Transducer Placement Option for Ultrasonic Lamb Wave Structural Health Monitoring (SHM) on Damage Tolerant Aircraft Substructure

In this paper, we review two transducer placement options to locate and quantify damage in primary aircraft structures using ultrasonic Structural Health Monitoring (SHM). The first placement approach concerns a known expected damage location, for example a fatigue crack growth from rivet hole. The location of such a damage can already be predicted by fracture mechanics and therefore the focus of this SHM system design is to determine the damage size. For this approach, we have developed our previous work in finite-element (FE) modelling of a damage tolerant aluminum fuselage by introducing an artificial crack into the structural FE model and assessed its influence on the Lamb wave propagation. Image processing was performed by subtracting the wave propagation image of the damaged from the undamaged structure. A second category of damage occurs at locations that cannot be predicted by fracture mechanics, such as impact damage from hail. This type of damage requires the SHM system to both locate and assess the size of the damage and this is heavily influenced by the positioning of the transducers. Optimal sensor placement (OSP) techniques tend to rely on assessment using the probability of detection (POD) parameter. In this work, we propose an alternative placement method which maximizes the detectability of the transducer coverage area based on the pulse-echo technique without relying on the POD parameter, by determining the fitness function based on sensor coverage area for single and multiple sensors and random damage locations. Results from both these approaches are compared in this paper, with a perspective towards the overall design of SHM systems.