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Vol. 8. Issue 3.
Pages 2662-2673 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2662-2673 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.04.005
Open Access
Low velocity impact behaviour and post-impact characteristics of kenaf/glass hybrid composites with various weight ratios
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Muhammad F. Ismaila, Mohamed T.H. Sultana,b,c,
Corresponding author
thariq@upm.edu.my

Corresponding author.
, Ahmad Hamdana, Ain U.M. Shaha,b, Mohammad Jawaidb
a Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia
b Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia
c Aerospace Malaysia Innovation Centre (944751-A), Prime Minister's Department, MIGHT Partnership Hub, Jalan Impact, 63000 Cyberjaya, Selangor Darul Ehsan, Malaysia
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Tables (6)
Table 1. Force and displacement values from the tensile testing.
Table 2. Hybrid composites fabricated with different ratio.
Table 3. Height of the striker for low velocity impact testing.
Table 4. Data tabulation for low velocity impact testing.
Table 5. Damage area of impacted specimens with respect to different impact energy levels.
Table 6. Force and displacement values from the compression after impact testing.
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Abstract

The aim of this work was to analyze the effects of hybridizing kenaf and glass fibre to develop hybrid composites with varying weight ratios on the low velocity impact response and the post-impact properties of the obtained composites. Four main process had been carried out in this study, which were the fabrication of composites, the low velocity impact testing, the dye penetrant evaluation on the impacted composites and the compression testing on the impacted samples after the dye penetrant evaluation. This research was motivated by the increasing demand for lightweight, cost-effective and environmentally friendly materials to be applied at an industrial level. In this paper, natural kenaf fibre was hybridized with synthetic glass fibre in an attempt to create an attractive material for the composite industries. The materials were fabricated in seven samples with varying weight percentage ratios of the fibres, while the glass fibre was used as the outermost layer for each formulation. A sample made entirely from kenaf fibre and another one entirely from glass fibre were also included for comparison. The formulation that demonstrated the best tensile performance – that with the weight percentage ratio of 25% kenaf fibre and 75% glass fibre – was then subjected to low velocity impact tests. Four impact energy levels of 10J, 20J, 30J and 40J were applied to study the propagation of impact in the composite with the optimum formulation. The closed curve on the graph plotting force versus displacement indicated the success of the specimen in absorbing the dissipated energy up to 40J. The dye penetrant test was performed to investigate the damage area progression, and it revealed that a higher energy level will produce greater damage. Compression after impact tests indicated that the compression damage decreased as the impact energy was increased. Considering that the hybrid composite with the weight ratio of 25% kenaf fibre and 75% glass fibre approached the performance of the material made entirely from glass fibre, it may be concluded that it can be employed for product development in environmentally friendly technologies.

Keywords:
Kenaf
Glass
Hybrid composites
Low velocity impact
Compression after impact
Dye-penetrant test
Non-destructive testing
Full Text
1Introduction

The production of next generation materials from renewable sources is an increasingly important field of research and development. The demand for stronger, stiffer and more lightweight materials is essential in most industries. The fibres used in the composites industry can be divided into two categories, namely natural fibres and synthetic fibres. Natural fibres (mostly derived from trees and plants) are considered to be environmentally friendly and therefore, in recent years, they have become the raw materials of choice to be used by many industries, including the aerospace, automotive, marine, military and defense industries. Actually, according to findings, natural fibres were utilized more than 3000 years ago as reinforcement for materials in many countries [1]. Most research nowadays emphasises the development of new natural fibre based composites that exhibit the best performance in terms of mechanical properties, i.e. tensile, flexural, and impact strength [1–4]. The use of natural fibres as a renewable material reduces the use of synthetic fibres [5,6], which thus contributes to preserving natural fossil resources.

Kenaf plant or Hibiscus cannabinus L., is an annual herbaceous plant originating from West Africa [7] and well spread to Asia via Egypt. There are several varieties of kenaf plant which have different flowering schedules. The production of six to ten tonnes of dry fibre per acre per year means that kenaf plant has a high harvestable yield in comparison with other plants [8]. Kenaf plant grows quickly and is a renewable resource. Hence, it may guarantee a continuous supply of source material with low production and manufacturing cost. Kenaf plant contains two fibre types, long and short. The plant's stalk consists of inner core fibre surrounded by outer bast fibre [8]. Kenaf fibres can be used as reinforcing fibres for composites. Kenaf sheets have similar anisotropic mechanical properties to composite sheets. The tensile strength and Young's modulus of kenaf fibres are lower but still comparable to those of composites [8,9]. Therefore, kenaf fibres have good potential to replace current reinforcement materials in high-performance biodegradable composites [8,9]. Several research studies on kenaf polymer composites have already been conducted, such as analysis studies on the hybridization effect [10] and resin application. Kenaf fibre may be utilized in composite materials by providing essential solutions for new developments in materials industries.

Plastic matrices can be reinforced by fine glass fibre and processed into fibre reinforced polymers. Those matrices may be polyester, vinylester, epoxy or polypropylene. The compositional range for each element of the glass fibre determines its significant characteristics, including the presence of inorganic oxides during the manufacturing process of the glasses [11]. Different glass fibres may be distinguished by letter designation, as compliant with ASTM specifications. Glass fibre exhibits both stiffness and strength when under tension, but not under compression, because of the buckling of the typically long and narrow fibres [12]. However, this weakness can be overcome and controlled during manufacture by arranging the glass fibre layer permanently in the required direction. Glass fibre is often used in the automotive industry in the production of bumpers, crashboxes and other vehicle parts. Door, aircraft skins and other interior parts in the aerospace industry also comprise glass fibre as insulator.

In comparison with natural fibres, synthetic fibres are much more durable, stronger, easier to maintain, and washable. However, the applications of synthetic fibres, such as glass fibre, carbon fibre and Kevlar, are highly costly. Another disadvantage of synthetic fibre is that it is not a biodegradable material, and its use may cause environmental hazards and other detrimental effects [13]. For example, the burning of synthetic fibres may emit poisonous gases that pollute the air and the surroundings. The use of natural fibres as renewable materials will reduce the usage of synthetic fibres, thus preserving natural fossil resources and the environment.

The current use of composite materials in the transportation sector has resulted in significant cost reductions and improvements in material performance. This is because the composite materials used are lightweight and highly resistant to corrosion and fatigue. Their promising characteristics have attracted key players from the industries to support further research on composites. Airframes and vehicle parts made of composites can increase the operational range of the aircrafts and reduce fuel costs. The demand for lightweight and cost-effective materials, while also considering their environmental friendliness, can be met by implementing a hybrid composite material [12,14]. Today, major components in the production of boats, as well as in aerospace and automotive parts, are also made from this material [15,16]. The usage of composites has the potential for reducing the weight of the manufactured components by up to 80% compared with steel parts, and by 20% to 50% compared to aluminium components [17]. Besides this, due to concerns about materials moisture absorption and processing costs, new materials were introduced and applied in manufacturing, such as carbon fibre hybrid composites and glass fibre hybrid composites [18]. The aim of this paper is to investigate the ability of natural fibre (kenaf fibre) to partially replace the synthetic fibre (glass fibre) widely used as reinforcement in composites, by developing hybrid composites. Thus, the present work evaluates the effect of adding kenaf fibre to develop hybrid glass fibre epoxy composite laminates, while optimizing the kenaf weight ratio, on the properties of the obtained composites, including their damage characteristics.

1.1Hybrid composites

Hybrid composites generally represent the combination of three elements, and may be fibre reinforced composites, particle reinforced composites or polymer matrix composites [6,10,13]. Generally, hybrid composites comprising fibres exhibit very high strength, high stiffness, as well as high resistance to corrosion and fatigue damage, when compared to aluminium and metal alloy [19]. With the help of epoxy, rust and corrosion may be avoided or delayed since the epoxy is used for adhesion, but also for coating and protecting materials [16]. Since it allows easily fabricating materials with a smooth surface, its use can improve the aerodynamic performance of the manufactured components [19]. The application of hybrid fibre composites at an industrial level includes the manufacture a wind turbine [18]. Major components for the boat construction, aerospace and automotive industries have been also made from this material [15,16]. The replacement of synthetic fibre reinforced composites with hybrid fibre composites in Fibre-Reinforced Plastics (FRP) is becoming increasingly widespread [20]. This is due to the various advantages of these materials, such as their low density and high specific strength, as well as the renewability of natural fibres. The process of hybridization of kenaf with glass fibre as reinforcement may increase the potential of the obtained material in terms of better tensile performance, thereby allowing kenaf to compete with other conventional fibres in its class.

1.2Low velocity impact

Recently, increasing research has been conducted on low velocity impact damage in composites [21]. The impact response which can be elastic, plastic or fluid, or any combination of these, presents the damage to the composites [14]. Studies on impact dynamics focus on fracture and fragmentation. The most critical damage caused by impact consists in holes and cracks, which may reduce the material's strength [22]. The residual tensile strength also decreases substantially because of fibre cracks in the impact contact zone [14]. Another consequence of impact damage is delamination, a microscopic effect that is difficult to detect by visual inspection, but it decreases the residual compressive strength [5]. For testing purposes, impact velocities can be classified into four types: drop testing and pendulum testing for low velocity impact, intermediate velocity impact, ballistic testing for high velocity impact, and hyper velocity impact. Low velocity impact occurs at velocities below 10m/s and is likely to cause some dents and visible damage on the surface due to matrix cracking and fibre breaking, as well as delamination of the material [14]. Intermediate impact occurs in a velocity range from 10m/s to 50m/s. High velocity impact may range between 50m/s and 1000m/s, while hyper velocity impact has a range of 2000m/s to 5000m/s [22]. Delamination reduces the strength of composite materials [5,12,23]. Delamination only occurs at interfaces between layers that have different fibre orientations [5,23].

The principle of conservation of energy states that the energy absorbed by the sample will be equivalent to the energy that is produced to create the damage. Low velocity impacts rely on the principle of drop weight (Fig. 1) inspired by the first law of vibrations; the static mode [24]. The incident energy that is absorbed in composites can create some large fracture areas. As a result, both the strength and stiffness of the composite are reduced. The contact force duration is longer than the time required for the impact wave to reach its limit and return back to its original position. Maximum deflection occurs when the force curve returns to zero [25]. This phenomenon can also be described as an energy balance model, where the total energy of the system is conserved, while the higher vibration modes, friction, and other losses of energy are neglected [26]. Differences in low velocity impact response and impact damage are closely related to the difference in the dimensions of the sample and its method of production [27].

Fig. 1.

The principle of drop weight for low velocity impact testing.

(0.15MB).
1.3Compression after impact

Components subjected to the low velocity impact test may experience internal delamination, which is hazardous because the damage to the impact area is invisible [5,28]. Therefore, the components that have been damaged by low velocity impact and non-destructive testing will undergo another test named compression after impact (CAI) [29]. This is done to avoid undetectable damage by visual inspections, which will cause reduction in strength, and to assess the effect of the reinforcement by evaluating the performance of the composites [30]. NASA and Boeing (Fig. 2) use a similar approach in testing, where the sample is clamped both at top and bottom, and the specimen's thickness exceeds 3mm [24,31,32].

Fig. 2.

The schematic diagram of compression after impact testing [24,31,32].

(0.09MB).

In this study, the impact properties of kenaf/glass hybrid composites will be evaluated using both non-destructive and destructive methods, which are dye-penetrant and compression after impact respectively. These evaluation can describe the damages occurred on the impacted samples in terms of damage area and residual strength, which the residual strength was less reported previously.

2Experimental2.1Materials

Woven glass fibre E600 and woven kenaf fibres were utilized for this research. Type E-glass fibre with a mass of 600g/mm2 is very thin, and its hardness is considered to be low, while woven kenaf fibre has a thickness of 2±0.2mm and density of±1.2g/cm3. Both fibres were obtained from Aerospace Material Laboratory, Faculty of Engineering (UPM, Selangor, Malaysia). The resin used was epoxy, typed EpoxAmite 100 together with 102 Medium Hardener (Smooth-on, supplied by My East, Selangor, Malaysia). The actual composition of the epoxy resin used was Diglycidyl Ether of Bisphenol A (DGEBA), while the chemical name of the hardener used is polypropytriamine.

2.2Fabrication of composites

The resin and hardener were prepared at a 2:1 ratio for slow curing. The advantage of thermosetting is that the process temperature for the resin is lower, hence it requires a longer processing time, resulting in slow curing. This is why thermoset resin is widely used in hand lay-up processes. The rationale for the ratio stated is that slow curing should only be used at temperatures up to 80°C for 2h, unless an extremely long curing time is needed. The mixture of epoxies to the weight of the panel using a 35:65 percentage ratio was stirred constantly until the solution became non-viscous. The tensile strength testing was previously conducted to determine the stress–strain behaviour of the glass fibre and kenaf fibre hybrid composites. This stage provided clear information on which formulations presented the best performance in terms of tensile strength, based on ASTM D 3039/D3039M-17 [33,34]. The results were compared to identify the effect of weight percentage on the mechanical properties of the glass fibre and kenaf fibre hybrid composites, as illustrated in Fig. 3 and Table 1.

Fig. 3.

Force-displacement curves from the tensile testing of composites with varying glass to kenaf ratio.

(0.18MB).
Table 1.

Force and displacement values from the tensile testing.

Sample  Weight percentage ratio  Force (kN)  Displacement (mm) 
GS  100% glass fibre  48.30  5.53 
KS  100% kenaf fibre  11.50  3.40 
HS1  25% glass fibre+75% kenaf fibre  14.99  3.45 
HS2  30% glass fibre+70% kenaf fibre  20.71  3.63 
HS3  50% glass fibre+50% kenaf fibre  28.79  4.50 
HS4  70% glass fibre+30% kenaf fibre  36.23  4.89 
HS5  75% glass fibre+25% kenaf fibre  39.08  5.36 

With reference to the results of the tensile testing, sample GS shows the highest force and displacement as opposed to sample KS, which shows the lowest force and displacement before failure occurs. The tensile strength for all the types of samples, presented as a force versus displacement graph (Fig. 3), shows similar trends, rising up to a certain point before failure. In addition, the performance of sample HS5, which consists of hybridized 75% glass fibre and 25% kenaf fibre, compared to other weight ratios of hybrid composites, is almost similar to the performance of the 100% glass fibre material during tensile testing. Therefore, sample HS5 was selected to be submitted to low velocity impact tests.

The experimental procedure began with the fabrication of the samples, involving the use of 10 layers of fibre in each sample (Fig. 4). The specimen fabrication was conducted using the hand lay-up method. Each sample had a different weight composition of fibre (Table 2). The panel size was dictated by that of the plate, which had dimensions of 300mm×300mm. The first layer of glass fibre was then placed on the glass panel. All the samples had a glass fibre layer as the first layer and the last layer of the sample (Fig. 5). The arrangements varied due to the weight percentage of fibres in order to keep the outermost layer constant.

Fig. 4.

Layers of fibreglass and kenaf fibre in square panel shape.

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Table 2.

Hybrid composites fabricated with different ratio.

Sample  Weight percentage ratio 
GS  100% glass fibre 
KS  100% kenaf fibre 
HS1  25% glass fibre+75% kenaf fibre 
HS2  30% glass fibre+70% kenaf fibre 
HS3  50% glass fibre+50% kenaf fibre 
HS4  70% glass fibre+30% kenaf fibre 
HS5  75% glass fibre+25% kenaf fibre 
Fig. 5.

Stacking sequence of the hybrid composites with glass fibre at the outermost layer.

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Low velocity impact testing using a drop weight was carried out with different impact energies. A sample with the formulation that performed best in the tensile testing was carefully cut using a CNC machine (AC MECA, Johor, Malaysia) and grinded to obtain accurate dimensions and avoid cracking. The cured samples from the panel were cut to 100mm×150mm according to the requirements of the test. The size of the test specimens was determined by Boeing BSS 7260 [35,36] impact testing specifications, i.e., 101.6mm×152.4mm (±1mm to 3mm offset). Testing was conducted by varying the incident impact energy from 10J to 40J, as this is the force range for low velocity. The low velocity impact test was conducted using a drop test machine (Imatek Ltd, type 8000D, model D5000, Knebworth, UK). Contact force is the response force of the samples against the impactor from the impact energy (Eq. (1)). Therefore, the height of the striker from the sample depended on the value of the impact force (Table 3).

Table 3.

Height of the striker for low velocity impact testing.

Sample impact energy (J)  Height of striker (m) 
10  0.2 
20  0.4 
30  0.6 
40  0.8 

All the samples underwent testing with the four edges clamped to avoid any misalignment of the target area. The samples were observed to find out whether the striker penetrated the sample, or if it withstood the impact energy. The conservation of energy is related to the free downfall phenomenon. There are three scenarios according to which the free fall takes place: the first – free fall, stop and rebound, the second – free fall and stop, and the third one – free fall and perforation [6,37]. Then, each sample was analyzed via dye penetrant inspection analysis. Post-impact testing was performed to detect any damage that occurred as a consequence of the load testing conducted [38]. The post-impact damage can be analyzed using several non-destructive techniques, such as visual inspection, visible dye penetrant inspection, eddy current, radiography and C-Scan [29,39,40]. Non-visible impact damage can be analyzed using optical microscopy [41]. Dye penetrant inspection is classified as a non-destructive technique to detect the location of impact damage and determine the size and shape of delamination and matrix cracks. It is also known as liquid penetrant inspection and is considered to be a cost-effective method. The area of the damage will be bigger when the impact energy is higher, and any internal damage may not be visible to the naked eye. The visible colour contrast in the inspected materials may distinguish those damaged parts (Fig. 6). Dye penetrant inspection may be applied to both ferrous and non-ferrous materials, as well as to non-porous materials. This technique is widely used in the oil and gas, petrochemical, aerospace and automotive industries.

Fig. 6.

Basic principle in dye penetrant inspection (ECE Global).

(0.18MB).

The impact damage location was easily detected and the impact damage area was clearly drawn and measured. The dye penetrant used was Spotcheck SKL-SP2. It is a solvent removable (or post-emulsifiable), red-coloured contrast penetrant with profound penetrating characteristics that comply with the ASME B & PV Code Sec V and ASTM E1417 (2016) standards [42]. The dye penetrant managed to extend into the under layers of the damage region. The sample was first cleaned using thinner to wipe out all the dirt and marks. Since the sample's surface colour is brighter, developer spray was not used in the process. All the samples were sprayed in the impact area using the penetrant and left for 20min at room temperature. Finally, the excess red dye penetrant on the surface was cleaned away by using the thinner. The red parts that remained visible on the sample's surface showed the damaged area affected by the impact test.

The compression after impact test was carried out on the same samples using a servo-hydraulic machine (Shimadzu AGX, 300kN), according to ASTM D7137 (2017) [43]. The presence of bending during the compression test was determined by utilizing the data recorded by the test machine. The elastic modulus was calculated from the linear region. All the results are discussed in the next section.

3Result and discussion3.1Low velocity impact test

The physical impact of the test is clearly illustrated in Fig. 7. The raw data for this testing were obtained directly from the drop test data acquisition system (Table 4). Those data were then analyzed with regard to the impact and absorbed energy, peak force, peak deformation energy and elongation. Each of the following graphical representations is obtained from various impact test runs (Fig. 8). In this investigation, the relationship between the force and the displacement is evaluated.

Fig. 7.

Impacted specimen at different impact energy level (a) 10J, (b) 20J, (c) 30J and (d) 40J.

(0.41MB).
Table 4.

Data tabulation for low velocity impact testing.

Impact energy (J)  Peak force (kN)  Peak deformation (mm)  Distance specimen to striker (mm)  Peak deformation energy (J)  Absorbed energy (J) 
10  4.54  3.59  200  9.72  5.44 
20  6.21  5.06  400  19.25  11.34 
30  8.24  5.80  600  28.80  17.63 
40  9.31  7.38  800  38.71  23.23 
Fig. 8.

Force-displacement curves for HS5 sample.

(0.24MB).

By referring to the data from Table 4 and the graph in Fig. 8, it may be remarked that for the hybrid sample with the weight percentage of 75% glass fibre and 25% kenaf fibre, at an impact energy of 10J, the absorbed energy – reached 5.44J, the peak force – 4.54kN and the peak deformation – 3.59mm. Since the height of the striker relative to the specimen was 200mm, the results for the peak deformation energy had reached up to 9.72J. In addition, at the impact energy of 20J, the absorbed energy was 11.34J, the peak force was 6.21kN with a peak deformation of 5.06mm. When the height of the striker relative to the specimen was increased to 400mm, the results for the peak deformation energy ascended up to 19.25J. The impact energy of 30J caused an absorbed energy of 17.63J, a peak force of 8.24kN, and a peak deformation of 5.80mm. When the height of the striker relative to the specimen was again enhanced to 600mm, the resulting peak deformation energy was up to 28.80J. On the other hand, the impact energy of 40J caused an absorbed energy of 23.23J. The peak force was 9.31kN with the peak deformation of 7.38mm. If the height of the striker relative to the specimen was 800mm, the result of the peak deformation energy was up to 23.23J.

The closed curve on the graph of force against displacement indicates that during testing the striker did not penetrate the specimen and rebounded after hitting. In theory, an open curve suggests that the striker has penetrated the specimen. Therefore, it can be concluded that none of the samples experienced penetration, as the incident energy was fully transferred back to the specimens at the point where maximum displacement occurred. Hence, the first scenario of the free fall was realized. When the maximum displacement is achieved, the sample transfers elastically the excess impact energy back to the striker, where the bouncing phenomenon between the striker and the sample occurs. The rebounding of the striker when in contact with the sample is informative of the differences between the maximum impact energy and the absorbed energy. Small matrix fracture may occur in the samples, which may be visible [22]. The same trends happened for all impact energy values, ranging for 10J up to 40J. Thus, it can be assumed that sample HS5 with the weight percentage of 75% glass fibre and 25% kenaf fibre can withstand an impact energy up to 40J.

3.2Dye penetrant test

The samples damaged as a result of the low velocity impact testing were then analyzed by a non-destructive technique, namely dye penetrant inspection. A damage area progression curve was then plotted and the surface area subjected to damage was calculated (Table 5). Low velocity impact tests result in visible marks on the impacted surface of the sample [23,44]. Low velocity impacts from 10J to 40J, with increments of 10J, led to increments of the damage area. The damage from low velocity impacts starts with a matrix crack at the surface of the specimen, displaying some changes of colour, which reveal the location of the delamination. Using dye penetrant spray has clearly shown the damaged area and the size of the effects due to impact energy. As the incident impact energy was increased, the delaminated area also increased [40], as demonstrated by the fact that the impact energy of 40J caused a damage area of approximately 2885.94mm2, while an impact of 10J resulted in a damage area of 235.03mm2.

Table 5.

Damage area of impacted specimens with respect to different impact energy levels.

10
235.03MM2 
1447.38MM2 
20
30
2572.63MM2 
2885.94MM2 
40
3.3Compression after impact (CAI) test

In order to assess any reduction in strength due to possible damage that remained undetectable upon visual inspection of the samples subjected to low velocity impact tests, the samples further underwent compression after impact (CAI) tests. The effect of CAI can be clearly seen on the samples that undergo compression (Fig. 9). The uniform thickness of the samples of almost 65mm prevented the usage of anti-buckling plates to ensure pure compressive loading and stability of the displacement prior to failure. It may be remarked the absence of local crushing damage in the loading zone, which indicates that uniform residual strength values can be obtained [29]. Moreover, additional usage of tabs for larger thickness of samples will also assist in recording consistent data.

Fig. 9.

HS5 samples after compression after impact testing.

(0.1MB).

Table 6 tabulates the results of the CAI tests, while Fig. 10 shows the graphical representation of the CAI compressive load force and displacement for each impact energy level. At an impact energy level of 10J, the maximum compressive strength was recorded at 35.12kN prior to failure, with a displacement of 3.3mm. The sample subjected to an impact energy level of 20J recorded a compressive strength of 33.75kN with a displacement of 3.1mm, while the sample impacted at 30J showed a compressive strength of 29.78kN and a displacement of 3.5mm. The lowest compressive strength recorded was of 26.97kN corresponding to a displacement of 4.5mm. Comparing the samples, it can be concluded that a larger damage area contributes to a lower compressive force. Visible damage could be observed upon inspection with the naked eye, including cracks. It is well known that the samples had previously undergone damage caused by the low velocity impact, involving drop testing. Therefore, the compression after impact test increased the damage area and the cracks that had been initiated in the inner layers of the material, transferring to the front layers due to significant delamination. Hence, the reinforcement that increased the toughness and impact resistance of the composite matrix needs to be further investigated [25,29].

Table 6.

Force and displacement values from the compression after impact testing.

Sample HS5  Force (kN)  Displacement (mm) 
1035.12  3.30 
2033.75  3.10 
3029.78  3.50 
4026.97  4.50 
Fig. 10.

Compressive force-displacement curves from the compression after impact testing for HS5 samples.

(0.17MB).
4Conclusion

In previous studies, hybrid composites made up of 75% glass fibre and 25% kenaf fibre displayed best tensile properties, thus the combination was selected to be evaluated for its low velocity impact properties in the current study. It was shown that the hybrid composites can withstand the impact energy up to 40J with the peak impact load and absorbed energy increased with the increase in incident impact energy. The damage area evaluated from the dye penetrant testing proportionally increased as the energy absorbed increased. Less damage samples results in higher residual compressive strength evaluated from the compression after impact testing.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work is supported by UPM under GPB 9668200. The authors would like to express their gratitude and sincere appreciation to the Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia and Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia (HiCOE) for the close collaboration in this work.

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Copyright © 2019. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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