Image shows inspection of an advanced composite at Dassault Aviation, using an M2M system to drive the linear array probe. Used by permission.

Image shows a radiated acoustic beam using a sectorial probe. 64 elements are used to compute the delay laws and the acoustic field. The beam can be focused at any location in the specimen.

Less movement, faster inspection, better reliability

The advantages of phased-array systems include the ability to perform electronic scanning of the ultrasonic beam, which can reduce inspection times by eliminating or reducing the need move the probe. As illustrated in Figure 3, electronic scanning is accomplished by firing successive groups of elements in the array. A complete C-scan image can be obtained with a matrix phased array with the probe in a fixed position. The reliability of inspections can also be improved by reducing the need to move the probe. As is well known, good coupling between ultrasonic probes and the part undergoing inspection is crucial for good acoustic measurements. Each time the probe is moved, there is a risk of losing or degrading coupling. Thus, minimizing the number of times the probe is moved helps to maintain uniform conditions for multiple measurements.

Real-time imaging, easier interpretation

Phased arrays allow a broad spectrum of inspection strategies that improve performance, for example, sectorial scanning and focalization after reflection off the back surface of the test specimen. The most advanced phased-array systems include tools such as dynamic-depth focusing. With real-time imaging, inspections are easier to perform and the reliability of the measurements is also greatly improved. Because thousands of signals are captured and displayed at once, the struggle that operators often have in locating and visualizing defects on the screen is greatly reduced. In addition, the number of false alarms is diminished because of reduced operator dependence, and data recording and traceability are improved. Experimental results obtained using a sectorial scan are shown in Figure 4. Measurements were performed using 32 elements of a 64-element linear array with a frequency of 5 MHz. The test specimen was an aluminum reference block containing planar defects.

Applying delay laws to improve performance and simplify procedures

Figure 4: Example of sectorial scanning used for crack sizing. The solid rectangular line indicates the geometry of the test specimen under examination, including two parallel saw cuts. The corner echoes (labeled “a”) resulting from the cuts were easily detected using 32 elements of a linear array with a central frequency of 5 MHz. Diffraction from the crack tips (labeled “b”) is only observed when the beam is appropriately focused and directed.

Phased arrays can also replace an entire tool kit of conventional transducers. A single phased array used in conjunction with appropriate delay laws can reproduce the same acoustic beams achieved with numerous conventional probes, while also providing greater functionality. Using a phased-array controller that allows several types of delay laws per inspection, the results of several different sets of measurements that comprise a complete NDT procedure can be visualized simultaneously in real time.

Phased-array probes are composed of several piezoelectric crystals that can transmit/receive independently at different times. To focus the ultrasonic beam, time delays are applied to the elements to create constructive interference of the wavefronts, allowing the energy to be focused at any depth in the test specimen undergoing inspection. This principle is illustrated in Figure 1, where delay laws have been computed to focus the acoustic beam at a specified depth and angle. As shown in the figure, each element radiates a spherical wave at a specified time. The superposition of these wavelets results in an almost planar wavefront at the specified location.

Figure 1: Principle of phased-arrays; delay laws calculated to focus at a given depth and angle.

Figure 2: Examples of delay-laws and visualization of the radiated acoustic beam (displacement field). Calculations made using CIVA simulation software: (a) no delay-laws applied, (b) steering only, (c) depth focusing and (d) combined steering and depth focusing.

Figure 3: Example of electronic scanning. A subset of the elements in the array are used to generate a focused beam at normal incidence; this beam is then translated across the test specimen by firing subsequent groups of elements without moving the probe.

Before and after the targeted focal spot, wavefronts are spherically converging and diverging, respectively. A few examples of delay-law computation are displayed in Figure 2. When no delay laws are applied (Figure 2a), the resulting ultrasonic beam is unfocused and is equivalent to the beam generated by a conventional flat transducer. The natural “pseudo focalization” evident in the image corresponds to the near-field distance of the probe. The configuration illustrated in Figure 2b results in the same ultrasonic beam that would be generated by a conventional flat transducer used in conjunction with a wedge. In this case, there is no focusing of the ultrasonic energy; the applied delay laws result in steering of the ultrasonic beam. Figures 2c and 2d are the same configurations as illustrated in 2a and 2b, respectively, except that the delay laws have been modified to focus the acoustic energy at a specified depth. In both images (2c and 2d), it is evident that the focal spot is narrower and more localized. To obtain the same results with a conventional probe would require using a specially designed crystal shaped to obtain the desired focal point.

Phased arrays can reduce inspection times by eliminating or reducing the need for mechanical scanning by taking advantage of the ability to perform electronic scanning (see animation at left). Electronic scanning is accomplished by firing successive groups of elements in the array. Eliminating or reducing mechanical scanning also increases the reliability of the measurements by eliminating changes in (or loss of) coupling, which is a risk each time the probe is moved.

Whereas a conventional probe has one focal length and one orientation, a single phased-array probe allows the user to change the shape and focal point of the ultrasonic beam to optimize each inspection. The acoustic energy can be focused, and delay laws can be applied to steer the acoustic beam. Dynamic-depth focusing allows measurements to be made at several depths in the same amount of time as it takes to a single depth measurement using a conventional probe.

Phased arrays improve the reliability of the measurements and defect sizing can be improved using tools such as sectorial scanning (see figure below), or focalization after reflection off the back wall, two options available with M2M systems. A distinguishing feature of M2M systems is that the user can tune the beam, for example, to define any focal points in a CAD drawing.

Because of their flexibility, a phased-array probe can replace an entire toolbox of conventional ultrasonic probes. It can thereby simplify complex inspection procedures by eliminating the need for multiple probes, and the associated calibrations and setups.
Phased-arrays provide tremendous functionality including real-time imaging (see image below). Compared to measurements with conventional single-element probes, the detection and sizing of defects is much easier and more robust. Instead of struggling to find the optimal single signal that can be obtained with one element, a phased array allows hundreds of signals to be captured at once. The greatly improved efficiency makes it much easier to characterize defects and reduces the number of false alarms. When used in conjunction with simulation, inspection strategies can be optimized to improve detection.
Data recording and traceability are also greatly improved. For example, inspection data can be saved and compared to simulated results, helping to confirm whether or not there is a defect in the inspected structure.

Phased-arrays provide tremendous functionality including real-time imaging (see image at left). Compared to measurements with conventional single-element probes, the detection and sizing of defects is much easier and more robust. Instead of struggling to find the optimal single signal that can be obtained with one element, a phased array allows hundreds of signals to be captured at once. The greatly improved efficiency makes it much easier to characterize defects and reduces the number of false alarms. When used in conjunction with simulation, inspection strategies can be optimized to improve detection.
Data recording and traceability are also greatly improved. For example, inspection data can be saved and compared to simulated results, helping to confirm whether or not there is a defect in the inspected structure.