A MEMS Transducer for Ultrasonic Flaw Detection

Metal structures can fail because of fatigue crack propagation or because of section loss from corrosion. Regular inspection is required to intercept such failures, and in situ sensors would be a superior technology for that purpose. We are developing MEMS-based transducers for ultrasonic flaw detection, to be permanently mounted at critical locations on structures. Ultrasonic testing requires the emission of a short pulse of ultrasonic energy and the detection of echoes from nearby surfaces or flaws. In a permanently mounted device, phased-array detection will be needed to scan for flaws in off-axis locations. The device we envision will use a mm-scale piezoelectric emitter and an array of MEMS capacitive diaphragm detectors. We have designed and fabricated arrays of MEMS capacitive diaphragm transducers using the MUMPS process and we report on their performance as pulse-echo detectors in direct contact with metal specimens. Our chip contains 23 sensor elements, featuring nine elements in a linear array and 14 other elements to study alternative diaphragm designs. In the linear array, each element contains 180 hexagonal diaphragms with a leg length of 49 microns, a polysilicon diaphragm thickness of 2 microns, and a gap of 2 microns. The diaphragm diameter was chosen to obtain a lowest-mode resonant frequency of approximately 4 MHz for an unloaded device, corresponding to a ultrasonic wavelength which is well-suited for flaw detection in structural applications. The unloaded elements were characterized by capacitance-voltage measurements under ambient conditions and by admittance measurements in vacuum, verifying the expected mechanical performance of the diaphragm design. Performance of the detector array was studied by bonding the chip to a test specimen and applying an ultrasonic pulse using a commercial (PZT) ultrasonic transducer. One experiment reflects an on-axis excitation in which the pulse arrives uniformly at all eight detectors. Another experiment reflects an off-axis excitation in which the pulse arrival is delayed from one detector to the next. The delay permits accurate localization of the pulse source using phased array signal processing. The results establish that MEMS transducers can function as phased array detectors for monitoring flaw conditions in metal structures. INTRODUCTION Steel is used in buildings, bridges, pressure piping, and industrial construction, but the safe performance of any steel structure is threatened by section loss from corrosion or wear, by crack propagation from fatigue or cyclic loading, by weld failure from overload or seismic loading, or by other discontinuities. Such flaws can develop with time, and the continued service of major structures often requires confirmation that such flaws have not developed. Single Mode Measurement -25 25 75 125 175 225 0 5 10 15 20 25 30 35 40 Time (micros) A m pl itu de (G ai n = 30 ) Figure 1a. Pulse-echo flaw detection (from ref [1]) Figure 1b. Results using mm-scale PZT specimen + * Contact author: ijo@cmu.edu; 412-268-2950; Carnegie Mellon University, Department of Civil and Environmental Engineering, Pittsburgh, PA 15213; ** dg07@andrew.cmu.edu; 412-268-3707; Department of Electrical and Computer Engineering; *** akash@andrew.cmu.edu. Ultrasonic flaw detection [1] is a versatile technology for nondestructive evaluation, but it must typically be performed by skilled personnel. The principles of pulse-echo flaw detection are depicted in Figures 1a and 1b in a throughthickness geometry. Figure 1a depicts an ultrasonic pulse generated and transmitted into the material. A typical transducer frequency is 5 MHz frequency, corresponding to a 1.2-mm wavelength in steel, which is sufficiently short to resolve flaws at that same scale. The typical transducer is a piezoelectric device, a PZT (lead-zirconium-titanate) ceramic, with a diameter much greater than the wavelength. The ultrasonic pulse will reflect from the first boundary it encounters, which in an unflawed specimen is the back surface of the steel plate. The time at which the echo returns to the front surface reveals the total travel distance, equal to twice the thickness. Figure 1b records a measurement using a mm-scale PZT sample affixed to a brass plate (velocity of sound c = 4400 m/s) with a thickness of 9.8 mm, showing successive echo returns. The time from the pulse to the return of the echo is slightly under 5 μs, correctly approximating the thickness. Ultrasonic inspection can be used in this manner to measure thickness, which would reveal any section loss, or to reveal reflections that arrive prematurely, which would signal the presence of a flaw. Although ultrasonic flaw detection is quite versatile, there are two limitations that could be eliminated by the development of resident sensors: in current practice the inspection is performed manually, and is therefore subject to interpretation, and the process is often memoryless, which makes no use of the earlier signal history. We envision building a resident ultrasonic flaw detection system to be mounted at critical locations on metal structures, which would retain a signal history to allow signature analysis in the detection of developing flaws. We intend that the device be polled remotely using RF communications. This paper describes the design and initial testing of a MEMS capacitive (diaphragm-type) transducer array to function as the receiver in the flaw detection system. In order to scan a volume of material from a fixed position it is necessary for the transducer to function as a phased array, and experiments to demonstrate signal detection and phased array processing were a main purpose of this study.