Determination of interfacial materials constants of glass/epoxy composite

Abstract

Interface properties of composite materials are important because they determine the mechanical properties, the fracture properties and other functional properties. Although reinforced fibre and matrix possess good mechanical properties, if the interface has poor properties, the composite will have poor properties. Interfacial research roughly divides into chemical investigations and mechanical investigations. Chemical investigations have mainly discussed the chemical structure of the interface and/ or the interphase [1]. Mechanical investigations have developed estimation techniques: the single-filament tensile test, the pull-out test, and so on [2–4]. In spite of many efforts, interface problems remain to be solved because exchange of information between chemical and mechanical teams has yet to take off. The most exciting challenge for collaborative research is the use of surface treatment for changing the mechanical constants of the interface, e.g. elastic modulus and tensile strength. We present a determination method for interfacial materials constants using experimental results and finite element calculations on interface elements. We have clarified the effect of surface treatment on initial microfracture in plain woven fabric composites [5]. Initial microfracture is defined as the first fracture under tensile load and appears as a knee point in the tensile stress–strain relations, so the initial microfracture stress is the limitation of elastic regions. The initial microfracture can be detected as the occurrence of fracture in the transverse fibre bundles to the tensile load. Fig. 1 shows the appearance of the microfracture observed from the surface of single-layer plain woven fabric composites using an optical microscope. The crack was observed in the transverse fibre bundle. We also measured the effect of the surface treatment, such as concentration of silane coupling agents, and the treatment condition on the microfracture stress. A more precise detection method was developed by monitoring the fracture aspects using combined image analysis. The interfacial element was included for finite element modelling. Fig. 2 shows a finite element division of plain woven fabric composite. The fibre bundle was divided into two bar elements, and interfacial elements were set between fibre elements. At the intersection, resin elements were made for transmitting stress from fibre elements; surface resin elements were also made. The first fracture occurred at the interface element under tensile load. The von Mises equivalent stress was adopted as the failure criterion. The initial microfracture stress of each specimen was calculated from the stress at which the interfacial element failed. Materials used in this paper were plain woven glass cloth (Asahi) and epoxy resin (Ukashell). Two kinds of glass cloth were prepared: one was treated with silane coupling agent (Shinetu Chemical), the other was untreated. Single-ply composites were subjected to tensile load along the warp fibres. The initial microfracture stress of treated material was 67 MPa, whereas for untreated material it was 48 MPa; both values were obtained by experiment. Fig. 3 shows the variation of the calculated initial microfracture stress with the elastic modulus of