Effect of matrix toughness on the shear strength of brittle and rubber-modified glass-fiber/vinyl ester composites

Abstract

Due to its superior mechanical properties and corrosion resistance, vinyl ester resin is increasingly used as the matrix for fiber-reinforced composites instead of polyester [1]. Corrosion resistance in particular is an important attribute for composites used in applications such as marine structures, where the environment is highly corrosive. However, while vinyl ester is also tougher than polyester, with a mode I strain energy release rate (GIc) of 0.13 kJ/m2 compared to 0.06 kJ/m2 for isophthalic polyester [2], it is still relatively brittle. This brittleness makes the composite susceptibel to delamination through interlaminar fracture. In response to this delamination problem, recent work investigated interlaminar fracture toughness for unidirectionally reinforced glass-fiber composites with brittle and rubber-modified vinyl ester matrices [3, 4]. There was a considerable increase in toughness with a GIc of nearly 2 kJ/m2 reported for the toughest neat resin and, under mode I tensile loading, a GIc of 2.4 kJ/m2 for steady-state crack propagation in the composite counterpart [3]. However, under mode II shear loading, there was no significant matrix effect on composite interlaminar fracture toughness, GIIc [4]. The absence of a matrix effect on shear induced interlaminar fracture toughness is surprising, which led to the next step in the characterization of these composites; to study the matrix effect on the shear strength properties. This letter presents the results from short beam shear and compression induced shear tests, and discusses them in relation to the previously obtained fracture toughness results. The influence of loading pin diameter on short beam shear strength is also addressed. The composite laminates were made by hand lay-up using 24 plies of unidirectional E-glass fiber (Owens