Due to the extensive use of composite materials in aerospace structures, they are highly susceptible to various types of damage including barely visible impact damage (BVID) due to low-velocity impact events such as bird strikes and runway debris. Implementation of guided-wave based structural health monitoring (SHM) for BVID detection in composites has been extremely challenging due to the anisotropic nature of composites and the complex wave-damage interaction with impact damage. This paper presents a new methodology for the detection of BVID in composites using selectively pure shear horizontal (SH) guided wave mode generated by angle beam transducers (ABT) and received by phased array transducers (PAT). In this methodology, pure SH0 wave excitation was achieved using the angle beam transducers and variable angle wedges and after propagation in the composite it was received by the phased array transducer. First, the semi-analytical finite element (SAFE) method was used to generate the guided wave dispersion curves and the corresponding tuning angle for the SH0 mode excitation was obtained based on Snell’s law. Then, pitch-catch experiments using the ABT-PAT transducer pair were conducted to validate the pure SH0 mode excitation. After this, impact damage experiments were conducted on multiple quasi-isotropic carbon fiber reinforced polymer (CFRP) composite plates having different thicknesses by conforming to the ASTM D7136 standard to generate controlled impact damage size of 1". Next, impact damage detection experiments using pure SH0 mode at 500 kHz, were conducted using the ABT-PAT transducer pair. It was observed that pure SH0 mode was sensitive to the impact damage as a significant decrease in the signal amplitude and mode conversion was observed. With our experiments, we demonstrated the validity and usefulness of this technique for impact damage detection in composite structures. An invention disclosure describing the use of angle beam transducers for SH wave excitation method has been filed and is in the process of becoming a provisional patent.
Guided wave-based damage evaluation has been regarded as a promising method in the area of structural health monitoring. The main obstacle for the practical application of these guided wave-based monitoring methods is the reliability of damage evaluation under time-varying ambient conditions. In this paper, an analytical model and a semi-analytical finite element (SAFE) method are proposed to study the effect of temperature and load on guided wave propagation in an isotropic plate respectively including the dispersion curves and waveform. In the presented models, the temperature and load dependent elastic constants are considered to study the variations of guided wave properties. The result shows that the phase velocity gradually decreases with the incremental temperature. It is also observed that the phase velocity gradually decreases with the incremental load. Finally, the analytical model and SAFE method are validated through the experimental data. It shows that the results obtained from the theoretical model match well with the experimental results.
Carbon fiber reinforced polymer (CFRP) composites have been widely used in aerospace structures due to their high specific strength and stiffness, resistance to corrosion, and lightweight. However, it has posed new challenges for implementing guided wave-based structural health monitoring (SHM) techniques due to the general anisotropic behavior and complicated wave-damage interaction scenarios in composites. This paper presents a new methodology for detecting various types of composite damage such as simulated delamination and actual impact damage using pure shear horizontal (SH) wave generated by adjustable angle beam transducers. For the first time, angle beam transducers were successfully applied to excite pure SH0 wave in a 2-mm quasi-isotropic composite plate. The pure SH0 wave excitation was verified by a three-dimensional (3D) finite element (FE) simulation. SH0 wave propagation and interaction with the delamination were further investigated numerically and strong trapped waves within the delamination region were observed. Experimental validations were conducted to detect simulated delamination by Teflon insert using pure SH0 wave. A good match between the FE simulation and the experiment was achieved. Pure SH0 wave was also utilized to detect actual impact damage in the quasi-isotropic CFRP composite plate. It can be found that the SH0 wave is sensitive to both delamination and impact damage, and a significant amplitude drop is observed due to the presence of different types of damage. Both numerical and experimental results demonstrated the effectiveness of pure SH0-wave detection of various damage types in composites using angle beam transducers.
In this paper, numerical and experimental investigations of wave damage interaction in metals and composites were conducted. The frequency and direction dependent complex-valued wave damage interaction coefficients (WDIC) were used to model the scattering and mode conversion phenomena of guided wave interaction with damage. These coefficients were extracted from the harmonic analysis of small-size finite element (FE) model with non-reflective boundaries (NRB) and they are capable of describing the amplitude and phase of the scattered waves as a function of frequency and direction. Commercial finite element package ANSYS 17.0 was used to implement and realize the small-size FE model. First, the proposed method was used to extract the WDIC of a through hole in an aluminum plate. Then, the FEM WDIC was compared with the analytical model of A0 wave scattering at through hole, and the experimental validation was performed using scanning laser Doppler vibrometer (SLDV) measurements on an aluminum plate. It is shown that a good agreement between FEM WDIC and experimental WDIC is achieved. Finally, the harmonic analysis of small-size FE models was extended to extract the WDIC of composites. Through hole and delamination in a unidirectional composite plate were investigated. It can be observed that the scattering energy is mainly concentrated in the fiber directions when the A0 waves interact with the through hole.
In this paper, a semi-analytical finite element (SAFE) approach is presented to model the guided-wave propagations in composite plates. The theoretical framework is formulated using finite element method (FEM) to describe the material variation along the thickness direction and assuming analytical solutions in the wave propagation direction. As with any finite element approach, the convergence study is first performed to ensure the accuracy of the solution. Then, the dispersive curves are obtained in terms of phase velocity, group velocity, and steering angle. In general, a wave packet in composite plates with anisotropic characteristics does not travel in the same direction as the phase velocity, and the difference is defined as steering angle. Knowledge of these properties in composite plates is important in guided waves based SHM applications. Finally, it is experimentally validated using the scanning laser Doppler vibrometer (SLDV) measurements of guided wave packets generated by a piezoelectric wafer active sensor (PWAS) in a unidirectional carbon fiber reinforced polymer (CFRP) composite plate. It will be shown that the SAFE approach achieves a good agreement with experimental results.
Structural health monitoring (SHM) is in urgent need and must be integrated into the nuclear-spent fuel storage systems to guarantee the safe operation. The dry cask storage system (DCSS) is such storage facility, which is licensed for temporary storage for nuclear-spent fuel at the independent spent fuel storage installations (ISFSIs) for certain predetermined period of time. Gamma radiation is one of the major radiation sources near DCSS. Therefore, a detailed experimental investigation was completed on the gamma radiation endurance of piezoelectric wafer active sensors (PWAS) transducers for SHM applications to the DCSS system. The irradiation test was done in a Co-60 gamma irradiator. Lead Zirconate Titanate (PZT) and Gallium Orthophosphate (GaPO4) PWAS transducers were exposed to 40.7 kGy gamma radiation. Total radiation dose was achieved in two different radiation dose rates: (a) slower radiation rate at 0.1 kGy/hr for 20 hours (b) accelerated radiation rate at 1.233 kGy/hr for 32 hours. The total cumulative radiation dose of 40.7 kGy is equivalent to 45 years of operation in DCSS system. Electro-mechanical impedance and admittance (EMIA) signatures and electrical capacitance were measured to evaluate the PWAS performance after each gamma radiation exposure. The change in resonance frequency of PZT-PWAS transducer for both in-plane and thickness mode was observed. The GaPO4-PWAS EMIA spectra do not show a significant shift in resonance frequency after gamma irradiation exposure. Radiation endurance of new high-temperature HPZ-HiT PWAS transducer was also evaluated. The HPZ-HiT transducers were exposed to gamma radiation at 1.233 kGy/hr for 160 hours with 80 hours interval. Therefore, the total accumulated gamma radiation dose is 184 kGy. No significant change in impedance spectra was observed due to gamma radiation exposure.