A novel optical method for measuring disbond growth in bonded composite joints

Bonded joints, consisting of composite-composite or composite-metal joints, are used in various industries. The advantages of using bonded joints, as opposed to mechanically fastened joints, include weight saving (which is particularly important in aerospace applications) and the avoidance of stress concentrations around holes. The difficulty with bonded joints is, of course, that the joint cannot be disassembled and structural health monitoring of joint integrity is difficult. In this paper a new method for monitoring composite bonded joints using a sensor based on a chirped fibre Bragg grating (CFBG) will be described. The CFBG has been embedded within a shear lap joint manufactured using transparent glass fibre reinforced plastic (GFRP) adherends, and the transparency of the model joint allows a simple comparison to be made between the growth of the disbond and changes in the reflected spectrum of the CFBG. The joint has been subjected to a fatigue loading which both initiates disbonds between the adherends and then causes the disbonds to grow stably (up to a length of about 15 mm prior to fast fracture).The results show (a) that the initiation of a disbond can be detected, and (b) that the position of the disbond front can be determined to within about 2 mm. The results have been modelled using a combination of finite-element analysis and OptiGrating (fibre Bragg grating) software