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epoxy composite face-sheet and sandwich foam, coremat and honeycomb materials
International Journal of Impact Engineering 58 (2013) 31e38
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International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng
Impact damage characteristics in reinforced woven natural silk/epoxy composite face-sheet and sandwich foam, coremat and honeycomb materials A.U. Ude*, A.K. Ariffin, C.H. Azhari Department of Mechanical and Materials Engineering, Faculty of Engineering & Built Environment, The National University of Malaysia (UKM), Bangi 43600, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2012 Accepted 4 March 2013 Available online 14 March 2013
This research is interested in the impact toughness of a reinforced composite face-sheet and cores materials used in lightweight sandwich panels. It investigated the degrees of damage inflicted on the contact surface, through thickness and rear surface of the sandwich panels. The sandwich specimens were prepared in configurations of natural silk (NS)/Epoxy/Foam, NS/Epoxy/Coremat, NS/Epoxy/Honeycomb and reinforced NS/Epoxy serving as referral. For all experiments, drop weight impact test was carried out under low velocity impact energies of 32 J, 48 J and 64 J. Parameters measured were load bearing capability, energy absorption capability and damage fragmentation of the specimen with regards to increasing impact loads. Dominant deformation modes were seen as upper face-sheet compression failure, lower face-sheet delamination and lower face-sheet tensile failure. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Natural silk Impact response Sandwich Low velocity
1. Introduction The need of improved materials with high specific mechanical properties, high strength and stiffness to weight ratio has led to increasing use of sandwich composite materials. Sandwich structures of diverse configurations have been used in industries such as automotive, aerospace, sports, marine and equipments like military bullet-proof, body armours, etc. The main idea behind this type of structures is to combine two advantages of stiffness and strength of the thin face-sheet and lightweight of thicker flexible core to achieve superior material properties. To properly design a sandwich panel for engineering applications, a thorough perceptive and characterization of all essential materials in this case: face-sheet, adhesive and core as well as the whole sandwich structure under dynamic impact loading is indispensable. The reason being that sandwich composites are known to be susceptible to fail by dynamic load (impact), of foreign objects. During service life of sandwich composite structures, internal damages may be induced via, low velocity, like tool drops or even high velocity impact of foreign objects. The problem associated with low velocity impact is that often times while the surface of the composite structure may appear undamaged to visual inspections, internal damages has
* Corresponding author. Tel.: þ60 162843058. E-mail address: [email protected] (A.U. Ude). 0734-743X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijimpeng.2013.03.003
occurred [1]. The effect of these internal damages on strength and reliability of the composite structures can be detrimental and most often lead to a sudden catastrophic failure. Edgren et al. [2], Hazizan and Cantwell [3], and Nemes and Simmonds [4] have independently identified several failure modes associated to low velocity impact of sandwich structures like, face-sheet buckling, face-sheet delamination, debonding between face-sheet and core; cracks in face-sheet and core as possible sources of failures. This complex nature of sandwich composite on the effect of low velocity impact of foreign objects, has kept their investigation an attractive research area and have drawn much attention [5e10]. In recent years, fibres in textile forms [11] have been introduced in the fabrication of composites (fibre composites) due to their ability to resist impact damage more than the unidirectional fibres. Their textile structures, such as dimensional stability, subtle conformability, and profound moldability/tailorbility are seen as the reason behind their good impact resistance property [12]. Certainly, great deals of research have been done to understand the low velocity impact behaviours of sandwich composites panels [13e16]. Much attention have been given to study carbon, glass and other fibres composite face-sheet [17e20], none has been reported on Bombyx mori woven natural silk (NS) as a composite face-sheet. Having introduced B. mori woven natural silk (NS) as a novel composite reinforcement for face-sheet, it is then necessary to investigate its mechanic of failure under low velocity impact.
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The objective of this paper is to present B. mori natural silk (NS) as a novel composite face-sheet and analyse its impact resistance under low velocity, considering its environmental and mechanical properties. It is among the strongest fibres produced in nature, high specific-strength and high specific-stiffness; extremely elastic and resilient. Previous studies by Bledzki et al. [21], Craven et al. [22], and Perez-Rigueriro et al. [23] showed that B. mori silk is better than Kevlar or steel in terms of elongation at failure (Table 1). It has a good capacity to absorb energy and to dissipate energy in a very controlled manner as the silk deforms [23]. Therefore, because of these attributes and the interlacing of fibre bundles in woven fabrics composites which prevents the growth of damage and hence provides an increase in impact toughness, improved resistance to impact is expected. 2. Materials and methods 2.1. Specimen preparation The materials used in this research experiments are epoxy resin, type (DER 331), A Jointmine hardener: type (905-35) employed to facilitate curing; core materials were Honeycomb, Foam and Coremat, all were supplied by DkComposites Malice. Material properties are listed in Tables 2e4 below. B. mori plain woven natural silk fabric (supplied by Loxevi Silk Indonesia) has been used as the face-sheet material and the mechanical properties listed in Table 3. The sandwich composite specimens were prepared via hand-lay-up method. This method provided high quality composite samples plates with minimal defects. To create the sandwich samples, core materials were sandwiched between 1.5 mm thicknesses of epoxy resin reinforced B. mori woven natural silk fabric. Special care was taken to ensure that correct amount of epoxy was used in addition to being evenly spread out. The vacuum bagging was carefully spread over the sample ensuring no wrinkles would form when the vacuum was applied. As any wrinkle that form on the vacuum bagging will affect the surface finish of the sample. A rubber squeeze was used to remove the extra epoxy and trapped air. The mould was closed and the composite plate placed in a hydraulic press at a room temperature and at a pressure of 10 bar for 3 h. After being taken out from the hydraulic press, the plate was left to cure at a room temperature for 24 h before being removed from the mould. The plate was then cut into the specimen size for a drop weight impact test using a diamond cuter.
Table 2 Properties of DER 331 epoxy. Materials
Density (kg/m3)
Compressive strength (MPa)
Tensile strength (MPa)
Cure time (h)
Cure temperature ( C)
Epoxy DER 331
1084
131
63.6
9e12
23.9
suitable for a wide variety of applications requiring low to high impact energies. A tup insert, which was assumed to be perfectly rigid, with a hemispherical nose of 12.7 mm diameter was used. The testing machine has a force transducer with capacity of 22.24 kN. The total mass of the impactor used was 5.5 kg. The composite specimen with dimensions of 100 mm 100 mm square and 7 mm thickness (see Table 4) was clamped on a fixture along a circumference having a 76.2 mm diameter. Potential striking energies were obtained from adjusting the drop height and calculated using energy equation shown in Eq. (1).
Ei ¼ mgh
(1)
where Ei is impact energy, m is mass of impactor, g is gravity and h is height. In this experiment, the main structural variables of the sandwich composite were face-sheet material, core materials and the overall thickness of the panels. Both visual inspection and scanning electron microscopy were used to assess the level of damage inflicted in the materials after the impact event. The interest was to study the extent and degree of the different types of failure processes in the face-sheet and sandwich materials and relate these data to experimental conditions and the dimensions and other properties of the materials. The main reason behind the choice of an instrumented drop weight impact testing machine for this study was based on its ability to simulate closer to real-life impact conditions. Three specimens were tested in each configuration and the average values of impact test results recorded. Fig. 1 shows the schematics diagram of the experimental set-up. During the impact test using this machine, the resistive force exercised by specimen on the impactor was measured by a load cell as a function of time and stored. From this basic force e time information, the software calculated important parameters such as load-deformation, absorbed energy and velocity, which were used to characterise the impact event. 2.3. Impact energy
2.2. Impact testing The tests were performed using an instrumented drop weight testing system, Instron-Dynatup 9250 HV. It is a test system
Impact energy (Ei) and absorbed energy (Ea) are two important parameters to assess impact response and resistance of composite structures. If an impactor of mass m, impacts a composite panel specimen with a velocity vo, the impact energy of the impactor Ei can be calculated using:
Table 1 Mechanical property comparison of some synthetics and natural fibres. Material
Density (g/cm3)
Tensile strength (MPa)
Young’s modulus (GPa)
Elongation at failure (%)
Source
B. mori silk Spider silk Flax Hemp Jute Coir Sisal Cotton E-glass Carbon Kevlar49
1.3e1.8 1.3 1.5 1.48 1.3 1.2 1.5 1.5e1.6 2.7 1.8 1.44
650e750 1300e2000 345e1035 690 393e773 175 155e635 287e597 1200 4000 3600e4100
16 30 50 70 26.5 4.0e6.0 9.2e22.0 5.5e12.5 73 131 131
18e20 28e30 2.7e3.2 1.6 1.5e1.8 10.0 2.0e2.5 7.0e8.0 2.5 2.8 2.8
[23] [22] [21] [21] [21] [21] [21] [21] [21] [21] [22]
Ei ¼
mv20 2
(2)
Also, the kinematic energy KE(t) transferred from the impactor to the specimen (composite sandwich panels) can be calculated by:
Table 3 Properties of Bombyx mori plain woven natural silk fabric. Materials
Density (g/cm3)
Thickness (mm)
Modulus of elasticity (N m2)
Ultimate strength (N m2)
Elongation (%)
Bombyx mori
1.4
0.42
22
11
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A.U. Ude et al. / International Journal of Impact Engineering 58 (2013) 31e38
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Table 4 Properties of core materials. Materials
Mat. density (kg/m3)
Comp. strength (MPa)
Tensile strength (MPa)
Shear strength (MPa)
Coremat Honeycomb Foam
48 56 52
1.1 2.02 3.17
2.0
0.9 1.65
KEðtÞ ¼
4.27
mv20 mv2i 2 2
10e175
(3)
where the velocity of the impactor vi(t) can be obtained by
1 vi ðtÞ ¼ v0 m
Zt F dt
Elongation (%)
(4)
0
where vi is the impact velocity; v0, initial velocity, Fexp, experimental striking force. Since the experimental impact force expressed as Fexp is measured during the impact event, it is therefore, possible to estimate the impact energy which reaches the composite plate using (3) [16]. 3. Results and discussions Three specimens were tested in each impact energy category, the average values of the impact tests were plotted and the result compared. 3.1. Energy absorbing capability of the composite panels Fig. 2aec compares the energy absorption capability of the composite specimen under varied impact energies of 32 J, 48 J and 64 J, respectively. It showed the variation of the energy absorbed by the tested specimens. It can be seen that there is an increase in the total energy absorption by the sandwich specimens compared to the reinforced woven natural silk/epoxy composite specimen (control material). Although the total energy absorbed by sandwich coremat specimen and reinforced natural silk/epoxy specimen (control material) under 32 J impact energy was insignificant, the effect of sandwich over reinforced on the overall is very significant. The available energy for the impact test in the first energy category was 32 J. As can be seen in Fig. 3 total energy absorption of NS/ epoxy/foam and NS/epoxy/honeycomb specimen was 15.65 J and 15.5 J respectively, representing approximately half of the impact energy introduced into the specimens. The impact energies
Fig. 1. Photo of drop weight machine and schematic diagram of experimental set-up.
Average cell size (mm)
Thickness (mm)
Cell size (mm) 12.7
0.6
5 7 7
available for test in Figs. 4 and 5 were 48 J and 64 J. It was observed that the same specimens absorbed less energy under these impact energy categories. As it can be seen, the total absorbed energies were approximately 13 J and 11 J and 10.3 J and 10 J for each impact category, representing far less than half of the impact energies introduced into the specimen. This phenomenon is influenced by the type of failure mechanisms activated by each energy category during the impact event. The absorbed energies or sometimes referred as “released energy” because the failure mechanisms activated during the impact event releases energy to the environment [16]. However, the literature consider these release energies as a fraction of impact energy which is absorbed by the structure and is not transformed on the elastic energy. In other words the impact energy Ei(t) transferred from the object to the specimen (sandwich laminated composite panel) is absorbed by elastic deformation and the failure mechanisms activated (formation of damages). Energies absorbed up to the peak load is absorbed through elastic deformation of the composite specimen, elastic deformation on an energy profile is characterised by a linear profile, all other energies absorbed beyond this point is absorbed through failure mechanisms activated. Thus, each failure mechanism, e.g., matrix crack, intra-layer failures, delamination, fibre breakage etc, absorb a fraction of the impact energy. Consequently, the amount and the type of failure mechanisms activated will affect the total absorbed energy values. However, the amount and the type of failure mechanism activated depend on these factors: e Mechanical properties of fibre, core and matrix used for the manufacturing of the sandwich structure. e Shape of the impactor head. e Orientation of the fibre layers. e Geometry of the specimen. e Impact energy level. In the present study mechanical properties of fibre, core and matrix used for the manufacturing of the sandwich structure and impact energy level, were seen as the major causes of the variation in total energy absorption under different impact categories. It can be seen that energy absorption decreased as the impact load increased. It also revealed that sandwich specimens have better energy absorption capabilities than reinforced composites. In all categories of impact load tested in this experiment they performed better than the reinforced specimens. It is notable to state that foam sandwich composite was the best energy absorption specimen among the specimens tested. Analytically, the impact performance of composite structure can also be characterized be calculating the loss of kinetic energy of the impactor during impact. By measuring initial striking velocity and residual velocity, the energy absorption by the impacted specimen can be analysed using the following equation:
ETotal ¼
1 2 m vi v2r 2
(5)
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Fig. 2. Comparison of energyetime profiles (a) 32 J, (b) 48 J, (c) 64 J.
where ETotal is the energy dissipated by the target during impact, m is the mass of the impactor, vi is the initial velocity and vr is the final velocity. All other energies not absorbed by the specimen were release to the environment through the law of energy conversion. 3.2. Load bearing capability of the composite panels Loadetime profiles from the impact test are shown in Fig. 3aec. It compares the time the impactor was in contact with each specimen. Each of the specimens showed a distinct period of contact with the impactor. This is as a result of varied core materials used in fabricating the specimens. The loadetime profile was linear up to damage initiation point or the maximum load point (peak load). Initiation of failure/first peak point on the loadetime profile is designated to the maximum load (load carrying capability), this point also corresponding to the onset of material damage. Siow and Shim [24] called it the incipient point of damage. This damage is
usually matrix failure, very little or no visible damage is observed upon superficial inspection of the specimen at this point. Following a damage initiation, there is a decrease in material stiffness resulting to a drop in the loadetime profile. In all three impact energy categories, each specimen showed distinct incipient point of damage. The second highest peak in the loadetime profile, corresponds to the onset of circumferential fracture or complete failure (note: there may be other peaks in-between). At this point damages are visual, mostly characterised by fibre fracture, delamination and fibre breakage. Banks et al. [25] described it as classical mode of failure in composites. Combinations of this failure most often lead to penetration and perforation damages. Comparison of load bearing capability revealed NS/Epoxy/Coremat and NS/Epoxy/Foam specimen better than sandwich honeycomb and reinforced NS/epoxy. It was seen that NS/Epoxy/Coremat specimen had the highest load value of approximately 1.35 kN (see Fig. 3a). Foam and Honeycomb sandwich was 0.92 kN and 0.83 kN,
A.U. Ude et al. / International Journal of Impact Engineering 58 (2013) 31e38
35
Fig. 3. Comparison of loadetime profiles (a) 32 J, (b) 48 J, (c) 64 J.
while reinforced NS/epoxy specimen was 0.80 kN respectively. The specimen also depicts different load capability values in 48 J and 64 J impact load categories. The values of maximum load for sandwich composites involving different core materials depend on the core materials properties; since they all have the same facesheet/skin. Observation on time to reach peak load revealed that as impact load increases, time to reach complete failure decreases. For instance NS/Epoxy specimen (Fig. 3a) time to maximum load was 3.4 ms and 6.8 ms to reach complete failure under 32 J impact load, but was 2 ms and 1.6 ms under 48 J and 64 J impact load (Fig. 3b, c). The same phenomenon was observed in other specimens. This occurrence was attributed to the increase in velocity [24].
3.3. Load effect on structural degradation of composite panels Loadedeflection profiles of the impacted specimen compared in Fig. 4aec, showed the structural degradation history of the composite panel. The nature of initial rise in the loadedeflection profile correlates to the load bearing capabilities of the specimens. At a load level of 0.34 kN and deflection of approximately 1.5 mm (Fig. 4a) the loadedeflection profile shows a change in stiffness for NS/epoxy/honeycomb specimen, indicating structural degradation. For other specimens, the change in stiffness indicates that structural degradation takes place in a higher load level. In the case of foam and coremat specimen (at a load level of 0.8 kN and 1.35 kN and deflection of approximately 7 mm and 10.1 mm) a change in
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Fig. 4. Comparison of loadedeflection profiles (a) 32 J, (b) 48 J, (c) 64 J.
Fig. 5. Damage fragmentation of sandwich composite panels under 32 J impact load: (a) NS/Epoxy/Honeycomb; (b) NS/Epoxy/Foam; (c) NS/Epoxy/Coremat; (d) NS/Epoxy.
A.U. Ude et al. / International Journal of Impact Engineering 58 (2013) 31e38
37
Fig. 6. Damage fragmentation of sandwich composite panels under 48 J impact load: (a) NS/Epoxy/Honeycomb; (b) NS/Epoxy/Foam; (c) NS/Epoxy/Coremat; (d) NS/Epoxy.
stiffness occurs indicating structural degradation. It was seen that as the impact energy level increased to 48 J and 64 J (Fig. 4b, c), the change in deflection decreased. The maximum value of impact load where failure of the composites structures begins is higher for coremat and foam specimens than in honeycomb and NS/epoxy specimens. 4. Damage fragmentation Figs. 5e7, compare a typical fracture pattern of a specimen after impact loading. Fig. 5a showed damages incurred by NS/Epoxy/ Honeycomb specimen; there is approximately 25 mm diameter front damage, the depth of tup penetration is about 2 mm, matrix crack, fibre breakage and circumferential fracture lines were also observed. The colour change seen at the rear surface of the specimen showed that the damage through the thickness increase away from the impacted face and toward the tensile stressed back in form of a delamination of the face-sheet from the core. Fig. 5b (NS/ Epoxy/Foam) specimen, showed front damage of approximately 30 mm diameter, dominated by radial fracture lines. The rear surface showed a vertical crack of length 20 mm and radial fractures visible to physical observation. This sign suggest that the specimen may have incurred at least matrix crack or fabric fracture. In Fig. 5c and d, NS/Epoxy/Coremat and NS/Epoxy, both specimens were perforated and the shape of their damage fragmentation were similar (i.e. a combination of damages leading from matrix crack to fibre breakage) with severe pyramid perforated rear damages. Tear damage were also noticed in both specimens. All damages incurred by the specimens under 32 J impact load appeared to remain localised to the impact site.
A similar study reported by Anderson and Madenci [26] using graphite/epoxy as face-sheet while honeycomb and foam serves as sandwich, agreed with the results of this impact category. The damage in the specimens appears to remain more local to the impact site. Failure mechanisms were described in one of two ways: a localized spherical dent, or a face-sheet tear that runs from the centre of the impact area to the edge of the specimen. Several types of specimens experienced the two different failure mechanisms at the same impact energy level and possessed very different permanent indentation depths. Their report strongly supports the occurrence in both honeycomb and foam specimens in the present investigation (Fig. 5a, b). The damage areas of both honeycomb and foam were also localized to the impact site. The damages incurred by the specimens under 48 J impact load were more severe compared to damages under 32 J impact loads. The entire specimens suffered through thickness perforation damages. In Fig. 6a NS/Epoxy/Honeycomb, showed front surface area damage of 314.2 mm2. On the rear surface, radial pyramid perforation, a large area of delamination approximately 706.95 mm2 was observed. Delamination is an internal damage therefore; increase in delamination is evidence of much internal damage. This occurrence is attributed to increased impact load. Fig. 6b shows the damage areas of NS/Epoxy/Foam specimen. The front surface area damage was measured to be 314.2 mm2. A void indicating point of penetration and circumferential fracture around the point of impact were visible. On the rear surface the total damage area is approximately 490.9 mm2. Fig. 6c and d represents NS/Epoxy/Coremat and NS/Epoxy. These specimens showed similar surface damage fragmentation. Their front surface showed battered entering point of the tup while their rear face showed the pyramid protruded fracture
Fig. 7. Damage fragmentation of sandwich composite panels under 64 J impact load: (a) NS/Epoxy/Honeycomb; (b) NS/Epoxy/Foam; (c) NS/Epoxy/Coremat; (d) NS/Epoxy.
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A.U. Ude et al. / International Journal of Impact Engineering 58 (2013) 31e38
as well as tear damaged areas. A reported study by Gustin et al. [27] agrees with the present results. Evaluations of specimens under 64 J impact load were shown in Fig. 7aed. The fragmentation of failure in this category is similar to the previous under 48 J impact load but with more severe damages observed. For instance total front surface and rear surface area damages of NS/Epoxy/honeycomb specimen were measured as approximately 706.95 mm2 and 1256.8 mm2 respectively under 64 J impact load compared to 314.2 mm2 and 706.95 mm2 respectively recorded under 48 J impact load. This fragmentation results were expected as it agreed to previous reports in [27e29], damage areas increased as the impact load increased. Comparison with high velocity impact also shows that damage areas increases as the strike velocity increases [30]. Several damage modes have been observed in this experiment, under 32 J impact load (Fig. 5a, b), the main damage modes were delamination and matrix cracks rather than fibre fracture. Damage modes on other specimens in this energy category were fibre breakage accompanied by matrix cracks, delaminations, penetrations and perforations. For higher impact energies of 48 J and 64 J, splitting between fibre and matrix and fibre fractures were noticed around site of impact. 5. Conclusions An investigation has been conducted to study the impact response of B. mori woven natural fibre/Epoxy composite as a facesheet with honeycomb, foam and coremat as sandwich core materials. A comparison of the load bearing capabilities, energy absorption and failure mechanism under varied impact energies were assessed, the entire specimen suffered certain degree of damage. In terms of energy absorption, NS/Epoxy/Foam specimen showed more energy attenuation capability among other configuration. It was found that NS/Epoxy/Coremat possess better load bearing capability than other sandwich configurations. Physical examination of the specimens proved that fragmentation areas increased with increase in impact load. It was evident that most of the energies were absorbed through the formation of damages. This occurrence agrees with Dhakal et al. [17]. These findings are expected to be helpful in understanding the response of NS sandwich composite panels under impact loading. Acknowledgements Financial support for this research work was provided by MOSTI (project code 03-01-02-SF0075). Partial financial support by Universiti Kebangsaan Malaysia (The National University of Malaysia) Bangi is also gratefully acknowledged. References [1] Peng XQ, Cao J, Numerical determination of mechanical elastic constants of textile composites, in: 15th annual technical conference of the American Society for Composite, College Station, TX; 2000. [2] Hazizan MA, Cantwell WJ. The low velocity impact response of an aluminium honeycomb sandwich structure. Composites Part B: Engineering 2003;34: 679e87.
[3] Nemes JA, Simmonds KE. Low-velocity impact response of foam core sandwich composites. Journal of Composite Materials 1992;26:500e19. [4] Edgren F, Asp LE, Bull PH. Compressive failure of impacted NCF composite sandwich panels characterisation of the failure process. Journal of Composite Materials 2004;38:495e514. [5] Othman AR, Barton DC. Failure initiation and propagation characteristics of honeycomb sandwich composites. Composite Structures 2008;85:126e38. [6] Kress G, Winkler M. Honeycomb sandwich residual stress deformation pattern. Composite Structures 2009;89:294e302. [7] Mines RAW, Jones N. Approximate elasticeplastic analysis of the static and impact behaviour of polymer composite sandwich beams. Composites 1995;26:803e14. [8] Mines RAW, Worrall CM, Gibson AG. Low velocity perforation behaviour of polymer composite sandwich panels. International Journal of Impact Engineering 1998;21:855e79. [9] Nguyen MQ, Jacombs SS, Thomson RS, Hachenberg D, Scott ML. Simulation of impact on sandwich structures. Composite Structures 2005;67:217e27. [10] Lestari W, Qiao P. Damage detection of fiber-reinforced polymer honeycomb sandwich beams. Composite Structures 2005;67:365e73. [11] Chou TW, Ko F. Textile structural composites. New York: Elsevier Science Publishing; 1989. [12] Naik KN. Woven fabric composites. Lancaster: Technomic Publishing; 1994. [13] Mehmet A, Cesim A, Bulent MI, Ramazan K. An experimental investigation of the impact response of composite laminates. Composite Structures 2009;87: 307e13. [14] Dear JP, Lee H, Brown SA. Impact damage processes in composite sheet and sandwich honeycomb materials. International Journal of Impact Engineering 2005;32:130e54. [15] Sevkat E, Liaw B, Delale F, Raju BB. Drop-weight impact of plain-woven hybrid glassegraphite/toughened epoxy composites. Composites Part A: Applied Science and Manufacturing 2009;40:1090e110. [16] Tita V, de Carvalho J, Vandepitte D. Failure analysis of low velocity impact on thin composite laminates: experimental and numerical approaches. Composite Structures 2008;83:413e28. [17] Dhakal HN, Zhang ZY, Richardson MOW, Errajhi OAZ. The low velocity impact response of non-woven hemp fibre reinforced unsaturated polyester composites. Composite Structures 2007;81:559e67. [18] Basu G, Roy AN, Bhattacharyya SK, Ghosh SK. Construction of unpaved rural road using juteesynthetic blended woven geotextile e a case study. Geotextiles and Geomembranes 2009;27:506e12. [19] Sapuan SM, Maleque MA. Design and fabrication of natural woven fabric reinforced epoxy composite for household telephone stand. Materials & Design 2005;26:65e71. [20] Ude AU, Ariffin AK, Lashlem AA, Azhari CH. Drop weight impact response of woven natural Silk/Epoxy laminated composite plates. Australian Journal of Basic and Applied Sciences 2011;5:289e95. [21] Bledzki AK, Gassan J. Composites reinforced with cellulose based fibres. Progress in Polymer Science 1999;24:221e74. [22] Craven JP, Cripps R, Viney C. Evaluating the silk/epoxy interface by means of the microbond test. Composites Part A: Applied Science and Manufacturing 2000;31:653e60. [23] Perez-Rigueiro J, Viney C, Llorca J, Elices M. Mechanical properties of singlebrin silkworm silk. Journal of Applied Polymer Science 2000;75: 1270e7. [24] Siow YP, Shim VPW. An experimental study of low velocity impact damage in woven fibre composites. Journal of Composite Materials. 1998;32. [25] Banks WM, David-West OS, Alexander NV, Nash DH. Energy absorption and bending stiffness in CFRP laminates:The effect of 451 plies. Thin-walled Structures 2008;46:860e9. [26] Anderson T, Madenci E. Experimental investigation of low-velocity impact characteristics of sandwich composites. Composite Structures 2000;50: 239e47. [27] Gustin J, Joneson A, Mahinfalah M, Stone J. Low velocity impact of combination Kevlar/carbon fiber sandwich composites. Composite Structures 2005;69: 396e406. [28] Hosur MV, Jain K, Chowdhury F, Jeelani S, Bhat MR, Murthy CRL. Low-velocity impact response of carbon/epoxy laminates subjected to coldedry and colde moist conditioning. Composite Structures 2007;79:300e11. [29] Atas C, Sayman O. An overall view on impact response of woven fabric composite plates. Composite Structures 2008;82:336e45. [30] Wambua P, Vangrimde B, Lomov S, Verpoest I. The response of natural fibre composites to ballistic impact by fragment simulating projectiles. Composite Structures 2007;77:232e40.
Contents lists available at SciVerse ScienceDirect
International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng
Impact damage characteristics in reinforced woven natural silk/epoxy composite face-sheet and sandwich foam, coremat and honeycomb materials A.U. Ude*, A.K. Ariffin, C.H. Azhari Department of Mechanical and Materials Engineering, Faculty of Engineering & Built Environment, The National University of Malaysia (UKM), Bangi 43600, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2012 Accepted 4 March 2013 Available online 14 March 2013
This research is interested in the impact toughness of a reinforced composite face-sheet and cores materials used in lightweight sandwich panels. It investigated the degrees of damage inflicted on the contact surface, through thickness and rear surface of the sandwich panels. The sandwich specimens were prepared in configurations of natural silk (NS)/Epoxy/Foam, NS/Epoxy/Coremat, NS/Epoxy/Honeycomb and reinforced NS/Epoxy serving as referral. For all experiments, drop weight impact test was carried out under low velocity impact energies of 32 J, 48 J and 64 J. Parameters measured were load bearing capability, energy absorption capability and damage fragmentation of the specimen with regards to increasing impact loads. Dominant deformation modes were seen as upper face-sheet compression failure, lower face-sheet delamination and lower face-sheet tensile failure. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Natural silk Impact response Sandwich Low velocity
1. Introduction The need of improved materials with high specific mechanical properties, high strength and stiffness to weight ratio has led to increasing use of sandwich composite materials. Sandwich structures of diverse configurations have been used in industries such as automotive, aerospace, sports, marine and equipments like military bullet-proof, body armours, etc. The main idea behind this type of structures is to combine two advantages of stiffness and strength of the thin face-sheet and lightweight of thicker flexible core to achieve superior material properties. To properly design a sandwich panel for engineering applications, a thorough perceptive and characterization of all essential materials in this case: face-sheet, adhesive and core as well as the whole sandwich structure under dynamic impact loading is indispensable. The reason being that sandwich composites are known to be susceptible to fail by dynamic load (impact), of foreign objects. During service life of sandwich composite structures, internal damages may be induced via, low velocity, like tool drops or even high velocity impact of foreign objects. The problem associated with low velocity impact is that often times while the surface of the composite structure may appear undamaged to visual inspections, internal damages has
* Corresponding author. Tel.: þ60 162843058. E-mail address: [email protected] (A.U. Ude). 0734-743X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijimpeng.2013.03.003
occurred [1]. The effect of these internal damages on strength and reliability of the composite structures can be detrimental and most often lead to a sudden catastrophic failure. Edgren et al. [2], Hazizan and Cantwell [3], and Nemes and Simmonds [4] have independently identified several failure modes associated to low velocity impact of sandwich structures like, face-sheet buckling, face-sheet delamination, debonding between face-sheet and core; cracks in face-sheet and core as possible sources of failures. This complex nature of sandwich composite on the effect of low velocity impact of foreign objects, has kept their investigation an attractive research area and have drawn much attention [5e10]. In recent years, fibres in textile forms [11] have been introduced in the fabrication of composites (fibre composites) due to their ability to resist impact damage more than the unidirectional fibres. Their textile structures, such as dimensional stability, subtle conformability, and profound moldability/tailorbility are seen as the reason behind their good impact resistance property [12]. Certainly, great deals of research have been done to understand the low velocity impact behaviours of sandwich composites panels [13e16]. Much attention have been given to study carbon, glass and other fibres composite face-sheet [17e20], none has been reported on Bombyx mori woven natural silk (NS) as a composite face-sheet. Having introduced B. mori woven natural silk (NS) as a novel composite reinforcement for face-sheet, it is then necessary to investigate its mechanic of failure under low velocity impact.
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The objective of this paper is to present B. mori natural silk (NS) as a novel composite face-sheet and analyse its impact resistance under low velocity, considering its environmental and mechanical properties. It is among the strongest fibres produced in nature, high specific-strength and high specific-stiffness; extremely elastic and resilient. Previous studies by Bledzki et al. [21], Craven et al. [22], and Perez-Rigueriro et al. [23] showed that B. mori silk is better than Kevlar or steel in terms of elongation at failure (Table 1). It has a good capacity to absorb energy and to dissipate energy in a very controlled manner as the silk deforms [23]. Therefore, because of these attributes and the interlacing of fibre bundles in woven fabrics composites which prevents the growth of damage and hence provides an increase in impact toughness, improved resistance to impact is expected. 2. Materials and methods 2.1. Specimen preparation The materials used in this research experiments are epoxy resin, type (DER 331), A Jointmine hardener: type (905-35) employed to facilitate curing; core materials were Honeycomb, Foam and Coremat, all were supplied by DkComposites Malice. Material properties are listed in Tables 2e4 below. B. mori plain woven natural silk fabric (supplied by Loxevi Silk Indonesia) has been used as the face-sheet material and the mechanical properties listed in Table 3. The sandwich composite specimens were prepared via hand-lay-up method. This method provided high quality composite samples plates with minimal defects. To create the sandwich samples, core materials were sandwiched between 1.5 mm thicknesses of epoxy resin reinforced B. mori woven natural silk fabric. Special care was taken to ensure that correct amount of epoxy was used in addition to being evenly spread out. The vacuum bagging was carefully spread over the sample ensuring no wrinkles would form when the vacuum was applied. As any wrinkle that form on the vacuum bagging will affect the surface finish of the sample. A rubber squeeze was used to remove the extra epoxy and trapped air. The mould was closed and the composite plate placed in a hydraulic press at a room temperature and at a pressure of 10 bar for 3 h. After being taken out from the hydraulic press, the plate was left to cure at a room temperature for 24 h before being removed from the mould. The plate was then cut into the specimen size for a drop weight impact test using a diamond cuter.
Table 2 Properties of DER 331 epoxy. Materials
Density (kg/m3)
Compressive strength (MPa)
Tensile strength (MPa)
Cure time (h)
Cure temperature ( C)
Epoxy DER 331
1084
131
63.6
9e12
23.9
suitable for a wide variety of applications requiring low to high impact energies. A tup insert, which was assumed to be perfectly rigid, with a hemispherical nose of 12.7 mm diameter was used. The testing machine has a force transducer with capacity of 22.24 kN. The total mass of the impactor used was 5.5 kg. The composite specimen with dimensions of 100 mm 100 mm square and 7 mm thickness (see Table 4) was clamped on a fixture along a circumference having a 76.2 mm diameter. Potential striking energies were obtained from adjusting the drop height and calculated using energy equation shown in Eq. (1).
Ei ¼ mgh
(1)
where Ei is impact energy, m is mass of impactor, g is gravity and h is height. In this experiment, the main structural variables of the sandwich composite were face-sheet material, core materials and the overall thickness of the panels. Both visual inspection and scanning electron microscopy were used to assess the level of damage inflicted in the materials after the impact event. The interest was to study the extent and degree of the different types of failure processes in the face-sheet and sandwich materials and relate these data to experimental conditions and the dimensions and other properties of the materials. The main reason behind the choice of an instrumented drop weight impact testing machine for this study was based on its ability to simulate closer to real-life impact conditions. Three specimens were tested in each configuration and the average values of impact test results recorded. Fig. 1 shows the schematics diagram of the experimental set-up. During the impact test using this machine, the resistive force exercised by specimen on the impactor was measured by a load cell as a function of time and stored. From this basic force e time information, the software calculated important parameters such as load-deformation, absorbed energy and velocity, which were used to characterise the impact event. 2.3. Impact energy
2.2. Impact testing The tests were performed using an instrumented drop weight testing system, Instron-Dynatup 9250 HV. It is a test system
Impact energy (Ei) and absorbed energy (Ea) are two important parameters to assess impact response and resistance of composite structures. If an impactor of mass m, impacts a composite panel specimen with a velocity vo, the impact energy of the impactor Ei can be calculated using:
Table 1 Mechanical property comparison of some synthetics and natural fibres. Material
Density (g/cm3)
Tensile strength (MPa)
Young’s modulus (GPa)
Elongation at failure (%)
Source
B. mori silk Spider silk Flax Hemp Jute Coir Sisal Cotton E-glass Carbon Kevlar49
1.3e1.8 1.3 1.5 1.48 1.3 1.2 1.5 1.5e1.6 2.7 1.8 1.44
650e750 1300e2000 345e1035 690 393e773 175 155e635 287e597 1200 4000 3600e4100
16 30 50 70 26.5 4.0e6.0 9.2e22.0 5.5e12.5 73 131 131
18e20 28e30 2.7e3.2 1.6 1.5e1.8 10.0 2.0e2.5 7.0e8.0 2.5 2.8 2.8
[23] [22] [21] [21] [21] [21] [21] [21] [21] [21] [22]
Ei ¼
mv20 2
(2)
Also, the kinematic energy KE(t) transferred from the impactor to the specimen (composite sandwich panels) can be calculated by:
Table 3 Properties of Bombyx mori plain woven natural silk fabric. Materials
Density (g/cm3)
Thickness (mm)
Modulus of elasticity (N m2)
Ultimate strength (N m2)
Elongation (%)
Bombyx mori
1.4
0.42
22
11
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Table 4 Properties of core materials. Materials
Mat. density (kg/m3)
Comp. strength (MPa)
Tensile strength (MPa)
Shear strength (MPa)
Coremat Honeycomb Foam
48 56 52
1.1 2.02 3.17
2.0
0.9 1.65
KEðtÞ ¼
4.27
mv20 mv2i 2 2
10e175
(3)
where the velocity of the impactor vi(t) can be obtained by
1 vi ðtÞ ¼ v0 m
Zt F dt
Elongation (%)
(4)
0
where vi is the impact velocity; v0, initial velocity, Fexp, experimental striking force. Since the experimental impact force expressed as Fexp is measured during the impact event, it is therefore, possible to estimate the impact energy which reaches the composite plate using (3) [16]. 3. Results and discussions Three specimens were tested in each impact energy category, the average values of the impact tests were plotted and the result compared. 3.1. Energy absorbing capability of the composite panels Fig. 2aec compares the energy absorption capability of the composite specimen under varied impact energies of 32 J, 48 J and 64 J, respectively. It showed the variation of the energy absorbed by the tested specimens. It can be seen that there is an increase in the total energy absorption by the sandwich specimens compared to the reinforced woven natural silk/epoxy composite specimen (control material). Although the total energy absorbed by sandwich coremat specimen and reinforced natural silk/epoxy specimen (control material) under 32 J impact energy was insignificant, the effect of sandwich over reinforced on the overall is very significant. The available energy for the impact test in the first energy category was 32 J. As can be seen in Fig. 3 total energy absorption of NS/ epoxy/foam and NS/epoxy/honeycomb specimen was 15.65 J and 15.5 J respectively, representing approximately half of the impact energy introduced into the specimens. The impact energies
Fig. 1. Photo of drop weight machine and schematic diagram of experimental set-up.
Average cell size (mm)
Thickness (mm)
Cell size (mm) 12.7
0.6
5 7 7
available for test in Figs. 4 and 5 were 48 J and 64 J. It was observed that the same specimens absorbed less energy under these impact energy categories. As it can be seen, the total absorbed energies were approximately 13 J and 11 J and 10.3 J and 10 J for each impact category, representing far less than half of the impact energies introduced into the specimen. This phenomenon is influenced by the type of failure mechanisms activated by each energy category during the impact event. The absorbed energies or sometimes referred as “released energy” because the failure mechanisms activated during the impact event releases energy to the environment [16]. However, the literature consider these release energies as a fraction of impact energy which is absorbed by the structure and is not transformed on the elastic energy. In other words the impact energy Ei(t) transferred from the object to the specimen (sandwich laminated composite panel) is absorbed by elastic deformation and the failure mechanisms activated (formation of damages). Energies absorbed up to the peak load is absorbed through elastic deformation of the composite specimen, elastic deformation on an energy profile is characterised by a linear profile, all other energies absorbed beyond this point is absorbed through failure mechanisms activated. Thus, each failure mechanism, e.g., matrix crack, intra-layer failures, delamination, fibre breakage etc, absorb a fraction of the impact energy. Consequently, the amount and the type of failure mechanisms activated will affect the total absorbed energy values. However, the amount and the type of failure mechanism activated depend on these factors: e Mechanical properties of fibre, core and matrix used for the manufacturing of the sandwich structure. e Shape of the impactor head. e Orientation of the fibre layers. e Geometry of the specimen. e Impact energy level. In the present study mechanical properties of fibre, core and matrix used for the manufacturing of the sandwich structure and impact energy level, were seen as the major causes of the variation in total energy absorption under different impact categories. It can be seen that energy absorption decreased as the impact load increased. It also revealed that sandwich specimens have better energy absorption capabilities than reinforced composites. In all categories of impact load tested in this experiment they performed better than the reinforced specimens. It is notable to state that foam sandwich composite was the best energy absorption specimen among the specimens tested. Analytically, the impact performance of composite structure can also be characterized be calculating the loss of kinetic energy of the impactor during impact. By measuring initial striking velocity and residual velocity, the energy absorption by the impacted specimen can be analysed using the following equation:
ETotal ¼
1 2 m vi v2r 2
(5)
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Fig. 2. Comparison of energyetime profiles (a) 32 J, (b) 48 J, (c) 64 J.
where ETotal is the energy dissipated by the target during impact, m is the mass of the impactor, vi is the initial velocity and vr is the final velocity. All other energies not absorbed by the specimen were release to the environment through the law of energy conversion. 3.2. Load bearing capability of the composite panels Loadetime profiles from the impact test are shown in Fig. 3aec. It compares the time the impactor was in contact with each specimen. Each of the specimens showed a distinct period of contact with the impactor. This is as a result of varied core materials used in fabricating the specimens. The loadetime profile was linear up to damage initiation point or the maximum load point (peak load). Initiation of failure/first peak point on the loadetime profile is designated to the maximum load (load carrying capability), this point also corresponding to the onset of material damage. Siow and Shim [24] called it the incipient point of damage. This damage is
usually matrix failure, very little or no visible damage is observed upon superficial inspection of the specimen at this point. Following a damage initiation, there is a decrease in material stiffness resulting to a drop in the loadetime profile. In all three impact energy categories, each specimen showed distinct incipient point of damage. The second highest peak in the loadetime profile, corresponds to the onset of circumferential fracture or complete failure (note: there may be other peaks in-between). At this point damages are visual, mostly characterised by fibre fracture, delamination and fibre breakage. Banks et al. [25] described it as classical mode of failure in composites. Combinations of this failure most often lead to penetration and perforation damages. Comparison of load bearing capability revealed NS/Epoxy/Coremat and NS/Epoxy/Foam specimen better than sandwich honeycomb and reinforced NS/epoxy. It was seen that NS/Epoxy/Coremat specimen had the highest load value of approximately 1.35 kN (see Fig. 3a). Foam and Honeycomb sandwich was 0.92 kN and 0.83 kN,
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35
Fig. 3. Comparison of loadetime profiles (a) 32 J, (b) 48 J, (c) 64 J.
while reinforced NS/epoxy specimen was 0.80 kN respectively. The specimen also depicts different load capability values in 48 J and 64 J impact load categories. The values of maximum load for sandwich composites involving different core materials depend on the core materials properties; since they all have the same facesheet/skin. Observation on time to reach peak load revealed that as impact load increases, time to reach complete failure decreases. For instance NS/Epoxy specimen (Fig. 3a) time to maximum load was 3.4 ms and 6.8 ms to reach complete failure under 32 J impact load, but was 2 ms and 1.6 ms under 48 J and 64 J impact load (Fig. 3b, c). The same phenomenon was observed in other specimens. This occurrence was attributed to the increase in velocity [24].
3.3. Load effect on structural degradation of composite panels Loadedeflection profiles of the impacted specimen compared in Fig. 4aec, showed the structural degradation history of the composite panel. The nature of initial rise in the loadedeflection profile correlates to the load bearing capabilities of the specimens. At a load level of 0.34 kN and deflection of approximately 1.5 mm (Fig. 4a) the loadedeflection profile shows a change in stiffness for NS/epoxy/honeycomb specimen, indicating structural degradation. For other specimens, the change in stiffness indicates that structural degradation takes place in a higher load level. In the case of foam and coremat specimen (at a load level of 0.8 kN and 1.35 kN and deflection of approximately 7 mm and 10.1 mm) a change in
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Fig. 4. Comparison of loadedeflection profiles (a) 32 J, (b) 48 J, (c) 64 J.
Fig. 5. Damage fragmentation of sandwich composite panels under 32 J impact load: (a) NS/Epoxy/Honeycomb; (b) NS/Epoxy/Foam; (c) NS/Epoxy/Coremat; (d) NS/Epoxy.
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Fig. 6. Damage fragmentation of sandwich composite panels under 48 J impact load: (a) NS/Epoxy/Honeycomb; (b) NS/Epoxy/Foam; (c) NS/Epoxy/Coremat; (d) NS/Epoxy.
stiffness occurs indicating structural degradation. It was seen that as the impact energy level increased to 48 J and 64 J (Fig. 4b, c), the change in deflection decreased. The maximum value of impact load where failure of the composites structures begins is higher for coremat and foam specimens than in honeycomb and NS/epoxy specimens. 4. Damage fragmentation Figs. 5e7, compare a typical fracture pattern of a specimen after impact loading. Fig. 5a showed damages incurred by NS/Epoxy/ Honeycomb specimen; there is approximately 25 mm diameter front damage, the depth of tup penetration is about 2 mm, matrix crack, fibre breakage and circumferential fracture lines were also observed. The colour change seen at the rear surface of the specimen showed that the damage through the thickness increase away from the impacted face and toward the tensile stressed back in form of a delamination of the face-sheet from the core. Fig. 5b (NS/ Epoxy/Foam) specimen, showed front damage of approximately 30 mm diameter, dominated by radial fracture lines. The rear surface showed a vertical crack of length 20 mm and radial fractures visible to physical observation. This sign suggest that the specimen may have incurred at least matrix crack or fabric fracture. In Fig. 5c and d, NS/Epoxy/Coremat and NS/Epoxy, both specimens were perforated and the shape of their damage fragmentation were similar (i.e. a combination of damages leading from matrix crack to fibre breakage) with severe pyramid perforated rear damages. Tear damage were also noticed in both specimens. All damages incurred by the specimens under 32 J impact load appeared to remain localised to the impact site.
A similar study reported by Anderson and Madenci [26] using graphite/epoxy as face-sheet while honeycomb and foam serves as sandwich, agreed with the results of this impact category. The damage in the specimens appears to remain more local to the impact site. Failure mechanisms were described in one of two ways: a localized spherical dent, or a face-sheet tear that runs from the centre of the impact area to the edge of the specimen. Several types of specimens experienced the two different failure mechanisms at the same impact energy level and possessed very different permanent indentation depths. Their report strongly supports the occurrence in both honeycomb and foam specimens in the present investigation (Fig. 5a, b). The damage areas of both honeycomb and foam were also localized to the impact site. The damages incurred by the specimens under 48 J impact load were more severe compared to damages under 32 J impact loads. The entire specimens suffered through thickness perforation damages. In Fig. 6a NS/Epoxy/Honeycomb, showed front surface area damage of 314.2 mm2. On the rear surface, radial pyramid perforation, a large area of delamination approximately 706.95 mm2 was observed. Delamination is an internal damage therefore; increase in delamination is evidence of much internal damage. This occurrence is attributed to increased impact load. Fig. 6b shows the damage areas of NS/Epoxy/Foam specimen. The front surface area damage was measured to be 314.2 mm2. A void indicating point of penetration and circumferential fracture around the point of impact were visible. On the rear surface the total damage area is approximately 490.9 mm2. Fig. 6c and d represents NS/Epoxy/Coremat and NS/Epoxy. These specimens showed similar surface damage fragmentation. Their front surface showed battered entering point of the tup while their rear face showed the pyramid protruded fracture
Fig. 7. Damage fragmentation of sandwich composite panels under 64 J impact load: (a) NS/Epoxy/Honeycomb; (b) NS/Epoxy/Foam; (c) NS/Epoxy/Coremat; (d) NS/Epoxy.
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as well as tear damaged areas. A reported study by Gustin et al. [27] agrees with the present results. Evaluations of specimens under 64 J impact load were shown in Fig. 7aed. The fragmentation of failure in this category is similar to the previous under 48 J impact load but with more severe damages observed. For instance total front surface and rear surface area damages of NS/Epoxy/honeycomb specimen were measured as approximately 706.95 mm2 and 1256.8 mm2 respectively under 64 J impact load compared to 314.2 mm2 and 706.95 mm2 respectively recorded under 48 J impact load. This fragmentation results were expected as it agreed to previous reports in [27e29], damage areas increased as the impact load increased. Comparison with high velocity impact also shows that damage areas increases as the strike velocity increases [30]. Several damage modes have been observed in this experiment, under 32 J impact load (Fig. 5a, b), the main damage modes were delamination and matrix cracks rather than fibre fracture. Damage modes on other specimens in this energy category were fibre breakage accompanied by matrix cracks, delaminations, penetrations and perforations. For higher impact energies of 48 J and 64 J, splitting between fibre and matrix and fibre fractures were noticed around site of impact. 5. Conclusions An investigation has been conducted to study the impact response of B. mori woven natural fibre/Epoxy composite as a facesheet with honeycomb, foam and coremat as sandwich core materials. A comparison of the load bearing capabilities, energy absorption and failure mechanism under varied impact energies were assessed, the entire specimen suffered certain degree of damage. In terms of energy absorption, NS/Epoxy/Foam specimen showed more energy attenuation capability among other configuration. It was found that NS/Epoxy/Coremat possess better load bearing capability than other sandwich configurations. Physical examination of the specimens proved that fragmentation areas increased with increase in impact load. It was evident that most of the energies were absorbed through the formation of damages. This occurrence agrees with Dhakal et al. [17]. These findings are expected to be helpful in understanding the response of NS sandwich composite panels under impact loading. Acknowledgements Financial support for this research work was provided by MOSTI (project code 03-01-02-SF0075). Partial financial support by Universiti Kebangsaan Malaysia (The National University of Malaysia) Bangi is also gratefully acknowledged. References [1] Peng XQ, Cao J, Numerical determination of mechanical elastic constants of textile composites, in: 15th annual technical conference of the American Society for Composite, College Station, TX; 2000. [2] Hazizan MA, Cantwell WJ. The low velocity impact response of an aluminium honeycomb sandwich structure. Composites Part B: Engineering 2003;34: 679e87.
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