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Thickness effect on repeated impact response of woven fabric composite plates
Composites: Part B 49 (2013) 80–85
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Thickness effect on repeated impact response of woven fabric composite plates Cesim Atas a,⇑, Bulent Murat Icten a, Mümin Küçük b a b
_ Dokuz Eylül University, Department of Mechanical Engineering, 35397 Buca, Izmir, Turkey _ Ege University, Ege Higher Vocational School, 35100 Bornova, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 13 September 2012 Received in revised form 14 December 2012 Accepted 23 January 2013 Available online 29 January 2013 Keywords: A. Glass fibers A. Plates B. Impact behavior C. Damage mechanics D. Mechanical testing
a b s t r a c t This paper presents an experimental investigation on the repeated impact response of woven E-glass/ epoxy composites with various thicknesses. Energy profile diagrams of the samples, the variation of perforation thresholds with thickness and the variation of absorbed energy with repeat numbers are provided. Considering varied energy levels, the impact numbers causing complete perforation of the specimens are also depicted. Along with some images of the perforated samples for both single impact and repeated impact cases, the contour plots of the damage expansion with increasing impact numbers are also provided for better understanding. It is found that the perforation threshold/energy for single impact varies linearly with thickness for the chosen composite plates. Considering different energy levels, the impact numbers corresponding complete perforation of the specimens with different thicknesses, i.e. layer numbers, are also provided. It is found that the data points of the each thickness, using power regression, may be written as; Ei ¼ aNbr , where Ei stands for impact energy, Nr for the ‘‘repeat number of impact to perforation’’, while a and b are the constants. The equations found enable to predict the number of impacts for perforation (Nr) under smaller impact energies, without testing. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced laminated composites have been extensively used, especially in the field of aerospace and marine industries, due to their inherent advantages like high strength to weight ratio. However, these materials are also susceptible to transverse impact loads resulting in various damage modes such as local permanent deformations, fiber breakage, delamination and matrix cracking. Since they cause considerable reduction in structural stiffness, the impact response of laminated composites has been an important area of research for a long time. In contrast to the most commonly used single impact tests, little work has been reported on repeated impact response of composite plates. However, there are many realistic cases that repeated impacts are of high importance. Ships and offshore structures subjected to repeated impacts from hard objects other than waves can be given as an example [1]. So, this topic needs further investigation. Some of the important works [2–19] regarding repeated impacts are discussed here. In former studies, impact fatigue of the thermoset composites was investigated by researchers [2,3]. Hosur and co-authors [4] have investigated the response of stitched/unstitched woven glass/epoxy composites under single and repeated low velocity impact loading experimentally. Repeated drop weight impact tests over a range of incident energies ⇑ Corresponding author. Tel.: +90 232 301 92 14; fax: +90 232 301 92 04. E-mail address: [email protected] (C. Atas). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.01.019
were carried out on glass, carbon and aramid epoxy composites [5,6]. It was found that, with increase in drop numbers, peak load decreases while total energy increases until failure. In that work a mathematical relationship between impact energy and number of impact has been proposed. De Morais et al. [7,8] have evaluated repeated low energy impact damages and examined the influence of laminate thickness on the resistance to repeated low energy impacts for glass, carbon and aramid fabrics reinforced composites. The results showed that below a certain energy level the cross section of the laminate is the most relevant variable that determines the impact resistance. They concluded that when the impact energy was increased, fiber characteristics became relevant. Further references may be found in literature [9–13] regarding damage characterization and damage mechanisms of fiber reinforced composites under repeated/multiple impact loading. Belingardi and coauthors [14] have performed repeated impact tests on thick glass reinforced laminates manufactured by hand lay-up and vacuum infusion processes. They evaluated impact response in terms of damage progression, evolution of the peak force and of stiffness loss as a function of impact number, and by calculating the damage index. Low velocity repeated impacts of unidirectional carbon fiber reinforced polyetherimide (PEI) composites were performed by using a pendulum type instrumented impact tester at energy levels ranging 0.54–0.94 J to examine impact-fatigue properties [15]. An analytical model to describe the life time of composite materials subjected to repeated impact loadings was also presented. Atas and Sevim [16] have conducted an experimental work on single
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dedicated software suitable for CEAST instruments, managing the complete test procedure, all the quantities required can be handled from both a numerical and a graphical point of view. And, therefore, force–time, force–deflection, absorbed energy–time, etc. were created. Considering force values taken from piezoelectric force sensor versus the aforementioned time intervals, incremental calculation of acceleration followed by velocity and deflection can be done based on the Newton’s second law and kinematic equations. Afterwards, absorbed energy can be obtained from integrating the area bounded by the force–deflection curve and the deflection axis. The actual initial velocity of the impactor right before contact with sample was determined by a velocity sensor. By rewriting Newton’s second law in alternative forms, that is linear impulse–linear momentum and work–kinetic energy, the required quantities could also be incrementally calculated easily.
impact and repeated impact responses of sandwich composites with PVC foam core and balsa wood core. They have investigated the damage process of the sandwich composites by crossexamining load–deflection curves, energy profile diagrams and the damaged specimens. Repeated impact response of composites continues to be one of research topics of interest [17–19]. In this work, repeated impact response of woven E-glass/epoxy composites with various thicknesses is investigated. Considering different plate thicknesses, the variation of absorbed energy with repeat number, the variation of perforation threshold with thickness and the energy profile diagrams are given. For different energy levels, the repeated impact numbers until complete perforation of the specimens with different thicknesses are also provided. Some images of the perforated samples for both single impact and repeated impact cases are given for comparison. In order to have an opinion about damage expansion with increasing impact numbers, for a given impact energy, the contour plots of the damage areas corresponding impact energy of 20 J are also provided. In the light of the mentioned figures and images, helpful discussions and comparisons are made in the following.
3. Results and discussion Thickness effect on repeated impact response of woven fabric composite plates is examined experimentally. Force–deflection curves contain significant information about the damage process in an impact event. The deflection term here indicates the motion of the impactor and accordingly the deflection of the impacted surface of the sample during contact between sample and impactor. There are two basic types, closed curve and open curve [20]. A closed curve consists of an ascending section of loading and a descending section combining loading and unloading in general. If the descending section is completely a softening curve, open curve, the force–deflection curve may represent either penetration or perforation cases. As indicated before, the absorbed energy can be determined from area enveloped by the force–deflection curves. As an example, the force–deflection and the energy–time diagrams of the 16-layer samples single-impacted at 20 J are given in Fig. 1a and b. From Fig. 1a, it is seen that the force–deflection curve has ascending and descending sections. The slope of the ascending section of force–deflection curves is named as the impact bending stiffness. The highest value reached in force–deflection curves is called as the peak force. Returning toward the origin of the diagram following descending section indicates the rebounding of the impactor from the specimen surface after impact. Impact energy (Ei) and absorbed energy (Ea) are shown in Fig. 1b. Variation of the absorbed energy versus impact energy, called as energy profile diagram [20] for different thicknesses, i.e. different layer numbers, is given in Fig. 2. The variation of perforation energy (Pr) versus increasing thickness (t), in case of single impact, is given in Fig. 3. It is seen that the perforation threshold varies al-
2. Material and impact testing Composite material was manufactured from 500 g/m2 woven Eglass fabrics and CY225 epoxy resin mixed with HY225 hardener. The woven fabric was of balanced plain weave and there were two warp and fill yarns per 10 mm. The gap between adjacent yarns was 1 mm while the thickness of the fabric was 0.48 mm, approximately. A hot-lamination press was used in fabrication of laminated plates with different number of identical layers. The composite plates were cured at 120 °C for 2 h under a pressure of 0.25 MPa. The nominal thicknesses of the specimens were measured as 2.70, 3.35, 4.05, 5.05 and 5.75 mm for the plates having 8, 10, 12, 16, and 18 layers, respectively. It can be stated that thickness varies almost linearly with layer numbers. The fiber volume fractions of all the plates were found as 58 ± 2%. The variation is assumed to be negligible. The impact tests were performed by using a drop weight impact testing machine, CEAST-Fractovis Plus. The impactor used was of a hemispherical nose with 12.7 mm diameter and is connected to a force transducer having loading capacity of 22.4 kN. The total impact mass including impactor nose, force transducer and crosshead was 5.02 kg. The specimens were fixed by a pneumatic fixture with a 76.2 mm hole diameter. The impact testing machine was of an advanced data acquisition system especially designed for instrumented impact. The system can acquire 16,000 points at a frequency of up to 2 MHz, from a piezoelectric force sensor. Thus, it is possible to collect data for very small time intervals. Thanks to
(a)
(b)
10
25
16 layer Ei =20 J
20
.
Ei =20 J
Energy (J)
Contact force (kN)
16 layer 8 6 4
15 10
Ei
Ea
5
2
0
0 0
1
2
3
4
Deflection (mm)
5
6
0
1
2
3
4
Time (msec)
Fig. 1. (a) Contact force–deflection and (b) energy–time curves of 16-layer samples for single impact cases.
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(a)
240
12
16 layer Ei=20 J
Contact force (kN) .
8 layers 10 layers
200
Absorbed energy (J)
12 layers 16 layers
160
18 layers
120
10
Nr =133 10th
60th
2nd
8
100th 1st
6
133th 4 2 0
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0
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3
4
5
6
7
8
9
10
Deflection (mm) 40
0
0
40
80
120
160
200
240
Impact energy (J) Fig. 2. Energy profiles of the plates with different layer numbers.
200
Absorbed Energy (J) .
(b)
16 layer Ei=20 J
20
Nr =133 133th 1st
15
100th
60th
2nd
10
10th
5 0 0
least squares
1
2
3
4
5
6
7
8
9
10
Time (msec) 120
Fig. 4. Force–deflection and energy–time diagrams of 16-layer woven glass–epoxy composites under 20 J repeated impact loading.
80
P r = 40,057 t - 67,68 2
R = 0,9999 40
0 2
3
4
5
6
Thickness, t (mm) Fig. 3. Variation of the perforation threshold with thickness.
most linearly with thickness, for the woven glass–epoxy composites used. The force–deflection and the energy–time diagrams of the 16-layer samples at 20 J are given in Fig. 4, for repeated impact loading. As seen from the figure, the bending stiffness of the samples decreases gradually with the increase of repeat number. However, the ascending section of the first impact consists of two parts contrary to the following impacts. The curve corresponding first impact reaches a peak value at the end of the first part, resulting in indentation at the point of contact between impactor nose and sample. After that point, the curve varies with a smaller slope at the ascending section; the second part. Similar observation is made for the other stacking sequences. Due to matrix cracks and indentation, the peak force of the first impact seems to be smaller than the subsequent impacts until a certain impact number, for instance 100th impact in the figure, at which sample experiences drastic fiber damages. That is, as the impact number increases, delaminations and fiber breakages increases to a large extent, resulting in the reduction of stiffness and contact force. Variation of the absorbed energy with repeat number is given in Fig. 4b. Considering different energy levels, the impact numbers until complete perforation of the specimens with different thick-
nesses/layer numbers are also provided, Fig. 5. It is found that the data points of each thickness, using power regression, may be written as; Ei ¼ aN br where Ei stands for impact energy, Nr for the ‘‘repeat number of impact until perforation’’, while a and b are the constants. The equations of curve fitting and corresponding R-squared values are provided in the figure. For smaller impact energies, such as 5 J and the total number of tests (Nr) increases significantly. For example, samples with eight layers (t = 2.70 mm) subjected to 5 J were perforated after 238th impact. For thicker samples much more tests are needed till perforation. Thus, the equations found enable to predict Nr under smaller impact energies, without testing. For example; for Ei = 5 J the Nr would be equal to 749, 1150, 3488 and 4868 for 10, 12, 16 and 18 layers respectively, using equations given in Fig. 5.
140 8 layers 10 layers 12 layers 16 layers 18 layers
120
Impact energy, Ei (J)
Perforation energy, Pr (J)
data points 160
25
100 80 60 40 20 0 0
20
40
60
80
100
120
140
Number of repeated impacts (N r ) Fig. 5. Repeat numbers of impacts to perforation for given impact energies.
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C. Atas et al. / Composites: Part B 49 (2013) 80–85
24
12
Absorbed energy (J)
22
Peak contact force (kN)
E i =20J 8 layers 10 layers 12 layers 16 layers
20 18 16 14 12 10
10 8 6
Ei=20J
4
8 layers 10 layers
2
12 layers 16 layers
0 0
8 0
20
40
60
80
100
120
140
20
40
60
80
100
120
140
Number of repeated impacts (Nr )
Number of repeated impacts (Nr ) Fig. 7. The peak contact force corresponding individual impacts under 20 J. Fig. 6. The absorbed energies corresponding individual impacts under 20 J.
Repeated impact (20J)
40 J
20 J, Nr =5
60 J
20 J, Nr =16
100 J
20 J, Nr =36
100 J
20 J, Nr =133
16 layers
12 layers
10 layers
8 layers
Single impact
Fig. 8. Impact induced damage areas of samples with 8, 10, 12 and 16 layers for both single impact and repeated impact (under 20 J) cases.
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Figs. 6 and 7 indicate, respectively, dissipated energy and contact force values for successive impacts at 20 J until perforation of the samples. With a careful examination, it can be seen from Fig. 6 that the amount of absorbed energy drops suddenly after a few impacts followed by a quite constant variation, and then an increase. As the number of layers or thickness increases, e.g. 16-layer, the horizontal part of the curve implying constant energy absorption gets larger. This means that for each impact at this region, almost the same amount of energy is dissipated through damage of samples, especially in the form of delaminations and matrix cracks. After a certain repeat number, catastrophic damages take place resulting in dramatic increases in the curves. On the other hand, as expected, the variation of peak contact force with impact number, Fig. 7, shows an opposite variation characteristic, in comparison with Fig. 6.
In order to have an opinion about damage expansion in composite samples with various thicknesses, some images of the damaged samples are provided in Figs. 8 and 9. For different thicknesses, impact induced damage areas for single impact and repeated impact cases undergoing perforation are shown in Fig. 8. For repeated impact cases, in this figure, perforated sample images of 20 J are provided except for the samples with 18 layers, t = 5.75 mm. This is because the repeated impacts of samples with 18 layers were conducted for impact energies higher than 20 J, Fig. 9. As seen in figures, the overall damage areas under single impacts increase compared to repeated impacts, particularly for layer numbers of 12, 16 and 18. It implies that single impacts under higher energies result in larger delaminations while repeated impacts with smaller energies lead to localized fiber breakages rather than delaminations.
200 J
25 J, Nr =83
Fig. 9. Impact induced damage areas of samples with 18 layers for both single impact and repeated impact (under 25 J) cases.
8 layers
Ei=20 J
10 layers
E i =20 J 5th 10 th
th
5
16
12 layers
Ei=20 J
16 layers
Ei=20 J 20 th
10 th th 20
th
th
30 th 36
Fig. 10. Overall damage expansions with increasing repeat number.
60 th th 100 th 133
C. Atas et al. / Composites: Part B 49 (2013) 80–85
For comparison and better understanding, the overall damage expansions with increasing repeat numbers are also depicted in Fig. 10. As noted earlier for Fig. 5, the total numbers of impacts under impact energy of 20 J until perforation are found respectively as 5, 16, 36 and 133 for samples with 8, 10, 12 and 16 respectively. The figure enables to have an opinion about step by step damage expansion in the samples. 4. Conclusions An experimental investigation of the thickness effect on repeated impact response of woven E-glass/epoxy composites has been done in this work. The following results are obtained from the investigation: The perforation threshold/energy for single impact varies linearly with thickness for the chosen composite plates. For varied impact energy levels, the impact numbers causing complete perforation of the samples can be represented by some equations, Ei ¼ aNbr , and accordingly curves using power regression methods. They enable to predict the number of impacts till perforation (Nr) for too small impact energies, without testing. For the perforation case, increase of thickness as in 16-layer and 18-layer samples, single impacts results in larger damage areas compared to repeated impacts.
References [1] Zhu L, Faulkner D. Damage estimate for plating of ships and platforms under repeated impacts. Mar Struct 1996;9:697–720. [2] Azouaoui K, Rechak S, Azari Z, Benmedakhene S, Laksimi A, Pluvinage G. Modelling of damage and failure of glass/epoxy composite plates subject to impact fatigue. Int J Fatigue 2001;23:877–85. [3] Roy R, Sarkar BK, Bose NR. Impact fatigue of glass fiber–vinylester resin composites. Composites: Part A 2001;32:871–6.
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[4] 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. [5] Sugun BS, Rao RMVGK. Low-velocity impact characterization of glass, carbon and kevlar composites using repeated drop tests. J Reinf Plast Compos 2004;23:1583–99. [6] Sugun BS, Rao RMVGK. Impactor mass effects in glass–epoxy composites subjected to repeated drop tests. J Reinf Plast Compos 2004;23:1547–60. [7] De Morais WA, Monteiro SN, d’Almeida JRM. Evaluation of repeated low energy impact damage in carbon–epoxy composite materials. Compos Struct 2005;67:307–15. [8] De Morais WA, Monteiro SN, d’Almeida JRM. Effect of the laminate thickness on the composite strength to repeated low energy impacts. Compos Struct 2005;70:223–8. [9] Chakraborty D, Kumar M. Response of laminated FRP composites under multiple impact loading. J Reinf Plast Compos 2005;24:1457–77. [10] Baucom JN, Zikry MA, Rajendran AM. Low-velocity impact damage accumulation in woven S2-glass composite systems. Compos Sci Technol 2006;66:1229–38. [11] Azouaoui K, Ouali N, Ouroua Y, Mesbah A, Boukharouba T. Damage characterisation of glass/polyester composite plates subjected to low-energy impact fatigue. J Sound Vib 2007;308:504–13. [12] Icten BM. Repeated impact behavior of glass/epoxy laminates. Polym Compos 2009;32:1562–9. [13] 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. [14] Belingardi G, Cavatorta MP, Paolino DS. Repeated impact response of hand layup and vacuum infusion thick glass reinforced laminates. Int J Impact Eng 2008;35:609–19. _ Günay V. On the life time prediction [15] Bora MÖ, Çoban O, Sinmazçelik T, Cürgül I, of repeatedly impacted thermoplastic matrix composites. Mater Des 2009;30:145–53. [16] Atas C, Sevim C. On the impact response of sandwich composites with cores of balsa wood and PVC foam. Compos Struct 2010;93:40–8. [17] Sevkat E, Liaw B, Delale F, Raju BB. Effect of repeated impacts on the response of plain-woven hybrid composites. Composites: Part B 2010;41:403–13. [18] Azouaoui K, Azari Z, Pluvinage G. Evaluation of impact fatigue damage in glass/ epoxy composite laminate. Int J Fatigue 2010;32:443–52. [19] Aurrekoetxea J, Sarrionandia M, Mateos M, Aretxabaleta L. Repeated low energy impact behaviour of self-reinforced polypropylene composites. Polym Testing 2011;30:216–21. [20] Atas C, Liu D. Impact response of woven composites with small weaving angles. Int J Impact Eng 2008;35:80–97.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Thickness effect on repeated impact response of woven fabric composite plates Cesim Atas a,⇑, Bulent Murat Icten a, Mümin Küçük b a b
_ Dokuz Eylül University, Department of Mechanical Engineering, 35397 Buca, Izmir, Turkey _ Ege University, Ege Higher Vocational School, 35100 Bornova, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 13 September 2012 Received in revised form 14 December 2012 Accepted 23 January 2013 Available online 29 January 2013 Keywords: A. Glass fibers A. Plates B. Impact behavior C. Damage mechanics D. Mechanical testing
a b s t r a c t This paper presents an experimental investigation on the repeated impact response of woven E-glass/ epoxy composites with various thicknesses. Energy profile diagrams of the samples, the variation of perforation thresholds with thickness and the variation of absorbed energy with repeat numbers are provided. Considering varied energy levels, the impact numbers causing complete perforation of the specimens are also depicted. Along with some images of the perforated samples for both single impact and repeated impact cases, the contour plots of the damage expansion with increasing impact numbers are also provided for better understanding. It is found that the perforation threshold/energy for single impact varies linearly with thickness for the chosen composite plates. Considering different energy levels, the impact numbers corresponding complete perforation of the specimens with different thicknesses, i.e. layer numbers, are also provided. It is found that the data points of the each thickness, using power regression, may be written as; Ei ¼ aNbr , where Ei stands for impact energy, Nr for the ‘‘repeat number of impact to perforation’’, while a and b are the constants. The equations found enable to predict the number of impacts for perforation (Nr) under smaller impact energies, without testing. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced laminated composites have been extensively used, especially in the field of aerospace and marine industries, due to their inherent advantages like high strength to weight ratio. However, these materials are also susceptible to transverse impact loads resulting in various damage modes such as local permanent deformations, fiber breakage, delamination and matrix cracking. Since they cause considerable reduction in structural stiffness, the impact response of laminated composites has been an important area of research for a long time. In contrast to the most commonly used single impact tests, little work has been reported on repeated impact response of composite plates. However, there are many realistic cases that repeated impacts are of high importance. Ships and offshore structures subjected to repeated impacts from hard objects other than waves can be given as an example [1]. So, this topic needs further investigation. Some of the important works [2–19] regarding repeated impacts are discussed here. In former studies, impact fatigue of the thermoset composites was investigated by researchers [2,3]. Hosur and co-authors [4] have investigated the response of stitched/unstitched woven glass/epoxy composites under single and repeated low velocity impact loading experimentally. Repeated drop weight impact tests over a range of incident energies ⇑ Corresponding author. Tel.: +90 232 301 92 14; fax: +90 232 301 92 04. E-mail address: [email protected] (C. Atas). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.01.019
were carried out on glass, carbon and aramid epoxy composites [5,6]. It was found that, with increase in drop numbers, peak load decreases while total energy increases until failure. In that work a mathematical relationship between impact energy and number of impact has been proposed. De Morais et al. [7,8] have evaluated repeated low energy impact damages and examined the influence of laminate thickness on the resistance to repeated low energy impacts for glass, carbon and aramid fabrics reinforced composites. The results showed that below a certain energy level the cross section of the laminate is the most relevant variable that determines the impact resistance. They concluded that when the impact energy was increased, fiber characteristics became relevant. Further references may be found in literature [9–13] regarding damage characterization and damage mechanisms of fiber reinforced composites under repeated/multiple impact loading. Belingardi and coauthors [14] have performed repeated impact tests on thick glass reinforced laminates manufactured by hand lay-up and vacuum infusion processes. They evaluated impact response in terms of damage progression, evolution of the peak force and of stiffness loss as a function of impact number, and by calculating the damage index. Low velocity repeated impacts of unidirectional carbon fiber reinforced polyetherimide (PEI) composites were performed by using a pendulum type instrumented impact tester at energy levels ranging 0.54–0.94 J to examine impact-fatigue properties [15]. An analytical model to describe the life time of composite materials subjected to repeated impact loadings was also presented. Atas and Sevim [16] have conducted an experimental work on single
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C. Atas et al. / Composites: Part B 49 (2013) 80–85
dedicated software suitable for CEAST instruments, managing the complete test procedure, all the quantities required can be handled from both a numerical and a graphical point of view. And, therefore, force–time, force–deflection, absorbed energy–time, etc. were created. Considering force values taken from piezoelectric force sensor versus the aforementioned time intervals, incremental calculation of acceleration followed by velocity and deflection can be done based on the Newton’s second law and kinematic equations. Afterwards, absorbed energy can be obtained from integrating the area bounded by the force–deflection curve and the deflection axis. The actual initial velocity of the impactor right before contact with sample was determined by a velocity sensor. By rewriting Newton’s second law in alternative forms, that is linear impulse–linear momentum and work–kinetic energy, the required quantities could also be incrementally calculated easily.
impact and repeated impact responses of sandwich composites with PVC foam core and balsa wood core. They have investigated the damage process of the sandwich composites by crossexamining load–deflection curves, energy profile diagrams and the damaged specimens. Repeated impact response of composites continues to be one of research topics of interest [17–19]. In this work, repeated impact response of woven E-glass/epoxy composites with various thicknesses is investigated. Considering different plate thicknesses, the variation of absorbed energy with repeat number, the variation of perforation threshold with thickness and the energy profile diagrams are given. For different energy levels, the repeated impact numbers until complete perforation of the specimens with different thicknesses are also provided. Some images of the perforated samples for both single impact and repeated impact cases are given for comparison. In order to have an opinion about damage expansion with increasing impact numbers, for a given impact energy, the contour plots of the damage areas corresponding impact energy of 20 J are also provided. In the light of the mentioned figures and images, helpful discussions and comparisons are made in the following.
3. Results and discussion Thickness effect on repeated impact response of woven fabric composite plates is examined experimentally. Force–deflection curves contain significant information about the damage process in an impact event. The deflection term here indicates the motion of the impactor and accordingly the deflection of the impacted surface of the sample during contact between sample and impactor. There are two basic types, closed curve and open curve [20]. A closed curve consists of an ascending section of loading and a descending section combining loading and unloading in general. If the descending section is completely a softening curve, open curve, the force–deflection curve may represent either penetration or perforation cases. As indicated before, the absorbed energy can be determined from area enveloped by the force–deflection curves. As an example, the force–deflection and the energy–time diagrams of the 16-layer samples single-impacted at 20 J are given in Fig. 1a and b. From Fig. 1a, it is seen that the force–deflection curve has ascending and descending sections. The slope of the ascending section of force–deflection curves is named as the impact bending stiffness. The highest value reached in force–deflection curves is called as the peak force. Returning toward the origin of the diagram following descending section indicates the rebounding of the impactor from the specimen surface after impact. Impact energy (Ei) and absorbed energy (Ea) are shown in Fig. 1b. Variation of the absorbed energy versus impact energy, called as energy profile diagram [20] for different thicknesses, i.e. different layer numbers, is given in Fig. 2. The variation of perforation energy (Pr) versus increasing thickness (t), in case of single impact, is given in Fig. 3. It is seen that the perforation threshold varies al-
2. Material and impact testing Composite material was manufactured from 500 g/m2 woven Eglass fabrics and CY225 epoxy resin mixed with HY225 hardener. The woven fabric was of balanced plain weave and there were two warp and fill yarns per 10 mm. The gap between adjacent yarns was 1 mm while the thickness of the fabric was 0.48 mm, approximately. A hot-lamination press was used in fabrication of laminated plates with different number of identical layers. The composite plates were cured at 120 °C for 2 h under a pressure of 0.25 MPa. The nominal thicknesses of the specimens were measured as 2.70, 3.35, 4.05, 5.05 and 5.75 mm for the plates having 8, 10, 12, 16, and 18 layers, respectively. It can be stated that thickness varies almost linearly with layer numbers. The fiber volume fractions of all the plates were found as 58 ± 2%. The variation is assumed to be negligible. The impact tests were performed by using a drop weight impact testing machine, CEAST-Fractovis Plus. The impactor used was of a hemispherical nose with 12.7 mm diameter and is connected to a force transducer having loading capacity of 22.4 kN. The total impact mass including impactor nose, force transducer and crosshead was 5.02 kg. The specimens were fixed by a pneumatic fixture with a 76.2 mm hole diameter. The impact testing machine was of an advanced data acquisition system especially designed for instrumented impact. The system can acquire 16,000 points at a frequency of up to 2 MHz, from a piezoelectric force sensor. Thus, it is possible to collect data for very small time intervals. Thanks to
(a)
(b)
10
25
16 layer Ei =20 J
20
.
Ei =20 J
Energy (J)
Contact force (kN)
16 layer 8 6 4
15 10
Ei
Ea
5
2
0
0 0
1
2
3
4
Deflection (mm)
5
6
0
1
2
3
4
Time (msec)
Fig. 1. (a) Contact force–deflection and (b) energy–time curves of 16-layer samples for single impact cases.
5
6
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(a)
240
12
16 layer Ei=20 J
Contact force (kN) .
8 layers 10 layers
200
Absorbed energy (J)
12 layers 16 layers
160
18 layers
120
10
Nr =133 10th
60th
2nd
8
100th 1st
6
133th 4 2 0
80
0
1
2
3
4
5
6
7
8
9
10
Deflection (mm) 40
0
0
40
80
120
160
200
240
Impact energy (J) Fig. 2. Energy profiles of the plates with different layer numbers.
200
Absorbed Energy (J) .
(b)
16 layer Ei=20 J
20
Nr =133 133th 1st
15
100th
60th
2nd
10
10th
5 0 0
least squares
1
2
3
4
5
6
7
8
9
10
Time (msec) 120
Fig. 4. Force–deflection and energy–time diagrams of 16-layer woven glass–epoxy composites under 20 J repeated impact loading.
80
P r = 40,057 t - 67,68 2
R = 0,9999 40
0 2
3
4
5
6
Thickness, t (mm) Fig. 3. Variation of the perforation threshold with thickness.
most linearly with thickness, for the woven glass–epoxy composites used. The force–deflection and the energy–time diagrams of the 16-layer samples at 20 J are given in Fig. 4, for repeated impact loading. As seen from the figure, the bending stiffness of the samples decreases gradually with the increase of repeat number. However, the ascending section of the first impact consists of two parts contrary to the following impacts. The curve corresponding first impact reaches a peak value at the end of the first part, resulting in indentation at the point of contact between impactor nose and sample. After that point, the curve varies with a smaller slope at the ascending section; the second part. Similar observation is made for the other stacking sequences. Due to matrix cracks and indentation, the peak force of the first impact seems to be smaller than the subsequent impacts until a certain impact number, for instance 100th impact in the figure, at which sample experiences drastic fiber damages. That is, as the impact number increases, delaminations and fiber breakages increases to a large extent, resulting in the reduction of stiffness and contact force. Variation of the absorbed energy with repeat number is given in Fig. 4b. Considering different energy levels, the impact numbers until complete perforation of the specimens with different thick-
nesses/layer numbers are also provided, Fig. 5. It is found that the data points of each thickness, using power regression, may be written as; Ei ¼ aN br where Ei stands for impact energy, Nr for the ‘‘repeat number of impact until perforation’’, while a and b are the constants. The equations of curve fitting and corresponding R-squared values are provided in the figure. For smaller impact energies, such as 5 J and the total number of tests (Nr) increases significantly. For example, samples with eight layers (t = 2.70 mm) subjected to 5 J were perforated after 238th impact. For thicker samples much more tests are needed till perforation. Thus, the equations found enable to predict Nr under smaller impact energies, without testing. For example; for Ei = 5 J the Nr would be equal to 749, 1150, 3488 and 4868 for 10, 12, 16 and 18 layers respectively, using equations given in Fig. 5.
140 8 layers 10 layers 12 layers 16 layers 18 layers
120
Impact energy, Ei (J)
Perforation energy, Pr (J)
data points 160
25
100 80 60 40 20 0 0
20
40
60
80
100
120
140
Number of repeated impacts (N r ) Fig. 5. Repeat numbers of impacts to perforation for given impact energies.
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24
12
Absorbed energy (J)
22
Peak contact force (kN)
E i =20J 8 layers 10 layers 12 layers 16 layers
20 18 16 14 12 10
10 8 6
Ei=20J
4
8 layers 10 layers
2
12 layers 16 layers
0 0
8 0
20
40
60
80
100
120
140
20
40
60
80
100
120
140
Number of repeated impacts (Nr )
Number of repeated impacts (Nr ) Fig. 7. The peak contact force corresponding individual impacts under 20 J. Fig. 6. The absorbed energies corresponding individual impacts under 20 J.
Repeated impact (20J)
40 J
20 J, Nr =5
60 J
20 J, Nr =16
100 J
20 J, Nr =36
100 J
20 J, Nr =133
16 layers
12 layers
10 layers
8 layers
Single impact
Fig. 8. Impact induced damage areas of samples with 8, 10, 12 and 16 layers for both single impact and repeated impact (under 20 J) cases.
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Figs. 6 and 7 indicate, respectively, dissipated energy and contact force values for successive impacts at 20 J until perforation of the samples. With a careful examination, it can be seen from Fig. 6 that the amount of absorbed energy drops suddenly after a few impacts followed by a quite constant variation, and then an increase. As the number of layers or thickness increases, e.g. 16-layer, the horizontal part of the curve implying constant energy absorption gets larger. This means that for each impact at this region, almost the same amount of energy is dissipated through damage of samples, especially in the form of delaminations and matrix cracks. After a certain repeat number, catastrophic damages take place resulting in dramatic increases in the curves. On the other hand, as expected, the variation of peak contact force with impact number, Fig. 7, shows an opposite variation characteristic, in comparison with Fig. 6.
In order to have an opinion about damage expansion in composite samples with various thicknesses, some images of the damaged samples are provided in Figs. 8 and 9. For different thicknesses, impact induced damage areas for single impact and repeated impact cases undergoing perforation are shown in Fig. 8. For repeated impact cases, in this figure, perforated sample images of 20 J are provided except for the samples with 18 layers, t = 5.75 mm. This is because the repeated impacts of samples with 18 layers were conducted for impact energies higher than 20 J, Fig. 9. As seen in figures, the overall damage areas under single impacts increase compared to repeated impacts, particularly for layer numbers of 12, 16 and 18. It implies that single impacts under higher energies result in larger delaminations while repeated impacts with smaller energies lead to localized fiber breakages rather than delaminations.
200 J
25 J, Nr =83
Fig. 9. Impact induced damage areas of samples with 18 layers for both single impact and repeated impact (under 25 J) cases.
8 layers
Ei=20 J
10 layers
E i =20 J 5th 10 th
th
5
16
12 layers
Ei=20 J
16 layers
Ei=20 J 20 th
10 th th 20
th
th
30 th 36
Fig. 10. Overall damage expansions with increasing repeat number.
60 th th 100 th 133
C. Atas et al. / Composites: Part B 49 (2013) 80–85
For comparison and better understanding, the overall damage expansions with increasing repeat numbers are also depicted in Fig. 10. As noted earlier for Fig. 5, the total numbers of impacts under impact energy of 20 J until perforation are found respectively as 5, 16, 36 and 133 for samples with 8, 10, 12 and 16 respectively. The figure enables to have an opinion about step by step damage expansion in the samples. 4. Conclusions An experimental investigation of the thickness effect on repeated impact response of woven E-glass/epoxy composites has been done in this work. The following results are obtained from the investigation: The perforation threshold/energy for single impact varies linearly with thickness for the chosen composite plates. For varied impact energy levels, the impact numbers causing complete perforation of the samples can be represented by some equations, Ei ¼ aNbr , and accordingly curves using power regression methods. They enable to predict the number of impacts till perforation (Nr) for too small impact energies, without testing. For the perforation case, increase of thickness as in 16-layer and 18-layer samples, single impacts results in larger damage areas compared to repeated impacts.
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