Modelling of Ultrasonic Array Signals in Anisotropic Media

There has been a steady increase of composites and anisotropic materials in primary aircraft structures over the years. This increase is driven by the high strength to weight ratio of such materials leading to lighter and more efficient aircraft. As the uptake of such materials keeps increasing so does the complexity in geometry and manufacture of the parts which use these materials. As in the case of any structural component, these structures suffer from defects during manufacture and from damage inservice and have to be tested using nondestructive methods regularly. A plethora of NDT techniques exist for testing aircraft structures with ultrasonic NDT being a staple in the industry. Testing using single element transducers is being replaced by phased arrays as phased arrays can be used in different testing configurations such as beam steering or using phased arrays to capture the signals and then post process the data to form an image. Phased array testing of isotropic materials has been carried out for a number of years with a lot of research being devoted to the testing of such materials. The testing of isotropic materials is relatively less complicated than for anisotropic materials due to the constant material properties throughout the material, the types of defects or failures which such structures suffer and the effect of the material properties on the ultrasonic beam propagating through it or on the output signals. This leads to a simpler interpretation and easier understanding of results when such structures are tested. On the other hand testing of anisotropic materials is complicated by the fact that the material properties are not the same in every direction. The anisotropic nature of such materials has an effect on the ultrasonic beam propagation and output signals which makes the interpretation and understanding of the output more difficult. The layered structure of the composite materials also leads to multiple reflections and reverberations of the layers during inspection which are properties of the laminate, array parameters etc. leading to noise in the output signals and noise in the image. Due to this ultrasonic NDT remains a bottleneck in the further implementation of composites in aircraft structures. Understanding the effect of these various parameters experimentally would require dozens of experiments with different isolated parameters. To overcome the need for this enormous experimental campaign, modelling and simulations can be carried out to help understand these effects. There has been progress in the NDT community on the adoption of modelling methodologies to simulate the predict the response of the inspected material to the wave passing through it and the output signals which are generated. The numerical models already developed have been applied to a variety of scenarios and to different complex geometries but become quite computationally expensive as the material and inspection procedure complexity increases and take hours of runtime when run on personal computers. Some analytical and semi-analytical models which have been applied are restricted either in geometry, require the numerical evaluation of multiple integrals, are computationally expensive or do not take into consideration the array parameters or are singular when interacting with different geometries.