Roger L. Royer

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.

Ultrasonic Guided Wave Imaging Techniques in Structural Health Monitoring

Ultrasonic guided waves, due to their capability of interrogating a large structure from a single sensor position, have been proven as a promising tool for structural health monitoring. In this article, we present two imaging approaches of utilizing guided wave leave-in-place sensors for early detection of defects in plate-like structures. The first approach is based on a guided wave tomographic technique, in which the region surrounded by a sparse sensor array is monitored. The second one is a phased array approach, in which sensors are attached to a structure in a compact format to form an array. The region subjected to inspection and monitoring is the region outside the array. Both techniques have shown excellent capability of determining damage size, location, and severity.

Ultrasonic Lamb wave tomography in structural health monitoring

Variations of Lamb wave propagation reflect some changes in effective thickness and material properties caused by such structural flaws as corrosion, fatigue cracks, disbonds and voids that can then be mapped via a reconstructed tomographic image. Ultrasonic Lamb wave tomography can be used to evaluate structural integrity based on the variations in features extracted from measurements made by a transducer array from a reference point in time. In this paper, several tomographic imaging techniques, such as the filtered backprojection algorithm, the algebraic reconstruction technique and the reconstruction algorithm for probabilistic inspection of damage are compared, and the advantages and drawbacks of these methods, as well as practical considerations such as reconstruction fidelity, quality, efficiency and the minimum number of sensors required for each array geometry, are discussed, and some application examples are given.

Ultrasonic De-Icing of Wind-Tunnel Impact Icing

Ultrasonic excitation has proven to provide ice-interface transverse shear stresses exceeding the adhesion strength of freezer ice to various metals, promoting instantaneous ice delamination. Wind-tunnel impact ice presents challenges that are not encountered when removing freezer ice. The low-power, nonthermal ultrasonic de-icing concept is investigated under impact-icing conditions in an icing wind tunnel. In this research effort, ultrasonic actuator disks excite isotropic plates and airfoil-shaped structures that are representative of helicopter leading-edge protection-cap shapes. Off-the-shelf ultrasonic actuators are used to create ice-interface shear stresses sufficient to promote instantaneous ice delamination of thin layers of impact ice (less than 3 mm thick). A steel plate of 30.48 cm x 30.48 cm x 1 mm was actuated by three lead zirconate titanate disks excited at their ultrasonic radial mode. The ultrasonic vibration introduced transverse shear stresses that prevented ice formation on top of the actuator locations for a fraction of the power required with electrothermal systems used in helicopter rotor blades (0.18 W/cm 2 vs 3.8 W/cm 2 ). Experiments also showed ice delamination in areas of the plates where transverse shear stresses were concentrated. As ice thicknesses reached a critical value of approximately 1.2 mm, ice debonded from those steel-plate areas. A model of the three disk actuated steel plate was created and correlated with experimental results observed during impact-icing test experiments. Both, the predicted ultrasonic modes of the system and the ice-shedding areas agreed with experimental results. In addition, a second set of experiments involving NACA 0012 airfoil-shaped structures were conducted. Actuators located on the top and bottom surfaces of the leading-edge cap were actuated with an input power as low as 200 W (32 kHz ultrasonic mode). Thin layers of ice (less than 2 mm thick) constantly delaminated from the leading edge of the airfoil on those regions where stress concentrations were predicted.

Instantaneous De-Icing of Freezer Ice via Ultrasonic Actuation

A low-power, nonthermal, ultrasonic de-icing system is introduced as a potential de-icing system for helicopter rotor blades. In this research effort, ultrasonic actuators excite isotropic plates and airfoil-shaped structures that are representative of helicopter leading-edge protection caps. The system generates delaminating ultrasonic transverse shear stresses at the interface of accreted ice, debonding thin ice layers (less than 3 mm thick) as they form on the isotropic host structure. A finite element model of the proposed actuator and the isotropic structures with accreted ice guides the selection of the actuator prototypes. Several actuator-isotropic plate structures are fabricated and tested under freezer ice conditions. Test results demonstrate that radial resonance disk actuators (28-32 kHz) create ultrasonic transverse shear stresses capable of instantaneously delaminating ice layers. The finite element modeling predicts the delamination patterns of the accreted ice layers. Models also predict (within 15%) the required input voltage to promote instantaneous ice debonding. At environment temperatures of -20°C, the system delaminates 2.5-mm-thick ice layers with power input densities as low as 0.07 W/cm 2 (0.5 W/in. 2 ).

A Portable Hybrid Ultrasound-Eddy Current NDI System for Metal Matrix Composite Track Shoes

Abstract : Track shoes made of Metal Matrix Composite (MMC) are light in weight, and can resist high temperature and wear. Defects such as disbond, cracks and porosity can be introduced during the manufacturing process and while in service. Presented in this paper is a portable nondestructive inspection (NDI) system to automatically inspect the tank track shoes for disbond, cracks and porosity defects. The work focuses on the inspection of the track shoe center spline where MMC inserts are attached to the aluminum substrate. A hybrid approach has been developed where an array of broadband high frequency ultrasonic transducers operating in a pulse/echo mode are utilized to detect disbond, and a scanning eddy current probe array is used to detect cracks and porosity. The inspection results agree quite well with immersion ultrasonic C-scan images and destructive tests.

Wireless ultrasonic guided wave tomography for corrosion monitoring

Structural Integrity of predetermined critical zones in a structure is of growing interest in the non-destructive testing (NDT) and structural health monitoring (SHM) communities. Quite often the presence of defects does not imply the end of life of the underlying structure and it could be economical to continue using the structure until the damage severity reaches a point where it can no longer be used. For structures like pipelines and aircraft in which a failure can be catastrophic, it is extremely important to monitor continuously any defects on the structure. Leave in place sensors provide a convenient way to embed the sensors permanently on the structure to monitor periodically and to establish its integrity. Wireless sensing units provide a robust means to regularly monitor a structure and return the data to a central data collection infrastructure. In this study we explore the design of a wireless tomographic imaging system that uses Lamb wave propagation characteristics on a structure to map accurately the material loss zones due to corrosion in the area enclosed by the sensors. The wireless unit has an actuator to excite the piezoceramic sensors and the computational capability to interrogate the signals in real time and to communicate the information. The capability of the system is illustrated by testing it on an aluminum plate with simulated corrosion damage and the results are presented.