Sem.org SEM X Int Cong s101p03 Low Velocity Impact Combination Kevlar Carbon Fiber Sandwich

8/2/2019 Sem.org SEM X Int Cong s101p03 Low Velocity Impact Combination Kevlar Carbon Fiber Sandwich

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LOW VELOCITY IMPACT OF COMBINATION KEVLAR/CARBON FIBERSANDWICH COMPOSITES

Jeremy Gustin, Graduate Student, [email protected]

Aaran Joneson, Graduate Student, [email protected] Mahinfalah1, Associate Professor, [email protected] Stone, Associate Professor, [email protected]

Department of Mechanical Engineering and Applied MechanicsNorth Dakota State University

Fargo, ND 58105Tel: 701-231-8839 Fax: 701-231-8913

ABSTRACTImpact, compression after impact, and tensile stiffness properties of carbon fiber and Kevlar combination sandwich composites

were investigated in this study. The different samples consisted of impact-side facesheets having different combinations ofcarbon fiber/Kevlar and carbon fiber/hybrid. The bottom facesheets remained entirely carbon fiber to maintain the high overallflexural stiffness of the sandwich composite. The focus of this research was to determine if any improvement in impactproperties existed as a result of replacing the impact-side facesheet layers of carbon fiber with Kevlar or hybrid. Impact testswere conducted on different sample types to obtain information about absorbed energy and maximum impact force. Also,compression after impact tests were conducted to determine the reduction in compressive strength when comparing impactedto non-impacted samples. The elastic moduli of carbon fiber, Kevlar, and hybrid was determined from tensile testing. Thisdata was used to characterize the reduction in stiffness from replacing carbon fiber layers with the Kevlar or hybrid layers. Theexperimental data in its entirety helps define the benefits and disadvantages of replacing carbon fiber layers with Kevlar orhybrid.

INTRODUCTIONSandwich composites have become increasingly popular for applications including aerospace, marine/offshore industries, andground transportation [1-2]. They are favored for their high specific strength and stiffness, corrosion resistance, tailorability,and stability [3]. Sandwich composites are very suitable for lightweight structures requiring high in-plane and flexural stiffness

[4]. However, applications prone to impact are limited because of a relatively poor resistance to localized impact loading [5-10]. This is an important concern since a reduction in stiffness and residual strength occurs after impact.

Carbon fiber reinforced, epoxy matrix composites have emerged as a very promising class of structural materials with highstatic strength and stiffness properties [11]. The core, which is typically a low strength, lightweight material, is responsible fortransmitting shear forces between the facesheets [12]. A vital element of the sandwich construction is the effective bondbetween the facesheets and the core material. The joint must be stiff and strong, as well as tough in order to allow thesandwich structure to sustain high loads over a long service life [13].

Kevlar was added to the impact-side facesheet in an attempt to improve upon the impact properties. Kevlar composites havebeen extensively utilized as lightweight armor structures in applications ranging from military helmets to large scale vehiclesystems such as aircraft, spacecraft, land vehicles, and naval vessels [14-15]. Reinforcement of composites with Kevlar fiberscan significantly improve the impact damage tolerance if used in place of or in conjunction with graphite fibers [16-18]. Bunselland Harris [19] suggested that once the failure strain of the brittle layers (carbon fiber) is reached in an interlaminar hybrid, theload can be transferred to the ductile layers (Kevlar) if bonding between the laminates is sufficient.

In this study, sandwich composite plates with different combinations of Kevlar/carbon fiber and hybrid/carbon fiber impact-sidefacesheets were subjected to low velocity impacts at energies ranging from 5 J to those that caused complete samplepenetration. On the impact-side facesheet, the very top 1, 2, 3, or 4 layers of carbon fiber were replaced with Kevlar or hybrid.Low energy impacts were considered since even in the absence of fiber breakage, the laminate mechanical performance canbe drastically affected [20-21]. These experimental impact tests are necessary to determine impact force and energyperformance. Current sandwich composite impact theory does not consider impact energies that result in facesheet cracking,

1 Corresponding author.


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which is very common. For impacts without facesheet cracking, Hoo Fatt and Parks [3,22] used equivalent single and multidegree-of-freedom systems to predict low velocity impact response and damage initiation impact forces.

Compression tests were conducted to determine the remaining compressive strength after impact. In-plane compression isthe critical load for impact-damaged specimens, since strength reductions are the largest under this type of loading [23]. Thetype of compression after impact failure depends on the impact damage. Davies et al [24] discussed the correlation betweenthree types of impact damage and the resulting compression after impact failure. For an impact resulting in a permanentindent to the facesheet, compressive loads can increase local bending and push the skin further into the core causing failure.For impacts that cause internal delamination, compressive loads may cause localized buckling where the skin is not supported

by the foam. For impacts where the top facesheet and core are penetrated, the bottom facesheet may experience massivedebonding. The combination of the bottom facesheet debonding and penetration of the top facesheet would significantly limitthe compressive strength. Compressive failure would likely occur as a result of top facesheet cracking from stressconcentrations and/or bottom facesheet buckling.

Tensile tests were performed to characterize how the addition of Kevlar or hybrid to the impact-side facesheet would decreasethe stiffness compared to facesheets with only carbon fiber. Understanding how the stiffness is affected is necessary indetermining potential applications for sandwich composites incorporating carbon fiber with Kevlar or hybrid.

SAMPLE CONSTRUCTIONA hand-layup method shown in Figure 1 was used to construct the samples. The major components required for this methodare a vacuum pump, vacuum bagging, spiral tubing and sealant tape. The spiral tubing insured a uniform vacuum across thesample and prevented epoxy from pooling on the sample side with less vacuum. This would have occurred on the sidewithout spiral tubing. If epoxy pooling occurs, the facesheet thickness is not uniform. The facesheet would be the thickest onthe sample side without the spiral tubing. The core of the sandwich composite consisted of polyurethane foam filled

honeycomb. The honeycomb structure was constructed out of kraft paper. The foam filled honeycomb sheets, purchased fromMGI, had the properties indicated in Table 1. The carbon fiber, Kevlar, and hybrid fabric properties are listed in Table 2. Theepoxy consisted of F-82 resin and TP-41 hardener, which was allowed to cure under a 600 mm Hg vacuum for a minimum of 9hours. The cured properties of the epoxy, purchased from Eastpointe Fiberglass, are listed in Table 3.

Aluminum SheetCarbon Fiber Fabric / Core

To Vacuum

Pump

Vacuum

Bagging Sealant Tape

Vacuum Tubing Spiral Tubing

Figure 1. Sample construction setup

Table 1. Properties of MGI Canada MIKOR honeycomb sheets

Density 112 kg / m3

Compressive Strength 2.02MPa

Cell Size 12.7 mm

Shear Strength ToRibbon 1.65 MPa

Cell Thickness 0.3175 mm

Honeycomb Thickness 25.4 mm


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Table 2. Fabric properties

Hybrid

Carbon Fiber Kevlar 49(3K Carbon Fiber/1500 Denier

Kevlar)

Yarn Type 3 K

Weave Type Plain 4 HS 2 x 2 Twill

Area Density 193 g / m2

169 g / m2

183 g / m2

Thickness 0.3048 mm 0.254 mm 0.305 mm

Count (Rows/Tows Per Inch) 12.5 x 12.5 17 x 17 12.5 x 12.5

Table 3. Properties of Eastpointe Fiberglass epoxy

Density 1084 kg / m3

Compressive Strength 131 MPa

Tensile Strength 63.6 MPa

Cure Time 9-12 Hours

Cure Temperature 75 F

The hand-layup method provided high quality samples with minimal defects. To create the foam filled core samples, a layer ofepoxy was applied before each layer of fabric was placed. Special care was taken to insure the correct amount of epoxy was

used in addition to being evenly spread out. After the four layers were placed, the vacuum bagging was carefully spread overthe sample insuring no wrinkles would form when the vacuum was applied. Any wrinkles that form on the vacuum bagging willaffect the surface finish of the sample. A rubber squeegee was used to remove the extra epoxy and trapped air.

As shown in Table 2, the addition of Kevlar and hybrid to the facesheets actually reduced the overall weight of the sandwichcomposite compared to the entirely carbon fiber samples. The Kevlar and hybrid fabrics had a lower areal densities comparedto carbon fiber. The different combinations of 100 mm x 100 mm samples used in this study are listed in Table 4 with theircorresponding weights. The sample type description corresponds to the impact-side facesheet information. The bottomfacesheets remained four layers of carbon fiber for all sample types. The abbreviations for the sample types shown in thistable are used throughout the paper.

Table 4. Properties of Hybrid Carbon Fiber/Kevlar

Abb. Sample TypeWeight

(g)

CF 4 Carbon Fiber Layers 60.8

1K 1 Kevlar - 3 Carbon Fiber Layers 59.7



4K 4 Kevlar Layers 58.4

1H 1 Hybrid - 3 Carbon Fiber Layers 59.4



4H 4 Hybrid Layers 59.3

TEST METHODAn Instron Dynatup drop tower, Model 9250HV, was used for impact testing. This machine is capable of impacting samples atenergies of up to 826 J utilizing a spring-assist. For this study, all samples were impacted with a 7.25 kg drop weight. Since

the drop weight was not changed, the different impact energies were achieved by adjusting the drop height. A pneumaticclamping fixture, with a 76.2 mm (3in) diameter opening, secured each sample during impact. The air pressure was set to 0.4MPa, which was well below the 2.02 MPa compressive strength of the foam filled honeycomb core. The samples wereimpacted with a 12.7 mm (0.5in) diameter striker, constructed out of high strength steel. Impulse software was used in orderto display and store the impact data.

The compression and tensile tests were conducted using a 50 kip MTS fatigue test system. The compression testing fixturewas designed similar to a Boeing Model CU-CI fixture [25]. This fixture is specifically designed with side supports to preventbuckling during compression testing. For this study, the side supports were used for all compression tests.


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LOW VELOCITY IMPACT RESULTSFor the entire set of samples tested, partial/total facesheet penetration occurred for the 510 J impacts, total facesheet andpartial core penetration for the 15 J impacts, and partial/total bottom facesheet penetration for the 2045 J impacts. Ingeneral, the initial force-time spike resulted from the top facesheet impact response, the following drop and slow increaseresulted from the foam core crushing, and the final force-time spike resulted from the bottom facesheet impact response.

Some deviation in the force-time results occurred because of the non-homogeneous nature of the foam filled honeycomb core.For some samples, the striker directly impacted the honeycomb structure after penetration of the top facesheet. The additionalsupport for the top facesheet resulted in higher forces and more absorbed energy compared to when the striker missed the

honeycomb structure. In addition, the bottom facesheet damage varied depending on whether the honeycomb structure wasimpacted, as can be seen by comparing the two 25 J impacts shown in Figures 2a and 2b. When the honeycomb structurewas impacted, the loading was more distributed over the bottom facesheet and higher forces were required to createdelamination and a larger cracking diameter. The low density of the core was the main reason that the honeycomb structurehad such a significant affect on impact response. Lower forces were required to shear the facesheet when there was littlesupport from the honeycomb structure. A higher density core would have likely reduced these affects. The top facesheetcracking diameter, Figure 2c, did not show any significant change regarding the amount of honeycomb structure impacted.

a b cFigure 2. 25 J Impact damage of CF sample a) Rear damage-honeycomb structure not impacted, b)

Rear damage-honeycomb structure impacted c) Front impact damage

The force-time curves shown in the appendix characterized how the maximum forces changed when Kevlar or hybrid wereadded to the facesheet. For each sample type, the force-time curves were split into two impact energy groupings: those thatdid not penetrate the bottom facesheet (5-15 J) and those that did (20-45 J). As shown in Table 5, the average maximumforces for the Kevlar samples (1K-4K) showed significant improvement over the CF samples. The hybrid samples (1H-4H)showed even more dramatic improvement when considering samples 2H-4H. The 1H samples were comparable to the CFsamples.

Table 5. Average Maximum Force Data

Avg. Maximum Force (kN)/ Avg. Maximum Force (kN)/

Sample % Increase over CF % Increase over CF

Type (5-15 J) (20-45 J)

CF 1.26 1.33

1K 1.41 11.9% 1.48 11.3%

2K 1.37 8.7% 1.45 9.0%

3K 1.35 7.1% 1.44 8.3%

4K 1.35 7.1% 1.43 7.5%

1H 1.27 0.9% 1.32 -0.8%

2H 1.49 18.2% 1.63 22.5%

3H 1.44 14.3% 1.46 9.7%

4H 1.72 36.5% 1.67 25.6%

The maximum forces developed for the top facesheet varied when considering the carbon fiber, Kevlar, and hybrid sampletypes. The difference in maximum forces was likely a result of different failure modes. Each sample type had different tensileand shear properties. The CF samples had the high tensile strength, but lacked the shear strength needed to achieve theimpact performance of the Kevlar and hybrid samples. The hybrid samples experienced the highest maximum forces, whichwas likely a result of the combination of high tensile and shear properties. The facesheet penetration damage, shown inFigure 3, illustrates the shear and tensile failure modes for the different sample types. The carbon fiber samples clearlyexperienced shear failure, which resulted in cracking that corresponded to the diameter of the striker. The Kevlar sample hada tensile failure mode, which is shown by the bending at the striker perimeter and tearing at the center of impact. The impact


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damage to the top facesheet of the hybrid showed the failure characteristics of both the CF and 4K samples. The carbon fiberstrands experienced shear failure/cracking and the Kevlar strands tensile failure/tearing.

a b cFigure 3. 15 J Impact Damage to a) CF,b) 4K, and c) 4H

ABSORBED IMPACT ENERGYThe addition of Kevlar and hybrid to layers 1, 2, 3, and 4 of the top facesheet resulted in higher absorbed energies comparedto the CF samples as can be seen in Figures 4 and 5. All of the sample types were able to absorb impact energies up to 30 J.However, after 30 J the impact performance for the different sample types varied. This performance can be seen in Table 6,which lists the average absorbed energies of the 35-45 J impacts. These impacts resulted in complete sample penetration.

No significant difference in impact performance was seen when considering the entire set of Kevlar samples, which consist ofreplacing layers 1-4 of the top facesheet as shown in Table 6. The additional layers of Kevlar were not required to achieve theimproved impact performance. In fact, the 1K samples had the highest overall absorbed energy, reaching 36 J. This

represents a 14% increase over the 31.5 J reached by the CF samples.

Just like the Kevlar samples, only minor improvements were seen after adding the first layer of hybrid. This impact datavalidates the benefits of replacing only 1 layer of CF with Kevlar or hybrid. The Kevlar or hybrid layer provides additionalprotection against impact, while the remaining CF layers maintain the high stiffnessof the sandwich composite.

Figure 4. Absorbed vs. Impact Energy for CF all Kevlar sample types


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Figure 5. Absorbed vs. Impact Energy for CF and all hybrid sample types

Table 6. Avg. Absorbed Energy of 35 45J impact energies

Sample

Type Avg. Absorbed Energy (J)

CF 31

1K 34.9

2K 33.2

3K 34.5

4K 33.5

1H 32.4

2H 31.9

3H 33.1

4H 32.8

COMPRESSION TEST RESULTSThe maximum compressive stress resulting in failure for the different sample types is shown in Figures 6 and 7. Thecompression data is the average of two tests conducted at each impact energy. The non-impacted CF sample had the highestcompressive strength at 15.1 MPa. This was approximately 11.5% and 8.6% higher than the average of the non-impactedKevlar and hybrid samples, respectively. Although the general trend was decreased compressive strength with increasedimpact energy, some variation was observed. When delamination occurred to the bottom facesheet, the maximumcompressive strength was greatly reduced. During testing, the delamination that occurred during impact quickly spread andcaused the bottom facesheet to buckle. If a higher density, stronger core were used, the facesheet buckling would have likelyoccurred at much higher compressive loads.

The impact damage consisted of either facesheet crushing, cracking, or delamination. As can be seen, the samples followeda trend of decreasing compressive strength as impact energy increased. The facesheet crushing occurred on samples withminimal or no initial impact damage shown in Figure 8a. The facesheet cracking occurred from the impact hole across thesample, perpendicular to the compressive load, as can be seen in Figure 8b. A combination of facesheet cracking andfacesheet delamination occurred on some of samples impacted at high energies. This likely occurred as a result of the striker

partially or totally penetrating the bottom facesheet. During these impacts, the bottom facesheet experienced both crackingand delamination, which increased once the compressive load was applied.

The non-impacted compressive strength, the average compressive strength for 35-45 J impacts, and the percent decrease incompressive strength when comparing impacted to non-impacted samples can be seen in Table 7. Some deviation occurredwhen considering the non-impacted samples. The deviation between the different sample types can likely be attributed toirregularities between the facesheet/core bond and also the sporadic nature of facesheet crushing/cracking. In addition, whenthe carbon fiber layers were replaced by 2-4 layers of the Kevlar in the facesheet, loading eccentricities might have occurred.Considering the overall compressive strength data, the addition of Kevlar or hybrid to the facesheet did not seem to alter thecompressive strength significantly. In fact, considering all the data, 1K-4K and 1H-4H, the non impacted compressive strengthaveraged less than a 10% drop. A noticeable difference between the different sample types was the percent decrease in


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compressive strength when comparing impacted to non-impacted samples. The better impact performance of the Kevlar andhybrid samples seemed to reduce the decrease in compressive strength when compared to the CF samples.

Figure 6. Max. Compressive Stress vs. Impact Energy

Figure7. Max. Compressive Stress vs. Impact Energy

Table 7. Compression Testing Data

Impacted

Non-Impacted Comp. Strength (MPa) % Decrease

Sample Type Comp. Strength (MPa) 35-45 J (Average) Non Impacted-Impacted

CF 15.1 6.5 57

1K 13.2 6.0 55

2K 13.7 6.3 543K 12.4 7.0 44

4K 14.8 7.7 48

1H 13.1 8.3 37

2H 13.9 8.0 42

3H 13.6 7.9 42

4H 15.0 6.7 55


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a b

Figure 8. a) Facesheet crushing during compression testing damage of 2H non impacted sample and b) Facesheetcracking from buckling during compression testing of 2H 30 J sample

MODULUS OF ELASTICITYThe modulus of elasticity for the carbon fiber, Kevlar, and hybrid were determined to characterize how the stiffness wasreduced by the addition of Kevlar. ASTM standard D3039 was followed for determining the elastic moduli [26]. Table 8 liststhe E1 and E2 values for the different sample types. Rule of mixtures was applied to determine how E1 and E2 of thefacesheet changed when layers of carbon fiber were replaced with Kevlar or hybrid [27].

Table 8. Elastic Modulii for the different sample types

Facesheet E1 (GPa) E2 (GPa)

Carbon Fiber 52 52

Kevlar 31 31

Hybrid(Kevlar E2 Direction) 45 26

1K 47.4 47.4

2K 42.5 42.5

3K 37 37

4K 31 31

1H 50.25 45.5

2H 48.5 39

3H 46.75 32.5

4H 45 26

CONCLUSIONCarbon fiber sandwich composites have relatively low impact properties. In an attempt to improve upon the impact propertieswhile maintaining the high stiffness, lightweight nature of the carbon fiber, Kevlar or hybrid were added to the facesheet. Theimpact and compression after impact data characterized how adding Kevlar or hybrid to the facesheet improved the sandwichcomposite performance during and after impact. The elastic moduli data helped characterize the loss in stiffness resultingform the addition of Kevlar or hybrid to the facesheet. The following conclusions were drawn from this study:

1. The addition of Kevlar to the facesheet improved the maximum absorbed energy and average maximum impact forceof the 1K-4K samples by approximately 10% compared to the CF samples.

2. The addition of hybrid to the facesheet improved the maximum absorbed energy of the 1K-4K samples byapproximately 5% and the average maximum impact force by approximately 14% compared to the CF samples.

3. The addition of Kevlar or reduced the reduction in compression after impact strength when considering non-impactedsamples and those that experienced complete striker penetration. However, the compression strength of the non-impacted samples was the highest for the CF samples.

4. The elastic moduli, E1 and E2, were reduced when Kevlar or hybrid were added to the facesheet. However, thereduction can be minimized to around 10% by replacing only one layer of carbon fiber with Kevlar or hybrid.

5. The advantages and disadvantages of using 1K samples over CF are:Advantagea. 12.5% higher average absorbed energy (35 45 J)b. 11.9% higher average maximum force

Disadvantagec. 12.7% lower non-impacted compressive strengthd. Reduction in stiffness (9% decrease in E1 and E2)


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6. The advantages and disadvantages of using 1H samples over CF are:Advantage

a. 4.5% higher average absorbed energy (35 45 J)Disadvantage

b. 13% lower non-impacted compressive strengthc. Properties are not orthotropicd. Reduction in stiffness (3.3% decrease in E1 and 12.5% decrease in E2)

ACKNOWLEDGEMENTS

The National Science Foundation, under Grant 0082832, and the NASA Space Grant Fellowship Program provided funding forthis research. Their support is greatly appreciated.

APPENDIX

4 Layers Carbon Fiber (CF)

(a) (b)Figure A1. Force vs. Time Response for CF: (a) 5-15 J; (b) 20-45 J

3 Layers Carbon Fiber 1 Layer Kevlar (1K)

(a) (b)Figure A2. Force vs. Time Response for 1K: (a) 5-15 J; (b) 20-45 J


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2 Layers Carbon Fiber 2 Layers Kevlar (2K)


3 Layers Carbon Fiber 1 Layer Kevlar (3K)


4 Layers Kevlar (4K)



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3 Layers Carbon Fiber 1 Layer Hybrid (1H)

(a) (b)Figure A7. Force vs. Time Response for 1H: (a) 5-15 J; (b) 20-45 J

2 Layers Carbon Fiber 2 Layers Hybrid (2H)


1 Layer Carbon Fiber 3 Layers Hybrid (3H)



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4 Layer Hybrid (4H)



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Sem.org SEM X Int Cong s101p03 Low Velocity Impact Combination Kevlar Carbon Fiber Sandwich

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