Fibre reinforced polymers (FRP) have been replacing traditional materials, such as steel and aluminium, in critical load bearing applications in a wide range of industries including aerospace, automotive and renewable energy generation. As a result, more and more of the primary load bearing structures will be comprised of composite materials. When used in industry, composite parts are typically drilled and bolted together. This damages the ?bre reinforcement and polymer matrix, creating stress concentrations and possible initiation sites for fracture. A better way to join composite parts is to use a structural adhesive, as this results in an improved stress distribution across the entire joint. There are several different composite joint systems used in indistry. Two types of joints, co-cured and secondary bonded, are investigated in this work. Co-cured joints are produced by curing the composite prepreg and adhesive in a single processing operation. Secondary joints are produced by adhesive bonding of pre-cure laminates.

The aim of this work was to investigate the fracture behaviour of three composite joint systems; one co-cured (denoted as CC) and two secondary bonded (SHH and SHL). The joints were evaluated under mode I, mode II and mixed-mode loading. Two observations were made. Firstly, the mode II fracture toughness was typically 5-6 times greater than that of mode I (see Figure 1). The fracture mechanisms associated with this enhanced toughness were investigated experimentally through the use of microscopy (see Figure 2), and numerically through the use of fracture simulations. Secondly, it was found that the co-cured joints failed interfacially due to pre-bond moisture in the prepreg.

_{Figure 1: Fracture toughness, Gc, as function of percentage mode II for three joint systems.}

_{Figure 2: SEM image of mode II fracture surface of CC joint.}

The composite prepreg was treated with an atmospheric pressure plasma in an attempt to improve the toughness of the co-cured joint. The mode I fracture toughness of the co-cured joint was improved by approximately 15% (see Figure 3).

_{Figure 3: Mode I fracture toughness of co-cured joint before and after plasma treatment.}

Sponsors:

• Embark Initiative - (opens in a new window)website
• Cytec Industries - (opens in a new window)website

Contact details:

• Prof. Alojz Ivankovic (opens in a new window)alojz.ivankovic@ucd.ie