Optimize Synthetic Aperture Focusing

Advanced signal analysis techniques assist ultrasonic testing to maximize the information collected.

Ultrasonic testing of materials can be done in several ways using different types of instruments, ranging from thickness gages for localized measurements to multi-axis automated C-Scan scanners for imaging of complex parts. While these instruments can satisfy the needs of most applications, some situations require advanced signal analysis techniques in order to maximize the information collected from the tested structure.

One of the problems related to structure inspection is the correct identification of the ultrasonic echoes and time-of-flight related to defects. Differentiating between echoes from defects and from the nondefected surrounding areas of the material can be challenging, particularly when complex geometries are involved. In addition, time-of-flight or phase information of a single echo produced by a defect is usually insufficient to correctly determine the origin of this echo in space, and completing defect sizing is impossible. However, multiple echoes acquired from different probe locations along with numeric or synthetic focusing could contain enough information to localize defects in materials and display clearer cross-sectional B-scans and full C-Scan images.

Synthetic focusing concept focuses the echoes recorded from multiple probe locations at predefined focal spots. Correct delays add the recorded echoes on defected pixels, creating constructive interferences (top) while delays corresponding to a focal spot on a nondefected pixel results in randomly constructive and destructive interferences and weaker signals (bottom). Source: TecScan Systems Inc.

Synthetic aperture focusing is an imaging technique based on the principle of phase matching. It numerically synchronizes the time-of-flight collected from series of echoes using correct delays which are calculated from the inspected material properties. Because of ultrasonic beam spreading, a defect can be detected with series of echoes at different probe positions or apertures. Subsequently, different time-of-flight information is obtained for each echo in the aperture. Therefore, correct synthetic focusing of the echoes collected during a C-Scan inspection can maximize defect information such as its position within the material, shape, size and in some cases improve the detection of defects. This type of numerical focusing of data can be done off-line and final results are similar to the electronic focusing achieved by phased-array systems.

Synthetic aperture focusing and radar rely on the same theoretical basis, which uses scanning aperture to locate defects or objects from time-of-flight information. A defect (red) can be detected from multiple transducers because of beam divergence (blue lines). Source: TecScan Systems Inc.

Basic Principles

A single ultrasonic echo can be represented by multiple properties such as its center frequency, bandwidth, amplitude or energy, as well as time-of-flight or phase. All of these characteristics can provide valuable information about defect size or material attenuation, for example. However, when it comes to defect localization, only time-of-flight or phase are considered valuable information. Unfortunately, ultrasonic beams are never perfectly unidirectional and a defect can be the source of an echo for multiple probe positions. A single A-Scan is therefore generally insufficient to determine the exact location of a defect because a given time-of-flight can only provide information about the absolute distance from the transducer. However, a correct comparison of the echoes of a defect recorded from multiple probe positions can provide this information.

The principles of synthetic aperture focusing rely on the same basis as radar or phased-array imaging. In the case of the radar, sweeps of electromagnetic waves are sent in the space to cover a predetermined aperture. The electromagnetic waves are then reflected if they meet an electrically leading surface or if an obstacle is in the propagation direction. The position of this obstacle is then obtained by comparing the echoes recorded within the aperture.

In synthetic aperture focusing of ultrasounds, a transducer emitting ultrasonic waves is moved along the test piece in pulse-echo mode to create a scanning aperture. If a defect is present in the aperture, echoes can be generated for multiple scanning positions, allowing to locate the defect. While a perfect transducer may be unidirectional to easily locate defects, synthetic apertures are optimized by beam divergence. Defect location is obtained by correctly delaying the echoes recorded from multiple probe locations in order to synchronize their phase and to average the resulting A-Scans. The area of interest of the structure under evaluation is separated into a matrix of positions and the delays are calculated in order to focus at each individual pixel of the B-Scan or C-Scan.

If a defect is present within a reconstructed pixel, the phase of the corresponding echoes match and a constructive summation, or constructive interference, occurs. On the other hand, if the focal spot is located in a nondefected pixel, the echoes are not in phase and the summation will be randomly constructive and destructive. The result of this process is a pixelized representation of the structure, where focusing is done on each pixel and defects are identified by high amplitude of echoes. Synthetic focusing on B-Scans-time vs. probe position-results in a depth vs. probe position image, while a complete X-Y-Z positioning is done with synthetic focusing on C-Scans.

Synthetic focusing on B-Scans or C-Scans has advantages. First, the signal-to-noise ratio is increased, which helps in identifying the echoes. The imaging resolution also is improved, leading to better defect mapping and sizing possibilities. Another benefit is the presentation of the results, where the 3-D positioning of echoes tend to ease the interpretation of the results.

The principles on which this focusing method is based are compliant with small defect detection. In fact, the ideal defect for an optimal image reconstruction is a small one. The whole phase matching process of multiple detections requires the defect to reflect ultrasonic energy toward the transducer. Small defects respond in such a way, mainly because of the wave diffraction phenomena.

However, large reflecting surfaces need to be either oriented correctly so that the waves reflect back in the correct direction, or rough enough to create diffuse reflections, for which orientation is not as important. For example, defects such as flaws will in most cases produce diffuse reflections and tip diffraction, both effects being favorable to synthetic focusing. On the other hand, surfaces which mainly produce specular reflections-reflection angle corresponding to the incidence angle-need to be properly oriented to reflect ultrasonic waves to the transducer, reducing the number of echoes that can be recorded for each point of this type of surface and the subsequent phase construction possibilities. This particular property has a certain interest since reflections from small defects can be discriminated from larger reflectors such as the boundaries of a tested piece.

This 2-D reconstruction of a calibration block from synthetic aperture focusing (front) is compared to the original B-Scan (back) using a 10 MHz focused transducer focused on the block surface showing the improved lateral resolution. Red lines indicate the true boundaries of the calibration block. Source: TecScan Systems Inc.

Practical Synthetic Aperture Focusing

One of the best qualities of synthetic focusing is that it can be complementary to conventional ultrasonic scanning systems. As an analysis tool, synthetic aperture focusing adds focusing advantages to scanners not equipped with phased-array capabilities. For example, a conventional immersion scanner with a single element could be used to image a sample with the advantages of synthetic focusing. Furthermore, focusing at large depths using a single transducer can hardly be achieved and the use of common focused transducers is limited to certain depths. Synthetic aperture focusing allows high-resolution 2-D and 3-D imaging of structures using a single probe, which is suitable for situations where C-Scan imaging is involved.

In the particular case of immersion testing using single element, the selection of the transducer type influences the ability to do optimal beam focusing. Keeping in mind the benefit of a widely divergent beam to better locate a defect, two choices of transducers are possible. Small transducer elements contribute to enlarge the beam divergence, however at the cost of ultrasonic power. Furthermore, generating a divergent ultrasonic beam in the couplant (water) does not leave much ultrasonic power to detect defects in the material, considering the beam spread in water and the larger beam spread in the inspected structure because of refraction. However, focused transducers can generate a divergent beam in the test piece while minimizing the energy loss. By focusing the ultrasounds directly on the material surface, a quasi-point source is created on the surface of the test piece, resulting in a divergent beam generated directly in the material. This methodology allows optimized ultrasound penetration in the inspected structure because its energy is concentrated in the focal spot of the transducer, while generating a sufficiently wide beam within the structure.

In general, synthetic aperture focusing also can be applied to multiple ultrasonic inspection fields. It is already widely used as a complement analysis tool for time-of-flight diffraction inspection. Hidden or inaccessible sections of structures also can be inspected and imaged when inspected using divergent transducers. Angle-beam inspection of welds also could benefit from synthetic focusing by considering the echoes originating from multiple wave paths within the structure to do focusing.

Advanced Testing

Advanced testing for aerospace components such as inspection of composite materials also could benefit from the use of focusing techniques. With a good knowledge of the wave velocity behavior within the composite structure-slowness curves-focusing is possible and smaller defects could be detected due to the better signal-to-noise ratio.

In the past few years, the feasibility to use synthetic aperture focusing in the field of long range ultrasonic inspection-guided waves inspection-has been done successfully. This technique presents a good potential for the imaging of structures such as aircraft wing skins and hidden sections of fuselage components.

As radar technology collects reflected echoes and adds them in a manner to signal the presence of moving targets in the space, synthetic focusing collects echoes from targets within the inspection area and produces better image reconstruction of defects compared to conventional B-Scan or C-Scan images. Such clear images allow better target and defect interpretation and good signal-to-noise ratio. Applying the presented synthetic focusing technique to advanced inspection of material also could lead to higher lateral resolution in the inspected material, providing inspectors with better sizing of defects apparent in the produced synthetic B-Scan or C-Scan images. NDT

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