Journal Articles

The acoustic emission (AE) waveforms from a fatigue crack advancing in a thin metallic plate possess diverse and complex spectral signatures. In this article, we analyze these waveform signatures in coordination with the load level during cyclic fatigue. The advancing fatigue crack may generate numerous AE hits while it grows under fatigue loading. We found that these AE hits can be sorted into various groups based on their AE waveform signatures. Each waveform group has a particular time-domain signal pattern and a specific frequency spectrum. This indicates that each group represents a certain AE event related to the fatigue crack growth behavior. In situ AE-fatigue experiments were conducted to monitor the fatigue crack growth with simultaneous measurement of AE signals, fatigue loading, and optical crack growth measurement. An in situ microscope was installed in the load-frame of the mechanical testing system (MTS) to optically monitor the fatigue crack growth and relate the AE signals with the crack growth measurement. We found the AE signal groups at higher load levels (75%–85% of maximum load) were different from the AE signal groups that happened at lower load levels (below 60% of load level). These AE waveform groups are highly related to the fatigue crackrelated AE events. These AE signals mostly contain the higher frequency peaks (100 kHz, 230 kHz, 450 kHz, 550 kHz). Some AE signal groups happened as a clustered form that relates a sequence of small AE events within the fatigue crack. They happened at relatively lower load level (50%–60% of the maximum load). These AE signal groups may be related to crack friction and micro-fracture during the friction process. These AE signals mostly contain the lower frequency peaks (60 kHz, 100 kHz, 200 kHz). The AE waveform based analysis may give us comprehensive information of the metal fatigue.

The accuracy of the stiffness matrix used as input in dispersion curve algorithm determine the accuracy of the predicted wave speeds. Common practice is to use standard mechanical testing procedures to determining the 𝐸11,𝐸22,𝐺12 and 𝜈12. The other engineering constants are then based on assumptions such as: 𝐸33=𝐸22. The engineering constants are converted to the stiffness matrix and used as input. Due to this approach the dispersion curves can vary significantly from those obtained experimentally.

In this research the stiffness matrix components are determined non-destructively using a newly introduced ultrasonic immersion technique, the LAMSS approach. The LAMSS approach utilizes the symmetry planes within an orthotropic transversely isotropic material and the critical angle approach to divide the stiffness matrix retrieval process into several steps to reduce the complexity of the process and increase the accuracy of the solution.

As last, the predicted group velocity dispersion curves obtained using a stiffness matrix based on mechanical testing and the ultrasonic immersion technique are compared to experimentally obtained velocities.

This paper presents cylindrical coordinate solutions of axis symmetric circular crested elastic waves that appear due to sudden energy release during incremental crack propagation in a plate. Axis symmetric assumption decouples the elastic wave problem to Lamb (P+SV) and shear (SH) horizontal waves. Helmholtz decomposition principle was used to decompose displacement field in to unknown scalar and vector potentials; and body force vectors to known excitation scalar and vector potentials respectively. Therefore, Navier–Lame equations yield a set of four inhomogeneous wave equations of unknown potentials , Hr, Hθ , Hz and known excitation potentials A∗, B∗ r , B∗ θ , B∗ z . There are two types of potentials exist in a plate for axis symmetric circular crested Lamb wave: pressure potentials , A∗ and shear potentials Hθ , B∗ θ . Inhomogeneous wave equations for and Hθ were solved due to generalized excitation potentials A∗ and B∗ θ in a form, suitable for numerical calculation. The theoretical formulation shows that elastic waves generated in a plate using excitation potentials follow the Rayleigh-Lamb equations. The resulting solution is a series expansion containing the superposition of all the Lamb wave modes existing for the particular frequency-thickness combination under consideration. In addition, bulk wave solution is also recovered due to the effect of the excitation potentials. The numerical studies modeled the two-dimensional (2D) (circular crested) AE elastic wave propagation in order to simulate the out-of-plane displacement that would be recorded by an AE sensor placed on the plate surface at some distance away from the source. Parameter studies were performed to evaluate: (a) the effect of the pressure and shear potentials; (b) the effect of the thickness-wise location of the excitation potential sources varying from mid-plane to the top surface (source depth effect); (c) the effect of peak time (d) the effect of propagating distance away from the source. 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, A0 peak signals

Piezoelectric wafer active sensors have been widely used for Lamb-wave generation and acquisition. For selective preferential excitation of a certain Lamb-wave mode and rejection of other modes, the piezoelectric wafer active sensor size and the excitation frequency should be tuned. However, structural damping depends on the structure material and the excitation frequency and it will affect the amplitude response of piezoelectric wafer active sensor–excited Lamb waves in the structure, that is, tuning curves. Its influence on the piezoelectric wafer active sensor tuning reflects the effect of structural health monitoring configuration considered in the excitation. Therefore, it is important to have knowledge about the effect of structural damping on the tuning between piezoelectric wafer active sensor and Lamb waves. In this article, the analytical tuning solution of undamped media is extended to damped materials using the Kelvin–Voigt damping model, in which a complex Young’s modulus is utilized to include the effect of structural damping as an improvement over existing models. This extension is particularly relevant for the structural health monitoring applications on high-loss materials, such as metallic materials with viscoelastic coatings and fiber-reinforced polymer composites. The effects of structural damping on the piezoelectric wafer active sensor tuning are successfully captured by the improved model, with experimental validations on an aluminum plate with adhesive films on both sides and a quasi-isotropic woven composite plate using circular piezoelectric wafer active sensor transducers.

Guided wave attenuation in composites due to material damping is strong, anisotropic, and cannot be neglected. Material damping is a critical parameter in selection of a particular wave mode for long-range structural health monitoring in composites. In this article, a semi-analytical finite element approach is presented to model guided wave excitation and propagation in damped composite plates. The theoretical framework is formulated using finite element method to describe the material behavior in the thickness direction while assuming analytical expressions in the wave propagation direction along the plate. In the proposed method, the Kelvin–Voigt damping model using a complex frequencydependent stiffness matrix is utilized to account for anisotropic damping effects of composites. Thus, the existing semianalytical finite element approach is being extended to include material damping effect. Theoretical predictions are experimentally validated using scanning laser Doppler vibrometer measurements of guided wave propagation generated by a circular piezoelectric wafer active sensor transducer in a unidirectional carbon fiber reinforced polymer composite plate. The proposed method achieves good agreement with the experimental results.

The validation of structural health monitoring (SHM) systems for aircraft is complicated by the extent and number of factors that the SHM system must demonstrate for robust performance. Therefore, a time- and cost-efficient method for examining all of the sensitive factors must be conducted. In this paper, we demonstrate the utility of using the simulation modeling environment to determine the SHM sensitive factors that must be considered for subsequent experiments, in order to enable the SHM validation. We demonstrate this concept by examining the effect of SHM system configuration and flaw characteristics on the response of a signal from a known piezoelectric wafer active sensor (PWAS) in an aluminum plate, using simulation models of a particular hot spot. We derive the signal responses mathematically and through the statistical design of experiments, we determine the significant factors that affect the damage indices that are computed from the signal, using only half the number of runs that are normally required. We determine that the transmitter angle is the largest source of variation for the damage indices that are considered, followed by signal frequency and transmitter distance to the hot spot. These results demonstrate that the use of efficient statistical design and simulation may enable a cost- and time-efficient sequential approach to quantifying sensitive SHM factors and system validation.

This paper addresses an analytical and experimental analysis based on the physics of the Lamb wave propagation and interaction with the discontinuity. An analytical method called “complex modes expansion with vector projection (CMEP)” is used to calculate the scattering coefficients (amplitude of the out-of-plane velocity) of Lamb wave modes from geometric discontinuities. Two cases are considered in this research: (a) a plate with a pristine stiffener and (b) a plate with a cracked stiffener. Complex-valued scattering coefficients are calculated from 50 kHz to 350 kHz for A0 incident waves. Scatter coefficients are compared for both cases to identify a suitable frequency range to excite Lamb waves using a piezoelectric wafer active sensor (PWAS) to detect the crack. The frequencydependent complex-valued scatter coefficients are then inserted into the global analytical model. The global analytical solution predicts time domain scattered signal from the discontinuity. The crack can be detected by comparing the waveforms for pristine stiffener and cracked stiffener. An experiment was conducted for both pristine stiffener and cracked stiffener to compare with the analytical results. A long PWAS was placed parallel to the waveform to create straight crested Lamb wave modes in the plate. Antisymmetric Lamb wave modes were selectively excited by using two PWAS transducers placed on opposite sides of the plate and energized by out-of-phase signals. A single-point laser Doppler vibrometer (LDV) was used to measure the out-of-plane velocity of scattered Lamb waves on the plate. The obtained experimental results agree well with the analytical predictions.

This paper focuses on impact localization of composite structures, which possess more complexity in the guided wave propagation due to the anisotropic behavior of composite materials. In this work, a composite plate was manufactured by using a compression molding process with proper pressure and temperature cycle. Eight layers of woven composite prepreg were used to manufacture the composite plate. A structural health monitoring (SHM) technique was implemented with piezoelectric wafer active sensors (PWAS) to detect and localize the impact on the plate. There were two types of impact event that were considered in this paper (a) low energy impact event (b) high energy impact event. Two clusters of sensors recorded the guided acoustic waves generated from the impact. The acoustic signals were then analyzed using a wavelet transform based time-frequency analysis. The proposed SHM technique successfully detected and localized the impact event on the plate. The experimentally measured impact locations were compared with the actual impact locations. An immersion ultrasonic scanning method was used to visualize the composite plate before and after the impact event. A high frequency 10 MHz 1-inch focused transducer was used to scan the plate in the immersion tank. Scanning results showed that there was no visible manufacturing damage in the composite plate. However, clear impact damage was observed after the high-energy impact event.

To predict dispersion curves it is common to use different solution approaches depending on the material type, isotropic or composite, of the medium in which the wave propagates. The two different solution methods are defined in different domains, frequency–wavespeed domain for isotropic materials, and wavenumber–wavespeed domain for composites which can lead to difficulties, and unsatisfying results when predicting the dispersion curves for hybrid laminates which contain both isotropic and composite materials. This article, therefore, proposes a unified formulation defined in the wavenumber–wavespeed domain for both isotropic and composite materials. The unified formulation, simple, and mathematically straightforward formulation, utilizes Christoffel’s equation for a lamina to obtain the eigenvalues and eigenvectors. The eigenvalues and eigenvectors are then used to set up the field matrix from which the dispersion curves could be retrieved. Once the dispersion curves were obtained the waves are grouped using a modeshape analysis. A spline algorithm is applied to obtain a continuous solution from a rough domain which was used to reduce computational time. In addition, this article highlights the challenges faced in the numerical process, and provides a discussions of the methods used to overcome these obstacles.

The aerospace industry continues to increase the use of adhesives for structural bonding due to the increased joint efficiency (reduced weight), even distribution of the load path and decrease in stress concentrations. However, the limited techniques for verifying the strength of adhesive bonds has reduced its use on primary structures and requires an intensive inspection schedule. This paper discusses a potential structural health monitoring (SHM) technique for the detection of disbonds through the in situ inspection of adhesive joints. This is achieved through the use of piezoelectric wafer active sensors (PWAS), thin unobtrusive sensors which are permanently bonded to the aircraft structure. The detection method utilized in this study is the electromechanical impedance spectroscopy, a local vibration method. This method detects disbonds from the change in the mechanical impedance of the structure surrounding the disbond. This paper will discuss how predictive modeling can provide valuable insight into the inspection method, and provide better results than purely empirical methods will provide. A method for identifying the appropriate frequency range and sensor locations is presented, and the method was verified experimentally using a large aluminum test article, and included both pristine and disbond coupons.