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### Research Papers

ASME J Nondestructive Evaluation. 2018;2(1):011001-011001-12. doi:10.1115/1.4041068.

This paper presents gamma radiation effects on resonant and antiresonant characteristics of piezoelectric wafer active sensors (PWAS) for structural health monitoring (SHM) applications to nuclear-spent fuel storage facilities. The irradiation test was done in a Co-60 gamma irradiator. Lead zirconate titanate (PZT) and Gallium Orthophosphate (GaPO4) PWAS transducers were exposed to 225 kGy gamma radiation dose. First, 2 kGy of total radiation dose was achieved with slower radiation rate at 0.1 kGy/h for 20; h then the remaining radiation dose was achieved with accelerated radiation rate at 1.233 kGy/h for 192 h. The total cumulative radiation dose of 225 kGy is equivalent to 256 years of operation in nuclear-spent fuel storage facilities. Electro-mechanical impedance and admittance (EMIA) signatures were measured after each gamma radiation exposure. Radiation-dependent logarithmic sensitivity of PZT-PWAS in-plane and thickness modes resonance frequency $(∂(fR)/∂( logeRd))$ was estimated as 0.244 kHz and 7.44 kHz, respectively; the logarithmic sensitivity of GaPO4-PWAS in-plane and thickness modes resonance frequency was estimated as 0.0629 kHz and 2.454 kHz, respectively. Therefore, GaPO4-PWAS EMIA spectra show more gamma radiation endurance than PZT-PWAS. Scanning electron microscope (SEM) and X-ray diffraction method (XRD) was used to investigate the microstructure and crystal structure of PWAS transducers. From SEM and XRD results, it can be inferred that there is no significant variation in the morphology, the crystal structure, and grain size before and after the irradiation exposure.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;2(1):011002-011002-11. doi:10.1115/1.4041122.

In recent years, nonlinear vibro-acoustic methods have shown potential to identify defects which are difficult to detect using linear ultrasonic methods. However, these methods come with their own challenges such as frequency dependence, requirement for a high excitation amplitude, and difficulties in distinguishing nonlinearity from defect with nonlinearity from other sources to name a few. This paper aims to study the dependence of nonlinear vibro-acoustic methods for detection of delaminations inside a composite laminate, on the excitation methods and excitation frequencies. It is shown that nonlinear vibro-acoustic methods are highly frequency dependent and commonly used excitation signals which utilize particular values of excitation frequencies might not always lead to a clear distinction between intact and delaminated regions of the specimen. To overcome the frequency dependence, signals based on frequency sweep are used. Interpretation of output response to sweep signals to identify damage is demonstrated using an earlier available approach, and a simpler approach is proposed. It is demonstrated that the damage detection with sweep signal excitations is relatively less dependent on excitation frequency than the conventional excitation methods. The proposed interpretation technique is then applied to specimens with delamination of varying sizes and with delaminations at different depths inside the laminate to demonstrate its effectiveness.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;2(1):011003-011003-6. doi:10.1115/1.4041567.
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A linear sensor arrangement is presented as a means of measuring the three-dimensional grain angle of wood. The measurement principle is based on an optical characteristic of a wood surface where the microscopic cell structure causes preferential reflection of light perpendicular to the wood grain. This response is notable among the several other techniques for measuring wood grain angle in that it enables identification of diving (out-of-plane) angle in addition to the surface (within-plane) angle. The basic measurement principle has been previously investigated using a circular array of light sensors to measure the spatial distribution of the light reflected from a wood surface. That procedure works reasonably well for surface points near the center of the circle and for modest dive angles. The linear sensor arrangement investigated here is designed to extend measurement functionality so as to be able to measure grain angle at any point along a central line and over a greater range of dive angle. A prototype scanner system is presented together with example experimental results for clear wood samples and for a face knot sample.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;2(1):011004-011004-8. doi:10.1115/1.4041717.

Sometimes, nondestructive evaluation (NDE) or structural health monitoring methods commonly used in engineering structures are used for the betterment of consumer goods. A classic example is the use of sensor systems to monitor the pressure and the quality of car tires. In this paper, we present a nondestructive method to characterize tennis balls. The International Tennis Federation (ITF) specifies which characteristics a tennis ball must have in order to be commercialized. One of these characteristics is bounciness and the standardized method to measure it is the rebound test, where a ball is released from 2.54 m onto a smooth rigid surface and, in order to be approved, the ball must bounce within a certain range. This test can be staged by manufacturers and testing authorities but the equipment necessary to perform it is not readily available to the average consumer. In the study presented in this paper, an empirical method based on the propagation of highly nonlinear solitary waves (HNSWs) is proposed to establish whether a given ball conforms the specifications set by the ITF in terms of bounciness and allowed deformation. The experiments conducted in this study aim to discover a correlation between some features of the waves and the values obtained with the rebound test and the compression test in which the deformation of the ball under a known load is measured. The presence of such correlations could represent a viable alternative to establish the conformity of tennis balls. Based on the empirical evidences collected in this study, a possible new standard is suggested.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2019;2(1):011005-011005-9. doi:10.1115/1.4042176.

This research investigates the application of sum-of-squares (SOS) optimization method on finite element model updating through minimization of modal dynamic residuals. The modal dynamic residual formulation usually leads to a nonconvex polynomial optimization problem, the global optimality of which cannot be guaranteed by most off-the-shelf optimization solvers. The SOS optimization method can recast a nonconvex polynomial optimization problem into a convex semidefinite programming (SDP) problem. However, the size of the SDP problem can grow very large, sometimes with hundreds of thousands of variables. To improve the computation efficiency, this study exploits the sparsity in SOS optimization to significantly reduce the size of the SDP problem. A numerical example is provided to validate the proposed method.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2019;2(1):011006-011006-8. doi:10.1115/1.4042259.

Heat-resistant composites, such as ceramic matrix composites and heat-resistant carbon fiber reinforced plastics (CFRPs), are expected to be used for aircraft engine parts. The development of reliable heat-resistant composite materials requires the use of nondestructive test techniques for evaluating the progression of damage during material testing at elevated temperatures. Furthermore, structural health monitoring (SHM) technologies that operate under harsh environments are expected to be realized for monitoring heat-resistant composite structures. To provide potential solutions for the establishment of such technologies, this research developed a heat-resistant ultrasonic sensor based on a regenerated fiber-optic Bragg grating (RFBG). First, we fabricated an RFBG by annealing a normal fiber-optic Bragg gratings (FBG) sensor. Because the RFBG exhibits high heat resistance at temperatures of 1000 °C, the sensor achieved stable ultrasonic detection at an elevated temperature. In addition, we attempted to use a π-phase-shifted FBG (PSFBG) as the seed grating to construct an ultrasonic sensor with enhanced performance. As a result, the regenerated phase-shifted fiber-optic Bragg grating (R(PS)FBG) sensor possessed a very short effective gauge length and achieved a broad frequency response to ultrasonic waves with frequencies greater than 1.5 MHz. The broadband detectability enables the R(PS)FBG sensor to acquire an accurate response to ultrasonic waves. Hence, we believe the regenerated Bragg grating-based ultrasonic sensors can contribute to establishing an effective nondestructive evaluation method for composite materials, thereby enabling a structural health monitoring system for a composite-made structure operating under extreme high-temperature environments.

Commentary by Dr. Valentin Fuster