AUTOMOTIVE WINDSHIELD - PEDESTRIAN HEAD IMPACT: ENERGY ABSORPTION CAPABILITY OF INTERLAYER MATERIAL

Transcription

1 International Journal of Automotive Technology, Vol. 12, No. 5, pp (2011) DOI /s Copyright 2011 KSAE /2011/ AUTOMOTIVE WINDSHIELD - PEDESTRIAN HEAD IMPACT: ENERGY ABSORPTION CAPABILITY OF INTERLAYER MATERIAL J. XU 1,2)*, Y. B. LI 1), X. CHEN 2,4,5)*, D. Y. GE 3), B. H. LIU 1), M. Y. ZHU 1) and T. H. PARK 5) 1) State Key Laboratory of Automotive Safety & Energy, Department of Automotive Engineering, Tsinghua University, Beijing , China 2) Department of Earth and Environmental Engineering, Columbia University, 500 W 120th Street, New York, NY 10027, USA 3) School of Aerospace, Tsinghua University, Beijing , China 4) School of Aerospace, Xi an Jiaotong University, Xi an , China 5) Department of Civil & Environmental Engineering, Hanyang University, Seoul , Korea (Received 27 May 2010; Revised 17 December 2010) ABSTRACT During accident, the interlayer of windshield plays an important role in the crash safety of automotive and protection of pedestrian or passenger. The understanding of its energy absorption capability is of fundamental importance. Conventional interlayer material of automotive windshield is made by Polyvinyl butyral (PVB). Recently, a new candidate of high-performance nanoporous energy absorption system (NEAS) has been suggested as a candidate for crashworthiness. For the model problem of pedestrian head impact with windshield, we compare the energy absorption capabilities of PVB and NEAS interlayers, in terms of the contact force, acceleration, velocity, head injury criteria, and energy absorption ratio, among which results obtained from PVB interlayers are validated by literature references. The impact speed is obtained from virtual test field in PC-CRASH, and the impact simulations are carried out using explicit finite element simulations. Both the accident speed and interlayer thickness are varied to explore their effects. The explicit relationships established among the energy absorption capabilities, impact speed, and interlayer material/thickness, are useful for safety evaluation as well as automotive design. It is shown that the NEAS interlayer may absorb more energy than PVB interlayer and it may be a competitive candidate for windshield interlayer. KEY WORDS : Energy absorption, PVB, NEAS, Windshield, Impact, Pedestrian protection 1. INTRODUCTION One of the most pressing issues in automotive technology and industry is crash safety (Atahan) which includes both active safety (avoiding danger via electronic control and mechanical manipulation) and passive safety (human protection through energy absorption during accident) (Cunto and Saccomanno, 2008, Toy and Hammitt, 2003). Windshield plays an important role in real-world accidents: when a pedestrian is hit by a car the impact usually takes place on the windshield this accounts for about half of the deaths of the pedestrians during accident (Xu and Li, 2009a) which is usually due to head impact with windshield (Otte, 1999). Notably, pedestrians are the first largest category of motor vehicle deaths and counts for about 24.78% of total traffic accident fatalities (2010) whereas in Korea, the percentage is even higher: approximately 40% of all traffic-related fatalities in 2004 (Oh et al., 2008). Meanwhile, when a vehicle crashes into another one with sharp deceleration, the passenger may be *Corresponding author. xichen@columbia.edu injured by bumping into the windshield (Xu and Li, 2009c, Xu et al., 2009), although that can be lessened by the widespread usage of three-point/two point seatbelt. Unfortunately, for the purpose of pedestrian protection during accident, little effective modification and improvement has been made on the energy absorption capability of windshield (by contrast, for bumper and engine hood for example, progressive failure of energy absorption design has been well developed (Shahbeyk and Abvabi, 2009; Shin et al., 2008; Lee et al., 2008). As one of the hardest and most dangerous compartment in the vehicle front, the windshield as a primary pedestrian injury source has gained increasing concern from automotive and mechanical researchers. Previous studies mainly focused on the type and mechanism of windshield damage (or fracture) under different impact conditions. For example, Zhao and Dharani (Zhao et al., 2005; Zhao et al., 2006a; Zhao et al., 2006b) suggested a continuum damage mechanics model and simulated the damage and failure of the windshield upon featureless headform impact. Timmel and Kolling (Timmel et al., 2007) used a smeared model based on an explicit finite element solver to compute the dynamic 687

2 688 J. XU et al. response of the windshield before and after fracture. While these results are very useful to clarify the damage mechanism of windshield, they do not provide sufficient information in terms of the energy absorption characteristics upon impact with pedestrian s head. To fulfill both high impact-resistance and high energy absorption capability, polymeric foams and honeycombs have been widely applied (usually in sandwich structures in combination with high stiffness face sheets) in transportation tools (LJ and MF, 1999). Pyttel and Weyer (2003) tested glassy polymers at different temperatures and loading rates, and the established constitutive relation was verified using finite element analysis (FEA). Yasuki (Yasuki and Kojima, 2009) introduced a new FEA method for aluminum honeycomb. Currently, the PVB interlayer material (Valera and Demarquette, 2008), which is widely used in windshields, absorbs limited energy upon large deformation (while the outer glass layer resists impact to some extent) (Xu et al., 2011; Xu et al., In press-a). However, the energy absorption capability of traditional foams and honeycombs are still quite low, on the order of 0.1~10 J/g (Jo and Kim, 2009; Stobener and Rausch, 2009). Recently, we have proposed the nanoporous energy absorption system (NEAS), which is a high-performance protection and/or damping system. It utilizes the principle that as a nanoporous material is immersed in a non-wetting second-phase (e.g. a liquid or gel-like solution), the second phase can be forced to infiltrate the otherwise energetically unfavorable nanopore when an external mechanical pressure exceeds a critical threshold (Chen et al., 2006; Liu et al., 2008; Qiao et al., 2009; Liu et al., 2009; Chen et al., 2008; Zhao et al., 2009). The ultralarge specific surface area of the nanoporous material (~1000 m 2 /g) can therefore absorb the external mechanical energy into interface energy, with an overall energy absorption capability in excess of 100 J/g. The NEAS is therefore a potential choice of energy-absorbing windshield interlayer material. The potential performance windshield interlayer made by NEAS needs to be validated using numerical simulation. In particular, a comparison between the energy absorption characteristics of conventional PVB and new NEAS interlayer may highlight the advantage and possible suitability of NEAS as potential candidate for windshield interlayer. In this paper, we first set up a virtual pedestrian-vehicle accident simulation environment in PC-CRASH, from which the real-world pedestrian-vehicle accident speed is converted to head-windshield impact speed and equivalent mass. Next, the model problem of spherical head impacting the laminated windshield is simulated via FEA. The representative stress-strain relations of PVB material and NEAS are input into the interlayer material model. Contact force of the windshield plate, acceleration and velocity of the head, and their distributions in time history are comprehensively and parametrically studied. In addition, several explicit formulae regarding the design variables of windshield are extracted to provide a better understanding of windshield safety design. 2. SIMULATION OF REAL-WORLD ACCIDENT BETWEEN PEDESTRIAN AND VEHICLE In pedestrian-vehicle accident, pedestrian s kinetic movement/trajectory is complex due to the different frontend shapes of vehicles, contact points and pedestrian s height (Xu and Li, 2009b). Nevertheless, the initial impact speed to the windshield, point of impact on the windshield, and the equivalent mass of foreign object, are the three major factors responsible for windshield damage and pedestrian injury (Xu and Li, 2009a). As a first step of this research, we will convert the real-world pedestrian-vehicle accident speed (for typical impact conditions) to headwindshield impact speed and equivalent mass (for simplicity, the effect of point of impact is not discussed in this paper) Virtual Pedestrian-vehicle Accident Test Field A virtual pedestrian-vehicle accident field is setup based on PC-CRASH (Xu and Fan, 2007), a commercial software widely used for traffic accident reconstruction, under different conditions, i.e. different speeds of vehicles. A typical modern passenger car serves as the model vehicle with mass M v = 1310 kg, and dimensions L v W v H v =4480mm 1850 mm 1420 mm. The windshield on is measured as a b h = 1435 mm 634 mm 4.76 mm where a, b and c refer to the length, width and thickness respectively (the curvature of windshield is included). The pedestrian is modeled via multi-rigidbody method including head, torso, pelvic, upper limber, lower limber and foot parts with typical height H p = 1.75 m and weight H p =75 kg **. The coefficients of stiffness, friction, and contact are kept as default values in the software, and the tire-road coefficient is µ = 0.8. The initial condition of the pedestrian is stationary facing to the vehicle, and the vehicle is traveling at a certain speed and it takes full brake (i.e. deceleration is set as 0.8 g) as soon as it hits the pedestrian. Figure 1 shows the schematic model and in this study, pedestrian head are all hit the center of the windshield normally Simulation Results Our simulations show that the impact between head and windshield is almost normal. As the pedestrian-vehicle **The weight of pedestrian is negligible compared with vehicle mass, and it plays a minor role in our analysis below. On the other hand, the pedestrian s height and the front-end shape of vehicle will moderately influence the head-windshield impact speed. In this study, we keep the pedestrian s height fixed at a typical value and derive the relationship between pedestrian-vehicle accident speed and head-windshield impact speed (see Figure 2, Equation (1)-(2)). When the accident conditions such as the specifics of the car and pedestrian height are changed, by following the same approach one can simulate the detailed head-windshield impact speed

3 AUTOMOTIVE WINDSHIELD - PEDESTRIAN HEAD IMPACT 689 Figure 1. Virtual simulation setup for pedestrian-vehicle testing field. accident speed v p-v is varied, the head-windshield impact speed v h-w and the equivalent projectile (head) mass M e are investigated for the current model accident conditions. The results are given in Figure 2. According to Euro-NCAP, in a standard pedestrian protection test, the vehicle moves toward a stationary pedestrian at a speed of 40 km/h (2009), so we choose a reference accident speed v 0 = 40 km/h. With the definition of normalized variable Ω = v h-w /v 0, Ξ = v p-v /v 0 and M = M e /4.5 (standard industrial headform mass M 0 = 4.5 kg (2009)), the data in Figure 2 can be fitted as Ω = Ξ e Ξ 12.36lnΞ/Ξ 2 (1) M = Ξ Ξ Ξ 0.5 (2) Comparing the trends of the two relations, it is concluded that as the initial pedestrian-vehicle accident speed v p-v increases, the head-windshield impact speed v h-w also increases which agrees with the results of Wood s research in Ref. (Wood, 1998), and the difference between v h-w and v p-v becomes smaller; M e at the meantime increases except for the cases of between 30 km/h and 40 km/h, and that above 90 km/h. That s mainly because of the body inertia effects are quite different on the head in the cases below 40 km/h and above 90 km/h. In general, during a high-speed accident, the pedestrian hits the windshield far before it accelerates to the same speed as the vehicle. Therefore, when the factors of impact angle and impact point are excluded, a higher accident speed can increase both the impact speed between head and windshield and the equivalent impact mass. It is a long-lasting headache for people to get the relationship between v h-w and v p-v in real-world accident since no sensor can be installed in pedestrian s head. The only solution for the validation is through sled test within crash labs. Unfortunately however, the contact force, position and duration of head vs. windshield in real-world are quite different from those test resutls in labs, evidenced by realworld accidents site-investigations of Schmucker (Schmucker et al., 2010) and Tollon (Thollon et al., 2007). In the following analysis, impact simulations with the data pair of (M e, v h-w ) obtained above are employed to investigate Figure 2. Relationship between pedestrian-vehicle accident speed and impact speed between head- windshield and equivalent head mass. the energy-absorption capability of the interlayer. 3. FINITE ELEMENT MODELING OF HEAD- WINDSHIELD IMPACT A featureless spherical headform is used to model the human head (a rigid ball, the same one as that used in PC- CRASH), a common approach in automotive industry testing and evaluation; the model head has a radius R =90 mm (Zhao et al., 2005). The head Young s modulus is E = 6.5 GPa. The head mass is changeable through the change of density to represent the different equivalent masses of impactor (the variation of M e conforms with that of v h-w in Figure 2, for a given v p-v ). The windshield dimension is the same as that in PC-CRASH (Figure 1), except that it now has the tri-layer structure. The outer glass sheets are assigned with the typical soda-lime glass properties with density ρ = 2500 kg/m 3, Young s modulus E = 70 GPa, Poisson s ratio v =.022 (Mencik and Kralove, 1992). Cracking in glass is brittle and has the mode I energy release rate G I =10 J/m 2 (Mencik and Kralove, 1992), so energy dissipated by each fully developed crack (longest crack) is about e 1 = G I l 2h glass 0.028J, where l is the crack length and h glass is the thickness of mono glass sheet. Assuming that the maximum number of cracks is 100 (which is far more than realistic), the total dissipated energy by cracking is only about 2.8J, much less than the total kinetic energy and energy absorbed by interlayer and supporting materials obviously. Therefore, cracking of the glass is not considered. The properties are assumed to be rate independent (Holand and Beall, 2002) which is a common practice in dealing with glass. The thicknesses of two glass sheets are the same, i.e. 2 mm each. The PVB interlayer is modeled as a viscoelastic material with parameters: the short time shear modulus G 0 = 0.33 GPa, long time shear modulus G = GPa, bulk modulus K = 20 GPa and decay time β =12.6s -1 (Zhao et al., 2006a). The material parameters mentioned above fully goes accordance with the experimental data in Ref.(Xu et

4 690 J. XU et al. al., 2011). The standard thickness of PVB interlayer used in automotive industry is 0.76 mm; in this study, the thickness is also varied to explore its effect on energy absorption. For the NEAS interlayer, based on the testing results using ZSM-5 (Zhao et al., 2009), the pressurevolume response measured from the experiment of such a typical NEAS is converted to an equivalent stress-strain curve and used as the constitutive relation. The thickness of the NEAS interlayer is also varied. The boundary condition of the windshield is assumed to be fixed (clamped), qualitatively consistent with the framesetup in practice (Yuan and Li, 2005). The plate and head are modeled with eight-node linear brick elements, and a mesh convergence study is carried out to determine the proper mesh density and friction coefficient between head and windshield. The mesh density is determined to be in size of 1 mm 1 mm. The contact between the head and plate obeys the Coulomb s friction law with coefficient f =0.1 (since according to our accident scene investigation, a small slip exists between head and windshield). Calculation time step is 0.1 ms. 4. RESULTS AND DISCUSSIONS First of all, four indices are defined to evaluate the energy absorption capability of the interlayer quantitatively: F max : Maximum contact force occurred during the impact t Fdt 0 α : Force uniformity, i.e. α = F t max max, where t max is the time head detaches from windshield β : Energy absorption ratio, i.e. ( v 2 h w v' 2 h w ) v 2 h w, 2 where v' h w is the speed at the moment the head detaches from windshield plate. HIC : Head injury criterion, a standard indicator adopted in automotive industry, i.e. t HIC = max ( t 2 t 1 ) at ()dt (Gadd, 1966). t 2 t 1 (t 1-t 2) t 1 where a is the acceleration of head, and t 1 and t 2 are two instants during impact and we choose t 1 t 2 =0.15 ms These indices reveal the energy absorption capability of a structure/material in a comprehensive way Parametric Study: Impact Speed Contact force analysis Figure 3 shows the force-displacement curve when the accident speed v p-v equals to 10 m/s, 20 m/s and 40 m/s, which cover typical range of the pedestrian-vehicle accidents (Xu and Li, 2009c). The windshield quickly deforms subject to the impact of pedestrian s head via compression of the interlayer, followed by bending of the glass sheets (denoted as hard contact stage in Figure 3), leading to a high contact force. As the NEAS is more Figure 3. Contact force in time history under the accident speed of 10 m/s. Table 1. Maximum force and force uniformity at different accident speeds for both PVB and NEAS interlayers. v p-v =10 ms v p-v =20 ms v p-v =40 ms F max (kn) α F max (kn) ductile, the first stage of interlayer compression is longer than that of PVB interlayer so that more impact energy is absorbed. Moreover, NEAS also mitigates the peak contact force during the hard contact stage which is extremely beneficial to head protection (Marjoux et al., 2008). Meanwhile, if the impact speed and impactor mass increase, the contact time becomes shorter. Table 1 shows that the contact force in the case of PVB interlayer is about 8.9%, 13.05% and 14.98% higher than that of NEAS interlayer under three accident speeds. Moreover, the variation of the contact force with NEAS interlayer is also smoother (meaning smaller acceleration of the head) compared with the case of PVB; this can be further confirmed from Figure Acceleration analysis The time history of acceleration in Figure 4 shows that the NEAS interlayer provides gentler and slighter acceleration for the impacting head. The peak acceleration of head in NEAS laminated windshield is about 50% of that in PVB laminated one. The HIC value can be calculated from the acceleration time history curve, which equals to , and for PVB laminated windshield, respectively, when the accident speed is 10 m/s, 20 m/s and 40m/s; the results are similar to Ref. (Xu et al., In press-b) and all of them are above the non-severe head injury threshold of HIC=1000 (Gadd, 1966). If the interlayer is substituted by NEAS, the HIC values soon decrease to , and in these three cases. In α F max (kn) NEAS PVB α

5 AUTOMOTIVE WINDSHIELD - PEDESTRIAN HEAD IMPACT 691 Figure 4. Acceleration in time history under the accident speed of 10 m/s. general, in real-world accidents, pedestrian has a large possibility to die over the accident speed of 50 km/h (Giavotto, 2004; Maki et al., 2003; Xu and Li, 2009a). Our analyses show that with the possible use of NEAS interlayer, the HIC value can be below the threshold when the accident speed is below about 40 km/h, a considerable enhancement for vehicle crash safety. In order to obtain more appreciation of the relationship between head injury and accident speed v p-v, a parametric study is conducted. With the same normalization definition of Ξ = v p-v /v 0 and H=HIC/HIC 0 where HIC 0 =1000 is the threshold of severe head damage, results can be fitted as Ξ Ξ lnΞ for NEAS interlayer HIC = Ξ Ξ lnΞ for PVB interlayer shown in Figure 5. As remarked earlier, if the pedestrian s height and vehicle specifications are changed, the relationship between HIC and v p-v may undergo moderate variations. The result may provide useful insight not only on the estimation of pedestrian head injury during a certain accident, but also on the establishment of crash test standards or regulations. (3) Figure 6. Velocity changes in time history at different accident speeds for PVB and NEAS interlayers Velocity analysis Figure 6 demonstrates the velocity change of head with different interlayers at different accident speeds. The head velocity decreases faster in the approaching stage and increases slower in the detaching stage in the NEAS laminated windshield system. The total energy of the headwindshield system E total equals to the kinetic energy of head before impact, and becomes E total = E d-head +E d-windshield + E k-head +E k-windshield after impact, where E d-head and E d-windshield are the deformation (strain) energy of head and windshield; E k-head and E k-windshield are the kinetic energy of head and windshield, respectively. Since the deformation of head and windshield is small; and the kinetic energy of the head is more dominant compared with other energy components (see Figure 7, in the case of PVB interlayer), β is employed as a direct indicator to evaluate how much kinetic energy is dissipated by the windshield plate, shown in Figure 8. β values for NEAS laminated windshield are 37.65%, 57.10% and 41.25%, while they are 24.48%, 49.13% and 28.75% in the PVB counterpart (and the results for PVB interlayer is consistent with ref. (Zhao et al., 2006b). Therefore, NEAS can mitigate the foreign object impact Figure 5. HIC value calculated at different accident speeds for both PVB and NEAS interlayers and their non-linear fitting curves. Figure 7. Kinetic energy comparison between head and windshield during the impact process, for accident speeds at 72 km/h.

6 692 J. XU et al. Figure 8. Energy absorption efficient β of at different accident speeds for PVB and NEAS interlayers and their non-linear fitting curves. energy more efficiently. With the same normalization definition of Ξ = v p-v /v 0, the fitting formula can take the following form, Ξ Ξ for NEAS interlayer Ξ Ξ β = Ξ Ξ for PVB interlayer Ξ Ξ (4) shown in Figure 8. At around 70km/h of accident speed, both PVB and NEAS can dissipate the most percentage of incident kinetic energy. If the impact speed is lower, NEAS can perform much better than PVB in terms of energy absorption, thanks to its ductility Parametric Study: Thickness of Interlayer The interlayer of windshield nowadays is standardized, i.e. the thicknesses of PVB interlayer in passenger cars and trucks are 0.76 mm and 1.52 mm respectively. In this section, we fix the accident speed at a typical value of v p- v = 20 m/s(except in section 4.2.2) and investigate the effect brought by the interlayer thickness Contact force analysis Figure 9 shows the time history of contact force: With the thickness of interlayer increases, F max decreases and α increases in both PVB and NEAS laminated windshields, indicating that the present design of the thickness of interlayer is not sufficient in terms of pedestrian s protection. Since impact mostly induces localized deformation (i.e. the energy absorption mechanism is prominent only in a small region near the impact point), the overall thickness effect on the contact force is not very significant; the thick PVB layer can only reduce the peak contact force by 11.78% and that for NEAS is 22.63%, despite of the 8 times of thickness increase Acceleration analysis With the increase of interlayer thickness, the acceleration Figure 9. Contact force variation during impact process at different interlayer thicknesses, for accident speeds at 72 km/h. become smoother and that will be reflected in the HIC value. We normalize HIC as H = HIC/HIC 0. The dimensionless interlayer thickness is Γ = h/h 0 where h 0 = 0.76 mm, the standard thickness for cars. As aforementioned, the accident speed in the standard pedestrian protection test is 40 km/h (which means that the pedestrian is usually safe below that accident speed), hereby we choose the accident speed v p-v = 50 km/h to check the HIC variation under different interlayer thicknesses. Figure 10 illustrates the relationship between H and Γ, which may be fitted as Γ Γ lnΓ for NEAS interlayer H = Γ Γ lnΓ for PVB interlayer According to Equation (5), for NEAS interlayer, theoretically, only under the condition that h 4.8 mm, can we keep the HIC value below 1000 while it is much more difficult to make the HIC value below safety threshold if the interlayer is made of PVB (for v p-v = 50 km/h). Similar results like the above equation can be established for other accident speeds and conditions, which will be useful for reducing the critical thickness of the interlayer at which the injury is below the threshold. (5)

7 AUTOMOTIVE WINDSHIELD - PEDESTRIAN HEAD IMPACT 693 laminated windshield, indicating a potential service as an interlayer. 5. CONCLUSION Figure 10. Relationship between normalized HIC value and interlayer thickness, for accident speeds at 72 km/h Velocity analysis Similarly, β can be correlated with the interlayer thickness (for NEAS and PVB) to provide a quick estimation on the requirement thickness of the interlayer for a given energy mitigation criterion (shown in Figure 11 for v p-v = 72 km/h): Γ 0.5 for NEAS interlayer β = Γ 0.5 for PVB interlayer Again, for other accident speeds, similar results like Equation (6) can be established which would facilitate the decision making of automotive safety engineer for choosing the right thickness of windshield interlayer for a specific interlayer material. In addition, since the glass sheet is kept the same, the ability of NEAS laminated windshield would have an equivalent ability to prevent occupant from ejection, thus NEAS laminated windshield has a superior pedestrian/ occupant energy absorption ability and similar occupant ejection capability compared to the traditional PVB (6) We employed numerical simulations to study energy absorption ability of windshield interlayer materials. The performance of a traditional material, PVB, is compared with a new material, nanoporous energy absorption system (NEAS). First, the relationship between the accident speed between pedestrian and automobile, v p-v, is bridged with the human head-windshield impact speed v h-w and equivalent head mass M e, for typical impact conditions using PC- CRASH. The deduced combinations of v p-v and M e are then employed to simulate head-windshield impact using FEA, focusing on several energy absorption characteristics using NEAS or PVB as an interlayer; the interlayer thickness is also varied to explore its effect. Four indices are firstly defined and used to quantitatively evaluate the energy absorption performance, and they are explicitly related to the accident speed, which may provide a quick and effective guidance of automotive safety design and evaluation. Results indicate that NEAS can perform better in providing pedestrian protection than PVB, especially during high-speed impact accidents and thus NEAS could be an ideal candidate for current laminated windshield design. In addition, the thickness of interlayer should be increased both for PVB and NEAS since current PVB laminated windshield cannot provide sufficient protection for pedestrian against windshield impact in order to reach a better safety goal. Further research should be carried out in two major directions, e.g. one is to verify the relationship between v h-w and v p-v in real-world accident scenes and the other one is to fully utilize the NEAS (including solving the sealing problem) or modify current PVB interlayer to provide a better pedestrian protection in terms of real-world application. ACKNOWLEDGEMENT This work is financially supported by National Natural Science Foundation of China (NSFC) under the grant No , State Key Laboratory of Automotive Safety & Energy, Tsinghua University under grant No. ZZ and Doctoral Fund of Ministry of Education of China under grant No Y. Li and X. Chen appreciate the founding from Tsinghua University under the International Cooperation Project. J. Xu appreciates China Scholarship Council (CSC) to financially sponsor his study at Columbia University through joint Ph.D. program. X. Chen is supported by the National Science Foundation (CMMI ), NSFC ( ) and a World Class University (WCU) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology of Korea (R ). REFERENCES Figure 11. Relationship between Energy absorption efficient β and normalized thickness with their non-linear fittings. Euro NCAP (2009). Assessment Protocol-Pedestrian Protection.

8 694 J. XU et al. Atahan, A. O. (2010). Vehicle crash test simulation of roadside hardware using LS-DYNA: A literature review. Int. J. Heavy Veh. Syst. 17, 1, Chen, X., Cao, G., Han, A., Punyamurtula, V. K., Liu, L., Culligan, P. J., Kim, T. and Qiao, Y. (2008). Nanoscale fluid transport: Size and rate effects. Nano Letters, 8, Chen, X., Surani, F. B., Kong, X. G., Punyamurtula, V. K. and Qiao, Y. (2006). Energy absorption performance of a steel tube enhanced by a nanoporous material functionalized liquid. Appl. Phys. Lett., 89, Cunto, F. and Saccomanno, F. F. (2008). Calibration and validation of simulated vehicle safety performance at signalized intersections. Accident Analysis and Prevention 40, 3, Gadd, C. W. (1966). Use of a Weighted impulse Criterion for Estimating Injury Hazard. SAE. Giavotto, V. (2004). Compatibility of Vehicles with Safety Barriers - Head Ejection Through Side Windows. Transportation Research Board. Washington. Holand, W. and Beall, G. H. (2002). Glass Ceramic Technology. Wiley-Blackwell. Berlin. Jo, H. C. and Kim, Y. E. (2009). A study on the influence of the seat and head restraint foam stiffnesses on neck injury in low speed offset rear impacts. Int. J. Precision Engineering and Manufacturing 10, 2, Lee, J. W., Shin, M. K., Yoon, K. H. and Park, G. J. (2008). An orthogonal-array-based design-of-experiments method for designing a vehicle hood and bumper structure. Proc. Institution of Mechanical Engineers Part D-J. Automobile Engineering, 222, D2, Liu, L., Chen, X., Lu, W., Han, A. and Qiao, Y. (2009). Infiltration of electrolytes in molecular-sized nanopores. Phys. Rev. Lett. 102, 18, Liu, L., Qiao, Y. and Chen, X. (2008). Pressure-driven water infiltration into carbon nanotube: The effect of applied charges. Appl. Phys. Lett., 92, Lj, G. and Mf, A. (1999). Cellular Solids. Cambridge University Press. Cambridge. Maki, T., Kajzer, J., Mizuno, K. and Sekine, Y. (2003). Comparative analysis of vehicle-bicyclist and vehiclepedestrian accidents in Japan. Accident Analysis and Prevention 35, 6. Marjoux, D., Baumgartner, D., Deck, C. and Willinger, R. (2008). Head injury prediction capability of the HIC, HIP, SIMon and ULP criteria. Accident Analysis and Prevention 40, 3, Mencik, J. and Kralove, H. (1992). Strength and Fracture of Glass and Ceramics. Elsevier Science Publisher. Amsterdam. Oh, C., Kang, Y. S., Youn, Y. and Konosu, A. (2008). Development of probabilistic pedestrian fatality model for characterizing pedestrian-vehicle collisions. Int. J. Automotive Technology 9, 2, Otte, D. (1999). Severity and Mechanism of Head Impacts in Car-to-pedestrian Accidents. IRCOBI. Sitges. Spain. Pyttel, T. and Weyer, S. (2003). Crash simulation with glassy polymers - constitutive model and application. Int. J. Crashworthiness 8, 5, Qiao, Y., Liu, L. and Chen, X. (2009). Pressurized liquid in nanopores: A modified laplace-young equation. Nano Lett. 9, 3, Research Institution of Traffic Management of Public Safety Ministry (2010). Traffic Accident Annual Census of People's Republic of China. Beijing. Schmucker, U., Beirau, M., Frank, M., Stengel, D., Matthes, G., Ekkernkamp, A. and Seifert, J. (2010). Real-world car-to-pedestrain-crash data from an urban center. J. Trauma Management and Outcomes 4, 2, 1 6. Shahbeyk, S. and Abvabi, A. (2009). A numerical study on the effect of accident configuration on pedestrian lower extremity injuries. Scientia Iranica Trans. a-civil Engineering 16, 5, Shin, M. K., Yi, S. I., Kwon, O. T. and Park, G. J. (2008). Structural optimization of the automobile frontal structure for pedestrian protection and the low-speed impact test. Proc. Institution of Mechanical Engineers Part D-J. Automobile Engineering, 222, D12, Stobener, K. and Rausch, G. (2009). Aluminium foampolymer composites: Processing and characteristics. J. Materials Science 44, 6, Thollon, L., Jammes, C., Behr, M., Amoux, P. J., Cavallero, C. and Brunet, C. (2007). How to decrease pedestrain injuries: Conceptual evolutions starting from 137 crash tests. J. Trauma, 62, Timmel, M., Kolling, S., Osterrieder, P. and Bois, P. A. D. (2007). A finite element model for impact simulation with laminated glass. Int. J. Impact Eng. 34, 8, Toy, E. L. and Hammitt, J. K. (2003). Safety impacts of SUVs, vans, and pickup trucks in two-vehicle crashes. Risk Anal. 23, 4, Valera, T. S. and Demarquette, N. R. (2008). Polymer toughening using residue of recycled windshields: PVB film as impact modifier. Eur. Polym. J., 44, Wood, D. P. (1998). Impact and movement of pedestrian in frontal collisions with vehicles. Proc. Institute of Mechanical Engineers, Part D, Automotive Engineering, 202, Xu, H. and Fan, Y. (2007). Simulation Research on Form and Kinematics Law of Contact Process for Automobilepedestrian Collision based on the Coupling of PC- CRASH and MADYMO. American Society of Civil Engineering. Chengdu, Sichuan. Xu, J. and Li, Y. (2009a). Crack analysis in PVB laminated windshield impacted by pedestrian head in traffic accident. Int. J. Crashworthiness 14, 1, Xu, J. and Li, Y. (2009b). Review on pedestrian-vehicle impact accident reconstruction technology. Automotive Engineering 31, 11, Xu, J. and Li, Y. (2009c). Study of damage in windshield

9 AUTOMOTIVE WINDSHIELD - PEDESTRIAN HEAD IMPACT 695 glazing subject to impact by a pedestrian's head. Proc. Inst. Mech. Eng. Part D-J. Automob. Eng. 223, 1, Xu, J., Li, Y., Ge, D., Liu, B. and Zhu, M. (2011). Experimental investigation on constitutive behavior of PVB under impact loading. Int. J. Impact Eng. 38, 2 3, Xu, J., Li, Y., Liu, B., Zhu, M. and Ge, D. (2011). Experimental study on mechanical behavior of PVB laminated glass under quasi-static and dynamic loadings. Composite B, 42, Xu, J., Li, Y., Lu, G. and Zhou, W. (2009). Reconstruction model of vehicle impact speed in pedestrian-vehicle accident. Int. J. Impact Eng 36, 6, Xu, J., Zhu, M., Liu, B. and Li, Y. (In press-b). Investigation on dynamic response of PVB laminated windshield in low impact speed. ACTA Armamentaria, 31, Yasuki, T. and Kojima, S. (2009). Application of aluminium honeycomb model using shell elements to offset deformable barrier model. Int. J. Crashworthiness 14, 5, Yuan, X. and Li, S. M. (2005). Analysis of rectangular thin plate vibration under different support conditions. Aeroengine 31, 3, Zhao, J., Culligan, P. J., Germaine, J. T. and Chen, X. (2009). Experimental study on energy dissipation of electrolyte in nanopores. Langmuir, 25, Zhao, S., Dharani, L. R., Chai, L. and Barbat, S. D. (2006a). Analysis of damage in laminated automotive glazing subjected to simulated head impact. Eng. Fail. Anal. 13, 4, Zhao, S., Dharani, L. R., Chai, L. and Barbat, S. D. (2006b). Dynamic response of laminated automotive glazing impacted by spherical featureless headform. Int. J. Crashworthiness 11, 2, Zhao, S., Dharani, L. R., Liang, X., Chai, L. and Barbat, S. D. (2005). Crack initiation in laminated automotive glazing subjected to simulated head impact. Int. J. Crashworthiness 10, 3,