Custom made electrostatic transducers were used for ultrasonic investigations of damaged CFRP. These investigations resulted in identification of non-linear ultrasonic responses due to the presence of damage. The broadband nature of electrostatic transducers is essential for this work to scan a wide range of frequencies to identify non-linear responses. These non-linear response frequencies were used with an electrostatic transducer to induce thermosonic heating at the damage which was measured with an IR camera. The use of non-linear response frequencies reduces the power required to produce thermosonic heating. Travelling guided waves (Lamb waves) allow excitation of the damage at a distance from the excitation source allowing large samples to be studied. This provides a completely non-contact measurement for NDE which shows promise for future composite inspection systems.
A commercially-available air coupled piezoelectric array was used to induce thermosonic heating in damaged CFRP. Themosonic heating of the damage has been observed with an IR camera. The intensity of the thermosonic heating decreased with the increased distance of the ultrasonic excitation from the damage, as would be expected with energy dissipation into the CFRP. The use of an array allowed scanning of the focal point across a sample to locate and image areas of damage, without moving either the array or the camera. This scanning capability could increase the speed at which composites can be inspected, reducing the current laborious contact transducer methods.
This work proposes a novel technique for the localization of low-velocity impacts in composites without a-priori knowledge of the mechanical properties nor the speed of propagating waves, thus overcoming current limitations of existing impact localization methods. The proposed algorithm is based on the estimation of the power of acoustic emissions generated by impacts on a composite plate instrumented with embedded piezo-transducers. The signal power values calculated at sparse sensor locations are interpolated over the sample by using radial basis function networks. The impact coordinates on the specimen surface are estimated by a center-of-gravity method based on the interpolated power values. Experimental tests were performed by using both an instrumented impact hammer and a drop tower. The results obtained showed the validity of the presented approach, which was able to identify the impact locations with high level of accuracy.
This paper proposes a novel nonlinear ultrasonically stimulated thermography technique for a quick and reliable assessment of material damage in carbon fibre reinforced plastic (CFRP) composite materials. The proposed nondestructive evaluation (NDE) method requires narrow sweep ultrasonic excitation using contact piezoelectric transducers in order to identify dual excitation frequencies associated with the damage resonance. High-amplitude signals and higher harmonic generation are necessary conditions for an accurate identification of these two input frequencies. Dual periodic excitation using high- and low-frequency input signals was then performed in order to generate frictional heating at the crack location that was measured by an infrared (IR) camera. To validate this concept, an impact damaged CFRP composite panel was tested and the experimental results were compared with traditional flash thermography. A laser vibrometer was used to investigate the response of the material with dual frequency excitation. The proposed nonlinear ultrasonically modulated thermography successfully detected barely visible impact damage in CFRP composites. Hence, it can be considered as an alternative to traditional flash thermography and thermosonics by allowing repeatable detection of damage in composites.
This paper is aimed at developing a theoretical model able to predict the generation of nonlinear elastic effects associated to the interaction of ultrasonic waves with the steady-state nonlinear response of local defect resonance (LDR). The LDR effect is used in nonlinear elastic wave spectroscopy to enhance the excitation of the material damage at its local resonance, thus to dramatically increase the vibrational amplitude of material nonlinear phenomena. The main result of this work is to prove both analytically and experimentally the generation of novel nonlinear elastic wave effects, here named as nonlinear damage resonance intermodulation, which correspond to a nonlinear intermodulation between the driving frequency and the LDR one. Beside this intermodulation effect, other nonlinear elastic wave phenomena such as higher harmonics of the input frequency and superharmonics of LDR frequency were found. The analytical model relies on solving the nonlinear equation of motion governing bending displacement under the assumption of both quadratic and cubic nonlinear defect approximation. Experimental tests on a damaged composite laminate confirmed and validated these predictions and showed that using continuous periodic excitation, the nonlinear structural phenomena associated to LDR could also be featured at locations different from the damage resonance. These findings will provide new opportunities for material damage detection using nonlinear ultrasounds.
Two methods have been evaluated in order to locate barely visible impact damage (BVID) in a composite stiffener panel. A nonlinear thermosonics technique and a nonlinear laser vibrometer technique were evaluated. Damaged regions were excited using a piezo shaker in both methods. Evaluation of the damaged regions was done by first determining the second and third order nonlinear harmonic response of the damaged regions. This was then used to determine the excitation frequency. By evaluating the presence of nonlinear responses in the output signal it is possible to excite the damaged structure at frequencies that give high heat generation and high displacements at the damaged regions. The results showed that both methods can be used to locate damaged regions, although it was shown that the stiffener impedes the propagation of the exciting wave and that these tests should be carried out in-between stiffeners in order to maximise the excitation and heating of damaged regions. Furthermore, both methods allowed for excitation of damaged regions over a large area.
This paper presents a damage detection and localization technique based on nonlinear elastic waves propagation in a damage composite laminate. The proposed method relies on the time of arrival estimation of the second harmonic nonlinear response obtained with second order phase symmetry analysis filtering and burst excitation. The Akaike Information Criterion approach was used to estimate the arrival times measured by six receiver transducers. Then, a combination of Newton’s method and unconstrained optimization was employed to solve a system of nonlinear equations in order to obtain the material damage coordinates. To validate this methodology, experimental tests were carried out on a damaged composite plate. The results showed that the technique allows calculating the damage position with high accuracy (maximum error ~5 mm).
Nonlinear ultrasonic non-destructive evaluation (NDE) methods can be used for the identification of defects within adhesive bonds as they rely on the detection of nonlinear elastic features for the evaluation of the bond strength. In this paper the nonlinear content of the structural response of a single lap joint subjected to ultrasonic harmonic excitation is both numerically and experimentally evaluated to identify and characterize the defects within the bonded region. Different metallic samples with the same geometry were experimentally tested in order to characterize the debonding between two plates by using two surface bonded piezoelectric transducers in pitch-catch mode. The dynamic response of the damaged samples acquired by the single receiver sensor showed the presence of higher harmonics (2nd and 3rd) and subharmonics of the fundamental frequencies. These nonlinear elastic phenomena are clearly due to nonlinear effects induced by the poor adhesion between the two plates. A new constitutive model aimed at representing the nonlinear material response generated by the interaction of the ultrasonic waves with the adhesive joint is also presented. Such a model is implemented in an explicit FE software and uses a nonlinear user defined traction-displacement relationship implemented by means of a cohesive material user model interface. The developed model is verified for the different geometrical and material configurations. Good agreement between the experimental and numerical nonlinear response showed that this model can be used as a simple and useful tool for understanding the quality of the adhesive joint.
SMArt Thermography exploits the electrothermal properties of multifunctional smart structures, which are created by embedding shape memory alloy (SMA) wires in traditional carbon fibre reinforced composite laminates (known as SMArt composites), in order to detect the structural flaws using an embedded source. Such a system enables a built-in, fast, cost-effective and in-depth assessment of the structural damage as it overcomes the limitations of standard thermography techniques. However, a theoretical background of the thermal wave propagation behaviour, especially in the presence of internal structural defects, is needed to better interpret the observations/data acquired during the experiments and to optimise those critical parameters such as the mechanical and thermal properties of the composite laminate, the depth of the SMA wires and the intensity of the excitation energy. This information is essential to enhance the sensitivity of the system, thus to evaluate the integrity of the medium with different types of damage. For this purpose, this paper aims at developing an analytical model for SMArt composites, which is able to predict the temperature contrast on the surface of the laminate in the presence of in-plane internal damage (delamination-like) using pulsed thermography. Such a model, based on the Green’s function formalism for one-dimensional heat equation, takes into account the thermal lateral diffusion around the defect and it can be used to compute the defect depth within the laminate. The results showed good agreement between the analytical model and the measured thermal waves using an infrared (IR) camera. Particularly, the contrast temperature curves were found to change significantly depending on the defect opening.
Literature offers a quantitative number of diagnostic methods that can continuously provide detailed information of the
material defects and damages in aerospace and civil engineering applications. Indeed, low velocity impact damages can
considerably degrade the integrity of structural components and, if not detected, they can result in catastrophic failure
conditions. This paper presents a nonlinear Structural Health Monitoring (SHM) method, based on ultrasonic guided
waves (GW), for the detection of the nonlinear signature in a damaged composite structure. The proposed technique,
based on a bispectral analysis of ultrasonic input waveforms, allows for the evaluation of the nonlinear response due to
the presence of cracks and delaminations. Indeed, such a methodology was used to characterize the nonlinear behaviour
of the structure, by exploiting the frequency mixing of the original waveform acquired from a sparse array of sensors.
The robustness of bispectral analysis was experimentally demonstrated on a damaged carbon fibre reinforce plastic
(CFRP) composite panel, and the nonlinear source was retrieved with a high level of accuracy. Unlike other linear and
nonlinear ultrasonic methods for damage detection, this methodology does not require any baseline with the undamaged
structure for the evaluation of the nonlinear source, nor a priori knowledge of the mechanical properties of the specimen.
Moreover, bispectral analysis can be considered as a nonlinear elastic wave spectroscopy (NEWS) technique for
materials showing either classical or non-classical nonlinear behaviour.
In 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.
The purpose of this paper is to analyse the possibility to manufacture and verify the self-sensing capability of
composite materials plates with an embedded network of NiTi shape memory alloys (SMA) used as
transducers for structural integrity. Firstly, the thermo-electrical material properties of SMAs were
investigated to assess their capability to sense strain within. The results showed that the electrical resistance
variation provided by the shape memory alloys network enables a built in and fast assessment of the stress
distribution over the entire structure. Then, by transmitting a low amperage current, results in an electric and
thermal flow through the entire SMA network. Using an IR Camera it is possible to capture the emitted
thermal waves from the sample and create an image of the thermal field within the material. Consequently,
analysing the behaviour of the heating curves on different points of the sample, it is possible to identify
potential variation in the apparent temperature of the composite, leading to the identification of damages
within the composite structure.
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