Fundamental wave amplitude difference imaging for detection and characterization of embedded cracks
An ultrasonic technique for imaging nonlinear scatterers, such as partially-closed cracks, buried in a medium has been recently proposed. The method called fundamental wave amplitude difference (FAD) consists of a sequence of acquisitions with different subsets of elements for each line of the image. An image revealing nonlinear scatterers in the medium is reconstructed line by line by subtracting the responses measured with the subsets of elements from the response obtained with all elements transmitting. In order to get a better insight of the capabilities of FAD, two metallic samples having a fatigue or thermal crack are inspected by translating the probe with ultrasonic beam perpendicular (i.e. parallel) to the crack direction which is the most (i.e. less) favorable case. Each time, the responses of the linear scatterers (i.e. conventional image) and nonlinear scatterers (i.e. FAD image) are compared in term of intensity and spatial repartition. FAD exhibits higher detection specificity of the crack with a better contrast than conventional ultrasound imaging. Moreover, we observe that both methods give complementary results as nonlinear and linear scatterers are mostly not co-localized. In addition, we show experimentally that FAD resolution in elevation and lateral follows the same rule as the theoretical resolution of conventional ultrasonic technique. Finally, we report that FAD gives the possibility to perform parametric studies which let the opportunity to address the physical mechanisms causing the distortion of the signal. FAD is a promising and reliable tool which can be directly implemented on a conventional open scanner ultrasound device for real-time imaging. This might contribute to its fast and wide spread in the industry.
Improvement of the total focusing method using an inverse problem approach
Imaging using the total focusing method (TFM) is a popular tool, which is becoming a standard, for nondestructive testing and evaluation. From full matrix capture data, it consists in focusing at each point of a defined reconstruction zone. It is generally more efficient than conventional phased array focusing, which only focuses at a few points. Despite its good image quality, TFM suffers from a lack of resolution and contrast, in particular in the case of close defects or in scattering materials. The TFM algorithm can be formalized as a linear operation on the data. The contribution of this paper is then to include a sparsity- inducing penalization to the TFM procedure. The reconstructed image is therefore supposed to contain only a few non zero values, corresponding to flaws or geometry. The final image is obtained by minimizing a penalized least-squares criterion within an iterative procedure. A first example uses data acquired from two close side drilled holes (SDH) in an aluminum block. The proposed algorithm shows good echo separation whereas the reference method suffers from overlapping, demonstrating resolution improvement. A second example comes from a stainless steel specimen, with high scattering noise level. The proposed method reduces the scattering noise and improves the contrast.
Total focusing method for ultrasonic imaging
This work deals with advanced and fast imaging techniques using phased array probes for non destructive evaluation or medical imaging. These methods employ a large amount of summations in order to focus at each pixel of the reconstruction image, which often represent a prohibitive computational cost. We present two acceleration methods, i.e. GPU computation and a migration approach. The GPU computing uses massively parallel computations. The migration approach works in the wavenumber domain and permits a significant improvement in terms of image quality. In this paper, we demonstrate the benefits of these techniques with experimental data captured from an aluminum block containing artificial flaws.
Total focusing method imaging for flaw characterization in homogeneous media
Phased array imaging using multi-element probes is an efficient technique to detect and characterize flaws in industrial components. In particular, the total focusing methods are advanced approaches that optimally focus at each point of the reconstruction zone. They generally outperform conventional imaging in terms of reconstruction quality and computational cost. They need all transmitter-receiver pair signals of an array of transducers. Then, the post-processing is performed by computing the proper propagation times at each reconstruction point and by applying coherent summations over all elements. A migration approach working in the wavenumber domain is analogous for total focusing. In this paper, we show experimental results of flaw characterization in homogeneous media using those total focusing methods. We demonstrate that the migration approach leads to a better signal to noise ratio and a better resolution for side drilled hole and horizontal slit reconstruction. On the other hand, the standard total focusing method achieves a better imaging of angled slits
Fast total focusing method for ultrasonic imaging
Synthetic aperture focusing technique (SAFT) and total focusing method (TFM) have become popular tools in the field of ultrasonic non destructive testing. In particular, they are employed for detection and characterization of flaws. From data acquired with a transducer array, those techniques aim at reconstructing an image of the inspected object from coherent summations. In this paper, we make a comparison between the standard technique and a migration approach. Using experimental data, we show that the developed approach is faster and offers a better signal to noise ratio than the standard total focusing method. Moreover, the migration is particularly effective for near-surface imaging where standard methods used to fail. On the other hand, the migration approach is only adapted to layered objects whereas the standard technique can fit complex geometries. The methods are tested on homogeneous pieces containing artificial flaws such as side drilled holes.
Full Matrix Capture with a Customizable Phased Array Instrument
In recent years, a technique known as Full Matrix Capture (FMC) has gained some headway in the NDE community for phased array applications. It’s important to understand that FMC is the method that the instrumentation acquires the ultrasonic signals, but further post-processing is required in software to create a meaningful image for a particular application. Having a flexible software interface, small form factor, excellent signal-to-noise ratio per acquisition channel on a 64/64 or 128/128 phased array module with FMC capability proves beneficial in both industrial implementation and in further investigation of post-processing techniques. This paper will provide an example of imaging with a 5MHz linear phased array transducer with 128 elements using FMC and a popular post-processing algorithm known as Total Focusing Method (TFM).
Phased Array UT Platform for Customizing Dedicated and Automated NDT Applications
Small form factor, excellent signal-to-noise ratio (SNR), fast data throughput and an easy to integrate electronics are just some of the important aspects for automated phased array inspection systems. Furthermore, a software application programming interface (API) is crucial for creating a dedicated and simple software front end. This paper will present how a family of phased array ultrasonic modules can meet the demands of the most unique and innovative industrial and research based phased array applications.