Materials Science and Engineering C 39 (2014) 253–258
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
An experimental-finite element analysis on the kinetic energy absorption capacity of polyvinyl alcohol sponge Alireza Karimi, Mahdi Navidbakhsh ⁎, Reza Razaghi School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846, Iran Tissue Engineering and Biological Systems Research Lab, School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846, Iran
a r t i c l e
i n f o
Article history: Received 30 January 2014 Received in revised form 22 February 2014 Accepted 2 March 2014 Available online 12 March 2014 Keywords: Polyvinyl alcohol sponge Helmet Finite element Kinetic energy absorption Energy loss
a b s t r a c t Polyvinyl alcohol (PVA) sponge is in widespread use for biomedical and tissue engineering applications owing to its biocompatibility, availability, relative cheapness, and excellent mechanical properties. This study reports a novel concept of design in energy absorbing materials which consist in the use of PVA sponge as an alternative reinforcement material to enhance the energy loss of impact loads. An experimental study is carried out to measure the mechanical properties of the PVA sponge under uniaxial loading. The kinetic energy absorption capacity of the PVA sponge is computed by a hexahedral finite element (FE) model of the steel ball and bullet through the LS-DYNA code under impact load at three different thicknesses (5, 10, 15 mm). The results show that a higher sponge thickness invokes a higher energy loss of the steel ball and bullet. The highest energy loss of the steel ball and bullet is observed for the thickest sponge with 160 and 35 J, respectively. The most common type of traumatic brain injury in which the head subject to impact load causes the brain to move within the skull and consequently brain hemorrhaging. These results suggest the application of the PVA sponge as a great kinetic energy absorber material compared to commonly used expanded polystyrene foams (EPS) to absorb most of the impact energy and reduces the transmitted load. The results might have implications not only for understanding of the mechanical properties of PVA sponge but also for use as an alternative reinforcement material in helmet and packaging material design. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Polyvinyl alcohol (PVA) sponges are currently in widespread use for the removal and management of diffuse fluids/blood at surgical site [1]. They are also contemplated as the most attractive biomedical polymers owing to a combination of qualities, such as biocompatibility [2–5], highly hydrophilicity [6–8], excellent mechanical strength and flexibility [4–7,9,10], thermal stability and absence of toxicity [11], availability, and relative cheapness [12]. However, the application of these versatile biomaterials has been limited to ophthalmic, plastic, and hand surgeries as a biocompatible biodegradable material. Recently, Karimi et al. [13] characterized the mechanical properties of a fabricated PVA sponge for tissue engineering applications. Their results showed the Young's modulus and maximum stress of 40 and 9.79 MPa for PVA sponge, respectively. Further tests were also carried out to measure the Young's modulus of the PVA sponge at higher strain rates. The results revealed the Young's modulus of 4.28, 208.33, and 187.51 MPa at the strain rates of 1, 20, and 100 mm/min, respectively [14]. The Young's modulus of the PVA sponges was also measured ⁎ Corresponding author at: School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846, Iran. Tel.: +98 21 77209027; fax: +98 21 73021585. E-mail address: [email protected] (M. Navidbakhsh).
http://dx.doi.org/10.1016/j.msec.2014.03.009 0928-4931/© 2014 Elsevier B.V. All rights reserved.
under longitudinal (38.91 MPa) and circumferential (33.34 MPa) loads. The maximum stress, in addition, in the longitudinal direction was 17.90% greater than that of the circumferential direction [15]. The mechanical behavior of PVA sponge has shown to be similar to rubber-like materials, such as time-dependent viscoelastic behavior which can be formulated by the visco-hyperelastic approach under low strain uniaxial loading [16–19]. Considering both the advantage of biocompatibility and suitable mechanical properties of the PVA sponges, they can be used as an alternative reinforcement material to enhance the mechanical properties of the materials for biomedical or industrial applications. The suitable mechanical properties of the PVA sponges especially under fast strain rates would also enable them to be used as an energy absorber material for helmet design. However, a critical barrier to the use of the PVA sponge as an energy absorber material is a lack of knowledge on its kinetic energy absorption capacity. Among the energy absorbing materials available in the market, expanded polystyrene foams (EPS) are often used for the design of the helmet liners [20,21], due to their capability of providing multidirectional resistance to impacts, combined with light weight and relatively low costs of production and excellent kinetic energy absorption capacity [22]. A way to improve the energy absorption properties of current helmets could be the use of non-conventional materials capable of higher energy absorption, while keeping the accelerations transmitted to the head at a safe level.
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This study is aimed to introduce a novel concept of design in helmet materials which consists in the use of PVA sponge as an alternative reinforcement material to enhance the kinetic energy absorption capacity of commonly used expanded polystyrene foams. Finite element (FE) analysis of the sponge and steel ball or bullet is executed through the LS-DYNA code under impact load to compute the kinetic energy absorption capacity of the PVA sponge at three different thicknesses (5, 10, 15 mm). The kinetic energy of the steel ball and bullet is also computed after penetration in the PVA sponge.
meshed with 27,018, 54,036, and 81,054 8-noded hexagonal elements plus 31,017, 59,241, and 87,465 nodes for 5, 10, and 15 mm thickness, respectively. An FE model of a 7.62 mm pointed-nose bullet (7.92 g) was meshed with 5440 8-noded hexagonal elements and 6490 nodes. Since the bullet consisted of a carbon steel core and a copper jacket, the bullet and the steel ball were both classified as low-carbon steel materials in this analysis. The material model for the sponge, steel ball, and bullet was *MAT_PIECEWISE_LINEAR_PLASTICITY, which was *MAT type 24 in LS-DYNA version 970. The material properties of the PVA sponge, steel ball, and bullet are listed in Table 1.
2. Materials and methods 2.4. Numerical simulation 2.1. Materials and specimen preparation The preparation of the polyvinyl alcohol sponge has been thoroughly described in our previous studies [15,16]. Briefly, to prepare the PVA aqueous solution, 2 g of PVA (molecular weight = 40,000, SigmaAldrich) was dissolved in 100 ml of distilled water at 50 °C under stirring at 400 rpm for 6 h. The polymer solution was then cast into cylindrical molds and freeze dried in order to obtain PVA spongy matrix. To improve its stability in water, the above sponge was cross-linked by exposure to the vapors of a glutaraldehyde aqueous solution (25%) at 37 °C for 24 h. After rinsed with distilled water, the sponge was freeze dried again. The final solution was poured into Petri dishes and allowed to stand at room temperature (25–30 °C) until crosslinking was completed (48 h). 2.2. Axial measurements The initial dimensions of all specimens were measured precisely. The tensile test was performed using a uniaxial tensile test apparatus adapted for testing biological specimens used in our previous studies [23–29]. All tests were performed at 25 °C and each sample was tested only once. A low strain rate of 5 mm/min which is typical for surgical procedures and gives more insight into tissue behavior was employed by the action of an axial servo motor [30–32]. In order to make sure about a firm fixation of samples between the jaws of the machine a small tensile pre-load of 0.05 N was applied to each specimen. Moreover, rough sandpaper was used between the jaw and sample to assure no slip boundary. Preconditioning of spongy tissues has become a common procedure in tensile testing to assess the history dependence of spongy tissues. Therefore, ten cyclic preconditioning with a suitable pre-load based on experimental results was applied to each PVA sponge sample before any measurement begin. The sample's length was measured after the application of the pre-load. This also helped minimize the bending effect caused by the weight of each specimen. 2.3. Finite element model A 3D FE model of the PVA sponge was established using the explicit dynamics finite element code LS-DYNA 970 (LSTC, Livermore, CA, United States). The kinetic energy absorption capacity of the PVA sponges at different thicknesses, including 5, 10, and 15 mm, was computed by FE analyses. The mechanical properties of the PVA sponge [13] and the steel ball or bullet [33] were assigned to the FE models. To obtain greater computational precision, the hexahedral element was used for the sponge, steel ball, and bullet. To achieve a dynamic separation and spatter effect, the elements in the PVA sponge and angle region were selected for node release. The properties of boundary element of the surface mesh for the sponge were used to determine the boundary between the sponge and fixed area. The final model of the PVA sponge along a steel ball was meshed with 4842, 9684, and 14,526 8-noded hexagonal elements besides 6377, 11,591, and 16,805 nodes for 5, 10, and 15 mm thickness, respectively. An FE model of a 6.3 mm steel ball (1.03 g) was meshed with 6984 8-noded hexagonal elements and 7939 nodes. The final model of the PVA sponge along a bullet was
The numerical simulations were performed using the nonlinear explicit FE code LS-DYNA version 970. In the current simulation, the elements of the sponge surfaces region of the FE model were fixed in the X, Y, and Z directions. An entire simulation lasted 80 μs, with time increments of 1 μs. The post-processing software (LS-PREPOST of LS-DYNA) simulated and measured the stress distribution in each region when the model was hit by the steel ball or bullet. During the simulation, three different thicknesses for the sponge were used. The steel ball and bullet also were shot with the same angle. An impact velocity of 734 m/s was used for the steel ball and bullet. The kinetic energy of the steel ball and bullet when they penetrated the sponge at three different thicknesses (5, 10, and 15 mm) was calculated using FE modeling results. The energy loss from the projectile was determined using the formula ΔE = m(v21 − v22)/2, where ΔE is the energy loss, v1 is the impact velocity, and v2 is the residual velocity, to study the degree of damage that was produced when the steel ball or bullet penetrated the sponge. Therefore, the energy loss was calculated to investigate the damage efficiency of the sponge. 2.5. Statistical analysis Data were first analyzed by analysis of variance (ANOVA); when statistical differences were detected, Student's t-test for comparisons between groups was performed using SPSS software version 16.0 (SPSS Inc., Chicago, IL, USA). Data are reported as mean ± std at a significance level of p b 0.05. 3. Results and discussion PVA sponges possess many attractive features, such as biocompatibility, thermal stability, availability, and relative cheapness. However, the application of these versatile biomaterials is limited to biomedical and tissue engineering [34]. The excellent mechanical strength of PVA sponges enables them to be used as an energy absorber material for biomedical and industrial applications. Computing the kinetic energy of PVA sponges is, therefore, an important part of a comprehensive evaluation of their kinetic energy absorption capacity. With that in mind, the purpose of this study was to quantify the kinetic energy of a PVA sponge intended for use as an energy absorber in helmet and packaging material design. The kinetic energy absorption capacity of PVA sponge was computed through gunshot finite element modeling by the steel ball and bullet.
Table 1 Material properties of the finite element models. Material properties
Bullet and steel ball (low-carbon steel)
Polyvinyl alcohol sponge
Young's modulus (MPa) Poisson's ratio Yield stress (MPa) Failure strain (%)
210,000 0.284 260 0.330
40 0.499 9.790 0.660
A. Karimi et al. / Materials Science and Engineering C 39 (2014) 253–258
Fig. 1. A PVA sponge under uniaxial force.
Fig. 2. Schematic FE model of (a) steel ball and (b) steel bullet.
A PVA specimen during uniaxial tensile test is illustrated in Fig. 1. The uniaxial tensile test machine was consisted of a fixed and moveable jaw which provides a constant strain rate. The kinetic energy absorption capacity of PVA sponge is studied using gunshot with the steel ball and bullet. The schematic FE model of (a) steel ball and (b) bullet is indicated in Fig. 2. The dynamic mechanism of the PVA sponge being penetrated by a 6.3 mm steel ball and a 7.62 mm bullet at different sponge thicknesses is simulated. The dynamic changes in the stress distribution Fig. 3. The FE modeling for polyvinyl alcohol sponge damage and stress distribution after the steel ball penetrated at three different thicknesses, including (a) 5, (b) 10, and (c) 15 mm.
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in each sponge region are also simulated. The damage to the sponge and the distribution when the steel ball penetrated through the sponge at impact velocity of 734 m/s at three different thicknesses is presented in Fig. 3. Fig. 4 is also showed the damage to the sponge and the distribution when the bullet penetrated through the sponge at impact velocity of 734 m/s at three different thicknesses. The kinetic energy and energy loss for both the steel ball and bullet are computed. The kinetic energy for both (a) steel ball and (b) bullet is presented in Fig. 5. A
histogram representation of the energy loss for both the (a) steel ball and (b) bullet is indicated in Fig. 6. The results show that a higher thickness of the sponge leads to a higher energy loss of the steel ball and bullet. When the thickness for both the steel ball and bullet is the same, the energy loss of the steel ball is greater than that of the bullet. Therefore, the energy loss of the steel ball is always significantly greater than that of the bullet at the same thickness (Fig. 6). The highest energy loss of the steel ball and bullet is observed for the thickest sponge with 160 and 35.51 J, respectively. Hence, the damage efficiency of the steel ball is much greater than that of the bullet. However, the bullet causes more severe damage to the sponge than the steel ball in the PVA sponge models (Figs. 3 and 4). According to the law of conservation of energy, the energy that the projectile loses when it hits the sponge is absorbed by the impacted sponge. Thus, the more energy that the projectile loses as it penetrates the sponge, the more damage it causes. The results suggest the application of PVA sponge as a great kinetic energy absorber material compared to commonly used energy absorbing ones for helmet design. Traumatic brain injury is generally considered as a signature injury of the people using motorcycle, with costly and life-altering long-term effects. Hence, there is an urgent need to combat this problem by designing/developing a more effective helmet [35]. The main helmet components are the foam liner and the shell. Basically, the function of the foam is to absorb most of the impact energy, and to distribute the impact load on a wider foam area thus increasing the foam liner energy absorption capacity. Usually manufacturers design their helmets based on experimental verification. During the experimental verification the helmet must absorb the energy of the impact [36]. Among the energy absorbing materials available in the market, expanded polystyrene foams (EPS) are often considered for the design of the helmet liners [20,21], due to their capability of providing multidirectional resistance to impacts, combined with light weight and relatively low costs of production [22]. The compressive Young's modulus and maximum stress of 6.5–39 and 0.51–1.50 MPa have been reported for the current foam materials in helmet design (EPS) [20,22] as a function of thickness and density. The enhancement of the energy absorption properties of motorbike helmets could significantly improve the safety of riders [22]. Thus, the impact energy absorption by helmets is of vital importance to the safety of motorcyclists during accidents [37]. Our previous results showed the excellent mechanical strength of PVA sponge with the Young's modulus and maximum stress of 40 and 9.79 MPa, respectively [13]. Our results also showed the longitudinal and circumferential Young's modulus of 38.91 and 33.34 MPa, respectively. The maximum stress for longitudinal direction was 17.90% more than that of circumferential direction [15]. Interestingly, the faster strain rate for the PVA sponge caused a higher Young's modulus and maximum stress [14]. The cyclic mechanical tests revealed that the PVA sponge successfully retains its initial Young's modulus and maximum stress with less than 5% differences [14]. During the service life of a helmet, environmental factors such as pre-compression strain might lead to energy absorption ability degradation in the polystyrene foam which can be considerably minimized by reinforcement of PVA sponge [38]. It has also been reported that the energy loss of 100 J is acceptable for the energy absorber material to protect the head against impact loads [36]. Therefore, the PVA sponge can be considered as an alternative reinforcement material for helmet design to control the effect of injury by increasing the energy loss of the steel ball or bullet [39,40]. 4. Conclusions
Fig. 4. The FE modeling for polyvinyl alcohol sponge damage and stress distribution after the bullet penetrated at three different thicknesses, including (a) 5, (b) 10, and (c) 15 mm.
This study computed the kinetic energy absorption capacity of the polyvinyl alcohol sponge by finite element modeling of gunshot. The results showed the beneficial kinetic energy absorption capacity of the PVA sponge compared to commonly use polystyrene foams which have been considered as the most suitable material in helmet design. The suitable kinetic energy absorption capacity of the PVA sponge enables it to be used as a novel alternative reinforcement material in
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Fig. 5. A comparative representation of the kinetic energy at different thicknesses. (a) Kinetic energy of the steel ball. (b) Kinetic energy of the bullet.
Fig. 6. A comparative histogram representation of the energy loss at different thicknesses. (a) The steel ball. (b) The bullet. ⁎p b 0.05 compared to 5 mm thickness.
helmet and packaging material design. The successful establishment of sponge gunshot model provides a new and effective method for studying the damage mechanisms of gunshot to the sponge and, more generally, to the helmet. Conflict of interest We declare that we have no conflicts of interest. Role of the funding source The sponsor of the study had no role in study design, data collection, data analysis, data interpretation, or writing of the report. The corresponding author had full access to all the data in the study and had final responsibility for the decision to submit for publication. Acknowledgments The authors acknowledge the funding supports of this work by the Iran University of Science and Technology. References [1] W. Korteweg, G.P. Korteweg, Methods for producing surgical sponge device and product thereof, vol. US006711879B2Ultracell Medical Technologies of Connecticult, Inc, North Stonington, CT (US), U.S.A, 2004. [2] Y. Shi, D. Xiong, Wear 305 (2013) 280–285. [3] V. DiTizio, G.W. Ferguson, M.W. Mittelman, A.E. Khoury, A.W. Bruce, F. DiCosmo, Biomaterials 19 (1998) 1877–1884. [4] D. Zhang, K. Chen, L. Wu, D. Wang, S. Ge, J. Bion, J. Bionic Eng. 9 (2012) 234–242.
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[29] A. Karimi, A. Shojaei, M. Navidbakhsh, B. Beigzadeh, S. Faghihi, Characterizing the Mechanical Properties of Rat Brain Tissue in Tension (SJFM), 18 (2013) 221–226. [30] A. Karimi, M. Navidbakhsh, A. Motevalli Haghi, S. Faghihi, Proc. Inst. Mech. Eng. H 227 (2013) 609–614. [31] A. Karimi, M. Navidbakhsh, M. Alizadeh, A. Shojaei, Artery Res. (2014), http://dx.doi. org/10.1016/j.artres.2014.02.001, http://www.sciencedirect.com/science/article/pii/ S1872931214000222 . [32] A. Karimi, M. Navidbakhsh, H. Yousefi, A. Motevalli Haghi, S.J. Adnani Sadati, Perfusion (2014), http://dx.doi.org/10.1177/0267659114522088, Published online 11 February 2014.
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Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
An experimental-finite element analysis on the kinetic energy absorption capacity of polyvinyl alcohol sponge Alireza Karimi, Mahdi Navidbakhsh ⁎, Reza Razaghi School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846, Iran Tissue Engineering and Biological Systems Research Lab, School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846, Iran
a r t i c l e
i n f o
Article history: Received 30 January 2014 Received in revised form 22 February 2014 Accepted 2 March 2014 Available online 12 March 2014 Keywords: Polyvinyl alcohol sponge Helmet Finite element Kinetic energy absorption Energy loss
a b s t r a c t Polyvinyl alcohol (PVA) sponge is in widespread use for biomedical and tissue engineering applications owing to its biocompatibility, availability, relative cheapness, and excellent mechanical properties. This study reports a novel concept of design in energy absorbing materials which consist in the use of PVA sponge as an alternative reinforcement material to enhance the energy loss of impact loads. An experimental study is carried out to measure the mechanical properties of the PVA sponge under uniaxial loading. The kinetic energy absorption capacity of the PVA sponge is computed by a hexahedral finite element (FE) model of the steel ball and bullet through the LS-DYNA code under impact load at three different thicknesses (5, 10, 15 mm). The results show that a higher sponge thickness invokes a higher energy loss of the steel ball and bullet. The highest energy loss of the steel ball and bullet is observed for the thickest sponge with 160 and 35 J, respectively. The most common type of traumatic brain injury in which the head subject to impact load causes the brain to move within the skull and consequently brain hemorrhaging. These results suggest the application of the PVA sponge as a great kinetic energy absorber material compared to commonly used expanded polystyrene foams (EPS) to absorb most of the impact energy and reduces the transmitted load. The results might have implications not only for understanding of the mechanical properties of PVA sponge but also for use as an alternative reinforcement material in helmet and packaging material design. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Polyvinyl alcohol (PVA) sponges are currently in widespread use for the removal and management of diffuse fluids/blood at surgical site [1]. They are also contemplated as the most attractive biomedical polymers owing to a combination of qualities, such as biocompatibility [2–5], highly hydrophilicity [6–8], excellent mechanical strength and flexibility [4–7,9,10], thermal stability and absence of toxicity [11], availability, and relative cheapness [12]. However, the application of these versatile biomaterials has been limited to ophthalmic, plastic, and hand surgeries as a biocompatible biodegradable material. Recently, Karimi et al. [13] characterized the mechanical properties of a fabricated PVA sponge for tissue engineering applications. Their results showed the Young's modulus and maximum stress of 40 and 9.79 MPa for PVA sponge, respectively. Further tests were also carried out to measure the Young's modulus of the PVA sponge at higher strain rates. The results revealed the Young's modulus of 4.28, 208.33, and 187.51 MPa at the strain rates of 1, 20, and 100 mm/min, respectively [14]. The Young's modulus of the PVA sponges was also measured ⁎ Corresponding author at: School of Mechanical Engineering, Iran University of Science and Technology, Tehran 16846, Iran. Tel.: +98 21 77209027; fax: +98 21 73021585. E-mail address: [email protected] (M. Navidbakhsh).
http://dx.doi.org/10.1016/j.msec.2014.03.009 0928-4931/© 2014 Elsevier B.V. All rights reserved.
under longitudinal (38.91 MPa) and circumferential (33.34 MPa) loads. The maximum stress, in addition, in the longitudinal direction was 17.90% greater than that of the circumferential direction [15]. The mechanical behavior of PVA sponge has shown to be similar to rubber-like materials, such as time-dependent viscoelastic behavior which can be formulated by the visco-hyperelastic approach under low strain uniaxial loading [16–19]. Considering both the advantage of biocompatibility and suitable mechanical properties of the PVA sponges, they can be used as an alternative reinforcement material to enhance the mechanical properties of the materials for biomedical or industrial applications. The suitable mechanical properties of the PVA sponges especially under fast strain rates would also enable them to be used as an energy absorber material for helmet design. However, a critical barrier to the use of the PVA sponge as an energy absorber material is a lack of knowledge on its kinetic energy absorption capacity. Among the energy absorbing materials available in the market, expanded polystyrene foams (EPS) are often used for the design of the helmet liners [20,21], due to their capability of providing multidirectional resistance to impacts, combined with light weight and relatively low costs of production and excellent kinetic energy absorption capacity [22]. A way to improve the energy absorption properties of current helmets could be the use of non-conventional materials capable of higher energy absorption, while keeping the accelerations transmitted to the head at a safe level.
254
A. Karimi et al. / Materials Science and Engineering C 39 (2014) 253–258
This study is aimed to introduce a novel concept of design in helmet materials which consists in the use of PVA sponge as an alternative reinforcement material to enhance the kinetic energy absorption capacity of commonly used expanded polystyrene foams. Finite element (FE) analysis of the sponge and steel ball or bullet is executed through the LS-DYNA code under impact load to compute the kinetic energy absorption capacity of the PVA sponge at three different thicknesses (5, 10, 15 mm). The kinetic energy of the steel ball and bullet is also computed after penetration in the PVA sponge.
meshed with 27,018, 54,036, and 81,054 8-noded hexagonal elements plus 31,017, 59,241, and 87,465 nodes for 5, 10, and 15 mm thickness, respectively. An FE model of a 7.62 mm pointed-nose bullet (7.92 g) was meshed with 5440 8-noded hexagonal elements and 6490 nodes. Since the bullet consisted of a carbon steel core and a copper jacket, the bullet and the steel ball were both classified as low-carbon steel materials in this analysis. The material model for the sponge, steel ball, and bullet was *MAT_PIECEWISE_LINEAR_PLASTICITY, which was *MAT type 24 in LS-DYNA version 970. The material properties of the PVA sponge, steel ball, and bullet are listed in Table 1.
2. Materials and methods 2.4. Numerical simulation 2.1. Materials and specimen preparation The preparation of the polyvinyl alcohol sponge has been thoroughly described in our previous studies [15,16]. Briefly, to prepare the PVA aqueous solution, 2 g of PVA (molecular weight = 40,000, SigmaAldrich) was dissolved in 100 ml of distilled water at 50 °C under stirring at 400 rpm for 6 h. The polymer solution was then cast into cylindrical molds and freeze dried in order to obtain PVA spongy matrix. To improve its stability in water, the above sponge was cross-linked by exposure to the vapors of a glutaraldehyde aqueous solution (25%) at 37 °C for 24 h. After rinsed with distilled water, the sponge was freeze dried again. The final solution was poured into Petri dishes and allowed to stand at room temperature (25–30 °C) until crosslinking was completed (48 h). 2.2. Axial measurements The initial dimensions of all specimens were measured precisely. The tensile test was performed using a uniaxial tensile test apparatus adapted for testing biological specimens used in our previous studies [23–29]. All tests were performed at 25 °C and each sample was tested only once. A low strain rate of 5 mm/min which is typical for surgical procedures and gives more insight into tissue behavior was employed by the action of an axial servo motor [30–32]. In order to make sure about a firm fixation of samples between the jaws of the machine a small tensile pre-load of 0.05 N was applied to each specimen. Moreover, rough sandpaper was used between the jaw and sample to assure no slip boundary. Preconditioning of spongy tissues has become a common procedure in tensile testing to assess the history dependence of spongy tissues. Therefore, ten cyclic preconditioning with a suitable pre-load based on experimental results was applied to each PVA sponge sample before any measurement begin. The sample's length was measured after the application of the pre-load. This also helped minimize the bending effect caused by the weight of each specimen. 2.3. Finite element model A 3D FE model of the PVA sponge was established using the explicit dynamics finite element code LS-DYNA 970 (LSTC, Livermore, CA, United States). The kinetic energy absorption capacity of the PVA sponges at different thicknesses, including 5, 10, and 15 mm, was computed by FE analyses. The mechanical properties of the PVA sponge [13] and the steel ball or bullet [33] were assigned to the FE models. To obtain greater computational precision, the hexahedral element was used for the sponge, steel ball, and bullet. To achieve a dynamic separation and spatter effect, the elements in the PVA sponge and angle region were selected for node release. The properties of boundary element of the surface mesh for the sponge were used to determine the boundary between the sponge and fixed area. The final model of the PVA sponge along a steel ball was meshed with 4842, 9684, and 14,526 8-noded hexagonal elements besides 6377, 11,591, and 16,805 nodes for 5, 10, and 15 mm thickness, respectively. An FE model of a 6.3 mm steel ball (1.03 g) was meshed with 6984 8-noded hexagonal elements and 7939 nodes. The final model of the PVA sponge along a bullet was
The numerical simulations were performed using the nonlinear explicit FE code LS-DYNA version 970. In the current simulation, the elements of the sponge surfaces region of the FE model were fixed in the X, Y, and Z directions. An entire simulation lasted 80 μs, with time increments of 1 μs. The post-processing software (LS-PREPOST of LS-DYNA) simulated and measured the stress distribution in each region when the model was hit by the steel ball or bullet. During the simulation, three different thicknesses for the sponge were used. The steel ball and bullet also were shot with the same angle. An impact velocity of 734 m/s was used for the steel ball and bullet. The kinetic energy of the steel ball and bullet when they penetrated the sponge at three different thicknesses (5, 10, and 15 mm) was calculated using FE modeling results. The energy loss from the projectile was determined using the formula ΔE = m(v21 − v22)/2, where ΔE is the energy loss, v1 is the impact velocity, and v2 is the residual velocity, to study the degree of damage that was produced when the steel ball or bullet penetrated the sponge. Therefore, the energy loss was calculated to investigate the damage efficiency of the sponge. 2.5. Statistical analysis Data were first analyzed by analysis of variance (ANOVA); when statistical differences were detected, Student's t-test for comparisons between groups was performed using SPSS software version 16.0 (SPSS Inc., Chicago, IL, USA). Data are reported as mean ± std at a significance level of p b 0.05. 3. Results and discussion PVA sponges possess many attractive features, such as biocompatibility, thermal stability, availability, and relative cheapness. However, the application of these versatile biomaterials is limited to biomedical and tissue engineering [34]. The excellent mechanical strength of PVA sponges enables them to be used as an energy absorber material for biomedical and industrial applications. Computing the kinetic energy of PVA sponges is, therefore, an important part of a comprehensive evaluation of their kinetic energy absorption capacity. With that in mind, the purpose of this study was to quantify the kinetic energy of a PVA sponge intended for use as an energy absorber in helmet and packaging material design. The kinetic energy absorption capacity of PVA sponge was computed through gunshot finite element modeling by the steel ball and bullet.
Table 1 Material properties of the finite element models. Material properties
Bullet and steel ball (low-carbon steel)
Polyvinyl alcohol sponge
Young's modulus (MPa) Poisson's ratio Yield stress (MPa) Failure strain (%)
210,000 0.284 260 0.330
40 0.499 9.790 0.660
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Fig. 1. A PVA sponge under uniaxial force.
Fig. 2. Schematic FE model of (a) steel ball and (b) steel bullet.
A PVA specimen during uniaxial tensile test is illustrated in Fig. 1. The uniaxial tensile test machine was consisted of a fixed and moveable jaw which provides a constant strain rate. The kinetic energy absorption capacity of PVA sponge is studied using gunshot with the steel ball and bullet. The schematic FE model of (a) steel ball and (b) bullet is indicated in Fig. 2. The dynamic mechanism of the PVA sponge being penetrated by a 6.3 mm steel ball and a 7.62 mm bullet at different sponge thicknesses is simulated. The dynamic changes in the stress distribution Fig. 3. The FE modeling for polyvinyl alcohol sponge damage and stress distribution after the steel ball penetrated at three different thicknesses, including (a) 5, (b) 10, and (c) 15 mm.
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in each sponge region are also simulated. The damage to the sponge and the distribution when the steel ball penetrated through the sponge at impact velocity of 734 m/s at three different thicknesses is presented in Fig. 3. Fig. 4 is also showed the damage to the sponge and the distribution when the bullet penetrated through the sponge at impact velocity of 734 m/s at three different thicknesses. The kinetic energy and energy loss for both the steel ball and bullet are computed. The kinetic energy for both (a) steel ball and (b) bullet is presented in Fig. 5. A
histogram representation of the energy loss for both the (a) steel ball and (b) bullet is indicated in Fig. 6. The results show that a higher thickness of the sponge leads to a higher energy loss of the steel ball and bullet. When the thickness for both the steel ball and bullet is the same, the energy loss of the steel ball is greater than that of the bullet. Therefore, the energy loss of the steel ball is always significantly greater than that of the bullet at the same thickness (Fig. 6). The highest energy loss of the steel ball and bullet is observed for the thickest sponge with 160 and 35.51 J, respectively. Hence, the damage efficiency of the steel ball is much greater than that of the bullet. However, the bullet causes more severe damage to the sponge than the steel ball in the PVA sponge models (Figs. 3 and 4). According to the law of conservation of energy, the energy that the projectile loses when it hits the sponge is absorbed by the impacted sponge. Thus, the more energy that the projectile loses as it penetrates the sponge, the more damage it causes. The results suggest the application of PVA sponge as a great kinetic energy absorber material compared to commonly used energy absorbing ones for helmet design. Traumatic brain injury is generally considered as a signature injury of the people using motorcycle, with costly and life-altering long-term effects. Hence, there is an urgent need to combat this problem by designing/developing a more effective helmet [35]. The main helmet components are the foam liner and the shell. Basically, the function of the foam is to absorb most of the impact energy, and to distribute the impact load on a wider foam area thus increasing the foam liner energy absorption capacity. Usually manufacturers design their helmets based on experimental verification. During the experimental verification the helmet must absorb the energy of the impact [36]. Among the energy absorbing materials available in the market, expanded polystyrene foams (EPS) are often considered for the design of the helmet liners [20,21], due to their capability of providing multidirectional resistance to impacts, combined with light weight and relatively low costs of production [22]. The compressive Young's modulus and maximum stress of 6.5–39 and 0.51–1.50 MPa have been reported for the current foam materials in helmet design (EPS) [20,22] as a function of thickness and density. The enhancement of the energy absorption properties of motorbike helmets could significantly improve the safety of riders [22]. Thus, the impact energy absorption by helmets is of vital importance to the safety of motorcyclists during accidents [37]. Our previous results showed the excellent mechanical strength of PVA sponge with the Young's modulus and maximum stress of 40 and 9.79 MPa, respectively [13]. Our results also showed the longitudinal and circumferential Young's modulus of 38.91 and 33.34 MPa, respectively. The maximum stress for longitudinal direction was 17.90% more than that of circumferential direction [15]. Interestingly, the faster strain rate for the PVA sponge caused a higher Young's modulus and maximum stress [14]. The cyclic mechanical tests revealed that the PVA sponge successfully retains its initial Young's modulus and maximum stress with less than 5% differences [14]. During the service life of a helmet, environmental factors such as pre-compression strain might lead to energy absorption ability degradation in the polystyrene foam which can be considerably minimized by reinforcement of PVA sponge [38]. It has also been reported that the energy loss of 100 J is acceptable for the energy absorber material to protect the head against impact loads [36]. Therefore, the PVA sponge can be considered as an alternative reinforcement material for helmet design to control the effect of injury by increasing the energy loss of the steel ball or bullet [39,40]. 4. Conclusions
Fig. 4. The FE modeling for polyvinyl alcohol sponge damage and stress distribution after the bullet penetrated at three different thicknesses, including (a) 5, (b) 10, and (c) 15 mm.
This study computed the kinetic energy absorption capacity of the polyvinyl alcohol sponge by finite element modeling of gunshot. The results showed the beneficial kinetic energy absorption capacity of the PVA sponge compared to commonly use polystyrene foams which have been considered as the most suitable material in helmet design. The suitable kinetic energy absorption capacity of the PVA sponge enables it to be used as a novel alternative reinforcement material in
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Fig. 5. A comparative representation of the kinetic energy at different thicknesses. (a) Kinetic energy of the steel ball. (b) Kinetic energy of the bullet.
Fig. 6. A comparative histogram representation of the energy loss at different thicknesses. (a) The steel ball. (b) The bullet. ⁎p b 0.05 compared to 5 mm thickness.
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