Didem Ozevin

Resonant capacitive MEMS acoustic emission transducers

We describe resonant capacitive MEMS transducers developed for use as acoustic emission (AE) detectors, fabricated in the commercial three-layer polysilicon surface micromachining process (MUMPs). The 1 cm square device contains six independent transducers in the frequency range between 100 and 500 kHz, and a seventh transducer at 1 MHz. Each transducer is a parallel plate capacitor with one plate free to vibrate, thereby causing a capacitance change which creates an output signal in the form of a current under a dc bias voltage. With the geometric proportions we employed, each transducer responds with two distinct resonant frequencies. In our design the etch hole spacing was chosen to limit squeeze film damping and thereby produce an underdamped vibration when operated at atmospheric pressure. Characterization experiments obtained by capacitance and admittance measurements are presented, and transducer responses to physically simulated AE source are discussed. Finally, we report our use of the device to detect acoustic emissions associated with crack initiation and growth in weld metal.

MEMS acoustic emission transducers designed with high aspect ratio geometry

In this paper, micro-electro-mechanic systems (MEMS) acoustic emission (AE) transducers are manufactured using an electroplating technique. The transducers use a capacitance change as their transduction principle, and are tuned to the range 50?200?kHz. Through the electroplating technique, a thick metal layer (20??m nickel?+?0.5??m gold) is used to form a freely moving microstructure layer. The presence of the gold layer reduces the potential corrosion of the nickel layer. A dielectric layer is deposited between the two electrodes, thus preventing the stiction phenomenon. The transducers have a measured quality factor in the range 15?30 at atmospheric pressure and are functional without vacuum packaging. The transducers are characterized using electrical and mechanical tests to identify the capacitance, resonance frequency and damping. Ultrasonic wave generation using a Q-switched laser shows the directivity of the transducer sensitivity. The comparison of the MEMS transducers with similar frequency piezoelectric transducers shows that the MEMS AE transducers have better response characteristics and sensitivity at the resonance frequency and well-defined waveform signatures (rise time and decay time) due to pure resonance behavior in the out-of-plane direction. The transducers are sensitive to a unique wave direction, which can be utilized to increase the accuracy of source localization by selecting the correct wave velocity at the structures.

Detecting and locating damage initiation and progression in full-scale sandwich composite fuselage panels using acoustic emission

In this study, acoustic emission was evaluated as a supplementary nondestructive testing method for detecting damage initiation and progression, identifying the site of damage, and anticipating ultimate fracture in notched full-scale honeycomb sandwich composite fuselage panels using redundant arrays of different acoustic emission sensor models. Each panel contained different damage scenarios and was subjected to combinations of quasi-static hoop and longitudinal loads. Damage progression and location were characterized with various inspection techniques, and the acoustic emission results were correlated with photogrammetric strain fields. Applying post-test signal processing, acoustic emission accurately detected notch tip damage initiation and tracked its progression to ultimate failure.

Geometry-based spatial acoustic source location for spaced structures

This study presents a new source localization methodology to identify the spatial location of active flaws on spaced structures using the acoustic emission (AE) method. The methodology uses the geometric boundaries and the local coordinate system to find the shortest direct paths from the acoustic source to the AE sensors. The structural elements are modeled as 1D so that the source location is determined using linear equations in the local coordinate system. The local source location is converted into the global coordinate system to determine the source location in space. The approach has been successfully demonstrated using computer and laboratory simulations. The use of linear equations to solve the spatial location of the acoustic source in multi-dimensional space reduces the computational load as compared to the conventional 2D and 3D location algorithms.

Steel bridge fatigue crack detection with piezoelectric wafer active sensors

Piezoelectric wafer active sensors (PWAS) are well known for its dual capabilities in structural health monitoring, acting as either actuators or sensors. Due to the variety of deterioration sources and locations of bridge defects, there is currently no single method that can detect and address the potential sources globally. In our research, our use of the PWAS based sensing has the novelty of implementing both passive (as acoustic emission) and active (as ultrasonic transducers) sensing with a single PWAS network. The combined schematic is using acoustic emission to detect the presence of fatigue cracks in steel bridges in their early stage since methods such as ultrasonics are unable to quantify the initial condition of crack growth since most of the fatigue life for these details is consumed while the fatigue crack is too small to be detected. Hence, combing acoustic emission with ultrasonic active sensing will strengthen the damage detection process. The integration of passive acoustic emission detection with active sensing will be a technological leap forward from the current practice of periodic and subjective visual inspection, and bridge management based primarily on history of past performance. In this study, extensive laboratory investigation is performed supported by theoretical modeling analysis. A demonstration system will be presented to show how piezoelectric wafer active sensor is used for acoustic emission. Specimens representing complex structures are tested. The results will also be compared with traditional acoustic emission transducers to identify the application barriers.

The integration of superlattices and immersion nonlinear ultrasonics to enhance damage detection threshold

We demonstrate the enhancement of immersion nonlinear ultrasonic testing (NLUT) by exploiting superlattices (SLs). NLUT can detect sub-wavelength micro-structural changes in solids by measuring the fundamental and second harmonic frequencies. The amplitude of second harmonic frequency increases with the presence of defects or other heterogeneities. The immersion NLUT is beneficial as water provides a consistent coupling condition; however, water generates high non-linearity that can mask the weak non-linearity originated from the micro-structural features in solids. In this research, SLs are proposed to remove the non-linearity arisen from water and experimental instruments. The SLs made of a periodic arrangement of composite layers can provide a band gap to restrict the propagation of a specific range of frequencies between transmitter and receiver. The periodic arrangement of solid-fluid layers is numerically designed and experimentally adapted to the immersion NLUT. Our results imply that the periodic array of 100 μm thick glass and 100 μm thick water layers provides a band gap that blocks 4.5 MHz (the second harmonic frequency), while this periodic structure passes 2.25 MHz (the first harmonic frequency). The improvement in the sensitivity of the NLUT is demonstrated through detecting the micro-structural changes associated with plastic deformation in aluminum 1100 specimens. It is revealed that the proposed methodology enhances the damage detection sensitivity of immersion NLUT by an order of magnitude as compared to the current practice.We demonstrate the enhancement of immersion nonlinear ultrasonic testing (NLUT) by exploiting superlattices (SLs). NLUT can detect sub-wavelength micro-structural changes in solids by measuring the fundamental and second harmonic frequencies. The amplitude of second harmonic frequency increases with the presence of defects or other heterogeneities. The immersion NLUT is beneficial as water provides a consistent coupling condition; however, water generates high non-linearity that can mask the weak non-linearity originated from the micro-structural features in solids. In this research, SLs are proposed to remove the non-linearity arisen from water and experimental instruments. The SLs made of a periodic arrangement of composite layers can provide a band gap to restrict the propagation of a specific range of frequencies between transmitter and receiver. The periodic arrangement of solid-fluid layers is numeric...

On the applicability of acoustic emission to identify modes of damage in full-scale composite fuselage structures

The acoustic emission method was applied during the testing of six full-scale sandwich composite aircraft fuselage panels containing through-the-thickness notches. The panels were subjected to different combinations of quasi-static internal pressure, the corresponding hoop loads, and longitudinal loads. The applicability of conventional acoustic emission signal feature analysis to identify the dominant modes of failure and extraneous emission in large composite structures was investigated. It was concluded that no clear distinction could be made among the different failure mechanisms based on the conventional acoustic emission signal features alone. Further, emission generated by fretting, either among fracture surfaces or of loading fixtures, has acoustic emission signal waveform features that are similar to those of damage-generated emission signals.

The design and calibration of particular geometry piezoelectric acoustic emission transducer for leak detection and localization

Pipeline leak detection using an acoustic emission (AE) method requires highly sensitive transducers responding to less attenuative and dispersive wave motion in order to place the discrete transducer spacing in an acceptable approach. In this paper, a new piezoelectric transducer geometry made of PZT-5A is introduced to increase the transducer sensitivity to the tangential direction. The finite element analysis of the transducer geometry is modeled in the frequency domain to identify the resonant frequency, targeting 60 kHz, and the loss factor. The numerical results are compared with the electromechanical characterization tests. The transducer response to wave motion generated in different directions is studied using a multiphysics model that couples mechanical and electrical responses of structural and piezoelectric properties. The directional dependence and the sensitivity of the transducer response are identified using the laser-induced load function. The transducer response is compared with a conventional thickness mode AE transducer under simulations and leak localization in a laboratory scale steel pipe.

Development of a MEMS device for acoustic emission testing

Acoustic emission testing is an important technology for evaluating structural materials, and especially for detecting damage in structural members. Significant new capabilities may be gained by developing MEMS transducers for acoustic emission testing, including permanent bonding or embedment for superior coupling, greater density of transducer placement, and a bundle of transducers on each device tuned to different frequencies. Additional advantages include capabilities for maintenance of signal histories and coordination between multiple transducers. We designed a MEMS device for acoustic emission testing that features two different mechanical types, a hexagonal plate design and a spring-mass design, with multiple detectors of each type at ten different frequencies in the range of 100 kHz to 1 MHz. The devices were fabricated in the multi-user polysilicon surface micromachining (MUMPs) process and we have conducted electrical characterization experiments and initial experiments on acoustic emission detection. We first report on C(V) measurements and perform a comparison between predicted (design) and measured response. We next report on admittance measurements conducted at pressures varying from vacuum to atmospheric, identifying the resonant frequencies and again providing a comparison with predicted performance. We then describe initial calibration experiments that compare the performance of the detectors to other acoustic emission transducers, and we discuss the overall performance of the device as a sensor suite, as contrasted to the single-channel performance of most commercial transducers.

New Leak Localization Approach in Pipelines Using Single-Point Measurement

Pipeline failures cause significant environmental and property damage and fatalities every year. Current methods have a lack of sensitivity to pinpoint the leak location with sufficient accuracy when the leak is at the stable regime. The acoustic emission (AE) method relies on propagating elastic waves caused by the leak turbulence. The current AE leak detection methodology relies on two neighbor sensors, and requires either a long wiring system or a wireless system with clock synchronization, which raises many other challenges. In this paper, the new leak localization approach is proposed to detect the leak from a single point measurement with two sensors designed to be sensitive to the wave motions generated in pipes at two orthogonal directions (radial and axial). The approach is demonstrated on a laboratory-scale steel pipe in comparison with the conventional two-neighbor sensor approach. The result indicates that the proposed localization approach successfully pinpoints the leak location with good accuracy, which enables the implementation of single-point wireless acoustic emission sensing in long-range pipeline networks.