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Inspection of fastener holes in pitch-catch mode (2/2)  



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

From industrial constraints compiled from the variety of geometrical models used on aircrafts, as well as from technicians’ requests for ease of use, the ultrasonic phased-array probe has been optimized with simulation tools. Displayed in Figure 1 is the CAD drawing of the outside shape of the probe designed to fit between fasteners, as well as solve positioning issues raised by NDT inspectors. Experiments have been carried out on a real part, taken from a grounded jetfighter. Several sizes of EDM notches were created in several locations of the spar (see Figure 2). Measurements were performed using an M2M phased-array controller to drive the optimized probe. The first step of the procedure is to obtain the image of the fastener hole. The axis of the probe is aligned with the fastener diameter. While the optimized focal laws are applied, the image of the fastener hole is displayed in the sectorial scan (see left picture in Figure 3). The echo visible in the scan is the reflection off the fastener hole in the spar. The probe is then moved laterally without rotating its scanning axis. The scan is now performed at the tangent to the fastener hole to detect the presence of cracks. When there are no defects over the thickness of the spar, no echoes are picked up by the scan and the image displayed on the live sectorial scan remains unchanged. When cracks are present, they act as reflectors and their echoes are picked up by the tandem inspection strategy. Thus, a crack located at half the thickness of the spar will produce the scan displayed in the middle picture in Figure 3. Another example of crack detection is given in the right hand picture in Figure 3. In this case the crack is located just underneath the skin in the spar.

Figure 2: Photograph of spar undergoing inspection and experimental setup. Courtesy of DASSAULT AVIATION.

The sensitivity of the developed procedure is 0.5mm-crack detection at any depth of the spar. The inspection is flexible enough to cover a large range of thicknesses. Simulation helped to find a optimal compromise between the space between the transmitting and receiving parts of the probe and their apertures. The space between the transmission and the reception subsets of elements can be electronically controlled. The two subsets are set close together for thin-layers inspections and wide apart for thick zones. The size of both the transmitting and receiving subsets can also be adjusted for maximum resolution. For instance, since a larger aperture is needed for long sound paths, the active part used to focus after reflection off the bottom surface can be increased electronically. Knowing beforehand the geometries of the structures to be inspected, all the adapted configurations are loaded onto the phased-array system and applied automatically. This method has proven reliable and easy to use by inspectors. Their feedback is being taken into account to finalize the procedure and the visualization interface.

Figure 3: Experimental results. The left picture illustrates the echo off the fastener’s vertical axis. The center picture shows the image of a middle crack. The right picture displays the image of a crack located underneath the skin. Courtesy of DASSAULT AVIATION.



The extensive capabilities of phased-array systems promise to improve inspectability and resolution for many nondestructive inspection applications. Studies at DASSAULT AVIATION demonstrate the advantages of phased arrays that include electronic focusing, scanning and beam steering, as well as real-time imaging. These features in turn enable inspection procedures that are faster, easier, and more reliable. Modeling stands to be an increasingly important tool for both specifying hardware and for determining optimal inspection strategies. Simulations performed with CIVA software illustrate how modeling is used to characterize the beam radiated into structures undergoing inspection, as well as the response of the radiated field to defects. This allows resolution limits and the minimum size of a detectable defect to be determined, as well as the coverage zone. Although phased-arrays and modeling do not eliminate the need for experimental validation, they can reduce the number of required tests. Moreover, both qualitative and quantitative characteristics of an NDT procedure can be evaluated using modeling tools such as CIVA. These modeling and parametric studies are often a necessary step in designing green-light/red-light solutions used in the field. Modeling also helps to find the optimal tradeoffs between performance and cost, while also meeting field constraints. For the fastener-hole inspection challenge, simulations have been used to optimize the probe design in conjunction with the NDT strategy (choice of focal-laws to be applied). While applying the developed inspection, a relatively simple modus operandi ensures the detection of 0.5mm-cracks located anywhere in the spar thickness.

For a better sizing ability and also to improve the ease of data interpretation, the fastener-hole echo should have the same amplitude from the top interface at the skin to the bottom surface at the spar. Using the latest development in the phased-array M2M electronics, shot-specific gain can be applied in order to ensure that the echoes off the fastener-hole are seen consistently throughout the thickness of the spar. Future plans call for implementing this feature to improve detection and sizing.

[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.  

[3] Roy O., Mahaut S. and Casula O., “Development of a smart flexible transducer to inspect component of complex geometry: modeling and experiments”, Review of Quantitative Nondestructive Evaluation Vol. 21, ed. by D. O. Thompson and D. E. Chimenti, American Institute of Physics, 2002.

See complete article at ASNT website. Contacting authors: Guillaume Neau*, Emmanuel Guilliorit**, Luc Boyer** and Herve Tretout**

*M2M NDT, inc.
1700 Montgomery Street,
Suite 102 94111
San Francisco, USA
+1 415 500 5280   
e-mail: g.neau@m2m-ndt.com 

1, avenue du Parc, 
Argenteuil 95100, France 
+33 134 118 706, 
e-mail: emmanuel.guilliorit@dassault-aviation.f