R. Farlow

Applications of through-air ultrasound for rapid NDE scanning in the aerospace industry

This work describes two separate applications of air-coupled ultrasonic nondestructive evaluation (NDE) for rapid inspection of aerospace components. First, air-coupled transducer arrays are used for through transmission scanning of relatively large preproduction test samples. In this case a focused array system was designed to facilitate faster inspection rates and at the same time match the resolution and performance of available water jet apparatus. The second approach is geared toward in situ inspection and involves single-sided scanning using a dual transducer configuration for generation of surface, shear, and Lamb waves within the test specimen. In both cases, the difficulties of practical air-coupled inspection are discussed and the design solutions presented for each application. Piezocomposite transducer technology, in conjunction with narrow-band low-noise electronics, constitutes the basis for achieving the required signal to noise ratio (SNR) to ensure robust operation within the industrial environment. A number of scan images, performed on realistic samples, are shown and the results compared with those obtained using alternative methods. Excellent image quality is demonstrated from real time scanning and without recourse to any off-line data processing.

Micromachining of a piezocomposite transducer using a copper vapor laser

A 1-3 piezocomposite transducer with front face dimensions of 2/spl times/2 mm has been micromachined using a copper vapor laser. The device consists of PZT5A piezoceramic pillars with a 65-/spl mu/m pitch suspended in a low viscosity thermosetting polymer. The kerf width is 13 /spl mu/m, and the transducer thickness is 170 /spl mu/m, making the device suitable for ultrasonic reception at frequencies close to 10 MHz.

The design of embedded transducers for structural health monitoring applications

This paper describes a theoretical and experimental investigation concerning embedded piezoelectric transducers employed principally for condition monitoring of engineering composites. Both interdigital transducers (IDTs) and plate transducers are investigated with the aim of assessing their efficiency as uni-modal Lamb wave transmitters. The IDT configuration comprises a piezocomposite layer sandwiched between two flexible printed circuit boards, where the interdigital electrode spacing corresponds to the wavelength of the desired Lamb wave mode. The alternative configuration comprises a thin piezoceramic plate for which the lateral dimensions are chosen to efficiently couple energy into the desired mode. For both types of transducer, finite element models have been successfully employed to establish the design requirements for generating the zero order symmetrical mode (So) without simultaneously generating the zero order anti-symmetrical mode (Ao), which exhibits strong velocity dispersion. In this investigation the Ao mode is regarded as coherent noise. Generation of a pure So mode is shown to require positioning of the transducer at a depth which is exactly half way between the top and bottom faces of the plate-like structure within which it is embedded. For structural monitoring, the plate-type transducer is shown to be more suitable than the IDT. A scanning laser vibrometer was used to verify many of the theoretical findings.

Advances in air coupled NDE for rapid scanning applications

Results obtained during experiments with a state of the art NDE system are presented. These results demonstrate the sensitivity that can be achieved with modem piezoceramic composite transducers. One set of experiments was concerned with the through transmission inspection of composite sections. These included thick honeycomb structures such as those used in the aircraft industry. The work involved a study of the influence of multiple reflections as a function of transducer airgap separation. Also investigated were the generation and detection of shear waves, Rayleigh waves and Lamb waves. The Lamb wave technique has been used to demonstrate the feasibility of producing real-time images of defects in thin plates of various materials including carbon fibre and aluminium

Comparison of different piezoelectric materials for the design of embedded transducers for structural health monitoring applications

This paper describes the design requirements for embedded piezoelectric plate transducers suitable for generating efficient, uni-modal Lamb wave signals in plate structures. Three piezoelectric materials are considered, modified lead titanate, lead zirconate titanate, and polyvinylidene difluoride, in conjunction with different 1-3 piezoelectric composite configurations. Finite element modelling was used to predict the various Lamb wave modes generated by transducers embedded in a hard-set epoxy plate. Accordingly, the design requirements for successful generation of the zeroth order symmetric (S/sub 0/) Lamb wave mode have been identified. It is shown that the transducer position and the orientation within the host structure, and the appropriate choice of the excitation frequency of the Lamb wave, relative to the structural resonances, are the crucial design parameters for the success of Lamb wave mode selectivity. The amplitude of the Lamb wave signal can be optimised by appropriate choice of the transducer lateral dimension. To validate the theoretical findings, transducers were embedded in both hard-set epoxy and uni-directional carbon fibre plates. Successful generation of the S/sub 0/ mode is demonstrated by means of a scanning laser vibrometer.

The minimum signal force detectable in air with a piezoelectric plate transducer

A theoretical analysis based on the Johnson noise equation and an established transducer model has revealed a simple mathematical expression for the minimum signal force detectable in air with an open–circuit piezoelectric plate transducer operating in its thickness mode. A significant finding is that, except for any frequency dependence associated with a transducers intrinsic losses, the minimum detectable signal force is independent of frequency. By contrast, the sensitivity (e.g. volts per unit signal force) is known to be a strong function of frequency, with the principal peak being at the plates fundamental thickness resonance. The results are explained by showing that the new equation for minimum detectable force (MDF) is equivalent to the mechanical version of the Johnson noise equation. Both the Johnson noise equation and its mechanical equivalent are consistent with a generalized theory of thermal noise, which is sometimes referred to as the fluctuation–dissipation theorem. It is now evident that the mechanical equivalent of the Johnson noise equation provides a useful starting point from which many other device–specific MDF equations may be derived with relative ease. This approach is not restricted to piezoelectric transducers and can be applied regardless of whether the device is intended for operation in a solid, liquid or gaseous medium.

Micromachined unimorphs and bimorphs

A frequency doubled copper vapour laser (CVL) operating at an ultraviolet wavelength of 271 nm has been used to cut miniature cantilevers in laminated composites incorporating at least one layer of the ferroelectric polymer polyvinylidene fluoride (PVDF). Devices have been fabricated from PVDF with thickness values in the range 9 /spl mu/m to 38 /spl mu/m. Laser vibrometer measurements show that the fundamental resonance frequencies of these devices correspond almost exactly with values obtained from mechanical beam theory. Ultrasonic transducers consisting of bimorph and unimorph arrays are currently being designed with the aid of a finite element model, which is not discussed.

Embedded ultrasonic sensors for monitoring structural integrity

The in situ monitoring of structural integrity in large structures requires a substantial number of sensors. This presents problems especially due to the complexity of their electrical connections. This paper describes a monitoring system based on embedded ultrasonic sensors, which are interfaced using a license-exempt telemetry band to a remote data acquisition system. The sensors themselves are designed for the inspection of carbon fiber composite plates and involve the embedding of piezoelectric and piezocomposite components as well as acoustic fiber waveguides.

An evaluation of different piezoelectric materials for ‘SMART’ structural monitoring applications: The issue of structural integrity in the host structure and mechanical compatibility of embedded transducers

A two-dimensional, finite-element model has been developed for calculating the performance of an embedded, pre-encapsulated, plate transducer for generating So Lamb waves in a uni-directional carbon-fiber plate. For optimum performance, the plate must be centrally positioned and tolerance to deviations away from this point is dependent upon the design of the embedded transducer. Another model was used to predict the relative values of the inter-laminar stresses developed at the inter-faces of adjacent plies of the plate and the stress concentrations around the embedded device under a bending load.

Comparison of piezoceramic, piezopolymer and single crystal materials for use in micromachined bimorph structures

Miniature bimorph and unimorph structures provide a novel approach to the generation of ultrasonic signals in a gaseous medium such as air. It is shown that such devices can be constructed from piezoceramic, piezopolymer and single crystal materials. Fabrication involves the use of a micromachining system based on a copper vapour laser. The fundamental wavelength of this laser is 511nm, however its versatility is greatly enhanced by the utilisation of a frequency-doubling cell that facilitates operation at ultraviolet wavelengths. It is shown that the performance of a device fabricated form any one of the previously mentioned materials can be accurately predicted using mutually consistent displacement and natural frequency equations that are the product of a recently developed theory.