Quasi-static penetration resistance behavior of glass fiber reinforced thermoplastic composites

Composites: Part B 43 (2012) 3391–3405

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Quasi-static penetration resistance behavior of glass fiber reinforced thermoplastic composites Ömer F. Erkendirci a,b, Bazle Z. (Gama) Haque b,⇑ a b

Gaziantep University, Gaziantep Technical College, 27310 Gaziantep, Turkey University of Delaware Center for Composite Materials (UD-CCM), University of Delaware, Newark, DE 19716, USA

a r t i c l e

i n f o

Article history: Received 1 November 2011 Received in revised form 2 January 2012 Accepted 10 January 2012 Available online 20 January 2012 Keywords: A. Glass fibers A. Thermoplastic resin D. Mechanical testing E. Compression molding Quasi-static punch shear

a b s t r a c t Quasi-static penetration resistance of a composite structure represents the energy dissipating capacity of the structure under transverse loading without dynamic and rate effects. In this paper, a comparative study of the quasi-static penetration resistance behavior of S-2 Glass/SC-15, S-2 Glass/HDPE and E-Glass/HDPE composite systems with varying thicknesses, i.e., 1.4–8.4-mm, is presented using the Quasi-Static Punch Shear Test (QS-PST) methodology developed earlier. The penetration resistance behavior is usually presented by a series of force–displacement graphs at different support conditions, the integral of which is the energy dissipated by the composite during the quasi-static penetration at corresponding support conditions. The penetration energy varies with the diameter of the support span which is usually higher than the punch diameter, and also with the thickness of the composite laminate. During QS-PST experiments, a flat punch of diameter 7.6-mm with a range of support spans 8.89–50.8mm has been used to obtain varying support span to punch diameter ratios (i.e., SPR = DS/DP = 1.16, 1.33, 1.67, 2.00, 2.33, 2.67, etc.). In order to compare the penetration resistance behavior of three different material systems, the S-2 Glass/SC-15, S-2 Glass/HDPE and E-Glass/HDPE composites of identical layer counts are used and the S-2 Glass/SC15 composite system is considered as the baseline. Composite plate specimens are sectioned after the test and then dipped into an ink–alcohol solution to study the damage mechanisms at different SPRs. Non-linear penetration stiffness and an average penetration resistance force are defined to quantify the average penetration resistance of each material. S-2 Glass and E-Glass reinforced HDPE composite material showed lower stiffness, lower peak force, higher deflection, lower damage area, and lower energy dissipation as compared to the baseline. A detailed comparison of results is presented. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Composite materials have been used in military and other industrial applications intensely because of their energy dissipation capabilities, damage tolerance, and light weight. In addition to their excellent quasi-static properties such as high specific stiffness and strength, these composite structures perform well under various types of impact loading due to their inherent high fracture toughness [1–19]. Thermoset composite materials play crucial role of dissipating energy due to various inter-laminar and intralaminar damage mechanisms such as matrix cracking, tensile fiber failure, transverse shear failures, and delamination, and are critical in determining the properties of composites for low-velocity impact and ballistic applications [3–18]. Due to their high temperature processing requirements, thermoplastic composites are less

⇑ Corresponding author. Tel.: +1 302 831 0248. E-mail address: [email protected] (B.Z. (Gama) Haque). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2012.01.053

used in high velocity impact applications [20–30]. Absence of matrix cracking and high delamination resistance, however, demands the use of thermoplastic composites in ballistic applications. Impact performance and other properties of glass/epoxy composites have been studied and investigated extensively in recent years [5–15] and many technical articles about impact on composites, are available. The main goal in optimizing these materials for use in ballistic applications is to understand their characteristic properties and to evaluate their energy dissipating impact damage mechanisms. Gama and Gillespie [18] developed a Quasi-Static Punch Shear Test (QS-PST) methodology to quantify and partition the penetration energy into elastic and dissipated energies as a function of penetration displacement and support spans. Based on this QS-PST experimental methodology, a ‘Quasi-Static Penetration Model’ of ballistic penetration is developed to mimic different phases of ballistic penetration. The quasi-static energy dissipation due to material damage is shown to be 69% of the total ballistic limit energy for the S-2 Glass/SC15 composite. Also, Gama et al. [8,9,12,13] studied the punch shear behavior of thick-section

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Nomenclature SPR DS DP AD HDPE HC

qC #L

vf

support span to punch diameter ratio support span diameter punch diameter areal density high density poly-ethylene specimen thickness specimen density number of layers in a laminate fiber volume fraction

composites under quasi-static, low velocity, and ballistic impact loading and identified the energy dissipating damage mechanisms during quasi-static punch shear of thick-section composites. However, penetration resistance behavior of fiber reinforced thermoplastic composite system is not available in the literature, and is the motivation for the present research. This research builds upon the earlier development of a QuasiStatic Punch Shear Test (QS-PST) methodology in the study of penetration mechanics of composite materials [18]. The QS-PST experimental methodology uses several support spans with a constant diameter punch to mimic ballistic impact damage mechanisms and is also able to quantify the QS-penetration energy. Previous QS-PST experiments were performed on a baseline plain weave (PW, 5  5 tows/in., 24 oz/yd2) S-2 glass/SC15 composite system with a 12.7-mm diameter punch. The main objective of the present study is to compare the penetration resistance behavior of glass/polyethylene composites with the baseline glass/epoxy composites. In achieving this primary goal, plain weave S-2 Glass and E-Glass fabrics of different layer counts are compression molded using high density poly-ethylene (HDPE) films. Baseline PW S-2 glass/SC15 composites of equivalent layer counts are also fabricated using vacuum assisted resin transfer molding (VARTM) process. Composite laminates of different thickness thus obtained are tested following the QS-PST methodology using a 7.62-mm diameter flat punch at a wide range of support span to punch diameter ratio i.e., at SPR = DS/DP = 1.16, 1.33, 1.67, 2.00, 2.33, 2.67, etc. Penetration mechanics behavior of these materials is quantified in terms of energy dissipation, perforation stiffness, and energy dissipating damage mechanisms. 2. Experimental 2.1. Material and material processing Baseline PW S-2 glass/SC-15 composites are made from PW S-2 glass fabric (5  5 tows/in., 24 oz/yd2) cut into 25-in.  18-in. (635-mm  457-mm) sheets. The panels of various layer counts, i.e., 2-layers (2L), 3L, 4L, 6L, 8L, and 12L (with stacking sequence [0]N); are infused with SC-15 resin using the vacuum assisted resin transfer molding (VARTM) process. The panels are cured at room temperature (RT, 23 °C, 73 °F) for 8 h and post-cured at 115 °C (240°F) for 4 h under vacuum. PW S-2 glass/HDPE and E-Glass/HDPE composites are fabricated using the same fabric of various layer counts (i.e., 2L–12L) by a compression molding method. During compression molding, commercial HDPE films (thickness = 0.07-mm) are laid on the top, bottom, and in between each glass layer following the stacking sequence presented in Table 1. Glass fabrics and HDPE films are cut into 305-mm by 305-mm (12-in.  12-in.) sheets. Stacking sequence presented in Tables 1 and 2 is followed and the stacks of glass fiber and HDPE films are centered on the bottom platen of the hot press (Fig. 1a). The

F x ED Fmax Kmax dmax LPST b1 h2

resistance force displacement dissipated energy maximum force maximum stiffness displacement at maximum load QS-PST length scale dimensionless thickness parameter dimensionless inverse thickness parameter

platens are closed and the hot press plates are heated to 356°F and the compression pressure is set to 2.69 MPa (390 psi). Once the temperature of the platens reaches 356 °F, the compression pressure is increased to 13.44 MPa (1950 psi) and held constant for 5-min for the first glass layer and an additional 3-min for each additional glass layers. After this first compression cycle (Fig. 1b), the platen temperature is reduced to 302 °F, while the pressure is maintained at 13.44 MPa until the temperature reaches 302 °F. At 302 °F platen temperature, the pressure is increased to 40.33 MPa (5850 psi) and held for a total of 5-min (for all layers) followed by cooling to room temperature (75 °F). Once the platen temperature reaches 75 °F, the glass fiber reinforced thermoplastic composite laminate is taken out of the compression molding frame. The thickness, dimensions, and mass of the thermoplastic composite laminates are measured to calculate the density and arealdensity of the composite materials. Density and fiber volume fraction of the composite panels are measured following ASTM standard ASTM D2584. The areal-densities of the composite materials are calculated from the measured density and thickness of the composite panels:

AD ¼ qC HC

ð1Þ

where qC is the average composite density and HC is the average thickness of the composite laminates. Mass and geometric properties of all composite laminates are shown in Tables 3 and 4.

Table 1 Stacking sequence of S-2 Glass/HDPE composite laminates. Material

S-2 Glass/HDPE

# of L 2L 3L 4L 6L 8L 12L

Stacking sequence [HDPE4/S2G1/HDPE4]S (HDPE4/S2G1/HDPE8)/S2G1/(HDPE8/S2G1/HDPE4) [HDPE4/S2G1/HDPE4]2S [HDPE4/S2G1/HDPE4]3S [HDPE4/S2G1/HDPE4]4S [HDPE4/S2G1/HDPE4]6S

Table 2 Stacking sequence of E-Glass/HDPE composite laminates. Material

E-Glass/HDPE

# of L 2L 3L 4L 6L 8L 12L

Stacking sequence [HDPE4/EG1/HDPE4]S (HDPE4/EG1/HDPE8)/EG1/(HDPE8/EG1/HDPE4) [HDPE4/EG1/HDPE4]2S [HDPE4/EG1/HDPE4]3S [HDPE4/EG1/HDPE4]4S [HDPE4/EG1/HDPE4]6S

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Fig. 1. Compression molding hot press and the temperature profile.

Table 3 Mass and Geometric Properties of PW S-2 Glass/HDPE and PW S-2 Glass/SC-15 Composite Laminates. S-2 Glass/HDPE

S-2 Glass/SC-15

vf

cm3)

AD (kg/ m2)

1.47 1.40 1.47 1.49 1.40 1.47

2.17 3.05 4.10 6.51 7.64 12.42

0.53 0.55 0.58 0.60 0.56 0.55

# L

qC (g/

2 3 4 6 8 12

HC (mm)

# L

qC (g/

AD (kg/ m2)

vf

cm3)

HC (mm)

1.48 2.18 2.80 4.36 5.46 8.43

2 3 4 6 8 12

1.57 1.67 1.67 1.69 1.75 1.79

2.19 3.51 4.54 7.01 9.33 13.48

0.54 0.49 0.47 0.51 0.46 0.50

1.40 2.10 2.77 3.95 5.34 7.52

Table 4 Mass and geometric properties of E-Glass/HDPE composite laminates. E-Glass/HDPE #L

qC (g/cm3)

AD (kg/m2)

vf

HC (mm)

2 3 4 6 8 12

1.43 1.43 1.43 1.42 1.43 1.44

2.19 3.35 3.91 5.74 8.36 11.92

0.564 0.565 0.586 0.595 0.598 0.603

1.24 2.09 2.84 4.27 5.62 8.25

2.2. Quasi-Static Punch Shear Testing (QS-PST) A Quasi-Static Punch Shear Test (QS-PST) methodology has been developed for studying the energy dissipating damage mechanisms and penetration resistance behavior of thick section composites [1–3]. QS-PSTs are performed using a custom made steel fixture which consists of a square bottom support plate, a matching top cover plate, a punch guide, and a punch (Fig. 2). Two QSPST test fixtures are used, i.e., (i) the Small QS-PST Fixture (Fig. 2a) and (ii) the Medium QS-PST Fixture (Fig. 2b). The Small QS-PST Fixture has a bottom support plate of dimension 101.6mm  101.4-mm  38.1-mm (4-in.  4-in.  1.5-in.) with a centered hole of diameter 76.2-mm (3-in.) bored 25.4-mm (1-in.) deep, and is capable of housing many support rings of various diameters. There is also a 25.4-mm diameter hole in the support plate which provides access from the rear side of the support plate. Around the perimeter of the support plate are eight 1/4–16 bolt holes to secure the cover and the support plates while clamping the composite specimen between them. Fig. 2a shows a schematic cross-sectional diagram of the ‘Small QS-PST Fixture’. The inner hole diameter of the punch guide is 12.70 + 0.01-mm through which a two-step cylindrical punch of shank diameter 12.70-mm can slide through. The neck of the cylindrical punch may contain cylindrical punch heads of diameters ranging from DP = 7.62– 12.70-mm. The length of the neck and shank of the two-step punch can be determined from the thickness of the composite specimen

Punch Shank

Punch Shank

Punch Guide

Punch Guide Punch Neck

Punch Neck

Punch Head

Punch Head Cover Plate

Specimen

Cover Plate Specimen

DP HC

DS

DP

Support Cylinder HC

Support Plate

Through Hole

DS

Through Hole

(a) Small QS-PST Fixture

(b) Medium QS-PST Fixture

Fig. 2. Schematic diagram of the cross-section of the QS-PST fixtures.

Support Cylinder Support Plate

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and the desired penetration depth. Fig. 2b shows a similar schematic of the ‘Medium QS-PST Fixture’. The overall in-plane dimension of the ‘Medium’ fixture is 152.4-mm  152.4-mm  50.8-mm

(6-in.  6-in.  2-in.) with a centered hole of diameter 101.6-mm (4-in.) bored 25.4-mm (1-in.) deep, and is very similar to the ‘Small’ fixture. In both the fixtures, concentric rings (thickness

Fig. 3. The Small and Medium QS-PST Fixtures with the driving nose.

30 1.16, 13.25 1.33, 10.87 1.67, 10.53 2.00, 10.17 2.33, 16.30 2.67, 15.27 3.00, 17.65 3.37, 13.57 5.00, 16.16 6.67, 16.65 SPR, ED (kJ)

4 Layer S-2 Glass/SC-15 DP=7.62 mm

10 8 6

Resistance Force, F, kN

Resistance Force, F, kN

12

4 2

1.16, 112.52 1.33, 100.26 1.67, 83.84 2.00, 88.91 2.33, 104.45 2.67, 98.49 3.00, 103.77 3.37, 102.20 5.00, 99.46 6.67, 109.66 SPR, ED (kJ)

12 Layer S-2 Glass/SC-15 DP=7.62 mm

25 20 15 10 5

0

0 0

5

10

15

20

25

30

0

5

Displacement,x,mm

10

15

20

25

30

Displacement,x,mm

(a) 4L PW S-2 Glass/SC15

(b) 12L PW S-2 Glass/SC15

Fig. 4. Resistance force–displacement behavior of PW S-2 Glass/SC15 composites.

30 1.16, 19.38 1.33, 22.85 1.67, 18.10 2.00, 30.35 2.33, 34.87 2.67, 44.05 3.00, 42.17 3.37, 41.70 5.00, 50.99 6.67, 61.85 SPR, ED (kJ)

4 Layer S-2 Glass/HDPE DP= 7.62 mm

10 8 6

Resistance Force, F, kN.

Resistance Force, F, kN.

12

4

1.16, 70.95 1.33, 68.91 1.67, 90.89 2.00, 95.38 2.33, 86.91 2.67, 80.77 3.00, 88.98 3.37, 85.58 5.00, 137.61 6.67,167.15 SPR, ED (kJ)

12 Layer S-2 Glass/HDPE DP= 7.62 mm

25

20

15

10

5

2 0

0 0

5

10

15

20

Displacement, x, mm

(a) 4L PW S-2 Glass/HDPE

25

30

0

5

10

15

20

Displacement, x, mm

(b) 12L PW S-2 Glass/HDPE

Fig. 5. Resistance force–displacement behavior of PW S-2 Glass/HDPE composites.

25

30

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30 1.16, 8.15 1.33, 8.71 1.67, 9.61 2.00, 16.21 2.33, 15.41 2.67, 15.34 3.0 , 18.62 3.37, 15.18 5.00, 24.06 6.67, 37.34 SPR, ED (kJ)

4 Layer E Glass/HDPE DP= 7.62 mm

10 8 6

Resistance Force, F, kN.

Resistance Force, F, kN.

12

4

1.16, 47.64 1.33, 43.76 1.67, 43.93 2.00, 40.95 2.33, 42.11 2.67, 43.13 3.00, 40.13 3.37, 33.52 5.00,47.43 6.67, 60.94 SPR, ED (kJ)

12 Layer E Glass/HDPE DP= 7.62 mm

25

20

15

10

2

5

0 0

10

20

0

30

0

5

10

15

20

25

Displacement, x, mm

Displacement, x, mm

(a) 4L PW E-Glass/HDPE

(b) 12L PW E-Glass/HDPE

30

Fig. 6. Resistance force–displacement behavior of PW E-Glass/HDPE composites.

40

40 S2 Glass/SC 15 S2 Glass/HDPE E Glass/HDPE

S2 Glass/SC 15 S2 Glass/HDPE E Glass/HDPE

35

Displacement, xF, mm

Resistance Force, Fmax, kN

35 30 25 20 15

30 25 20 15

10

10

5

5

0

0 0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

SPR

SPR

(a) Maximum Force at Failure, 12L.

(b) Final Displacement at the End of Penetration, 12L.

Fig. 7. Comparison of maximum force at failure and maximum displacement at the end of penetration for three different materials systems for 12L laminates.

Fig. 8. Rear surface penetration damage behavior of baseline SC15 and HDPE composite systems made from 8L of PW S-2 Glass fabrics at SPR = 5.0.

25.4-mm) of different inner and outer diameters can be assembled to cover a wide range of support span diameters in the range,

7.7-mm < DS < 101.6-mm. The ratio between the support span diameter and the punch head diameter is termed as ‘SPR’, a value

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2L

3L

4L

6L

8L

12L

(a) PW S-2 Glass/SC15

(b) PW S-2Glass/HDPE

Fig. 9. Quasi-static punch shear damage of S-2 Glass/SC15 and S-2 Glass/HDPE specimens. Number of layers: 2L, 3L, 4L, 6L, 8L and 12L, SPR = 5.00.

which can vary in the range, 1.01 < SPR = DS/DP < 13.33 for the 7.62-mm punch head. The punch shank is loaded by a driving nose which consists of an adapter with a lock ring threaded to fit the 150-kN load cell used in these experiments (Fig. 3). Threaded into the lower end of the adapter is a larger punch with a 1 in. (25.4-mm) diameter with rounded edges. This punch is aligned with the 0.50-in. punch shank before the tests are begun. By using different diameter support span-to-punch ratios (SPRs), one can test under sheardominated loading, bending-dominated loading, and a combination of the two. QS-PST experiments are performed at ten different SPRs, i.e., at SPRs = 1.16, 1.33, 1.67, 2.0, 2.33, 2.67, 3.0, 3.37, 5.0 and 6.67 in this study. An Instron 4484 universal testing machine with a 150-kN load cell is used in quasi-static tests (Fig. 2). Displacement control tests are performed at a cross-head displacement rate of 0.05-in./min (1.27-mm/min). The load and cross-head displacement data are acquired using the Blue Hill control and data acquisition software using a data collection rate of 10 data per second. After the tests are completed, selected SPR samples are sectioned in half, along the center of penetration using a vertical band saw. After sectioning, specimens are dipped in a solution consisting of blue ink and alcohol for 1 min each, and then placed in an oven at 80 °C until dry for observing QS-PS damage mechanisms. For the baseline S-2 Glass/SC15 specimens, a slot grinder with diamond coated

grinding disk is used for sectioning. The specimens are inked in the same fashion as described above, and dried in an oven at 80 °C.

3. Results and discussion Experiments are carried out under QS-PST loading conditions to test the penetration resistance force for various laminate thicknesses as a function of punch displacement. Results of the QS-PST of S-2 Glass/HDPE, E-Glass/HDPE, and S-2 Glass/SC-15 composites are presented. At first, force–displacement (F–x) data will be presented in order to compare the raw testing results of each material. Force–displacement data will be further reduced to calculate penetration stiffness and dissipated energy as a function of punch displacement, and other parameters. 3.1. Quasi-Static Penetration Resistance In order to determine the penetration resistance of S-2 Glass/ HDPE, E-Glass/HDPE, and S-2 Glass/SC-15 composites, QS-PST experiments are carried on composite laminates made from 2, 3, 4, 6, 8 and 12 layers of PW S-2 glass fabric, and at SPR = DS/ DP = 1.16, 1.33, 1.67, 2.0, 2.33, 2.67, 3.0, 3.37, 5.0 and 6.67. A laminate made from ‘n’ layers of fabric is denoted as ‘‘nL.’’

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10000

10000 ED = Q θR 2

ED = Q θR 2 2.57

Dissipated Energy, ED, kJ.

Dissipated Energy, ED, kJ.

q

Q = q1β1 2 = 100.5 β1 R = r1β1+r2 = - 0.310 β1 + 0.366 Power Law Model

100

-1

UB: ED = 1200 θ2 -1

LB: ED = 120 θ2 ------------------12 L 8L 6L 3L 4L 2L # Layers S-2 Glass/SC15

1

0.01 0.01

Q = q1β1q2 = 42.46 β12.16 R = r1β1+r2 = 0.7 Power Law Model

100

-0.8

UB: ED = 1200 θ2 -0.8

LB: ED = 80 θ2 --------------------12 L 8L 6L 4L 3L 2L # Layers S-2 Glass/HDPE

1

β1 = HC/DP θ2 = DPDS/HC2

1

100

10000

0.01 0.01

1

Dimensionless Thickness Parameter, θ2.

100

10000

Dimensionless Thickness Parameter, θ2.

(a) S-2 Glass/SC-15

(b) S-2 Glass/HDPE

10000 ED = Q θR 2

Dissipated Energy, ED, kJ.

q

2.26

Q = q1β1 2 = 25.49 β1 R = r1β1+r2 = - 0.869 β1 + 1.03 Power Law Model

100

-0.7

LB: ED = 500 θ2

-0.7 ED = 40 θ2

1

0.01 0.01

LB: ------------------12 L 8L 6L 4L 3L 2L # Layers E-Glass/HDPE

1

100

10000

Dimensionless Thickness Parameter, θ2.

(c) E-Glass/HDPE Fig. 10. Quasi-static energy dissipation as a function of h2.

3.1.1. Force–displacement data Figs. 4–6 show the penetration resistance force–displacement (F–x) plot for three different materials at two selected thicknesses, i.e. for 4L and 12L laminates, at the same range of force and displacement for visual comparison. The rest of the F–x data is presented in Appendix A (Figs. A.1–A.4) for 2L, 3L, 6L, and 8L. Force–displacement behavior of PW S-2 glass/SC15 composites at two different thicknesses, i.e. 4L and 12L, shows a ‘knee’ after the initial linear segment, signifying the initial matrix damage which has been reported in our previous work [12,13]. After the initial matrix damage, the force–displacement shows a second-linear segment up to the maximum force signifying ‘compression-shear’ dominated damage mechanisms followed by progressive force drop to a near zero value, due to ‘tension-shear’ dominated damage mechanisms as described in Ref. [13]. The area under the force–displacement curve represents the work done during the penetration process, which is also termed as the energy dissipated by the material by the different damage mechanisms:

W ¼ ED ¼

Z

xF

FðxÞdx

ð2Þ

0

where xF is the final displacement after which no appreciable load is carried. Fig. 4 also shows the value of ED for all tests performed at different SPRs, which will be analyzed for all materials investigated.

Force–displacement behavior of the PW S-2 Glass/HDPE composite system (Fig. 5) is very different than the PW S-2 Glass/ SC15 composite system. The initial linear region followed by the ‘knee’ behavior of the PW S-2 glass/SC15 system is absent in the case of the PW S-2 glass/HDPE system, and instead a non-linear hardening behavior followed by a linear-segment up to the maximum force is noticed. This behavior is unique to the thermoplastic HDPE composite system and is believed to be dominated by ‘non-linear membrane tension-shear’ deformation of the entire laminate. At the maximum force, some non-linearity is also observed. We can assume that up to the maximum force there is no fiber damage, thus the deformation is non-linear elastic. However, progressive shear cutting of fibers start at and around the maximum force followed by a progressive decrease in force to a near zero value, signifying the dissipation of energy through progressive fiber failure mechanisms under tension-shear damage around the periphery of the punch. In the case of the baseline S-2 glass/SC15 system, we observe a tendency of decreasing maximum force as a function of increasing SPR for all thicknesses. On the other hand, for the PW S-2 glass/HDPE system, the maximum force remains almost constant for increasing SPRs for a specific laminate thickness. This is because of the lower transverse shear stiffness of the HDPE system. The final displacement (xF) at the end of the penetration process for PW S-2 glass/ HDPE system is found to be much more than the baseline S-2 glass/SC15 system. The overall penetration mechanics behavior

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150 S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 1.16

Dissipated Energy, ED, kJ.

Dissipated Energy, ED, kJ.

150

100

50

5

10

15

S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 2.00

100

50

5

20

Areal Density, AD, Kg/m2

(a) SPR = 1.16

20

15

20

150 S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 2.33

Dissipated Energy, ED, kJ.

Dissipated Energy, ED, kJ.

15

(b) SPR = 2.00

150

100

50

5

10

15

S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 3.37

100

50

20

5

10

Areal Density, AD, Kg/m2

Areal Density, AD, Kg/m2

(c) SPR = 2.33

(d) SPR = 3.37

150

200 S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 5.00

Dissipated Energy, ED, kJ.

Dissipated Energy, ED, kJ.

10

Areal Density, AD, Kg/m2

100

50

5

10

15

Areal Density, AD,

20

Kg/m2

(e) SPR = 5.0

S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 6.67

150

100

50

5

10

15

20

Areal Density, AD, Kg/m2

(f) SPR = 6.67

Fig. 11. Quasi-static energy dissipation as a function of areal-density.

of the HDPE system is unique and is very much different than the baseline SC15 epoxy system in terms of damage modes and deformation mechanisms. The PW E-glass/HDPE composite system presented in Fig. 6 shows similar penetration mechanics behavior as the PW S-2 glass/HDPE system which was expected because the deformation mechanics is dominated by the HDPE matrix material. The E-Glass/HDPE system also showed lower maximum force as compared to S-2 Glass/SC15 and S-2 Glass/HDPE systems and was also

expected due to the lower tensile strength of E-glass fibers (Fig. 7a). In addition, the final displacement (xF) at the end of the penetration process for the E-glass/HDPE system is found to be less than the S-2 glass/HDPE system and also less than the baseline S-2 glass/SC15 system (Fig. 7b). 3.1.2. Quasi-static penetration damage mechanisms QS-PST at various support spans is conducted to study the quasi-static penetration behavior of baseline PW S-2 Glass/SC-15, PW

3399

20 18

Favg (x), kN k(x)=18*exp(-x/1.2)

6.67 5.00 3.37 3.00 2.67 2.33 2.00 1.67 1.33 1.16, 13.25 SPR 4L S-2 Glass/SC 15

16 14 12 10 8 6 4 2 0 0

2.5

5.0

7.5

10.0

12.5

15.0

Penetration Displacement, x, mm.

Non-Linear Penetration Stiffness, k(x), kN/mm.

Non-Linear Penetration Stiffness, k(x), kN/mm.

Ö.F. Erkendirci, B.Z. (Gama) Haque / Composites: Part B 43 (2012) 3391–3405

40 Favg (x), kN k(x)=40exp(-x/1.75)

35

6.67 5.00 3.37 3.00 2.67 2.33 2.00 1.67 1.33 1.16 SPR

30 25 20

12L S-2 Glass/SC 15

15 10 5 0

0

5

(a) 4L S-2 Glass/SC15

10 15 20 25 Penetration Displacement, x, mm.

30

(b) 12L S-2Glass/SC15

Favg (x), kN k(x) = 12*exp(-x/1.85) 6.67 5.00 3.37 3.00 2.67 2.33 2.00 1.67 1.33 1.16 SPR 4L S-2 Glass/HDPE

8

6

4

2

0 0

5

10

15

20

25

30

Non-LinearPenetrationStiffness,k(x),kN/mm.

10

20.0 Favg (x), kN k(x)=21*exp(-x/2.15) 6.67 5.00 3.37 3.00 2.67 2.33 2.00 1.67 1.33 1.16 SPR 12L S-2 Glass/HDPE

17.5 15.0 12.5 10.0 7.5 5.0 2.5 0

0

5

10

15

20

25

PenetrationDisplacement,x,mm.

PenetrationDisplacement,x,mm.

(a) 4L S-2 Glass/HDPE

(b) 12L S-2 Glass/HDPE

30

5 Favg (x), kN k(x)=7*exp(-x/1.7) 6.67 5.00 3.37 3.0 2.67 2.33 2.00 1.67 1.33 1.16 SPR 4L E-Glass/HDPE

4

3

2

1

0 0

5

10

15

PenetrationDisplacement, x, mm.

(a) 4L E-Glass/HDPE

20

Non-Linear Penetration Stiffness, k(x), kN/mm.

Fig. 13. Non-linear penetration stiffness behavior of S-2 Glass/HDPE composite system.

Non-Linear Penetration Stiffness, k(x), kN/mm.

Non-LinearPenetrationStiffness,k(x),kN/mm.

Fig. 12. Non-linear penetration stiffness behavior of baseline S-2 Glass/SC15 composite system.

10 Favg (x), kN k(x)=10.5*exp(-x/2.05) 6.67 5.00 3.37 3.00 2.67 2.33 2.00 1.67 1.33 1.16 SPR 12L E-Glass/HDPE

8

6

4

2

0

0

5

10

15

PenetrationDisplacement, x, mm.

(b) 12L E-Glass/HDPE

Fig. 14. Non-linear penetration stiffness behavior of E-Glass/HDPE composite system.

20

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Ö.F. Erkendirci, B.Z. (Gama) Haque / Composites: Part B 43 (2012) 3391–3405

60

3.5 0.788

k0(β1)S2G/SC15 = 40.42 β1 . 0.669 k0(β1)S2G/HDPE = 21.26 β1 . 0.453 k0(β1)E-G/HDPE = 10.60 β1

50

E-Glass/HDPE x0 S2-Glass/HDPE x0 S2-Glass/SC 15 x0

3.0 .

x0(β1)S2G/HDPE = 1.99 (1 - exp (-β1/0.120) . x0(β1)EG/HDPE = 1.89 (1 - exp (-β1/0.126)

2.5

x0(β1).

k0(β1).

40

x0(β1)S2G/SC15 = 1.76 (1 - exp (-β1/0.238)

S-2 Glass/SC 15 k0 S2-Glass/HDPE k0 E-Glass/HDPE k0

30

2.0 1.5

20 1.0 10 0

0.5

0

0.2

0.4

0.6

0.8

1.0

1.2

0

1.4

0

0.2

0.4

0.6

(a) Parameter

0.8

β1.

DimensionlessThickness, β1.

k0 (β1 )

(b) Parameter

1.0

1.2

1.4

x0 (β1 )

30

Average Penetration Resistance Force, Favg(x, β1), kN.

Average Penetration Resistance Force, Favg(x, β1), kN.

Fig. 15. Parameters of Non-Linear Penetration Stiffness Envelope (NL-PRE) curves.

12L 8L 6L 4L 3L 2L #L, S-2Glass/SC15

25 20 15 10 5 0 0

2

4

6

8

10

12

14

16

30 12L 8L 6L 4L 3L 2L #L, S-2Glass/HDPE

25 20 15 10 5 0 0

2

4

Displacement, x, mm.

6

8

10

12

14

16

Displacement, x, mm.

(a) S-2 Glass/SC-15

(b) S-2 Glass/HDPE

30

125

Average Dissipated Energy,ED, kJ.

Average Penetration Resistance Force, Favg (x, β1), kN.

Fig. 16. Average Penetration Resistance Force (APRF) of S-2 Glass/SC15 and S-2 Glass/HDPE Materials Systems.

12L 8L 6L 4L 3L 2L #L, E-Glass/HDPE

25 20 15 10 5

0

0

2

4

6

8

10

12

14

16

Displacement, x, mm.

(a) E-Glass/HDPE

SC15/S2Glass HDPE/S2Glass HDPE/EGlass

100

75

50

25

0 0

0.25

0.50

0.75

1.00

1.25

Dimensionless thickness parameter, β1.

(b) Average Dissipated Energy from APRF Curves

Fig. 17. Average Penetration Resistance Force (APRF) of E-Glass/HDPE and dissipated energy of all materials systems.

S-2 Glass/HDPE and PW E-Glass/HDPE composites. The QS damage mechanisms vary as a function of support span ratio and laminate thickness. While the QS damage mechanisms of

S-2 glass/HDPE and E-Glass/HDPE composite systems are similar, for brevity, the damage mechanisms of only the S-2 glass/SC15 and S-2 glass/HDPE composite system are presented.

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Ö.F. Erkendirci, B.Z. (Gama) Haque / Composites: Part B 43 (2012) 3391–3405

Fig. 8 shows the rear surface damage of a 8L laminate made from SC15 and HDPE matrix and S-2 glass fabrics and tested at SPR = 5.0. Matrix cracks and brittle fiber fracture has been observed in the case of the SC15 system, however, the matrix crack mode is naturally absent in the HDPE system. The HDPE system, being thermoplastics and not wetting all the fiber bundles, showed a pull out of fiber bundles instead of brittle fracture. The lower degree of impregnation of HDPE system is thus attributed to the damage modes associated with the HDPE system. Both the SC15 and HDPE laminates of different thicknesses tested at SPR = 5.0 are sectioned and dipped into an ink-alcohol solution and the optical photographs of the damage are presented in Fig. 9. Thinner SC15 composite specimens (2L, 3L, and 4L) showed membrane deformation and punch shear failure localized at the periphery of the punch tip. However, thicker sections (6L, 8L, and 12L) showed all three damage mechanisms, i.e. initial damage, compression shear, and tension shear, as have been reported earlier in Ref. [13]. However, at all thicknesses, a permanent shear deformation cone is observed in the HDPE composite system due to its lower transverse shear stiffness and strength as compared to the baseline SC15 epoxy system. In addition, local punch shear and clean shear cutting of fibers are also visible. The earlier discussion of two penetration phases of ‘membrane tension-shear deformation’ of the entire laminate maintained by the support span and local ‘tension-shear dominated failure at the periphery of the punch’ is thus somehow justified.

3.2. Quasi-static energy dissipation We define a dimensionless thickness parameter and an inverse thickness parameter as follows:

b1 ¼ HC =DP

ð3Þ

h2 ¼ DP DS =H2C ¼ SPR=b21

ð4Þ

Energy dissipated by the material under quasi-static punch shear loading is calculated using Eq. (2) and is presented in Fig. 10 as a function of dimensionless inverse thickness parameter, h2, for all three material systems. In log–log plots, dissipated energy for different material thicknesses is found to be separated as different groups, which tells us that these data sets are also a function of laminate thickness. Thus, we can also express the dissipated energy as: ED = f(b1, h)

ED ¼ Q hR2 where the parameters Q and R are functions of b1 only. q

Q ¼ q1 b12 and R ¼ r 1 þ r 2 b1

ð6Þ

Fig. 10 shows the values of these parameters, q1, q2, r1, and r2; as determined from the statistical analysis of the experimental data and is as well presented below: f0:3660:310b1 g

ED jS-2 Glass=SC15 ¼ 100:5b2:57 h2 1

6

ð7aÞ

10 1.16, 4.74 1.33, 4.11 1.67, 2.64 2.00, 3.08 2.33, 6.61 2.67, 6.62 3.00, 5.73 3.37, 4.00 5.00, 6.73 6.67, 5.34 SPR, ED (Kj)

4

3

8

2 Layer S-2 Glass/SC-15 DP= 7.62 mm

2

1.16, 9.83 1.33, 8.10 1.67, 7.32 2.00, 7.22 2.33, 14.80 2.67, 12.97 3.00, 13.81 3.37, 11.03 5.00, 17.06 6.67, 12.95 SPR, ED (kJ)

9

Resistance Force, F, kN.

5

Resistance Force, F, kN.

ð5Þ

7 6 5 4

3 Layer S-2 Glass/SC-15 DP=7.62 mm

3 2

1

1 0 0

2

4

6

8

10

12

14

16

18

0 0

20

2

4

(a) 2L S-2 Glass/SC15

10

12

14

16

18

20

20 1.16, 33.10 1.33, 27.96 1.67, 22.99 2.00, 28.27 2.33, 35.46 2.67, 35.14 3.00, 33.75 3.37, 34.51 5.00, 38.29 6.67, 44.53 SPR, ED (kJ)

16 14 12 10 8

16

6 Layer S-2 Glass/SC-15 DP=7.62 mm

6

14 12 10 8

4

2

2 10

15

20

Displacement, x mm.

(c) 6L S-2 Glass/SC15

25

30

8 Layer S-2 Glass/SC-15 DP=7.62 mm

6

4

5

1.16, 53.41 1.33, 47.94 1.67, 39.65 2.00, 45.85 2.33, 58.09 2.67, 60.26 3.00, 52.54 3.37, 51.72 5.00, 55.64 6.67, 64.79 SPR, ED (kJ)

18

Resistance Force, F, kN.

18

Resistance Force, F, kN

8

(b) 3L S-2 Glass/SC15

20

0 0

6

Displacement, x mm.

Displacement,x mm.

0 0

5

10

15

20

Displacement, x mm.

(b) 8L S-2 Glass/SC15

Fig. A.1. Force–displacement behavior of 2L, 3L, 6L and 8L S-2 Glass/SC15 composites.

25

30

3402

Ö.F. Erkendirci, B.Z. (Gama) Haque / Composites: Part B 43 (2012) 3391–3405 f0:7g

ED jS-2 Glass=HDPE ¼ 42:46b12:16 h2

ð7bÞ

h2f1:030:869b1 g ED jE-Glass=HDPE ¼ 25:49b2:26 1

ð7cÞ

Eq. (7) provides the energy dissipation of a material under quasistatic punch shear loading as a function of laminate thickness, punch diameter, and support span diameter. This is required to map the energy dissipation charts for a material as shown in Fig. 10. It is also interesting to note that the energy dissipation of a material for all the support spans and laminate thicknesses defined by h2 and b1, can be bound by an upper and a lower limit curve, as shown in Fig. 10 by ‘UB’ and ‘LB’. We can also express these bounds with a range of values as follows:

  1200 < ED  h2 S-2 1200 h0:8 2 500 h0:7 2

< Glass=SC15

   < ED  

<

S-2 Glass=HDPE

   < ED  

< E-Glass=HDPE

120 h2

ð8aÞ

80

ð8bÞ

h0:8 2

40

ð8cÞ

h0:7 2

Another way of comparing the dissipated energy for different materials is to express it as a function of areal density, AD, as presented by Eq. (1) for each specific SPR (Fig. 11).

Since the S-2 Glass/HDPE composites show large deformation behavior, the QS energy dissipation is found to be higher for all SPRs up to a certain areal-density/thickness of the laminate as compared to the baseline S-2 Glass/SC15 composites. Even though the deformation behavior of E-Glass/HDPE system is similar to the Baseline S-2 Glass/SC15 system, due to their low maximum load capacity (Fig. 7), E-Glass/HDPE composite system show lower energy dissipation than the baseline S-2 Glass/SC15 composite systems. Some exceptions are found for SPRs = 2.33 and 3.37, where at higher areal-densities, the baseline S-2 Glass/SC15 is found to have higher energy dissipation as compared to S-2 Glass/HDPE composite systems. 3.3. Non-linear penetration stiffness and average penetration resistance behavior Earlier QS-PST on baseline S-2 Glass/SC16 composites using a right circular cylinder punch of diameter, DP = 12.7-mm, revealed that a ‘Quasi-Static Envelope’ curve can be plotted combining the force–displacement plots at several SPRs, e.g. SPR = 1.1–4.0 [14]. However, the HDPE composite systems did not show such a ‘QS-Envelope’ behavior and thus the quasi-static approximation of material’s ability to dissipate energy under ballistic impact conditions cannot be predicted. In order to formulate an average penetration resistance behavior of these groups of composites, a new definition of ‘Non-Linear Penetration Stiffness (NL-PS)’ is formulated. If the force obtained under QS-PST loading as a function

6

10

2 Layer S-2 Glass/HDPE DP= 7.62 mm

4

3

8

2

1.16, 10.00 1.33, 11.67 1.67, 11.15 2.00, 19.46 2.33, 26.14 2.67, 28.42 3.00, 30.86 3.37, 24.31 5.00, 31.18 6.67, 35.57 SPR, ED (kJ)

3 Layer S-2 Glass/HDPE DP= 7.62 mm

9

Resistance Force, F, kN.

Resistance Force, F, N.

5

1.16, 5.25 1.33, 5.84 1.67, 6.21 2.00, 8.62 2.33, 11.22 2.67, 18.98 3.00, 15.25 3.37, 11.02 5.00, 15.83 6.67, 25.06 SPR, ED (kJ)

7 6 5 4 3 2

1

1 0

0

2

4

6

8

10

12

14

16

18

0 0

20

2

4

(a) 2L S-2 Glass/HDPE

10

12

14

16

18

20

(b) 3L S-2 Glass/HDPE 1.16, 26.02 1.33, 33.57 1.67, 38.10 2.00, 49.95 2.33, 46.71 2.67, 51.60 3.00, 66.39 3.37, 69.53 5.00, 87.03 6.67, 110.31 SPR, ED (kJ)

16 14 12 10

16

8 6

14 12 10 8 6

4

4

2

2 5

10

15

20

Displacement, x mm.

(c) 6L S-2 Glass/HDPE

25

30

1.16, 38.24 1.33, 41.23 1.67, 43.12 2.00, 59.92 2.33, 62.87 2.67, 58.12 3.00, 62.40 3.37, 68.25 5.00, 103.38 6.67, 146.54 SPR, ED (kJ)

8 Layer S-2 Glass/HDPE DP= 7.62 mm

18

Resistance Force, F, N.

6 Layer S-2 Glass/HDPE DP= 7.62 mm

18

Resistance Force, F, kN

8

20

20

0 0

6

Displacement, x mm.

Displacement, x, mm.

0

0

5

10

15

20

Displacement, x, mm.

(b) 8L S-2 Glass/HDPE

Fig. A.2. Force–displacement behavior of 2L, 3L, 6L and 8L S-2 Glass/HDPE composites.

25

30

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Ö.F. Erkendirci, B.Z. (Gama) Haque / Composites: Part B 43 (2012) 3391–3405

of penetration displacement x is expressed as, F(x), then the NL-PS can be defined as:

FðxÞ x

ð9Þ

And then the penetration resistance force can be expressed as:

FðxÞ ¼ kðxÞ  x

ð10Þ

Figs. 12–14 show the ‘Non-Linear Penetration Stiffness’ for two different laminate thicknesses, i.e. 4L and 12L for all three composite materials systems. These figures are generated by transforming Figs. 4–6 using Eq. (9). It is interesting to note that in Figs. 4–6, none of the force–displacement plots show any self-similar penetration resistance behavior except Fig. 4b for 12L baseline S-2 Glass/SC15 composites. However, in the k(x)  x stiffness space, all of Figs. 12–14 show some sort of self-similar penetration resistance behavior, which is defined by a single ‘Envelope’ curve representing the locus of all k(x)  x curves at different SPRs for one specific material thickness. Such an ‘Envelope’ curve can be described by the ‘Non-Linear Penetration Stiffness Envelope (NL-PRE)’ behavior of the material at a specific thickness and can have a functional form of:

 kNL-PRE ðx; b1 Þ ¼ k0 ðb1 Þ  exp 

x x0 ðb1 Þ



ð11Þ

where k0(b1) and x0(b1) are modeling parameters as a function of dimensionless thickness b1. The Non-Linear Penetration Resistance Envelope (NL-PRE) curves are determined for all composite laminate thicknesses and

 F av g ðx; b1 Þ ¼ k0 ðb1 Þ  x  exp 

ð12Þ

k0 ðb1 Þ ¼ j1 bj1 2

ð13Þ

where j1 and j2 are fitting parameters, values of which are shown in Fig. 15a for all three materials systems studied. Fig. 15b shows the plot of x0(b1) for three different materials which shows an exponential behavior of the form:

   b x0 ðb1 Þ ¼ v1 1  exp  1

ð14Þ

v2

where v1 and v2 are fitting parameters, values of which are shown in Fig. 15b for all three materials systems. The functional forms of k0(b1) and x0(b1), as outlined in Eqs. (13) and (14), allow one to predict the NL-PRE curves and thereby the APRF curves for the

6

10 1.16, 3.86 1.33, 3.75 1.67, 3.59 2.00, 6.84 2.33, 4.79 2.67, 9.08 3.00, 5.22 3.37, 4.55 5.00, 9.26 6.67, 14.79 SPR, ED (kJ)

4

3

9

2

1.16, 5.46 1.33, 6.65 1.67, 8.55 2.00, 9.21 2.33, 9.53 2.67, 12.32 3.00,8.42 3.37,8.91 5.00, 16.24 6.67, 20.11 SPR, ED (kJ)

3 Layer E Glass/HDPE DP= 7.62 mm

8

Resistance Force, F, kN.

2 Layer E Glass/HDPE DP= 7.62 mm

5

Resistance Force, F, kN.

 x x0 ðb1 Þ

which is also presented in Figs. 12–14 (see the legend for F ðxÞ av g ). Since both k0(b1) and x0(b1) are functions of b1, these parameters are presented in Fig. 15 as a function of b1. Fig. 15a shows the plot of k0(b1) for three different materials which shows a power law behavior of form:

7 6 5 4 3 2

1

1 0

0

5

10

15

0 0

20

5

(a) 2L E-Glass/HDPE

20

20

18

1.16, 14.95 1.33, 15.11 1.67, 15.85 2.00, 20.03 2.33, 21.83 2.67, 22.96 3.00, 19.03 3.37, 17.84 5.00, 27.63 6.67, 34.98 SPR, ED (kJ

6 Layer E Glass/HDPE DP= 7.62 mm

16

Resistance Force, F, kN.

15

(b) 3L E-Glass/HDPE

20

14 12 10 8 6

16 14 12 10 8 6 4

2

2 5

10

15

20

Displacement, x, mm

(c) 6L E-Glass/HDPE

25

30

1.16, 24.27 1.33, 22.11 1.67, 20.02 2.00, 24.23 2.33, 22.70 2.67, 24.73 3.00, 22.94 3.37, 24.72 5.00, 36.65 6.67, 44.19 SPR, ED (kJ

8 Layer E Glass/HDPE DP= 7.62 mm

18

4

0 0

10

Displacement, x, mm.

Displacement, x, mm.

Resistance Force, F, kN.

kðxÞ ¼

for all three materials systems, and are also presented in Figs. 12–14 (see the legend for k(x)). Using Eqs. (11) and (10), one can determine the Average Penetration Resistance Force (APRF) curve which has the functional form:

0 0

5

10

15

20

Displacement, x, mm

(b) 8L E-Glass/HDPE

Fig. A.3. Force–displacement behavior of 2L, 3L, 6L and 8L E-Glass/HDPE composites.

25

30

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Ö.F. Erkendirci, B.Z. (Gama) Haque / Composites: Part B 43 (2012) 3391–3405

different thicknesses of the three composite material systems presented in this study. The Average Penetration Resistance Force (APRF) given by Eq. (12) is presented in Figs. 16a and b, and 17a for the three material systems with the same range of values in both the co-ordinates for visual comparison. The average dissipated energy (ADE) calculated by integrating these plots is presented in Fig. 17b. The APRF curves are equivalent to the ‘QS-Envelope’ curves reported earlier in Ref. [13], and represent the materials capacity of dissipating energy under quasi-static punch shear loading. It has also been shown that this energy is also the quasi-static equivalent of the materials capacity of energy dissipation under high velocity impact and penetration loading at ballistic limit velocity. Fig. 17b shows that for thinner composite laminates (b1 < 0.50) the energy dissipation capacity of the S-2 Glass/SC15 and S-2 Glass/HDPE systems are almost same, however, the HDPE system showed lower energy dissipation capacity than the SC15 system at higher thicknesses, i.e. b1 > 0.50. In the case of the E-Glass/HDPE composite system, the overall energy dissipation capacity is less than half of that for the baseline S-2 Glass/SC15 system. 4. Summary In this study, the quasi-static penetration resistance behavior of three different material systems, i.e., PW S-2 Glass/SC-15, PW S-2 Glass/HDPE and PW E-Glass/HDPE composites with varying

thicknesses, i.e., Hc = 1.4–8.4-mm, were investigated. PW S-2 Glass/SC-15 laminates have been investigated and used as a baseline material in the past. Tests were conducted at a range of support diameter-to-punch diameter ratios (SPRs), to quantify the energy dissipation capacity of the different material systems. It has been identified that the force–displacement behavior of HDPE composites differs from the baseline SC15 epoxy composites, and a quasi-static penetration resistance envelope can be constructed. In order to overcome this problem, non-linear penetration stiffness and an average penetration resistance force for each laminate thickness are defined. This new definition allowed the quantification of quasi-static average dissipated penetration energy and the comparison between the HDPE composites and the baseline SC15 epoxy composites. For the S-2 Glass/HDPE and E-Glass/HDPE composite materials, no initial damage was observed, contrary to what has been found for S-2 Glass/SC-15 composites. This was expected because of the visco-elastic–plastic behavior of HDPE resin system. S-2 Glass/HDPE and E-Glass/HDPE composite materials also exhibited a larger effective displacement for complete energy dissipation, dissipating more energy in the case of thinner laminates and dissipating less energy for thicker composites as compared to baseline. The maximum load carrying capacities are found to vary with test SPRs, and for the HDPE composite systems are lower than the baseline composites in general. The low velocity impact and ballistic impact behavior of HDPE composites remains as the future work.

100

S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 1.67

Dissipated Energy, ED, kJ.

Dissipated Energy, ED, kJ.

150 S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 1.33

50

5

10

15

100

50

20

5

2

15

20

15

20

Areal Density, AD, Kg/m

(a) SPR = 1.33

(b) SPR = 1.67

150

150 S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 3.00

Dissipated Energy, ED, kJ.

S2 Glass/SC-15 S2 Glass/HDPE E Glass/HDPE SPR= 2.67

Dissipated Energy, ED, kJ.

10 2

Areal Density, AD, Kg/m

100

50

5

10

15 2

Areal Density, AD, Kg/m

(a) SPR = 2.67

20

100

50

5

10 2

Areal Density, AD, Kg/m

(b) SPR = 3.00

Fig. A.4. Quasi-static energy dissipation as a function of areal-density.

Ö.F. Erkendirci, B.Z. (Gama) Haque / Composites: Part B 43 (2012) 3391–3405

Acknowledgment This research was supported by the Scientific and Technological _ Research Council of Turkey (TÜBITAK). Appendix A. Force–displacement data for remaining materials See Figs. A.1–A.4.

[14]

[15]

[16] [17] [18]

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