1,721,029 research outputs found
Phononic crystal sensing devices for enhanced nonlinear ultrasonic inspection
No full textNonlinear ultrasonic methods have proven to be far more sensitive than conventional linear ultrasounds to early detection of damage such as micro- or partially closed cracks, by measuring small nonlinear ultrasonic waves generated by the defect. However, the efficiency of nonlinear ultrasonic testing and their large-scale use in industrial applications is still limited by undesired instrumentation effects caused by high-power ultrasonic signal transmission, which may mask nonlinear ultrasonic waves and prevent damage detection. This paper aims to overcome the challenge of accurate detection of nonlinear ultrasonic signals by designing and developing new portable and compact phononic crystal (PC) waveguide transducers, which enhance the performance of nonlinear ultrasonic methods by achieving unprecedented sensitivity and reliability. These innovative PC sensing devices are material-based tunable filters that allow natural propagation of nonlinear ultrasonic waves and suppression of undesired instrumentation effects. PC waveguide transducers can be integrated directly on the monitored component and are easily fabricated using additive manufacturing (3D printing) processes. Their design and optimisation is driven by a pioneering theoretical model based on perturbative couple-mode theory of guided wave propagation in structures with periodic corrugated surface profiles. Experimental nonlinear ultrasonic tests confirmed these theoretical predictions and indicated that PC sensing device are able to enhance the sensitivity of nonlinear ultrasonic inspection for various materials and components
CFRP composites with embedded PZT transducers for nonlinear ultrasonic inspection of space structures
No full textSpacecraft structures are made of carbon fibre reinforced plastic (CFRP) composites due to their high strength-to-weight ratio. However, material damage such as micro-cracks and delamination are likely to occur during spacecraft fabrication, assembly or on-orbit due to hypervelocity debris impacts. In the latter case, satellite components are visually inspected during time-consuming and risky astronauts’ extravehicular activities. Hence, there is a need for real-time monitoring of cracks in spacecraft composites, especially for future manned missions. The integration of piezoelectric lead zirconate titanate (PZT) transducers in CFRP composites is a possible solution for the development of “smart” structures capable of (i) providing in-situ ultrasonic monitoring of damage, and (ii) preventing the direct exposure of PZTs to the harsh outer space. In a previous study, the use of a woven E-glass fibre fabric layer between the PZT and the CFRP plies was proposed as a suitable technique for electrical insulation of embedded PZTs with no effect on the interlaminar properties of the composite. Nonlinear ultrasonic experiments on artificially delaminated CFRP plates revealed that the damage sensitivity based on the second harmonic generation was nearly two times higher than with conventionally surface-bonded PZTs. In this study, nonlinear ultrasonic experiments on CFRP test samples with both artificial (in-plane delamination) and real impact damage proved the capability of the proposed embedded PZTs to detect multiple defects of various dimensions. The ultrasonic response of damaged specimens was studied against that of a pristine one, and damage detection was achieved based on the generation of second harmonics at specific input signal frequencies. In addition, by scanning the material response with a laser Doppler vibrometer it was verified that for each of the chosen driving frequencies, the area on the sample’s surface at which the out-of-plane vibrational velocity was higher matched the position of the associated damage. Based on the results of this study, the novel sensor embedding technique has the potential to be used for in-service monitoring of composite spacecraft components and other critical engineering structures
Acoustic emission source localization and velocity determination of the fundamental mode A<sub>0</sub> using wavelet analysis and a Newton-based optimization technique
Full text linkThis paper investigates the development of an in situ impact detection monitoring system able to identify in real-time the acoustic emission location. The proposed algorithm is based on the differences of stress waves measured by surface-bonded piezoelectric transducers. A joint time-frequency analysis based on the magnitude of the continuous wavelet transform was used to determine the time of arrival of the wavepackets. A combination of unconstrained optimization technique associated with a local Newton's iterative method was employed to solve a set of nonlinear equations in order to assess the impact location coordinates and the wave speed. With the proposed approach, the drawbacks of a triangulation method in terms of estimating a priori the group velocity and the need to find the best time-frequency technique for the time-of-arrival determination were overcome. Moreover, this algorithm proved to be very robust since it was able to converge from almost any guess point and required little computational time. A comparison between the theoretical and experimental results carried out with piezoelectric film (PVDF) and acoustic emission transducers showed that the impact source location and the wave velocity were predicted with reasonable accuracy. In particular, the maximum error in estimation of the impact location was less than 2% and about 1% for the flexural wave velocity
Imaging non-classical elastic nonlinearities using reciprocal time reversal and phase symmetry analysis
No full textIn this research work, an imaging method of the nonlinear signature in a reverberant complex anisotropic structure with hysteretic behaviour is reported. The proposed technique relies on a combination of phase symmetry analysis with frequency modulation excitation and nonlinear time reversal, and it is applied to a number of waveforms containing the nonlinear impulse responses of the medium. Phase symmetry analysis was used to characterize the third order nonlinearity of the structure due to delamination and cracks, by exploiting its invariant properties with the phase angle of the input waveforms. Then, a "virtual" reciprocal time reversal imaging process, using only two sensors in pitch-catch mode, was used to "illuminate" the damage. Taking advantage of multiple linear scattering, this methodology allows achieving the optimal focalization at the nonlinear source by a compensation of the distortion effects in a dissipative medium. The robustness of this technique was experimentally demonstrated on a damaged sandwich panel undergone to low-velocity impact loading. The nonlinear source was retrieved with a high level of accuracy with little computational time (less than 1 sec). Its minimal processing requirements make this method a valid alternative to the traditional nonlinear elastic wave spectroscopy techniques for materials showing either classical or non-classical nonlinear behaviour. © 2012 SPIE
Impact detection in anisotropic materials using a time reversal approach
Full text linkThis article presents an in situ imaging method able to detect in real-time the impact source location in reverberant complex composite structures using only one passive sensor. This technique is based on the time reversal acoustic method applied to a number of waveforms stored in a database containing the impulse response (Green's function) of the structure. The proposed method allows achieving the optimal focalization of the acoustic emission source in the time and spatial domain as it overcomes the drawbacks of other ultrasonic techniques. This is mainly due to the dispersive nature of guided Lamb waves as well as the presence of multiple scattering and mode conversion that can degrade the quality of the focusing, causing poor localization. Conversely, using the benefits of a diffuse wave field, the imaging of the source location can be obtained through a virtual time reversal procedure, which does not require any iterative algorithms and a priori knowledge of the mechanical properties and the anisotropic group speed. The efficiency of this method is experimentally demonstrated on a stiffened composite panel. The results showed that the impact source location can be retrieved with a high level of accuracy in any position of the structure (maximum error was less than 3%)
Fatigue testing and damage evaluation using smart CFRP composites with embedded PZT transducers
No full textIn aerospace applications, carbon fibre reinforced plastic (CFRP) composite parts are prone to fibre-breakage, matrix cracking and delamination as a result of manufacturing and assembly errors, or in-service impacts. The idea of developing “smart” composites with built-in sensing networks for real-time ultrasonic inspection of aircraft parts has become very popular, to minimise equipment purchase costs and service delays associated with conventional non-destructive testing techniques. Recently, the authors proposed a novel design of “smart” CFRP plates including internal piezoelectric (PZT) transducers without any compromise on the compressive, flexural or interlaminar shear strength of the material. The sensors were embedded between the composite layers and covered with glass fibre patches for electrical insulation from the carbon fibres. A series of nonlinear ultrasonic experiments proved the suitability of this internal sensor configuration for detecting material damage, based on the second harmonic generation method. In this paper impacted CFRP samples with the same glass fibre insulated PZTs (G-specimens) were subject to fatigue testing and the number of cycles to failure (∼675,000) was found to be equal to that of impacted samples without sensors (P-specimens). Ultrasonic experiments on G-specimens showed that the acoustic nonlinearity was increased by almost two orders of magnitude up to 480,000 cycles based on the ratio of second-to-fundamental harmonic amplitude. This confirmed that the unique layout of embedded transducers could not only be used for material damage detection, but it was also capable of monitoring the damage evolution under repeated loading. The capacitance of the PZTs remained constant (∼1.54 nF) during ultrasonic experimentation, verifying their functionality for at least 70% of the fatigue life
A new algorithm for acoustic emission localization and flexural group velocity determination in anisotropic structures
No full textThis paper presents a new in situ Structural Health Monitoring (SHM) system able to identify the location of acoustic emission (AE) sources due to low-velocity impacts and to determine the group velocity in complex composite structures with unknown lay-up and thickness. The proposed algorithm is based on the differences of stress waves measured by six piezoelectric sensors surface bonded. The magnitude of the Continuous Wavelet Transform (CWT) squared modulus was employed for the identification of the time of arrivals (TOA) of the flexural Lamb mode (A0). Then, the coordinates of the impact location and the flexural wave velocity were obtained by solving a set of non-linear equations through a combination of global Line Search and backtracking techniques associated to a local Newton’s iterative method. To validate this algorithm, experimental tests were conducted on two different composite structures, a quasi-isotropic CFRP and a sandwich panel. The results showed that the impact source location and the group speed were predicted with reasonable accuracy (maximum error in estimation of the impact location was approximately 2% for quasi-isotropic CFRP panel and nearly 1% for sandwich plate), requiring little computational time (less than 2 s)
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