This paper presents a two-stage numerical model of a meander coil electromagnetic acoustic transducer transmitter operating on the Lorentz force principle. This model was used to predict the ultrasonic A-scan signals for bulk (longitudinal and shear) and guided (Rayleigh) waves in isotropic homogeneous materials. The simulation results agree with the experimental results. The grating effects of the meandering coil, for different coil spacings, are also discussed in this paper.

A two-stage finite element model of a meander coil electromagnetic acoustic transducer transmitter

Process Analytical Technology for the Food Industry

A pair of noncontact Electromagnetic Acoustic Transducers (EMATs) has been used for thickness measurements and imaging of metallic plates. This was performed using wide bandwidth EMATs and pulse-compression signal processing techniques, using chirp excitation. This gives a greatly improved signal-to-noise ratio for air-coupled experiments, increasing the speed of data acquisition. A numerical simulation of the technique has confirmed the performance. Experimental results indicate that it is possible to perform noncontact ultrasonic imaging and thickness gauging in a wide range of metal plates. An accuracy of up to 99% has been obtained for aluminum, brass, and copper samples. The resolution of the image obtained using the pulse compression approach was also improved compared to a transient pulse signal from conventional pulser/receiver. It is thus suggested that the combination of EMATs and pulse compression can lead to a wide range of online applications where fast time acquisition is required.

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Application of pulse compression signal processing techniques to electromagnetic acoustic transducers for noncontact thickness measurements and imaging

Transient ultrasonic Rayleigh wave generation by pulsed electromagnetic coils operating via the Lorentz force mechanism is investigated. Optimal coil design is calculated by considering the entire ultrasonic generation system including the pulse generator, coil shape and size, and the electromagnetic coupling between coil and metal sample. The study is carried out through theoretical modelling and comparison with experimental measurements taken using an 80 MHz bandwidth Michelson laser interferometer as a detector. The time dependent Lorentz force generated by the electromagnetic acoustic transducer (EMAT) is modelled using finite element calculations, and the resultant ultrasonic field is calculated using an efficient numerical algorithm. Excellent correlation is observed between the simulated waveforms and experimental measurements. This demonstrates how the approach reported here will greatly assist in optimising the design of generating EMAT for specific applications.

Optimising ultrasonic wideband Rayleigh wave generation by pulsed electromagnetic coils

The evaluation of adhesively bonded structures has been a challenge over the several decades that these structures have been used. Applications within the aerospace industry often call for particularly high performance adhesive bonds. Several techniques have been proposed for the detection of disbonds and cohesive weakness but a reliable NDE method for detecting interfacial weakness (also sometimes called a kissing bond) has been elusive. Different techniques, including ultrasonic, thermal imaging and shearographic methods, have been proposed; all have had some degree of success. In particular, ultrasonic methods, including those based upon shear and guided waves, have been explored for the assessment of interfacial bond quality. Since 3-D guided shear horizontal (SH) waves in plates have predominantly shear displacement at the plate surfaces, we conjectured that SH guided waves should be influenced by interfacial conditions when they propagate between adhesively bonded plates of comparable thickness. This paper describes a new technique based on SH guided waves that propagate within and through a lap joint. Through mechanisms we have yet to fully understand, the propagation of an SH wave through a lap joint gives rise to a reverberation signal that is due to one or more reflections of an SH guided wave mode within that lap joint. Based upon a combination of numerical simulations and measurements, this method shows promise for detecting and classifying interfacial bonds. It is also apparent from our measurements that the SH wave modes can discriminate between adhesive and cohesive bond weakness in both Aluminum-Epoxy-Aluminum and Composite-Epoxy-Composite lap joints. All measurements reported here used periodic permanent magnet (PPM) Electro-Magnetic Acoustic Transducers (EMATs) to generate either or both of the two lowest order SH modes in the plates that comprise the lap joint. This exact configuration has been simulated using finite element (FE) models to describe the SH mode generation, propagation and reception. Of particular interest is that one SH guided wave mode (probably SH0) reverberates within the lap joint. Moreover, in both simulations and measurements, features of this so-called reverberation signal appear to be related to interfacial weakness between the plate (substrate) and the epoxy bond. The results of a hybrid numerical (FE) approach based on using COMSOL to calculate the driving forces within an elastic solid and ABAQUS to propagate the resulting elastic disturbances (waves) within the plates and lap joint are compared with measurements of SH wave generation and reception in lap joint specimens having different interfacial and cohesive bonding conditions.

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An EMAT-based shear horizontal (SH) wave technique for adhesive bond inspection

Directional characteristics of electromagnetic acoustic transducers

The directivity patterns of a wide range of electromagnetic acoustic transducers (EMATs) have been determined experimentally using a second, spiral coil EMAT as the detector around a cylindrical surface. A qualitative comparison to theory is presented, together with a discussion of the generation mechanisms involved. In addition, the use of a dipolar shear wave EMAT detector with a pulsed laser source is presented and some applications are discussed.

J.K. Hu

Papers

Directional characteristics of electromagnetic acoustic transducers

Directional characteristics of electromagnetic acoustic transducers