Fracture toughness of the nano-particle reinforced epoxy composite
Introduction
Thermoset polymers have been widely used for engineering components, adhesives and matrix for fiber-reinforced composites due to their good mechanical properties compared to those of thermoplastic polymers. However, since they are usually brittle and vulnerable to crack, ductile thermoplastic materials such as micro-sized rubber or nylon particles are added to the polymers to increase their fracture toughness, which compromises the strength of thermoset polymers.
The addition of rigid micro-scale fillers to polymers often increases its strength, but decreases the toughness since the fillers or agglomerates may induce stress concentration, which initiates cracks and make them become larger than the critical crack size that causes failure. Therefore, it is a good way to reinforce the polymers with nano-particles in order to increase the fracture toughness without sacrificing the mechanical strength of the polymers because well-dispersed nano-particles are much smaller than the critical crack size to initiate failure. Thus, they provide an avenue for simultaneously toughening and strengthening polymers [1].
Nano-particle reinforced polymer composites have been widely studied and some researchers already studied the improvement of the fracture toughness of polymers. Carbon nanotubes (CNT) have shown a high potential to improve the mechanical properties of polymers as well as electrical properties [2], [3]. Gojny et al. [4] reported that DWCNT (double walled CNT) could increase both tensile strength and fracture toughness. Florian et al. [5] studied the influence of different carbon nanotubes on the tensile properties as well as fracture toughness and explained the contribution of nanomechanical mechanisms to enhancement of the fracture toughness. However, CNT has not been widely used to improve the mechanical properties because of its high material cost. Since the first discovery by Toyota researchers on the reinforcing effect of relatively cheap nanoclay on the polymeric material [6], many researchers have focused on it: Weiping et al. [7] reported that nanoclay could increase the fracture toughness of epoxy by 2.2 and 5.8 times. Lei et al. [8] studied the dependences of Young’s modulus and fracture toughness on clay concentration using the tensile and 3-point bending methods. Qi et al. [9] investigated the effect of several nanoclay additives, which were mixed with DGEBA epoxy resin using a mechanical stirrer, on tensile modulus, tensile strength and fracture toughness of the nanocomposite. Ho et al. [10] increased the tensile strength and Vickers’ hardness value of the epoxy using nanoclay mixed by mechanical stirring method. Carbon black also has been used for reinforcement of polymers, which has been mainly used for reinforcing elastomer and for UV protection, electromagnetic interference shielding and anti-static shielding of polymers [11]. Novak [12] studied the electro-conductive HDPE/CB composite with improved toughness. However, mechanical reinforcing effects of the carbon black have not been widely studied yet.
As mentioned above, most of researchers have been interested in the mechanical properties of nanocomposites at the room temperature. However, only few researchers studied the cryogenic properties of epoxy and its composite [13], [14], [15].
A study on the nanocomposite is important since it can affect the structural characteristics of a composite structure when it is used as a matrix of the laminates or the reinforcement of a foam core. It was reported that the characteristics of a composite structure could be improved when the nanoclay reinforced epoxy was used as a matrix of laminates. Antonio et al. [16] improved the damping coefficient and the energy dissipation characteristic of the glass/epoxy composite using nanoclay particles. Hosur et al. [17] improved the impact characteristic of the composite sandwich structure using the nanoclay infused foam.
In this study, carbon black and nanoclay were mixed with epoxy to investigate their toughening effect. The fracture toughness was measured using the single edge notched bend (SENB) specimens with respect to the particle content at the room (25 °C) and cryogenic temperature (−150 °C). In order to investigate their toughening mechanism, the fracture surface was also observed with SEM.
Section snippets
Materials
The epoxy matrix used in this study was the modified bisphenol-A type epoxy resin (YD-114F, Kukdo Chemical, Korea) and polyetheramine hardener (JEFFAMINE D-230, Huntsman, US) were used as a curing agent. The proper mixing ratio was 10:3. The conductive carbon black (Ketjenblack EC-300J, Ketjen Black International Co., Japan) and the nanoclay (Cloisite 93A, Southern Clay Products, US), which was natural montmorillonite modified with quaternary ammonium salt, were used as reinforcements. Table 1
Experiments
The mode I fracture toughness, KIc, was determined by the 3-point bending method with SENB specimens and the fixture as shown in Fig. 2. The tests were performed using INSTRON 5583 (INSTRON corp., MA, USA) equipped with an insulation chamber. The crosshead speed was 10 mm/min as recommended by ASTM D5045-99, which was fast enough to prevent the viscoelastic behavior of the epoxy [20].
The KIc values were determined using the following relationship [20]:
Effect of the residual stress around the crack tip and the crack length
ASTM recommends either inserting a fresh razor blade by tapping or sliding a new razor blade across the notch root to initiate a pre-crack [20]. However, it can generate large amount of residual stress around the crack tip. Though many researchers have suggested the fracture toughness of polymeric materials, they have not inspected the crack tip visually or investigating the effect of the residual stress. In addition, it is not easy to make the specimen with the same crack length. In order to
Conclusion
In this work, the toughening effect of carbon black (Ketjenblack EC-300J, Ketjen Black International Co., Japan) and nanoclay (Cloisite 93A, Southern Clay Products, US) on the modified bisphenol-A type epoxy resin (YD-114F, Kukdo Chemical, Korea) was investigated at the room (25 °C) and cryogenic (−150 °C) temperatures.
At the room temperature, the carbon black (CB) of 3.0 wt% could increase the KIc value by 23% on the average due to the toughening mechanisms of nano-scale crack branching and
Acknowledgements
This work has been supported by Ministry of Science and Technology in part by NRL (TBP), and BK21. Their supports are gratefully acknowledged.
References (22)
- et al.
Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties
Polymer
(1999) - et al.
Formation of percolating networks in multi-wall carbon nanotube-epoxy composites
Compos Sci Technol
(2004) - et al.
Carbon nanotube-reinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content
Compos Sci Technol
(2004) - et al.
Investigation of the mechanical properties of DGEBA-based epoxy resin with nanoclay additives
Compos Struct
(2006) - et al.
Mechanical properties of epoxy-based composites using nanoclays
Compos Struct
(2006) - et al.
Hybrid electro-conductive composites with improved toughness filled by carbon black
Carbon
(2005) - et al.
Molecular design of an epoxy for cryogenic temperatures
Cryogenics
(1995) - et al.
Application of the positron annihilation method for evaluation of organic materials for cryogenic use
Cryogenics
(1995) - et al.
Cryogenic properties of SiO2/epoxy nanocomposites
Cryogenics
(2005) - et al.
Processing of nanoclay filled sandwich composites and their response to low velocity impact loading
Compos Struct
(2008)
Compos Sci Technol
Cited by (361)
Enhancing bond performance of CFRP-steel epoxy-bonded interface by electrospun nanofiber veils
2024, Thin-Walled StructuresTopology as a limiting factor for mechanical properties in disordered networks
2024, Cell Reports Physical ScienceRapid and scalable synthesis of novel carboxylated aramid nanofibers for simultaneously improving the strength and toughness of carbon fiber/epoxy laminates
2024, Composites Science and TechnologyFracture mechanics of polymer concretes: A review
2023, Theoretical and Applied Fracture Mechanics