Fracture surface analysis on nano-SiO₂/epoxy composite

Abstract

Fracture surface morphologies of nano-SiO₂/epoxy composite with different weight percentage of SiO₂ are investigated using scanning electron microscopy. Two types of curing agent, dimethylbenzanthracene (DMBA) and methyltetrahydrophthalic anhydride (MeTHPA), are individually used for preparing the composites. It is found that the fracture surface morphology of the composite cured by DMBA shows as radial striations, which suggests a rapid brittle fracture mode, while the fracture surface morphology of the composite cured by MeTHPA shows as regularly spaced ‘rib’ markings, which indicates a stick–slip motion during the fracture process. Furthermore, the uniaxial tensile behavior under constant loading rate and ambient temperature are investigated. It is shown that the elastic modulus of the composite cured by DMBA firstly increases, and then decreases with the mass fraction of nano-SiO₂ particles, but the elongation of the composite cured by MeTHPA is reversed with increasing fraction of nano-SiO₂ particles. For nano-SiO₂/epoxy composite cured with MeTHPA that possesses a suitable fraction of nano-SiO₂, an excellent synthetic mechanical property on elastic modulus and elongation is obtained.

Introduction

Due to their increasing engineering applications, epoxy resin and epoxy based composites have been extensively studied in recent years [1], [2], [3], [4], [5]. The fracture surface morphology analysis is of an important aspect for study on fracture patterns and their corresponding fracture mechanism. There are different fracture surface morphologies corresponding to different deformation and fracture mechanism. For the poly(methyl methacrylate) (PMMA) enduring loads, the craze fracture at crack tip commonly occurs at the middle of craze fibril under a slow crack propagation velocity, the crack propagates along the midrib of craze fibril, consequently, the fracture surface shows as a smooth mirror [6], and a little amount of energy is needed to produce new crack surfaces. In the loading process, the crack propagation velocity increases, and a series of regularly spaced ‘rib’ markings that are perpendicular to the direction of fracture propagation emerge on the fracture surface [7], [8]. The transition between the fracture surface morphology of smooth mirror and that of ‘rib’ marking occurs abruptly [9]. The characteristic morphology showing as conic-shaped patterns is investigated on the fracture surfaces of polymers with low crack propagation velocity [8], [10], [11], [12], while some irregular ‘mackerel’ or ‘patch’ are observed on the fracture surfaces of polymers under high crack propagation velocities [9]. In this paper, the tensile fracture surfaces of nano-SiO₂/epoxy composite with different SiO₂ contents are observed using scanning electron microscopy (SEM). Two different curing agents, dimethylbenzanthrancene (DMBA) and methyltetrahydrophthalic anhydride (MeTHPA), are individually used in the preparation of the composites. The uniaxial tensile behavior under constant loading rate and ambient temperature are investigated to bring more powerful understanding of the effects of curing agent and weight percentage of nano-SiO₂ particles on the fracture surface patterns and some mechanical properties of the composites.