Toward the 3D characterisation of GLARE and other fibre-metal laminate composites

Fibre-metal laminates such as GLARE (alternating glass-fibre composite and aluminium layers) are seeing increasing usage on critical aircraft structures due to their enhanced fatigue resistance compared with unreinforced metal. They can be inspected for overall quality using through-transmission ultrasound, but it is very difficult to determine the depth or nature of any defect in the structure in order to assess its importance or severity. As a result, manufacturing scrap rates are higher than desirable and designed components are heavier in order to mitigate risk due to inadequate information. Defect-depth information is buried in the ultrasonic response but is difficult to extract due to the high reflection coefficients of the interfaces and the variable glass-fibre layer thicknesses. This paper presents the potential for using model-based multi-dimensional optimisation to determine the layer thicknesses and depth locations of anomalies in the ultrasonic response due to delaminations or porosity. Numerical (FEM) and analytical methods are presented to model the ultrasonic response of fibre-metal laminates, calculated as the steady-state harmonic response of the layered medium. These frequency-domain responses can be used to determine the individual layer thicknesses and depth locations of anomalies by multi-dimensional optimisation. Investigations on the accuracy and the limitations of the method for the 3D characterisation of laminates will be presented. In addition, the evaluated frequency-domain responses show that the high reflection coefficients in combination with the periodic arrangement of the layup effectively mimic the behaviour of a one-dimensional phononic crystal. In the through-transmission ultrasound response, stop bands arise where the transmission is close to zero. None of the resonance frequencies of a laminate - even one with a finite number of layers - can lie within a stop band. However, the presence of a defect in a layer, or different material properties or thickness, can cause the defect modes, i.e. eigenmodes, to shift into the expected stop bands. This might open new possibilities in the nondestructive testing of fibre-metal laminates, which will be elaborated in the presented paper.