Materials Science and Engineering A 528 (2011) 6173–6178 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Correlation between aspect ratio of MWCNTs and mixed mode fracture of epoxy based nanocomposites M.R. Ayatollahi a,∗ , S. Shadlou a , M.M. Shokrieh b a Fatigue and Fracture Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran b Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran article info Article history: Received 23 January 2011 Received in revised form 10 April 2011 Accepted 26 April 2011 Available online 4 May 2011 Keywords: MWCNT diameter Epoxy Nanocomposite Mixed mode loading Fracture toughness abstract The effects of multi-walled carbon nanotube (MWCNT) diameter on fracture behavior of epoxy based nanocomposites under mixed mode I/II loading were investigated. Four types of MWCNTs with different diameters were used. It was found that the aspect ratio of filler has huge impact on the fracture behavior of nanocomposite. Regardless of their dimensions, MWCNTs were discovered to be more effective under shear loading conditions rather than normal loading conditions. The fracture surfaces of all specimens were studied and the observed features were explained based on the possible fracture mechanisms. Finally, the experimentally obtained fracture loads were compared with the theoretical estimates of three fracture criteria. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Despite an increasing attempt for commercialization of poly- mer/carbon nanotube (CNT) nanocomposites, different important issues related to the fracture behavior and fracture mechanisms of nanocomposites are not still clear. Almost all of the previ- ous researchers have only investigated pure mode I fracture of nanocomposites [1–6]. However, in the service conditions the crack extension usually initiates from defects which can be directed at arbitrary angles relative to the loading direction providing mixed mode fracture. Mixed mode fracture in polymers has been stud- ied but only for pure materials and not nanocomposites [7–9]. In addition, some researchers have used different fracture criteria to estimate mixed mode fracture toughness of polymers [10]. For instance, in [11,12] a generalized maximum tangential stress cri- terion was employed for analyzing mixed mode fracture in PMMA and dental resin materials. The effect of CNT dimensions on the fracture properties of nanocomposites is another important issue which requires a care- ful investigation. This is because depending on the synthesis method and other parameters involved, the CNT diameter may practically vary from several nm to several hundred nm. This prob- ∗ Corresponding author. Tel.: +98 21 7724 0201; fax: +98 21 7724 0488. E-mail address: m.ayat@iust.ac.ir (M.R. Ayatollahi). lem is more serious when CNTs are produced industrially in large quantities. A few researchers have investigated the effect of CNT dimensions on different properties of nanocomposites [13,14]. It has been shown that, in general, with increasing the aspect ratio of CNTs, the mechanical properties of nanocomposites increase [15,16]. For example, Zhang et al. [17] studied the effect of multi- walled carbon nanotube (MWCNT) dimension on fatigue behavior of epoxy/MWCNTs nanocomposites. Their results indicated that longer MWCNTs with smaller diameters (i.e. higher aspect ratios) were well suited for the reduction of fatigue crack growth rate. In the present research, epoxy based nanocomposites contain- ing MWCNTs with four different aspect ratios were prepared and their mixed mode fracture behavior were studied. The trend of results was justified using parameters and mechanisms involved in different mode mixities. In addition, the relation between these mechanisms and the observed features of fracture surface was stud- ied. Moreover, the experimental results were compared with the theoretical estimations of three well-known fracture criteria. 2. Experiment 2.1. Materials and specimen preparation The epoxy resin ML-506 (Bisphenol F) and the curing agent HA-11 (Polyamine) were used together with MWCNTs of dif- ferent diameters namely 20 < d < 30, 30 < d < 40, 40 < d < 50 and 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.04.082