Thomas R. Hay

A comparison of embedded sensor Lamb wave ultrasonic tomography approaches for material loss detection

Computerized tomography (CT) algorithms have been used mainly in the medical field but their powerful capabilities are being exploited more and more in industrial applications. This paper demonstrates that the technology is capable of detecting material loss on real aircraft components using embedded piezoelectric sensors on hidden surfaces. The work is novel in more than one respect. Firstly, it demonstrates that Lamb wave ultrasonic tomography can be used to accurately map material loss on an exposed aircraft surface with sensors embedded on the structures hidden surface. Hidden, in this case, refers to the surface that is not exposed to the atmosphere—the underneath of an aircraft wing, for example. Secondly, it compares tomographic images generated by fan-beam back projection and the signal difference coefficient methods, showing clearly that the latter are more sensitive to material loss.

Flexible PVDF comb transducers for excitation of axisymmetric guided waves in pipe

Flexible PVDF pipe comb transducers are easy to install by wrapping around any size pipe. It is possible to mechanically couple these transducers to the pipe thereby eliminating the need to bond electrodes to the film and adhesively couple the transducer to the pipe. The simple fabrication process, installation, and affordability of these transducers makes them realistic candidates for condition-based monitoring of some critical pipeline applications. These transducers are capable of exciting lower order axisymmetric modes with minimal radial displacement and maximum axial displacement as well as modes with both surface displacement components. This versatility is extremely important since under certain loading conditions, modes with significant radial displacement are almost completely attenuated.

Wireless Acoustic Emission Sensor Network for Structural Monitoring

The paper presents a prototype wireless system for the detection of active fatigue cracks in aging railways bridges in real-time. The system is based on a small low-cost sensor node, called an AEPod, that has four acoustic emission (AE) channels and a strain channel for sensing, as well as the capability to communicate in a wireless fashion with other nodes and a base station. AEPods are placed at fracture-critical bridge locations. The strain sensor detects oncoming traffic and triggers the AEPod out of its hibernation mode. As the train stresses the fracture-critical member, acoustic emission and strain data are acquired. The data are compressed and filtered at the AEPod and transmitted off the bridge using cell-phone communication.

Rapid Inspection of Composite Skin-Honeycomb Core Structures with Ultrasonic Guided Waves

Guided wave inspection of composite skin-honeycomb core structures is an efficient and sensitive alternative to other common inspection methods. This paper shows that sweeping experimentally through the dispersion curves is an effective way to experimentally locate guided wave modes sensitive to skin-core delamination. Composite skin-Nomex honeycomb core specimens were developed with simulated delaminated areas. The delaminated areas were detected with guided waves and confirmed with conventional ultrasonic testing methods. Calculated phase velocity dispersion curves are given to define the practical phase velocity and frequency ranges. Example wave structures in this range are given to illustrate the change in sensitivity as frequency is swept for a given mode.

Fouling detection in the food industry using ultrasonic guided waves

Ultrasonic guided waves can propagate over long distances in process pipe and have demonstrated excellent sensitivity to viscous coatings on the inner surface of piping commonly used in the processing of dairy products. This technology shows excellent potential to help upgrade plant maintenance from a time based to a condition based schedule. The design, fabrication, and installation of new piezopolymer guided wave sensors is discussed with emphasis on their affordability.

Flexible piezopolymer ultrasonic guided wave arrays

Ultrasonic guided wave technology is being applied to a variety of gas and liquid transmission pipeline inspection applications. There are a variety of promising transduction techniques used to excite longitudinal, torsional, and flexural modes in pipe. Some of the more common methods include electromagnetic-acoustic, magnetostrictive, and piezoceramic array transducers. The objective of the work presented in this paper was to develop an array design that is simpler to manufacture and attach to pipelines compared to the current piezoceramic design. The design considerations for a flexible piezopolymer-based array are discussed in this paper along with the basic principles behind the selection of the array element width and spacing. The performance of a piezoceramic and piezopolymer array, with identical element spacing and width, are compared at four different frequencies. Tests were undertaken on a carbon steel pipe with a simulated defect. Evaluation of the different arrays was performed in terms of the defect response, in terms of amplitude, of the lower-order axisymmetric modes. It is shown that while the piezopolymer array provides comparable sensitivity to the piezoceramic array, the amplitude of the signals reflected from the simulated defect are 30 dB lower compared to those generated using the piezoceramic array.

Interfacing guided-wave ultrasound with wireless technology

Guided wave ultrasound is a very powerful and reliable nondestructive testing technique. The emerging smart structure health monitoring strategies demand a wireless sensor for most applications. Passive sensor interfacing with wireless technology is advanced due mainly to the mW power requirements of such sensors. Guided wave sensors, on the other hand, are active sensors that require orders of magnitude more power than the typical passive sensor. Consequently, the design of the sensor, embedded electronics, and adjacent power source become more complicated. Sensor accessories can be minimized by locating zones on the phase dispersion curves where modes are efficiently generated. In this paper, this concept formulated via the source influence phenomenon. Experimentation focuses on quantifying the activation power requirements in different zone of the dispersion curves.

Railway Bridge Structural Health Monitoring Using Wireless Acoustic Emission Sensor Network

Acoustic emission monitoring of fracture-critical components of load-bearing structures such as railway bridges, provides a valuable input to condition-based maintenance planning and the allocation of maintenance resources. This paper presents a prototype wireless acoustic emission (AE) sensor network system for the structural health monitoring of aging steel railway bridges. It involves an AE sensor node, consisting of four AE channels and one strain gage input channel. Advances in data acquisition and communications instrumentation and the application of the fundamentals of fracture mechanics and artificial intelligence in signal processing have led to the development of a comprehensive, integrated system for acoustic emission monitoring. An active fatigue crack, when stressed due to the load of the on-coming freight train, emits AE. The strain gage channel senses the oncoming train and wakes up the AE channels from their hibernation mode. The wireless instrument digitizes the information, analyzes, compresses, and then transmits the filtered information to a central processing station for review by bridge engineers.Copyright

Self-sustaining Wireless Acoustic Emission Sensor System for Bridge Monitoring

A novel approach to structural monitoring of bridges is presented. Acoustic emission sensing has been constrained to hardwired systems up till now because the processing of high bandwidth sensor data on multiple channels requires a lot of energy. The presented prototype wireless system for the real-time detection of active fatigue cracks in bridges overcomes this problem by utilizing a low-power Flash FPGA for signal processing, a novel vibration energy harvester and a sophisticated sleep scheduler.

Characterization of Work Hardened Layers in Rails

As part of an infrastructure subject to increased magnitude and frequency of loads, railroad track systems require regular inspection to assure high reliability for the safety of the public and passengers and the safe and efficient movement of goods. Unattended work hardened layers on the surface of the rails are operational hazards due to the inevitability of rolling contact fatigue cracks. A novel method utilizing the higher order surface waves, also called as Sezawa waves, has been used to detect and characterize the work hardened layer in rails. The technique involves generating Sezawa waves using contact ultrasonic transducers and monitoring the cut-off frequency of these waves. Results from hardness tests and metallographic analyses on the work hardened layers are also reported. The tests using the Sezawa wave technology demonstrated the ability to interrogate and resolve traffic-hardened layers in the depth ranges of 0–1 mm, 1–3 mm, and 4–7 mm. The minimum Brinell Hardness (HB) gradient, between the hardened layer and underlying rail, required to support Sezawa wave generation was also investigated and determined to be greater than 20 HB.Copyright