## 1 Introduction  
Within the aerospace industry there is a constant drive to produce more environmentally friendly aircraft. Although at present the aerospace sector is not a major contributor to greenhouse emissions, historical trends show that global air travel is doubling every fifteen years [1]. One way of reducing the environmental impact of such an increase in air travel is to produce aircraft that are lighter in mass through the use of advanced composite materials and construction techniques which provide improved strength and stiffness to weight ratios when compared to traditional materials and methods. Aircraft structures are typically constructed from thin load bearing skins with stiffeners attached to provide the structure with the required amount of rigidity [2]. There are two predominant methods of attaching the stiffeners to the skin; mechanical fasteners such as rivets, or bonding with an adhesive. Traditionally, mechanical fasteners have been used in primary structure of the aircraft (_i.e._, the structure in which failure would be catastrophic) because of their ability to transfer loads between components with predictable performance. However mechanical fasteners create high stress concentrations which can lead to crack initiation [3; 4]. Adhesively bonded metallic joints have been used in the secondary structure of aircraft for the past fifty years [5]. Bonding stiffeners to the aircraft's skin is advantageous when compared to riveting as it can reduce the mass of the joints, distribute the stresses more evenly and be more resistant to environmental effects (such as temperature, humidity and vibration) which an aircraft experiences [6]. Adhesively bonding stiffeners is also less labour intensive and more cost effective than using mechanical fasteners as it does not require the structure to go through repetitive drilling operations [7]. Adhesive bonding is particularly suited to composite structures where the use of mechanical fasteners presents additional difficulties. Due to the hardness and high tensile strengths of some composites, drilling can cause rapid deterioration of the drill bits [8] resulting in high machining costs and thus skilled operatives are required to carry out the operation. Drilling can also cause damage to the composite such as interlaminar crack propagation, micro cracking, fibre breakage, fibre pull-out, matrix cracking, thermal damage and delaminations. The presence of the hole also forms a reduction in the strength of the material due to the fibres no longer being continuous. Though adhesives bonds have been used in the secondary structure without arrestment fasteners (_i.e.,_ fasteners that would still bear the load should the adhesive fail) they are rarely used in the primary structure without the use of arrestment fasteners due to the lack of confidence in the reliability of the joint [9]. From the experience of the Royal Australian Air Force (RAAF) 53% of defects detected in aircraft structures such as the F-111 were found to be bond failures [10]. Many cases of adhesive bond failure have been found to be the result of improper application of the adhesive or insufficient surface preparation [9]_i.e.,_ human error in the manufacture of the joint. Though it has been proven that bonded joints are more resistant to fatigue loading as well as being able to withstand higher peak loading [10], the degradation of the adhesive layer over time is not yet fully understood [11]. This uncertainty in the condition of the bond throughout the aircraft's in-service life requires monitoring using non-destructive techniques to ensure airworthiness. Traditional non-destructive (NDT) techniques can be used to inspect the structural integrity of bonds on aircraft structures. However with the increasing size of commercial aircraft, the influence of human error and pressures from aircraft operators to reduce maintenance times [12], alternative solutions to traditional NDT techniques have to be found to enable the application of airworthy adhesively bonded stiffeners.