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Impact and post impact (CAI) behavior of stitched woven–knit hybrid composites
Accepted Manuscript Impact and post impact (CAI) behavior of stitched woven-knit hybrid composites Alaattin Aktaş, Mehmet Aktaş, Fatih Turan PII: DOI: Reference:
S0263-8223(14)00234-7 http://dx.doi.org/10.1016/j.compstruct.2014.05.024 COST 5705
To appear in:
Composite Structures
Please cite this article as: Aktaş, A., Aktaş, M., Turan, F., Impact and post impact (CAI) behavior of stitched wovenknit hybrid composites, Composite Structures (2014), doi: http://dx.doi.org/10.1016/j.compstruct.2014.05.024
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Impact and post impact (CAI) behavior of stitched woven-knit hybrid composites Alaattin Aktaşa, Mehmet Aktaşb*, Fatih Turana a
Istanbul University, Department of Mechanical Engineering, 34320, Istanbul, Turkey b
Usak University, Department of Mechanical Engineering, 64200, Usak, Turkey
Abstract In this study, the effect of stitch pattern on the impact and post impact (CAI) behavior of eight ply woven-knit hybrid composite plates consisting of 2D woven fabrics as outer layers and rib knitted fabrics as inner layers was investigated. Layers of the woven-knit hybrid composite plates were sewed in circular, square and diamond shape through thickness in order to prevent delamination and slippage between the layers. Impact and post impact testing were carried out in order to determine the impact and post impact resistance of three different stitched composite structures. Results showed that the impact strength of composite laminate increased 5.5%, 11% and 22% in the case of square, diamond, and circular stitching, respectively. It was also found that the woven-knit hybrid composites with square pattern have the highest CAI strength.
Keywords: Fabrics/textiles, glass fibers, impact behavior, damage mechanics, stitching.
*Corresponding author: Tel: +90-276-221-2136, Fax: +90-276-221-2137 E-mail: [email protected]
1. Introduction
A structural composite is material system consisting of two or more phases on a macroscopic scale, whose mechanical performance and properties are designed to be superior those of the constituent materials acting independently. The high stiffness, high strength, and low density characteristics make composites highly desirable in aerospace, automotive, defense, and sport industries [1]. However, these materials are susceptible to impact damages. Therefore, the impact behavior of laminated composites has been an important research area for a long time [2-4]. Composite materials dissipate most of the energy created by an impact via some fracture mechanisms such as matrix cracks, delaminations, fiber fracture, fiber–matrix debonding and fiber pull-out. Among fracture mechanisms, delamination has a serious potential to degrade the compressive mechanical properties of the material that causes catastrophic failure of the material abruptly [5]. The delamination growth caused by impact can be characterized by fracture toughness under modes I – II – III and mixed mode loading conditions. Therefore, the interlaminar fracture toughness can be used as a controller of the delamination level in composite structures [6].
Some enhancements of the delamination resistance have been proposed such as matrix toughening [7], adoption of high-strain/high-strength fibers [8], and through thickness reinforcement [9]. Stitching offers a potential to increase impact delamination resistance and improve interlaminar strength and fracture toughness [10 -15].
Some researches [10, 11, 16] have concluded that the through thickness reinforcement reduces the delamination area caused by impact and such reduction has been directly related to the higher interlaminar fracture toughness of stitched laminates. Numerous studies have also conducted out-of-plane impact tests for the stitched laminates, and most of the results confirmed that the out-of-plane stitching improved the impact damage resistance of the FRP laminates [17, 18]. In the case of three dimensional composites, stitched laminates possess greatly improved translaminar properties compared to woven laminates [19].
Compression after impact (CAI) performance remains an important design criterion in the use of composite structures in order to investigate damage tolerance caused by impact loading. Therefore, a number of studies have been reported in the literature in this direction [20 – 26]. Various studies have shown that stitching improves the CAI strength of composite materials owing to capability to reduce impact – induced delamination and improve
interlaminar strength [27-31]. However, the effect of the stitch pattern on the impact and post impact behavior of composite materials has not been considered.
Textile fabrics such as woven and knitted fabrics have long been known as prime reinforcement for composite application owing to their attractive intra- and inter-laminar strengths, damage tolerance, and lower cost and versatile design potential [32]. Hybrid composites have been commonly used in many applications such as automotive and aviation industries due to their high strength, low weight, good corrosion resistance and fatigue life. In this study, hybrid composite plates composed of woven and knitted fabrics were stitched through thickness in square, circular, and diamond stitch pattern. The reason to combine the knitted and woven layers as reinforcement of composite plates is to enhance the energy absorption capability and to determine the optimum impact resistance of knitted and woven composite structures. In the previous study [33], the authors have investigated the effect of stacking sequence on the impact behavior of sequentially stacked woven (2D)-knit (Rib and Milano) fabric glass/epoxy hybrid composites. Woven and knit fabric layers were sequentially stacked in six different variations to fabricate eight ply woven-knit hybrid composites. Results showed that specimens having outer layer of woven fabric exhibited better impact properties than that of the specimens having outer layer of knitted fabric. Thus, in this study, the effect of the stitch pattern on the impact and post impact (CAI) behavior of eight ply woven-knit glass/epoxy hybrid composites in which 2D woven fabric are used as outer layers and rib knitted fabrics are used as inner layers was investigated.
2. Experimental Study
2.1 Specimen Preparation
Rib fabrics (R) are fabricated from 136 tex slightly twisted E-glass yarn using a seven gauge V bed knitting machine. The architecture and photograph of rib and double layer (2D) woven fabrics are shown in Figure 1. Areal weight, course density and wale density of the rib knitted fabrics are 385 g/m2, 4 course/cm and 3.5 wale/cm, respectively. Fabrication of 2 double layer woven fabrics (2D) was mentioned in previous study [34]. The areal weight, warp density and weft density of the 2D woven fabrics are 380 g/m2, 16.5 warp/cm and 18 weft/cm, respectively. The stacking sequences of woven-knitted fabric layers are [2D/2D/R/R/R/R/2D/2D]= [2D2R2]S. In this
notation, “2D” and “R” are referred to as “double layer woven fabric” and “rib fabric”, respectively. The reason for [2D2R2]S woven-knit hybrid composite to be chosen for stitching is that because its impact resistance is the highest [33]. Then square, circular, and diamond stitch patterns through thickness using136 tex glass yarn was applied between woven and knitted fabrics in [2D2R2]S woven-knit hybrid composite as shown in Figure 2. The almost circular stitch pattern was actually a polygon that has high concentrated linear edges and vertices.
The eight plies of layered [2D2R2]S woven-knit hybrid composite plates that consist of outer layer 2D woven fabric and inner layer of rib knitted fabric were produced by hand lay-up technique at the Composite Manufacturing Laboratory of Usak University. Epoxy used in this study as matrix material was based on CY225 resin and HY 225 hardener. Layered composite plates were produced using a hot lamination press. Curing of composite plates was achieved by retaining specimens under constant pressure of 8 MPa and at 110 ºC for 100 min. Curing was followed by cooling the specimens to room temperature under the same pressure. The thickness of the resulting composite plates was approximately 3mm. Composite plates were cut using diamond tip saw in order to obtain specimens of 100×150 mm2 for impact testing.
2.2 Impact Tests
The low-velocity impacts on the specimens were created using Fractovis Plus impact test machine, which is a drop weight tower, in Composite Research Center at Department of Mechanical Engineering, Dokuz Eylül University. Testing machine has a force transducer with capacity of 22.24 kN. The hemispherical tip of the impactor was of 12 mm diameter and the total impact mass including impactor nose, force transducer, and crosshead was 5.027 kg. The composite specimens with dimensions of 100 mm by 150 mm were clamped on a fixture along a circumference having a 76.2 mm diameter. Four trials were performed at each energy level and layer fabric. The contact force between the impactor and samples, impact velocity and energy, and central deflection of the specimens were recorded as a function of time using a software program called VisualImpact. The impact force value at each time step, F(t), were recorded by data acquisition system (DAS). The specimen deflection was calculated by double integrating the force – time curve F(t) as
δi = ∫ ∫ i
F (t ) − gM total 2 dt M total
(1)
where δi is deflection of the specimen up to point i, F(t) is force acquired by DAS, g is gravity acceleration and Mtotal is total impact mass. The velocity up to point i, was derived from a single integration of force-time curve F(t) as
vi = ∫ i
F (t ) − gM total dt M total
(2)
2.3 Compression after Impact (CAI) Tests
The compression after impact (CAI) tests of stitched woven-knit hybrid composite specimens were conducted at room temperature by using UTEST universal tensile machine with 50 kN load capacity at Department of Mechanical Engineering, Usak University. For determination of the CAI strength of woven-knit glass/epoxy hybrid composite plates, a CAI test fixture is fabricated in accordance with the Boeing CAI test fixture (ASTM D 7137). The fixture is fully adjustable to accommodate small variations in specimen width and thickness. During CAI tests, the specimens are clamped at the top and bottom edges. To prevent buckling of the specimen under compressive load, a lateral support was provided. Compressive test was conducted at a displacement rate of 1 mm/min. During the CAI tests, the force versus displacement history was recorded with a data acquisition system. The CAI strength was obtained from the maximum load divided by the cross sectional area.
3. Results and Discussion
3.1 Impact Behavior of Stitched Woven-Knit Hybrid Composites
The impact testing was carried out in order to investigate the effect of stitch pattern on the impact behavior of [2D2R2]S woven-knit hybrid composites. Impact tests were performed until complete perforation of the specimens. Table 1 shows the energy levels used for stitched and unstitched woven-knit hybrid specimens. The maximum energy levels for each stitch pattern and unstitched case correspond to the energy at which perforation occurs.
Figures 3 – 6 show the contact force – time and contact force - deflection graphs for diamond, square and circular stitch pattern, and unstitched woven-knit hybrid composites, respectively. It can be clearly from the figures that
stitched woven-knit hybrid composites exhibit better impact performance compared to unstitched woven-knit hybrid composites. It is also observed that the perforation threshold energy of woven-knit hybrid composites with diamond, square, circular stitch pattern is 5.5%, 11%, and 22% greater than that of the unstitched woven-knit hybrid composites, respectively. The contact force – deflection graphs (Figures 3b – 5b) show that the perforation threshold energy of woven-knit hybrid composites with square and circular stitch pattern is 5.26% and 15.79% higher than that of the woven-knit hybrid composites with diamond stitched pattern, respectively. These results indicate that circular stitch pattern gives the highest ability to the composite specimens to keep their overall integrity preventing the delaminations and crack propagations that occur during perforation. This ability might be attributed to the fact that circular stitch pattern occupies more space through the composite specimens providing less unstitched area and resulting more dense structure in the composite specimens than that of square and diamond stitch pattern.
The maximum contact force versus impact energy curves of [2D2R2]S woven-knit hybrid composites are given in Figure 7a for better understanding of the stitch patterns on the impact behavior of the glass/epoxy woven-knit hybrid composite materials. It is clear that the maximum contact force – impact energy curve of the stitched wovenknit hybrid composites can be divided into three main regions. Since delamination and matrix cracks occur, the maximum contact force increases rapidly in the first region. In the second region, not only delamination and matrix cracks but also fiber cracks occur so that the maximum contact force does not change significantly. In the last region, it is seen that the maximum contact force decreases because of catastrophic failure of the knitted composites as a result of perforation. According to the maximum impact energy levels at which perforation occurs, specimens with circular stitch pattern has greater perforation threshold energy than that of specimens with diamond and square stitch pattern. This indicates that the specimen with circular stitch pattern can absorb more energy than that of specimens with diamond and square stitch pattern. It is also not easy to say something clear about the maximum contact force of the stitched woven-knit hybrid composites. This behavior suggests that stitching has almost no effect on the impact resistance of the composite specimens until perforation occurs at which delaminations and crack propagations are in critical level.
The contact time versus impact energy curves of stitched [2D2R2]S woven-knit hybrid composites are given in Figure 7b. It should be noted that the contact time is almost same for each stitched woven-knit hybrid composites
until the impact energy reaches perforation threshold energy. The maximum deflection versus impact energy curves of stitched woven-knit hybrid composites are given in Figure 7c. It should be noticed that the maximum deflections of stitched woven-knit hybrid composites increase linearly with increasing impact energy up to the energy at which perforation occurs. After this point the curve exhibits very steep angle due to perforation that cause failure in the stitched woven-knit hybrid composite. Figure 7d shows the permanent deflection versus impact energy curves for each woven-knit hybrid composites. Although the maximum deflection - impact energy curves and permanent deflection – impact energy curves show some similarity the permanent deflection is always lower than that of the maximum deflection as expected due to the rebounding effect. It appears that stitch pattern has almost no effect on the maximum and permanent deflections of the composite specimens.
Figure 8 shows the energy profiling diagram of stitched woven-knit hybrid composites. Area under the contact force-deflection curve gives energy absorbed by the impacted specimen. This energy profiling diagram represents the relation between the impact energy and absorbed energy. Dashed line in the figure represents the equal energy line which is the line that represents the equivalence between the impact energy that impactor has and energy absorbed by the samples. The gap between solid line and dashed line is equal to the excessive impact energy. The excessive energy is retained in the impactor and used to rebound the impactor from the specimen at the end of an impact event [34-35]. Higher excessive energy means less energy absorbed by the stitched composites. It is evident from the results that energy absorption capability of specimen with circular stitch pattern is higher than that of specimens with diamond and square stitch pattern since specimen with circular stitch pattern exhibit higher perforation threshold energy than that of specimens with diamond and square stitch pattern. Also it can be seen that the maximum excessive energy occurs as 6.3 J at the impact energy of 35 J, 5.4 J at the impact energy of 20 J, and 5.1 J at the impact energy of 20 J for woven-knit hybrid composites with circular, diamond, and square stitch pattern, respectively. It should be noted that the maximum excessive energy occurs in the rebounding region.
3.3 Compression after Impact (CAI) Behavior
The first load reaching to nonlinear part of the force-deflection curve is accepted as the critical CAI load [4, 20]. Afterwards, the CAI strength of the specimens is calculated by dividing the critical CAI load to the cross-sectional area of the samples. The CAI strength versus impact energy curves for the woven-knit hybrid composites are shown
in Figure 9. From Figure 9, it can be seen that the CAI strength of woven-knit hybrid composites decreases with increasing impact energy as expected. It is also clear that the woven-knit hybrid composites with square stitch pattern possess the highest CAI strength. Until the impact energy of 30 J, CAI strength of the woven-knit hybrid composites with circular stitch pattern is higher than that of the woven-knit hybrid composites with diamond stitch pattern. The opposite trend is observed after 30 J.
3.4 Damage Mechanism
When a foreign object impacts on a composite laminate, several damage modes including delamination, edge delamination, fiber splitting, fiber cracking and matrix cracking can occur in the composite laminate. These damage modes depend on the impact parameters such as the geometry and mass of the impactor, impact energy and dimension of composite laminate. A high intensity light source behind the specimen was used in order to observe these damage mechanisms of the impacted woven-knit hybrid composites. It is seen that delamination and matrix cracks occur as dominant damage mechanisms until the impact energy of 25 J while fiber cracks and fiber splitting are the dominant damage mechanisms at the impact energies higher than 25 J. Figures 10 – 12 show the impact damage of woven-knit hybrid composites with diamond, square, and circular stitch pattern at three different impact energies, respectively. Figures show that no delamination occurs in the last layer of the woven-knit hybrid composites. Fiber cracks are seen in each layer except the last layer and therefore the last layer is only bended. It is also observed that delamination and matrix cracking are the main damage mechanism at the impact energy levels of rebounding. However, the fiber splitting and delamination are the main damage mechanisms at the impact energy levels of penetration and the fiber cracking and edge delamination are the dominant damage mechanisms at the impact energy levels of perforation.
The impact and CAI damage of the stitched woven-knit hybrid composites are given in Figures 13 – 15. It can be seen from these figures that generally the CAI damage starts around the impact damage and progress up to edges of the specimens. However, it is evident from the figures that CAI damage grows from the edge of the samples and progress through the impact damage for the stitched woven-knit hybrid composites being damaged at even low impact energies.
4. Conclusion
This paper presents an experimental investigation on the impact and post-impact (CAI) behavior of the stitched woven-knit hybrid composite materials based on glass/epoxy. The concluding remarks can be summarized as follows: •
The deflection of the stitched woven-knit hybrid composites increases with increasing impact energy.
•
The perforation threshold energy of woven-knit hybrid composites with diamond, square, and circular- stitch pattern is 5.5%, 11%, and 22% greater than that of the unstitched woven-knit hybrid composites, respectively.
•
Impact damage depends on the impact energy that the impactor has.
•
Delamination and matrix cracks occur as dominant damage mechanisms until the impact energy of 25 J while fiber cracks and fiber splitting are the dominant damage mechanisms at the impact energies higher than 25 J.
•
CAI strength of the stitched composites decreases with increasing impact energy. In addition, square stitched woven-knit hybrid composites have the highest CAI strength.
•
CAI damage starts around the impact damage zone and progress up to the edge of the stitched composite plates.
Acknowledgements
This study was sponsored by The Scientific and Technological Research Council of Turkey (TUBITAK), (Project No: 108M128). Partial financial support by Pul-tech FRP, in Usak-Turkey, was also gratefully acknowledged.
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LIST OF FIGURES Figure 1 The architecture and photograph of (a) rib knitted and (b) double layer (2D) woven fabrics. Figure 2 The schematic of circular (a), square (b), diamond (c) stitch pattern, and woven-knit hybrid composite with through thickness stitching (d). Figure 3 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with diamond stitch pattern at various impact energies. Figure 4 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with square stitch pattern at various impact energies. Figure 5 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with circular stitch pattern at various impact energies. Figure 6 The contact force – time (a) and contact force – deflection (b) curves for unstitched woven-knit hybrid composites at various impact energies. Figure 7 The maximum contact force (a), the contact time (b), the maximum deflection (c), and the permanent deflection (d) versus impact energy curves of woven-knit hybrid composites. Figure 8 The energy profiling diagram of woven-knit hybrid composites. Figure 9 The CAI strength versus impact energy curves for the woven-knit hybrid composites. Figure 10 The impact damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c). Figure 11 The impact damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c). Figure 12 The impact damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c). Figure 13 The CAI damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c). Figure 14 The CAI damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c). Figure 15 The CAI damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c).
(a)
(b) Figure 1 The architecture and photograph of (a) rib knitted and (b) double layer (2D) woven fabrics.
a)
c)
b)
d)
Figure 2 The schematic of circular (a), square (b), diamond (c) stitch pattern, and woven-knit hybrid composite with through thickness stitching (d).
(a)
(b)
Figure 3 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with diamond stitch pattern at various impact energies.
(a)
(b)
Figure 4 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with square stitch pattern at various impact energies.
(a)
(b)
Figure 5 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with circular stitch pattern at various impact energies.
(a)
(b)
Figure 6 The contact force – time (a) and contact force – deflection (b) curves for unstitched woven-knit hybrid composites at various impact energies.
(a)
(b)
(c)
(d)
Figure 7 The maximum contact force (a), the contact time (b), the maximum deflection (c), and the permanent deflection (d) versus impact energy curves of woven-knit hybrid composites.
Figure 8 The energy profiling diagram of woven-knit hybrid composites.
Figure 9 The CAI strength versus impact energy curves for the woven-knit hybrid composites.
Delamination Stitching yarn
Matrix cracking 25 J Front
25 J Back
(a)
Stitching yarn Fiber splitting
45 J Front
45 J Back
(b)
Fiber cracking
Fiber cracking
Stitching yarn
47.5 J Front
47.5 J Back
(c) Figure 10 The impact damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c).
Delamination
Matrix cracking
25 J Front
25 J Back
(a)
Fiber splitting
Matrix cracking 45 J Front
45 J Back
(b)
Edge delamination
Fiber cracking
No delamination
50 J Front
50 J Back
(c) Figure 11 The impact damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c).
Matrix cracking 25 J Front
25 J Back
(a)
Fiber splitting
Matrix cracking 45 J Back
45 J Front
(b)
Fiber cracking
Stitching yarns
55 J Back
55 J Front
(c) Figure 12 The impact damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c).
CAI damage
Impact damage
25 J Back
25 J Front
(a)
Impact damage
CAI damage
45 J Front
45 J Back
(b)
CAI damage Impact damage
47.5 J Front
47.5 J Back
(c) Figure 13 The CAI damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c).
CAI damage
Impact damage
25 J Back
25 J Front
(a)
CAI damage
45 J Front
Impact damage
45 J Back
(b)
CAI damage Impact damage 50 J Back
50 J Front
(c) Figure 14 The CAI damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c).
CAI damage Impact damage 25 J Front
25 J Back
(a)
CAI damage Impact damage 45 J Front
45 J Back
(b)
CAI damage
Impact damage
55 J Back
55 J Front
(c) Figure 15 The CAI damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c).
LIST OF TABLES Table 1 Energy levels used for the impact of woven-knit hybrid composites.
Table 1 Energy levels used for the impact of woven-knit hybrid composites. Stitch pattern
Impact Energy (J)
Diamond
5, 10, 15, 20, 25, 30, 35, 40, 45, 47.5
Square
5, 10, 15, 20, 25, 30, 35, 40, 45, 50
Circular
5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55
Unstitched
5, 10, 15, 20, 25, 30, 35, 37.5, 40, 42.5, 45
S0263-8223(14)00234-7 http://dx.doi.org/10.1016/j.compstruct.2014.05.024 COST 5705
To appear in:
Composite Structures
Please cite this article as: Aktaş, A., Aktaş, M., Turan, F., Impact and post impact (CAI) behavior of stitched wovenknit hybrid composites, Composite Structures (2014), doi: http://dx.doi.org/10.1016/j.compstruct.2014.05.024
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Impact and post impact (CAI) behavior of stitched woven-knit hybrid composites Alaattin Aktaşa, Mehmet Aktaşb*, Fatih Turana a
Istanbul University, Department of Mechanical Engineering, 34320, Istanbul, Turkey b
Usak University, Department of Mechanical Engineering, 64200, Usak, Turkey
Abstract In this study, the effect of stitch pattern on the impact and post impact (CAI) behavior of eight ply woven-knit hybrid composite plates consisting of 2D woven fabrics as outer layers and rib knitted fabrics as inner layers was investigated. Layers of the woven-knit hybrid composite plates were sewed in circular, square and diamond shape through thickness in order to prevent delamination and slippage between the layers. Impact and post impact testing were carried out in order to determine the impact and post impact resistance of three different stitched composite structures. Results showed that the impact strength of composite laminate increased 5.5%, 11% and 22% in the case of square, diamond, and circular stitching, respectively. It was also found that the woven-knit hybrid composites with square pattern have the highest CAI strength.
Keywords: Fabrics/textiles, glass fibers, impact behavior, damage mechanics, stitching.
*Corresponding author: Tel: +90-276-221-2136, Fax: +90-276-221-2137 E-mail: [email protected]
1. Introduction
A structural composite is material system consisting of two or more phases on a macroscopic scale, whose mechanical performance and properties are designed to be superior those of the constituent materials acting independently. The high stiffness, high strength, and low density characteristics make composites highly desirable in aerospace, automotive, defense, and sport industries [1]. However, these materials are susceptible to impact damages. Therefore, the impact behavior of laminated composites has been an important research area for a long time [2-4]. Composite materials dissipate most of the energy created by an impact via some fracture mechanisms such as matrix cracks, delaminations, fiber fracture, fiber–matrix debonding and fiber pull-out. Among fracture mechanisms, delamination has a serious potential to degrade the compressive mechanical properties of the material that causes catastrophic failure of the material abruptly [5]. The delamination growth caused by impact can be characterized by fracture toughness under modes I – II – III and mixed mode loading conditions. Therefore, the interlaminar fracture toughness can be used as a controller of the delamination level in composite structures [6].
Some enhancements of the delamination resistance have been proposed such as matrix toughening [7], adoption of high-strain/high-strength fibers [8], and through thickness reinforcement [9]. Stitching offers a potential to increase impact delamination resistance and improve interlaminar strength and fracture toughness [10 -15].
Some researches [10, 11, 16] have concluded that the through thickness reinforcement reduces the delamination area caused by impact and such reduction has been directly related to the higher interlaminar fracture toughness of stitched laminates. Numerous studies have also conducted out-of-plane impact tests for the stitched laminates, and most of the results confirmed that the out-of-plane stitching improved the impact damage resistance of the FRP laminates [17, 18]. In the case of three dimensional composites, stitched laminates possess greatly improved translaminar properties compared to woven laminates [19].
Compression after impact (CAI) performance remains an important design criterion in the use of composite structures in order to investigate damage tolerance caused by impact loading. Therefore, a number of studies have been reported in the literature in this direction [20 – 26]. Various studies have shown that stitching improves the CAI strength of composite materials owing to capability to reduce impact – induced delamination and improve
interlaminar strength [27-31]. However, the effect of the stitch pattern on the impact and post impact behavior of composite materials has not been considered.
Textile fabrics such as woven and knitted fabrics have long been known as prime reinforcement for composite application owing to their attractive intra- and inter-laminar strengths, damage tolerance, and lower cost and versatile design potential [32]. Hybrid composites have been commonly used in many applications such as automotive and aviation industries due to their high strength, low weight, good corrosion resistance and fatigue life. In this study, hybrid composite plates composed of woven and knitted fabrics were stitched through thickness in square, circular, and diamond stitch pattern. The reason to combine the knitted and woven layers as reinforcement of composite plates is to enhance the energy absorption capability and to determine the optimum impact resistance of knitted and woven composite structures. In the previous study [33], the authors have investigated the effect of stacking sequence on the impact behavior of sequentially stacked woven (2D)-knit (Rib and Milano) fabric glass/epoxy hybrid composites. Woven and knit fabric layers were sequentially stacked in six different variations to fabricate eight ply woven-knit hybrid composites. Results showed that specimens having outer layer of woven fabric exhibited better impact properties than that of the specimens having outer layer of knitted fabric. Thus, in this study, the effect of the stitch pattern on the impact and post impact (CAI) behavior of eight ply woven-knit glass/epoxy hybrid composites in which 2D woven fabric are used as outer layers and rib knitted fabrics are used as inner layers was investigated.
2. Experimental Study
2.1 Specimen Preparation
Rib fabrics (R) are fabricated from 136 tex slightly twisted E-glass yarn using a seven gauge V bed knitting machine. The architecture and photograph of rib and double layer (2D) woven fabrics are shown in Figure 1. Areal weight, course density and wale density of the rib knitted fabrics are 385 g/m2, 4 course/cm and 3.5 wale/cm, respectively. Fabrication of 2 double layer woven fabrics (2D) was mentioned in previous study [34]. The areal weight, warp density and weft density of the 2D woven fabrics are 380 g/m2, 16.5 warp/cm and 18 weft/cm, respectively. The stacking sequences of woven-knitted fabric layers are [2D/2D/R/R/R/R/2D/2D]= [2D2R2]S. In this
notation, “2D” and “R” are referred to as “double layer woven fabric” and “rib fabric”, respectively. The reason for [2D2R2]S woven-knit hybrid composite to be chosen for stitching is that because its impact resistance is the highest [33]. Then square, circular, and diamond stitch patterns through thickness using136 tex glass yarn was applied between woven and knitted fabrics in [2D2R2]S woven-knit hybrid composite as shown in Figure 2. The almost circular stitch pattern was actually a polygon that has high concentrated linear edges and vertices.
The eight plies of layered [2D2R2]S woven-knit hybrid composite plates that consist of outer layer 2D woven fabric and inner layer of rib knitted fabric were produced by hand lay-up technique at the Composite Manufacturing Laboratory of Usak University. Epoxy used in this study as matrix material was based on CY225 resin and HY 225 hardener. Layered composite plates were produced using a hot lamination press. Curing of composite plates was achieved by retaining specimens under constant pressure of 8 MPa and at 110 ºC for 100 min. Curing was followed by cooling the specimens to room temperature under the same pressure. The thickness of the resulting composite plates was approximately 3mm. Composite plates were cut using diamond tip saw in order to obtain specimens of 100×150 mm2 for impact testing.
2.2 Impact Tests
The low-velocity impacts on the specimens were created using Fractovis Plus impact test machine, which is a drop weight tower, in Composite Research Center at Department of Mechanical Engineering, Dokuz Eylül University. Testing machine has a force transducer with capacity of 22.24 kN. The hemispherical tip of the impactor was of 12 mm diameter and the total impact mass including impactor nose, force transducer, and crosshead was 5.027 kg. The composite specimens with dimensions of 100 mm by 150 mm were clamped on a fixture along a circumference having a 76.2 mm diameter. Four trials were performed at each energy level and layer fabric. The contact force between the impactor and samples, impact velocity and energy, and central deflection of the specimens were recorded as a function of time using a software program called VisualImpact. The impact force value at each time step, F(t), were recorded by data acquisition system (DAS). The specimen deflection was calculated by double integrating the force – time curve F(t) as
δi = ∫ ∫ i
F (t ) − gM total 2 dt M total
(1)
where δi is deflection of the specimen up to point i, F(t) is force acquired by DAS, g is gravity acceleration and Mtotal is total impact mass. The velocity up to point i, was derived from a single integration of force-time curve F(t) as
vi = ∫ i
F (t ) − gM total dt M total
(2)
2.3 Compression after Impact (CAI) Tests
The compression after impact (CAI) tests of stitched woven-knit hybrid composite specimens were conducted at room temperature by using UTEST universal tensile machine with 50 kN load capacity at Department of Mechanical Engineering, Usak University. For determination of the CAI strength of woven-knit glass/epoxy hybrid composite plates, a CAI test fixture is fabricated in accordance with the Boeing CAI test fixture (ASTM D 7137). The fixture is fully adjustable to accommodate small variations in specimen width and thickness. During CAI tests, the specimens are clamped at the top and bottom edges. To prevent buckling of the specimen under compressive load, a lateral support was provided. Compressive test was conducted at a displacement rate of 1 mm/min. During the CAI tests, the force versus displacement history was recorded with a data acquisition system. The CAI strength was obtained from the maximum load divided by the cross sectional area.
3. Results and Discussion
3.1 Impact Behavior of Stitched Woven-Knit Hybrid Composites
The impact testing was carried out in order to investigate the effect of stitch pattern on the impact behavior of [2D2R2]S woven-knit hybrid composites. Impact tests were performed until complete perforation of the specimens. Table 1 shows the energy levels used for stitched and unstitched woven-knit hybrid specimens. The maximum energy levels for each stitch pattern and unstitched case correspond to the energy at which perforation occurs.
Figures 3 – 6 show the contact force – time and contact force - deflection graphs for diamond, square and circular stitch pattern, and unstitched woven-knit hybrid composites, respectively. It can be clearly from the figures that
stitched woven-knit hybrid composites exhibit better impact performance compared to unstitched woven-knit hybrid composites. It is also observed that the perforation threshold energy of woven-knit hybrid composites with diamond, square, circular stitch pattern is 5.5%, 11%, and 22% greater than that of the unstitched woven-knit hybrid composites, respectively. The contact force – deflection graphs (Figures 3b – 5b) show that the perforation threshold energy of woven-knit hybrid composites with square and circular stitch pattern is 5.26% and 15.79% higher than that of the woven-knit hybrid composites with diamond stitched pattern, respectively. These results indicate that circular stitch pattern gives the highest ability to the composite specimens to keep their overall integrity preventing the delaminations and crack propagations that occur during perforation. This ability might be attributed to the fact that circular stitch pattern occupies more space through the composite specimens providing less unstitched area and resulting more dense structure in the composite specimens than that of square and diamond stitch pattern.
The maximum contact force versus impact energy curves of [2D2R2]S woven-knit hybrid composites are given in Figure 7a for better understanding of the stitch patterns on the impact behavior of the glass/epoxy woven-knit hybrid composite materials. It is clear that the maximum contact force – impact energy curve of the stitched wovenknit hybrid composites can be divided into three main regions. Since delamination and matrix cracks occur, the maximum contact force increases rapidly in the first region. In the second region, not only delamination and matrix cracks but also fiber cracks occur so that the maximum contact force does not change significantly. In the last region, it is seen that the maximum contact force decreases because of catastrophic failure of the knitted composites as a result of perforation. According to the maximum impact energy levels at which perforation occurs, specimens with circular stitch pattern has greater perforation threshold energy than that of specimens with diamond and square stitch pattern. This indicates that the specimen with circular stitch pattern can absorb more energy than that of specimens with diamond and square stitch pattern. It is also not easy to say something clear about the maximum contact force of the stitched woven-knit hybrid composites. This behavior suggests that stitching has almost no effect on the impact resistance of the composite specimens until perforation occurs at which delaminations and crack propagations are in critical level.
The contact time versus impact energy curves of stitched [2D2R2]S woven-knit hybrid composites are given in Figure 7b. It should be noted that the contact time is almost same for each stitched woven-knit hybrid composites
until the impact energy reaches perforation threshold energy. The maximum deflection versus impact energy curves of stitched woven-knit hybrid composites are given in Figure 7c. It should be noticed that the maximum deflections of stitched woven-knit hybrid composites increase linearly with increasing impact energy up to the energy at which perforation occurs. After this point the curve exhibits very steep angle due to perforation that cause failure in the stitched woven-knit hybrid composite. Figure 7d shows the permanent deflection versus impact energy curves for each woven-knit hybrid composites. Although the maximum deflection - impact energy curves and permanent deflection – impact energy curves show some similarity the permanent deflection is always lower than that of the maximum deflection as expected due to the rebounding effect. It appears that stitch pattern has almost no effect on the maximum and permanent deflections of the composite specimens.
Figure 8 shows the energy profiling diagram of stitched woven-knit hybrid composites. Area under the contact force-deflection curve gives energy absorbed by the impacted specimen. This energy profiling diagram represents the relation between the impact energy and absorbed energy. Dashed line in the figure represents the equal energy line which is the line that represents the equivalence between the impact energy that impactor has and energy absorbed by the samples. The gap between solid line and dashed line is equal to the excessive impact energy. The excessive energy is retained in the impactor and used to rebound the impactor from the specimen at the end of an impact event [34-35]. Higher excessive energy means less energy absorbed by the stitched composites. It is evident from the results that energy absorption capability of specimen with circular stitch pattern is higher than that of specimens with diamond and square stitch pattern since specimen with circular stitch pattern exhibit higher perforation threshold energy than that of specimens with diamond and square stitch pattern. Also it can be seen that the maximum excessive energy occurs as 6.3 J at the impact energy of 35 J, 5.4 J at the impact energy of 20 J, and 5.1 J at the impact energy of 20 J for woven-knit hybrid composites with circular, diamond, and square stitch pattern, respectively. It should be noted that the maximum excessive energy occurs in the rebounding region.
3.3 Compression after Impact (CAI) Behavior
The first load reaching to nonlinear part of the force-deflection curve is accepted as the critical CAI load [4, 20]. Afterwards, the CAI strength of the specimens is calculated by dividing the critical CAI load to the cross-sectional area of the samples. The CAI strength versus impact energy curves for the woven-knit hybrid composites are shown
in Figure 9. From Figure 9, it can be seen that the CAI strength of woven-knit hybrid composites decreases with increasing impact energy as expected. It is also clear that the woven-knit hybrid composites with square stitch pattern possess the highest CAI strength. Until the impact energy of 30 J, CAI strength of the woven-knit hybrid composites with circular stitch pattern is higher than that of the woven-knit hybrid composites with diamond stitch pattern. The opposite trend is observed after 30 J.
3.4 Damage Mechanism
When a foreign object impacts on a composite laminate, several damage modes including delamination, edge delamination, fiber splitting, fiber cracking and matrix cracking can occur in the composite laminate. These damage modes depend on the impact parameters such as the geometry and mass of the impactor, impact energy and dimension of composite laminate. A high intensity light source behind the specimen was used in order to observe these damage mechanisms of the impacted woven-knit hybrid composites. It is seen that delamination and matrix cracks occur as dominant damage mechanisms until the impact energy of 25 J while fiber cracks and fiber splitting are the dominant damage mechanisms at the impact energies higher than 25 J. Figures 10 – 12 show the impact damage of woven-knit hybrid composites with diamond, square, and circular stitch pattern at three different impact energies, respectively. Figures show that no delamination occurs in the last layer of the woven-knit hybrid composites. Fiber cracks are seen in each layer except the last layer and therefore the last layer is only bended. It is also observed that delamination and matrix cracking are the main damage mechanism at the impact energy levels of rebounding. However, the fiber splitting and delamination are the main damage mechanisms at the impact energy levels of penetration and the fiber cracking and edge delamination are the dominant damage mechanisms at the impact energy levels of perforation.
The impact and CAI damage of the stitched woven-knit hybrid composites are given in Figures 13 – 15. It can be seen from these figures that generally the CAI damage starts around the impact damage and progress up to edges of the specimens. However, it is evident from the figures that CAI damage grows from the edge of the samples and progress through the impact damage for the stitched woven-knit hybrid composites being damaged at even low impact energies.
4. Conclusion
This paper presents an experimental investigation on the impact and post-impact (CAI) behavior of the stitched woven-knit hybrid composite materials based on glass/epoxy. The concluding remarks can be summarized as follows: •
The deflection of the stitched woven-knit hybrid composites increases with increasing impact energy.
•
The perforation threshold energy of woven-knit hybrid composites with diamond, square, and circular- stitch pattern is 5.5%, 11%, and 22% greater than that of the unstitched woven-knit hybrid composites, respectively.
•
Impact damage depends on the impact energy that the impactor has.
•
Delamination and matrix cracks occur as dominant damage mechanisms until the impact energy of 25 J while fiber cracks and fiber splitting are the dominant damage mechanisms at the impact energies higher than 25 J.
•
CAI strength of the stitched composites decreases with increasing impact energy. In addition, square stitched woven-knit hybrid composites have the highest CAI strength.
•
CAI damage starts around the impact damage zone and progress up to the edge of the stitched composite plates.
Acknowledgements
This study was sponsored by The Scientific and Technological Research Council of Turkey (TUBITAK), (Project No: 108M128). Partial financial support by Pul-tech FRP, in Usak-Turkey, was also gratefully acknowledged.
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LIST OF FIGURES Figure 1 The architecture and photograph of (a) rib knitted and (b) double layer (2D) woven fabrics. Figure 2 The schematic of circular (a), square (b), diamond (c) stitch pattern, and woven-knit hybrid composite with through thickness stitching (d). Figure 3 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with diamond stitch pattern at various impact energies. Figure 4 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with square stitch pattern at various impact energies. Figure 5 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with circular stitch pattern at various impact energies. Figure 6 The contact force – time (a) and contact force – deflection (b) curves for unstitched woven-knit hybrid composites at various impact energies. Figure 7 The maximum contact force (a), the contact time (b), the maximum deflection (c), and the permanent deflection (d) versus impact energy curves of woven-knit hybrid composites. Figure 8 The energy profiling diagram of woven-knit hybrid composites. Figure 9 The CAI strength versus impact energy curves for the woven-knit hybrid composites. Figure 10 The impact damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c). Figure 11 The impact damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c). Figure 12 The impact damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c). Figure 13 The CAI damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c). Figure 14 The CAI damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c). Figure 15 The CAI damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c).
(a)
(b) Figure 1 The architecture and photograph of (a) rib knitted and (b) double layer (2D) woven fabrics.
a)
c)
b)
d)
Figure 2 The schematic of circular (a), square (b), diamond (c) stitch pattern, and woven-knit hybrid composite with through thickness stitching (d).
(a)
(b)
Figure 3 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with diamond stitch pattern at various impact energies.
(a)
(b)
Figure 4 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with square stitch pattern at various impact energies.
(a)
(b)
Figure 5 The contact force – time (a) and contact force – deflection (b) curves for woven-knit hybrid composites with circular stitch pattern at various impact energies.
(a)
(b)
Figure 6 The contact force – time (a) and contact force – deflection (b) curves for unstitched woven-knit hybrid composites at various impact energies.
(a)
(b)
(c)
(d)
Figure 7 The maximum contact force (a), the contact time (b), the maximum deflection (c), and the permanent deflection (d) versus impact energy curves of woven-knit hybrid composites.
Figure 8 The energy profiling diagram of woven-knit hybrid composites.
Figure 9 The CAI strength versus impact energy curves for the woven-knit hybrid composites.
Delamination Stitching yarn
Matrix cracking 25 J Front
25 J Back
(a)
Stitching yarn Fiber splitting
45 J Front
45 J Back
(b)
Fiber cracking
Fiber cracking
Stitching yarn
47.5 J Front
47.5 J Back
(c) Figure 10 The impact damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c).
Delamination
Matrix cracking
25 J Front
25 J Back
(a)
Fiber splitting
Matrix cracking 45 J Front
45 J Back
(b)
Edge delamination
Fiber cracking
No delamination
50 J Front
50 J Back
(c) Figure 11 The impact damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c).
Matrix cracking 25 J Front
25 J Back
(a)
Fiber splitting
Matrix cracking 45 J Back
45 J Front
(b)
Fiber cracking
Stitching yarns
55 J Back
55 J Front
(c) Figure 12 The impact damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c).
CAI damage
Impact damage
25 J Back
25 J Front
(a)
Impact damage
CAI damage
45 J Front
45 J Back
(b)
CAI damage Impact damage
47.5 J Front
47.5 J Back
(c) Figure 13 The CAI damage of the woven-knit hybrid composites with diamond stitch pattern at 25 J (a), 45 J (b), and 47.5 J (c).
CAI damage
Impact damage
25 J Back
25 J Front
(a)
CAI damage
45 J Front
Impact damage
45 J Back
(b)
CAI damage Impact damage 50 J Back
50 J Front
(c) Figure 14 The CAI damage of the woven-knit hybrid composites with square stitch pattern at 25 J (a), 45 J (b), and 50 J (c).
CAI damage Impact damage 25 J Front
25 J Back
(a)
CAI damage Impact damage 45 J Front
45 J Back
(b)
CAI damage
Impact damage
55 J Back
55 J Front
(c) Figure 15 The CAI damage of the woven-knit hybrid composites with circular stitch pattern at 25 J (a), 45 J (b), and 55 J (c).
LIST OF TABLES Table 1 Energy levels used for the impact of woven-knit hybrid composites.
Table 1 Energy levels used for the impact of woven-knit hybrid composites. Stitch pattern
Impact Energy (J)
Diamond
5, 10, 15, 20, 25, 30, 35, 40, 45, 47.5
Square
5, 10, 15, 20, 25, 30, 35, 40, 45, 50
Circular
5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55
Unstitched
5, 10, 15, 20, 25, 30, 35, 37.5, 40, 42.5, 45