1,721,055 research outputs found
Design of subwavelength wide bandwidth sound absorbers by inverse convolutional neural networks
No full textMicroperforated panel sound absorber metamaterials are crucial for noise reduction in various applications. This study leverages a convolutional neural network (CNN) machine learning model to optimise these metamaterials for maximum absorption strength and bandwidth range. The model allows for inverse optimisation of sound absorption performance. A desired absorption response can be supplied as input, and the network returns the necessary geometry parameters to achieve the target characteristic. Metamaterials were optimised to provide over 90 % absorption at target frequencies between 0–1000 Hz. Theoretical predictions were validated experimentally via impedance tube testing. The model achieved no less than 70 % absorption over a 923 Hz range (548–1471 Hz) with a material thickness of 41 mm, and 70 % absorption over 1000 Hz (470–1470 Hz) with a thickness of 57 mm. A case study for an automotive/energy application targeted 50 % absorption between 500–1000 Hz at a thickness of less than 25 mm. Experimental results showed 50 % absorption between 506–1032 Hz at 23 mm thickness. These findings demonstrate the potential of CNN models in optimising sound absorber metamaterials, offering significant improvements in noise reduction with minimal material thickness. The proposed methodology offers significant potential for lightweight applications in various noise-reduction scenarios, including automotive, aerospace, energy, and architectural acoustics.</p
Factors affecting the imaging of the impact location with inverse filtering and diffuse wave fields
Full text linkReciprocal time reversal (inverse filtering) of acousto-ultrasonic fields is a very efficient technique to focus elastic waves through reverberant isotropic and anisotropic media. Such a methodology relies on the correlation of the experimental Green's function that is acquired by a set of receiver sensors from a limited number of impact sources. However, although heterogeneities and discontinuities within the structural response can be compensated by the inverse filtering process, environmental effects such as temperature variations as well as incoherent noise measurements and the finite number of excitation sources may degrade the quality of time reversal focusing. The scope of this article was to study the factors affecting the impact location imaging using the reciprocal time reversal method in the presence of complex diffuse wave fields. Particularly, a signal-stretch strategy was developed to compensate the temperature changes before remitting the back-propagated wave field at the focus point. Then, in order to investigate the imaging performance and the sensitivity of the proposed methodology, different sets of libraries with reduced input signals were created and tested. Finally, different configurations of the receiver piezoelectric sensors were used to perform the reciprocal time reversal method. To validate this research work, two geometrically complex composite structures, that is, a composite tail rotor blade and a stiffened composite panel, were used. Results showed that both the temperature compensation and the signal processing with the reduced time traced signals and receiver sensors allowed obtaining an accurate identification of the impact events
Tensile and fatigue testing of impacted smart CFRP composites with embedded PZT transducers for nonlinear ultrasonic monitoring of damage evolution
Full text linkUltrasonic systems based on 'smart' composite structures with embedded sensor networks can reduce both inspection time and costs of aircraft components during maintenance or in-service. This paper assessed the tensile strength and fatigue endurance of carbon fibre reinforced plastic (CFRP) laminates with embedded piezoelectric (PZT) transducers, which were covered with glass fibre patches for electrical insulation. This sensor layout was proposed and tested by the authors in recent studies, proving its suitability for nonlinear ultrasonic detection of material damage without compromising the compressive, flexural or interlaminar shear strength of the 'smart' CFRP composite. In this work, CFRP samples including PZTs (G-specimens) were tested against plain samples (P-specimens), and their mean values of tensile strength and fatigue cycles to failure were found to be statistically the same (910 MPa and 713 000 cycles) using the one-way analysis of variance method. The same tests on P- and G-specimens with barely visible impact damage (BVID) showed that the corresponding group means were also the same (865 MPa and 675 000 cycles). Nonlinear ultrasonic experiments on impacted G-samples demonstrated that embedded PZTs could monitor the growth of BVID during fatigue testing, for a minimum of 480 000 cycles. This was achieved by calculating an increase of nearly two orders of magnitude in the ratio of second-to-fundamental harmonic amplitude. Finally, PZT transducers were confirmed functional under cyclic loading up to ∼70% of sample's life, since their capacitance remained constant during ultrasonic testing
Nonlinear elastic imaging using reciprocal time reversal and third order symmetry analysis
Full text linkThis paper presents a nonlinear imaging method for the detection of the nonlinear signature due to impact damage in complex anisotropic solids with diffuse field conditions. The proposed technique, based on a combination of an inverse filtering approach with phase symmetry analysis and frequency modulated excitation signals, 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 by exploiting its invariant properties with the phase angle of the input waveforms. Then, a “virtual” reciprocal time reversal imaging process, using only one broadcasting transducer and one receiving transducer, was used to insonify the defect taking advantage of multiple linear scattering as mode conversion and boundary reflections. The robustness of this technique was experimentally demonstrated on a damaged sandwich panel, and the nonlinear source, induced by low-velocity impact loading, was retrieved with a high level of accuracy. 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 behavior
Acoustic emission localization in complex dissipative anisotropic structures using a one-channel reciprocal time reversal method
Full text linkThis paper presents an imaging method for the localization of the impact point in complex anisotropic structures with diffuse field conditions, using only one passive transducer. The proposed technique is based on the reciprocal time reversal approach (inverse filtering) applied to a number of waveforms stored into a database containing the experimental Green's function of the structure. Unlike most acoustic emission monitoring systems, the present method exploits the benefits of multiple scattering, mode conversion, and boundaries reflections to achieve the focusing of the source with high resolution. Compared to a standard time reversal approach, the optimal refocusing of the back propagated wave field at the impact point is accomplished through a virtual imaging process. The robustness of the inverse filtering technique is experimentally demonstrated on a dissipative stiffened composite panel and the source position can be retrieved with a high level of accuracy in any position of the structure. Its very simple configuration and minimal processing requirements make this method a valid alternative to the conventional imaging Structural Health Monitoring systems for the acoustic emission source localization
Dislocation detection of gas turbine materials using a nonlinear ultrasound modulation technique
No full textIndustrial gas turbines are used for generating electricity or driving other turbomachinery with the continuous development goal of further increasing machine efficiency. This is primarily achieved by raising the pressure ratio generated in the compressor and by increasing the turbine inlet temperature. Consequently, the hot gas components in gas turbines are subjected to extreme loads and the need for non-destructive testing and structural health monitoring techniques is becoming increasingly important to maintain these components. An important indicator for assessing the structural integrity is the determination of the initial plastic deformation. In this paper, a new method for the detection of plasticity was developed, which is based on a nonlinear ultrasonic two-frequency excitation. The one-dimensional wave equation was solved with a two-frequency excitation and combined with the expanded dislocation theory. As a result, various nonlinearity parameters were defined, showing a clear increasing or a decreasing behaviour with increasing plastic strain. This was experimentally proven with flat tensile specimen made of stainless steel and Inconel 718 (metal plates and additively manufactured). The new indicators allow the possibility to efficiently detect the initial plastic deformation in gas turbine components
Impact localization on a composite tail rotor blade using an inverse filtering approach
Full text linkThis article presents an in situ structural health monitoring imaging system for the localization of impacts on a composite complex structure such as a tail rotor blade. Unlike conventional plate-like panels, this composite structure presents a strong anisotropy and inhomogeneous elastic nature due to the presence of both glass fibre and carbon fibre, a geometrically complex shape due to the curvature of the blade's airfoil section and variations in the mechanical behaviour due to local changes in the thickness. The proposed imaging technique is based on the inverse filtering or reciprocal time reversal approach applied to the waveforms originated from a point of the structure of unknown location (impact source) and a number of signals stored in a database containing the experimental Green's function of the medium. Unlike other ultrasonic impact localization methods, the present technique allows achieving the optimal focalization of the impact point in the spatial and time domain, by taking advantage of multiple linear scattering and a small number of receiver sensors
Nonlinear imaging method using second order phase symmetry analysis and inverse filtering
Full text linkThis paper presents a nonlinear imaging method based on nonlinear elastic guided waves, for the damage detection and localisation in a composite laminate. The proposed technique relies on the study of the structural nonlinear responses by means of a combination of second order phase symmetry analysis (PSA) with chirp excitation and inverse filtering (IF) method. PSA was used to exploit the invariant properties of the propagating elastic waves with the phase angle of the pulse compressed chirp signals, in order to characterise the second order nonlinear behaviour of the medium. Then, the IF approach was applied to a library of second order nonlinear responses to obtain a two-dimensional image of the damage. The experimental tests carried out on an impact damage composite sample were compared to standard C-scan. The results showed that the present technique allowed achieving the optimal focalisation of the nonlinear source in the spatial and time domain, by taking advantage of multiple scattering and a small number of receiver sensors
Nonlinear ultrasonic inspection of smart carbon fibre reinforced plastic composites with embedded piezoelectric lead zirconate titanate transducers for space applications
Full text linkCarbon fibre reinforced plastic composites used in spacecraft structures are susceptible to delamination, debonds and fibre cracking that may arise during manufacturing, assembly or in-service operations (e.g. caused by debris impacts in near-Earth orbital spaceflights). Therefore, in situ and real-time health monitoring is necessary to avoid time-consuming and unsafe visual inspections performed either on-ground or during extra vehicular activities. In this article, a recently created ‘smart’ carbon fibre reinforced plastic composite structure with embedded piezoelectric lead zirconate titanate transducers was used to detect multiple areas of artificial delamination and real impact damage of different size using nonlinear ultrasound. The electrical insulation of embedded piezoelectric lead zirconate titanate transducers was achieved by interlacing a dry layer of woven glass fibre fabric between the sensor and the carbon fibre reinforced plastic plies before curing. Damage detection was successfully demonstrated using both second harmonic generation and nonlinear modulation (sidebands) of the measured ultrasonic spectrum. The material nonlinear response at the second harmonic and sidebands frequencies was also measured with a laser Doppler vibrometer to validate the nonlinear ultrasonic tests and provide damage localisation. Experimental results revealed that the proposed configuration of embedded piezoelectric lead zirconate titanate transducers can be utilised for on-board ultrasonic inspection of spacecraft composite parts.</p
A bistable impact resistant sandwich carbon-fibre reinforced core
Full text linkThe interest of aerospace and automotive fields in the improvement of energy absorption properties of composite materials has stimulated researchers to design new typologies of cores of sandwich-structures under impact events. In this work, a sandwich structure based on pre-stressed bistable carbon fibre composite core is proposed. The core, named Circular Prestressed Carbon Core (CPCC), takes advantage of the asymmetric fibres orientation and the bistability-concept to enhance energy dissipation and reduce the damage extension during impacts. The impact campaign was conducted both experimentally and numerically to analyse the mechanical properties of the core and to study the contribution of the residual stresses. Experimental results were compared with an aerospace-grade aluminium honeycomb-core and a not-prestressed version of the core. Results proved its unique impact characteristics, showing, an improvement of the maximum-force up to 171 % and 23 % in comparison with aluminium honeycomb and not-prestressed CPCC, respectively. The specific properties of the CPCC were considerably higher than the aluminium sandwich, with peak force values up to +75 %, displacement reduced up to −71 %, and non-destructive evaluation (NDE) test showed a reduction of the projected damaged area by ∼20 %. These results demonstrates that the CPCC structure constitutes a novel high-performance sandwich core for improved impact resistance
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