- Email: [email protected]
Impact response of fiber metal laminate sandwich composite structure with polypropylene honeycomb core
Composites: Part B 43 (2012) 1433–1438
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Impact response of fiber metal laminate sandwich composite structure with polypropylene honeycomb core C.Y. Tan, Hazizan Md. Akil ⇑ School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 9 May 2011 Received in revised form 18 July 2011 Accepted 22 August 2011 Available online 28 August 2011 Keywords: A. Laminates A. Honeycomb B. Impact behavior Fiber metal laminates
a b s t r a c t Fiber metal laminates (FMLs) were used as skin on polypropylene honeycomb core to form a sandwich structure. Impact response was measured by conducting a series of low-velocity impact test. Impact force and the force time history were recorded and analyzed. It was found that the maximum impact load increased up to a threshold value at which it plateaus while the energy absorption in the structure increased with increasing impact energy. Post-impact optical image showed a change in damage area with increasing impact energy. The impact damage threshold energy for the sandwich structure was clearly shown in the range of impact energy between 7.84 J and 11.76 J where damages including delamination of the skins and global bending of the structure were observed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Sandwich structures are composed of two thin but stiff material as skins bonded to a thick but lightweight material as core. This presents a structure with properties for high bending stiffness with overall low density. Sandwich structures are usually used as an alternative material to achieve the same structural performance as conventional materials with less weight. In this study, polypropylene honeycomb core were used because of its excellent strength to weight ratio, corrosion resistant, high energy absorption and do not absorb water. Muzzy et al. [1] reported that impact response varied with different types of cores and skins used. Comparison between a few types of polypropylene honeycomb core, paper core and foam cores were done. However, results showed that the impact responses of panels with different cores were not conclusive. Paulius et al. [2] also investigated the behavior of composite facesheets and polypropylene hexagonal honeycomb core under impact loading. They found out that the honeycomb core absorbed between 50% and 95% energy of all sandwich structure while the top facesheet only contributed 7–35% and bottom facesheet the least. Although thermoplastic honeycomb core showed some excellent results on energy absorbing properties it had yet to receive wide interest on its impact response compared to metallic or aramid core materials. Its susceptibility to damage due to impact loading is still less widely reported.
⇑ Corresponding author. Tel.: +60 45996161. E-mail addresses: [email protected] (C.Y. Tan), hazizan@en g.usm.my (H.Md. Akil). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.08.036
Metal-composite systems such as fiber–metal laminates (FMLs) are based on layers of fiber-reinforced composite materials and metal had gained interest from a wide range of engineering sectors due to its superior impact and fatigue properties compared to conventional material systems [3,4]. At present, systems such as GLARE (glass fiber/aluminum) are available and widely used in aircraft body constructions. Besides GLARE, other configuration such as ARALL (aramid fiber/aluminum) and CALL (carbon fiber/aluminum) are also available. However these are epoxy-based fiber–metal laminates, which come with a number of limitations such as long processing cycles and low interlaminar fracture toughness. To overcome these problems, several FMLs based thermoplastic matrices have been developed and tested [5–7]. As a result, glass fiber reinforced polypropylene FML had shown good impact resistance to both high and low velocity impact [6]. It is known that impact response of a sandwich structure depends on both of the skin and core materials used. Various researchers have shown the influence of different skin and core materials to its impact response. Park et al. [8] had shown that the impact response was greatly influenced by core thickness and the effect of core thickness varied with the facesheet materials. Foo et al. [9] and Erickson et al. [10] also concluded that denser/ stiffer cores shows greater peak loads during an impact event. Besides materials properties, the test specimen geometry and types of support systems used during impact test also played an important part in determining the impact response of the sandwich structure. Hazizan and Cantwell [11] reported that impact forces are significantly higher in plates than in simply supported beams due to higher flexural rigidity in plates. They also found that by reducing the support span, higher impact force will be recorded
1434
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
at a given energy due to the increasing flexural stiffness of the specimen. The purpose of this research is to investigate the impact properties of combined polypropylene honeycomb core and fiber–metal laminates based on glass fiber reinforced polypropylene skins. A series of low-velocity impact test were performed using a drop type instrumented impact testing machine and resulting impact damages were inspected using optical microscope. Damage area measurements were then taken and compared under various drop heights. The impact response parameters of the sandwich structure were identified using the force and energy histories. Both FML and PP honeycomb core are ductile in nature and the sandwich structure was expected to absorb energy more readily. 2. Materials and methods 2.1. Materials The polypropylene honeycomb sheet’s (PP8T40F) cell size and density are 8 mm and 56.1 kg/m3, respectively. The sheet is supplied with both sides laminated with non-woven polyester tissue in 40 g/m2. The thickness of the core materials is 20 mm. The facesheets used consist of a fiber metal laminate sheet. Fiber metal laminates used in this study were based on an aluminum (1100) and a glass-fiber-reinforced thermoplastic prepreg (Atenply GFPP45 from Jonam Composites, UK). The properties of the materials used were shown in Tables 1–3. 2.2. FML skin and its fabrication The fiber metal laminates were produced using a frame mold by stacking the prepreg, the bonding adhesive and the aluminum plies with dimensions of 200 140 mm. Aluminum plies which needed to be bonded were sanded prior to stacking. Configuration of this FML skin was consisting of a single ply of prepreg sandwiched by two plies of aluminum sheet. The stack was loaded into a hot press at 180 °C and soaked for 10 min. It was then compressed under a pressure of 0.4 MPa for 20 min before being cooled slowly to room temperature. 2.3. Sandwich structure fabrication The FMLs were bonded to the top and bottom of the honeycomb core with similar sizes using an Araldite Rapid Resin with an Araldite Rapid Hardener. The properties of resin and hardener were shown in Table 4. The fabricated sandwich structure was placed under weights for 1 day to ensure a complete curing of the adhesive at room temperature. Fig. 1 shows the schematic diagram of sandwich structure configuration adopted in this study. 2.4. Drop weight impact test Impact tests were conducted using an instrumented drop weight impact machine. Here, a 2.1 kg carriage with a 25 mm
Table 1 Selected mechanical properties of sandwich core material: PP8T40F. Property
Value 3
Density (kg/m ) Compressive strength (MPa) Compression modulus (MPa) Bending strength (MPa) Bending modulus (MPa) Pulling shear strength (MPa)
56.1 2.05 65.4 18.5 1.02 103 0.53
Table 2 Properties of unidirectional glass fiber reinforced thermoplastic composite prepreg tape: AtenPly GPP45. Property
Value
Density (kg/m3) Fiber content (Wt%/V%) Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa)
1.65 70/45 452 21.8 323 16
Table 3 Properties of metallic skin – Aluminum 1100. Property
Value
Thickness (mm) Density (kg/m3) Elastic modulus (GPa) Tensile strength (MPa)
0.5 2.71 103 70 110
Table 4 Properties of adhesive – AralditeÒ Rapid. Property
Value 3
Density (Resin) (kg/m ) Density (Hardener) (kg/m3) Cure time (min) Cure temperature (°C) Flexural strength (MPa) Flexural modulus (MPa)
2.25 0.94 15–30 18–27 46 1654.4
diameter hemi-spherical indenter was released from heights of up to 1.0 m. Impact energy levels up to 20 J were achieved by varying the drop height. The specimens were supported on two 12 mm diameter steel cylinders positioned on movable right angle supports. The impact force history during the test was measured using a piezo-electric load cell located just above the impactor tip. The signals from the load cell were then recorded and stored by computer using data acquisition software. The schematic of the setup is shown in Fig. 2. Test for each impact energy were repeated three sets. However, only average peak load versus each impact energy was shown. The force–time histories, absorbed energy, energy– time histories and contact duration for drop-weight impacts onto composites were based on single result from the three sets of data. This is because it would be much appropriate to relate the results based on post-impact damages on the composite.
3. Results and discussion The low velocity impact performance of FML skin with polypropylene honeycomb sandwiched composite were evaluated using various impact-sensitive parameters such as impact force–time history, peak load at various impact velocity, contact duration and total absorbed energy. Post-impact damage was evaluated using optical microscope and measurements of damage area were taken and compared with absorbed energy. It should be noted here that the custom instrumented drop weight impact tester could only measure the force versus time directly, thus the impact velocity was calculated based on conservation of energy, where all the potential energy was converted into kinetic energy prior to impact on the sandwich structure. By varying the drop height, we could get a series of impact velocities and impact energy. Fig. 3 shows the load–time histories recorded by varying the drop height of the impact test. At impact energy of 3.92 J and
1435
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
Fig. 1. Schematic diagram of a sandwich structure configuration.
Motor for height control
Magnet for release of indenter and carriage PC for data analysis Piezo-electric load cell
Stainless steel indenter
Three point bend support system
Fig. 2. Schematics of the drop weight impact tester.
7.84 J, the loading and unloading was relatively smooth by reaching the peak around 4 ms as compared to impact energies at 11.76 J and above. This could be compared with a typical load–time history data shown in Fig. 4. It showed smooth loading and unloading of the load on a sample which showed none or little damage. The Fmax showed the maximum force which is the highest point of the graph and t which indicated the time when maximum force occurs. At impact energy of 11.76 J, the loading part was quite similar with the lower impact energy reaching maximum force at almost the same time around 4 ms, but during the unloading part
it showed a sudden load drop at around 5 ms. This might indicate that the specimen was reaching its threshold impact energy. However, impact energy of 15.68 J and 19.62 J both showed the time taken to reach the maximum force was much earlier at around 2.5 ms and sudden load drops seemed to be occurring earlier at around 3 ms. This indicated that the structure had suffered serious damage and according to Herup and Palazotto [12], as impact energy increased, eventually a point was reached where the load history curve started to show an oscillation which was no longer smooth, instead, a major load drop occurred and was followed by
Fig. 3. Time histories of the contact forces of drop-weight impacts onto composites.
1436
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
Fig. 4. Typical load–time histories for a drop-weight impact test.
multiple cycles of loading and partial unloading. This major load drop could be related to damages occurred on the specimen such as delamination, core crushing and also bending of the structure. Fig. 5a shows the variation of peak load with impact energy for this sandwich structure. The result showed a rise in load with
increasing impact energy. An examination on the specimens showed that at impact energy above 11.76 J, the damage started to extend to the side of the specimen with slight delamination. At impact energy of 15.68 J and 19.62 J, specimens showed severe damage across the width of the surface and suffered bending. Some previous studies [7,13] on peak load versus impact energy showed that for sandwich structures, the damage threshold energy usually occurred at the starting point where the maximum impact force started to reach plateau. In this study, we could see that at 19.62 J, the peak load reached approximately 1700 N which was quite similar at impact energy of 11.76 J and 15.68 J, compared to load at impact energy of 3.92 J which is 1200 N and 7.84 J at 1500 N, this shows that the sandwich structure had reach its impact damage threshold energy. Fig. 6a shows energy time history for this study. According to Sevkat et al. [14], there are usually three interaction modes between the composite panel and the impactor depending on the level of impact energy. First is when the energy absorbed by the composite is very little, the impactor bounces back. Second is when most of the energy is being absorbed by the composite through various modes of damage, thus no rebound occurs. Finally in the case of high energy level, perforation can be observed. Fig. 6b shows a typical energy curve for drop weight impact test where rebound of the impactor occurs. In this study, most of the
Fig. 5. (a) Maximum peak load under various impact energy. (b) Absorbed energy on composite specimen under various impact energy.
Fig. 6. (a) Energy–time histories for drop weight impact test of composite specimens at various energy. (b) A typical energy curve for drop weight impact test when rebound of the impactor occurs.
1437
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
Impact damage threshold energy
Fig. 9. Damage area measured under various impact energy. Fig. 7. Contact duration at different impact energy.
energy–time histories shown are similar to Fig. 6b, except for impact energy of 15.68 J and 19.62 J. At both of the high impact energy, the composite structure suffered bending which had caused minimum rebound energy recorded. Fig. 5b shows that energy absorption of the composite increased with increasing impact energy. The contact durations of impact test are being summarized in Fig. 7. We noticed that the contact duration increased slightly with the increase in impact energy at region I. However, the contact duration at impact energy of 15.68 J and 19.62 J (region II) increased sharply as compared to those in region I. Based on Paolo and Kedward [15] summary on contact duration plot, in the first region, contact duration is supposed to be constant due to their structures stiffness. Somehow, the contact duration plot in this study (Fig. 7.) differs. This might be due to the fact that the combination of metallic skin materials and single-ply fiber reinforced pre-preg used in this study were ductile in nature with low stiffness thus resulting in slight increase in contact duration of the indenter with the sandwich structure. The sharp increase of the
Impact Energy
3.92J
7.84J
contact duration in region II was likely due to severe damage in the sandwich structure. This had been reported [14] where the higher the damage, the longer the time impactor will be in contact with the specimen. So, in this case, the specimen which had suffered bending at both impact energy levels of 15.68 J and 19.62 J, can be the reason of longer contact duration. As shown in Fig. 8, increasing impact energy will increase the damage of the impact area. Based on measurement software provided on the optical microscope, a simple graph with impact energy versus damage area was being plotted. Based on Fig. 9, the damage area measured for the composite sandwich structure shows an overall increasing trend when given higher impact energy. However, under impact energy below approximately 8 J, it was found that the damage area did not vary much compared to higher impact energy levels. It was noted that the damage area result was somehow related to the result of force–time history during impact. An increase in damage area showed a sudden load drops under the force–time history graph, indicating that the impact threshold energy had been reached. From extrapolation lines in Fig. 9, it is estimated that the sandwich structure’s impact threshold energy might occur around 13–14 J of impact energy.
11.76J
15.68J
19.62J
Top View
No Impact
3.92 J 7.84 J Side View 11.76 J 15.68 J
19.62 J
Fig. 8. Top view damage area obtained by optical microscope and side view of damaged specimen.
1438
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
4. Conclusion Low-velocity impact response and energy absorption capacity in polypropylene honeycomb sandwich structure with simple fiber–metal laminate skin was identified. Lower impact energy damage showed only indentation around the impact center, but once the impact threshold energy was reached, it showed a sudden drop in recorded impact force and post-impact examination on the composite showing delamination of the skins and global bending of the structure. There was an increase in energy absorption of the composite specimen with an increase in impact energy. However under impact energy of 15.68 J and 19.62 J, the structure showed severe core crushing and bending globally, thus it could be concluded that the impact energy absorption capacity was set at its impact threshold energy which was under impact energy of 11.76 J with 9.12 J of energy being absorbed. Results from the damage area measurements however, showed a possibly higher than 11.76 J critical damage energy where it was estimated to fall in the range of 13–14 J.
Acknowledgements The first author acknowledges the financial assistance under the fellowship scheme of Universiti Sains Malaysia and the Postgraduate Research Grant Scheme (USM-RU-PRGS – Grant No.: 1001/PBAHAN/8043039) that has resulted in this article. References [1] Muzzy J, Pfaendtner J, Shaw B, Holty D. Thermoplastic composite sandwich panels. In: Proceedings of the automotive composites conference, SPE; 2001.
[2] Paulius G, Daiva Z, Vitalis L, Marian O. Experimental and numerical study of impact energy absorption of safety important honeycomb core sandwich structures. Mater Sci 2010;16/2:119–23. [3] Alderliesten RC. Fiber–metal laminates: an introduction. In: Volt A, Gunnick JW, editors. Kluwer Academic Publishers.; 2001. [4] Vlot AD, Kroon E, la Rocca G. Impact response of fiber metal laminates. Key Eng Mater 1998;235/76:141–3. [5] Abdullah MR, Cantwell WJ. The impact resistance of polypropylene-based fibre–metal laminates. Compos Sci Technol 2006;66:1682–93. [6] Reyes G, Cantwell WJ. The mechanical properties of fiber–metal laminates based on glass fiber reinforced polypropylene. Compos Sci Technol 2000;60:1085–94. [7] Reyes German. Mechanical behaviour of thermoplastic FML-reinforced sandwich panels using an aluminum foam core: experiments and modeling. J Sandwich Struct Mater 2010;12/1:81–96. [8] Park JH, Ha SK, kang KW, Kim CW, Kim HS. Impact damage resistance of sandwich structure subjected to low velocity impact. J Mater Pro Tech 2008;201/1–3:425–30. [9] Foo CC, Seah LK, Chai GB. Low-velocity impact failure of aluminum honeycomb sandwich panels. Compos Struct 2008;85/1:20–8. [10] Erickson MD, Kallmeyer AR, Kellogg KG. Effect of temperature on the lowvelocity impact behaviour of composite sandwich panels. J Sandwich Struct Mater 2005;7:245–64. [11] Hazizan MA, Cantwell WJ. The low velocity impact response of an aluminum honeycomb sandwich structure. Compos: Part B 2003;34:679–87. [12] Herup EJ, Palazotto AN. Low-velocity impact damage initiation in graphite/ epoxy/nomex honeycomb-sandwich plates. Compos Sci Tech 1997;57/ 12:1581–98. [13] Hazizan MA, Cantwell WJ. The low velocity impact response of foam-based sandwich structures. Compos: Part B 2002;33/3:193–204. [14] Sevkat E, Liaw B, Delale F, Raju BB. Drop-weight impact of plain-woven hybrid glass-graphite/toughened epoxy composites. Compos: Part A 2009;40/ 8:1090–110. [15] Feraboli Paolo, Kedward Keith T. A new composite structure impact performance assessment program. Compos Sci Tech 2006;66(12):1336–47.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Impact response of fiber metal laminate sandwich composite structure with polypropylene honeycomb core C.Y. Tan, Hazizan Md. Akil ⇑ School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 9 May 2011 Received in revised form 18 July 2011 Accepted 22 August 2011 Available online 28 August 2011 Keywords: A. Laminates A. Honeycomb B. Impact behavior Fiber metal laminates
a b s t r a c t Fiber metal laminates (FMLs) were used as skin on polypropylene honeycomb core to form a sandwich structure. Impact response was measured by conducting a series of low-velocity impact test. Impact force and the force time history were recorded and analyzed. It was found that the maximum impact load increased up to a threshold value at which it plateaus while the energy absorption in the structure increased with increasing impact energy. Post-impact optical image showed a change in damage area with increasing impact energy. The impact damage threshold energy for the sandwich structure was clearly shown in the range of impact energy between 7.84 J and 11.76 J where damages including delamination of the skins and global bending of the structure were observed. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Sandwich structures are composed of two thin but stiff material as skins bonded to a thick but lightweight material as core. This presents a structure with properties for high bending stiffness with overall low density. Sandwich structures are usually used as an alternative material to achieve the same structural performance as conventional materials with less weight. In this study, polypropylene honeycomb core were used because of its excellent strength to weight ratio, corrosion resistant, high energy absorption and do not absorb water. Muzzy et al. [1] reported that impact response varied with different types of cores and skins used. Comparison between a few types of polypropylene honeycomb core, paper core and foam cores were done. However, results showed that the impact responses of panels with different cores were not conclusive. Paulius et al. [2] also investigated the behavior of composite facesheets and polypropylene hexagonal honeycomb core under impact loading. They found out that the honeycomb core absorbed between 50% and 95% energy of all sandwich structure while the top facesheet only contributed 7–35% and bottom facesheet the least. Although thermoplastic honeycomb core showed some excellent results on energy absorbing properties it had yet to receive wide interest on its impact response compared to metallic or aramid core materials. Its susceptibility to damage due to impact loading is still less widely reported.
⇑ Corresponding author. Tel.: +60 45996161. E-mail addresses: [email protected] (C.Y. Tan), hazizan@en g.usm.my (H.Md. Akil). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.08.036
Metal-composite systems such as fiber–metal laminates (FMLs) are based on layers of fiber-reinforced composite materials and metal had gained interest from a wide range of engineering sectors due to its superior impact and fatigue properties compared to conventional material systems [3,4]. At present, systems such as GLARE (glass fiber/aluminum) are available and widely used in aircraft body constructions. Besides GLARE, other configuration such as ARALL (aramid fiber/aluminum) and CALL (carbon fiber/aluminum) are also available. However these are epoxy-based fiber–metal laminates, which come with a number of limitations such as long processing cycles and low interlaminar fracture toughness. To overcome these problems, several FMLs based thermoplastic matrices have been developed and tested [5–7]. As a result, glass fiber reinforced polypropylene FML had shown good impact resistance to both high and low velocity impact [6]. It is known that impact response of a sandwich structure depends on both of the skin and core materials used. Various researchers have shown the influence of different skin and core materials to its impact response. Park et al. [8] had shown that the impact response was greatly influenced by core thickness and the effect of core thickness varied with the facesheet materials. Foo et al. [9] and Erickson et al. [10] also concluded that denser/ stiffer cores shows greater peak loads during an impact event. Besides materials properties, the test specimen geometry and types of support systems used during impact test also played an important part in determining the impact response of the sandwich structure. Hazizan and Cantwell [11] reported that impact forces are significantly higher in plates than in simply supported beams due to higher flexural rigidity in plates. They also found that by reducing the support span, higher impact force will be recorded
1434
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
at a given energy due to the increasing flexural stiffness of the specimen. The purpose of this research is to investigate the impact properties of combined polypropylene honeycomb core and fiber–metal laminates based on glass fiber reinforced polypropylene skins. A series of low-velocity impact test were performed using a drop type instrumented impact testing machine and resulting impact damages were inspected using optical microscope. Damage area measurements were then taken and compared under various drop heights. The impact response parameters of the sandwich structure were identified using the force and energy histories. Both FML and PP honeycomb core are ductile in nature and the sandwich structure was expected to absorb energy more readily. 2. Materials and methods 2.1. Materials The polypropylene honeycomb sheet’s (PP8T40F) cell size and density are 8 mm and 56.1 kg/m3, respectively. The sheet is supplied with both sides laminated with non-woven polyester tissue in 40 g/m2. The thickness of the core materials is 20 mm. The facesheets used consist of a fiber metal laminate sheet. Fiber metal laminates used in this study were based on an aluminum (1100) and a glass-fiber-reinforced thermoplastic prepreg (Atenply GFPP45 from Jonam Composites, UK). The properties of the materials used were shown in Tables 1–3. 2.2. FML skin and its fabrication The fiber metal laminates were produced using a frame mold by stacking the prepreg, the bonding adhesive and the aluminum plies with dimensions of 200 140 mm. Aluminum plies which needed to be bonded were sanded prior to stacking. Configuration of this FML skin was consisting of a single ply of prepreg sandwiched by two plies of aluminum sheet. The stack was loaded into a hot press at 180 °C and soaked for 10 min. It was then compressed under a pressure of 0.4 MPa for 20 min before being cooled slowly to room temperature. 2.3. Sandwich structure fabrication The FMLs were bonded to the top and bottom of the honeycomb core with similar sizes using an Araldite Rapid Resin with an Araldite Rapid Hardener. The properties of resin and hardener were shown in Table 4. The fabricated sandwich structure was placed under weights for 1 day to ensure a complete curing of the adhesive at room temperature. Fig. 1 shows the schematic diagram of sandwich structure configuration adopted in this study. 2.4. Drop weight impact test Impact tests were conducted using an instrumented drop weight impact machine. Here, a 2.1 kg carriage with a 25 mm
Table 1 Selected mechanical properties of sandwich core material: PP8T40F. Property
Value 3
Density (kg/m ) Compressive strength (MPa) Compression modulus (MPa) Bending strength (MPa) Bending modulus (MPa) Pulling shear strength (MPa)
56.1 2.05 65.4 18.5 1.02 103 0.53
Table 2 Properties of unidirectional glass fiber reinforced thermoplastic composite prepreg tape: AtenPly GPP45. Property
Value
Density (kg/m3) Fiber content (Wt%/V%) Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa)
1.65 70/45 452 21.8 323 16
Table 3 Properties of metallic skin – Aluminum 1100. Property
Value
Thickness (mm) Density (kg/m3) Elastic modulus (GPa) Tensile strength (MPa)
0.5 2.71 103 70 110
Table 4 Properties of adhesive – AralditeÒ Rapid. Property
Value 3
Density (Resin) (kg/m ) Density (Hardener) (kg/m3) Cure time (min) Cure temperature (°C) Flexural strength (MPa) Flexural modulus (MPa)
2.25 0.94 15–30 18–27 46 1654.4
diameter hemi-spherical indenter was released from heights of up to 1.0 m. Impact energy levels up to 20 J were achieved by varying the drop height. The specimens were supported on two 12 mm diameter steel cylinders positioned on movable right angle supports. The impact force history during the test was measured using a piezo-electric load cell located just above the impactor tip. The signals from the load cell were then recorded and stored by computer using data acquisition software. The schematic of the setup is shown in Fig. 2. Test for each impact energy were repeated three sets. However, only average peak load versus each impact energy was shown. The force–time histories, absorbed energy, energy– time histories and contact duration for drop-weight impacts onto composites were based on single result from the three sets of data. This is because it would be much appropriate to relate the results based on post-impact damages on the composite.
3. Results and discussion The low velocity impact performance of FML skin with polypropylene honeycomb sandwiched composite were evaluated using various impact-sensitive parameters such as impact force–time history, peak load at various impact velocity, contact duration and total absorbed energy. Post-impact damage was evaluated using optical microscope and measurements of damage area were taken and compared with absorbed energy. It should be noted here that the custom instrumented drop weight impact tester could only measure the force versus time directly, thus the impact velocity was calculated based on conservation of energy, where all the potential energy was converted into kinetic energy prior to impact on the sandwich structure. By varying the drop height, we could get a series of impact velocities and impact energy. Fig. 3 shows the load–time histories recorded by varying the drop height of the impact test. At impact energy of 3.92 J and
1435
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
Fig. 1. Schematic diagram of a sandwich structure configuration.
Motor for height control
Magnet for release of indenter and carriage PC for data analysis Piezo-electric load cell
Stainless steel indenter
Three point bend support system
Fig. 2. Schematics of the drop weight impact tester.
7.84 J, the loading and unloading was relatively smooth by reaching the peak around 4 ms as compared to impact energies at 11.76 J and above. This could be compared with a typical load–time history data shown in Fig. 4. It showed smooth loading and unloading of the load on a sample which showed none or little damage. The Fmax showed the maximum force which is the highest point of the graph and t which indicated the time when maximum force occurs. At impact energy of 11.76 J, the loading part was quite similar with the lower impact energy reaching maximum force at almost the same time around 4 ms, but during the unloading part
it showed a sudden load drop at around 5 ms. This might indicate that the specimen was reaching its threshold impact energy. However, impact energy of 15.68 J and 19.62 J both showed the time taken to reach the maximum force was much earlier at around 2.5 ms and sudden load drops seemed to be occurring earlier at around 3 ms. This indicated that the structure had suffered serious damage and according to Herup and Palazotto [12], as impact energy increased, eventually a point was reached where the load history curve started to show an oscillation which was no longer smooth, instead, a major load drop occurred and was followed by
Fig. 3. Time histories of the contact forces of drop-weight impacts onto composites.
1436
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
Fig. 4. Typical load–time histories for a drop-weight impact test.
multiple cycles of loading and partial unloading. This major load drop could be related to damages occurred on the specimen such as delamination, core crushing and also bending of the structure. Fig. 5a shows the variation of peak load with impact energy for this sandwich structure. The result showed a rise in load with
increasing impact energy. An examination on the specimens showed that at impact energy above 11.76 J, the damage started to extend to the side of the specimen with slight delamination. At impact energy of 15.68 J and 19.62 J, specimens showed severe damage across the width of the surface and suffered bending. Some previous studies [7,13] on peak load versus impact energy showed that for sandwich structures, the damage threshold energy usually occurred at the starting point where the maximum impact force started to reach plateau. In this study, we could see that at 19.62 J, the peak load reached approximately 1700 N which was quite similar at impact energy of 11.76 J and 15.68 J, compared to load at impact energy of 3.92 J which is 1200 N and 7.84 J at 1500 N, this shows that the sandwich structure had reach its impact damage threshold energy. Fig. 6a shows energy time history for this study. According to Sevkat et al. [14], there are usually three interaction modes between the composite panel and the impactor depending on the level of impact energy. First is when the energy absorbed by the composite is very little, the impactor bounces back. Second is when most of the energy is being absorbed by the composite through various modes of damage, thus no rebound occurs. Finally in the case of high energy level, perforation can be observed. Fig. 6b shows a typical energy curve for drop weight impact test where rebound of the impactor occurs. In this study, most of the
Fig. 5. (a) Maximum peak load under various impact energy. (b) Absorbed energy on composite specimen under various impact energy.
Fig. 6. (a) Energy–time histories for drop weight impact test of composite specimens at various energy. (b) A typical energy curve for drop weight impact test when rebound of the impactor occurs.
1437
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
Impact damage threshold energy
Fig. 9. Damage area measured under various impact energy. Fig. 7. Contact duration at different impact energy.
energy–time histories shown are similar to Fig. 6b, except for impact energy of 15.68 J and 19.62 J. At both of the high impact energy, the composite structure suffered bending which had caused minimum rebound energy recorded. Fig. 5b shows that energy absorption of the composite increased with increasing impact energy. The contact durations of impact test are being summarized in Fig. 7. We noticed that the contact duration increased slightly with the increase in impact energy at region I. However, the contact duration at impact energy of 15.68 J and 19.62 J (region II) increased sharply as compared to those in region I. Based on Paolo and Kedward [15] summary on contact duration plot, in the first region, contact duration is supposed to be constant due to their structures stiffness. Somehow, the contact duration plot in this study (Fig. 7.) differs. This might be due to the fact that the combination of metallic skin materials and single-ply fiber reinforced pre-preg used in this study were ductile in nature with low stiffness thus resulting in slight increase in contact duration of the indenter with the sandwich structure. The sharp increase of the
Impact Energy
3.92J
7.84J
contact duration in region II was likely due to severe damage in the sandwich structure. This had been reported [14] where the higher the damage, the longer the time impactor will be in contact with the specimen. So, in this case, the specimen which had suffered bending at both impact energy levels of 15.68 J and 19.62 J, can be the reason of longer contact duration. As shown in Fig. 8, increasing impact energy will increase the damage of the impact area. Based on measurement software provided on the optical microscope, a simple graph with impact energy versus damage area was being plotted. Based on Fig. 9, the damage area measured for the composite sandwich structure shows an overall increasing trend when given higher impact energy. However, under impact energy below approximately 8 J, it was found that the damage area did not vary much compared to higher impact energy levels. It was noted that the damage area result was somehow related to the result of force–time history during impact. An increase in damage area showed a sudden load drops under the force–time history graph, indicating that the impact threshold energy had been reached. From extrapolation lines in Fig. 9, it is estimated that the sandwich structure’s impact threshold energy might occur around 13–14 J of impact energy.
11.76J
15.68J
19.62J
Top View
No Impact
3.92 J 7.84 J Side View 11.76 J 15.68 J
19.62 J
Fig. 8. Top view damage area obtained by optical microscope and side view of damaged specimen.
1438
C.Y. Tan, H.Md. Akil / Composites: Part B 43 (2012) 1433–1438
4. Conclusion Low-velocity impact response and energy absorption capacity in polypropylene honeycomb sandwich structure with simple fiber–metal laminate skin was identified. Lower impact energy damage showed only indentation around the impact center, but once the impact threshold energy was reached, it showed a sudden drop in recorded impact force and post-impact examination on the composite showing delamination of the skins and global bending of the structure. There was an increase in energy absorption of the composite specimen with an increase in impact energy. However under impact energy of 15.68 J and 19.62 J, the structure showed severe core crushing and bending globally, thus it could be concluded that the impact energy absorption capacity was set at its impact threshold energy which was under impact energy of 11.76 J with 9.12 J of energy being absorbed. Results from the damage area measurements however, showed a possibly higher than 11.76 J critical damage energy where it was estimated to fall in the range of 13–14 J.
Acknowledgements The first author acknowledges the financial assistance under the fellowship scheme of Universiti Sains Malaysia and the Postgraduate Research Grant Scheme (USM-RU-PRGS – Grant No.: 1001/PBAHAN/8043039) that has resulted in this article. References [1] Muzzy J, Pfaendtner J, Shaw B, Holty D. Thermoplastic composite sandwich panels. In: Proceedings of the automotive composites conference, SPE; 2001.
[2] Paulius G, Daiva Z, Vitalis L, Marian O. Experimental and numerical study of impact energy absorption of safety important honeycomb core sandwich structures. Mater Sci 2010;16/2:119–23. [3] Alderliesten RC. Fiber–metal laminates: an introduction. In: Volt A, Gunnick JW, editors. Kluwer Academic Publishers.; 2001. [4] Vlot AD, Kroon E, la Rocca G. Impact response of fiber metal laminates. Key Eng Mater 1998;235/76:141–3. [5] Abdullah MR, Cantwell WJ. The impact resistance of polypropylene-based fibre–metal laminates. Compos Sci Technol 2006;66:1682–93. [6] Reyes G, Cantwell WJ. The mechanical properties of fiber–metal laminates based on glass fiber reinforced polypropylene. Compos Sci Technol 2000;60:1085–94. [7] Reyes German. Mechanical behaviour of thermoplastic FML-reinforced sandwich panels using an aluminum foam core: experiments and modeling. J Sandwich Struct Mater 2010;12/1:81–96. [8] Park JH, Ha SK, kang KW, Kim CW, Kim HS. Impact damage resistance of sandwich structure subjected to low velocity impact. J Mater Pro Tech 2008;201/1–3:425–30. [9] Foo CC, Seah LK, Chai GB. Low-velocity impact failure of aluminum honeycomb sandwich panels. Compos Struct 2008;85/1:20–8. [10] Erickson MD, Kallmeyer AR, Kellogg KG. Effect of temperature on the lowvelocity impact behaviour of composite sandwich panels. J Sandwich Struct Mater 2005;7:245–64. [11] Hazizan MA, Cantwell WJ. The low velocity impact response of an aluminum honeycomb sandwich structure. Compos: Part B 2003;34:679–87. [12] Herup EJ, Palazotto AN. Low-velocity impact damage initiation in graphite/ epoxy/nomex honeycomb-sandwich plates. Compos Sci Tech 1997;57/ 12:1581–98. [13] Hazizan MA, Cantwell WJ. The low velocity impact response of foam-based sandwich structures. Compos: Part B 2002;33/3:193–204. [14] Sevkat E, Liaw B, Delale F, Raju BB. Drop-weight impact of plain-woven hybrid glass-graphite/toughened epoxy composites. Compos: Part A 2009;40/ 8:1090–110. [15] Feraboli Paolo, Kedward Keith T. A new composite structure impact performance assessment program. Compos Sci Tech 2006;66(12):1336–47.