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.

Bercli offers a package deal for customers who purchase the complete version of CIVA together with a phased-array system.

Contact us for more information: