Cracks occur most often near the bottom of the tank where there is a change in wall thickness. Simulations using CIVA were performed to determine the optimal probe configuration and inspection strategy. One proposed solution is to use a shear-wave inspection at 45 degrees using a linear array with a wedge (see below), combined with electronic scanning. As a first step, the beam profile is optimized using CIVA to gain maximum resolution in the area of interest. The number of elements to be fired at the same time and the focusing delay laws are deduced from analysis of the beam calculations.

Once the beam is optimized, wave-defect interactions are calculated (see animation above). A reference case is simulated (usually a calibrated side-drilled hole) and a parametric study is conducted to determine the effect of crack orientation. Here the crack is rotated from –20 to +20 degrees, with a 10º step. Results are shown in the plot below, which shows echo amplitude versus crack orientation.

By correlating these simulation results to experimental measurements on the calibrated defect, the sensitivity of the procedure can be inferred; i.e., the inclination range that can be detected. Note that similar parametric studies can be performed to determine the smallest resolvable defect, and the coverage zone.

Practical example:

For equipment with a 80-dB dynamic range, let’s suppose that the calibrated defect is observed experimentally at a 50dB gain with an echo at 100% of the screen height and a noise below 5% of the screen height.

Let’s now assume that any full-height echo measurement is acceptable as long as the noise level is below 30% of the screen height.

As a first approach, the remaining dynamic range to keep the measurement acceptable is:

20xlog (30/5) = 15.5 dB

Any echo within a 15.5dB range of the side-drilled-hole echo will therefore be accepted. Going back to the simulation results that show that any echo from a rotated crack (within ± 20º) remains within 10 dB of the reference echo (see plot), we can conclude that all defects with rotations within this range will be detected using the proposed inspection strategy.

The conventional inspection technique requires the fastener to be removed. The challenge in developing an easier and less expensive inspection strategy is to design a technique that can be used from the skin side, that does not require removal of the fastener, and that provides the same or better resolution than the conventional method (see Figure 1).

Figure 1: Generic aircraft fastener joining the skin and spar. Large stresses in the fixation area make it a potential site for crack initiation.

The phased-array concept is to use a large linear array in a pseudo-tandem configuration, in which different elements of the same probe are used for transmission and reception [2]. The transmitting elements are phased to achieve a focused incident wave along a sectorial scan path after reflection off the bottom surface (see Figure 2).

Figure 2: Transmission delay laws are computed to focus shear waves after reflection off the bottom surface.

The reception elements are phased to perform a focused sectorial scan (see Figure 3).

Figure 3: Reception delay laws are computed to focus shear waves directly beneath the probe wedge.

The convolution between the transmitted and received signals defines the active focal spot for the measurements (see Figure 4).

Figure 4: Resolved focal points using separate transmission and reception delay laws. The entire thickness is covered by the inspection procedure. The linear array replaces a large number of tandem probes (one for each thickness).

The objective of combining the pitch-catch mode and sectorial scanning in this way is to optimize both the zone coverage and the consistency of the resolution. The optimized inspection strategy uses a 96-element linear array at 5MHz (see animation in Figure 5).

Figure 5: Ray tracing of the focal points using separate transmission and reception delay laws. The entire thickness of the part is covered by the inspection procedure.

Advantages:

• Requires only one probe for the complete inspection.
• No dead zones.
• Tunable focusing for transmission and reception.
• Adaptable to any geometry.
• Consistent resolution through the whole thickness.

The first inspection strategy considered used the same number of elements (40) for both transmission and reception. Simulation results calculated using CIVA (see Figure 6) show good consistency of the focal-spot size throughout the thickness, with a 7dB-amplitude variation between the top and bottom focal spots and a 4mm-diameter focal spot at -6dB. The results also show a lack of resolution in transmission underneath the front surface and in reception at the bottom surface.

Figure 6: 40-element configuration. Beam visualization shows sufficient resolution for crack detection.

Using these results, a new strategy was designed that uses 54 elements for transmission and 42 elements for reception. The elements are phased in both transmission and reception to improve resolution. Results (see Figures 7, 8 and 9) show better resolution throughout the entire thickness (2mm-diameter focal spot at -6dB), as well as better consistency (2.5 dB variation from top to bottom). The linear array replaces a large number of tandem probes (one for each thickness).

Figure 7: Visualization of the focused beam after reflection off the bottom surface using 54 elements for transmission.

Figure 8: Visualization of the focused beam using 54 elements for reception.

[1] Neau G., Hopkins D., Tretout H, and Boyer L., “Phased-array applications for aircraft maintenance, manufacturing and development”, Aerospace Testing Expo 2006, UKIP Media & Events 2006.

[2] Mahaut S., Chatillon S., Raillon-Picot R. and Calmon P., “Simulation and application of dynamic inspection modes using ultrasonic phased arrays”, Review of Quantitative Nondestructive Evaluation Vol. 23, ed. by D. O. Thompson and D. E. Chimenti, American Institute of Physics, 2004.

Figure 1: T-section stiffener. Expected defects are located inside the stiffener. The corner piece prevents inspection using only normally incident waves.

Without the corner part, the inspection could be performed with good accuracy using normally incident pressure waves. However, with the corner piece, pressure waves at normal incidence cannot sample every zone of the stiffener, and the shadow zone (or silent zone) is too large to be acceptable. The inspection strategy must therefore to be adapted to be able to detect defects that lie underneath the corner piece.

One solution is to use a linear phased-array probe. By performing a sectorial scan and using delay laws to steer the acoustic beam, the full width of the part can be inspected and the formerly silent zone is eliminated. As illustrated in Figure 2, cracks lying underneath the corner part can be detected using this procedure. In this figure, the inspector can observe the multiple reflections of the thin part (occurring at normal incidence, on the left side of the sectorial scan), as well as other structural reflections (at high steering angles, right side of the sectorial scan).

Figure 2: Sectorial scan superimposed on a cross section showing the stiffener geometry. The sectorial scan allows cracks lying beneath the corner piece to be detected.

Image depicts crack detection in an aluminum sample using an M2M system, an Imasonic probe and a displacement encoder. Courtesy of Dassault Aviation.

Our largest system is a fully parallel 128-channel system. With some development, we can provide a fully parallel 256-channel system.

Our product brochures can provide you with the technical specifications of each system.

• Multi-X and Multi-2000
• Multi-2000 Pocket

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.