Non-destructive ultrasonic testing: increased speed and sensitivity

Citation

(1998), "Non-destructive ultrasonic testing: increased speed and sensitivity", Sensor Review, Vol. 18 No. 1. https://doi.org/10.1108/sr.1998.08718aaf.007

Non-destructive ultrasonic testing: increased speed and sensitivity

Non-destructive ultrasonic testing: increased speed and sensitivity

Non-destructive ultrasonic testing is used widely in industry in France and especially in the fields of nuclear technology, aeronautics and transport. Parts are usually tested under water.

Defects are detected by the way in which they reflect ultrasound waves. A piezo-electric transducer emits a burst of sound. In reception mode it picks up the echoes from any impurities in the material. By moving the transducer and running data acquisition and processing software it is possible to obtain a cartographic printout of the material being examined.

The diversity of parts to be examined, the complexity of their shape, the great thicknesses often involved and the fact that parts can actually be made up of several different materials led NDT Systems SA to design modular test equipment. The ultrasound beam is controlled electronically and the width of the beam, the focal point and the angle can all be pre-set according to requirements.

The system is based on networks of transducers designed by the Imasonic company to emit and receive ultrasound. A network can consist of up to 128 piezo-electric and polymer transducers arranged in rows, rings, etc.

The more elements are activated the larger the beam width (or aperture). The focal point can be adjusted by programming in emission delays from one element to the next. Any lack of symmetry in the transmission delay curve changes the direction of the beam. In this way hard-to-reach zones can be inspected without the need to physically move the transducer modules.

A few months ago the French Iron and Steel Research Institute (IRSID) ­ the USINOR group's joint research centre ­ acquired NDTs "All-in" system to increase test productivity without compromising on accuracy. Steel plate output from continuous casting are inspected using samples which are planed off to spread out any inclusions. Defects measuring as little as 80 µm can be detected. A transducer consists of in-line modules holding a total of 128 components running at 15MHz. The equipment has an electronic scanning mechanism which works by activating successive groups of components. At each pass the transducer sweeps a 50 turn-wide band. Inspecting samples in this way is eight times faster than using the traditional spot method.

Auto-focusing on a defect

If the shape of the parts changes during an inspection ­ for example, if cracks are detected under an irregular coating or on inspection of misaligned welds ­ the above-mentioned focusing techniques will fail to produce satisfactory results.

The FAUST system

To meet the increasingly stringent requirements of safety bodies, the Atomic Energy Commissariat (AEC) has developed the FAUST system, a French acronym for adaptive tomographic ultrasound focusing. This system can inspect parts of varying shapes and sizes. First, the multi-component transducer spots the defect in the mid-definition range. Depending on the response and using a theoretical situation model, it then adjusts its behaviour in real time, changing the depth of field and signal amplitude to obtain the best possible signal-to-noise ratio. This is the first intelligent transducer. The AEC has granted operating licences for this process to NDT Systems SA and Intercontrôle.

Double wave reflection

The Waves and Acoustics Laboratory at the University of Paris has adopted a wholly different approach. In association with the National Company for Aeroengine Design and Manufacture (SNECMA), the researchers have produced a system for identifying defects in titanium engine parts. The problem in this specific case resides in locating faults which reflect relatively little sound in a generally noisy structure. This requires concentrating an ultrasound beam on a suspected defect in a heterogeneous material irrespective of the shape and size of the part concerned.

Imasonic and the Philips Electronics Laboratory designed networks of 128-component transducers positioned in matrices 60-120 mm across. The aspherical surface of the array was carefully calculated to counteract any aberrations caused by passing the sound through titanium and water, acoustically two very different media.

Three transducers used for double wave reflection. The two on the left were made by Imasonic. The one on the right was made by Philips Electronics Laboratory

The zone inspected is targeted by the central components. The signals reflected back by any impurities are collected by all the receivers. They then send them back along the same paths in the reverse order to that in which they were picked up. The waves again converge on the defect. This enables the device to refocus more accurately on impurities. The Corelec-designed electronics on the prototype system helped identify defects measuring just 0.4 mm across in a 250-mm part, at a frequency of 5 MHz.

Piezo-composite transducers

In ultrasound technology, system performance is largely dependent on the specifications of the transducer. Transducers are now capable of working in transmit and receive mode. Performance is further enhanced in the 250kHz-20MHz range if they are manufactured from a composite material.

Tiny cylindrical ceramic rods are bonded in a polymer matrix to give the transducers a lower acoustic impedance than ordinary ceramic and a higher coupling coefficient. This technology makes it possible to design multi-component transducers (up to 128) in row, ring and matrix configurations. Although highly sensitive and having a large bandwidth, each component still benefits from a high level of acoustic insulation which ensures top-quality electronic focusing.

Moreover, the material structure can be optimised depending on the intended application because characteristics such as acoustic impedance and coupling coefficient vary according to the amount of ceramic in the composite.

Expert analysis of irradiated materials using an acoustic microscope

The materials used in the building of a nuclear reactor, steels for the most part, are exposed to radiation. In time this can cause distension and creep and can damage macroscopic properties such as hardness and corrosion resistance.

Irradiated materials are analysed in the "high activity" cells of the Irradiated Materials Inspection department (SCMI) at the Chinon nuclear site. A special, remote-controlled acoustic microscope has been built for use in these cells by the Acoustique Metrologie Company and Montpellier University's Interface Analysis and Nanophysics Laboratory. All the equipment in the protected area has been carefully designed to be handled by robots and to withstand the hostile environment.

The acoustic microscope here is used mostly to obtain acoustic signatures, data whose interpretation reveals valuable information about longitudinal, transverse, surface propagation speeds.

The signature is obtained by varying the distance between the transducer and the sample. The transducer is made up of a zinc oxide piezo-electric component and a glass delay line out of which the lens is machined. It works with a wide aperture. The bandwidth is very low and is centred on a single frequency. The curves plotted represent the interference between the waves reflected by the sample to the focal point and the waves sent back again to the transducer via the material/liquid interface (Rayleigh waves).

The results obtained on elements taken from the core of a 900MW pressurised water reactor helped characterise the acoustic parameters of damaged materials. Thus, for a particular type of part it can be shown that the surface speed (Rayleigh waves), which is dependent on Young's modulus, drops off considerably the more radiation the sample has been exposed to.