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Local displacement of core in two-layer sandwich composite structures subjected to low velocity impact
Composite Structures 71 (2005) 53–60 www.elsevier.com/locate/compstruct
Local displacement of core in two-layer sandwich composite structures subjected to low velocity impact Dazhi Jiang, Dongwei Shu
*
School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Available online 5 November 2004
Abstract Traditional single layer sandwich structures consist of two face sheets and a core. With an additional sheet, called internal sheet in this paper, inserted into the core, a two-layer sandwich panel is then formed. Two-layer panels with polymer matrix composite laminated face sheets and a thin internal sheet subjected to low velocity impact are studied in this paper. The commercial LSDYNA3D software is used to model and analyze the dynamic problem, and attention is focused on the local displacement of a honeycomb core under the point of impact in the conditions of various locations of the internal sheet and of different levels of impact energy. Simulated results reveal that the local displacement of the core along the direction of the impact has been decreased significantly by introducing the internal sheet into a traditional single sandwich structure and by reducing the space between the internal sheet and the impacted face sheet. This result suggests a new sandwich structure to reduce the local crash/buckling of the core under the point of the impact. 2004 Elsevier Ltd. All rights reserved. Keywords: Sandwich structures; Finite element analysis; Composite laminates; Low velocity impact; Local crash
1. Introduction Sandwich structures with polymer matrix composite laminated face sheets and lightweight core material are being utilized increasingly as primary load-carrying components in aircraft and aerospace structures. Serving in this capacity, these structures are subjected to impacts such as tool drops, hail, bird strikes, and runway debris. With enough energy, the localized impact loadings can induce localized internal damage in the face sheets, in the core, or in the interface between the face sheets and the core, or even in all of them. It was well known that mechanical properties of the sandwich structures will be degraded significantly due to the internal damage. This type of impact damage has been demonstrated to substantially reduce the tensile [1], *
Corresponding author. Tel.: +65 67904440; fax: +65 67911859. E-mail address: [email protected] (D. Shu).
0263-8223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2004.09.019
compressive [2], and bending [3] strengths of the sandwich structures. Abrate [4,5] reported that based on great reviews, the critical failure modes are: (a) core buckling; (b) delamination in the impacted face sheet; (c) core cracking; (d) matrix cracking, and (e) fiber breakage in the facings. In most cases of the damage occurred, local crash of the core under the point of the impact was commonly observed experimentally [6] in such kind structures. These permanent failures will lead to not only the degradation of the mechanical properties of the sandwich structures but also other problems for these sandwich structures, i.e., accelerating the process of absorbing moisture of the core, which is a critical weakness for the core of paper honeycomb. Impact crush performance of the core in a traditional single layer sandwich structures was determined largely by honeycomb core density, cell size of the core, material properties of the core, and the panel configuration
54
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
subjected to impact loads. Nettles and Hodge [7] observed that for glass phenolic honeycomb cores at very low energy levels, core buckling took place under the impact point. For higher-energy impacts, core cracking was observed when the glass fiber reinforcement was broken. Low-velocity impact damage on sandwich plates introduced significant reductions in tensile, compressive, shear, and bending strengths [2,8–11]. Mine et al. [12] studied the modes of failure of various sandwich beams, and concluded that the best mode of failure for energy absorption was upper skin failure and subsequent stable core crushing. However, one of the primary disadvantages of sandwich structures is the dissimilarity of skin and core materials. The skin-core interface is a potential weakness for delamination under localized impact loading, and causes relatively poor impact resistance [13–16]. For purpose of increasing the delamination resistance of the skin–core interface, Vaidya et al. [17] developed an integrated hollow core sandwich composite panel, which provided enhanced low velocity impact resistance. By introducing an additional sheet, called internal sheet in this paper, into the core, a two-layer sandwich structures is then formed. This new structure is expected to be of better resistance to the local crash comparing with the single layer sandwich structure because of the increase of stiffness in direction of the impact. However, there is few information of the impact response on this structure in literatures so far. In this paper we try to investigate effects of the internal sheet involved and locations of the internal sheet in the core on local displacements of the core under the point of the impact. That displacement plays an important role on determining the local crash and buckling of the core under the point of the impact. On the other hand, shear stresses in the interface between the impacted face sheet and the core and in the core layer should be enhanced. Therefore, the two-layer sandwich structures could improve the ability to against the delamination between the face sheets and the core, and this will be discussed in our next paper.
Fig. 1. Illustration of the impact on a two-layer sandwich structure.
Table 1 Properties of the laminates and the core materials [18] Materials
Laminates
Core materials
Steel projectile
Density (kg/m3)
1100.0
2800.0
7800.0
YoungÕs modulus (GPa)
Shear modulus (GPa)
PoissonÕs ratio
Ex
50.0
0.00295
Ey Ez
50.0 9.5
0.00295 2.128
200.0
Gxy
5.43
0.738 · 10
Gyx Gxz
3.26 5.43
0.4 0.4
mxy myz mxz
0.163 0.0458 0.0263
0.99 0.000457 0.000457
3
n/a
0.3
Table 2 Flexural stiffness of the single and two-layer sandwich structures HC1 (mm)
25.0 (single)
12.0
9.0
6.0
3.0
Dxx (GPa) Dyy (GPa) Dzz (GPa)
1.036 1.036 0.330
1.037 1.037 0.330
1.058 1.058 0.333
1.123 1.123 0.343
1.231 1.231 0.359
2. Approach The analyzed system consists of two parts as shown in Fig. 1. One is the projectile, which is a hemispherical steel tip with diameter of 6.35 mm. The initial velocity of the projectile before striking is 2.3 m/s and is the same for all cases investigated in this study. A lumped mass will be associated on the nodes on top of the finite element mesh of the projectile in order to get a definite kinetic energy of the projectile. Another is the target, which is a two-layer sandwich structure consisting of five components, marked top face sheet, top core, internal sheet, bottom core and bottom face sheet, respectively, in sequence from top to bottom. The top face sheet and the bottom face sheet are a same graphite fiber
Fig. 2. Mesh of the finite element of the projectile and the target (cross section on central plane).
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
55
6E+4
1.0
HC=15.0mm Impact Energy (J)
Experi. data [18]
0.8
4E+4
Contact Force (N)
Contact Force (KN)
LS-Dyna FEM
0.6
0.4
7.01 23.25 43.56 84.17
2E+4
124.77 165.38 205.99
0
0.2
-2E+4
0.0 0.000
0.002
0.004
0.006
0.000
0.008
0.002
Time (sec)
0.004
0.006
0.008
0.010
Time (sec)
Fig. 3. Calculated contact force compared with experimental data [18].
reinforced epoxy resin matrix composite laminate (CFRP) with ply sequences of [0/45/90/ 45/0]s. The thickness of the laminate is 1.25 mm. The internal sheet is also a CFRP composites laminate but with ply sequences of [0/90/90/0]s and thickness of 1.0 mm. Dimensions of the target in x–y plane are 76.4 · 76.4 in mm. The total thickness of the core is kept 25.0 mm and the thickness of the top core is marked by HC1 as shown
Fig. 5. Contact forces of the target with HC1 = 3.0 mm under different impact energy.
in Fig. 1. Four cases, corresponding to HC1 = 3.0 mm, 6.0 mm, 9.0 mm and 12.0 mm, respectively, are investigated. Properties of the laminate and the core material are listed in Table 1 [18]. For purpose of comparison, a single layer sandwich structure with a uniform core with height of HC = 25.0 mm is also involved. Normalized flexural stiffness D of the single layer sandwich
2E+4
5E+4
Impact Energy: 43.56J 1.5E+4
HC=25.0mm
4E+4
Peak Contact Force (N)
Contact Force (N)
HC1=12.0mm HC1=9.0mm 1E+4
HC1=6.0mm HC1=3.0mm
5E+3
0
3E+4
2E+4
1E+4
-5E+3
0 0.000
0.002
0.004
0.006
0.008
0.010
Time (sec)
Fig. 4. Contact forces of the five configuration targets under impact energy of 43.56 J.
0.0
50.0
100.0
150.0
200.0
250.0
Impact Energy (J)
Fig. 6. Peak contact forces of two-layer sandwich structure with HC1 = 3.0 mm.
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D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
structure and the two-layer sandwich structures with various values of HC1 is listed in Table 2. In physical model, the projectile is treated as a rigid body because of the natural of the low velocity impact involved in this study. The face sheets, internal sheet and the core are considered orthotropic elastic bodies during the entire process of the impact, no damage and failure are involved at this moment for the purpose of estimation of the maximum displacement in the target. For the core of a honeycomb, an equivalent orthotropic elastic solid of it is considered for simplifying the problem of meshing in finite element analysis. The target is clamped along its four edges. A surface-to-surface contact condition for the projectile and the target is adopt in the analysis, which means that no perforation or penetration is formed during the process of the impact.
Fig. 2 shows a mesh net for the projectile and the target. It is formed by the pre-process component with LS-DYNA3D software. The local displacement of the core during the impact is characterized by the displacement of the node under the point of impact, which is marked as Node: B as shown in Fig. 2. The displacement of the Node: B represents the deformation of the element of the core under the point of the impact during impact. And the crash probably happens in the location of the element. During the following investigation, an attention is focused on the displacement of the node. Meanwhile, deflection of the target is characterized by the displacement of the Node: C located on the central point of the bottom surface of the bottom face sheet, as shown in Fig. 2.
0.8
0.0
Displacement (mm)
Displacement (mm)
0.4
-0.2
Node: B (E=7.01J) HC=25.0mm
-0.4
0.0
Node: B (E=23.25J)
-0.4
HC=25.0mm
HC1=12.0mm
HC1=12.0mm
HC1=9.0mm
-0.8
HC1=9.0mm
HC1=6.0mm
HC1=6.0mm
HC1=3.0mm
HC1=3.0mm
-1.2
-0.6 0.000
0.001
0.002
(a)
0.003
0.004
0.000
0.005
0.004
(b)
Time (sec)
0.008
0.012
Time (sec) 1.0
1.0
Displacement (mm)
Displacement (mm)
0.5
0.0
Node: B (E=43.56J)
-0.5
HC=25.0mm
-1.0
Node: B (E=84.17J) HC=25.0mm
-1.0
HC1=12.0mm
HC1=12.0mm
HC1=9.0mm
HC1=9.0mm
HC1=6.0mm
HC1=6.0mm
HC1=3.0mm
HC1=3.0mm
-1.5
-2.0 0.000
(c)
0.0
0.004
0.008
Time (sec)
0.012
0.000
(d)
0.004
0.008
0.012
Time (sec)
Fig. 7. Displacement history at Node: B in sandwich structures under different impact energy: (a) E = 7.01 J, (b) E = 23.25 J, (c) E = 43.56 J, (d) E = 84.17 J, (e) E = 124.77 J, (f) E = 165.38 J, (g) E = 205.99 J.
2.0
2.0
1.0
1.0
Displacement (mm)
Displacement (mm)
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
0.0
Node: B (E=124.77J)
-1.0
HC=25.0mm
57
0.0
Node: B (E=165.38J)
-1.0
HC=25.0mm HC1=12.0mm
HC1=12.0mm
-2.0
-2.0
HC1=9.0mm
HC1=9.0mm HC1=6.0mm
HC1=6.0mm
HC1=3.0mm
HC1=3.0mm
-3.0
-3.0 0.000
(e)
0.004
0.008
0.000
0.012
0.004
(f)
Time (sec)
0.008
0.012
Time (sec)
2.0
Displacement (mm)
1.0
0.0
Node: B (E=205.99J)
-1.0
HC=25.0mm HC1=12.0mm HC1=9.0mm
-2.0
HC1=6.0mm HC1=3.0mm
-3.0 0.000
0.004
(g)
0.008
0.012
Time (sec)
Fig. 7 (continued) -0.5
Node: B
3. Results and discussion
Contact force between the projectile and the target is an important parameter, which can be used to estimate the absorbing energy of the target during impact. To validate the presented finite element model for the sandwich structures under transverse impact, a single-layer sandwich structure with a core with thickness of 15 mm is first simulated, and the calculated contact force is compared with the experimental data obtained by Chee [18] as shown in Fig. 3. CheeÕs experiment was conducted by using DYNAUP 8250 impact testing machine. The specimen was clamped around its edges. The projectile is a 6.35 mm diameter steel rod with
Peak Displacement (mm)
3.1. Contact forces
HC=25.0mm
-1.0
HC1=12.0mm HC1=9.0mm HC1=6.0mm
-1.5
HC1=3.0mm
-2.0
-2.5
-3.0 0.0
50.0
100.0
150.0
200.0
250.0
Impact Energy (J)
Fig. 8. Peak displacement at Node: B in sandwich structures under different impact energy.
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D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
hemispherical tip of diameter 6.35 mm. The projectile has weight of 2.65 kg and an initial velocity of 1.0 m/s before striking on the specimen. The calculated results come out in the same conditions with that of the experiment. Compared with the experimental data the calculated contact force shows a little bit higher peak value and short duration. That is probably due to the slight local crash of the core during the impact. For the two-layer sandwich structures, the contact forces for five configuration targets under same impact energy are shown in Fig. 4. The simulated results show that there is no significant difference of the contact forces among the five types of sandwich structures. That means that the total absorbing energy of the individual configuration of the sandwich structures is almost the same under same level of impact energy. This is because that the flexural stiffness Dzz for the targets is almost the
same, as listed in Table 2. The results keep the same for other impact energy levels. However, for a particular configuration of the target, i.e., the configuration corresponding to HC1 = 3.0 mm, the absorbed energy will increase with the impact energy, as shown in Fig. 5. Peak contact forces of the configuration varying with the impact energy are shown in Fig. 6. The results shown in Fig. 6 reveal that the ability of absorbing energy of the target at this point is decreasing with the impact energy. The absorbed energy at this moment is actually the elastic strain energy of the target since no damage and any other kinds of failure, crash or buckling in local and whole structure are involved in current investigation. The total elastic strain energy stored in a particular target is limited this is why we get results shown in Fig. 6. That means that the additional impact energy will be absorbed by the damage, the crash and the buckling of 0.6
0.00
Node: C (E=23.25J)
Displacement (mm)
Displacement (mm)
HC=25.0mm
0.4
-0.05
-0.10
Node: C (E=7.01J)
-0.15
HC=25.0mm
HC1=12.0mm HC1=9.0mm HC1=6.0mm
0.2
HC1=3.0mm
0.0
HC1=12.0mm HC1=9.0mm
-0.20
-0.2
HC1=6.0mm HC1=3.0mm
-0.25
-0.4 0.000
0.001
0.002
(a)
0.003
0.004
0.005
0.000
0.008
0.012
Time (sec) 1.5
0.8
Node: C (E=43.56J)
Node: C (E=84.17J)
HC=25.0mm
0.4
HC=25.0mm
1.0
HC1=12.0mm
Displacement (mm)
Displacement (mm)
0.004
(b)
Time (sec)
HC1=9.0mm HC1=6.0mm HC1=3.0mm
0.0
HC1=12.0mm HC1=9.0mm HC1=6.0mm
0.5
HC1=3.0mm
0.0
-0.4 -0.5
-0.8
-1.0 0.000
(c)
0.004
0.008
Time (sec)
0.012
0.000
(d)
0.004
0.008
0.012
Time (sec)
Fig. 9. Deflection of sandwich structures under different impact energy: (a) E = 7.01 J, (b) E = 23.25 J, (c) E = 43.56 J, (d) E = 84.17 J, (e) E = 124.77 J, (f) E = 165.38 J, (g) E = 205.99 J.
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60 2.0
2.0
Node: C (E=165.38J)
Node: C (E=124.77J)
HC=25.0mm
HC=25.0mm
HC1=12.0mm
1.0
HC1=12.0mm
Displacement (mm)
Displacement (mm)
1.0
HC1=9.0mm HC1=6.0mm HC1=3.0mm
0.0
HC1=9.0mm HC1=6.0mm HC1=3.0mm
0.0
-1.0
-1.0
-2.0
-2.0 0.000
(e)
59
0.004
0.008
0.012
0.000
0.004
0.008
0.012
Time (sec)
(f)
Time (sec) 2.0
Displacement (mm)
1.0
0.0
Node: C (E=205.99J) HC=25.0 mm HC1=12.0 mm
-1.0
HC1=9.0 mm HC1=6.0 mm HC1=3.0 mm
-2.0 0.000
0.004
(g)
0.008
0.012
Time (sec)
Fig. 9 (continued)
the sandwich structures under high impact energy. Therefore, more or serious damage, crash and buckling of the sandwich structures are expected in the high impact energy. 3.2. Local displacement of the core As mentioned previously, the local crash and buckling of the core are controlled by the local displacement of the element of core under the point of impact. In order to evaluate the deformation of the core element displacement of the Node: B, as shown in Fig. 2, is collected and shown in Fig. 7. As shown in Fig. 7, the displacement of the node will reach its maximum (in compression) in the first pulse of the response and the peak displacement is expected to control the local crash and buckling of the deformed
element of the core. It is apparent that the peak displacement of the node has been reduced significantly by the internal sheet involved. In summary for this point, Fig. 8 shows the peak displacements of the node in sandwich structures under different impact energy. It can be seen from Fig. 8 that with increase of the impact energy, the less the values of HC1, which is the space between the impacted face sheet and the internal sheet defined early, the more efficient of the internal sheet for reduction of the displacement of the node. Compared with the results addressed in the section of contact forces, this result has a big challenge in preventing damage and crash, especially in the cases with high impact energy. As addressed in the section of contact forces, the absorbed energy by the different targets are almost the same under a definite level of impact energy. However, the local displacement is reduced for all targets with
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D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
impact energy on a local area of the impact. The simulated results also revealed that the internal sheet involved has no significant effects on the contact forces and the deflection of the sandwich structures, no matter what the locations of the internal sheet and the impact energy are.
0.0
Node: C
Peak Displacement (mm)
HC=25.0mm -0.4
HC1=12.0mm HC1=9.0mm HC1=6.0mm
References
HC1=3.0mm
-0.8
-1.2
-1.6 0.0
50.0
100.0
150.0
200.0
250.0
Impact Energy (J) Fig. 10. Peak deflection of the sandwich structures under different impact energy.
the internal sheet. The mechanism of this effect is believed to be that the impact energy has been spread to whole structure element under the internal sheet by the internal sheet, instead of concentrating at a local area under the impacted point. 3.3. Deflection of sandwich structures The deflection of a sandwich structure is characterized by the displacement of a node located at the central point of the bottom of the bottom face sheet. That node is marked as Node: C as shown in Fig. 2. The deflections of sandwich structures under different impact energy are shown in Fig. 9, and the peak deflections are summarized and shown in Fig. 10. From these figures, a conclusion can be drawn that the internal sheet involved has no significant effects on the deflection of the sandwich structures, no matter what the locations of the internal sheet and the impact energy are. This result can be explained well by the point of the flexural stiffness of the targets.
4. Conclusions Two-layer sandwich panels promise to reduce the local displacement of the core significantly. Therefore, the local crash of the core under the point of the impact is expected to be reduced, especially in the cases with high impact energy. The internal sheet involved in the twolayer sandwich panels has effort to spread the impact energy to the whole panels, instead of concentrated the
[1] Rhodes MD. Impact fracture of composite sandwich structures. In: AIAA/ASME/SAE 16th structures, structural dynamics, and materials conference. New York: AIAA; 1975. p. 75–748. [2] Akay M, Hanna R. A comparison of honeycomb-core and foamcore carbon-fiber/epoxy sandwich panels. Composites 1990;21:325–31. [3] Caldwell MS, Borris PW, Falabella R. Impact damage tolerance testing of bonded sandwich panels. In: Michelove LD, editor. 22nd international SAMPE conference. Covina, CA: Society for the Advancement of Material and Process Engineering; 1990. p. 509–20. [4] Abrate S. Localized impact on sandwich structures with laminated facings. Appl Mech Rev 1997;50:69–82. [5] Abrate S. Impact on composite structures. Cambridge: Cambridge University Press; 1998. [6] Cantwell WJ, Hazizan MA. The low velocity impact response of an aluminum honeycomb sandwich structure. Composites: Part B 2003;34:679–87. [7] Nettles AT, Hodge AJ. Impact testing of glass/phenolic honeycomb panels with graphite/epoxy face sheets. In: Proceedings of 35th international SAMPE symposium and exhibition, Anaheim, CA, April 2-5, 1990. p. 1430–40. [8] Cantwell WJ, Dirat C, Davies P. A comparative study of the mechanical properties of sandwich materials for nautical construction. SAMPE J 1994;30(4):45–51. [9] Ishai O, Hiel CC. Damage tolerance of composite sandwich panels with interleaved form core. J Compos Technol Res 1992;14(3):155–68. [10] Weeks CA, Sun CT. Multi-core composite laminates. J Adv Mater 1994;25(3):28–37. [11] Chen CH, Chen MY, Chen JP. Residual shear strength and compressive strength of composite sandwich structure after low velocity impact. In: International SAMPE symposium and exhibition, vol. 36, Part 1. 1991. p. 932–43. [12] Mines RAW, Worral CM, Gibson AG. The static and impact behavior of polymer composite sandwich beams. Composites 1994;25(2):95–110. [13] Herup EJ, Palazotto AN. Low-velocity impact damage initiation in graphite/epoxy/nomex honeycomb-sandwich plates. Compos Sci Technol 1997;57:1581–98. [14] Anderson T, Madenci E. Experimental investigation of low velocity impact characteristic of sandwich composites. Compos Struct 2000;50:239–47. [15] Hazizan MA, Cantwell WJ. The low velocity impact response of foam-based sandwich structures. Composites: Part B 2002;33:193–204. [16] Palazotto AN, Herup EJ, Gummadi LNB. Finite element analysis of low-velocity impact on composite sandwich plates. Compos Struct 2000;49:209–27. [17] Vaidya UK, Hosur MV, Earl D, Jeelani S. Impact response of integrated hollow core sandwich composite panels. Composites: Part A 2000;31:761–72. [18] Chee PY. Energy absorption characteristics of sandwich panel subjected to impact. Master thesis. Nanyang Technological University. 2004.
Local displacement of core in two-layer sandwich composite structures subjected to low velocity impact Dazhi Jiang, Dongwei Shu
*
School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Available online 5 November 2004
Abstract Traditional single layer sandwich structures consist of two face sheets and a core. With an additional sheet, called internal sheet in this paper, inserted into the core, a two-layer sandwich panel is then formed. Two-layer panels with polymer matrix composite laminated face sheets and a thin internal sheet subjected to low velocity impact are studied in this paper. The commercial LSDYNA3D software is used to model and analyze the dynamic problem, and attention is focused on the local displacement of a honeycomb core under the point of impact in the conditions of various locations of the internal sheet and of different levels of impact energy. Simulated results reveal that the local displacement of the core along the direction of the impact has been decreased significantly by introducing the internal sheet into a traditional single sandwich structure and by reducing the space between the internal sheet and the impacted face sheet. This result suggests a new sandwich structure to reduce the local crash/buckling of the core under the point of the impact. 2004 Elsevier Ltd. All rights reserved. Keywords: Sandwich structures; Finite element analysis; Composite laminates; Low velocity impact; Local crash
1. Introduction Sandwich structures with polymer matrix composite laminated face sheets and lightweight core material are being utilized increasingly as primary load-carrying components in aircraft and aerospace structures. Serving in this capacity, these structures are subjected to impacts such as tool drops, hail, bird strikes, and runway debris. With enough energy, the localized impact loadings can induce localized internal damage in the face sheets, in the core, or in the interface between the face sheets and the core, or even in all of them. It was well known that mechanical properties of the sandwich structures will be degraded significantly due to the internal damage. This type of impact damage has been demonstrated to substantially reduce the tensile [1], *
Corresponding author. Tel.: +65 67904440; fax: +65 67911859. E-mail address: [email protected] (D. Shu).
0263-8223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2004.09.019
compressive [2], and bending [3] strengths of the sandwich structures. Abrate [4,5] reported that based on great reviews, the critical failure modes are: (a) core buckling; (b) delamination in the impacted face sheet; (c) core cracking; (d) matrix cracking, and (e) fiber breakage in the facings. In most cases of the damage occurred, local crash of the core under the point of the impact was commonly observed experimentally [6] in such kind structures. These permanent failures will lead to not only the degradation of the mechanical properties of the sandwich structures but also other problems for these sandwich structures, i.e., accelerating the process of absorbing moisture of the core, which is a critical weakness for the core of paper honeycomb. Impact crush performance of the core in a traditional single layer sandwich structures was determined largely by honeycomb core density, cell size of the core, material properties of the core, and the panel configuration
54
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
subjected to impact loads. Nettles and Hodge [7] observed that for glass phenolic honeycomb cores at very low energy levels, core buckling took place under the impact point. For higher-energy impacts, core cracking was observed when the glass fiber reinforcement was broken. Low-velocity impact damage on sandwich plates introduced significant reductions in tensile, compressive, shear, and bending strengths [2,8–11]. Mine et al. [12] studied the modes of failure of various sandwich beams, and concluded that the best mode of failure for energy absorption was upper skin failure and subsequent stable core crushing. However, one of the primary disadvantages of sandwich structures is the dissimilarity of skin and core materials. The skin-core interface is a potential weakness for delamination under localized impact loading, and causes relatively poor impact resistance [13–16]. For purpose of increasing the delamination resistance of the skin–core interface, Vaidya et al. [17] developed an integrated hollow core sandwich composite panel, which provided enhanced low velocity impact resistance. By introducing an additional sheet, called internal sheet in this paper, into the core, a two-layer sandwich structures is then formed. This new structure is expected to be of better resistance to the local crash comparing with the single layer sandwich structure because of the increase of stiffness in direction of the impact. However, there is few information of the impact response on this structure in literatures so far. In this paper we try to investigate effects of the internal sheet involved and locations of the internal sheet in the core on local displacements of the core under the point of the impact. That displacement plays an important role on determining the local crash and buckling of the core under the point of the impact. On the other hand, shear stresses in the interface between the impacted face sheet and the core and in the core layer should be enhanced. Therefore, the two-layer sandwich structures could improve the ability to against the delamination between the face sheets and the core, and this will be discussed in our next paper.
Fig. 1. Illustration of the impact on a two-layer sandwich structure.
Table 1 Properties of the laminates and the core materials [18] Materials
Laminates
Core materials
Steel projectile
Density (kg/m3)
1100.0
2800.0
7800.0
YoungÕs modulus (GPa)
Shear modulus (GPa)
PoissonÕs ratio
Ex
50.0
0.00295
Ey Ez
50.0 9.5
0.00295 2.128
200.0
Gxy
5.43
0.738 · 10
Gyx Gxz
3.26 5.43
0.4 0.4
mxy myz mxz
0.163 0.0458 0.0263
0.99 0.000457 0.000457
3
n/a
0.3
Table 2 Flexural stiffness of the single and two-layer sandwich structures HC1 (mm)
25.0 (single)
12.0
9.0
6.0
3.0
Dxx (GPa) Dyy (GPa) Dzz (GPa)
1.036 1.036 0.330
1.037 1.037 0.330
1.058 1.058 0.333
1.123 1.123 0.343
1.231 1.231 0.359
2. Approach The analyzed system consists of two parts as shown in Fig. 1. One is the projectile, which is a hemispherical steel tip with diameter of 6.35 mm. The initial velocity of the projectile before striking is 2.3 m/s and is the same for all cases investigated in this study. A lumped mass will be associated on the nodes on top of the finite element mesh of the projectile in order to get a definite kinetic energy of the projectile. Another is the target, which is a two-layer sandwich structure consisting of five components, marked top face sheet, top core, internal sheet, bottom core and bottom face sheet, respectively, in sequence from top to bottom. The top face sheet and the bottom face sheet are a same graphite fiber
Fig. 2. Mesh of the finite element of the projectile and the target (cross section on central plane).
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
55
6E+4
1.0
HC=15.0mm Impact Energy (J)
Experi. data [18]
0.8
4E+4
Contact Force (N)
Contact Force (KN)
LS-Dyna FEM
0.6
0.4
7.01 23.25 43.56 84.17
2E+4
124.77 165.38 205.99
0
0.2
-2E+4
0.0 0.000
0.002
0.004
0.006
0.000
0.008
0.002
Time (sec)
0.004
0.006
0.008
0.010
Time (sec)
Fig. 3. Calculated contact force compared with experimental data [18].
reinforced epoxy resin matrix composite laminate (CFRP) with ply sequences of [0/45/90/ 45/0]s. The thickness of the laminate is 1.25 mm. The internal sheet is also a CFRP composites laminate but with ply sequences of [0/90/90/0]s and thickness of 1.0 mm. Dimensions of the target in x–y plane are 76.4 · 76.4 in mm. The total thickness of the core is kept 25.0 mm and the thickness of the top core is marked by HC1 as shown
Fig. 5. Contact forces of the target with HC1 = 3.0 mm under different impact energy.
in Fig. 1. Four cases, corresponding to HC1 = 3.0 mm, 6.0 mm, 9.0 mm and 12.0 mm, respectively, are investigated. Properties of the laminate and the core material are listed in Table 1 [18]. For purpose of comparison, a single layer sandwich structure with a uniform core with height of HC = 25.0 mm is also involved. Normalized flexural stiffness D of the single layer sandwich
2E+4
5E+4
Impact Energy: 43.56J 1.5E+4
HC=25.0mm
4E+4
Peak Contact Force (N)
Contact Force (N)
HC1=12.0mm HC1=9.0mm 1E+4
HC1=6.0mm HC1=3.0mm
5E+3
0
3E+4
2E+4
1E+4
-5E+3
0 0.000
0.002
0.004
0.006
0.008
0.010
Time (sec)
Fig. 4. Contact forces of the five configuration targets under impact energy of 43.56 J.
0.0
50.0
100.0
150.0
200.0
250.0
Impact Energy (J)
Fig. 6. Peak contact forces of two-layer sandwich structure with HC1 = 3.0 mm.
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D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
structure and the two-layer sandwich structures with various values of HC1 is listed in Table 2. In physical model, the projectile is treated as a rigid body because of the natural of the low velocity impact involved in this study. The face sheets, internal sheet and the core are considered orthotropic elastic bodies during the entire process of the impact, no damage and failure are involved at this moment for the purpose of estimation of the maximum displacement in the target. For the core of a honeycomb, an equivalent orthotropic elastic solid of it is considered for simplifying the problem of meshing in finite element analysis. The target is clamped along its four edges. A surface-to-surface contact condition for the projectile and the target is adopt in the analysis, which means that no perforation or penetration is formed during the process of the impact.
Fig. 2 shows a mesh net for the projectile and the target. It is formed by the pre-process component with LS-DYNA3D software. The local displacement of the core during the impact is characterized by the displacement of the node under the point of impact, which is marked as Node: B as shown in Fig. 2. The displacement of the Node: B represents the deformation of the element of the core under the point of the impact during impact. And the crash probably happens in the location of the element. During the following investigation, an attention is focused on the displacement of the node. Meanwhile, deflection of the target is characterized by the displacement of the Node: C located on the central point of the bottom surface of the bottom face sheet, as shown in Fig. 2.
0.8
0.0
Displacement (mm)
Displacement (mm)
0.4
-0.2
Node: B (E=7.01J) HC=25.0mm
-0.4
0.0
Node: B (E=23.25J)
-0.4
HC=25.0mm
HC1=12.0mm
HC1=12.0mm
HC1=9.0mm
-0.8
HC1=9.0mm
HC1=6.0mm
HC1=6.0mm
HC1=3.0mm
HC1=3.0mm
-1.2
-0.6 0.000
0.001
0.002
(a)
0.003
0.004
0.000
0.005
0.004
(b)
Time (sec)
0.008
0.012
Time (sec) 1.0
1.0
Displacement (mm)
Displacement (mm)
0.5
0.0
Node: B (E=43.56J)
-0.5
HC=25.0mm
-1.0
Node: B (E=84.17J) HC=25.0mm
-1.0
HC1=12.0mm
HC1=12.0mm
HC1=9.0mm
HC1=9.0mm
HC1=6.0mm
HC1=6.0mm
HC1=3.0mm
HC1=3.0mm
-1.5
-2.0 0.000
(c)
0.0
0.004
0.008
Time (sec)
0.012
0.000
(d)
0.004
0.008
0.012
Time (sec)
Fig. 7. Displacement history at Node: B in sandwich structures under different impact energy: (a) E = 7.01 J, (b) E = 23.25 J, (c) E = 43.56 J, (d) E = 84.17 J, (e) E = 124.77 J, (f) E = 165.38 J, (g) E = 205.99 J.
2.0
2.0
1.0
1.0
Displacement (mm)
Displacement (mm)
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
0.0
Node: B (E=124.77J)
-1.0
HC=25.0mm
57
0.0
Node: B (E=165.38J)
-1.0
HC=25.0mm HC1=12.0mm
HC1=12.0mm
-2.0
-2.0
HC1=9.0mm
HC1=9.0mm HC1=6.0mm
HC1=6.0mm
HC1=3.0mm
HC1=3.0mm
-3.0
-3.0 0.000
(e)
0.004
0.008
0.000
0.012
0.004
(f)
Time (sec)
0.008
0.012
Time (sec)
2.0
Displacement (mm)
1.0
0.0
Node: B (E=205.99J)
-1.0
HC=25.0mm HC1=12.0mm HC1=9.0mm
-2.0
HC1=6.0mm HC1=3.0mm
-3.0 0.000
0.004
(g)
0.008
0.012
Time (sec)
Fig. 7 (continued) -0.5
Node: B
3. Results and discussion
Contact force between the projectile and the target is an important parameter, which can be used to estimate the absorbing energy of the target during impact. To validate the presented finite element model for the sandwich structures under transverse impact, a single-layer sandwich structure with a core with thickness of 15 mm is first simulated, and the calculated contact force is compared with the experimental data obtained by Chee [18] as shown in Fig. 3. CheeÕs experiment was conducted by using DYNAUP 8250 impact testing machine. The specimen was clamped around its edges. The projectile is a 6.35 mm diameter steel rod with
Peak Displacement (mm)
3.1. Contact forces
HC=25.0mm
-1.0
HC1=12.0mm HC1=9.0mm HC1=6.0mm
-1.5
HC1=3.0mm
-2.0
-2.5
-3.0 0.0
50.0
100.0
150.0
200.0
250.0
Impact Energy (J)
Fig. 8. Peak displacement at Node: B in sandwich structures under different impact energy.
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D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
hemispherical tip of diameter 6.35 mm. The projectile has weight of 2.65 kg and an initial velocity of 1.0 m/s before striking on the specimen. The calculated results come out in the same conditions with that of the experiment. Compared with the experimental data the calculated contact force shows a little bit higher peak value and short duration. That is probably due to the slight local crash of the core during the impact. For the two-layer sandwich structures, the contact forces for five configuration targets under same impact energy are shown in Fig. 4. The simulated results show that there is no significant difference of the contact forces among the five types of sandwich structures. That means that the total absorbing energy of the individual configuration of the sandwich structures is almost the same under same level of impact energy. This is because that the flexural stiffness Dzz for the targets is almost the
same, as listed in Table 2. The results keep the same for other impact energy levels. However, for a particular configuration of the target, i.e., the configuration corresponding to HC1 = 3.0 mm, the absorbed energy will increase with the impact energy, as shown in Fig. 5. Peak contact forces of the configuration varying with the impact energy are shown in Fig. 6. The results shown in Fig. 6 reveal that the ability of absorbing energy of the target at this point is decreasing with the impact energy. The absorbed energy at this moment is actually the elastic strain energy of the target since no damage and any other kinds of failure, crash or buckling in local and whole structure are involved in current investigation. The total elastic strain energy stored in a particular target is limited this is why we get results shown in Fig. 6. That means that the additional impact energy will be absorbed by the damage, the crash and the buckling of 0.6
0.00
Node: C (E=23.25J)
Displacement (mm)
Displacement (mm)
HC=25.0mm
0.4
-0.05
-0.10
Node: C (E=7.01J)
-0.15
HC=25.0mm
HC1=12.0mm HC1=9.0mm HC1=6.0mm
0.2
HC1=3.0mm
0.0
HC1=12.0mm HC1=9.0mm
-0.20
-0.2
HC1=6.0mm HC1=3.0mm
-0.25
-0.4 0.000
0.001
0.002
(a)
0.003
0.004
0.005
0.000
0.008
0.012
Time (sec) 1.5
0.8
Node: C (E=43.56J)
Node: C (E=84.17J)
HC=25.0mm
0.4
HC=25.0mm
1.0
HC1=12.0mm
Displacement (mm)
Displacement (mm)
0.004
(b)
Time (sec)
HC1=9.0mm HC1=6.0mm HC1=3.0mm
0.0
HC1=12.0mm HC1=9.0mm HC1=6.0mm
0.5
HC1=3.0mm
0.0
-0.4 -0.5
-0.8
-1.0 0.000
(c)
0.004
0.008
Time (sec)
0.012
0.000
(d)
0.004
0.008
0.012
Time (sec)
Fig. 9. Deflection of sandwich structures under different impact energy: (a) E = 7.01 J, (b) E = 23.25 J, (c) E = 43.56 J, (d) E = 84.17 J, (e) E = 124.77 J, (f) E = 165.38 J, (g) E = 205.99 J.
D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60 2.0
2.0
Node: C (E=165.38J)
Node: C (E=124.77J)
HC=25.0mm
HC=25.0mm
HC1=12.0mm
1.0
HC1=12.0mm
Displacement (mm)
Displacement (mm)
1.0
HC1=9.0mm HC1=6.0mm HC1=3.0mm
0.0
HC1=9.0mm HC1=6.0mm HC1=3.0mm
0.0
-1.0
-1.0
-2.0
-2.0 0.000
(e)
59
0.004
0.008
0.012
0.000
0.004
0.008
0.012
Time (sec)
(f)
Time (sec) 2.0
Displacement (mm)
1.0
0.0
Node: C (E=205.99J) HC=25.0 mm HC1=12.0 mm
-1.0
HC1=9.0 mm HC1=6.0 mm HC1=3.0 mm
-2.0 0.000
0.004
(g)
0.008
0.012
Time (sec)
Fig. 9 (continued)
the sandwich structures under high impact energy. Therefore, more or serious damage, crash and buckling of the sandwich structures are expected in the high impact energy. 3.2. Local displacement of the core As mentioned previously, the local crash and buckling of the core are controlled by the local displacement of the element of core under the point of impact. In order to evaluate the deformation of the core element displacement of the Node: B, as shown in Fig. 2, is collected and shown in Fig. 7. As shown in Fig. 7, the displacement of the node will reach its maximum (in compression) in the first pulse of the response and the peak displacement is expected to control the local crash and buckling of the deformed
element of the core. It is apparent that the peak displacement of the node has been reduced significantly by the internal sheet involved. In summary for this point, Fig. 8 shows the peak displacements of the node in sandwich structures under different impact energy. It can be seen from Fig. 8 that with increase of the impact energy, the less the values of HC1, which is the space between the impacted face sheet and the internal sheet defined early, the more efficient of the internal sheet for reduction of the displacement of the node. Compared with the results addressed in the section of contact forces, this result has a big challenge in preventing damage and crash, especially in the cases with high impact energy. As addressed in the section of contact forces, the absorbed energy by the different targets are almost the same under a definite level of impact energy. However, the local displacement is reduced for all targets with
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D. Jiang, D. Shu / Composite Structures 71 (2005) 53–60
impact energy on a local area of the impact. The simulated results also revealed that the internal sheet involved has no significant effects on the contact forces and the deflection of the sandwich structures, no matter what the locations of the internal sheet and the impact energy are.
0.0
Node: C
Peak Displacement (mm)
HC=25.0mm -0.4
HC1=12.0mm HC1=9.0mm HC1=6.0mm
References
HC1=3.0mm
-0.8
-1.2
-1.6 0.0
50.0
100.0
150.0
200.0
250.0
Impact Energy (J) Fig. 10. Peak deflection of the sandwich structures under different impact energy.
the internal sheet. The mechanism of this effect is believed to be that the impact energy has been spread to whole structure element under the internal sheet by the internal sheet, instead of concentrating at a local area under the impacted point. 3.3. Deflection of sandwich structures The deflection of a sandwich structure is characterized by the displacement of a node located at the central point of the bottom of the bottom face sheet. That node is marked as Node: C as shown in Fig. 2. The deflections of sandwich structures under different impact energy are shown in Fig. 9, and the peak deflections are summarized and shown in Fig. 10. From these figures, a conclusion can be drawn that the internal sheet involved has no significant effects on the deflection of the sandwich structures, no matter what the locations of the internal sheet and the impact energy are. This result can be explained well by the point of the flexural stiffness of the targets.
4. Conclusions Two-layer sandwich panels promise to reduce the local displacement of the core significantly. Therefore, the local crash of the core under the point of the impact is expected to be reduced, especially in the cases with high impact energy. The internal sheet involved in the twolayer sandwich panels has effort to spread the impact energy to the whole panels, instead of concentrated the
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