Effect of hybridization on the mode II fracture toughness properties of flax/vinyl ester composites
1F. A. Almansour* a, 2H. N. Dhakala, 3Z. Y. Zhanga and4H. Ghasemnejadb
,,
aAdvanced Polymer and Composites (APC) Research Group,
School of Engineering,
University of Portsmouth, Anglesea Road, Anglesea Building, Portsmouth, Hampshire, PO1 3DJ, U.K.
bSchool of Aerospace and Aircraft Engineering, Kingston University, London, SW15 3DW, U.K.
Abstract
In this study, flax fiberreinforced and flax/basalt hybridized vinyl ester composites were producedand their interlaminar fracture toughness (mode II) behavior was investigated using the three-point bend end-notched flexural (3ENF) testing. From the results, the average of the maximum values for each group of specimen obtained for critical strain energy release rate GIICand stress intensity factor KIIfor flax/vinyl ester specimens were 1940 J/m2 and 134 kPam0.5. Similarly, GIIC and KII values recorded for hybridizedspecimens were 2173 J/m2 and 178 kPam0.5 respectively.The results for the flax/basalt hybridized composites exhibited an improved fracture toughness behavior compared to flax/vinyl ester composites without hybridization. The cohesive zonemodelling (CZM) was also used to predict the delamination crack propagation in mode-II in laminated composite structures. After the experimental study, the 3ENF specimens were modelled and simulated using ANSYS. The CZM/FEA results were in reasonable agreement with the experimental results.
Keywords:
Flax fibers, Hybrid composites, Mode II,Interlaminar fracture toughness.
1* Corresponding author Tel: + 44 (0) 23 9284 2586; fax: + 44 (0) 23 9284 2351.
E-mail:(F.A.Almansour)
Introduction
Over the past few decades, many research studies conducted in the field of polymer composites have focused on developing sustainable composites by replacing many of the conventional composite materials in various applicationsdue to ecological issues related to climate change, the greenhouse effect and CO2 emissions. Therefore, the development and use of sustainable polymer composites not only provide environmentally-friendly sustainable materials but also provide composite materials with high-strength-to weight ratio, biodegradability, lower energy requirements for processing and lower cost, compared to existing synthetic reinforcements such as glass and carbon fibers[1–4].
In the last few years, there has been a substantial increase in the use of natural fibers as reinforcement in composites, particularly in the automotive, construction and packaging industries. Some of the common natural bastfibers, which have been used in both thermoset and thermoplastic composites, are flax, jute, hemp, and kenaf. This is due to acceptable properties that include low density, low cost and renewability[5, 6].
Despite these advantages, applications of natural fiber composites have been restricted to non-structural applications, mainly in automotive interior components. There are many reported works on natural fiber composites as far as their mechanical (tensile, flexural and impact) , thermal and environmental behaviors are concerned [7–12]. However, there are limited reported works relating to the investigation of fracture toughness behavior of natural fiber compositematerials[13–16]. For these composites to be used in structural components, an understanding of fracture toughness is extremely important.
Interlaminar crack growth in composite laminates can be investigated by using Linear Elastic Fracture Mechanics (LEFM), which is through the stress intensity analysis, providedspecific conditions are met. These conditions depend on the attendance of all basic ideal conditions analyzed in LEFM in which every material is elastic except at a point near the crack tips [17, 18].
The stress intensity methods are characterized by the strain release rate, G, which is dependent on the geometry of the crack body and the loadings and constraints that the material has been subjected to. In addition, from a Fracture Mechanics point of view, crack propagation takes place when the energy release rate prevails upon the fracture toughness of the material[19].
In the study of the fracture toughness, one has to consider two important factors: the critical stress intensity factor (Kc) and the critical strain energy release rate (Gc). The former is a local parameter or a function of stress, strain and displacement near the crack tip and the latter describes a measure of energy necessary for crack initiation [20]. The KIc which characterizes at a critical stress state can be calculated as it depends on the crack length [Eq. 1.1]. Therefore, it is important to determine the fracture toughness of materials as cracks grow under stresses at the crack tip with catastrophic consequences.
[Eq. 1.1]
Where α is crack length at the outset and σf is an applied stress.
There are several tests that have been carried out in order to measure the interlaminar strain energy release rate of fiber reinforced polymer-matrix (FRP) composite materials for different modes: mode I (tensile/opening), mode II (shear), mode III (tearing shear), and mixed Mode I/II [21, 22]. Mode I has been intensively studied using the Double Cantilever Beam Test (DCB) and is universally accepted, but there are fewer tests that have been done or proposed to validate results in mode II. However, there are three fundamental tests used in mode II to measure the strain energy release rate, GIIc. The most common test is the End Notched Flexure (ENF), but it is not yet approved by American Society for Testing and Materials (ASTM). It was developed for wood fracture characterization[23, 24]. For plain strain fracture toughness in mode I/II, the strain energy can be related to the stress intensity factor as shown by [Eq. 1.2]:
[Eq. 1.2]
Different approaches have been attempted to improve fracture toughness and delamination resistance of composite structures: this demands an understanding of composite failure mechanisms [15, 25–29]. Zhang et al. found that interlaminar fracture toughness and interlaminar shear strength of flax/glass fiber reinforced hybrid polymer composites (HFRP) were higher than glass fiber reinforced polymer composites (GFRP):this led to improvements in the mechanical properties due to the excellent hybrid performance [30]. Earlier works done on fracture energy for both mode I and mode II focused on interleaving under impact loading of composites [31]. Hamer et al. [32] showed that nanofibrilmat interleaved carbon / epoxy laminates were responsible for improvements in interlaminar fracture toughness (IFT) of 255-322% compared to noninterleaved. Further work done by [14] reported that the “toughness is dominated by the fiber volume fraction, rather than the reinforcement architecture”, and also the addition of woven fabric results in all cases increasing the fracture toughness of composites. Speranzini and Agnetti[33] concluded that solid wood reinforced FRP in natural fibers of basalt, flax and hemp exhibited higher strength and stiffness than those without hybridization.
Hybrid composites are made from a combination of more than one type of fiber reinforced in the same matrix. A present, there is significant interest in enhancing the mechanical properties of natural fiber reinforced composites with the use of hybrid materials.Limited published works exist on the hybridization of glass, carbon and basalt fibers into natural fiber composites[34–36]. Compared to synthetic fibers such as glass, basalt fibers possess lower global warming potential compared as these fibers are produced from commonly occurring rock that comes from the nature and their specific properties are superior to that of glass fibers.
In recent years, the emergence of composite materials made from glass and carbon fibers have extended the study of fracture toughness in order to understand the behavior of thesematerials. Despite numerous works relating to interlaminar fracture toughness on glass and carbon composites, very few researchers have studied the fracture toughness of natural fiber composites using both woven and non-woven natural fiber reinforced composites and biocomposites.
In addition, despite the benefits that hybrid materials offer, there are hardly any reported works involving the improvement of fracture toughness behavior of natural fiber composites using basalt fiberhybridization.In this study, needle punched non-woven flax fibers and basalt fibers were used as reinforcements to fabricate the hybrid composite laminates in order to investigate the influence of flax fiber reinforcement and basalt fiberhybridization on the mode II fracture toughness these composites. The fracture surface of flax/basalt hybrid composites was studied by scanning electronic microscopy (SEM). The mechanical propertiesof flax fiber reinforced vinyl ester composites and flax/basalt hybrid composites were investigated: these include interlaminar fracture toughness and interlaminar shear strength.
2. Materials
2.1 Materials
Vinyl ester resin and a curing catalyst, methyl ethyl ketone peroxide (MEKP), was used for flax composite laminate fabrication, which were supplied by GRP Ltd, UK. The reinforcement used was needle punched non-woven flax fibers; aerial weight of 330g/m2 supplied by Ecotecnilin Limited. The basalt fiberin the form of woven fabric (BAS 220.1270.P) was obtained fromBasaltex-flocart NV (Belgium); aerial weight of 220g/m2. The comparison of physical and mechanical properties of different natural fibers, glass fibers and basalt fibers are presented in Table 1.
2.2 Composite fabrication
The composite laminates were prepared by the combination of hand lay-up and the compression moldingtechnique.The curing catalyst, methyl ethyl ketone peroxide (MEKP),was mixed with the vinyl ester matrix at a concentration of 1.5 wt.% followed by 3 minutes of manually stirring for uniform mixing. A steel mold with required dimensions was prepared and coated with mold release agent for easy removal of the samples. The flax fiberlayerswere placed on the mold and matrix catalyst mixturewas poured on the fibers. A small hand roller was used to aligned the fibers and impregnate the resin into the fibers and then compacted using compression molding (Bipel JRD A10391) at a hydraulic pressure of 1 MPaat a temperature of 50 °C for 90 minutes followed by a post curing at 85 °C for 180 min in a fan assisted oven. Flax fiber mats were dried at in an oven at 100 °C for 60 min to remove the storage moisture content prior to molding. Two types of laminates (flax/vinyl ester and basalt/flax/vinyl ester) were prepared to investigate the fracture toughness behavior. The flax/vinyl ester laminates were made with 4 layers of non-woven mat. Whereas the hybrid basalt/flax/vinyl ester laminates was fabricated using similar techniqueby adding2 layers of basalt fabric on top and bottom of 3 layers of flax core. The fiber volume fraction was approximately 23% and 25% for flax/vinyl ester and basalt/flax/vinyl ester hybrid composites, respectively in a mold plate of 4mm thickness. The void content was approximately 5% and was calculated according to ASTM D2734-94. In order to initiate delamination, a Teflon release film was placed at laminate mid-thickness.
2.3 Specimen preparation
The experimental work performed included the end-notched flexure (ENF) test. All the specimens for testing were cut from the composite laminates using a diamond saw; the geometry and dimensions of the specimens are shown in Fig.1.
The nominal specimen dimensions were width (w) 20mm, thickness (b) 4mm, initial crack length (ao) 50mm and the total specimen length 130mm. Four specimens of each type were tested and average was taken.
2.4. Mode II interlaminar fracture (IFT) testing
The end-notched flexure (ENF) specimen appears to be the most frequently used test method to measure mode II critical strain energy release rate (GIIc). For ENF testing, the specimen was placed in a three-point bending fixture with a span length (L) of 80mm. In this test, the load was introduced by flexural forces to produce a crack from the insert. The crack then extended as a result of shear forces at the crack tip. The geometry of the three-point end-notched flexure (3ENF) specimen and loading are shown in Fig. 2. The3ENF test set up details in Zwick-Roell machine is shown in Fig. 3.
2.5 Mode II Data reduction method
The three-point-end-notched flexure (3ENF) test is used to measure the interlaminar fracture toughness under in-plane shear deformation. In order to evaluate the load-point compliance, the equation of the critical energy release rate can be found by classical simple beam theory (SBT) [37].
[Eq.2.1]
[Eq.2.2]
The compliance equation is given by;
[Eq.2.3]
The interlaminar fracture toughness can be obtained from [Eqs.2.1- 2.3]:
[Eq. 2.4]
Where:
P: maximum load for unstable crack propagation
: loading point displacement
a: crack length measured from the outer pin
L: half span of 3ENF specimen
b: beam width
An indirect approach has been used to calculate the stress intensity factor KIC since it does not depend on whichever mode is being use
[Eq.2.5]
2.6 Scanning electron microscopy (SEM)
The fractured surfaces of the composite specimens were studied using SEM JSM 6100 model at room temperature. After adhering to SEM stubs, a thin layer of gold/palladium was applied to the specimens before to SEM study. The SEM micrographs of the fractured surfaces provide important information on how the specimens fail in relation to fiber-matrix adhesion and resulting failure mechanisms.
3. Results and discussion
3.1 Evaluation of the load deflection curves
Typical load vs. displacement curves for the flax/VE and flax/basalt/VE 3ENF specimens are shown in Fig. 4. The various test results on these two types of specimens are also presented in Table 2. It can be seen from the figure that the graph for flax/VE shows a linear zone between 0 and 125 N, without any evidence of crack growth or damage within this range. It can also be observed that nonlinear behavior became prominent after the applied force exceeded 125 N. After that, the slope of the curve shows more inclined towards the x-axis, showing a gradual force drop after reaching its peak value, i.e. 362 N, followed by a significant drop.
The plot of load-displacement responses of the flax/basalt hybrid ENF test specimens show a linear region between 0 and 200 N. It can be seen that the curves leaned more towards the load-axis, which is an indication of increase stiffness and requires a greater load to cause the same displacement in the case of flax alone specimens. For example, when average displacement, δ is 2 mm, the average load for flax alone would be 166 N whereas for the flax/basalt hybrid composite, it would is around 296 N. The maximum load recorded for the average of flax/VE sample was 362N whereas the maximum load recorded for the average of flax/basalthybridize sample was 427 N. As it can be seen in Fig. 4, hybrid specimen showed significant increase in load with basalt fiber hybridization closer to its peak value, indicating a stable crack growth. It also shows that the load deflections curvehybrid specimen with an appearance of a plateau which is an indication of increase in toughness with the basalt fiberhybridization.
3.2 Comparison of fracture toughness (GIIc)and stress intensity factor (KII)
The mode II fracture toughness of flax fiber reinforced vinyl ester composite and basalt fiberhybridizedspecimens is shown in Fig. 5. Similarly, the stress intensity factor of flax and basalt hybridized composites are shown in Fig. 6.
It can be seen from Fig. 5 that the fracture toughness value is significantly increased for flax/basalt hybrid composites. For example, the average value of fracture toughness, GIIC of non-hybridized flax specimens were 1940.40 J/m2. In contrast, the average value of GIIC of basalt hybridized flax composite specimens were 2172.94 J/m2. The fracture toughness increased approximately by 12% with basalt fiberhybridization. The fracture energy,GIIc, for the flax/basalt hybrid composite is higher than that of flax/VE and greater load is needed to produce nearly the same displacement. Therefore, the combination of basalt and flax were provided a better resistance to crack propagation than flax alone when both specimens were placed under the same crosshead displacement at a rate of 2mm/min.
Fig. 6 shows the stress intensity factor for both flax alone and flax basalt hybridized composite specimens.The average stress intensity factor, KII, of non-hybridized flax specimens and basalt hybridized specimens were 133.74 kPam0.5and 177.84 kPam0.5 respectively, obtained from similar experimental conditions. It can be seen that the average value for KII for basalt hybrid sample is higher, increase approximately by 33% than those for flax/VE. This observation demonstrates that the hybrid sample shows increased resistance to crack propagation than that of flax alone.
Figures 7 and 8 show the average values of flexural strength and modulus of flax alone and flax/basalt hybridized specimens. An improvement in flexural strength can be seen with the hybridization of basalt fiber (Fig. 7). When considering the fracture toughness as well as flexural strength and modulus, the flax fiber/basalt hybridized specimens exhibited a far superior performance compared to the flax/VE composites without basalt fiberhybridization. The average flexural strength and modulus of flax alone samples were 137 MPa and 14.28 GPa, respectively. The average flexural strength and modulus values of basalt hybridized flax composite specimens were approximately 161 MPa and 23GPa, respectively. This exhibits the remarkable flexural strength and modulus improvement (approximately by 18% and 62%, respectively). The improvement in fracture toughness and flexural properties is attributed mainly to enhanced resistance to crack propagation as a result of hybrid effect. Similar observations towards the improvement of flexural properties as a result of basalt fiber hybridization have been reported by the work of Sarasini et al. [36]and Dhakal et al. [38]. The results for both sample type show some variation, Table 2 (high standard deviation),which can be explained by the fact that the arrangement of the fibers is not uniformly distributed in the process of laminate fabrication which could contribute to the presence of air pockets or even micro-fractures which are not easily seen with the naked eye.Another important factor that significantly affects the consistency of results is fiber defects including kink bands and crack running along the fiber bundles of natural fiber composites such as flax.