Sébastien Robert

Ultrasonic imaging of defects in coarse-grained steels with the decomposition of the time reversal operator

In the present work, the Synthetic Transmit Aperture (STA) imaging is combined with the Decomposition of the Time Reversal Operator (DORT) method to image a coarse grained austenitic-ferritic steel using a contact transducer array. The highly heterogeneous structure of this material produces a strong scattering noise in ultrasound images. Furthermore, the surface waves guided along the array interfere with the bulk waves backscattered by defects. In order to overcome these problems, the DORT method is applied before calculating images with the STA algorithm. The method consists in analyzing in the frequency domain the singular values and singular vectors of the full array transfer matrix. This paper first presents an analysis of the singular values of different waves contained in the data acquisition, which facilitates the identification of the subspace associated with the surface guided waves for filtering operations. Then, a filtered matrix is defined where the contribution of structural noise and guided waves are reduced. Finally, in the time domain, the STA algorithm is applied to this matrix in order to calculate an image with reduced structural noise. Experiments demonstrate that this filtering improves the signal-to-noise ratio by more than 12 dB in comparison with the STA image before filtering.

Adaptive ultrasonic imaging with the total focusing method for inspection of complex components immersed in water

In this paper, we propose an ultrasonic adaptive imaging method based on the phased-array technology and the synthetic focusing algorithm Total Focusing Method (TFM). The general principle is to image the surface by applying the TFM algorithm in a semi-infinite water medium. Then, the reconstructed surface is taken into account to make a second TFM image inside the component. In the surface reconstruction step, the TFM algorithm has been optimized to decrease computation time and to limit noise in water. In the second step, the ultrasonic paths through the reconstructed surface are calculated by the Fermats principle and an iterative algorithm, and the classical TFM is applied to obtain an image inside the component. This paper presents several results of TFM imaging in components of different geometries, and a result obtained with a new technology of probes equipped with a flexible wedge filled with water (manufactured by Imasonic).

Surface estimation methods with phased-arrays for adaptive ultrasonic imaging in complex components

Immersion ultrasonic testing of structures with complex geometries may be significantly improved by using phased-arrays and specific adaptive algorithms that allow to image flaws under a complex and unknown interface. In this context, this paper presents a comparative study of different Surface Estimation Methods (SEM) available in the CIVA software and used for adaptive imaging. These methods are based either on time-of-flight measurements or on image processing. We also introduce a generalized adaptive method where flaws may be fully imaged with half-skip modes. In this method, both the surface and the back-wall of a complex structure are estimated before imaging flaws.

Real time nondestrutive testing of composite aeronautical structures with a self-adaptive ultrasonic technique

In aeronautical industry, composite structures under testing often have complex and variable geometries. In this case, an optimal use of ultrasonic transducer arrays requires specific algorithms in electronic systems in order to achieve rapid and reliable inspections. To fulfil such requirements, a new real time and adaptive technique is presented. This technique is based on an iterative algorithm that does not require a prior knowledge of the geometrical and acoustical properties of the component undergoing inspection. All different parts of one given component (with flat, concave, convex parts...) can be controlled using a same transducer array, such as a conventional linear array with a flat shape. In this paper, the adaptive processing is demonstrated and illustrated through acquisitions performed with different typical aircraft composite structures.

Plane wave imaging for ultrasonic inspection of irregular structures with high frame rates

Ultrasonic imaging with high frame rates is of great interest to perform fast inspections and to reduce NDE costs. In this communication, we propose a new fast imaging method which is derived from the medical Plane Wave Imaging (PWI). The PWI method is applied to different immersion-testing configurations (specimens with plane or complex surfaces) and to different imaging modes (imaging with direct or half-skip wave paths) according to the type of defects (point-like or extended crack-types defects).

Ultrasonic Imaging in Highly Attenuating Materials With Hadamard Codes and the Decomposition of the Time Reversal Operator

In this paper, defects in a high density polyethylene pipe are imaged with the total focusing method. The viscoelastic attenuation of this material greatly reduces the signal level and leads to a poor signal-to-noise ratio (SNR) due to electronic noise. To improve the image quality, the decomposition of the time reversal operator method is combined with the spatial Hadamard coded transmissions before calculating images in the time domain. Because the Hadamard coding is not compatible with conventional imaging systems, this paper proposes two modified coding methods based on sparse Hadamard matrices with +1/0 coefficients. The SNRs expected with the different spatial codes are demonstrated, and then validated on both simulated and experimental data. Experiments are performed with a transducer array in contact with the base material of a polyethylene pipe. In order to improve the noise filtering procedure, the singular values associated with electronic noise are expressed on the basis of the random matrix theory. This model of noise singular values allows a better identification of the defect response in noisy experimental data. Finally, the imaging method is evaluated in a more industrial inspection configuration, where an immersion array probe is used to image defects in a butt fusion weld with a complex geometry.

Ultrasonic imaging in coarse-grained stainless steels by total focusing method

In the present work, the Total Focusing Method (TFM) is used to image flaws in coarse-grained steels with a contact phased-array probe. In order to reduce the noise introduced by the heterogeneous structure, as well as artifacts due to surface guided waves, the Decomposition of the Time Reversal Operator method is performed before calculating TFM images.