A comparative study on low-velocity impact response of fabric composite laminates

Materials and Design 50 (2013) 750–756

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A comparative study on low-velocity impact response of fabric composite laminates Diantang Zhang a, Ying Sun a,⇑, Li Chen a, Ning Pan a,b a b

Key Laboratory of Advanced Textile Composites, Tianjin Polytechnic University, Ministry of Educational, Tianjin 300387, China Division of Textiles and Clothing, Biological and Agricultural Engineering Department, University of California, Davis, CA 95616, United States

a r t i c l e

i n f o

Article history: Received 3 January 2013 Accepted 8 March 2013 Available online 24 March 2013 Keywords: Composite laminates Ultrahigh Molecular Weight Polyethylene Low velocity impact Single-ply 3D orthogonal woven fabric

a b s t r a c t Impact behaviors at low velocity of composite laminates reinforced with fabrics of different architectures are investigated. Unidirectional prepreg, 2D woven and 3D orthogonal fabrics, all formed of Ultrahigh Molecular Weight Polyethylene (UHMWPE) filaments, were selected as reinforcements to form composite laminates using hot pressing technology. Low velocity impact tests were conducted using a dropweight impact equipment at the energy level of 35 J. A three-coordinate measuring device was employed to determine the volume of plastic deformation and surface dent diameter. The results show that the composite laminates of single-ply 3D orthogonal woven fabric exhibit better energy absorbed capacity and impact damage resistance as compared to those of unidirectional and 2D plain-woven fabric. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Composite laminates are increasingly used in aerospace, military, civil engineering and automobile industries due to their inherently light weight, high strength and stiffness and tailorability. In those service conditions, transverse impact at low velocity is the normal form of loading which can cause internal damage such as delamination, matrix cracking, local permanent deformation and fiber breakage, leading to a reduction of load carrying capacity of the composite structures. Furthermore, catastrophic failure may occur when the composite laminates are serviced in such damaged state [1–5]. Hence, there have been many studies focusing on the improvement of damage tolerance and energy absorption. So far, methods of achieving the goals mainly include the design of fiber reinforcements and the use of tougher resin matrix [6,7]. Once the constitutive materials are selected, it is the most important to understand how single-layer fabric architectures affect the low-velocity impact response of composite laminates becomes essential for better design. Unidirectional prepreg and 2D plain-woven fabric are two typical single-layer architectures used in textile composite laminates. The low-velocity impact behaviors of such composite laminates have been studied extensively [7–22]. For example, Bibo and Hogg [8] reviewed the role of reinforcement architecture on the impact damage mechanisms of composite laminates, and investigated the relationships between the specific fabric architectures and the damage development processes. Aktas et al. [9] discussed the ⇑ Corresponding author. Tel.: +86 13920379205; fax: +86 2224528448 4. E-mail address: [email protected] (Y. Sun). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.03.044

damage modes and damage processes in unidirectional glass/ epoxy composite laminates under low-velocity impact loading. Baucom and Zikry [10] studied the effects of reinforcement geometry on the damage progression in 2D plain-woven composite laminates under the drop-weight impact loading. Belingardi [11] proposed two energy absorption parameters and two relevant characteristic values of the impact force history to describe the impact behavior of the material by testing 2D plain-woven reinforced composite laminates. Naik et al. [12,13] used a 3D transient finiteelement analysis code to examine the responses of unidirectional and 2D plain-woven composite laminates subjected to low velocity impact. Their results confirmed that woven fabric laminates are more resistant to impact damage than that of unidirectional laminates. Kim and Sham [7] studied the potential advantages of 2D plain-woven fabric composite laminates as opposed to unidirectional composite laminates from the relationship of the microstructure and the mechanical properties. More recently, Evci and Gulgec [14] analyzed the impact growth and occurance of Hertzian failure of unidirectional E-Glass, 2D plain-woven E-Glass and 2D plain-woven Aramid composite laminates. Fan et al. [15] developed a finite element model to predict the impact behaviors of glass fiber composites subjected to low velocity impact loading. Literature reviews reveals that unidirectional composite laminates have shown excellent energy absorption capacity under low-velocity impact loading, due to the in-plane fiber continuity and linearity, whereas serious delaminations are often the major concern. Although 2D plain-woven fabric composite laminates can offer better impact resistance just because that the two sets of yarns are interlaced in mutually perpendicular directions, it helps little in reducing the delamination of composite laminates.

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160

Cooling

Compacting

Preheating

140

10

Fig. 1. Schematic of single-ply 3D orthogonal woven fabric.

5

100

8

/min 0.4

80

5

/min

6

/min

60

4

Pressure (MPa)

Temperature (

)

120

40

Table 1 Parameters of different UHMWPE architecture fabrics.

2

20

Fabric type

Warp/weft/z-yarn density/ tows (cm)

Area density (g/ m2)

Thickness (mm)

UD 2D-P 3D-S

– 9.5/8.5/– 9.5/7/9.5

80 ± 5 210 ± 5 365 ± 5

0.10 0.28 0.55

0

0

20

40

60

80

100

120

140

0 180

Time (min) Fig. 4. Cure cycles for UHMWPE/LLDPE composite laminates by using hot pressing.

Searching for higher damage tolerance, higher energy absorption and better structural integrity of composite laminates, single-ply 3D orthogonal woven fabric is presented and studied in this paper. Single-ply 3D orthogonal woven fabric consists of three sets yarns, warp yarns, weft yarns and Z-yarns (see Fig. 1). It has two significant characteristics: (a) both the warp and weft yarns remain straight to the maximal degree so as to make the fabric achieve higher in-plane mechanical properties, and (b) the fracture toughness is improved due to the presence of the Z-yarns in the throughthickness direction. Thus, considerably enhanced impact properties of single-ply 3D woven composite laminates can be expected. To the best of the authors’ knowledge, little work has been done to examine the low-velocity impact response of such single-ply 3D woven composite laminates. Ultrahigh Molecular Weight Polyethylene (UHMWPE) fiber is known for their low density (0.97 g/cm3), high toughness fracture

Table 2 Specifications of UHMWPE composite laminates. Fabric architecture

Lay angle (°)

Number of fabric plies

Fiber volume fraction (%)

Thickness (mm)

UD

0/90 0/90 0/90

40 40 40

76.69 76.72 75.83

3.44 3.44 3.48

2D-P

– – –

13 13 13

78.48 78.49 78.04

3.50 3.50 3.52

3D-S

– – –

7 7 7

73.39 72.98 73.08

3.54 3.56 3.56

strain (3%) and excellent energy absorption capacity which make them top choice to form fibers in making the various reinforce-

Fig. 2. The surface photographs of fabrics: (a) UD, (b) 2D-P and (c) 3D-S.

Polyethylene film UD 2D-P

3D-S

(a)

160

(b)

(c)

Fig. 3. Schematic of stacking sequence: (a) UD composite laminates, (b) 2D-P composite laminates and (c) 3D-S composite laminates.

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Impactor Specimen Fixture

Fig. 5. Drop-weight impact test setup.

UHMWPE/LLDPE composite laminates were manufactured using the hot pressing technology. Firstly, a number of fabrics and films were stacked in different ways, as shown in Fig. 3. Then the laminated structures of fabrics and resin films were compacted to be the composite laminates using the hot-pressing method. The processing parameters including time, temperature, pressure, should be controlled strictly, as given in Fig. 4. Rectangular specimens for all the impact testing were cut to the final dimensions of 150 mm  100 mm according to the standard of ASTM: D7136. Three specimens were evaluated for each structure. Specifications of UHMWPE composite laminates are listed in Table 2. Here, the mean fiber volume fractions are about the same level for all three kinds of composite laminates. 2.2. Low-velocity impact test Low-velocity impact tests were carried out using a drop-weight testing system (Instron Dynatup 9250HV) illustrated in Fig. 5. It mainly consists of an impactor with a load transducer mounted on, attached to the crosshead to provide a drop-weight of 6.5042 kg for all tests. The impactor end was hemisphere shaped with a diameter of 12.7 mm. The four corners of a specimen were mounted to the fixture so the impact point was located in sample center. The initial impact energy was set to 35 J for all the composite laminates. The transient impact responses of the specimen included load and energy as function of time. Peak load, time to peak load and absorbed energy also were recorded. 2.3. Reverse scanning test

Fig. 6. Three-coordinate measuring device.

ments for composites. In the present study, the low-velocity impact behaviors of UHMWPE composite laminates reinforced with three kinds of single-layer fabric structures were investigated using the drop weight impact equipment at an energy level of 35 J. The single-layer fabric structures included unidirectional prepreg, 2D plain-woven fabric and single-ply 3D orthogonal woven fabric. Load and energy versus time responses were recorded, and the key impact parameters like peak load, time to peak load and absorbed energy were obtained and evaluated. Damage mechanisms were also analyzed from the digital images and reverse scanning graphs using a three-coordinate measuring device. 2. Experimental details 2.1. Materials and specimen preparation Three kinds of UHMWPE fabrics with different architectures, unidirectional prepreg (UD), 2D plain-woven fabric (2D-P), single-ply 3D orthogonal woven fabric (3D-S), were selected for the single layer of laminates in this study. UD and 2D-P were the products of Yizheng Chemical Fiber Company (China). 3D-S was fabricated using the 3D horizontal weaving machine at the Institute of Composite Materials of Tianjin Polytechnic University (China). The parameters of different UHMWPE architecture fabrics are shown in Table 1. The surface photographs of fabrics are shown in Fig. 2. Linear low density polyethylene (LLDPE) was used as the resin system in the form of film for all the composite laminates. The resin content of composite laminates was about 20% by weight. The

Reverse scanning tests were conducted to acquire the 3D topographies of composite laminates after the impact test by using a three-coordinate measuring device, as shown in Fig. 6. Then the volume of plastic deformation and dent diameter were calculated precisely using the Geomagic Studio software. 3. Results and discussions 3.1. Characteristics of impact response Figs. 7 and 8 show the typical load and energy versus time responses for all three kinds of specimens with similar fiber volume fraction at an incidental impact energy of 35 J, respectively. These curves can provide a comprehensive depiction of the damage initiation and growth, as well as changes of the specimen stiffness [3,10]. It can be noticed from Fig. 7a–c that there was a sudden load drop (point A) at the beginning of the strike for all specimens. The point A is named as Hertzian failure which denotes the incipient damage mainly in the form of interlaminar delamination [23– 25]. Sevkat et al. [26] suggested that the sudden load drop was an indication of an abrupt transition of the specimen from an intact state to a damaged state. However, as the impact continues, there were obviously different trends for different single-layer fabrics composite laminates (see Fig. 7). As shown in Fig. 7a, more load oscillations were observed after the point A for UD composite laminates. The occur of the point B and point C is may due to the expansion of the damage, which also results in the reduction of composite laminates stiffness [20]. The load decreased suddenly after reaching the peak level (Fmax). This result can also be found in Shyr and Pan [4]. They suggested that the trend could be attributed to the critical structure damage. Then there was a load redistribution of the surviving composite laminates until the impact load was removed. Unlike UD composite laminates, 2D-P specimens exhibited a much smoother curve in Fig. 7b with little fall of load at Point B before the peak load (Fmax). As compared to UD

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1.5

Fmax 1.5

Fmax

Load (kN)

Load (kN)

1.0

1.0

C

B 0.5

B

0.5

A A 0.0

0

2

4

6

8

10

0.0

12

0

2

4

Time (ms)

6

8

10

12

Time (ms)

(a)

(b) 2.0

Fmax

Load (kN)

1.5

1.0

0.5

A 0.0

0

2

4

6

8

10

12

Time (ms)

(c) Fig. 7. Load versus time response of composite laminates: (a) UD composite laminates, (b) 2D-P composite laminates and (c) 3D-S composite laminates.

and 2D-P composite laminates, the load–time curve of 3D-S composite laminates (Fig. 7c) was smooth other than the initial load drop at point A at the onset of the impact. Hence, it is demonstrated in [27] that the 3D-S specimens are able to withstand higher stress levels. As to the curves of energy versus time in Fig. 8, they indicated the impact absorbed energy by the samples in test and can be characterized with three distinct zones. This trend was also found in Refs. [14,27]. At the first stage (Zone I), the values of absorbed energy were relatively low. That can be attributed to the small dent and deformation along the thickness direction under the transverse impact load. At the second stage (Zone II), the energy-time curves exhibited an increase in slope, representing an augment in deflection, even internal damage. Thus, in this stage, the absorbed energy is mainly due to the increase of contact area between the impactor and the specimen. At the last stage (Zone III), the absorbed energy maintained a constant value. For better interpretation of the experimental results, the key impact parameters including peak load, time to peak load and absorbed energy for three kinds of samples are summarized in Fig. 9. Here, each value is the average of three samples tested. The peak load under the same impactor is in fact an indicator of the load buffering capacity of the composite laminates, directly related to the initial rigidity of specimens. From Fig. 9a, it was shown

that the peak loads of both UD specimens (1.52 kN) and 3D-S samples (1.53 kN) were higher than that of 2D-P specimens (1.12 kN). This result is consistent with the investigation in [14]. As mentioned before, the fibers in UD composite laminates, and the warp and weft yarns in 3D-S specimens all maintain a straight alignment inside the composite structure, leading to a higher initial in-plane stiffness. However, the warp and weft yarns in 2D-P specimens are undulated because of the interlacing each other. Hence, 2D-P composite laminates have lower in-plane stiffness comparing with other two kinds specimens. The numerical rankings in both Fig. 9a–c were consistent among the 3 type composites, suggesting that the initial in-plane stiffness also determines the capacity of the composite impact energy absorption. It should be noted that the absorbed energy of 3D-S composite laminates (21.34 J) exceeded that of UD (19.60 J) by 8.15% and that of 2D-P (16.68 J) by 21.83%. In Fig. 9b, it was interesting to see that ranking for the time to peak load is different, i.e., 3D-S > 2D-P > UD specimens. The main cause for this may lie in the structural complexity among the specimens. The unidirectional UD specimens possess clearly the simplest structures, followed by woven fabric reinforced 2D-P specimens, and then single-ply 3D orthogonal woven fabric reinforced 3D-S specimens. Richardson and Wisheart [28] indicate that the contact duration is long enough for the entire architecture to

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20

Zone I

Zone III

Zone II

Zone I

15

Energy (J)

Energy (J)

15

10

5

0

Zone III

Zone II

10

5

0

2

4

6

8

10

0

12

0

2

4

6

Time (ms)

Time (ms)

(a)

(b)

8

10

12

25

Zone I

Zone II

Zone III

20

Energy (J)

15

10

5

0

0

2

4

6

8

10

12

Time (ms)

(c) Fig. 8. Energy versus time response of composite laminates: (a) UD composite laminates, (b) 2D-P composite laminates and (c) 3D-S composite laminates.

respond to the impact loading and in consequence more energy may be absorbed. Thereby, the presence of Z-yarns results in an increased inherent energy absorption and time of the impact force transmission. 3.2. Damage analysis The failure modes of the specimens are illustrated in Fig. 10. The surface dent diameter and the volume of plastic deformation are listed in Table 3. Again each value is the average of three specimens tested. For the three specimens, it was seen clearly from the graphs that they resulted in different surface dent and plastic deformation. As shown in Table 3, both the surface dent diameter and the volume of plastic deformation of 2D-P composite laminates were the highest, followed by 3D-S composite laminates, and trailed by UD composite laminates. Moreover, there were serious delaminations in UD composite laminates, and less delamination damage in 2D-P composite laminates, while no visible damage was found in 3D-S composite laminates (see Fig. 10). The conclusion is consistent with the results of Fig. 7. That is the inflection points on the corresponding load–time curves suggest the damage generation and propagation [3,29].

From the above results, it can be concluded that 3D-S samples are superior to both UD and 2D-P ones in terms of energy absorbed and impact resistance. Up on the impactor contacts the surface of the composite samples, the stress waves spread along both the in-plane fiber direction and the through-thickness direction. The Z-yarn in 3D-S samples reinforces the system in the thickness direction, thus effectively preventing delamination from occurring. Furthermore, in 3D-S systems, the weft and warp yarns provide an energy dissipation in-plane path, and the Z-yarns do the same valuable thing along the through-thickness direction, thus leading to a much better impact resistance. In this paper, it demonstrates that 3D-S specimens are advantageous in structural application to resist low rate mechanical impact, mainly attributed to the existence of Z-yarns in the through-thickness direction. In addition, the tensile properties of E-glass composites reinforced with different fabrics, plain and single-ply 3D orthogonal woven fabric, were reported in Ref. [30]. Results of their study indicate that single-ply 3D orthogonal woven fabric composites showed higher in-plane ultimate failure stresses and strains as compared to plain woven fabric composites. Hence, further study is expected to examine how the manipulation and design of in-plane yarns and Z-yarns can optimize the energy dissipation, and damage initiation/evolution. Wider range of impact

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5

1.6 4

Time to Peak Load (ms)

1.4

1.0 0.8 0.6 0.4

3

2

1

0.2 0

0.0

UD composite laminates

2D-P composite laminates

3D-S composite laminates

UD composite laminates

2D-P composite laminates

(a)

3D-S composite laminates

(b) 25

20

Absorbed Energy (J)

Peak Load (kN)

1.2

15

10

5

0

UD composite laminates

2D-P composite laminates

3D-S composite laminates

(c) Fig. 9. Impact parameters of three kinds of UHMWPE composite laminates: (a) peak load, (b) time to peak load and (c) absorbed energy.

UD composite laminates

2D-P composite laminates

3D-S composite laminates

Front

Back

Front

Side Fig. 10. Failure modes and reverse scanning images for different architectures of composite laminates.

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Table 3 The value of surface dent diameter and plastic deformation volume. Fabric type

Dent diameter (mm)

Volume of plastic deformation (mm3)

UD 2D-P 3D-S

6.5 9.6 8.4

24606 65596 32382

force, other mechanical loading will be employed to further examine the behavior of 3D-S systems with more comprehensive performance metrics than just the impact energy value. 4. Conclusions The low-velocity impact tests on three types of UHMWPE/ LLDPE composite laminates reinforced respectively with unidirectional prepreg, 2D plain-woven fabric and single-ply 3D orthogonal woven fabric were performed in this paper. The influences of single-layer fabric structure on the impact responses of UHMWPE composite laminates at an energy level of 35 J were investigated. Following conclusions were drawn from the study: (1) Compared to both unidirectional and 2D plain-woven fabric reinforced specimens, single-ply 3D orthogonal woven fabric composite laminates exhibited better performance in impact energy absorption and delamination resistance. (2) Under low velocity impact, the initial in-plane stiffness is very important for the composite performance. As the warp and weft yarns in 2D-P samples are interlaced and thus more undulated, hence reducing its in-plane stiffness, 2D-P samples performed very poorly in tests. For instance ranking in terms of energy absorption was 21.34 J for 3D-S composite laminates, 19.60 J for UD composite laminates and 16.68 for 2D-P composite laminates. (3) Further study is desirable to examine how the manipulation and design of in-plane yarns and Z-yarns can optimize the energy dissipation, and damage initiation/evolution for 3DS systems.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant number 11102133) and the Science and Technology Research Project of Chinese Ministry of Education (Grant number 211007). References [1] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44(4):155–90. [2] Cantwell WJ, Morton J. The impact resistance of composite materials – a review. Composites 1991;22(5):347–62.

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