WIRELESS NONDESTRUCTIVE INSPECTION OF AIRCRAFT WING WITH ULTRASONIC GUIDED WAVES

A prototype wireless guided wave inspection system is developed to inspect layered structures such as aircraft wing. The system includes a stationary antenna and an active antenna as transceivers, an on-board antenna as transponder, and PVDF comb transducers for generating and receiving ultrasonic Lamb waves. Preliminary experiments on a 0.8mm aluminium plate with a 12mm long, 50% through-wall-depth crack clearly demonstrated its feasibility of defect detection. Potential applications on an E2 plane wing section are also presented. This system showed great potential of remote, leave-in-place and in-service defect detection and condition monitoring. Introduction: The flight environment of an aircraft is usually very harsh due to large changes in humidity, temperature, pressure, speed, and loading conditions. These effects cause a lot of stress to aircraft frame. As a result, corrosion, delamination, cracks, disbonds, and other failures creep in once the aircraft is in service for some time. Fault diagnosis and prognosis is very important in health monitoring or condition based maintenance of the aircraft structures. If one can measure the degradation of a component before it actually fails, it will provide ample time for maintenance engineers to schedule a repair, and to purchase or fabricate replacement components before the components actually fail. The end result will be lower cost and higher availability. Ultrasonic guided waves have been used in non-destructive inspection (NDI) of various defects in aircraft structures, with major advantages like fast scanning capabilities, low cost, and long range inspection. Small and reliable guided wave sensors can also be leave-in-place on the structures for online monitoring, which is more convenient for real time damage evaluation. All current data acquisition techniques, however, still rely on cable connections from a waveform generator to the inspection sensors, and then to a waveform displayer or oscilloscope for analysis. In this paper, we proposed a novel wireless system for structural integrity monitoring of an aircraft. The prototype system includes a passive antenna as a transmitter, an on-board antenna as transponder, a PVDF comb transducer for generating and receiving ultrasonic Lamb waves in a layered structure, and a portable active antenna as a receiver. A series of experiments on a 0.8mm thick aluminium plate with a 12mm long, 50% through-the-wall crack clearly showed its feasibility in defect detection. Conventional wired approach and semi-wireless approach are also presented for comparison. Some practical leave-on-board antennae for aircraft wings are also discussed, along with preliminary experiments. Approaches: One of the objectives of this study is that NDI sensors should be low-cost, compact, passive, conformable to the aircraft structure, and can be interrogated in a wireless manner. The wireless capability allows the aircraft to be monitored all the time. Boeing has performed an internal study, which concludes that 85% of maintenance effort is spent on tearing down and re-assembling the components. Only 15% of labour is spent on actual inspection. Hence the maintenance cost can be saved significantly by a wireless leave-in-place approach. For this objective to be materialized, we proposed a technical approach as shown in Figure 1. That is, a PVDF guided wave sensor is attached to the test structure, with its input/output connected to an on-board antenna to feed in interrogate electrical signal and send out its response. The interrogate signal can be transmitted by a transmission antenna through a cable connected to the signal generator, and the return signal captured by a receiving antenna cable connected to a signal receiver. In this case, both the transmission and receiving antennae can be moved around and collect information about the health status of the structure. The conventional wired approach does not use antennae as information transponder; usually co-axial cables are utilized to connect the signal generator, the sensor and the signal receiver together and it is the state-of-practice in industry. The choice of PVDF sensor is obvious since it is low-cost, compact, passive, conformable to the test structure, and easy installation. We will discuss more about its design and utilization in the next subsection. Ultrasonic guided wave Crack Signal generator Signal reciever PVDF comb transducer Radiated and returned signal FIGURE 1 The schematic view of wireless inspection system for crack detection. 45.8cm PVDF Comb Transducer Crack Excited Lamp Wave 15.3cm