials Letters 136 (2014) 130–132
Fig. 4. Crack in the samples of human enamel under diametral compression.
to the diametral tensile strength of dentin samples 40–50 MPa [3]. However, human dentin is not strictly a brittle substance in contrast with enamel under tension [3]. Comparison of the mechanical properties of human dentin and enamel under compression confirms that dentin is more elastic and plastic tissue than enamel [9,10]. Observation of cracks has shown that the trajectory of main crack lies between the contact points of the sample along the compression axis. Many satellite cracks are situated ahead of the main crack tip prior the failure of sample (Fig. 4). Such type of fracture behavior is inherent to a ductile material. The similar fracture behavior of human enamel was observed under fatigue testing, where the appearance of the satellite cracks precedes the failure of sample [8]. The same picture of crack growth takes place in human dentin, too [11]. However, dentin suppresses the crack growth more strongly in comparison with enamel [12]. It may be stated that under diametral compression the deformation behavior of human enamel is brittle, but its fracture behavior is close to a ductile solid. This contradiction may be explained by the scale effect. Diametral compression testing is the macroscopic experiment, where the plastic response of enamel is insignificant. On the contrary, under indentation, which is the deformation scheme for study of mechanical properties of materials on microscopic level, human enamel exhibits some plasticity [13,14]. Indeed, the plastic response of the organic component of enamel is able to suppress the growth of cracks in the sample. Besides, the bridging of cracks in enamel can occur on the intersection of the enamel rods, which is the structural feature of the intermediate enamel of a human tooth [8,15].
4. Conclusion Under diametral compression, the deformation behavior of human enamel is close to a brittle solid. The diametral tensile strength of human enamel is 39.97 4.7 MPa, its elastic modulus is 3.33 70.24 GPa and the total deformation is 1.370.1%. The crack trajectory is macroscopically straight and it lies between the contact points of the sample. There are the satellite cracks ahead the main crack in the enamel samples. Therefore, fracture behavior
of human enamel corresponds to fracture of a ductile solid in spite of the small total deformation. Acknowledgements The reported study was supported by RFBR, research project No. 14-08-31691 and by UrFU under the Framework Program of development of UrFU through the "Young scientists UrFU” competition. References [1] Berenbaum R, Brodie I. Measurement of the tensile strength of brittle materials. Br J Appl Phys 1959;10:281–7. [2] Procopia AT, Zavaliangos A, Cunningham JC. Analysis of the diametrical compression test and applicability to plastically deforming materials. J Mater Sci 2003;38:3629–39. [3] Lertchirakarn V, Palamara JEA, Messer HH. Anisotropy of tensile strength of root dentin. J Dent Res 2001;80(2):453–6. [4] Zaytsev D, Panfilov P. Deformation behavior of human dentin in liquid nitrogen: a diametral compression test. Mater Sci Eng, C 2014;42:48–51. [5] Thomaidisa S, Kakabouraa A, Muellerb WD, Zinelisc S. Mechanical properties of contemporary composite resins and their interrelations. Dent Mater 2013;29:132–41. [6] Lien W, Vandewalle KS. Physical properties of a new silorane-based restorative system. Dent Mater 2010;26:337–44. [7] Hannah CM. The tensile properties of human enamel and dentine I.A.D.R. 1970; Abstr. № 113; 1970. [8] Bajaj D, Nazari A, Eidelman N, Arola DD. A comparison of fatigue crack growth in human enamel and hydroxyapatite. Biomaterials 2008;29:4847–54. [9] Zaytsev D, Grigoriev S, Panfilov P. Deformation behavior of human dentin under uniaxial compression. Int J Biomater 2012. http://dx.doi.org/10.1155/ 2012/854539. [10] Zaytsev D, Panfilov P. Deformation behavior of human enamel and dentin– enamel junction under compression. Mater Sci Eng, C 2014;34:15–21. [11] Nalla RK, Kinney JH, Rotchie RO. Effect of orientation on the in vitro fracture toughness of dentin: the role of toughening mechanisms. Biomaterials 2003;24:3955–68. [12] Imbeni V, Kruzic JJ, Marshall GW, Marshall SJ, Ritchie RO. The dentin–enamel junction and the fracture of human teeth. Nat Mater 2005;4:229–32. [13] He LH, Swain MV. Enamel—a “metallic-like” deformable biocomposite. J Dent 2007;35:431–7. [14] Ang SF, Scholz T, Klocke A, Schneider GA. Determination of the elastic/plastic transition of human enamel by nanoindentation. Dent Mater 2009;25: 1403–10. [15] He LH, Swain MV. Understanding the mechanical behavior of human enamel from its structural and compositional characteristics. JMBBM 2008;1:18–29.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Deformation behavior of human enamel under diametral compression Dmitry Zaytsev n, Peter Panfilov Ural Federal University, Ekaterinburg, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 3 April 2014 Accepted 26 July 2014 Available online 14 August 2014
Deformation behavior of human enamel under diametral compression is examined. Enamel is a brittle substance under tensile stress: the deformation curve can be approximated by the straight line and the sample fails as soon as the maximal stress is reached. The diametral tensile strength of human enamel is 40 MPa, the elastic modulus is 3.5 GPa and the total deformation is 1.3%. The trajectory of crack is macroscopically straight and it lies between the contact points of the sample with the compression plates of testing machine. There is extensive cracking prior separation of the sample in its central part. Many satellite cracks are situated ahead the main crack tip. Therefore, fracture behavior of human enamel corresponds to a ductile solid. & 2014 Elsevier B.V. All rights reserved.
Keywords: Enamel Deformation Fracture Tension Diametral compression
1. Introduction Diametral compression test is the alternative method to the conventional tensile testing [1,2]. Compression load applies to the opposite ends of the tablet shape sample that induces the pure tensile stress in the diametral plane (Fig. 1). Such method allows estimating the tensile strength of both small-size samples and brittle materials. This technique was successfully applied to the estimation of the tensile properties of human dentin [3,4]. In addition, diametral compression testing is widely used for examination of the mechanical properties of the dental restorative materials [5,6]. The data on tensile properties of human enamel at static conventional testing is absent in a literature, because the volume of enamel in a human tooth is limited and, as a result, samples for the conventional tensile test cannot be manufactured. Therefore, diametral compression testing is the suitable technique for examination of deformation and fracture behavior of human enamel in the field of tensile stress. The diametral tensile strength of enamel was reported in the sole work (33–35 MPa); however, no any description of its deformation behavior was given [7]. Examination of fracture behavior of human enamel under fatigue test has shown that there are several mechanisms of fracture toughening in enamel, such as crack bridging, crack deflection and crack bifurcation [8]. Information on tensile properties of human enamel will be useful for manufacturers of dental materials, insomuch as the mechanical properties of restorative materials
n Correspondence to: Department of Physics, Institute of Natural Sciences, Ural Federal University, Lenin Avenue, 51, 620083 Ekaterinburg, Russia. Tel.: þ 7 343 261 5343; Mobile: þ 7 922 222 9455; fax: þ7 343 261 5978. E-mail address: [email protected] (D. Zaytsev).
http://dx.doi.org/10.1016/j.matlet.2014.07.189 0167-577X/& 2014 Elsevier B.V. All rights reserved.
should be close to the properties of tooth hard tissues. The aim of this work is examination of the deformation behavior of human enamel under diametral compression.
2. Experimental procedure Fifteen human molars and premolars without any visible damages were used in this work. They were extracted from the mature male and female subjects according to the medical diagnosis and the Ethic Protocol of the Urals State Medical University at Ekaterinburg. Fifteen samples for mechanical testing have been cut by means of the diamond saw with water irrigation from the lateral part of dental enamel. The shape of the samples was cylindrical/tablet with the diameter of 2.5 mm and 1.25 mm in height. The samples consist of the intermediate enamel. Enamel rods lie mainly along the height of tablet sample due to the choice of the geometry of cutting and the arrangement of the rods in enamel. Samples with other orientations of the enamel rods cannot be manufactured because of small thickness of the enamel in other directions. Therefore, such orientation of enamel rods is the sole one for the samples having such size. Shimadzu AGX50 kN testing machine was used for the mechanical testing (traverse rate was 0.1 mm/min). Processing of the results including the statistical analysis was carried out by the Trapezium—X package, which is the standard software of Shimadzu facilities. The diametral tensile stress and the diametral strain were calculated by σ ¼ 2F=π Dh and ε ¼ Δx=D, respectively, where F—applied force, D—diameter of samples, h—height of sample and Δx— movement of the traverse. Elastic modulus was calculated from the slope of the linear / elastic part of deformation curve. Back
D. Zaytsev, P. Panfilov / Materials Letters 136 (2014) 130–132
surface of the samples were examined with a help of the optical microscope in reflected light.
3. Results and discussion Under testing, the samples separate in the direction of applied loading on two equal parts as soon as the maximal stress is reached (Fig. 2). The deformation curve of enamel sample under
Fig. 1. Scheme of diametral compression.
131
diametral compression that is more close to the average curve is given in Fig. 3. The curve may be divided on two parts. The first part is nonlinear: it begins from the origin of coordinates and it finishes at 5 MPa and 0.2%. Further, the straight line can approximate the trend of the deformation curve. Nonlinear character of the deformation curve on the first part is caused by the imperfection of the geometry of samples. Therefore, intrinsic response of enamel on the tensile stress is linear in all range of loading. It may be concluded that deformation behavior of human enamel is brittle, insomuch as the deformation curve is the straight line and the sample fails when the maximal stress is reached. The diametral tensile strength is 39.97 4.7 MPa, the elastic modulus is 3.3370.24 GPa and the total deformation is 1.3 70.1%. The diametral tensile strength obtained in this work is higher than in Ref. [7] (33–35 MPa). This difference may be caused by the distinction of the orientation of enamel rods in the samples, because fracture behavior of enamel strongly depends on the arrangement of enamel rods in the sample [8]. Unfortunately, no orientation of the enamel rods in samples was given in Ref. [7]. In addition, the diametral tensile strength of enamel samples is close
Fig. 3. Deformation curve of human enamel under diametral compression.
Fig. 2. Back surfaces of the samples of human enamel for the diametral compression: a—prior the test; b—after the test.
132
D. Zaytsev, P. Panfilov / Materials Letters 136 (2014) 130–132
Fig. 4. Crack in the samples of human enamel under diametral compression.
to the diametral tensile strength of dentin samples 40–50 MPa [3]. However, human dentin is not strictly a brittle substance in contrast with enamel under tension [3]. Comparison of the mechanical properties of human dentin and enamel under compression confirms that dentin is more elastic and plastic tissue than enamel [9,10]. Observation of cracks has shown that the trajectory of main crack lies between the contact points of the sample along the compression axis. Many satellite cracks are situated ahead of the main crack tip prior the failure of sample (Fig. 4). Such type of fracture behavior is inherent to a ductile material. The similar fracture behavior of human enamel was observed under fatigue testing, where the appearance of the satellite cracks precedes the failure of sample [8]. The same picture of crack growth takes place in human dentin, too [11]. However, dentin suppresses the crack growth more strongly in comparison with enamel [12]. It may be stated that under diametral compression the deformation behavior of human enamel is brittle, but its fracture behavior is close to a ductile solid. This contradiction may be explained by the scale effect. Diametral compression testing is the macroscopic experiment, where the plastic response of enamel is insignificant. On the contrary, under indentation, which is the deformation scheme for study of mechanical properties of materials on microscopic level, human enamel exhibits some plasticity [13,14]. Indeed, the plastic response of the organic component of enamel is able to suppress the growth of cracks in the sample. Besides, the bridging of cracks in enamel can occur on the intersection of the enamel rods, which is the structural feature of the intermediate enamel of a human tooth [8,15].
4. Conclusion Under diametral compression, the deformation behavior of human enamel is close to a brittle solid. The diametral tensile strength of human enamel is 39.97 4.7 MPa, its elastic modulus is 3.33 70.24 GPa and the total deformation is 1.370.1%. The crack trajectory is macroscopically straight and it lies between the contact points of the sample. There are the satellite cracks ahead the main crack in the enamel samples. Therefore, fracture behavior
of human enamel corresponds to fracture of a ductile solid in spite of the small total deformation. Acknowledgements The reported study was supported by RFBR, research project No. 14-08-31691 and by UrFU under the Framework Program of development of UrFU through the "Young scientists UrFU” competition. References [1] Berenbaum R, Brodie I. Measurement of the tensile strength of brittle materials. Br J Appl Phys 1959;10:281–7. [2] Procopia AT, Zavaliangos A, Cunningham JC. Analysis of the diametrical compression test and applicability to plastically deforming materials. J Mater Sci 2003;38:3629–39. [3] Lertchirakarn V, Palamara JEA, Messer HH. Anisotropy of tensile strength of root dentin. J Dent Res 2001;80(2):453–6. [4] Zaytsev D, Panfilov P. Deformation behavior of human dentin in liquid nitrogen: a diametral compression test. Mater Sci Eng, C 2014;42:48–51. [5] Thomaidisa S, Kakabouraa A, Muellerb WD, Zinelisc S. Mechanical properties of contemporary composite resins and their interrelations. Dent Mater 2013;29:132–41. [6] Lien W, Vandewalle KS. Physical properties of a new silorane-based restorative system. Dent Mater 2010;26:337–44. [7] Hannah CM. The tensile properties of human enamel and dentine I.A.D.R. 1970; Abstr. № 113; 1970. [8] Bajaj D, Nazari A, Eidelman N, Arola DD. A comparison of fatigue crack growth in human enamel and hydroxyapatite. Biomaterials 2008;29:4847–54. [9] Zaytsev D, Grigoriev S, Panfilov P. Deformation behavior of human dentin under uniaxial compression. Int J Biomater 2012. http://dx.doi.org/10.1155/ 2012/854539. [10] Zaytsev D, Panfilov P. Deformation behavior of human enamel and dentin– enamel junction under compression. Mater Sci Eng, C 2014;34:15–21. [11] Nalla RK, Kinney JH, Rotchie RO. Effect of orientation on the in vitro fracture toughness of dentin: the role of toughening mechanisms. Biomaterials 2003;24:3955–68. [12] Imbeni V, Kruzic JJ, Marshall GW, Marshall SJ, Ritchie RO. The dentin–enamel junction and the fracture of human teeth. Nat Mater 2005;4:229–32. [13] He LH, Swain MV. Enamel—a “metallic-like” deformable biocomposite. J Dent 2007;35:431–7. [14] Ang SF, Scholz T, Klocke A, Schneider GA. Determination of the elastic/plastic transition of human enamel by nanoindentation. Dent Mater 2009;25: 1403–10. [15] He LH, Swain MV. Understanding the mechanical behavior of human enamel from its structural and compositional characteristics. JMBBM 2008;1:18–29.