During growth a crack will emit numerous pulses that cause waves to travel througout the structure. These can be picked up by an array of ultrasonic sensors at various distances from the crack. The electrical signals generated by the sensors encapsulate different arrival times from which the crack location can be identified. However, many different types of waves can travel in 'thick' structures such as wing spars at ultrasonic frequencies, and they travel at varying speeds. This complicates the inference of crack location from time of arrival information.

We have developed Semi-Analytical Finite Element (SAFE) computer models to predict how (and which) waves propagate in critical loadbearing substructures of aircraft, such as wing spars, at ultrasonic frequencies. The model is able to simulate the response at any position on the structure due to an assumed excitation elsewhere. We are able to use the model to quantify the variability in inferred existence and position of a crack position due to many factors such as sensor type and placement, frequency, form of excitation, and scattering of waves at reflecting boundaries.