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Advanced Materials Manufacturing & Characterization Vol 7 Issue 1 (2017)

Advanced Materials Manufacturing & Characterization journal home page: www.ijammc-griet.com

Microstructural Study and Evaluation of Few Mechanical Properties of Hybrid Composites Gangadhar M Kanaginahala, Madhav Murthyb a b

Department of Mechanical Engineering, BMSCE, Bengaluru, INDIA Department of Mechanical Engineering, BMSCE, Bengaluru, INDIA

Abstract Limitations of monolithic composite can be addressed by hybrid composites which are gaining importance in field of composites. An attempt has been made in making of hybrid composite comprising of bi-directional plain weaved Kevlar, S-glass and E-glass fibers reinforcements and epoxy matrix by vacuum bagging and hand layup methods. Few tests such as Hardness, Impact strength and Inter-laminar shear strength tests are conducted as per ASTM standards. Characterization of composites was carried out using SEM to analyze the material bonding, defects and failure behavior under load conditions. S-glass composite outperformed other composites performing better in all the test conditions but when combined with Kevlar the reduction in property is minimal and even microstructural study showed bonding of S-glass and Kevlar hybrids compared to S-glass is acceptable Keywords: Hybrid Composite, S-glass, Kevlar, E-glass, Mechanical Testing, SEM Characterization, Analysis

1.

Introduction

A particular component is made of a material, each component property is defined by the material used and the manufacturing process applied. Composite is a blend of two different properties of materials which are combined together to form a single part. In Fiber Reinforced Composite, Reinforcements are anisotropic (properties only in single direction) in nature, have high modulus of elasticity and carry the maximum load and Matrix protects the composite from environmental changes, flexible in nature but solidifies after curing. Main advantage of composites is they can be tailor made i.e. molded into any required shape and are light weight,

strength/stiffness to weight ratio is higher. MMCs, CMCs & PMCs are classification done based on matrix and based on reinforcements they are classified as short fibers, particulates etc. Synthetic Composites are made of glass, aramid, carbon etc. reinforcements and Natural Composites are made of wood, snail shell etc. Aramid fiber is less functional in compression but performs well in impact conditions and is not suitable for applications exposed to UV rays. Glass is composed of SiO 2, CaO etc. compounds in form of E-glass, S-glass and C-glass fibers which are used for specific applications. It’s Brittle and resistant to fire, flexible due to its variation in diameter and low modulus [1]. A section of composite displaying more than single type fiber is known as Hybrid composite. In Hybrid balanced approach is carried out between the cost and properties to meet the requirements. Mechanical properties vary according to the fiber type, stacking sequence, manufactured method and so on. Behavior & failure modes vary w.r.t intraply and interplay composites [2].

• Corresponing author: Gangadhar M Kanaginahal,E-mail address: [email protected] Doi: http://dx.doi.org/10.11127/ijammc2017.04.02Copyright@GRIET Publications. All rightsreserved.

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Nomenclature MMCs CMCs PMCs UV SiO2 CaO ILSS SEM Micro-CT CAI CMM EDAX ANOVA 2.

Metal Matrix Composites Ceramic Matrix Composites Polymer Matrix Composites Ultraviolet Silicon-dioxide Calcium Oxide Inter-laminar shear strength Scanning Electron Microscope Micro-Computed tomography Compression after impact Co-ordinate Measurement Machine Energy Dispersive Analysis of X-Rays Analysis of Variance

Literature Survey

The survey done here is based on raw materials used in manufacturing of composite and properties tested for determining the behaviour of composites under different load conditions followed by microstructural study. Hybrid Composites made by varying twill woven Kevlar/S-glass displayed friction between warp and weft direction and increase in stiffness was observed due to interlacing of fibers. SEM images showed that uneven resin formation may have an impact on properties [3]. Glass2/Kevlar8 hybrid composite showed better damping and tensile strength properties and reduction in property was observed when fibers were oriented from 0º to ±45° [4]. Analysis done by Micro-CT scan of Kevlar/Glass hybrid composite, the area under impact delaminates less than the adjacent areas [5]. Kevlar/Glass hybrid tested under low impact performed better at very low temperatures -50°C and higher i.e. at 75°C due to transformation of material into ductile form, CAI results were maximum at 25°C and low at 70°C [6].Varying the fiber volume of Kevlar in four-ply Fiberglass composite and stacking Urethane foam in between the plies, when tested under high impact at different temperatures, the damage area was more at -50°C and less at 120°C [7]. Kevlar/Glass hybrid soaked in sea water liquid, when tested Kevlar performed very low due to more moisture absorption which was validated by EDAX/SEM analysis [8], Fatigue tested Kevlar/Glass laminates showed similar results after soaked in dry air and tap water with increase in soaking period [9]. Kevlar/C-glass hybrid tested under nominal drop weight was studied by X-ray and CMM displayed Kevlar had more depth of penetration than Glass [10]. Silane treated fibers were tested under impact load conditions on Kevlar/Glass hybrid showed 80% increase in performance [11]. Siliconized resins displayed improvement in performance when tested for thermal stability and loss of material on Kevlar/Glass hybrid [12]. 3D braided Kevlar and Carbon hybrids tested under bending condition the results were linear for Carbon composite not for Kevlar composites. Kevlar didn’t perform well in flexural condition compared to Carbon but

hybrids outperformed Carbon composite as buckling of Kevlar was handled by Carbon, Kevlar performed well under impact conditions [13]. Varying the sequence of Carbon/E-Glass, hybrids when tested under tensile test Carbon performed better than E-Glass as crack formation was reduced by Carbon and addition of Carbon to Glass composite had linear increase in performance [14]. Glass/Carbon fibers combined by epoxy tested under tensile condition results were high for Carbon than Glass. Yield strength of a hybrid was similar to that of Carbon, Peak load & Ductility was higher for Carbon [15]. Forming pressure and coupler concentration were parameters studied on Glass/polypropyelene composite. At optimal pressure of 7MPa & 8wt% concentration composites performed better in flexural and tensile conditions & ANOVA analysis showed similar results as tested. SEM analysis displayed increase in the concentration reduced the bonding & wetting properties [16]. Polyamide resin combined with woven Glass was tested under tensile and fatigue conditions. In tensile and fatigue the fibers stacked along 45° didn’t perform well due to sharp rise in temperature during the testing observed by infrared camera. SEM analysis showed failures due to resin rich areas and crack propagation in transverse direction to length of fiber [17]. ILSS performance study was done on Carbon/Glass hybrid & maximum load was carried at same displacement for both hybrids having different stacking sequence [18]. Environmental effects was studied on Carbon/Glass hybrid, absorption and desorption of moisture has an effect on properties of hybrid as tested under ILSS conditions [19]. 3.

Experimental Procedure

Material (Reinforcements ) Kevlar (T714) S-glass 6580 E-glass (P92111)

Material (Resin) Epoxy GY 250

Material (Hardener) HY-951

Table 1: Material Properties Aerial Thicknes Tensile densit s Strength y (mm) (Pa) (gsm) 195 0.2 7700/820 0 193 0.15 7000/600 0 201 0.25 7500/500 0

Table 2: Matrix Properties Epoxy Epoxy Viscosity at Index Equivalent 25°C (eq/kg) (g/eq) [mPa-s] 5.3 – 5.4 183 – 189 10000 – 12000 Table 3: Hardener Properties Viscosity at Specific Gravity at 25°C 20°C [g/cm3] [mPa-s] 10-20 0.98

Modulus (GPa) 67.6/67. 6 29/27 -

Vapor Pressure at 20°C ≤ 0.01

Flash Point °C 110

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The reinforcements were cut into 300mm X 300mm dimensions & the resin was weighed as per requirement and hardener was added in ratio of 10:1 respectively. 4 laminates were made by vacuum bagging method as seen in Fig. 1(a), (b) of Kevlar and Sglass where percentage of Kevlar was varied as 100%, 68%, 34.8% and 0%. 2 laminates were made of E-glass and S-glass hybrid composite prepared by hand layup method. Kevlar

Metal Pad

Resin & Hardener Mixture

S-glass Mold

4.

(a)

Foam Silicon Sealant

Vacuum Applied

Vacuum Port

carried on International Equipment Izod/Charpy Impact Tester as per ASTM D256/D4812 standards. The specimens were cut into 63.5mm X 12.7mm X thickness and specimens were placed at the centre on the anvil. The maximum impact produced by the hammer is 21.68J and the results were plotted on graph as noted from digital scale. ILSS study gives the maximum bonding of fibers with matrix between the layers as they are subject to shear loading layers. Inter-laminar shear strength test was performed on INSTRON model 6025 machine as per ASTM D2344/D2344M-00 standards. The specimens were cut as per the standards maintain the length/thickness ratio as 4:1 and the loading was done at 2mm/min and results were plotted on graph. The microstructure of the composite is studied using SEM which gives the material formation and presence of any defects. SEM analysis is done on as cut surfaces and on Izod tested specimens study the material behaviour when subjected to loading conditions.

(b) Figure 1: Manufacturing of Composites (a) Hand Layup (b) Vacuum Bagging

The vacuum was applied at 90kPa for 1.5hours as seen in Fig. 1(b) and left to cure under room temperature for 24hours. The theory of applying vacuum is to squeeze out the trapped air and the gases that form during exothermic reaction which forms due to polymerization process where cross-linking of polymer takes place. Curing is a process where matrix tries to solidify bonding with fibers. During solidification the atoms in the matrix diffuse into the fibers through wetting. Due to uneven flow of resin and cooling of the parts it needs to be post-cured. Post-curing was done by keeping the laminates in oven for an hour at 80° to 100°. The temperature is to make sure not to reach the flash point of hardener and not to melt the matrix. The cured laminates were kept for 24hours at room temperature. E-glass/S-glass hybrids were manufactured by hand layup limiting the thickness approx. 3mm. They were cured at normal room temperature. The laminates were cut into specimens by Abrasive water jet cutting machine as per ASTM standards at pressure of 0.3MPa, abrasive flow rate 0.0067kg/sec and standoff distance 1.5mm. To study the surface hardness of material and the effect of heat treatment on the surface the hardness test was conducted on Rockwell Hardness Tester machine as per ASTM D785 standard following the L scale for study. For the study specimens were cut into 10mm X 10mm X thickness. Loading was done at 60kgf for 15 seconds, the results were plotted on graph. Izod impact test was

Results and Discussion

4.1 Izod Impact Testing as per ASTM D256/D4812 Failure mechanism of material under the impact can be ductile or brittle. Brittle materials show less deformation and the failed surface is flat. Ductile materials show cleavage failure and display high strength than brittle materials. Izod test is carried out by placing the specimen parallel to anvil i.e. along the lamina direction and this method will help in studying the bonding of material. An average of 5 specimens of each laminate is plotted on graph where S-glass/Kevlar hybrid has performed in par with the S-glass composite showing that the properties of hybrid can be accepted as seen in Fig. 2. Composite which shows highest Energy Absorption has undergone larger deformation. An advantage of Kevlar/S-glass hybrid is bonding of two types of material where Kevlar handles the Glass fiber under impact loading conditions avoiding catastrophic failure of material. Only E-glass/S-glass hybrid have shown brittle failure rest all composites have displayed ductile failure i.e. a cup-cone formation. S-glass composite has displayed better performance but S-glass/Kevlar hybrid will be a suitable option as the number of fiber layers used is less compared to S-glass Composite and even catastrophic failures can be avoided.

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the results plotted define the hardness of each composite subjected to the heat treatment as seen in Fig. 3. The Kevlar has the lowest value amongst all and S-glass has the highest. The amount of time allotted for manufacturing may not have been sufficient for Kevlar to bond with matrix resulting in a less value. 4.3 ILSS Testing as per ASTM D2344/D2344M-00

(a)

ILSS test is also known as short beam shear test because the specimen dimensions are small to the thickness when compared to three point bending test or flexural. The bonding between the matrix and reinforcements is the interface & it has been noted as significant area which is subjected to loading conditions such as tensile, compressive, shear etc. at higher rate. When the layers are subjected to shear loading the bonding of material will be under both compression and tensile forces. As studied from literature survey in ILSS testing Glass performs better than rest of materials. An average of 5 specimens of each laminate is tested and plotted on graph as seen in Fig. 4. Theresults also show that S-glass has the highest value followed by E-glass and Kevlar.

Void

(b) Figure 2: Izod Impact tested specimens a) Energy Absorbed b) Impact Resistance

4.2 Hardness Testing as per ASTM D785

Figure 4: Inter-Laminar Shear Strength tested specimens

4.4 SEM – Microstructural Study of As Cut Surfaces of composites

(a)

(b)

Figure 3: Rockwell Hardness Testing (HRL)

Figure 5: SEM image of Kevlar Composite specimen at a) 800x b) 5000x magnification

A heat treatment process solidifying the matrix and bonding with fibers is curing. Composites made of polymers act as insulators where maximum amount of heat is experienced on the surface of material and can be evaluated by hardness testing. An average of 5 specimens of each laminate is plotted on graph and

Fig. 5 shows Crazing a formation of resin in form of steep due to presence of micro-voids which initiate formation of crack. Cleans surfaces are the areas where the resin did not bond with the fiber as per requirement due to insufficient supply of resin or faster curing.

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Figure 8: SEM image of E-glass/S-glass Hybrid Composite specimen at a)100x b) 500x magnification

In E-glass/S-glass hybrid mis-alignment of fiber is observed in Fig. 8. At some areas resin is rich in content which initiates cracking around it due to variation in stress and matrix crazing is observed at some places. Poor resin areas are less observed except in Kevlar and hybrid composites.

(a) (b) Figure 6: SEM image of S-glassComposite specimen at b) 1500x magnification

a) 150x

4.5 SEM – Microstructural Study of Izod Tested Specimens of composites

Air trapped between the laminas during manufacturing leads to formation of voids as seen in Fig. 6, small voids are highly stressed areas which may lead to catastrophic failure. Presence of cracks after manufacturing may be due to resin shrinkage which may have an adverse affect on the properties which may lead to failure at early stages.

E-glass

Bonding of Kevlar and S-glass

S-glass

Kevlar

(a)

(b)

Figure 7: SEM image of Kevlar/S-glass Hybrid Composite specimen at a)100x b) 800x magnification

Alignment of fibers is visible in Kevlar/S-glass hybrid in Fig. 7, the bonding area shows the alignment of fibers and Kevlar looks like a cloth. Presence of voids is minimal but few small voids can be seen.

(a)

(a) (b) Figure 9: SEM image of Kevlar Composite specimen at a)20x b) 500x magnification

Kevlar has high stiffness property and it looks like a cloth material. Literature surveys have inferred that bonding of Kevlar with thermoset resins is a real challenge. As seen in Fig. 9 after the Izod impact single fiber filaments were observed on the surfaces. In Fig. 9(a) the black lines observed are cracks produced at centre due to impact. The cracks have travelled in a random manner between the laminas where the bonding between the matrix and fiber has failed in the given amount of manufacturing time. An uneven form of bonding has taken place as the propagation of crack is in a random fashion. The area at the centre has experienced the maximum force and has delaminated at higher rate than the adjacent areas. In Fig. 9(b) there is very little resin visible on the surfaces of fiber explaining the bonding process. The clean surfaces observed in Fig. 5 justify the amount of resin visible on the fiber surfaces. Single fibers are observed in Fig. 9(b) indicating the bonding between fiber and matrix in a lamina. A ductile failure of material is observed and there are no flat surfaces at the deformed areas. The amount of time required to bond Kevlar with epoxy was not sufficient.

(b)

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Figure 12: SEM image of S-glass Hybrid Composite specimen at a) 50x b) 500x magnification Uneven

(a)

(b)

resin of fiber Fig. 12 shows ductile failure of S-glass, a strong bonding and matrix is observed. The crack has followed a single path without affecting the layers adjacent to it and less fiber pull-out is seen compared to Fig. 9, 10, 11. At some areas radial cracking Large of fiber is observed where the failure of fiber is along diameter. Void of uniform resin on fiber shows better bonding of Presence materials. The cup-cone surfaces confirm the ductile failure of specimen under load. Fiber pull-out is minimum compared to other composites which explains the level of bonding formation.

Figure 10: SEM image of 68% Kevlar / 32% S-glass Hybrid Compositespecimen at a)20x b) 500x magnification

Single fiber failure (a) (b) Figure 13: SEM image of 31.25% E-glass / 68.75% S-glass Hybrid

(a) (b) Figure 11: SEM image of 34.8% Kevlar / 65.2% S-glass Hybrid Composite specimen at a)20x b) 500x magnification

Fig. 10, 11 shows the Izod tested images of S-glass/Kevlar hybrid composite. In both Fig. 10, 11 display complete failure of S-glass with minimal damage of Kevlar. But Kevlar handles failure of Sglass as seen in Fig. 10, 11 avoiding the catastrophic failure. In Fig. 10, 11 the fibers are held intact whereas crack has propagated through areas of poor bonding between laminas causing the failure esp. near the S-glass Kevlar bonding. Increasing the percentage of S-glass has shown better performance under loading may be due to its bonding and handling maximum load. Failure of fibers observed in Fig. 10, 11 is in form of bundles compared to failure seen in Fig. 9. Fiber pull-out is defined as fibers after failure detach from each other as single fibres which is minimum as observed.

Composite specimen at a) 20x b) 500x magnification

(a) (b) Figure 14: SEM image of 62.5% E-glass / 37.5% S-glass Hybrid Composite specimen at a) 20x b) 500x magnification

In Fig. 13 brittle failure is observed as flat surfaces are fomed at deformed area and Fig. 14 displays ductile failure. Both composites are made by hand layup due to which uneven resin is observed on the fibers as seen in Fig. 13(b). Fiber pull-out is observed in Fig. 13(b), 14(b) where single fibers are detached from the fiber bundle. Resin rich areas may have caused the failure of laminate at few places. Fiber bending is also seen at the impacted areas which infers during impact the material may be subjected to bending also [11,12,13,20]. 5.

(a)

(b)

Conclusion

Kevlar/S-glass hybrid has shown a performance in par with Sglass composite avoiding catastrophic failure under load conditions. The fiber thickness of S-glass was the key factor for

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outperforming other composites. ILSS test proved that Glass fiber can be the material relied upon when subjected to both tensile and compression loads. Impact study was significant in examining the bonding of matrix and fiber by SEM which has shown a fine bonding in S-glass composite and few defects such as clean surfaces where zero bonding has occurred in Kevlar. Izod specimen analysed under SEM have given more insight into the material behavior for e.g. specimens under impact loads sometimes have shown transfer of load to adjacent layers, fiber pull-outs are observed along with fiber bundles as the material failed. 6.

6.

7.

Future Scope

A differential approach of manufacturing and curing must be studied as general approach works have carried out in high numbers. Machining of composites must be studied in depth as some of their features are supportive or limit the compatibility with composites. Mode-I and Mode-II testings must be carried on each and every specimen or laminate made to have a overall study of performance. Impact and ILSS studies in depth can increase the importance of composites in more applications. Failure mechanism of composites needs to be studied as the mode of failures and deformation of the material is hard to predict.

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Acknowledgment

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I would like to express my gratitude to TEQIP (1.2.1) B.M.S.C.E. for funding the project, Dr. S. Srinivas, Associate Professor, Department of Mechanical Engineering, B.M.S.C.E for providing permission to use Water Jet Machine facility at B.M.S.C.E. & Dr. M. Ramachandra, Professor, Department of Mechanical Engineering, B.M.S.C.E for providing permission to use Scanning Electron Microscope facilities available at B.M.S.C.E.

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