Phased Arrays

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The inspection of titanium billet used for jet engine manufacture is an important step in assuring engine reliability.  Past studies performed by the FAA Engine Titanium Consortium (ETC) determined inspection specifications sufficient to reliably inspect most billets used in engine manufacture.  These specifications were implemented in a “multi-zone” inspection, which concurrently scans multiple focused transducers, each focused to an adjacent depth, so as to inspect the entire billet radius with a 5 MHz focused ultrasound beam no larger than 0.1 inch in diameter.

While proven effective, the multi-zone inspection requires the procurement of multiple sets of transducer, data acquisition, and fixturing hardware.  In production, special procedures are required to individually align of each of, say, a half dozen transducers.  To mitigate this cost and complexity, a challenge was put to CNDE by Pratt and Whitney to design a phased array transducer capable of meeting inspection requirements using a moderately priced commercially available phased array system.  In addition to sweeping a 0.1 inch diameter focus to the center of a 10 inch diameter billet, the design was also called upon to address an insidious transducer alignment problem.  When focusing to the center of the rotating billet, a small misalignment of the transducer can result in a core of the billet being left uninspected, as depicted in Fig.1.  Furthermore, because this beam deflection is so sensitive to misalignment, it was deemed questionable that sufficient alignment can be maintained throughout an actual billet inspection, due to inherent imperfections in billet roundness and associated billet surface following capabilities.  A robust means to mitigate this problem is to laterally sweep the beam.

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focus over the billet center (i.e. in the radial direction perpendicular to pulse propagation) multiple times per billet revolution, thereby assuring that the billet center is always inspected.  While straightforward in principle, asking the phased array system to electronically sweep the focus both in depth and lateral angle readily presses the design up against the limits of the available instrumentation capability.

Fortunately, the high sensitivity to misalignment which makes the lateral sweep necessary also makes a phased array design feasible, since the beam only has to be swept over a very small angle to achieve the required lateral beam displacement.  Noting this, a design was pursued which maintains the 0.1 inch beam diameter when sweeping the focus over the 5 inch depth, and maximizes the range of lateral sweep when using 64 phased array time delays.  Calling upon CNDE’s experience in optimizing array surface curvature and element layout, a design was produced which sweeps the beam over a 0.6 inch lateral range with a response variation < 3dB.  The element layout for this design is depicted in Fig.2.  Upon fabrication, it was experimentally verified that the design exceeds inspection requirements.  A factory demonstration of the array performance took place at West Penn Testing in April 2010 (Fig. 3). Future use in production was unanimously supported by engine manufactures and material suppliers in attendance.

 

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Figure 2.  Phased array element layout for 0.6 inch beam sweep in 10 inch billet.

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Figure 3.  Industry demonstration at West Penn Testing April 2010.