Scholarly article on topic 'Size Scale Effects on Post-impact Residual Strength of Hybrid Glass/Carbon/Epoxy Nano-composites'

Size Scale Effects on Post-impact Residual Strength of Hybrid Glass/Carbon/Epoxy Nano-composites Academic research paper on "Materials engineering"

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Procedia Materials Science
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{"Hybrid composite" / "post-impact residual strength" / "ply-level scaling" / "sub-laminate level scaling" / "Nanoclay ;"}

Abstract of research paper on Materials engineering, author of scientific article — N.S. Kavitha, Raghu.V. Prakash

Abstract Understanding the material response to macroscopic scaling along the thickness direction in fiber-reinforced composite materials is important to reduce the effects of scaling-up during design. This paper presents the effect of size-scale of composites obtained through two methods of scaling: a) ply-level (0gn/90cn)s and b) sub-laminate level (0g/90c)ns where, (n= 2,3,4), on the residual strength and post-impact response. The strength parameters were determined with and without nano-clay addition (Nanomer 1.30E, clay surface modified with 25-30 wt% octa-decylamine). The hybrid glass/carbon/epoxy composite and nano-composite (1.5% nano-clay of weight of epoxy) specimens were prepared using compression molding technique. The ultimate tensile strength of thickness scaled composite and nano-composite specimen was examined. Impact tests were done at two velocities, viz., 4.49 m/s and 3.78 m/s for both thickness scaled composite specimens with and without nano-composite addition. It was observed that ply-level scaled laminates absorbs more impact energy compared to sub-laminate level scaled specimens. The residual tensile strength was found out by conducting tensile test on the impacted specimens. The reduction in residual strength is more in nano-composite specimens compared to plain-composite material. The damage developed in the ply-level scaled specimen is more compared to sub-laminate level scaled specimens.

Academic research paper on topic "Size Scale Effects on Post-impact Residual Strength of Hybrid Glass/Carbon/Epoxy Nano-composites"

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Materials Science

Procedía

ELSEVIER

Procedía Materials Science 3 (2014) 2134 - 2141

www.elsevier.com/locate/procedia

20th European Conference on Fracture (ECF20)

Size scale effects on post-impact residual strength of hybrid glass/carbon/epoxy Nano-composites.

Kavitha N.S1 and Raghu.V.Prakash1, *

Machine Design Section, Department of Mechanical Engineering Indian Institute of Technology Madras, Chennai-600036, INDIA

Abstract

Understanding the material response to macroscopic scaling along the thickness direction in fiber-reinforced composite materials is important to reduce the effects of scaling-up during design. This paper presents the effect of size-scale of composites obtained through two methods of scaling: a) ply-level (0gn/90cn)s and b) sub-laminate level (0g/90c)ns where, (n= 2,3,4), on the residual strength and post-impact response. The strength parameters were determined with and without nano-clay addition (Nanomer 1.30E, clay surface modified with 25-30 wt% octa-decylamine). The hybrid glass/carbon/epoxy composite and nano-composite (1.5% nano-clay of weight of epoxy) specimens were prepared using compression molding technique. The ultimate tensile strength of thickness scaled composite and nano-composite specimen was examined. Impact tests were done at two velocities, viz., 4.49 m/s and 3.78 m/s for both thickness scaled composite specimens with and without nano-composite addition. It was observed that ply-level scaled laminates absorbs more impact energy compared to sub-laminate level scaled specimens. The residual tensile strength was found out by conducting tensile test on the impacted specimens. The reduction in residual strength is more in nano-composite specimens compared to plain-composite material. The damage developed in the ply-level scaled specimen is more compared to sub-laminate level scaled specimens.

© 2014 Elsevier Ltd. Open access under CC BY-NC-ND license.

Selectionandpeer-review underresponsibilityof theNorwegianUniversityof ScienceandTechnology(NTNU), Department ofStructuralEngineering

Keywords:Hybrid composite, post-impact residual strength, ply-level scaling, sub-laminate level scaling, Nanoclay;

* Corresponding author, Tel: +91-44-2257 4694, Fax:+91-44-2257 4652 E-mail address:raghuprakash@iitm.ac.in

2211-8128 © 2014 Elsevier Ltd. Open access under CC BY-NC-ND license.

Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering doi:10.1016/j.mspro.2014.06.345

1. Introduction

Fiber reinforced plastic (FRP) composite materials are widely used in load bearing, light-weight structures in view of their superior mechanical properties compared to metals. These light-weight structures experience high or low velocity impact loading during fabrication, assembly or maintenance operation due to accidental drop of tool, runway debris and hailstones. Some of the damages are difficult to detect with bare eyes and pose a threat during dynamic loading. The energy absorbed during impact is mainly dissipated by a combination of matrix damage, fiber-matrix debonding and fiber fracture that results in catastrophic failure of the structure if proper design precautions are not taken. In order to avoid this, there is a need to test the large scale structure to ensure that there is adequate residual strength of a damaged structure until the damage is detected. The effect of impact damage is studied through: a) Impact damage resistance, which deals with the response and damage caused by a certain impact, and b) Impact damage tolerance, which deals with the reduced strength and stability of the structure due to damage (Fig 1) [1].

Testing large scale structure is an expensive, time consuming and difficult affair, hence small scale specimens are used to predict the behavior of large scale structure. Change in mechanical properties while scaling from small scale specimen data to large scale structure is referred to as size-scale effect. The specimen thickness and the stacking sequence of the plies play an important role in damage modes. Ply level and sub-laminate scaling are two techniques used for constructing scale model composite structure at the macrostructure level scaling. In ply-level scaling, the baseline or laminate stacking sequence is "scaled-down" by simply decreasing the number of layers for each angular ply orientation. In sub-laminate level scaling, the laminate thickness is scaled by reducing the repeating baseline stacking sequence as a sub-laminate group (Fig 2).

Impact damage resistance Impact damage tolerance

Impact threat k 1 i Response fv Damage k. Residual properties ^ »V J

Impact tolerance

Fig 1:Impact damage divided into two sub problems [1]

Sublaminate level scaling Ply-level scaling

Baseline or standard Baseline or standard

specimen specimen

0° 0°

90* D" 0° 0°

0" 90° 0" 0° 0° 0°

90° 0" 90° 0° 0° 0°

0" 90° 0° 90" 90° 90°

90° 0° 90° 90° 90° 90°

0" 90° 90" 90° Ply-Z

90* Sub-3 Sub-2 90" Ply-3

Sub-4 Ply-4

Fig 2:General methods of thickness scaling in FRP composite'2

The thickness of the laminate is more sensitive to damage due to impact and static loads. Varying the scaling method changes the ability of laminate to store energy and impact resistance1-3-1. The delamination area and back face cracking decreases with increase in specimen's area. Increase in panel size does not affect the maximum static force but has an influence on damage [4]. The scaling effects have been studied by many researchers when the specimen is subjected to tensile, compression, flexural etc.A three dimensional analysis was used to develop the scaling laws for impact of marine composites. The elastic response was noted to scale well, whereas, size effect was observed in damage response. The similitude law is used to predict the elastic scaled structures [5]. The load carrying capacity of

hybrid composite increases compared to neat carbon/epoxy laminate with slight reduction in stiffness. The matrix toughness reduces the scaling effects. The material with the tougher resin suffers less impact damage and has better strength.

Polymer matrix can be toughened with the addition of functionalised Nanoclay[6], as it increases the interfacial shear strength between fibre and matrix and improvement in the mechanical properties [7]. The type of fibre and orientation plays an important role in mechanical properties. The fibre failure was found to occur at relatively low load and in thinner woven roving at high strain rate [8]. The level of impact energy, number of impacts, and the mass of the impactor significantly influences the residual strength degradation [9].

The residual strength has a significant effect with low temperature. The residual strength decreases with low temperature in tape carbon epoxy laminate; woven laminate has greater strength [10]. The preload increases the compression residual strength if the load actually approaches the initial buckling value, since the plate loses its stiffness -11-. It is noted that while scaling effects and scaling laws of different fibre composites have been studied, scaling effects of hybrid composites and residual strength are not studied, which is very important for structural integrity. The objective of this paper is to study the impact response, damage and residual tensile strength for two types of scaled composites and nano-composite.

2. Experimental

2.1 Material and fabrication technique

In this study, uni-directional glass and carbon fibre matt of 0.25 mm thickness were used as reinforcement materials, epoxy (LY556) as matrix material and hardener as curing agent. The hybrid laminates were fabricated according to Figure 2 by keeping glass fibre at 0 degree and carbon fibre at 90 degree orientation; compression molding was used for specimen preparation by applying pressure at 20 bars and temperature at 80 degree centigrade. The matrices for the nano-composites were prepared by mixing 0.5, 1, 1.5, 2, and 2.5 % of Nanoclay (Nanomer 1.30E, clay surface modified with 25-30 wt% octa-decylamine) to the epoxy resin by an electrical stirrer for 3 hours. During electric stir, shear force is applied on the clay particles during mixing; these dispersed nanoclay particles and exfoliated nano-composite gives good mechanical properties [12].

Standard specimens of size 25 mm x 250 mm x 4mm (Sub-4 and Ply -4) were prepared according to ASTM-D 3039standards (Fig 3b), and other specimens were scaled down from the baseline specimen thickness of 4 mm to 3 mm (Sub-3 and Ply-3) and 3 mm to 2 mm (Sub-2 and Ply-2). The laminates were cut according to the dimension by using vertical saw cutting machine for tensile and impact test.

2.2 Tensile testing:

Tensile test was done at a displacement rate of 1 mm/min using a MTS 810 servo hydraulic machine (Fig 3a) for both nanocomposite and composite specimens. The ultimate tensile strength was obtained from tensile test, which is useful to know the change in residual strength after impact. Tensile tests were done on specimens prepared with different percentages of nano-clay addition to understand the effect on tensile strength. It was observed that addition of 1.5% of Nanoclay provides the highest tensile strength and Young's modulus.

Fig 3: a) MTS 810 Servo hydraulic machine, b) tensile test specimens

Fig 4: Stress-strain curve for different percentage of Nanoclay added specimens

It was observed that the ply-level scaled baseline specimen had an ultimate tensile strength of 302.96 MPa; when the thickness was scaled down from 4 mm to 3 mm, the UTS increased by 12 to 14%, but when thickness was reduced from 4 mm to 2 mm, the UTS decreases by 7 to 13% in sub-laminate level scaling (Fig 5). The thinner specimens of sub-laminate scaling had higher tensile strength compared to thinner specimens in ply-level scaled laminate; hence, one could infer that there is scale effect as the thickness is reduced. Sub-laminate level scaled specimen had 16% higher tensile strength compared to ply-level scaled laminate. In [0/90]ns laminate, the 90 degree plies share the damage from 0 degree plies.

-Pty-2 ■My-3 •Plv-4

0.005 0.01 0.015 S train,mm/mm

0.01 0.02 0.03 Strain .шшшш

•Nanoply-2 ■Nancply-3 Nancply-4 •Nanosub-2 'Nanosub-3 Nanosub-4

Fig 5: Stress-strain curve for scaled a) composite specimens and b) nanocomposite. Ply-x indicates ply level scaled specimen, and Sub-x represents sub-laminate level scaled specimen.

Sub-laminate level nano-composite does not show any change in tensile strength compared to plain composite laminate; however, there is 36 % reduction in tensile strength in ply-level nanocomposite (Fig 5). In both types of scaled nanocomposites, thin specimen had less strength compared to thicker specimens, whereas failure strains are different with thickness.

2.3 Impact testing:

Impact tests were conducted at room temperature using a single impact drop weight impact machine, which consists of drop tower equipped with an impactor and a variable crosshead weight arrangement, a high speed data acquisition system, and a load transducer mounted in the impactor (Figure 6). The crosshead/ impactor weight was kept constant at 5.2 kg for all tests. Impact energy was varied by changing the drop height. The specimen support fixture is at the bottom of impact testing system; a stopper is provided at the bottom of the machine to stop the impactor after single impact. The shape of the impactor was semi-hemispherical with a diameter of 16 mm; the falling weight

is guided by two fricition less columns. A LVDT is placed at the bottom of the specimen to measure the deflection during impact. The impact tests were done at two velocities of 3.78 m/s and 4.49 m/s on both scaled nano-composite and plain-composite specimens.Transient response data during impact test, such as, velocity, deflection, load and energy as function of time is recorded for all specimens.

Figure 6: Low velocity impact test setup

The load-displacement response of the scaled sublaminate level and ply-level scaled nano-composite varied with thickness.When an impact load is applied to the composite plate, it absorbs the impact energy by elastic deformation (rebound of impactor) and plastic deformation. The energy absorbed energy by the plastic deformation remains as damage in the specimen. The load-displacement response for a specimen thickness of 3 mm sub-laminate level scaled nanocomposite specimens shows closed loop response. The area under the load-displacement curve represents the deformation energy that is initially transferred from the impactor to the specimen surface. The area included inside the loop refers to the energy absorbed during the impact. The maximum peak load is achieved in case of 3 mm thickness specimen at 3.78 m/s velocity in sub-laminate level scaled nano-composite. The maximum peak load for ply-level and sub-laminate level scaled nano-composite specimens are the same; however, the peak load decreases with increase in velocity (Figure 7(a), (b)). For thinner specimens, the load-displacement response is different and indicates penetration of impactor inside the specimen. The glass fiber has lower stiffness than carbon fiber, hence for thin specimen penetration of impactor may be taking place.

The peak load for specimens without addition of Nanoclay is different for both scaled composites; it is higher for ply-level scaled specimens compared to sub-laminate level scaled specimens. As the impact velocity increases, the peak load decreases with thickness (Figure 7(c), (d)). Rebouncing of load can be observed for 3 mm thickness and high velocity for both types of scaled composites. The thicker nano-composite specimen has more rebouncing but peak loads are same for two scaled specimens.

The damage in the specimen due to impact suggests that intra-ply matrix splitting cracks are created first, followed by de-lamination growth from these cracks. This delamination initiates at almost every interface through the specimen thickness, propagating from the cracks in the back plies of the interface. The delamination in the ±45 interface follows the matrix split in the +45 direction as shown in (Figure 8(a)). The impact damage mechanism in ply-level and sub-laminate scaled specimens is different at same velocity. The failure mode in hybrid composite is different from glass/epoxy or carbon /epoxy laminate. De-lamination failure was observed in glass/epoxy laminates while fibre fracture was observed in carbon/epoxy laminate. Combination of these two fibres results in change in failure mode and is based on the stacking sequence. The stacking plies of same fibre orientation increases the stress concentration at the adjacent interface, due to increase bending stiffness within that plies, which leads to delamination, hence delamination failure mode can be observed in ply-level scaled specimens (Figure 8(b)). The shape of the damage is circular in ply-level scaled specimens. If the fibre orientation is different between the adjoining plies, delamination occurs at the interface and will not propagate, and leads to fibre fracture. The delamination initiates when the transverse shear stress exceeds the critical value. The impact damage is found to be

more in thin specimens than in thick specimens. The damage is detected over the entire thickness whereas in thick specimens damage is restricted to top three or four layers. This impact damage reduces the impact force. In the sublaminate level specimens (Figure 8(c)), line damage can be observed along the width of the specimen, which failed by fibre failure because of shear stress between two dispersed plies. In sub laminate scaled specimen, the carbon and glass plies are dispersed, the carbon plies are more brittle and stronger; as a result, the fibre fracture happened when the specimens are subjected to impact load. The damage increases with increasing impact load, decreases with increase in specimen thickness.

NS2 4.49 щ/s NS3 4.49 щ/s NS4 4.49 щ/s NS2 3.78 щ/s NS3 3.78 щ/s NS4 3.78 щ/s

0 10 20 Displacement,mm

0 20 Displacement,mm

Sub-2 4.49 щ/s Sub-3 4.49 щ/s Sub-4 4.49 щ/s Sub-2 3.78 щ/s Sub-3 3.78 щ/s Sub-4 3.78 щ/s

300 250 §200 S150

NP2 4.49 щ/s NP3 4.49 щ/s NP4 4.49 щ/s NP2 3.78 щ/s NP3 3.78 щ/s NP4 3.78 щ/s

Displacement,mm

10 20 Displacement,mm

PLY-2 4.49 щ/s Piy-3 4.49 щ/s Piy-4 4.49 ni/s Ply 2 3.78 щ/s Ply-3 3.78 щ/s Ply-4 3.78 nVs

Figure 7 : Load displacement curve for: a) nanoclay added sub-laminate level scaled specimens, b) nanoclay added plylevel scaled specimens,c) sub-laminate level scaled specimens and d) ply-level scaled specimens.

Projectile Direction

0° Direction

Delamination Matrix crack

0/-45 Interface

■-45/45 Interface

Figure 8: a) Delamination orientation on laminate interfaces as observed by the de-ply technique.[6], b) sublaminate level scaled impact damaged specimen c) ply-level scaled impact damaged specimen.

The size of the damage depends on the impact energy absorbed by the specimens. In sub-laminate level scaled specimen, the impact energy absorbed by the speciemn is high for 3.78 m/s velocity for 2 mm and 3 mm thick specimens compared to 4.49 m/s velocity (Fig. 9). Nanoply in the figure legend represents laminates with nano-clay addition prepared by ply-level stacking while nanosub indicates samples with nano-clay addition prepared by sublaminate level stacking. As the thickness increases, the energy absorbed by the speciemn increases for 4.49 m/s velocity. In both types of scaled nano-composites, the impact energy decreases with thickness for 4.49 m/s velocity and increases for 3.78 m/s velocity. In ply-level scaled specimen, the energy absorption for 3 mm and 4 mm specimen has no effect on impact velocity and decreases with thickness. Thin specimen (2mm) has no effect on velocity in ply-level nancomposite specimen.

- Nan op ly 3 78 m/s

■ Nanosub 3.78 m/s

Nan op ly 4.49 m/s

Nanosub 4.49 m/s

Ply 3.78m/s

Sub 3.78m/s

—I— Ply 4.49 m/s 2 3 4 5

Sub 4.49 m/s

Thickness.mm

Fig 9 : Impact energy verses thickness of the specimens.

2.4 Residual strength

The tensile test was done on the impacted specimens to understand strength reduction after impact damage. Tensile test were done using MTS 810 servohydraulic machine of load rate of 1 mm/sec for all specimens.The reduction in residual strength for ply-level scaled laminate is 37 to 47%; as the thickness increases, residual strength decreases by 17 to 22%. In ply-level nancomposites the residual strength decreases by 40-60% (Figure 10 a-c). In sublaminate level scaled specimen, the decrease in residual strength is almost constant (50%) for different thickness of the specimen. Higher residual strengh variation (30-60%) was observed in sub-laminate scaled nano-composites.

3 Conclusions:

The behaviour of two types of scaling, viz., ply-level and sub-laminate scaled hybrid composite and nano-composite were examined. The following observations were made:

• In ply-level scaling, tensile strength decreases with thickness of the specimen, whereas it increases in sublaminate level scaled specimens.

• The tensile strength decreases with addition of nano-clay.

• The ply-level scaled specimen absorbs more impact energy compared to sub-laminate scaled specimen.

• The impact damage shape of the ply-level scaled specimen is circular and in sub-laminate specimen occurred along the width in the form of line.

• Reduction in residual strength is more in nano-clay added composite.

Acknowledgments:

The first author thanks the Dean, Office of International Relations, IIT Madras for providing the Boeing travel grant to present this work at ECF-20.

-Nanoply-2 -Nanosub-2 •Pfy-2 -Sub 2

■Nanoply-3 'Nanosub-3 ■Piy-3 - Sub-3

Impact velocity,m/s

Impact Velocity,m/s

• Isanoply-4

■ Nan osub -4

■ Fly-4 —--Sub4

Impact velocity,m/s

Fig 10:Residual strength variation with impact velocity for a) 2 mm,b) 3mm and c) 4mm thick specimen.

References:

1. Olsson R., "Impact and damage tolerance of composites - status and future work at FFA", FFA TN 1999-77, 1999, The Aeronautical Research Institute of Sweden, Bromma.

2. Karen E. Jackson, "Workshop on Scaling Effects in Composite Materials and Structures" NASA Conference Publication 3271, 1994.

3. Sutherland, L. S., C. Guedes Soares, "Scaling of impact on low fibre-volume glass-polyester laminates", Composites: Part A, 38,

(2007), 307-317.

4. Found, M. S., I. C. Howard, A. P. Paran, "Size effects in thin CFRP panels subjected to impact", Composite Structures, 35, (1997), 59-607.

5. Philippe Viot, "Scale effects on the response of composite structures under impact loading", Engineering Fracture Mechanics, 75,

(2008), 2725-2736.

6. Soutis, C., "Prediction of the post-impact compressive strength of CFRP laminated composites", Composites Science and Technology, 56, (1996), 677-684.

7. Nagalingam, R., S. Sundaram, B. Stanly, Jones Retnam, "Effect of nanoparticles on tensile, impact and fatigue properties of fibre reinforced plastics", Bulletin of Material Science, 33, (2010), 525-528.

8. Sutherland, L. S., C. Guedes Soares, "The effects of test parameters on the impact response of glass reinforced plastic using an experimental design approach", Composites Science and Technology, 63, (2003), 1-18.

9. Ernesto Guades, Thiru Aravinthan, "Residual properties of square FRP composite tubes subjected to repeated axial impact", Composite Structures, 95, (2013), 354-365.

10. Sanchez-Saez, S., E. Barbero, C. Navarro, "Compressive residual strength at low temperatures of composite laminates subjected to low-velocity impacts", Composite Structures, 85 (3), 2008, 226-232.

11. Zhang, X., G.A.O. Davies, D. Hitchings, "Impact damage with compressive preload and post-impact compression of carbon composite plates", International Journal of Impact Engineering, 22, (1999), 485-509.

12. Tri-Dung Ngo, Van-Suong Hoa, Minh-Tan Ton-That "Effect of shearing on dispersion, intercalation/exfoliation of clay in epoxy", 16th International Conference on Composite Materials, 2007, Kyoto, Japan.