Elsevier

Composite Structures

Volume 252, 15 November 2020, 112692
Composite Structures

An experimental and numerical investigation of highly strong and tough epoxy based nanocomposite by addition of MWCNTs: Tensile and mode I fracture tests

https://doi.org/10.1016/j.compstruct.2020.112692Get rights and content

Abstract

The present study investigates, through experimental and numerical approaches, the incorporation effect of different weight concentrations of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of epoxy. Tensile and mode I fracture tests were performed to investigate the effect of the addition of MWCNTs on Young’s modulus, Ultimate Tensile Strength (UTS), critical stress intensity factor (KIC) and critical strain energy release rate (GIC). Different carbon nanotubes (CNTs) contents were employed to compare the effect of the resulting microstructures (well-dispersed and agglomerated CNTs) on the mechanical properties. Field Emission Scanning Electron Microscopy (FESEM) and Scanning Electron Microscopy (SEM) were used for microstructural analysis and fractography. Experimental results showed that UTS was improved (28%) by incorporation of MWCNTs while the KIC and GIC were substantially increased by 192% and 614%, respectively. CNT pullout and crack bridging were the main contributing mechanisms in toughening the epoxy at low CNT contents (0.1 and 0.25 wt%). In contrast, a combination of crack bridging and crack branching was responsible for the resin toughening in the case of 0.5 wt% loading. The significant increase for KIC and GIC demonstrated the excellent performance of the dispersion approach used in this study. Finite Element modelling was used to provide a more robust analysis of the effect of CNT incorporation in tension tests and of the toughening mechanism of the nanocomposites in mode-I fracture tests.

Introduction

Nowadays, advanced polymer-based composites have been widely used in a variety of applications due to their advantages over conventional metallic materials including low weight, cost effectiveness, high corrosion resistance and high flexibility in fabrication as well as structural health monitoring [1], [2]. In this context, epoxy has widely attracted the attention of scientists for structural composite applications due to its excellent performance in various engineering applications such as adhesive bonding, coating, electronic industries and integrated circuit packaging as well as being used as the polymer-matrix for advanced laminated composites such as Carbon Fiber Reinforced Polymer (CFRP) [3], [4], [5]. However, the high crosslink density of the cured pristine epoxy resulted in unsatisfactory mechanical properties, in particular low fracture toughness, which limited its application due to high susceptibility to crack initiation, propagation and, thus, brittle fracture [6].

Numerous attempts were made to improve the poor mechanical properties of the pristine epoxy, tailoring its mechanical properties by introducing CNTs into the epoxy matrix [7], [8], [9], [10], [11], [12], [13], [14], [15]. This was mainly attributed to the promising characteristics of CNTs i.e. high aspect ratio, low density and outstanding mechanical properties [16]. Saboori et al. [17] investigated the fracture behavior of the epoxy reinforced with MWCNTs at different weight concentrations. They achieved a 19.5% increase in mode I fracture toughness of the nanocomposites at 0.5 wt% of MWCNT loading compared to the neat epoxy. In a comprehensive study made by Gojny et al. [18], the fracture toughness increased by 43% by incorporating 0.5 wt% of amino-functionalized DWCNTs into the epoxy matrix, whereas the enhancement effect on tensile strength was negligible. The significant improvement in fracture toughness was attributed to the toughening effect of CNTs via crack bridging and CNT pullout mechanisms [19].

It is worth noting that in most of the studies above, the addition of CNTs resulted in either negligible or a slight enhancement of the tensile strength of epoxy-based nanocomposites. Moreover, few studies even showed some reductions in tensile strength by introducing nanofillers into the epoxy matrix, while the fracture toughness increased [10], [18], [20], [21]. This was mainly attributed to the presence of aggregates, the formation of voids, and ineffective load transfer between the CNTs and the epoxy matrix, primarily when high CNT loadings were used [22], [23], [24], [25]. In fact, the brittleness of the epoxy caused the material to be very susceptible to imperfections, particularly in the form of bubbles and voids, resulting in a significant reduction of the mechanical properties.

The difficulty in predicting material properties in the presence of nanofillers paved the way to numerical simulations, aiming to provide a more robust explanation of the experimental trends. However, the structural modelling of nanocomposites is still challenging, especially when a larger scale with respect to the nanofiller is considered, which requires a macro-homogeneous approach [26], [27] that hardly retains sufficient details for obtaining reliable solutions. This is especially true for fracture test, and consequently, few studies have investigated the simulation of fracture properties of nanocomposites, as the use of microscale models would require excessive computational time [28], [29]. As a result, multiscale modelling can balance and bridge micro- and macroscale modelling strategies. Fereidoon et al. [30] utilized the global–local approach, similar to the multiscale approach, to mimic the fracture behavior of the nanofiller-reinforced epoxy, although the requirement of several parameters might hinder a widespread application of such a method. The replication of the fracture behaviors in nanocomposite is strongly driven by failure modes and the interaction of the constituents [19], [31]. The use of a numerical approach showed that the distribution of CNTs in the matrix could influence the performance of their reinforcement in fracture test, and an improved fracture toughness can be obtained when CNTs bridged the crack path [30]. A numerical approach in combination with experimental tests can therefore be used to describe the stress distribution and the toughening mechanism in fracture test. In particular, since mode-I fracture is mainly driven by the tensile stress normal to the crack surface, it seems reasonable to use the tensile properties to predict the mode I fracture behaviors [32], [33].

Therefore, this study was aimed to experimentally and numerically investigate the effect of addition of MWCNTs on the mechanical properties i.e. tensile strength and fracture toughness of the epoxy-based nanocomposites. Three different CNT contents, including 0.1 wt%, 0.25 wt% and 0.5 wt%, were used to thoroughly compare their microstructural effects, on tensile and fracture toughness properties. SEM and profilemetry analyses were used for microstructural characterization. For the experimental part, tensile and mode I fracture tests were performed in order to obtain Ultimate Tensile Strength (UTS), Young’s modulus (E), critical stress intensity factor (KIC) and critical strain energy release rate (GIC). In addition, the tensile test results obtained from the experiment with different weight fractions of CNTs were considered as the input for the calibration of macro-homogeneous material parameters to build a reliable and efficient model for the fracture test simulation considering the CNTs’ effect. This was particularly important in terms of accuracy of the numerical model developed in this study since the effect of CNT agglomeration has already been accounted for in the tensile specimens, i.e. higher CNT content, higher CNT agglomeration. As a matter of fact, with the aid of simulation, the deformation and toughening mechanisms could be adequately studied, being severely related to the state of CNT dispersion as well as the state of the CNT aggregates. Two types of simulated fracture behaviors results could be expected based on the material model from the tensile data: replicated and not-fitted simulation results. The former indicated the effective incorporation of CNTs in simultaneous tailoring of tensile and fracture toughness properties (at low CNT content), whereas the latter led to different effects of agglomerated CNTs on mechanical properties, i.e. an outstanding performance in fracture property but a decrease of tensile property.

Section snippets

Material

Nanocomposites were produced by adding MWCNTs to a low viscosity Bisphenol A diglycidyl ether (DGEBA) named Araldite LY556, cured with amine hardener XB3473 purchased from Hunstman. The matrix was an aerospace epoxy grade manifesting low viscosity which made it ideal candidate for CNT dispersion. Its mechanical properties, before addition of CNTs, were a Young’s modulus of 2820 MPa, ultimate tensile strength of 52 MPa, KIC of 0.77 MPam and GIC of 0.24 KJ/m2. The MWCNTs had 13–18 nm outer

Microstructural characterization

Fig. 5 shows FESEM images taken from the fracture surface of the dog-bone specimens. Specimens loaded at 0.1 wt% and 0.25 wt% of MWCNTs present a homogenous distribution of CNTs with respect to specimens loaded at 0.5 wt% showing the presence of aggregates. This is explained by the high CNT loading, which leads to the formation of CNT-bundles due to the fact that the viscosity of MWCNTs/epoxy mixture increases at a higher CNT content, thus proper dispersion becomes more difficult to achieve. It

Validation of the material model

The FE model was used to understand better the mechanical behavior and mechanism of the fracture tests. In our simulation, the tensile test results from nanocomposite with each weight fraction were directly used as tabular input data in the material model, Mat_024. Regarding the failure criterion, the maximum strain was herein used to determine the failure of the material under tension. As far as the Poisson ratio of both virgin and CNT-reinforced polymer is concerned, the same value, 0.3, was

Conclusion:

Tensile strength and fracture toughness of epoxy-based nanocomposites, loaded with MWCNTs at 0.1, 0.25 and 0.5 wt%, were experimentally and numerically investigated throughout this study. The following results were obtained:

  • CNTs were homogeneously dispersed at all CNT contents, even though the presence of agglomerates was increased by increasing the CNT content. The CNT content of 0.1 wt% and 0.25 wt% showed a more homogenous dispersion compared to 0.5 wt%, which was attributed to the high

CRediT authorship contribution statement

A. Esmaeili: Conceptualization, Methodology, Data curation, Investigation, Writing - original draft, Writing - review & editing. D. Ma: Methodology, Data curation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. A. Manes: Methodology, Supervision, Writing - original draft. T. Oggioni: Data curation. A. Jiménez-Suárez: Data curation. A. Urena: Data curation. A.M.S. Hamouda: Project administration. C. Sbarufatti: Methodology, Data curation, Supervision,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This publication was made possible by GSRA grant No. GSRA2-1-0609-14024 from the Qatar National Research fund (a member of Qatar foundation). The author D. Ma thanks the financial support from China Scholarship Council (CSC, No. 201706290032). Support by the Italian Ministry of Education, University and Research, through the project Department of Excellence LIS4.0 (Integrated Laboratory for Lightweight e Smart Structures), is acknowledged.

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