STUDYING THE EFFECT OF CHEMICAL TREATMENT AND ORIENTATION ON FRACTURE TOUGHNESS OF KENAF FIBRE-EPOXY COMPOSITES

: In this paper, the effect of surface modification and fibre arrangements of kenaf fibers on fracture toughness of epoxy composite was investigated. The chemical treatment of kenaf fibers (KFs) with 6 % NaOH was achieved, and composites with two different fibre arrangements (X and Y) directions were fabricated. Values of fracture toughness (KIc) measured of the compact tension (CT) specimens for both untreated kenaf fibre-reinforced epoxy (ut-KFRE) composites and treated kenaf fibre-reinforced epoxy (treated-KFRE) were much better than the neat epoxy. The K Ic value of the treated-KFRE composite in Y-fibre direction was the highest of 2.74 MPa.m1/2 while it was 1.45 MPa.m1/2 for the neat epoxy. Different toughening mechanisms were noticed in the fracture surfaces of the composites in relation to the fibre reinforcement planar, they are shear yielding and fibre splaying with the X-direction and broken fibers, fibre pullout and fibre delamination with the Y-fibre orientation. of on the of toughening and toughness of glass fibre-reinforced composites and that the and with ply up and it there study the alkaline treatment of kenaf and its arrangement (orientation) on crack path and fracture characteristics of composites. this study aims to comprehensively understand the role of treated fibres with NaOH in improving fracture toughness of And understanding the effect of fibre (arrangements) on the fracture


INTRODUCTION
In recent years, the development of sustainable or biodegradable materials representing at using natural fibre reinforced polymer composites in commercial and medical applications has attracted a remarkable attention from both academic perspective and industrial viewpoint. This attention has emerged since these materials can be useable instead of non-renewable petroleum-based materials, particularly petroleum-based plastic due to their lighter weight, cheap and provide better stiffness to weight than glass.
Additionally, by using natural fibres can preserve consumable resources such as petroleum, reducing landfill capacities, and alleviating a global warming impact due to diminishing carbon footprints that are generated by consumption of petroleum [1].
In order to improve the bonded interface between hydroxyl groups in natural fibres with matrix resin, thus different chemical, physical and mechanical modifications or treatments of the natural fibers have been implemented. The chemical treatments (e.g., alkaline treatment, silane treatment, isocyanate treatment, and acetylation) consider the most viable treatments, in which, reagents solution are normally used which contain functional groups that are capable of bonding with the hydroxyl group from the natural fibres. Amongst them, the alkaline treatment based on sodium hydroxide (NaOH) (or mercerization) is the most widely used for natural fibers, especially for kenaf fibre when reinforcing thermoplastics and/or thermosets. Via applying the alkaline chemical treatments ensures removal of lignin, hemicelluloses, pectin, wax and oils, etc. covering the external surface of the natural fibers, breaks and disperses hydrogen bonds in the network structure and increases the number of free hydroxyl groups on the surface of the fibres and therefore fibre reactivity [4,5].
Additionally, the chemical treatment of the natural fibres by alkali enhances the fibre's surface roughness, which in turn causes surface fibrillization and drastically improves mechanical interlocking and fibre-matrix adhesion.
Due to the growing demand of using natural fibres-polymer composites in structural applications, there is a need to thoroughly understand the fracture toughness and behaviour of these composites especially determining maximum loads applied to the component that can be carried before creating defects, initiating microcracking and causing local stress concentrations that result in catastrophic failure. Thus, numerous studies have been identified the fracture behaviour for different natural fibres-reinforced polymer composites, and others reported the importance of chemical treatments especially alkaline treatment with sodium hydroxide (NaOH) on mechanical and thermal properties of natural fibres/polymer composites [3,[6][7][8].
However, few works in the literature have conducted the influence of chemically treated natural fibres on the fracture behaviour and toughness of composites [7,[9][10][11][12].
Nosbi et al. [13] discussed the degradation of compressive properties of pultruded kenaf fibre reinforced unsaturated polyester composites. It was found that the strength and modulus of the composite are reduced and the compressive strain at failure recorded an increase due to the formation of hydrogen bonding between polymer molecules and cellulose fibers. Copyright  2017 Al-Qadisiyah Journal For Engineering Sciences. All rights reserved.
The effect of reinforced fibre arrangements on the mechanism of toughening and fracture toughness of glass fibre-reinforced composites and natural hemp fibre-reinforced PLA was presented by [14,15], respectively. They found that the fracture energy and toughness increases with increasing ply angle up to 40° and then it decreases. Nevertheless, there is no particular study has investigated the alkaline treatment of kenaf and its arrangement (orientation) on crack path and fracture characteristics of composites. Hence, this study aims to comprehensively understand the role of treated kenaf fibres with NaOH in improving fracture toughness of the composites. And understanding the effect of fibre orientations (arrangements) on the fracture resistance of the composites since the properties of the fibrous composite materials are strongly affected by the fibre properties and its microstructural parameters such as fibre length and diameter, fibre distribution, fibre orientation, volume fraction of the fibers and packing arrangement of the fibers.

MATERIAL AND ALKALI TREATMENT
Epoxy (DER331) and hardener (JOINTMINE 905-3S) with ratio of 2:1 were used as a polymer resin for kenaf fibre composites. After cleaning and extracting the kenaf fibres, portion of the prepared fibres were treated with 6 % sodium hydroxide (NaOH) solution for 24 h at room temperature, then washed by demineralised water and dried under sun light. The SEM micrographs for both untreated fibres and treated with NaOH are shown in Figure 1 a & b. Some specifications of the neat epoxy (NE) and untreated kenaf fibre reinforced epoxy (ut-KFRE) are listed in Table 1.

FRACTURE TOUGHNESS MEASUREMENT
The neat epoxy (NE), untreated and treated kenaf fibre reinforced epoxy (ut-KFRE) and (t-KFRE) with two different orientations were cut by a diamond saw from the cured composites to dimensions of compact tension (CT) geometry according to ASTM 5045 standard [16]. The holes were opened using a steel drill, the notch and pre-crack were cut precisely with a band saw and inserting a razor blade, respectively. Fracture toughness testing was carried out using a universal tensile machine model LR 50 K with a crosshead speed of  In order to satisfy the plain-strain conditions, the following size criteria are checked: where PQ is the peak load, B the specimen thickness (16 mm), the crack length (10 mm), W the specimen width (42 mm), and the yield stress of the material.

THE FRACTURED SURFACE STUDY
In order to understand the mechanism of fracture and deformation behaviour, the fractured surface of the composite from Mode I fracture test was examined by using Scanning Electron Microscopy (SEM) model Quanta 400 F. The samples were firstly coated with a thin layer of gold to improve the conductivity of the material before doing the test.

CHEMICAL TREATMENT
The SEM images for both untreated kenaf fibres (ut-KFs) and treated kenaf fibres (t-KFs) with 6 % alkali NaOH are shown in Figure 1 a & b. There is a noticeable difference between the surface morphologies for the ut-KFs and t-KFs in terms of smoothness and roughness. The characteristics of t-KFs surface considerably improved and became cleaner and smoother after the NaOH treatment compared to the ut-KFs. In addition, the fine fibres in the bundle are exposed after washing the outer layer of the fibres with NaOH while the impurities are clearly noticed in the surface morphology of ut-KFs. Hence, the 6 % NaOH treatment is adequate to eliminate the impurities from the natural kenaf surface determined in this study. The improved surface morphology would improve the level of interfacial adhesion between the fibre and the matrix under the applied load on the fabricated fiber-reinforced polymer composites.

FRACTURE TOUGHNESS
The load-displacement curves for the compact tension (CT) specimens made from neat epoxy, untreated kenaf fibre reinforced epoxy (ut-KFRE) and treated kenaf fibre reinforced epoxy (treated-KFRE) composites with two different X and Y fibre directions are presented in Figure 4. All CT fracture composite specimens performed better and carried higher tension loads before fracturing than neat epoxy. The composites reinforced by kenaf fibres in Y-direction conducted higher loads than these for the reinforced composites by X-direction regardless the considerations of alkali treatment method. This is an obvious indication since the neat epoxy (thermoset resin) is brittle in nature and the reinforced fibres contributed in absorb more energy and carry further load before fracturing. , which is almost double its value of 1.45 MPa.m 1/2 for epoxy as shown in Figure 5.

EFFECT OF FIBRE ORIENTATIONS
The K Ic values for the treated-KFRE (Y-direction) composites are superior compared with the neat epoxy and epoxy reinforced by X-fibre direction. This is due to the improved interfacial adhesion between the treated-KFs and epoxy which enhanced the stress-transfer from the resinous region to the fibers and consequently enhanced the K Ic of the treated-KFRE composites [17]. The improved treated-KFs and epoxy adhesion may also contribute to detain the growth of the delamination, which potentially occurs under the tensile loading.
The fracture surface morphology of the neat epoxy is shown in Figure 6. It is brittle fracture showing crazing region. However, with the improved interfacial bonding between the treated-KFs and epoxy, more energy is consumed by fibre before pull-out or broken (fracturing). Using the Y-fibre to reinforce the composites in this study is further improving its toughness where the applied stress is discreet by continuous treated-KFs that can subsequently affect the toughening mechanism of the treated-KFRE composites. A similar conclusion was revealed by Fiore et al. [18], it was revealed that the use of unidirectional (UD) kenaf fibres or randomly oriented treated-KFs increased the tensile strength and tensile modulus due to the improved compatibility between fibre and epoxy. The fibre arrangement in anisotropic materials such as polymer-based fibre composites is playing a notable role in changing their toughening mechanisms and mechanical properties since their properties are, in general, varying through each axis and depending on the fibre direction in which they are aligned. For instance, in term of un-treated fibres aligned in X-fibre direction, it was observed that the dominant toughening method is shear yielding of the resinous region and the progressive fibre splaying (see Figure 7).
The role of chemical modification on the fibre surface aligned in the same fibre direction emerged a little amount of treated KFs pull-out along with broken fibre due to the good interfaces between the fibre and matrix, as shown in Figure 8.
On the other hand, the KFRE composites reinforced by un-treated KFs oriented in 90° (Y-direction) exhibited besides the fibre pull-out toughening mechanism other two mechanisms are fibre splitting and fibre delamination as shown in Fig. 9. The reason of the appearance of such mechanisms and thereof the high fracture toughness of the composites reinforced in Y-fibre direction could be attributed to the large crackbridging zones formed by intact kenaf fibres in the crack wake. The fracture surface of the treated-KFRE composites reinforced in Y-direction exhibited a broken fibre as shown in Figure 9 and