An experimental and numerical investigation of highly strong and tough epoxy based nanocomposite by addition of MWCNTs: Tensile and 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 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.
References (44)
- et al.
Effects of carbon nanotube content on the mechanical and electrical properties of epoxy-based composites
Xinxing Tan Cailiao/New Carbon Mater
(2014) - et al.
Comparison to mechanical properties of epoxy nanocomposites reinforced by functionalized carbon nanotubes and graphene nanoplatelets
Compos Part B Eng
(2019) - et al.
A review of extending performance of epoxy resins using carbon nanomaterials
Compos Part B Eng
(2018) - et al.
On the mixed mode I/II delamination R-curve of E-glass/epoxy laminated composites
Compos Struct
(2017) - et al.
On the mixed mode I / II / III inter-laminar fracture toughness of cotton / epoxy laminated composites
Theor Appl Fract Mech
(2020) - et al.
Improvement of modulus, strength and fracture toughness of CNT/Epoxy nanocomposites through the functionalization of carbon nanotubes
Compos Part B Eng
(2017) - et al.
Dispersion stability of functionalized MWCNT in the epoxy-amine system and its effects on mechanical and interfacial properties of carbon fiber composites
Mater Des
(2016) - et al.
Effect of functionalization on the thermo-mechanical and electrical behavior of multi-wall carbon nanotube/epoxy composites
Carbon N Y
(2011) - et al.
Enhancing mode-I and mode-II fracture toughness of epoxy and carbon fibre reinforced epoxy composites using multi-walled carbon nanotubes
Mater Des
(2018) - et al.
Fracture toughness of epoxy/multi-walled carbon nanotube nano-composites under bending and shear loading conditions
Mater Des
(2011)
Highly tough and strain sensitive plasma functionalized carbon nanotube/epoxy composites
Compos Part A Appl Sci Manuf
Effects of carboxylated carbon nanotubes on the phase separation behaviour and fracture-mechanical properties of an epoxy/polysulfone blend
Compos Sci Technol
Effect of amino functionalized MWCNT on the crosslink density, fracture toughness of epoxy and mechanical properties of carbon-epoxy composites
Compos Part A Appl Sci Manuf
Enhanced fracture toughness of hierarchical carbon nanotube reinforced carbon fibre epoxy composites with engineered matrix microstructure
Compos Sci Technol
Elastic and mechanical properties of carbon nanotubes
Synth Met
Experimental fracture study of MWCNT/epoxy nanocomposites under the combined out-of-plane shear and tensile loading
Polym Test
Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites - A comparative study
Compos Sci Technol
Toughening mechanisms in polymer nanocomposites: From experiments to modelling
Compos Sci Technol
Tensile fracture and thermal conductivity characterization of toughened epoxy/CNT nanocomposites
Mater Sci Eng A
Reinforcing epoxy resin with nitrogen doped carbon nanotube: A potential lightweight structure material
J Mater Sci Technol
Experimental and numerical approach to study mechanical and fracture properties of high-density polyethylene carbon nanotubes composite
Mater Today Commun
Modeling of CNT-reinforced nanocomposite with complex morphologies using modified embedded finite element technique
Compos Struct
Cited by (26)
Thermomechanical behavior of a novel hybrid epoxy/ZnO nanocomposite adhesive in structural bonding: Experimental analysis and ANN modeling
2024, Colloids and Surfaces A: Physicochemical and Engineering AspectsA novel numerical method for stochastic study of fiber-reinforced composites with nanoparticles under impact loading
2023, International Journal of Impact EngineeringInterlaminar performance of 3D CNTs/carbon black film enhanced GFRP under low-temperature cycling
2023, Journal of Alloys and CompoundsHygrothermal degradation of MWCNT/epoxy brittle materials under I/II combined mode loading conditions: An experimental, micro structural and theoretical study
2023, Theoretical and Applied Fracture MechanicsThe effects of nano-additives on the mechanical, impact, vibration, and buckling/post-buckling properties of composites: A review
2023, Journal of Materials Research and TechnologyA multiscale model to predict fatigue crack growth behavior of carbon nanofiber/epoxy nanocomposites
2023, International Journal of Fatigue