The Blast Response of Sandwich Composites With a Functionally Graded Core and Polyurea Interlayer

Transcription

1 Proceedings of the IMPLAST 2010 Conference October Providence, Rhode Island USA 2010 Society for Experimental Mechanics, Inc. The Blast Response of Sandwich Composites With a Functionally Graded Core and Polyurea Interlayer Nate Gardner and Arun Shukla Dynamic Photomechanics Laboratory, Dept. of Mechanical, Industrial & Systems Engineering University of Rhode Island, 92 Upper College Road, Kingston, RI 02881, USA nate.gardner11@gmail.com ABSTRACT In the present study, the dynamic behaviors of two types of sandwich composites made of E-Glass Vinyl-Ester (EVE) face sheets and Corecell TM A-series foam with a polyurea interlayer were studied using a shock tube apparatus. The materials, as well as the core layer arrangements, were identical, with the only difference arising in the location of the polyurea interlayer. The foam core itself was layered based on monotonically increasing the acoustic wave impedance of the core layers, with the lowest wave impedance facing the shock loading. For configuration 1, the polyurea interlayer was placed behind the front face sheet, in front of the foam core, while in configuration 2 it was placed behind the foam core, in front of the back face sheet. A high-speed side-view camera system along with a high-speed back-view Digital Image Correlation (DIC) system was utilized to capture the real time deformation process as well as mechanisms of failure. Post mortem analysis was carried out to evaluate the overall blast performance of these two configurations. The energy redistribution behavior for both configurations during the blast loading process was also investigated. The results indicated that applying polyurea behind the foam core and in front of the back face sheet will reduce the back face deflection, particle velocity, and in-plane strain, thus improving the overall blast performance and maintaining structural integrity. INTRODUCTION Core materials play a crucial role in the dynamic behavior of sandwich structures when they are subjected to highintensity impulse loadings such as air blasts. Their properties assist in dispersing the mechanical impulse that is transmitted into the structure and thus protect anything located behind it [1-3]. Stepwise graded materials, where the material properties vary gradually or layer by layer within the material itself, were utilized as a core material in sandwich composites since their properties can be designed and controlled. Typical core materials utilized in blast loading applications are generally foam, due to its ability to compress and withstand highly transient loadings. However, this foam core lacks the ability to maintain structural integrity. In recent years, with its ability to improve structural performance and damage resistance of structures, as well as effectively dissipate blast energy, the application of polyurea to sandwich structures has become a new area of interest. The numerical investigation by Apetre et al. [4] on the impact damage of sandwich structures with a graded core (density) has shown that a reasonable core design can effectively reduce the shear forces and strains within the structures. Consequently, they can mitigate or completely prevent impact damage on sandwich composites. Li et al. [5] examined the impact response of layered and graded metal-ceramic structures numerically. He found that the choice of gradation has a great significance on the impact applications and the particular design can exhibit better energy dissipation properties. In their previous work, the authors experimentally investigated the blast resistance of sandwich composites with stepwise graded foam cores [6]. Two types of core configurations were studied and the sandwich composites were layered / graded based on the densities of the given foams, i.e. monotonically and non-monotonically. The results indicated that monotonically increasing the wave impedance of the foam core, thus reducing the wave impedance mismatch between successive foam layers, will introduce a stepwise core compression, greatly enhancing the overall blast resistance of sandwich composites. Although the behavior of polyurea has been studied [7-10], there have been no results past or present regarding the dynamic behavior of a functionally graded core with a polyurea interlayer. Tekalur et al. [11] experimentally studied the blast resistance and response of polyurea based layered composite materials subjected to blast

2 loading. Results indicated that sandwich materials prepared by sandwiching the polyurea between two composite skins had the best blast resistance compared to the EVE composite and polyurea layered plates. Dvorak et al. [12] experimentally and numerically investigated the blast resistance of sandwich plates with a polyurea interlayer under blast loading. Their results suggest that separating the composite face sheet from the foam core by a thin interlayer of polyurea can be very beneficial in comparison to the conventional sandwich plate design. The present study focuses on the blast response of sandwich composites with a functionally graded core and a polyurea (PU) interlayer. Two different core layer configurations were investigated, with the only difference arising in the location of the polyurea (PU) interlayer. The results will help to better understand the overall blast performance of sandwich composites with a functionally graded core system and PU interlayer under shock wave loading and provide a guideline for an optimal core design. The quasi-static and dynamic constitutive behaviors of the foam core materials, as well as the polyurea, were first studied using a modified SHPB device with a hollow transmitted bar. The sandwich composites were then fabricated and subjected to shock wave loading generated by a shock tube. The sandwich composites have identical core thickness, overall specimen geometry and areal densities, but different locations of the polyurea interlayer. The shock pressure profiles, real time deflection images, and post mortem images were carefully analyzed to reveal the mechanisms of dynamic failure of these sandwich composites. Digital Image Correlation (DIC) analysis was implemented to investigate the real time deflection, in-plane-strain, and particle velocity. The energy redistribution behavior during the blast loading process was carefully analyzed to better understand the energy absorption and reflection properties of the sandwich structures. 2. MATERIAL AND SPECIMEN 2.1 SKIN AND CORE MATERIAL The skin materials utilized in this study are E-Glass Vinyl Ester (EVE) composites comprised of.610 kg/m 2 (18 oz /yd 2 ) E-glass fibers and a vinyl-ester matrix. The plain weave woven roving E-glass fibers of the skin material consisted of 8 layers and were placed in a quasi-isotropic layout [0/45/90/-45] s. The core materials used in the present study are Corecell TM A series styrene foams manufactured by Gurit SP Technologies and a polyurea elastomer manufactured by Specialty Products Incorporated (SPI). The three types of Corecell TM A foam were A300, A500, and A800 with density 58.5, 92 and 150 kg/ m 3 respectively. The cell structures for the three foams are very similar and the only difference appears in the cell wall thickness and node sizes, which accounts for the different densities of the foams. The PU elastomer was Dragonshield-HT with an elongation percentage of 620% and density of 1000 kg/ m SANDWICH PANELS WITH STEPWISE GRADED CORE The sandwich panels were produced by VARTM-fabricated process. The panels were 102 mm (4 in) wide, 254 mm (10 in) long with 5 mm (.2 in) front and back skins. The core consisted of three layers of foam and a PU interlayer. The first two layers of the foam core were 12.7 mm (.5 in), while the third layer was 6.35 mm (.25 in) and the PU layer was 6.35 mm (.25 in) respectively. Fig. 1 shows the two core layer configurations and the shock wave loading direction. Configuration 1 consisted of a core gradation of PU/A300/A500/ A800 and configuration 2 consisted of a core gradation of A300/A500/A800/PU. Shock Wave Shock Wave (a) Configuration1 (b) Configuration2 Fig. 1 Specimen Configurations and loading direction

3 3. EXPERIMENT SETUP AND PROCEDURE 3.1 MODIFIED SPLIT HOPKINSON PRESS BARS WITH HOLLOW TRANSMITTER BAR Due to the low wave impedance of Corecell TM foam materials, core material tests were performed by a modified SHPB device with a hollow transmission bar. It has a mm (12 in)-long striker, 1600 mm (63 in)-long incident bar and 1447 mm (57 in)-long transmitter bar. All of the bars are made of a 6061 aluminum alloy. The nominal outer diameters of the solid incident bar and hollow transmission bar are mm (0.75 in). The hollow transmission bar has a mm (0.65 in) inner diameter. At the head and at the end of the hollow transmission bar, end caps made of the same material as the bar were press fitted into the hollow tube. By applying pulse shapers, the effect of the end caps on the stress waves can be minimized. The details of the analysis and derivation of equations can be found in ref [13]. The cylinderical specimens with a dimension Φ 10.2 mm (0.4 in) X 3.8 mm (0.15 in) were used for test. 3.2 SHOCK TUBE Fig. 2 shows the shock tube apparatus which was utilized to obtain a controlled blast loading, along with the muzzle detail and specimen. When the diaphragms located between the high pressure and low pressure sections rupture, this rapid release of gas creates a shock wave that travels down the tube and imparts dynamic loading on the specimen located in front of the muzzle. The final muzzle diameter is 76.2 mm (3 in). Two pressure transducers (PCB102A) are mounted at the end of the muzzle section 160 mm apart. The support fixtures ensure simply supported boundary conditions with a m (6 in) span. In the present study, a simply stacked diaphragm of 5 plies of 10 mil mylar sheets with a total thickness of 1.27 mm was utilized to generate an impulse loading on the specimen with an incident peak pressure of approximately 1 MPa and a wave speed of approximately 1050 m/s. For each configuration, at least three samples were tested. A high-speed side-view camera system along with a high-speed back-view Digital Image Correlation (DIC) system was utilized to capture the real time deformation process as well as mechanisms of failure. Both camera systems had an interframe time of 50 µs and the high speed photography system can be seen in Fig DIGITAL IMAGE CORRELATION (DIC) Digital Image Correlation (DIC) was utilized to obtain the real time response of the sandwich composites. A speckle pattern was placed on the back face sheet of the specimens. Two high speed digital cameras, Photron SA1, were placed behind the shock tube to capture the real time deformation and displacement of the sandwich composite, along with the speckle pattern. During the blast loading event, as the specimen bends, the cameras track the individual speckles on the back face sheet. Once the event is over, a graphical user interface was utilized to correlate the images from the two cameras and generate real time strains (in plane and out of plane), deflection and particle velocity. Shock tube Muzzle Detail and Specimen Fig. 2 Shock tube apparatus Fig. 3 High Speed Photography System

4 4. EXPERIMENTAL RESULTS AND DISCUSSION 4.1 DYNAMIC CONSTITUTIVE BEHAVIOR OF CORE MATERIALS Table1. Yield strength of core materials Core Layer A300 A500 A800 PU Quasi-Static Yield Stresses (MPa) High Strain- Rate Yield Stresses (MPa) Fig. 4 Quasi-static and high strain-rate behaviors of different types of Corecell TM A Foams and Dragonshield HT Polyurea Fig. 4 shows the quasi-static and high strain-rate behavior of the different types of Corecell TM A foams and Dragonshield-HT polyurea. The quasi-static and dynamic stress-strain responses have an obvious trend for the different types of foams. Lower density foam has a lower strength and stiffness, as well as a larger strain range for the plateau stress. The high strain-rate yield stresses and plateau stresses are much higher than the quasistatic ones for the same type of foams. The dynamic strength of A500 and A800 increases approximately 100% in comparison to their quasi-static strength, while A300 increases approximately 50%. Also it can be observed that the high strain-rate yield stress of Dragonshield-HT polyurea is much higher than its quasi-static yield stress. The dynamic strength increases 200% in comparison to its quasi-static strength. The improvement of the mechanical behavior from quasi-static to high strain-rates for all core materials used in the present study signifies their ability to absorb more energy under high strain-rate dynamic loading. Table 1 shows the quasi-static and high strain-rate yield stresses respectively. 4.2 RESPONSE OF SANDWICH COMPOSITES WITH GRADED CORES REAL TIME DEFORMATION The real time side view deformation image series of configuration 1 (PU/A300/A500/A800) and configuration 2 (A300/A500/ A800/PU) under shock wave loading are shown in fig. 5 respectively. The shock wave propagates from the right side of the image to the left side and some detailed deformation mechanisms are pointed out in the figures. It should be noted that the time frames used to represent the blast loading event are chosen in a manner so they can be correlated to the time in which these damage mechanisms were first observed. By using the time frame in which the deformation mechanism was first observed it allows for a better comparison between the two configurations in regards to their performance under blast loading. For configuration 1, the first core layer subjected to the shock wave loading is the polyurea (PU) interlayer. The core layer arrangement consists of a PU interlayer followed by the graded foam core. It can be observed that at t = 150 μs indentation failure has initiated. This is followed by delamination of the PU layer from the foam core at the bottom of the composite at t = 400 μs. At t = 550 μs delamination can be observed again at the top between the PU layer and foam core as well as core cracking. Also minimal core compression in the first foam core layer (A300) can be observed at this time. By t = 1150 μs, large core compression in the A300 foam is evident. Along with this compression, heavy core cracking propagating from the back face sheet towards the front face sheet as well as heavy interface delamination is visible. By t = 1800 μs heavy core cracking and interface delamination, are visible, along with compression in the core (A300 only).

5 In configuration 2, the first core layer that is subjected to the shock wave loading is the A300 foam layer. The core layer arrangement consists of the graded foam core followed by the PU interlayer. Indentation failure in configuration 2 is evident at t = 150 μs. Indentation failure is then followed by a stepwise compression of the core. By t = 650 μs the first layer of foam (A300) has compressed to a maximum, reaching its densification level, and the shock wave has now propagated into the second foam core layer (A500), initiating compression of this core layer. Also at this time, a core crack has initiated at the bottom of the specimen where the support is located. Skin delamination is evident between the front skin and the foam core, and is located at the bottom of the specimen. At t = 1150 μs minimal delamination is visible at the top of the composite between the first and second (A300 and A500) foam core layer. Also at this time the compression in the second foam core layer has increased. By t = 1800 μs heavy core compression can be observed (A300 and A500), along with very minimal core cracking and delamination. Both configurations exhibited a double-winged deformation shape which means both configurations were under shear loading. Unlike configuration 1 where the progression of damage was core cracking followed by interface delamination, the progression of damage in configuration 2 was core compression followed by core cracking. The difference between the two configurations and damage progression arises in the location of the PU interlayer. For configuration 2 the PU interlayer is located after the foam core, and thus the entire core, foam and PU respectively, is monotonically graded based on increasing wave impedance and therefore a stepwise core compression is visible. With the PU interlayer located in the front of the foam core (configuration 1), the core is non-monotonically graded, and thus the stepwise compression is not observed, instead heavy core cracking is evident. Configuration 2 (A300/A500/A800/PU) Configuration 1 (PU/A300/A500/A800) Fig. 5 Real Time side-view deformation of sandwich composites under shock wave loading The mid-point deflections of the front face (front skin), interface 1 (between first and second core layer), interface 2 (between second and third core layer), interface 3 (between third and fourth core layer), and the back face (back skin) for both configurations, directly measured from the real time side view images, are shown in fig. 6 respectively. It can be observed in fig. 6 that for both configurations the first core layer (A300) compresses approximately 75% of its original thickness (~10 mm of compression). For configuration 1, A300 is located between interface 1 and interface 2, while in configuration 2 A300 is located between the front face and interface 1. The major difference in deflection for the two configurations can be observed in the A500 layer. For configuration 1 (between interface 2 and interface 3), it follows the same trend as interface 2, interface 3, and the back face which means there was minimal compression in the A500 core layer. Unlike configuration 1, the A500

6 layer in configuration 2 (between interface 2 and interface 3) compresses approximately 25% of its original thickness (~3.5 mm of compression). With its ability to compress in a stepwise manner, configuration 2 has the ability to weaken the shock wave by the time it has reaches the back face of the specimen and thus reduces back face deflections. This phenomenon is evident in fig. 7, which shows a comparison between the back face deflections for both configurations. It is clear from the figure that configuration 2 exhibits 33% less back face deflection than configuration 1. (a) Configuration 1 (PU/A300/A500/A800) (b) Configuration 2 (A300/A500/A800/PU) Fig. 6 Mid-point Deflection of Both Configurations (A Typical Response) Fig. 7 Front Face Deflection of Both Configurations (Averaged) DIGITAL IMAGE CORRELATION (DIC) The real time response of the sandwich composites was generated using Digital Image Correlation and the results are shown in fig 8 - fig. 10. Utilizing the DIC software, full-field analysis as well as point inspection was implemented for both configuration 1 (PU/A300/A500/A800) and configuration 2 (A300/A500/A800/PU). Using the full-field analysis, data was generated across the entire area of the back face of the composite, while for point inspection, a single point was chosen in the center of the back face and the data was extrapolated. Fig. 8 and Fig. 9 show the full-field results for the out-of-plane deflection and in-plane-strain for both configurations. It can be seen in Fig. 8 that by t = 1800, the central region of the back face sheet of configuration 1 has deflected to approximately 32 mm, while the central region of the back face sheet of configuration 2 has deflected to approximately 22 mm. Therefore it is evident that the back face sheet of configuration 2 deflected approximately 33 % less than configuration 1. The in-plane-strain along the back face sheet of both configurations is shown in Fig. 9.It can be seen from the figure that the central region of the back face for configuration 1 exhibits an in-plane-strain of approximately.026, while the central region of configuration 2 exhibits an in-plane-strain of approximately Thus, it can be concluded that configuration 2 exhibits approximately 30% less in-plane-strain than configuration 1.

7 Configuration 2 A300/A500/A800/PU Configuration 1 PU/A300/A500/A800 Fig. 8 Full Field out-of-plane Deflection of Both Configurations Configuration 2 A300/A500/A800/PU Configuration 1 PU/A300/A500/A800 Fig. 9 Full Field in-plane-strain (ε yy ) of Both Configurations Fig. 10 shows back face out-of-plane velocity for both configurations utilizing the inspection of a single point in the center of the sandwich composite. It can be seen in the figure that the central point of configuration 1 reaches a maximum out-of-plane velocity of approximately 30,000 mm/s (30 m/s), while the central point of configuration 2 reaches a maximum out-of-plane velocity of approximately 25,000 mm/s (25 m/s). Therefore it can be concluded

8 that the central point of the back face of configuration 2 reaches a maximum out-of-plane velocity approximately 15% less than that of configuration POST MORTEM ANALYS After the shock event occurred, the damage patterns in the sandwich composites with a functionally graded foam core and PU interlayer were visually examined and recorded using a high resolution digital camera and are shown in fig.11. For configuration 1, there were two main cracks located at the support position. Delamination is visible between the PU layer and first layer of foam core, as well as between the bottom layer of foam core and back face sheet. Also compression was only observed in the A300 core layer. Unlike configuration 1, configuration 2 showed minimal core cracking and delamination. Configuration 2 also exhibited more compression in the core, especially in the first two layers of foam (A300 and A500). This means that configuration 2 is more flexible than configuration and therefore it showed much less permanent deformation Fig. 10 Out-Of-Plane Velocity of Both Configurations Utilizing Point Inspection Configuration 2 A300/A500/A800/PU Configuration 1 PU/A300/A500/A800 (a) Front face sheet (blast side) (b) Foam and PU core (c) Back face sheet Fig. 11 Visual Examination of Sandwich Composites After Being Subjected to High Intensity Blast Load ENERGY EVALUATION The energy redistribution behavior of both configurations was thoroughly analyzed using the methods described by Wang et. al [14]. Fig. 12 shows the total energy loss of both configuration 1 (PU/A300/A500/A800) and configuration 2 (A300/A500/A800/PU) during the blast loading event. Total energy loss is characterized as the difference between the incident and remaining energies of the gas, i.e. total amount of energy consumed by the specimen. It can be observed in the figure that at t = 1800 μs configuration 1 has a total of loss of approximately 14,000 J (14 KJ), while configuration 2 has a total energy loss of approximately 11,000 J (11 KJ). This indicates that configuration 1 has the ability to mitigate more energy, but results in heavier core damage as Fig. 12 Total Energy Loss in Both Configurations during Blast Loading Event

9 observed in Fig. 11 in section In regards to configuration 2, this arrangement allows for more energy to be reflected away from the specimen and thus less energy is transmitted into the specimen. This results in minimal core damage and better structural integrity. 6. Summary The following is the summary of the investigation: (1) The dynamic stress-strain response is significantly higher than the quasi-static response for every type of Corecell TM A foam studied. Both quasi-static and dynamic constitutive behaviors of Corecell TM A series foams (A300, A500, and A800) and polyurea interlayer show an increasing trend. The increase in the yield strength from quasi-static response to dynamic response, along with the longer stress plateau, indicates that these core materials show great potential in absorbing large amounts of energy. (2) Sandwich composites with different core arrangements, configuration 1 (PU/A300/A500/A800) and configuration 2 (A300/A500/A800/PU), were subjected to shock wave loading. The overall performance of configuration 2 is better than that of configuration 1. With the application of polyurea behind the foam core and in front of the back face sheet this core layer arrangement allows for stepwise core compression. Much larger compression is visible in the A300 and A500 core layers in this configuration than visible in configuration 1. This compression reduces the shock wave by the time it reaches the back face sheet and thus the overall deflection, in-plane strain, and velocity are reduced. (3) The methods used to evaluate the energy as described in ref [14] were implemented and the results analyzed. It was observed that the total energy loss in configuration 1 was higher than that of configuration 2. This means that configuration 1 has the ability to mitigate more energy, but this results in heavier core damage. Configuration 2 on the other hand allows for more energy to be reflected away from the specimen, and thus less energy is transmitted. This results in minimal core damage and better structural integrity. Acknowledgement The authors kindly acknowledge the financial support provided by Dr. Yapa D. S. Rajapakse, under Office of Naval Research (ONR) Grant No. N The authors acknowledge the support provided by the Department of Homeland Security (DHS) under Cooperative Agreement No ST-061-ED0002. Authors thank Gurit SP Technology and Specialty Products Incorporated (SPI) for providing the material as well as Dr. Stephen Nolet and TPI Composites for providing the facility for creating the composites used in this study. References [1] Xue, Z. and Hutchinson, J.W., Preliminary assessment of sandwich plates subject to blast loads. International Journal of Mechanical Sciences, 45, , [2] Fleck, N.A., Deshpande, V.S., The resistance of clamped sandwich beams to shock loading. Journal of Applied Mechanics, 71, , [3] Dharmasena, K.P., Wadley, H.N.G., Xue, Z. and Hutchinson, J.W., Mechanical response of metallic honeycomb sandwich panel structures to high-intensity dynamic loading. International Journal of Impact Engineering, 35 (9), , [4] Apetre, N.A., Sankar, B.V. and Ambur, D.R., Low-velocity impact response of sandwich beams with functionally graded core. International Journal of Solids and Structures, 43(9), , [5] Li, Y., Ramesh, K.T. and Chin, E.S.C., Dynamic characterization of layered and graded structures under impulsive loading. International Journal of Solids and Structures, 38(34-35), , [6] Wang, E. Gardner, N. and Shukla, A., The blast resistance of sandwich composites with stepwise graded cores. International Journal of Solids and Structures, 46, , [7] Yi, J., Boyce, M.C., Lee, G.F., and Balizer, E. Large deformation rate-dependent stress-strain behavior of polyurea and polyurethanes. Polymer, 47(1), , 2005.

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