failure as well as non-uniformity of a local stress distribution at the
interface. For this purpose, numerous models describing stress distributions
and interfacial failure in fibre–matrix systems were proposed.
Several stress-analysis schemes have been used to assess the energy release
rate G for initiation of an interfacial crack in a microbond specimen.
A comprehensive theory was developed by Nairn et al. [12–14].
The model assumes that the debonding zone extends when the energy
release rate reaches its critical value GIc; thus, GIc is equal to the interfacial
fracture toughness. The value of GIc reflects the energy-dissipation
capacity during the debonding process of fibre–matrix system. Recently,
the model was further modified by Scheer et al. [14] and applied to both
experimental and numerical finite-element (FE) analysis mainly for
glass and Kevlar fibres. In our study, the method will be applied for
the carbon fibre/epoxy system.
When using mixed sulphuric and nitric acids to modify carbon fibres
(CFs), the time of treatment is of considerable importance. Based on
current studies, the suggested time of surface treatment varies from a
few minutes to several hours for different applications. Han et al. [3]
pointed out that the tensile strength of CF decreased after 1 h of surface
treatment and reduced more than 50% after 2 h. Langston et al. [15] stated
that the best functionalization efficiency and highest oxygen concentration
was obtained with the surface treatment of 80 min. Wang et al.
[16] found that 15 min was preferred to pursue best electrical
conductivity of CFs. In this study, carbon fibres were treated by mixed
acids, and the surface treatment time was optimized by balancing the
levels of tensile strength of fibre, adhesive strength and fracture toughness
of CF/epoxy interface.