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On the impact response of sandwich composites with cores of balsa wood and PVC foam
Composite Structures 93 (2010) 40–48
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
On the impact response of sandwich composites with cores of balsa wood and PVC foam Cesim Atas a,*, Cenk Sevim b a b
_ Dokuz Eylül University, Department of Mechanical Engineering, 35100 Bornova-Izmir, Turkey _ AERO Wind Industry Inc., The Aegean Free Zone, 35410 Gaziemir-Izmir, Turkey
a r t i c l e
i n f o
Article history: Available online 26 June 2010 Keywords: Sandwich composite PVC foam Balsa wood Impact Damage
a b s t r a c t This paper presents an experimental investigation on impact response of sandwich composite panels with PVC foam core and balsa wood core. A number of tests were performed under various impact energies. Damage process of the sandwich composites is analyzed from cross-examining load–deflection curves, energy profile diagrams and the damaged specimens. The primary damage modes observed are; fiber fractures at upper and lower skins, delaminations between adjacent glass–epoxy layers, core shear fractures, and face/core debonding. After visual inspection of the top and bottom face-sheets, initial examination, damage mechanisms at the interior layers and cores were ascertained through destructive analysis, i.e. sectioning by an abrasive water-jet machine, of samples. In addition to the single impacts, repeated impact response of the samples is also investigated. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Structural sandwich composites have been widely used in many areas such as aircrafts, ship hulls, wind turbine blades, offshore oil platforms, bridge decks due to their superior structural capacity in carrying transverse loads, superior bending stiffness, low weight, and excellent thermal insulation and acoustic damping. They typically consist of surfacing plates (skins) and light-weight cores. The main duty of skins in a sandwich composite is carry the transverse load or bending moment while the core takes care of separating and fixing the skin, carrying the transverse shear load, and providing other structural or functional duties such as impact tolerance, radiation shielding, etc. [1]. One of the major concerns in the use of sandwich composites such in conventional polymer matrix composites is the impact induced damages which may occur during normal maintenance operations or during service conditions. Therefore, a number of studies in literature have been focused on the impact response of sandwich composites [2–11]. Among these, Hosur and co-authors [4] have presented a work on the manufacturing and low-velocity impact characterization of foam filled 3-D integrated core sandwich composites. Impact parameters like peak load, time to peak load, deflection at peak load, and absorbed energy were evaluated and compared for different types of hybrid face-sheets. Schubel et al. [6,7] have experimentally studied on the low-velocity impact and post impact behavior of composite sandwich panels consisting * Corresponding author. Tel.: +90 232 388 31 38; fax: +90 232 388 78 68. E-mail address: [email protected] (C. Atas). 0263-8223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2010.06.018
of woven carbon/epoxy face-sheets and a PVC foam core. Experimental results were compared with analytical and finite element model analysis to determine their effectiveness in predicting the indentation behavior of the sandwich panel. They have also compared the strength of the damaged and undamaged samples with each other and made useful discussions. Imielinska et al. [9] have investigated effect of manufacturing on impact damage behavior in E-glass/polyester–PVC foam core sandwich structures. Damage initiation and failure mechanisms were recorded by high-speed photography and related to the load–time plots. Ulven and Vaidya [10] have examined impact response of fire damaged E-glass/vinyl ester laminates and balsa wood core sandwich composites with Eglass/vinyl ester face-sheets. However, this topic still needs further investigations. Nowadays, among the most widely used cores, PVC foam and balsa wood have been utilized in manufacturing of wind turbine blades. So, in this study, low-velocity impact response of sandwich composite panels with PVC foam and balsa wood cores, and glass/ epoxy face-sheets has been examined. A number of tests were performed under various impact energies. As a matter of fact, the method, low-velocity impact, is not new. However, less work on impact response of sandwich composites is available in literature compared to the conventional laminated composites. The current paper presents a comparison between impact responses and damage mechanisms of sandwich composites with two different cores, balsa wood and PVC foam, which are being used for the similar purposes in the structures. Repeated impact response is also investigated. According to the authors’ knowledge, sufficient work is not available in the literature in this respect as well. The papers given
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in Refs. [12–16] are useful and guiding in understanding impact response of sandwich composites with mentioned cores. 2. Material preparation and impact testing Two different core materials were used in fabricating sandwich panels: PVC foam and balsa wood with densities of 62 kg/m3 and 157 kg/m3, respectively. With epoxy resin, the reinforcing material used in face-sheets was E-glass +45/45 biaxial stitch bonded noncrimp fabrics with an areal density of 780 g/m2. Both sandwich composites were manufactured by vacuum-assisted resin infusion process [17]. The stacking sequence of sandwich composites fabricated was [±45/core/±45]. Geometry of the core materials and fiber orientations of the biaxial E-glass fabric are given in Fig. 1. The nominal thicknesses of the samples were 11.5 mm and 10.30 mm for balsa and PVC foam core sandwiches, respectively. The impact tests were performed by using an instrumented drop weight testing system, CEAST-Fractovis Plus, which is suitable for a wide variety of applications requiring low to high impact energies. It was of an anti-rebound system to prevent multiple impacts on the specimen. With a hemispherical nose of 12.7 mm, capacity of the piezoelectric load cell used in tests was 22.4 kN. The total mass of the dropping weight was 5 kg. The specimens were fixed by a pneumatic fixture with a 76.2 mm hole diameter. 3. Experimental results and discussion PVC foam and balsa wood are among the most commonly used cores in sandwich composites, especially in wind turbine blades which needs to carry high bending loading. Along with single impact tests, in the present work, repeated impact response of this kind of sandwich panels is also investigated. The effect of the core type on the impact characteristics such as peak force and absorbed energy is examined. In former studies, it is shown that the damage process of composite plates can be constructed from comparing load–deflection curves, impact energy-absorbed energy diagrams and images of damaged specimens [18,19]. 3.1. Load–deflection curves and energy profile diagram A load–deflection curve in an impact event gives significant information about the damage process. The deflection in this study is defined as the distance that impactor travels during impact event. Namely, it indicates the deflection of the upper (impacted) surface of the samples during contact between sample and impactor. Based on the load–deflection curves, one can make an opinion about the damage formation in samples and the motion of the impactor during tests. There are two basic types, closed curve Fig. 2a and open curve Fig. 2c. A closed curve consists of an ascending section of loading and a descending section combining loading and unloading in general. Depending on the level of impact energy,
(c)
(b)
(a)
the descending section may have three different possibilities. It may be a pure rebounding curve representing the rebounding of the impactor from the specimen. It may contain partial softening of the specimen and partial rebounding of the impactor. If the descending section is completely a softening curve, the load– deflection curve may represent either penetration or perforation cases. As seen from the schematic illustration, Fig. 2b, in the penetration case, load value becomes zero at the end of softening section unlike complete perforation. Thus, in the study, besides to the visual observation, load–deflection curves were used to determine penetration cases. Load–deflection curves can also be utilized to carry out the picture of absorbed energy-impact energy variation, energy profile diagram. The energy profile diagrams of the sandwich composites with balsa wood and PVC foam are given in Fig. 3a and b. It shows the relation between absorbed energy (Ea) and impact energy (Ei) and is useful in understanding the overall energy absorption process. The absorbed energy can be determined from integrating the area bounded by the load–deflection curve and the deflection axis. The equal energy line between the impact energy and the absorbed energy is also shown on the diagram for comparison. Each data point in the diagrams represents the average values of the three tests repeated. At first sight, it is seen that all the data points are located right below the equal energy line, implying higher energy absorption due to the visible and invisible damages occurred in composite samples for all the impact energy levels. However, with a careful examination, the data points can be divided into five zones designated as A–E. From the test results it is noted that the energy lost during the impact process, like heat due to friction, vibration, etc. is negligible. Therefore, absorbed energy values especially at the penetration ranges represented by B and D in energy profile diagrams were obtained nearly equal to the impact energies, but not exactly the same. The difference is found as smaller than 1 J. So, the data points and equal energy line seem to be overlapped at these regions particularly. This is probably because the face-sheets are very thin and hence impact energies causing penetration or perforation are not high, resulting in smaller energy lost. The zoning of the energy profile seems to be assisting in description of the damage mechanisms during impacts. For both balsa and PVC core sandwich composites, these zones imply different damage mechanisms of the upper (top) face-sheets, core materials, lower (bottom) face-sheets and complete perforation of the composite stack. Analysis of the damage mechanisms, region to region, is given in the following section. However, briefly, it can be noted that zone A represents the impact energy levels resulting in minor fiber breakage and core damages. After these limited damages rebounding of the impactor from the impact surface takes place. In zone B, the impactor sticks into the upper face-sheet and core. In zone C, the impactor reaches to the bottom face-sheet causing limited damage and then rebounds from this face-sheet.
40 mm
25 mm
80 mm
45° 0°
50 mm
45°
Fig. 1. Geometry of the core materials; PVC foam (a), balsa wood (b), and fiber orientations of the biaxial E-glass fabric (c).
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(b)
(c)
penetration
Contact Force (kN)
Contact Force (kN)
rebounding
Contact Force (kN)
(a)
Deflection (mm)
Deflection (mm)
complete perforation
due to friction between impactor and sample
Deflection (mm)
Fig. 2. Typical load deflection curves in a low-velocity impact event.
(a)
70
60
Absorbed Energy (J)
to the difference between thicknesses, around 11.6% as mentioned earlier, of the two composites as well as different damage processes of the cores and face-sheets.
Equal energy line Balsa Wood
50
3.2. Damage process
40
Damage process of the sandwich composites may be reconstructed from cross-examining the corresponding load–deflection curves, the energy profile and the damaged specimens. The primary damage modes observed are; fiber fractures at top (impacted) and bottom (non-impacted) skins, delaminations between adjacent glass–epoxy layers, core shear fractures and face/core debonding. The failure processes in the impact-loaded sandwich composites were initially investigated by examining the front and rear surfaces of the damaged samples. After initial examination, damage mechanisms at the interior layers and cores were ascertained through destructive analysis, i.e. sectioning, of samples. In the following, load–deflection curves and images of the damaged samples for certain impact energy levels, corresponding to some data points given in Fig. 3a and b, are given for comparison and discussion. Fig. 4a–e shows load–deflection curves of the sandwich composites subjected to the impact energies of the 5 J, 15 J, 30 J, 45 J and 75 J, respectively. And, the corresponding images are given in Figs. 5–9. For all impact energies, it seems that the bending stiffness, the slope of the ascending section in load–deflection curves, of balsa core sandwich samples is higher than that of the PVC foam core samples. The engineering stress–strain curves under individual quasi-static compression loading for balsa wood and PVC foam used in the present study are given in Fig. 10. This figure includes significant tips to justify the discussions made below. The load–deflection curves for impact energies of 5 J and 15 J are given in Fig. 4a and b. The corresponding images of damaged samples are given in Figs. 5 and 6. Minor fiber fractures and matrix cracks were observed at impacted (top) face-sheets under impact energy of 5 J. As the impact energy is increased, e.g. to 15 J, delamination between ±45 glass/epoxy layers at top surfaces became clearer, especially in foam core samples. At the non-impacted (bottom) face-sheets of balsa core samples, face/core debonding and matrix cracks were observed. The total damage area at bottom surface increases with increase of impact energy as expected. However, no damage was observed at bottom face-sheets of foam core samples up to the end of region A given in energy profile diagram. Therefore, in Fig. 6, images of the bottom face-sheets for foam core sandwiches are not given. Cross-sectional view through centerline of the samples is also given in the figures. However, the scale of the cross-sectional view is larger, approximately 50%, than that of top and bottom face-sheets, for a better inspection. When the impact energy increased from zones A to B, top facesheets was perforated through fracture of the glass-fibers, resulting
30
20
B
A
10
D
C
E
0 0
10
20
30
40
50
60
70
80
Impact Energy (J)
(b)
70
Equal energy line 60
Absorbed Energy (J)
PVC foam 50
40
30 20
B
A
C
D
E
10 0 0
10
20
30
40
50
60
70
80
Impact Energy (J) Fig. 3. The energy profile diagrams of balsa wood core (a) and PVC foam core (b) composites.
As the impact energy is increased, impactor sticks into the bottom face-sheet, zone D. Finally, once the impact energy is further increased the perforation of the composite takes place, zone E. It seems that there are two penetration ranges [18], B and D, for either sandwich composite. However, it also seems that range D of balsa core sandwich composite is larger than that of the PVC foam implying a larger perforation threshold. This is likely owing
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(b)
(a)
foam
Displacement (mm)
Ei=15 J
foam
Displacement (mm)
(d)
(c)
balsa
Ei=30 J
balsa
Ei=45 J
Force (kN)
F o rce (k N )
balsa
Ei=5 J
Force (kN)
Force (kN)
balsa
foam
Displacement (mm)
foam
Displacement (mm)
(e) Force (kN)
balsa
Ei=75 J
foam
Displacement (mm) Fig. 4. Load–deflection curves of the sandwich composites for various impact energies.
in the penetration of the impactor into specimens. Fig. 4c shows the load–deflection curves of specimens subjected to the impact energy of 30 J corresponding to the beginning of region B. As seen in the figure, curves do not return back from the maximum deflection value implying no rebounding but penetration of the impactor as explained earlier. The images of the damaged samples for 30 J are given in Fig. 7. It seems that samples experience the first catastrophic damage from the beginning of zone B. In foam core sandwich, the delamination between ±45 glass/epoxy layers of top facesheet becomes clearer. As expected, the delamination is oriented in the fiber direction of the second layer (45°) from the top. However, in balsa core sandwich, the damage area seems to be concentrated around the impacted zone at top face-sheet. Along with Fig. 10, load–deflection curves of the impacted samples gives important information about the mentioned damage mechanisms at top face-sheets. As seen in Fig. 10, when compressed along the cell walls of the balsa wood [13], there is first a linear elastic response until point of the initial localized failure, buckling of the tubular cells, at the weakest sites accompanied by a drop in stress level. Then, progressive deformation occurs under almost a constant stress until densification regime that corresponds to the com-
pression of cell wall material itself, resulting in a steep increase in stress. However, the curve of the PVC foam increases in a nonlinear fashion up to the initial failure point and remains nearly constant horizontally until densification regime. It is seen that the failure stress of balsa wood is almost 20 times larger than that of PVC foam. That is, it seems that the balsa core samples are stiffer than PVC ones. The ascending sections of the load–deflection curves, indicated earlier, are also in good agreement with this conclusion. It results in higher peak force value but smaller deflection for balsa wood compared to PVC foam, for smaller energy levels such as 5 and 15 J in region A. Consequently, the higher deformation capability of PVC foam causes a larger delamination area formed between layers of top face-sheets due to stiffness mismatching. Therefore, on the contrary, the impact energy leads to the local damage formation rather than the delamination in balsa wood sandwiches. To the bottom side, the delamination area seems to be small for PVC foam. As indicated in the subsequent paragraphs, when the impact energy is increased, the delamination area gets larger to some extent followed by the fiber breakage, the dominant damage mode for higher impact energies at back face-sheets, due to bending. However, for balsa core sandwich, due to a poor interface
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C. Atas, C. Sevim / Composite Structures 93 (2010) 40–48
Ei= 5 J
10 mm
10 mm
Ei= 5 J
a 10 mm
10 mm
Ei= 15 Ja
delamination
matrix cracks and debonding
10 mm
Ei= 15 J
a–a view
a
Fig. 5. Impact induced damage in balsa core sandwich composites under impact energies of 5 J and 15 J.
a Ei= 5 J
10 mm
Ei= 15 J
10 mm
delamination
10 mm
fiber fractures
a
a–a view
Fig. 6. Impact induced damage in foam core sandwich composites under impact energies of 5 J and 15 J.
between balsa wood with the glass/epoxy layer compared with PVC foam core; the debonding between core and face-sheets becomes the dominant damage mode along with the fiber bending fractures as the impact energy increases. The circular whitish area at back surfaces hence represents the interface debonding to a large extent. As the impact energy increased from zones B to C, following the first penetration range, the rebounding of the impactor from the
bottom face-sheets was experienced. Fig. 4d shows the load– deflection curves of specimens subjected to the impact energy of 45 J, in region C. The images of the damaged samples for 45 J are given in Fig. 8. From Fig. 4d, it is obvious that the curves are of a mountain-like shape with two peaks, implying the contact of impactor with top and bottom glass/epoxy face-sheets, respectively. From the test results, it is noted that the second peak value for foam core sandwich samples is higher than first peak value
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Ei= 30 J
10 mm
Ei= 30 J
10 mm
fiber fractures
interface debonding
Ei= 30 J
10 mm
Ei= 30 J
10 mm
delamination fiber fractures
10 mm
Ei= 45 J
10 mm
Ei= 45 J
10 mm
skin/core debonding
Ei= 45 J
In-plane shear fracture of balsa
Fig. 7. Impact induced damage in balsa and foam core sandwich composites under impact energy of 30 J.
10 mm
In-plane shear fracture of foam
Ei= 45 J
Fig. 8. Impact induced damage in balsa and foam core sandwich composites under impact energy of 45 J.
10 mm
Ei= 75 J
Ei= 75 J
Ei= 75 J
10 mm
Ei= 75 J
10 mm
skin/core debonding
C. Atas, C. Sevim / Composite Structures 93 (2010) 40–48
transverse shear fracture
46
10 mm
Fig. 9. Impact induced damage in balsa and foam core sandwich composites under impact energy of 75 J.
16 Balsa wood
Stress (MPa)
14 12 10 8
Point of failure Point of densification
6
PVC Foam
4 2 0 0
0,1
0,2 0,3
0,4
0,5
0,6
0,7 0,8
0,9
1
Stroke Strain (mm/mm) Fig. 10. Engineering stress–strain curves for balsa wood and PVC foam used, in compression.
generally while those of the balsa core samples are of the nearly the same values. On the other hand, in two-peaked curves, it is also noted in general that the first peak load value of foam core samples were smaller than that of balsa core samples as in the lower impact energy cases result in one-peaked curves. The probable reason is explained in the preceding paragraph. However, to the second peak load value, it is found to be generally higher, sometimes nearly equal, for foam core samples compared to the balsa based ones. Probably one of the reason is the deformation characteristic of the foam core under compressive loading. That is, compressed foam core increases stiffness of the bottom face-sheets in comparison with balsa wood having lower stiffness after first failure due to buckling of tubular cells. In other words, along with the bottom face-sheets, the stiffness of the core at the end of compression also plays a significant role in the second peak load. The images of the damaged samples are given in Fig. 8. Cross-sectional views show
the core damages which are not visible in images of top and bottom surfaces. If the impact energy is increased to zone D the second penetration range in energy profile diagrams was experienced. Beyond that region, in region E, perforation of the samples takes place. Fig. 4e shows the load–deflection curves of specimens subjected to the impact energy of 75 J. It is seen that both curves are of open type, implying perforation and no rebounding of the impactor from samples. The images of the perforated samples subjected to impact energy of 75 J are given in Fig. 9. From the figures, perforation of the bottom face-sheets occurs rather than the in-plane shear fractures as observed in regions C and D. Besides to the bending fracture of the fibers resulting perforation of the bottom face-sheet, as seen in the cross-sectional view, balsa core samples experienced core/skin deboning as well as the transverse shear fracture of core. 3.3. Repeated impact response In contrast to the most commonly used single impact tests, little work like Refs. [20–23] has been reported on repeated impact response of composite plates. However, there are many realistic cases that repeated impacts are of high importance. Therefore, repeated impact response of the above mentioned composites were also examined experimentally. Fig. 11 gives the variation of the ‘‘repeat number of impact to perforation” (Rn) versus impact energy for balsa and PVC foam core sandwiches. For a better understanding, the Rn values of balsa and foam core samples versus impact energy are also provided in the figure as a table. These values stand for the number of experiments conducted for each impact energy level. It is seen that both kind of samples are of nearly the same number of impact resulting in the damage (perforation) for all impact energy levels except 5 J. For this impact energy, Ei = 5 J, Rn seems to be higher, about 15%, for foam core samples compared to the balsa ones. This difference would increase once the impact energy is decreased.
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11 Impact Energy (Joule) 10 15 20 25 30 40 60
5
50
Balsa 346 46 6 Foam 400 45 8
7 5
4 3
3 2
2 2
Balsa Wood
1 1
40 30 PVC Foam
20
PVC Foam
10.5
Absorbed Energy (J)
60 Rn
Impact Energy (J)
70
Balsa Wood
10 9.5 9 8.5 8
10
7.5 0 0
50
100 150 200 250 300 350 400 450
7 0
Repeat Number (Rn)
5
10
15
20
25
30
35
40
45
50
Repeat Number (Rn) Fig. 11. Variation of the ‘‘repeat number of impact to perforation” (Rn) versus varied impact energies.
As indicated earlier, balsa and foam core composites are not of the same damage mechanisms when subjected to impact loading. This case may be observed from the repeated impact tests as well. The variations of the peak contact force (Fc) and absorbed energy (Ea) versus repeat number are respectively given in Figs. 12 and 13, for impact energy of 10 J. From the figures, it is seen that up to the approximately Rn = 12, the Fc and Ea show opposite variation characteristics for balsa and foam cores. Beyond that they reach a quite constant value, especially Fc, and then through the approximately last five impacts they show again opposite variations. In that region a sudden decrease can be observed in Fc while Ea increases, implying the perforation of the samples. From the load– deflection curves, it is also noted that the stiffness of the samples decreased with Rn as expected. In Fig. 13, for some data points, it seems that the absorbed energy is higher than the impact energy value of 10 J, implying a contradiction. However, it should be noted that for exact or true impact energy value, the potential energy corresponding deflection of the plate is also to be considered. It is noted that the potential energy due to the deflection of the samples is around 0.7 J herein. On the other hand, the Rn–Ei curves given in Fig. 11 may be represented by equations:
Ei ¼ 43:5 R0:3858 with R2 ¼ 0:9361; for PVC foam: n Ei ¼ 46:7 R0:4053 with R2 ¼ 0:9379; for balsa wood: n
6
Peak Contact Force (kN)
PVC Foam energies. 5
Ei=10 J
Balsa Wood
4 3 2 1 0 0
5
10
15
20
25
30
35
40
45
50
Repeat Number (Rn) Fig. 12. Variation of the ‘‘peak contact force” versus ‘‘repeat number” of impacts to perforation for impact energy of 10 J.
Fig. 13. Variation of the ‘‘absorbed energy” versus ‘‘repeat number” of impacts to perforation for impact energy of 10 J.
Without testing, they enable to predict the number of impacts to perforation (Rn) under smaller impact energies. For example; for Ei = 2 J the Rn would be equal to 2930 and 2376 for PVC foam and balsa wood, respectively, while they would be 17,660 and 13,144 under impact energy of 1 J. 4. Conclusions This paper presents an experimental investigation on dropweight impact response of sandwich composite panels with PVC foam core and balsa wood core. From cross-examining load–deflection curves, energy profile and damaged specimens, damage processes are examined. The primary damage modes are found to be fiber fractures at top and bottom face-sheets, delaminations between adjacent glass–epoxy layers, transverse and in-plane shear fractures of core, and face/core debonding. Cross-sectional view of the damaged samples enabled to see the interior damage modes and damage extent. Other conclusions drawn from the work can be summarized as: Load–deflection curves of the both composites were of a mountain-like shape with two peaks under higher impact energies. It is noted that the second peak value for foam core sandwich samples is higher than first peak value generally while those of the balsa core samples are of the nearly the same values. On the other hand, in two-peaked curves, it is also noted that the first peak load value of foam core samples were smaller than that of balsa core samples as in the one-peaked curves. On the contrary, the second peak load value of foam core samples is found to be higher generally than that of balsa core ones. It is also seen from the load–deflection curves that the foam core sandwich is of lower bending stiffness. Based on Fig. 10, it is noted that balsa core samples are stiffer than PVC ones. It results in higher contact force but smaller deflection of the balsa wood sandwiches for smaller impact energy levels, before point of failure. This case implies a higher deformation capability of PVC foam samples, resulting in a larger delamination area formed between layers of top face-sheets due to stiffness mismatching. On the contrary, these impact energies lead to the local damage formation rather than the delamination in balsa wood sandwiches. At the non-impacted side, the delamination area seems to be small for PVC foam. When the impact energy is increased, the delamination area gets larger to some extent followed by the fiber breakage, the dominant damage mode for higher impact
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energies at back face-sheets, due to bending. However, for balsa core sandwich, due to a poor interface between balsa wood with the glass/epoxy layer compared with PVC foam core; the debonding between core and face-sheets becomes the dominant damage mode along with the fiber bending fractures as the impact energy increases. The circular whitish area at back surfaces hence represents the interface debonding to a large extent rather than the delaminations in face-sheets. The core damages due to the shear effects, another dominant damage mode, can also be observed from the cross-sectional views. For higher impact energy values, it is found that both sandwich composites are of nearly the same Rn, repeat number of impact to perforation. On the other hand, as the impact energy is decreased, the Rn increases for foam core samples compared to balsa wood composites.
Acknowledgements The authors gratefully acknowledge the help provided by AERO Wind Industry Inc. during this work and by an abrasive water jet cutting firm, TÜMER, in cutting damaged samples. References [1] Li G, Jones N. Development of rubberized syntactic foam. Composites: Part A 2007;38:1483–92. [2] Christopherson J, Mahinfalah M, Jazar GN, Aagaah MR. An investigation on the effect of a small mass impact on sandwich composite plates. Compos Struct 2005;67:299–306. [3] Gustin J, Joneson A, Mahinfalah M, Stone J. Low velocity impact of combination Kevlar/carbon fiber sandwich composites. Compos Struct 2005;69:396–406. [4] Hosur MV, Abdullah M, Jeelani S. Manufacturing and low-velocity impact characterization of foam filled 3-D integrated core sandwich composites with hybrid face sheets. Compos Struct 2005;69:167–81. [5] Ulven CA, Vaidya UK. Post-fire low velocity impact response of marine grade sandwich composites. Composites: Part A 2006;37:997–1004.
[6] Schubel PM, Luo J-J, Daniel IM. Low velocity impact behavior of composite sandwich panels. Composites: Part A 2005;36:1389–98. [7] Schubel PM, Luo J-J, Daniel IM. Impact and post impact behavior of composite sandwich panels. Composites: Part A 2007;38:1051–7. [8] Hosur MV, Mohammed AA, Jeelani S. Processing of nanoclay filled sandwich composites and their response to impact loading. J Reinf Plast Compos 2008;27:797–818. [9] Imielinska K, Guillaumat L, Wojtyra R, Castaings M. Effects of manufacturing and face/core bonding on impact damage in glass/polyester–PVC foam core sandwich panels. Composites: Part B 2008;39:1034–41. [10] Ulven CA, Vaidya UK. Impact response of fire damaged polymer-based composite materials. Composites: Part B 2008;39:92–107. [11] Xia F, Wu X. Work on low-velocity impact properties of foam sandwich composites with various face sheets. J Reinf Plast Compos. doi: 10.1177/ 0731684409102749 [12] Vural M, Ravichandran G. Dynamic response and energy dissipation characteristics of balsa wood: experiment and analysis. Int J Solids Struct 2003;40:2147–70. [13] Vural M, Ravichandran G. Failure mode transition and energy dissipation in naturally occurring composites. Composites: Part B 2004;35:639–46. [14] Truxel A, Avilés F, Carlsson LA, Grenestedt JL, Millay K. Influence of face/core interface on debond toughness of foam and balsa cored sandwich. J Sandwich Struct Mater 2006;8:237–58. [15] Mahfuz H, Thomas T, Rangari V, Jeelani S. On the dynamic response of sandwich composites and their core materials. Compos Sci Technol 2006;66:2465–72. [16] Tagarielli VL, Deshpande VS, Fleck NA. The high strain rate response of PVC foams and end-grain balsa wood. Composites: Part B 2008;39:83–91. [17] Goren A, Atas C. Manufacturing of polymer matrix composites using vacuum assisted resin infusion molding. Arch Mater Sci Eng 2008;34:117–20. [18] Atas C, Liu D. Impact response of woven composites with small weaving angles. Int J Impact Eng 2008;35:80–97. [19] Atas C, Sayman O. An overall view on impact response of woven fabric composite plates. Compos Struct 2008;82:336–45. [20] Hosur MV, Karim MR, Jeelani S. Experimental investigations on the response of stitched/unstitched woven S2-glass/SC15 epoxy composites under single and repeated low velocity impact loading. Compos Struct 2003;61:89–102. [21] David-West OS, Nash DH, Banks WM. An experimental study of damage accumulation in balanced CFRP laminates due to repeated impact. Compos Struct 2008;83:247–58. [22] Chakraborty D, Kumar M. Response of laminated FRP composites under multiple impact loading. J Reinf Plast Compos 2005;24:1457–77. [23] Baucom JN, Zikry MA, Rajendran AM. Low-velocity impact damage accumulation in woven S2-glass composite systems. Compos Sci Technol 2006;66:1229–38.
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
On the impact response of sandwich composites with cores of balsa wood and PVC foam Cesim Atas a,*, Cenk Sevim b a b
_ Dokuz Eylül University, Department of Mechanical Engineering, 35100 Bornova-Izmir, Turkey _ AERO Wind Industry Inc., The Aegean Free Zone, 35410 Gaziemir-Izmir, Turkey
a r t i c l e
i n f o
Article history: Available online 26 June 2010 Keywords: Sandwich composite PVC foam Balsa wood Impact Damage
a b s t r a c t This paper presents an experimental investigation on impact response of sandwich composite panels with PVC foam core and balsa wood core. A number of tests were performed under various impact energies. Damage process of the sandwich composites is analyzed from cross-examining load–deflection curves, energy profile diagrams and the damaged specimens. The primary damage modes observed are; fiber fractures at upper and lower skins, delaminations between adjacent glass–epoxy layers, core shear fractures, and face/core debonding. After visual inspection of the top and bottom face-sheets, initial examination, damage mechanisms at the interior layers and cores were ascertained through destructive analysis, i.e. sectioning by an abrasive water-jet machine, of samples. In addition to the single impacts, repeated impact response of the samples is also investigated. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Structural sandwich composites have been widely used in many areas such as aircrafts, ship hulls, wind turbine blades, offshore oil platforms, bridge decks due to their superior structural capacity in carrying transverse loads, superior bending stiffness, low weight, and excellent thermal insulation and acoustic damping. They typically consist of surfacing plates (skins) and light-weight cores. The main duty of skins in a sandwich composite is carry the transverse load or bending moment while the core takes care of separating and fixing the skin, carrying the transverse shear load, and providing other structural or functional duties such as impact tolerance, radiation shielding, etc. [1]. One of the major concerns in the use of sandwich composites such in conventional polymer matrix composites is the impact induced damages which may occur during normal maintenance operations or during service conditions. Therefore, a number of studies in literature have been focused on the impact response of sandwich composites [2–11]. Among these, Hosur and co-authors [4] have presented a work on the manufacturing and low-velocity impact characterization of foam filled 3-D integrated core sandwich composites. Impact parameters like peak load, time to peak load, deflection at peak load, and absorbed energy were evaluated and compared for different types of hybrid face-sheets. Schubel et al. [6,7] have experimentally studied on the low-velocity impact and post impact behavior of composite sandwich panels consisting * Corresponding author. Tel.: +90 232 388 31 38; fax: +90 232 388 78 68. E-mail address: [email protected] (C. Atas). 0263-8223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2010.06.018
of woven carbon/epoxy face-sheets and a PVC foam core. Experimental results were compared with analytical and finite element model analysis to determine their effectiveness in predicting the indentation behavior of the sandwich panel. They have also compared the strength of the damaged and undamaged samples with each other and made useful discussions. Imielinska et al. [9] have investigated effect of manufacturing on impact damage behavior in E-glass/polyester–PVC foam core sandwich structures. Damage initiation and failure mechanisms were recorded by high-speed photography and related to the load–time plots. Ulven and Vaidya [10] have examined impact response of fire damaged E-glass/vinyl ester laminates and balsa wood core sandwich composites with Eglass/vinyl ester face-sheets. However, this topic still needs further investigations. Nowadays, among the most widely used cores, PVC foam and balsa wood have been utilized in manufacturing of wind turbine blades. So, in this study, low-velocity impact response of sandwich composite panels with PVC foam and balsa wood cores, and glass/ epoxy face-sheets has been examined. A number of tests were performed under various impact energies. As a matter of fact, the method, low-velocity impact, is not new. However, less work on impact response of sandwich composites is available in literature compared to the conventional laminated composites. The current paper presents a comparison between impact responses and damage mechanisms of sandwich composites with two different cores, balsa wood and PVC foam, which are being used for the similar purposes in the structures. Repeated impact response is also investigated. According to the authors’ knowledge, sufficient work is not available in the literature in this respect as well. The papers given
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in Refs. [12–16] are useful and guiding in understanding impact response of sandwich composites with mentioned cores. 2. Material preparation and impact testing Two different core materials were used in fabricating sandwich panels: PVC foam and balsa wood with densities of 62 kg/m3 and 157 kg/m3, respectively. With epoxy resin, the reinforcing material used in face-sheets was E-glass +45/45 biaxial stitch bonded noncrimp fabrics with an areal density of 780 g/m2. Both sandwich composites were manufactured by vacuum-assisted resin infusion process [17]. The stacking sequence of sandwich composites fabricated was [±45/core/±45]. Geometry of the core materials and fiber orientations of the biaxial E-glass fabric are given in Fig. 1. The nominal thicknesses of the samples were 11.5 mm and 10.30 mm for balsa and PVC foam core sandwiches, respectively. The impact tests were performed by using an instrumented drop weight testing system, CEAST-Fractovis Plus, which is suitable for a wide variety of applications requiring low to high impact energies. It was of an anti-rebound system to prevent multiple impacts on the specimen. With a hemispherical nose of 12.7 mm, capacity of the piezoelectric load cell used in tests was 22.4 kN. The total mass of the dropping weight was 5 kg. The specimens were fixed by a pneumatic fixture with a 76.2 mm hole diameter. 3. Experimental results and discussion PVC foam and balsa wood are among the most commonly used cores in sandwich composites, especially in wind turbine blades which needs to carry high bending loading. Along with single impact tests, in the present work, repeated impact response of this kind of sandwich panels is also investigated. The effect of the core type on the impact characteristics such as peak force and absorbed energy is examined. In former studies, it is shown that the damage process of composite plates can be constructed from comparing load–deflection curves, impact energy-absorbed energy diagrams and images of damaged specimens [18,19]. 3.1. Load–deflection curves and energy profile diagram A load–deflection curve in an impact event gives significant information about the damage process. The deflection in this study is defined as the distance that impactor travels during impact event. Namely, it indicates the deflection of the upper (impacted) surface of the samples during contact between sample and impactor. Based on the load–deflection curves, one can make an opinion about the damage formation in samples and the motion of the impactor during tests. There are two basic types, closed curve Fig. 2a and open curve Fig. 2c. A closed curve consists of an ascending section of loading and a descending section combining loading and unloading in general. Depending on the level of impact energy,
(c)
(b)
(a)
the descending section may have three different possibilities. It may be a pure rebounding curve representing the rebounding of the impactor from the specimen. It may contain partial softening of the specimen and partial rebounding of the impactor. If the descending section is completely a softening curve, the load– deflection curve may represent either penetration or perforation cases. As seen from the schematic illustration, Fig. 2b, in the penetration case, load value becomes zero at the end of softening section unlike complete perforation. Thus, in the study, besides to the visual observation, load–deflection curves were used to determine penetration cases. Load–deflection curves can also be utilized to carry out the picture of absorbed energy-impact energy variation, energy profile diagram. The energy profile diagrams of the sandwich composites with balsa wood and PVC foam are given in Fig. 3a and b. It shows the relation between absorbed energy (Ea) and impact energy (Ei) and is useful in understanding the overall energy absorption process. The absorbed energy can be determined from integrating the area bounded by the load–deflection curve and the deflection axis. The equal energy line between the impact energy and the absorbed energy is also shown on the diagram for comparison. Each data point in the diagrams represents the average values of the three tests repeated. At first sight, it is seen that all the data points are located right below the equal energy line, implying higher energy absorption due to the visible and invisible damages occurred in composite samples for all the impact energy levels. However, with a careful examination, the data points can be divided into five zones designated as A–E. From the test results it is noted that the energy lost during the impact process, like heat due to friction, vibration, etc. is negligible. Therefore, absorbed energy values especially at the penetration ranges represented by B and D in energy profile diagrams were obtained nearly equal to the impact energies, but not exactly the same. The difference is found as smaller than 1 J. So, the data points and equal energy line seem to be overlapped at these regions particularly. This is probably because the face-sheets are very thin and hence impact energies causing penetration or perforation are not high, resulting in smaller energy lost. The zoning of the energy profile seems to be assisting in description of the damage mechanisms during impacts. For both balsa and PVC core sandwich composites, these zones imply different damage mechanisms of the upper (top) face-sheets, core materials, lower (bottom) face-sheets and complete perforation of the composite stack. Analysis of the damage mechanisms, region to region, is given in the following section. However, briefly, it can be noted that zone A represents the impact energy levels resulting in minor fiber breakage and core damages. After these limited damages rebounding of the impactor from the impact surface takes place. In zone B, the impactor sticks into the upper face-sheet and core. In zone C, the impactor reaches to the bottom face-sheet causing limited damage and then rebounds from this face-sheet.
40 mm
25 mm
80 mm
45° 0°
50 mm
45°
Fig. 1. Geometry of the core materials; PVC foam (a), balsa wood (b), and fiber orientations of the biaxial E-glass fabric (c).
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(b)
(c)
penetration
Contact Force (kN)
Contact Force (kN)
rebounding
Contact Force (kN)
(a)
Deflection (mm)
Deflection (mm)
complete perforation
due to friction between impactor and sample
Deflection (mm)
Fig. 2. Typical load deflection curves in a low-velocity impact event.
(a)
70
60
Absorbed Energy (J)
to the difference between thicknesses, around 11.6% as mentioned earlier, of the two composites as well as different damage processes of the cores and face-sheets.
Equal energy line Balsa Wood
50
3.2. Damage process
40
Damage process of the sandwich composites may be reconstructed from cross-examining the corresponding load–deflection curves, the energy profile and the damaged specimens. The primary damage modes observed are; fiber fractures at top (impacted) and bottom (non-impacted) skins, delaminations between adjacent glass–epoxy layers, core shear fractures and face/core debonding. The failure processes in the impact-loaded sandwich composites were initially investigated by examining the front and rear surfaces of the damaged samples. After initial examination, damage mechanisms at the interior layers and cores were ascertained through destructive analysis, i.e. sectioning, of samples. In the following, load–deflection curves and images of the damaged samples for certain impact energy levels, corresponding to some data points given in Fig. 3a and b, are given for comparison and discussion. Fig. 4a–e shows load–deflection curves of the sandwich composites subjected to the impact energies of the 5 J, 15 J, 30 J, 45 J and 75 J, respectively. And, the corresponding images are given in Figs. 5–9. For all impact energies, it seems that the bending stiffness, the slope of the ascending section in load–deflection curves, of balsa core sandwich samples is higher than that of the PVC foam core samples. The engineering stress–strain curves under individual quasi-static compression loading for balsa wood and PVC foam used in the present study are given in Fig. 10. This figure includes significant tips to justify the discussions made below. The load–deflection curves for impact energies of 5 J and 15 J are given in Fig. 4a and b. The corresponding images of damaged samples are given in Figs. 5 and 6. Minor fiber fractures and matrix cracks were observed at impacted (top) face-sheets under impact energy of 5 J. As the impact energy is increased, e.g. to 15 J, delamination between ±45 glass/epoxy layers at top surfaces became clearer, especially in foam core samples. At the non-impacted (bottom) face-sheets of balsa core samples, face/core debonding and matrix cracks were observed. The total damage area at bottom surface increases with increase of impact energy as expected. However, no damage was observed at bottom face-sheets of foam core samples up to the end of region A given in energy profile diagram. Therefore, in Fig. 6, images of the bottom face-sheets for foam core sandwiches are not given. Cross-sectional view through centerline of the samples is also given in the figures. However, the scale of the cross-sectional view is larger, approximately 50%, than that of top and bottom face-sheets, for a better inspection. When the impact energy increased from zones A to B, top facesheets was perforated through fracture of the glass-fibers, resulting
30
20
B
A
10
D
C
E
0 0
10
20
30
40
50
60
70
80
Impact Energy (J)
(b)
70
Equal energy line 60
Absorbed Energy (J)
PVC foam 50
40
30 20
B
A
C
D
E
10 0 0
10
20
30
40
50
60
70
80
Impact Energy (J) Fig. 3. The energy profile diagrams of balsa wood core (a) and PVC foam core (b) composites.
As the impact energy is increased, impactor sticks into the bottom face-sheet, zone D. Finally, once the impact energy is further increased the perforation of the composite takes place, zone E. It seems that there are two penetration ranges [18], B and D, for either sandwich composite. However, it also seems that range D of balsa core sandwich composite is larger than that of the PVC foam implying a larger perforation threshold. This is likely owing
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C. Atas, C. Sevim / Composite Structures 93 (2010) 40–48
(b)
(a)
foam
Displacement (mm)
Ei=15 J
foam
Displacement (mm)
(d)
(c)
balsa
Ei=30 J
balsa
Ei=45 J
Force (kN)
F o rce (k N )
balsa
Ei=5 J
Force (kN)
Force (kN)
balsa
foam
Displacement (mm)
foam
Displacement (mm)
(e) Force (kN)
balsa
Ei=75 J
foam
Displacement (mm) Fig. 4. Load–deflection curves of the sandwich composites for various impact energies.
in the penetration of the impactor into specimens. Fig. 4c shows the load–deflection curves of specimens subjected to the impact energy of 30 J corresponding to the beginning of region B. As seen in the figure, curves do not return back from the maximum deflection value implying no rebounding but penetration of the impactor as explained earlier. The images of the damaged samples for 30 J are given in Fig. 7. It seems that samples experience the first catastrophic damage from the beginning of zone B. In foam core sandwich, the delamination between ±45 glass/epoxy layers of top facesheet becomes clearer. As expected, the delamination is oriented in the fiber direction of the second layer (45°) from the top. However, in balsa core sandwich, the damage area seems to be concentrated around the impacted zone at top face-sheet. Along with Fig. 10, load–deflection curves of the impacted samples gives important information about the mentioned damage mechanisms at top face-sheets. As seen in Fig. 10, when compressed along the cell walls of the balsa wood [13], there is first a linear elastic response until point of the initial localized failure, buckling of the tubular cells, at the weakest sites accompanied by a drop in stress level. Then, progressive deformation occurs under almost a constant stress until densification regime that corresponds to the com-
pression of cell wall material itself, resulting in a steep increase in stress. However, the curve of the PVC foam increases in a nonlinear fashion up to the initial failure point and remains nearly constant horizontally until densification regime. It is seen that the failure stress of balsa wood is almost 20 times larger than that of PVC foam. That is, it seems that the balsa core samples are stiffer than PVC ones. The ascending sections of the load–deflection curves, indicated earlier, are also in good agreement with this conclusion. It results in higher peak force value but smaller deflection for balsa wood compared to PVC foam, for smaller energy levels such as 5 and 15 J in region A. Consequently, the higher deformation capability of PVC foam causes a larger delamination area formed between layers of top face-sheets due to stiffness mismatching. Therefore, on the contrary, the impact energy leads to the local damage formation rather than the delamination in balsa wood sandwiches. To the bottom side, the delamination area seems to be small for PVC foam. As indicated in the subsequent paragraphs, when the impact energy is increased, the delamination area gets larger to some extent followed by the fiber breakage, the dominant damage mode for higher impact energies at back face-sheets, due to bending. However, for balsa core sandwich, due to a poor interface
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C. Atas, C. Sevim / Composite Structures 93 (2010) 40–48
Ei= 5 J
10 mm
10 mm
Ei= 5 J
a 10 mm
10 mm
Ei= 15 Ja
delamination
matrix cracks and debonding
10 mm
Ei= 15 J
a–a view
a
Fig. 5. Impact induced damage in balsa core sandwich composites under impact energies of 5 J and 15 J.
a Ei= 5 J
10 mm
Ei= 15 J
10 mm
delamination
10 mm
fiber fractures
a
a–a view
Fig. 6. Impact induced damage in foam core sandwich composites under impact energies of 5 J and 15 J.
between balsa wood with the glass/epoxy layer compared with PVC foam core; the debonding between core and face-sheets becomes the dominant damage mode along with the fiber bending fractures as the impact energy increases. The circular whitish area at back surfaces hence represents the interface debonding to a large extent. As the impact energy increased from zones B to C, following the first penetration range, the rebounding of the impactor from the
bottom face-sheets was experienced. Fig. 4d shows the load– deflection curves of specimens subjected to the impact energy of 45 J, in region C. The images of the damaged samples for 45 J are given in Fig. 8. From Fig. 4d, it is obvious that the curves are of a mountain-like shape with two peaks, implying the contact of impactor with top and bottom glass/epoxy face-sheets, respectively. From the test results, it is noted that the second peak value for foam core sandwich samples is higher than first peak value
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C. Atas, C. Sevim / Composite Structures 93 (2010) 40–48
Ei= 30 J
10 mm
Ei= 30 J
10 mm
fiber fractures
interface debonding
Ei= 30 J
10 mm
Ei= 30 J
10 mm
delamination fiber fractures
10 mm
Ei= 45 J
10 mm
Ei= 45 J
10 mm
skin/core debonding
Ei= 45 J
In-plane shear fracture of balsa
Fig. 7. Impact induced damage in balsa and foam core sandwich composites under impact energy of 30 J.
10 mm
In-plane shear fracture of foam
Ei= 45 J
Fig. 8. Impact induced damage in balsa and foam core sandwich composites under impact energy of 45 J.
10 mm
Ei= 75 J
Ei= 75 J
Ei= 75 J
10 mm
Ei= 75 J
10 mm
skin/core debonding
C. Atas, C. Sevim / Composite Structures 93 (2010) 40–48
transverse shear fracture
46
10 mm
Fig. 9. Impact induced damage in balsa and foam core sandwich composites under impact energy of 75 J.
16 Balsa wood
Stress (MPa)
14 12 10 8
Point of failure Point of densification
6
PVC Foam
4 2 0 0
0,1
0,2 0,3
0,4
0,5
0,6
0,7 0,8
0,9
1
Stroke Strain (mm/mm) Fig. 10. Engineering stress–strain curves for balsa wood and PVC foam used, in compression.
generally while those of the balsa core samples are of the nearly the same values. On the other hand, in two-peaked curves, it is also noted in general that the first peak load value of foam core samples were smaller than that of balsa core samples as in the lower impact energy cases result in one-peaked curves. The probable reason is explained in the preceding paragraph. However, to the second peak load value, it is found to be generally higher, sometimes nearly equal, for foam core samples compared to the balsa based ones. Probably one of the reason is the deformation characteristic of the foam core under compressive loading. That is, compressed foam core increases stiffness of the bottom face-sheets in comparison with balsa wood having lower stiffness after first failure due to buckling of tubular cells. In other words, along with the bottom face-sheets, the stiffness of the core at the end of compression also plays a significant role in the second peak load. The images of the damaged samples are given in Fig. 8. Cross-sectional views show
the core damages which are not visible in images of top and bottom surfaces. If the impact energy is increased to zone D the second penetration range in energy profile diagrams was experienced. Beyond that region, in region E, perforation of the samples takes place. Fig. 4e shows the load–deflection curves of specimens subjected to the impact energy of 75 J. It is seen that both curves are of open type, implying perforation and no rebounding of the impactor from samples. The images of the perforated samples subjected to impact energy of 75 J are given in Fig. 9. From the figures, perforation of the bottom face-sheets occurs rather than the in-plane shear fractures as observed in regions C and D. Besides to the bending fracture of the fibers resulting perforation of the bottom face-sheet, as seen in the cross-sectional view, balsa core samples experienced core/skin deboning as well as the transverse shear fracture of core. 3.3. Repeated impact response In contrast to the most commonly used single impact tests, little work like Refs. [20–23] has been reported on repeated impact response of composite plates. However, there are many realistic cases that repeated impacts are of high importance. Therefore, repeated impact response of the above mentioned composites were also examined experimentally. Fig. 11 gives the variation of the ‘‘repeat number of impact to perforation” (Rn) versus impact energy for balsa and PVC foam core sandwiches. For a better understanding, the Rn values of balsa and foam core samples versus impact energy are also provided in the figure as a table. These values stand for the number of experiments conducted for each impact energy level. It is seen that both kind of samples are of nearly the same number of impact resulting in the damage (perforation) for all impact energy levels except 5 J. For this impact energy, Ei = 5 J, Rn seems to be higher, about 15%, for foam core samples compared to the balsa ones. This difference would increase once the impact energy is decreased.
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11 Impact Energy (Joule) 10 15 20 25 30 40 60
5
50
Balsa 346 46 6 Foam 400 45 8
7 5
4 3
3 2
2 2
Balsa Wood
1 1
40 30 PVC Foam
20
PVC Foam
10.5
Absorbed Energy (J)
60 Rn
Impact Energy (J)
70
Balsa Wood
10 9.5 9 8.5 8
10
7.5 0 0
50
100 150 200 250 300 350 400 450
7 0
Repeat Number (Rn)
5
10
15
20
25
30
35
40
45
50
Repeat Number (Rn) Fig. 11. Variation of the ‘‘repeat number of impact to perforation” (Rn) versus varied impact energies.
As indicated earlier, balsa and foam core composites are not of the same damage mechanisms when subjected to impact loading. This case may be observed from the repeated impact tests as well. The variations of the peak contact force (Fc) and absorbed energy (Ea) versus repeat number are respectively given in Figs. 12 and 13, for impact energy of 10 J. From the figures, it is seen that up to the approximately Rn = 12, the Fc and Ea show opposite variation characteristics for balsa and foam cores. Beyond that they reach a quite constant value, especially Fc, and then through the approximately last five impacts they show again opposite variations. In that region a sudden decrease can be observed in Fc while Ea increases, implying the perforation of the samples. From the load– deflection curves, it is also noted that the stiffness of the samples decreased with Rn as expected. In Fig. 13, for some data points, it seems that the absorbed energy is higher than the impact energy value of 10 J, implying a contradiction. However, it should be noted that for exact or true impact energy value, the potential energy corresponding deflection of the plate is also to be considered. It is noted that the potential energy due to the deflection of the samples is around 0.7 J herein. On the other hand, the Rn–Ei curves given in Fig. 11 may be represented by equations:
Ei ¼ 43:5 R0:3858 with R2 ¼ 0:9361; for PVC foam: n Ei ¼ 46:7 R0:4053 with R2 ¼ 0:9379; for balsa wood: n
6
Peak Contact Force (kN)
PVC Foam energies. 5
Ei=10 J
Balsa Wood
4 3 2 1 0 0
5
10
15
20
25
30
35
40
45
50
Repeat Number (Rn) Fig. 12. Variation of the ‘‘peak contact force” versus ‘‘repeat number” of impacts to perforation for impact energy of 10 J.
Fig. 13. Variation of the ‘‘absorbed energy” versus ‘‘repeat number” of impacts to perforation for impact energy of 10 J.
Without testing, they enable to predict the number of impacts to perforation (Rn) under smaller impact energies. For example; for Ei = 2 J the Rn would be equal to 2930 and 2376 for PVC foam and balsa wood, respectively, while they would be 17,660 and 13,144 under impact energy of 1 J. 4. Conclusions This paper presents an experimental investigation on dropweight impact response of sandwich composite panels with PVC foam core and balsa wood core. From cross-examining load–deflection curves, energy profile and damaged specimens, damage processes are examined. The primary damage modes are found to be fiber fractures at top and bottom face-sheets, delaminations between adjacent glass–epoxy layers, transverse and in-plane shear fractures of core, and face/core debonding. Cross-sectional view of the damaged samples enabled to see the interior damage modes and damage extent. Other conclusions drawn from the work can be summarized as: Load–deflection curves of the both composites were of a mountain-like shape with two peaks under higher impact energies. It is noted that the second peak value for foam core sandwich samples is higher than first peak value generally while those of the balsa core samples are of the nearly the same values. On the other hand, in two-peaked curves, it is also noted that the first peak load value of foam core samples were smaller than that of balsa core samples as in the one-peaked curves. On the contrary, the second peak load value of foam core samples is found to be higher generally than that of balsa core ones. It is also seen from the load–deflection curves that the foam core sandwich is of lower bending stiffness. Based on Fig. 10, it is noted that balsa core samples are stiffer than PVC ones. It results in higher contact force but smaller deflection of the balsa wood sandwiches for smaller impact energy levels, before point of failure. This case implies a higher deformation capability of PVC foam samples, resulting in a larger delamination area formed between layers of top face-sheets due to stiffness mismatching. On the contrary, these impact energies lead to the local damage formation rather than the delamination in balsa wood sandwiches. At the non-impacted side, the delamination area seems to be small for PVC foam. When the impact energy is increased, the delamination area gets larger to some extent followed by the fiber breakage, the dominant damage mode for higher impact
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energies at back face-sheets, due to bending. However, for balsa core sandwich, due to a poor interface between balsa wood with the glass/epoxy layer compared with PVC foam core; the debonding between core and face-sheets becomes the dominant damage mode along with the fiber bending fractures as the impact energy increases. The circular whitish area at back surfaces hence represents the interface debonding to a large extent rather than the delaminations in face-sheets. The core damages due to the shear effects, another dominant damage mode, can also be observed from the cross-sectional views. For higher impact energy values, it is found that both sandwich composites are of nearly the same Rn, repeat number of impact to perforation. On the other hand, as the impact energy is decreased, the Rn increases for foam core samples compared to balsa wood composites.
Acknowledgements The authors gratefully acknowledge the help provided by AERO Wind Industry Inc. during this work and by an abrasive water jet cutting firm, TÜMER, in cutting damaged samples. References [1] Li G, Jones N. Development of rubberized syntactic foam. Composites: Part A 2007;38:1483–92. [2] Christopherson J, Mahinfalah M, Jazar GN, Aagaah MR. An investigation on the effect of a small mass impact on sandwich composite plates. Compos Struct 2005;67:299–306. [3] Gustin J, Joneson A, Mahinfalah M, Stone J. Low velocity impact of combination Kevlar/carbon fiber sandwich composites. Compos Struct 2005;69:396–406. [4] Hosur MV, Abdullah M, Jeelani S. Manufacturing and low-velocity impact characterization of foam filled 3-D integrated core sandwich composites with hybrid face sheets. Compos Struct 2005;69:167–81. [5] Ulven CA, Vaidya UK. Post-fire low velocity impact response of marine grade sandwich composites. Composites: Part A 2006;37:997–1004.
[6] Schubel PM, Luo J-J, Daniel IM. Low velocity impact behavior of composite sandwich panels. Composites: Part A 2005;36:1389–98. [7] Schubel PM, Luo J-J, Daniel IM. Impact and post impact behavior of composite sandwich panels. Composites: Part A 2007;38:1051–7. [8] Hosur MV, Mohammed AA, Jeelani S. Processing of nanoclay filled sandwich composites and their response to impact loading. J Reinf Plast Compos 2008;27:797–818. [9] Imielinska K, Guillaumat L, Wojtyra R, Castaings M. Effects of manufacturing and face/core bonding on impact damage in glass/polyester–PVC foam core sandwich panels. Composites: Part B 2008;39:1034–41. [10] Ulven CA, Vaidya UK. Impact response of fire damaged polymer-based composite materials. Composites: Part B 2008;39:92–107. [11] Xia F, Wu X. Work on low-velocity impact properties of foam sandwich composites with various face sheets. J Reinf Plast Compos. doi: 10.1177/ 0731684409102749 [12] Vural M, Ravichandran G. Dynamic response and energy dissipation characteristics of balsa wood: experiment and analysis. Int J Solids Struct 2003;40:2147–70. [13] Vural M, Ravichandran G. Failure mode transition and energy dissipation in naturally occurring composites. Composites: Part B 2004;35:639–46. [14] Truxel A, Avilés F, Carlsson LA, Grenestedt JL, Millay K. Influence of face/core interface on debond toughness of foam and balsa cored sandwich. J Sandwich Struct Mater 2006;8:237–58. [15] Mahfuz H, Thomas T, Rangari V, Jeelani S. On the dynamic response of sandwich composites and their core materials. Compos Sci Technol 2006;66:2465–72. [16] Tagarielli VL, Deshpande VS, Fleck NA. The high strain rate response of PVC foams and end-grain balsa wood. Composites: Part B 2008;39:83–91. [17] Goren A, Atas C. Manufacturing of polymer matrix composites using vacuum assisted resin infusion molding. Arch Mater Sci Eng 2008;34:117–20. [18] Atas C, Liu D. Impact response of woven composites with small weaving angles. Int J Impact Eng 2008;35:80–97. [19] Atas C, Sayman O. An overall view on impact response of woven fabric composite plates. Compos Struct 2008;82:336–45. [20] Hosur MV, Karim MR, Jeelani S. Experimental investigations on the response of stitched/unstitched woven S2-glass/SC15 epoxy composites under single and repeated low velocity impact loading. Compos Struct 2003;61:89–102. [21] David-West OS, Nash DH, Banks WM. An experimental study of damage accumulation in balanced CFRP laminates due to repeated impact. Compos Struct 2008;83:247–58. [22] Chakraborty D, Kumar M. Response of laminated FRP composites under multiple impact loading. J Reinf Plast Compos 2005;24:1457–77. [23] Baucom JN, Zikry MA, Rajendran AM. Low-velocity impact damage accumulation in woven S2-glass composite systems. Compos Sci Technol 2006;66:1229–38.