Edison Cristofani

Nondestructive testing potential evaluation of a terahertz frequency-modulated continuous-wave imager for composite materials inspection

Abstract. The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.

A multibeam opportunistic SAR system

This paper discusses the implementation of a passive radar system designed to perform imaging of the ground using radar satellites as emitters of opportunity. The receiver consists in a 4-element microstrip antenna array and is able to receive the signals from most C-band Synthetic Aperture Radar satellites. The array antenna is used to attenuate the direct-path signal by null-steering. The successive processing steps are described with focus on the signal synchronization, signal separation and the image formation algorithm. The current results are presented and discussed.

Compressed sensing mm-wave SAR for non-destructive testing applications using side information

This paper evaluates the applicability of an innovative strategy for applying Compressed Sensing (CS) on Synthetic Aperture Radar (SAR) imaging, in the mm-wave range, using prior or structural side information. The studied technique adds the side information to the conventional CS minimization problem using an l1-l1 minimization approach, allowing for lower sub-Nyquist sampling than standard CS predicts. The applicability of this strategy on ultra-wideband SAR measurements is tested through simulations and real Non-Destructive Testing (NDT) experiments on a 3D-printed polymer object.

Applicability of compressive sensing on three-dimensional terahertz imagery for in-depth object defect detection and recognition using a dedicated semisupervised image processing methodology

Abstract. The quality control of composite multilayered materials and structures using nondestructive tests is of high interest for numerous applications in the aerospace and aeronautics industry. One of the established nondestructive methods uses microwaves to reveal defects inside a three-dimensional (3-D) object. Recently, there has been a tendency to extrapolate this method to higher frequencies (going to the subterahertz spectrum) which could lead to higher resolutions in the obtained 3-D images. Working at higher frequencies reveals challenges to deal with the increased data rate and to efficiently and effectively process and evaluate the obtained 3-D imagery for defect detection and recognition. To deal with these two challenges, we combine compressive sensing (for data rate reduction) with a dedicated image processing methodology for a fast, accurate, and robust quality evaluation of the object under test. We describe in detail the used methodology and evaluate the obtained results using subterahertz data acquired of two calibration samples with a frequency modulated continuous wave system. The applicability of compressive sensing within this context is discussed as well as the quality of the image processing methodology dealing with the reconstructed images.

In-depth high-resolution SAR imaging using Omega-k applied to FMCW systems

Imagery applied to non-destructive testing implicitly includes the ability of imaging defects or foreign inclusions inside materials. Within a certain frequency range some nonmetal materials become highly transparent and only media interfaces or defects will scatter signals back to the sensor. Three-dimensional in-depth imaging is therefore possible and synthetic aperture processing can be applied to compensate cost-efficiently several main disadvantages present in typical high-resolution microwave imagery systems using focused beams. This work investigates under the framework of the DOTNAC Project (an FP7 project funded by the European Commission) the possibilities of applying synthetic aperture radar processing to a high-resolution frequency-modulated continuous-wave system for non-destructive testing purposes. The Omega-k range migration algorithm is used to perform efficient range migration of the raw data. This paper shows in-depth SAR images from real composite materials including ad-hoc defects. Assessment of results as well as discussion on the proposed 3-D in-depth imaging system will be presented.

Continuous wave terahertz inspection of glass fiber reinforced plastics with semi-automatic 3-D image processing for enhanced defect detection

The increasing industrial application of fiber reinforced plastics demands developments of new techniques for non-destructive testing. The sub-terahertz frequency band has proved to be a noteworthy option for this task. Composite structures or laminates can be inspected for foreign inclusions, delaminations, debonds, etc., using sub-terahertz sensors during the manufacturing process or maintenance. In this contribution we present our results using a frequency modulated continuous wave terahertz imaging system in comparison with conventional NDT measurements on several different GFRP samples. Thereby a semi-automatic terahertz image processing approach for enhanced defect detection is applied.

Performance of 2D compressive sensing on wide-beam through-the-wall imaging

Compressive sensing has become an accepted and powerful alternative to conventional data sampling schemes. Hardware simplicity, data, and measurement time reduction and simplified imagery are some of its most attractive strengths. This work aims at exploring the possibilities of using sparse vector recovery theory for actual engineering and defense- and security-oriented applications. Conventional through-the-wall imaging using a synthetic aperture configuration can also take advantage of compressive sensing by reducing data acquisition rates and omitting certain azimuth scanning positions. An ultra-wideband stepped frequency system carrying wide beam antennas performs through-the-wall imaging of a real scene, including a hollow concrete block wall and a corner reflector behind it. Random downsampling rates lower than those announced by Nyquists theorem both in the fast-time and azimuth domains are studied, as well as downsampling limitations for accurate imaging. Separate dictionaries are considered and modeled depending on the objects to be reconstructed: walls or point targets. Results show that an easy interpretation of through-the-wall scenes using the l1-norm and orthogonal matching pursuit algorithms is possible thanks to the simplification of the reconstructed scene, for which only as low as 25% of the conventional SAR data are needed.

3-D radar image processing methodology for Non-Destructive Testing of aeronautics composite materials and structures

Recently, there has been a significant interest in employing high frequency radar imagery for many industrial oriented and security applications. The quality control of aeronautics composite multi-layered materials and structures through Non-Destructive Testing (NDT) is the main focus of this study. The main motivations for using this technology are: it allows penetration of most non-metal and non-polarized materials, it provides the ability for three-dimensional (3-D) imagery and in-depth information, and the millimeter waves (mmW) pose no health risk to the operator. The main goal of this research is to develop an integrated, semi-automatic and near real-time mode-operated image processing methodology for frequency-modulated continuous-wave (FMCW) millimeter wave images with center frequencies around 100 GHz and 300 GHz. The proposed methodology firstly foresees to process reflectance and transmittance 3-D imagery by extracting areas of interest and objects boundaries at two operated frequencies. Then, the detected areas are subjected to a multi-source database and integrated by a decision tree algorithm. The fused information is used to identify defects and in-homogeneities within the objects. Finally, the post-processing phase examines and evaluates the spatial accuracy of the extracted information.

Strategies for the calibration of an array of patch antennas in passive bistatic SAR imaging

One of the main challenges in opportunistic bistatic SAR imaging is the synchronisation of the receiver with the transmitter of opportunity. By adaptive spatial beamforming, we can retrieve the reference signal needed to perform the synchronisation in SAR imaging. This processing also attenuates the direct path interference. In this paper, a four-element patch array antenna is considered. In addition, this phased-array can be used to reject interferences coming from external sources. The calibration of the receiver is of utmost importance considering on the one hand the significant mutual coupling between the antenna elements and on the other hand the differences in phase and gain between the reception chains. This paper discusses two calibration strategies and compares their performance on experimental data.

Compressed Sensing mm-Wave SAR for Non-Destructive Testing Applications Using Multiple Weighted Side Information

This work explores an innovative strategy for increasing the efficiency of compressed sensing applied on mm-wave SAR sensing using multiple weighted side information. The approach is tested on synthetic and on real non-destructive testing measurements performed on a 3D-printed object with defects while taking advantage of multiple previous SAR images of the object with different degrees of similarity. The tested algorithm attributes autonomously weights to the side information at two levels: (1) between the components inside the side information and (2) between the different side information. The reconstruction is thereby almost immune to poor quality side information while exploiting the relevant components hidden inside the added side information. The presented results prove that, in contrast to common compressed sensing, good SAR image reconstruction is achieved at subsampling rates far below the Nyquist rate. Moreover, the algorithm is shown to be much more robust for low quality side information compared to coherent background subtraction.