Local Defect Resonance Frequency Research Articles

Far-sided defect recognition of FRP sandwich structures based on local defect resonance

This work provides a novel methodology to non-destructively detect far-sided defect of sandwich structures by local defect resonance-based acoustic activation. Sandwich panel specimens with 10 mm thick balsa wood core and 25 mm thick Nomex honeycomb core are investigated, and artificial facesheet-core disbond with a size of 30 mm × 30 mm is introduced to the specimens. Finite element modeling is constructed to investigate resonance frequency of defect region. Experimental apparatus is constructed to detect far-sided facesheet-core disbond defect, where vibration activation and measurement is performed at near-sided facesheet. It is shown that if the far-sided defect is activated at local defect resonance frequency, distinguishable nonlinear vibration characteristic could be observed from the near-sided facesheet, and the 3rd order superharmonic is especially significant. A stepped frequency sweep excitation approach is further developed for recognition of far-sided defect, which better fits automated defect detection process as local defect resonance frequency is not required to be preliminarily determined.

Efficient determination of local defect resonance frequencies from bicoherence plots using double excitations

Abstract In recent years, a lot of developments can be seen in the field of local defect resonance (LDR) based thermosonics which provide thermal image of a defect by exciting the structure at particular defect frequencies. LDR based thermosonics is found to be more promising compared to traditional thermosonics due to its high reproducibility and low power usage. However, fast and accurate detection of these LDR frequencies is a challenging task. In this context, this paper addresses a fast and reliable method of LDR frequency determination using bicoherence plots. Two exciters and one receiver PZTs (lead zirconate titanate) are used for the experiment. One exciter PZT is provided with chirp or Gaussian white noise signal input while a fixed frequency signal is given to the second exciter PZT. Intermodulation of LDR and excitation frequencies can be observed in frequency plots. LDR frequencies can be easily detected from bicoherence plots, while its detection from frequency plots is found to be cumbersome. The efficacy of bicoherence analysis in detection of LDR frequency is shown using an aluminium plate with flat bottom hole and three delaminated glass fibre reinforced composite (GFRP) plates. Second harmonic of LDR frequencies are evaluated from the bicoherence plots which can be used later for defect imaging.

Analytical evaluation of resonance frequencies for planar defects: Effect of a defect shape

Local Defect Resonance (LDR) is a promising approach to detect, localize, and visualise damage based on its resonant ultrasonic response. In this paper, the classical Rayleigh's method is applied for analytical evaluation of the fundamental frequencies of LDR for various shape planar defects in polymers and composite materials. A series of experiments implemented with simulated and realistic delaminations verifies the analytical formulas obtained and reveals the constraints for practical application of the theoretical approach. The case studies considered include some basic shapes of the defects to recognize the impact of the defect shape, size and position on LDR frequency. The expressions derived combined with the resonant imaging results address the reverse problem of characterizing the depth of the defect location.

Detection of local defect resonance frequencies using bicoherence analysis

The nonlinear acoustic wave spectroscopy and defectometry (NAWSD) technique is found to be an efficient tool for monitoring intricate and complex defects in composite structures due to its better resolution and accuracy compared to the traditional methods. Local defect resonance (LDR) is one of such methods based on amplifying vibrations at the defect regions only, resulting in defect-selective and high resolution image. In the present work, numerical as well as experimental approach has been presented in order to detect the LDR frequency of aluminium plate having circular flat bottom hole (FBH) and delaminated glass fibre reinforced polymer composite (GFRP) plates. First, a wave propagation study using explicit dynamic analysis is performed followed by, bicoherence analysis to detect the LDR frequency numerically. The LDR frequency modes are also verified by steady state dynamic analysis. A set of experiments are then carried out using Gaussian white noise as well as chirp signal to determine the LDR frequencies from the Fast Fourier Transform (FFT) and bicoherence plots. In every cases, the bicoherence peaks corresponds to the second harmonic of LDR frequency. The bicoherence plots are found to be robust and effective tool for detection of LDR frequencies compared to FFT plots. Bicoherence analysis based LDR frequency determination technique can be further utilized in delamination and disbond imaging of composite structures.

Detection and Characterization of Local Defect Resonances Arising from Delaminations and Flat Bottom Holes

The concept of local defect resonance (LDR) can be used to diagnose the presence of defects, such as delaminations and cracks, and enable their localisation. This paper describes an automated detection methodology of LDR frequencies using signals measured by laser Doppler vibrometry. The performance of the detection algorithm is validated, both numerically and experimentally, on an aluminum sample containing a flat bottom hole. Subsequently, the algorithm has been applied on a sample made of glass-fiber-reinforced polymer that contains a single circular delamination and a composite sandwich structure with a disbond. In all cases, the lowest order LDR frequencies were successfully identified.

Non-Destructive Evaluation of Kissing Bonds using Local Defect Resonance (LDR) Spectroscopy: A Simulation Study

With the growing demand from industry to optimize and further develop existing Non-Destructive Testing & Evaluation (NDT&E) techniques or new methods to detect and characterize incipient damage with high sensitivity and increased quality, ample efforts have been devoted to better understand the typical behavior of kissing bonds, such as delaminations and cracks. Recently, it has been shown experimentally that the nonlinear ultrasonic response of kissing bonds could be enhanced by using Local Defect Resonance (LDR) spectroscopy. LDR spectroscopy is an efficient NDT technique that takes advantage of the characteristic fre- quencies of the defect (defect resonances) in order to provide maximum acoustic wave-defect interaction. In fact, for nonlinear methodologies, the ultrasonic excitation of the sample should occur at either multiples or integer ratios of the characteristic defect resonance frequencies, in order to obtain the highest signal-to-noise response in the nonlinear LDR spectroscopy. In this paper, the potential of using LDR spectroscopy for the detection, localization and characterization of kissing bonds is illustrated using a 3D simulation code for elastic wave propagation in materials containing closed but dynamically active cracks or delaminations. Using the model, we are able to define an appropriate method, based on the Scaling Subtraction Method (SSM), to determine the local defect resonance frequencies of a delamination in a composite plate and to illustrate an increase in defect nonlinearity due to LDR. The simulation results will help us to obtain a better understanding of the concept of LDR and to assist in the further design and testing of LDR spectroscopy for the detection, localization and characterization of kissing bonds.

Impact damage detection in laminated composites by non-linear vibro-acoustic wave modulations

The paper presents an application of nonlinear acoustics for impact damage detection in composite laminates. Two composite plates were analysed. A low-velocity impact was used to damage one of the plates. Ultrasonic C-scan was applied to reveal the extent of barely visible impact damage. Finite element modelling was used to find vibration mode shapes of the plates and to estimate the local defect resonance frequency in the damaged plate. A delamination divergence study was performed to establish excitation parameters for nonlinear acoustics tests used for damage detection. Both composite plates were instrumented with surface-bonded, low-profile piezoceramic transducers that were used for the high-frequency ultrasonic excitation. Both an arbitrary frequency and a frequency corresponding to the local defect resonance were investigated. The low-frequency modal excitation was applied using an electromagnetic shaker. Scanning laser vibrometry was applied to acquire the vibro-acoustic responses from the plates. The study not only demonstrates that nonlinear vibro-acoustic modulations can successfully reveal the barely visible impact damage in composite plates, but also that the entire procedure can be enhanced when the ultrasonic excitation frequency corresponds to the resonant frequency of damage.