Journal Articles

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.

In this article, estimation of crack size, shape, and orientation was investigated numerically and experimentally using Lamb waves. A hybrid global–local approach was used in conjunction with the imaging methods for the numerical simulation. The hybrid global–local approach allowed fast and efficient prediction of scattering wave signals for Lamb wave interaction with crack from various incident directions. The simulation results showed the directionality effect of the scattering wave signals and suggested an optimum transmitter–sensor configuration. Two imaging methods were used: one involves the synthetic time reversal concept and the other involves Gaussian distribution function. Both imaging methods show very good agreement during simulations. Experiments were designed and conducted based on the simulated results. A network of eight piezoelectric wafer active sensors was used to capture the scattering waves from the crack. Both the pitch-catch and pulse-echo experimental modes were used. The directionality effect of incident Lamb waves on the imaging results was studied. The effect of summation, multiplication, and combined algorithms for each imaging method was studied. It was found that both methods can successfully predict the crack size and orientation. An attempt was made to use these imaging methods for detecting and sizing smaller sized damage (1- to 3-mm-diameter hole). It was found that these methods can successfully localize the hole, but size estimation was a bit challenging because of the smaller dimensions. The scattering waves for various hole sizes were studied.

The study of propagating, evanescent and complex wavenumbers of guided waves (GWs) in high-performance composites using a stable and robust semi-analytical finite element (SAFE) method is presented. To facilitate understanding of the wavenumber trajectories, an incremental material change study is performed moving gradually from isotropic aluminum alloy to carbon fiber reinforced polymer (CFRP) composites. The SAFE results for an isotropic aluminum alloy plate are compared with the exact analytical solutions, which shows that N = 20 SAFE elements across the thickness provides <0.5% error in the highest evanescent wavenumber for the given frequency-wavenumber range. The material change study reveals that reducing the transverse and shear moduli moves the wavenumber solution towards one similar to composite material. The comparison of the propagating, evanescent and complex wavenumber trajectories between composites and aluminum alloy show that antisymmetric imaginary Lamb wave modes always exist in composites although they may not exist in isotropic aluminum alloy at some frequencies. The wavenumber trajectories for a unidirectional CFRP plate show that the range of real wavenumber is much smaller than in the isotropic aluminum alloy. For laminated CFRP composite plates (e.g., unidirectional, off-axis, transverse, cross-ply and quasi-isotropic laminates), the quasi Lamb wave and shear horizontal (SH) wave trajectories are also identified and discussed. The imaginary SH wave trajectories in laminated composites are distorted due to the presence of ±45 plies. The convergence study of the SAFE method in various CFRP laminates indicates that sufficient accuracy can always be achieved by increasing the number of SAFE elements. Future work will address the stress-continuity between composite layers.

In this paper, some recent piezoelectric wafer active sensors (PWAS) progress achieved in our laboratory for active materials and smart structures (LAMSS) at the University of South Carolina: http: //www.me.sc.edu/research/lamss/ group is presented. First, the characterization of the PWAS materials shows that no significant change in the microstructure after exposure to high temperature and nuclear radiation, and the PWAS transducer can be used in harsh environments for structural health monitoring (SHM) applications. Next, PWAS active sensing of various damage types in aluminum and composite structures are explored. PWAS transducers can successfully detect the simulated crack and corrosion damage in aluminum plates through the wavefield analysis, and the simulated delamination damage in composite plates through the damage imaging method. Finally, the novel use of PWAS transducers as acoustic emission (AE) sensors for in situ AE detection during fatigue crack growth is presented. The time of arrival of AE signals at multiple PWAS transducers confirms that the AE signals are originating from the crack, and that the amplitude decay due to geometric spreading is observed.

This paper presents a new methodology for detecting and quantifying delamination in composite plates based on the high-frequency local vibration under the excitation of piezoelectric wafer active sensors. Finite-element-method-based numerical simulations and experimental measurements were performed to quantify the size, shape, and depth of the delaminations. Two composite plates with purpose-built delaminations of different sizes and depths were analyzed. In the experiments, ultrasonic C-scan was applied to visualize the simulated delaminations. In this methodology, piezoelectric wafer active sensors were used for the high-frequency excitation with a linear sine wave chirp from 1 to 500 kHz and a scanning laser Doppler vibrometer was used to measure the local vibration response of the composite plates. The local defect resonance frequencies of delaminations were determined from scanning laser Doppler vibrometer measurements and the corresponding operational vibration shapes were measured and utilized to quantify the delaminations. Harmonic analysis of local finite element model at the local defect resonance frequencies demonstrated that the strong vibrations only occurred in the delamination region. It is shown that the effect of delamination depth on the detectability of the delamination was more significant than the size of the delamination. The experimental and finite element modeling results demonstrate a good capability for the assessment of delamination with different sizes and depths in composite structure

The guided wave technique is commonly used in the health monitoring of thin-walled structures because the guided waves can propagate far in the structures without much energy loss. However, understanding of the wave propagation in bounded layered structures is still lacking. In this study, the Lamb wave field of single- and multi-layer plates excited by surface-mounted piezoelectric wafer active sensors is theoretically analyzed using the normal mode expansion method, which is based on the elastodynamic reciprocity relation and utilizes the orthogonality relations of the Lamb wave modes. The mode participation factors of Lamb wave in single- and multi-layer isotropic plates are derived. The time domain responses are obtained through the inverse Fourier transform of the structural response spectrum, which is obtained by multiplying the transfer function with the excitation frequency spectrum. The developed normal mode expansion method is first applied to an aluminum single-layer plate. The obtained analytical tuning curves and out-of-plane velocity of the plate are in good agreement with the numerical and experimental results. Finally, the analytical wave responses of an aluminum–adhesive–steel triple-layer plate are verified through comparison with the finite element analysis and experiment. The proposed normal mode expansion method provides a reliable and accurate calculation of the wave field in single- and multi-layer plates.

Dispersion curve, and displacement modeshapes of multilayer structures can be obtained using Semi-Analytical Finite Element (SAFE) method. Stress mode shapes calculated from SAFE were discontinuous at the interface, because SAFE method formulation does not apply stress continuity at the interface. A case study of 1 mm aluminum-1 mm steel double-layer plate is presented, showing the SAFE stress predictions at the interface. The results observed were discontinuity of out-of-plane stress modeshapes at the interface. Mesh refinement was performed at the interface to study the convergence of stress mode shapes at the interface, yet the stress discontinuity problem was not solved. A fine mesh at the interface was created by using variable mesh techniques. This also did not provide continuous stress mode shape at the interface. Therefore, in this paper, a novel hybrid SAFE-GMM (Global Matrix Method) approach was used to obtain stress mode shapes accurately and efficiently. GMM develops the displacement and stress equations for individual layers in a multilayered structure and assembles a global matrix by applying the boundary and interface continuity conditions. A case of practical importance, CFRP strengthened concrete structure was analyzed using the SAFE-GMM approach. The drawback of the SAFE technique to this case is also presented, and it shows that the hybrid SAFE-GMM approach gave the interface stresses accurately.

Fatigue cracking in sheet metal structures is a common problem during practical applications. It can cause a disastrous failure in the running condition of the structure if it is not addressed in time. A crack can potentially produce acoustic emission (AE) signals during the crack growth event as well as during rubbing/clapping of faying surfaces. All kind of AE signals from the crack provide useful information regarding the condition of the cracked structure. AE signals due to crack faying surface rubbing/clapping need to be identified and separated from AE signals due to other sources. This paper discusses the study of the AE signals generated due to rubbing/clapping of crack faying surfaces. The initial fatigue crack was generated in a sheet metal sample through fatigue loading. The sample was then vibrated at various frequencies using a vibration exciter, and AE signals were generated by vibration induced rubbing/clapping of crack faying surfaces. AE signal signature due to vibration induced rubbing/clapping was identified and studied. AE signal due to rubbing/clapping of sheet metal sample while undergoing an axial static load is also discussed in this paper. The sheet metal sample with fatigue crack was axially loaded using a test cell manufactured in-house for loading sheet metal samples. The samples were vibrated to excite AE signals, and the influence of static load on crack rubbing/clapping AE signals were studied. The vibration-induced crack faying surface motion generated a large number of AE signals. A comparison among these AE signals was performed by using the Pearson correlation of the signals. The first novelty of this research is experimental design and fabrication of an experimental setup for detecting vibration induced rubbing and clapping in fatigue cracked metallic structures. The second novelty of this research is the development of a novel correlation coefficient based approach for comparing a large number of AE signals.

The manufacturing process of carbon fiber reinforced polymer (CFRP) composite structures can introduce many characteristic defects and flaws such as fiber misorientation, fiber waviness, and wrinkling. Therefore, it becomes increasingly important to detect the presence of these defects at the earliest stages of development. Eddy current testing (ECT) is a nondestructive inspection (NDI) technique that has been proven quite effective in detection of damage in metallic structures. However, NDI of composite structures has mainly relied on other methods such as ultrasonic testing (UT) and X-ray to name a few and not much on ECT. In this paper, the authors explore the possibility of using ECT in NDI of CFRP composites by conducting simulations and experiments thereafter. This research is based on the fact that the CFRP displays some low-level electrical conductivity due to the inherent conductivity of the carbon fibers. This low-level conductivity may permit eddy current pathways to cause the flow of eddy currents in the CFRP composites that can be exploited for nondestructive damage detection. An invention disclosure describing our high-frequency ECT method has also been processed. First, the multiphysics finite element method (FEM) simulation was used to simulate the detection of various types of manufacturing flaws and operational damage in CFRP composites such as fiber misorientation, waviness, wrinkling, and so on. Thereafter, ECT experiments were conducted on CFRP specimens with various manufacturing flaws using the Eddyfi Reddy eddy current array (ECA) system.

This paper addresses the predictive simulation of acoustic emission (AE) guided waves that appear due to sudden energy release during incremental crack propagation. The Helmholtz decomposition approach is applied to the inhomogeneous elastodynamic Navier–Lame equations for both the displacement field and body forces. For the displacement field, we use the usual decomposition in terms of unknown scalar and vector potentials, U and H. For the body forces, we hypothesize that they can also be expressed in terms of excitation scalar and vector potentials, A and B. It is shown that these excitation potentials can be traced to the energy released during an incremental crack propagation. Thus, the inhomogeneous Navier–Lame equation has been transformed into a system of inhomogeneous wave equations in terms of known excitation potentials A* and B* and unknown potentials U and H. The solution is readily obtained through direct and inverse Fourier transforms and application of the residue theorem. A numerical study of the one-dimensional (1D) AE guided wave propagation in a 6 mm thick 304-stainless steel plate is conducted. A Gaussian pulse is used to model the growth of the excitation potentials during the AE event; as a result, the actual excitation potential follows the error function variation in the time domain. The numerical studies show that the peak amplitude of A0 signal is higher than the peak amplitude of S0 signal, and the peak amplitude of bulk wave is not significant compared to S0 and A0 peak amplitudes. In addition, the effects of the source depth, higher propagating modes, and propagating distance on guided waves are also investigated.