Minoo Kabir

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...

Accurate Source Localization Using Highly Narrowband and Densely Populated MEMS Acoustic Emission Sensors

The real time source detection and localization are the fundamental advantages of Acoustic Emission (AE) method. The multi-dimensional localization requires an array of sensors distributed and positioned strategically on the structure. The sensor positions are controlled by the attenuation characteristics of the structure in order to have minimum required wave arrivals to sensors (e.g., minimum three sensors detecting an event for 2D source localization) for an AE event. While minimum three sensors are sufficient for 2D source localization, redundant data collection increases the accuracy of source location, and cleans the data set from non-relevant signals. Additionally, accurate prediction of wave velocity results in better localization result. Current AE sensors are piezoelectric type, and while they are designed to exhibit resonant behavior, they have a certain bandwidth that opens the response to a range of frequencies. In this study, highly narrowband Micro-Electro-Mechanical Systems (MEMS) sensors are presented with more accurate source localization ability due to detecting single frequency, and potential to densely populate on structures due to lightweight, and lowcost properties. The MEMS AE sensors are tuned to a range of frequencies between 60 kHz – 200 kHz for different structural applications (e.g., 60 kHz for concrete and 150 kHz for steel). The accurate prediction of source localization is tested on aluminum plate, and wave velocity of particular frequency is obtained using the dispersion curves. The source localization ability of the MEMS AE sensors is compared with conventional piezoelectric sensors. The efficiency of MEMS AE sensors with densely positioned scheme is discussed to monitor large-scale structures. doi: 10.12783/SHM2015/372

The design, characterization, and comparison of MEMS comb-drive acoustic emission transducers with the principles of area-change and gap-change

Comb-drive transducers are made of interdigitized fingers formed by the stationary part known as stator and the moving part known as rotor, and based on the transduction principle of capacitance change. They can be designed as area-change or gap-change mechanism to convert the mechanical signal at in-plane direction into electrical output. The comb-drive transducers can be utilized to differentiate the wave motion in orthogonal directions when they are utilized with the outof- plane transducers. However, their sensitivity is weak to detect the wave motion released by newly formed damage surfaces. In this study, Micro-Electro-Mechanical System (MEMS) comb-drive Acoustic Emission (AE) transducer designs with two different mechanisms are designed, characterized and compared for sensing high frequency wave propagation. The MEMS AE transducers are manufactured using MetalMUMPs (Metal Multi-User MEMS Processes), which use electroplating technique for highly elevated microstructure geometries. Each type of the transducers is numerically modeled using COMSOL Multiphysics program in order to determine the sensitivity based on the applied load. The transducers are experimentally characterized and compared to the numerical models. The experiments include laser excitation to control the direction of the wave generation, and actual crack growth monitoring of aluminum 7075 specimens loaded under fatigue. Behavior and responses of the transducers are compared based on the parameters such as waveform signature, peak frequency, damping, sensitivity, and signal to noise ratio. The comparisons between the measured parameters are scaled according to the respective capacitance of each sensor in order to determine the most sensitive design geometry.

3D printed metamaterial design to focus wave energy in thin plates

Acoustic metamaterials are periodic and composite structures that can block, direct and strengthen propagating elastic waves. They are periodic elastic composites made of two or more materials with different elastic properties. The periodic structure can exhibit certain band gaps that are used to manipulate wave field. In this research, the periodic and composite structure is made of aluminum plate and rubber cylinders manufactured using 3D printing. The ability to block and redirect elastic waves is numerically and experimentally demonstrated. Wave field focusing reduces the wave attenuation, which allows increasing the distance of acoustic sensors for damage detection in large-scale structures.

Integration of periodic structure and highly narrowband MEMS sensor to enhance crack detection ability in steel structures

Acoustic emission method is a nondestructive evaluation method based on the propagation of elastic waves due to the sudden change in strain field caused by newly formed fracture surfaces. While the method has been successfully applied to many structures, the influence of friction emissions limits the diverse use of the method in large-scale structures. This research integrates the metamaterial geometry to block low frequency friction signals while allowing high frequency signals due to the crack growth. The phononic structure is composed of periodic arrangement of holes in a steel plate that prohibits propagation of elastic waves near the band gap of 60 kHz. The dispersion curve of the periodic structure is calculated using finite element modeling of a unit cell in COMSOL Multiphysics. As the band gap of the periodic structure is highly narrowband, the acoustic sensing is achieved by highly narrowband capacitive type Micro-Electro- Mechanical Systems (MEMS) sensors tuned to the desired stop band frequency. The integration of periodic plate design and MEMS sensors provides wave-field focusing to reduce wave attenuation, and prevent interference of secondary waves sources, such as friction, with the primary waveforms. The waveguiding feature of the designed structure is experimentally investigated and discussed in this paper.

Piezoelectric micromachined acoustic emission sensors for early stage damage detection in structures

Acoustic emission (AE) is a passive nondestructive evaluation (NDE) method that relies on the energy release of active flaws. The passive nature of this NDE method requires highly sensitive transducers in addition to low power and lightweight characteristics. With the advancement of micro-electro-mechanical systems (MEMS), acoustic emission (AE) transducers can be developed in low power and miniaturized. In this paper, the AE transducers operating in plate flexural mode driven piezoelectrically known as Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) are presented. The AE PMUTs are manufactured using PiezoMUMPS process by MEMSCAP and tuned to 46 kHz and 200 kHz. The PiezoMUMPs is a 5-mask level SOI (silicon-on-insulator) patterning and etching process followed by deposition of 0.5 micron Aluminum Nitride (AlN) to form piezoelectric layer to form the transducers. The AE transducers are numerically modeled using COMSOL Multiphysics software in order to optimize the performance before manufacturing. The electrometrical characterization experiments are presented. The efficiency of the proposed AE PMUTs compared to the conventional AE transducers in terms of power consumption, weight and sensitivity is presented.