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ScienceDirect Procedia Engineering 103 (2015) 287 – 293

The 13th Hypervelocity Impact Symposium

Stress Wave and Damage Propagation in Transparent Materials Subjected to Hypervelocity Impact N. Kawaia,c,*, S. Zamab,c, W. Takemotob,c, K. Moriguchib,c,†, K. Araib, S. Hasegawac, E. Satoc a

Institute of Pulsed Power Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan b Department of Mechanical Engineering, Hosei University, 3-7-2 Kajino, Koganei 184-8584, Japan c Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara 252-5210, Japan

Abstract Hypervelocity impact experiments have been conducted on transparent materials, SiO2 glass and polycarbonate, to observe directly the impact-induced damage process progressing inside materials. Stress wave propagation and damage evolution associated with hypervelocity impact are visualized by employing the Edge-on Impact technique coupled with an ultra-high-speed video camera. Recorded images clearly show how stress wave propagate and interact each other, and how damages form and propagate during hypervelocity-impact event. In the impacted SiO2 glass, the damages on the ballistic direction are formed associated with the propagation of a shear wave. And, the interaction of the shear wave with the longitudinal wave reflected from free surface initiates the drastic nucleation of damage points. In the impacted polycarbonate, in addition to penetration damage and spall fracture, the delamination-like fracture is initiated by the interaction of the reflected rarefaction waves from both side free surfaces. However, such delamination fracture does not extended inside the plastic deformed region induced by the penetration of the impacted projectile. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Curators of the University of Missouri On behalf of the Missouri University of Science and Technology Keywords: Hypervelocity impact; High-speed imaging; Wave propagation; Damage evolution; Transparent material

1. Introduction Hypervelocity impact phenomena have been studied in relate to the orbital debris problem in the field of space engineering. The impact of debris which are traveling at hypervelocity in space can endanger the structural integrity as well as the optical, thermal and electrical functionality of spacecraft. Against a backdrop of increasing number of space debris, impact-damage evaluation is a growing concern in spacecraft designing. Since large components of spacecraft are made of metal as represented by aluminum alloy, the impact-damage characteristic of metals have been extensively investigated [13]. Especially in relation to a Whipple bumper shield for micrometeoroids and space debris impact, the damage behavior of thin metallic plate subjected to hypervelocity impact has been examined by both in-situ and postmortem observation [4-6]. On the other hand, hypervelocity-impact studies conducted on glass, polymers and ceramics, which are often used for significant components in optical and thermal system of spacecraft, are very few in number comparing to the case of metals. In addition, these studies are predominantly based on postmortem observations and focused on developing equations to predict impact-damage geometry [1-2, 7-10]. While the end state of impact event is primal interest, damage formation processes can affect the overall outcome. Non-metal materials such as ceramics, glass and polymer are more vulnerable to damage by hypervelocity impact than metals. A further understanding of the impact-failure processes are necessary to

* K. Kawai. Tel.: +81-96-342-3299; fax: +81-96-342-3293 . E-mail address: [email protected] . † Present affiliation: Toyota Motor Corporation

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Curators of the University of Missouri On behalf of the Missouri University of Science and Technology

doi:10.1016/j.proeng.2015.04.049

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N. Kawai et al. / Procedia Engineering 103 (2015) 287 – 293

improve the material model and the fidelity of damage simulation for better designing of spacecraft. In this study, we performed high-speed imaging of hypervelocity-impact events on transparent materials, SiO2 glass and polycarbonate which are primary window and window cover materials for spacecraft [8], to visualize and investigate the impact-damage evolution and formation process in non-metal materials. The transparency of target materials allows for the observation of impact-related phenomena occurred inside the specimen. That kind of information can provide a valuable insight into the fundamental understanding of damage formation mechanism induced by the hypervelocity impact on glass, ceramics, and polymers. 2. Experimental Hypervelocity-impact experiments were conducted by launching a spherical projectile using a conventional two-stage light-gas gun [11], with the exception of one experiment conducted on polycarbonate using a vertical two-stage light-gas gun. The overview photos of both guns are shown Fig. 1. In the tests using the conventional gun, a 3.2-mm sphere was impacted to specimens by employing the sabot separation technique [11]. In the case of the experiment using the vertical gun, a 4.7-mm polycarbonate sphere was loaded without a sabot and lunched. The specimen sizes were 60x60x15 mm for SiO2 glass, and 80x40x30 mm for polycarbonate. In this study, we employed the Edge-on Impact (EOI) technique [12-14] to visualize the impact-damage evolution inside the specimen. In the EOI test, a projectile hits one edge of a specimen and damage evolution is observed by means of a high-speed camera of which shooting angle is set perpendicular to the impact plane. The schematics of experimental configurations are shown in Fig. 2. Stress wave propagation, damage nucleation and propagation can be observed via

Fig. 1. Photographs of (a) a conventional two-stage light-gas gun and (b) a vertical two-stage light-gas gun installed at ISAS/JAXA.

Fig. 2. Schematics of Edge on Impact test set-up. (a) transmitted light configuration for polycarbonate experiments (b) reflected light configuration for SiO2 glass experiments.


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shadowgraphy. A regular transmitted light shadowgraph set-up was adopted for polycarbonate experiments (Fig. 2(a)). In the case of SiO2 glass tests, high-speed shadowgraphy was performed in a reflected light configuration (Fig. 2(b)). In this configuration, a camera can detect the light reflected from front surface, which transfer the information of surface condition, and that reflected from rear surface, which pass through the specimen and reflect the internal condition. Consequently, it is possible to observe the internal damage and surface fracture, distinctively [15]. This set-up is useful for brittle materials of which the impact-damage easily propagates not only inside but also along the free surface. To obtain the appropriate intensity of the reflected light, a Pt-Pd layer less than 100 nm was deposited on the surface opposite to the illuminated surface. The impact event and ensuing damage evolution was recorded using an ultra-high-speed video camera (Shimadzu Corp., HPV-X). The flaming interval and exposure time were set at 1 μs and 200 ns, respectively. In the experiment using the vertical gun, another ultra-high-speed video camera (Shimadzu Corp., HPV-1) was also used to obtain the shadowgraph image in side and top views, simultaneously. The flaming interval and exposure time of HPV-1 were set at 2 μs and 250 ns, respectively. 3. Results and discussion 3.1. SiO2 glass Fig. 3 shows the series of high-speed video images of the SiO2 glass impacted by a 3.2-mm aluminum sphere at 3.01 km/s. A grid pattern of 10-mm wide was drown on the illuminated surface for post analysis of the obtained high-speed video images. On the images in this figure, the damaged region is represent as dark area because the reflected light intensity is reduced by light scattering at fracture surface inside material. However, the grid pattern drawn on the illuminated surface is still observed. This fact recognizes that this surface is not fractured and failure occurs inside material. Fig. 3(a) is the image captured at 4 μs after impact. The semispherical stress wave induced by impact is clearly observed, which is indicated as 1st wave in Fig. 3(a). Following that wave, the formation of the 2nd wave is also observed. In addition the continuous spreading of damaged region, the nucleation of damaged points is observed the forward part of continuous damage zone. Fig. 3(b) is still image at 8 μs after impact. The 1st and 2nd wave propagate forward and the part of spherical 1st wave reach the upper and lower surface and reflect into the glass. The reflected 1st wave is marked by the dashed line. Isolated damage points which denoted by circles appear at region where reflected 1st wave intersects with the 2nd wave. Fig. 3(c) shows the captured image at 13 μs after impact. At this time, the 1st wave reaches the rear surface and reflects back into SiO2 glass. The reflected wave induces the drastic formation of damage points. This damage-generating mechanism is similar to spall fracture [16]. However, it is different from usual spall fracture pattern which shows planer fracture surface [17-18]. The generation of these discrete spall damage pattern is expected to be related to the feature of amorphous materials which do not have grain boundary acting as crack path. Fig. 4 shows the time-path diagram of 1st wave, 2nd wave and damage front on the ballistic axis. The damage front means the front edge of damaged area including the isolated damage point. The velocities of 1st wave and 2nd wave can be derived by linear regression of these time-path data. The derived velocity of the 1st wave and 2nd wave is 5.71 km/s and 3.81 km/s, respectively. Each wave velocity corresponds to the longitudinal and transversal velocity of SiO2 glass, 5.90 km/s and 3.75 km/s. Therefore, the 1st wave and 2nd wave are assigned to be elastic compression wave and shear wave. From this time-distance history graph, it is noticed that the locus of the front edge of damaged zone correspond to the position of the 2nd wave up to 8 μs after impact. This result indicates that the nucleation of isolated damaged point preceding the continuous damage zone is induced by the shear wave. After 8 μs, the isolated damage points are linked with

Fig. 3. Series of reflected shadowgraphs of SiO2 glass impacted by a 3.2-mm aluminum sphere at 3.01 km/s.

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Fig. 4. Time-path plot of waves and fracture propagation in SiO2 glass depicted in Fig. 3.

the continuous damage zone and they continues to expand together. Its propagation velocity is also derived by linear regression of the time-path data after 8 μs. The derived damage velocity is 1.6 km/s and quite slower than that of shear wave. The transition of damage propagation velocity from transversal sound speed to lower one has been observed in the hypervelocity impact study conducted on basalt [19]. Although the authors have concluded that this transition is related to onset of instability of damage front, such phenomenon is not observed in this study. Sustained observations are needed to understand this transition mechanism. Fig. 4 also show that the generating time and place of spall-like fracture nearly correspond to the intersection point of the reflected 1st wave with 2nd wave. This fact indicates that the drastic nucleation of damage points is initiated by the interaction effect of the elastic rarefaction wave with the shear wave. The same conclusion can be applicable to the generation of damage points denoted in Fig. 3(b). 3.2. Polycarbonate Fig. 5 shows the still images extracted from the high-speed video images of the polycarbonate impacted by a 3.2-mm stainless steel sphere at 4.06 km/s and the postmortem photograph of specimen (Fig. 5(f)). The time on the each image denotes the time after impact. The propagation behavior of single semispherical wave induced by impact was clearly recorded. In the case of polycarbonate experiments, the impacted projectile penetrate into the specimen. Comparing Fig. 5(b)

Fig. 5. Damage evolution in a polycarbonate impacted by a 3.2-mm stainless steel sphere at 4.06 km/s. (a-e) the images obtained by transmitted shadowgraph. (f) the postmortem photograph of the impacted specimen.


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Fig. 6. Selected shadowgraph images of polycarbonate impacted by a 4.7-mm polycarbonate sphere at 6 km/s obtained in side view (a-c) and top view (eg), simultaneously. (d) and (h) are postmortem photographs in side view and top view, respectively.

with (a), it is recognized that the formation of some damages branched from the main penetration-failure zone, which are denoted by arrows in Fig. 5(b). The branched damages are caused by the brake-up of the projectile during penetration [20]. This result indicates that fracture behavior of projectile effects on the shape of the penetration damage on polymer materials. Fig. 5(b) also shows the formation of plastic deformation zone around the penetration-damaged area. The boundary of plastic deformed zone is represented by a dashed line in Fig. 5(b). The deformed zone is represented as slightly gray area because the transmitted light is refracted by the refractive index change associated with plastic deformation. The formation of plastic zone around a penetration hole was observed in the previous study to examine the penetration behavior of polycarbonate [21]. Fig. 5(c) shows the spall facture is induced by the rarefaction wave reflected from rear surface. In contrast to the case of SiO2 glass (see Fig. 3(c)), the spall fracture pattern shows usual planer shape [16-18]. As shown in Fig. 5(d), the spall fracture expands to both horizontal and vertical directions associated with the propagation of the rarefaction wave. In addition, a characteristic crescent-shaped fracture appear along the plastic deformed zone. As further passage of time (Fig. 5(e)), the crescent-shaped fracture continues to spread in-plane direction. However, this fracture does not extend into the plastic deformed zone. It is interesting that the impact-induced plastic deformation of polymer can interrupt the propagation of fracture. The postmortem specimen shows that the crescent-shaped fracture forms in relatively thin layer on center axis like delamination fracture. From the structure of fracture, this delamination-like fracture is expected to be initiated by the interaction of the reflected stress waves from both side free surfaces. To observe how stress wave interact and initiate fracture in side and top view, simultaneously, the hypervelocity impact test using the vertical gun was conducted on a polycarbonate. A 4.7-mm polycarbonate sphere was impacted at 6.00 km/s. The selected images obtained in this test are listed in Fig. 6 together with the photos of postmortem specimen (Fig. 6(d) and (h)). The side view and top view images aligned one below the other, allowing for a direct comparison. The time after impact of each pair is denoted in the side view images. Overall damage structure is similar to that shown in Fig. 5, except for the shape of penetration damage which is semispherical in this test. The formation of semispherical crater is caused that the polycarbonate projectile softer than stainless steel behaves like fluid under the shock compressed state achieved by impact. The series of images in Fig. 6 clearly demonstrates that the crescent-shaped fracture is initiated by the interaction of the reflected waves from both side free surfaces, as expected above. This result strongly indicates that the three-dimensional observation of wave propagation and interaction is important to understand the formation of hypervelocity-impact-induced

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damages. The high-speed visualization in two directions coupled with the vertical two-stage light gas gun, which was attempted in this study, is definitely one of promising methods to address that demand. 4. Conclusion Hypervelocity impact experiments were conducted on SiO2 glass and polycarbonate to investigate the impact-induced damage evolution of non-metal materials in hypervelocity regime. The Edge-on Impact technique coupled with an ultrahigh-speed video camera was employed to visualize stress wave and damage propagation in materials. The results of SiO2 experiment shows that the isolated damage points are initiated by the propagation of the shear wave in addition to the development of continuous damage zone spreading from the impact point. After linking the damage points with the continuous damage zone, the damage front propagates at velocity of 1.6 km/s which is slower than the shear wave velocity. Although the longitudinal wave itself does not initiate damages, the interaction of the reflected longitudinal wave from free surface with shear wave causes the drastic nucleation of damage points. In the case of polycarbonate, the projectile penetrates into material. As the result, damaged area is formed along the penetration path, and plastic deformed region is also formed around the penetration damage zone. In addition to usual spall facture induced by the rarefaction reflected form rear free surface, the crescent-shaped fracture is formed along the plastic deformed zone. This delamination-like fracture is initiated by the interaction of the reflected rarefaction waves from both side surfaces. Although this fracture widely spread into plane direction, it cannot extend inside the plastic deformed zone. This fact may propose the impact fracture interruption utilizing plastic deformation of polymers. Through this study, it is clearly recognized that the wave interaction is key issue to understand the damage formation process under hypervelocity impact event at least for glasses and polymers. The results of this study strongly suggest the usefulness of multidimensional high-speed visualization of impact event using transparent material to improve understanding the relationship between wave interaction and damage formation.

Acknowledgements This research was supported by the Space Plasma Laboratory, ISAS, JAXA.

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