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.
Despite demonstrated effectiveness in characterizing material properties or defect, the evaluation of material acoustic nonlinearity is highly prone to measurement contaminations introduced by various practical factors and the low robustness restricts its application. In order to obtain a precise quantification of the material acoustic nonlinearity in a robust manner, an approach based on the thermal fluctuations in nonlinear features of ultrasonic waves is developed. In this approach, the influence of temperature and defect on the interatomic distance is scrutinized analytically, and on this basis, the nonlinear features of ultrasonic waves linked with the temperature and defect is ascertained explicitly, whereby a thermal sensitivity index is proposed. With this thermal sensitivity index, the material acoustic nonlinearity can be evaluated without being affected by contaminations from practical sources, and therefore the defect which intensifies the material acoustic nonlinearity can be identified in a robust manner. Experimental validation corroborates the theoretical prediction, demonstrating that the proposed thermal sensitivity-based approach is capable of enhancing the robustness of material acoustic nonlinearity evaluation and defect characterization.
Based on the two-dimensional (2D)1 and three-dimensional (3D)2 analytical models previously developed for interpreting the contact acoustic nonlinearity (CAN) generated due to the modulation from a “breathing” crack in solid media on propagating guided ultrasonic waves (GUWs), this study proposes a new characterization approach, able to orientate a fatigue crack, even when the crack is at its embryo stage. CAN embodied in the scattered Lamb waves and shear horizontal (SH) waves converted from incident GUWs is extracted upon interaction with fatigue cracks, and the unique scattering pattern of CAN is associated with crack slant via the 3D analytical models, whereby the orientation of a fatigue crack can be pinpointed, without making a reference to the baseline signal. Experimental validation of the characterization approach is implemented, in which an undersized fatigue crack is orientated accurately and visualized in the image.
Extending a two-dimensional analytical framework previously developed for understanding contact acoustic nonlinearity (CAN) in a beam-like structure bearing a contact crack[1], this study reports an analytical model for interpreting CAN induced due to the modulation from a “breathing” crack in a plate-like structure on propagating guided ultrasonic waves (GUWs) in a three-dimensional (3-D) scenario. The “breathing” crack is considered, in a 3-D manner, as a second source to excite additional wave fields. Thorough investigation of the interaction between the probing GUWs and the “breathing” crack leads to explicit, analytical and full-field description of additional wave fields. In this study, influences of reflected and diffracted waves by the crack on the motion of crack surfaces are scrutinized, yielding a depiction of the “breathing” behavior of the crack, beneficial for quantifying the crack-induced source at double frequency, with which the crack-induced nonlinearity (i.e. second harmonic) can be evaluated quantitatively, in conjunction with the use of an elasto-dynamic method. A nonlinearity index is consequently defined to represent the severity of the “breathing” crack. Results obtained from the 3-D model are compared with those from a finite element simulation, to affirm good agreement. This model does not request a benchmarking process against baseline signals for evaluation of damage.
KEYWORDS: Temperature metrology, Waveguides, Structural health monitoring, Ferroelectric materials, Sensors, Composites, Damage detection, Electronic filtering, Digital filtering, Signal detection
A new temperature compensation technique combining optimal baseline selection and the filter based on Adaptive
Linear Neuron Network was developed to enhance the robustness and effectiveness of guided Lamb wave-based damage
detection. This paper focuses on three main issues for practically implementing the proposed method: (a) Establishment
of temperature compensation standard; (b) Parameters design of compensation filter; (c) Determination of temperature
gradient of baseline signals. Experiments were conducted on two stiffened composite plates to demonstrate the feasibility
of proposed method under a temperature range from -40°C to 80°C for compensating temperature effects. Results
showed that a reasonable temperature step for providing good temperature compensation can be up to 20°C in a baseline
dataset.
This paper presents an active monitoring method based on Lamb wave and wavelet transform to determine damage locations. The method compares damage signal with reference signal, namely undamage signal, and takes their algebraic difference as a signal caused by damage. In fact, by the analysis of Lamb wave signals recorded before and after damage, the differential signal is a reflect signal from damage which can be regarded as an acoustic emission signal. It is showed that the wavelet transform using the Gabor wavelet effectively decomposes the differential signal into its time-frequency components, and the peaks of the time-frequency distribution near the center frequency of exciting signal indicate the arrival times of waves. By calculation of time delays between the arrival of the differential and exciting signals, the damage localization points can be obtained. Our experimental results on fiber-glass materials which suffer a
delamination defect prove that the damage location is reliable.
Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper applies a new combined method of Kohonen self-organizing feature neural networks and active damage detection technique to composite health monitoring. The proposed method features a simple structure algorithm, supervision free self-study and lateral association, etc. According to such method and virtual instrument technique, an active damage detection system for composites is developed. This effective method for damage detection has been verified in the course of its practical use.
Simultaneous strain and temperature measurement for advanced 3D braided composite materials using fiber-optic sensor technology is demonstrated, for the first time. These advanced 3D braided composites can virtually eliminate the most serious problem of delamination for conventional composites. A tandem in-fiber Bragg grating (FBG)/extrinsic Fabry-Perot interferometric sensor (EFPI) system with improved accuracy has been used to facilitate simultaneous temperature and strain measurement in this work. The non- symmetric distortion of the optical spectrum of the FBG, due to combination of the FBG and the EFPI, is observed for the first time. Experimental and theoretical studies indicate that this type of distortion can affect the measurement accuracy seriously and it is mainly caused by the modulation of the periodic output of the EFPI. A simple method has been demonstrated to improve the accuracy for detection of the wavelength-shift of the FBG induced by temperature change. A strain accuracy of ~+/- 20(mu) (epsilon) and a temperature accuracy of ~+/- 1 degree(s)C have been achieved, which can meet the requirements for practical applications of 3D braided composites.
In this paper, special optical fibers are embedded into #-D braided carbon/epoxy composites to constitute intrinsic microbend fiber optic sensors for internal strain measurement. Samples with embedded optical fibers are braided through four-step method and processed through Resin Transfer Molding (RTM) process. Special measures are taken to prevent damage of the sensors. The microbend optic fiber sensor works well in a three-point bending experiment. The experiments show that the output intensity of the microbend sensor is proportional to the internal strain of the sample while the load increases. Similar results can be achieved as the load decreases. The results show that the embedded microbend fiber optic sensor is fit for internal strain measurement in 3-D braided composites.
In this article, a new method is put forward to research mechanical features of braided material using co-braided optic fiber sensor. Optic fiber sensors can be multi-braided into composites to measure various internal parameters, which overcomes the drawback of ordinary mechanical experimental methods and makes measurement more accurate and reliable. Followings are introduced in detail: the devising of carbon fiber braided composite specimens with co-braided optic, the AEFPI optic sensor adopted and the experiments to measure the tensile strain and thermal strain of specimens. It is the first time to use co-braided AEFPI sensor to research the mechanical features of brading composites and experiment results are satisfying.
In smart composite structures, piezoelectric patches are usually bonded to surfaces or embedded in structures for strain measurement and active control. This paper investigates applications of these patches for acousto-ultrasonic(AU) inspection of structure without special design or additional change of their shapes and configurations. By using these multi-functional built-in transducers, the smart composite has the potential to monitor a large area on the structure with AU method. Experiments were conducted on a fiber-epoxy composite beam. A PZT transmitter pasted on the beam was stimulated by a nanosecond pulse generator, reflected stress wave caused by edges of the beam was clearly observed. Artificial slots on the beam can be well located from the arriving time of the reflected waves, their size can be estimated from the shape and peak value of the waves reflected by or propagating through slot. Wavelet analysis was used to extract features from the measured signals. Local maxima of coefficients of Mexico hat wavelet decomposition were employed as features to describe arriving time and slope of signals. Features in different damage conditions were compared, results show these features can represent both the location and extent of the simulated damages.
The distributed strain sensor has significant application in real-time monitoring of the status of large and important engineering structures such as bridges and dams. In this paper, a quasi-distributed optical fiber sensor system is set up using OTDR technique. The strain sensors that base on a novel microbend configuration are designed and used to measure the local strains along the optical fiber. In experimental studies, we are concerned about two questions: the monitoring of the safety status and the strain measurement of the member to be measured. The experimental results are given.
Braided composites have advantages over conventional ply composites, which have limitation of brittle interlaminar. They are the hotspot of advanced material research in recent years. Although 3D braided composites have good performances, people do not know enough about them, whether in depth or in extent. In this paper a method in which optic fiber sensors embedded into 3D composites are used to study 3D braided composites is introduced. The braiding process of optic fiber sensors, optic performances of optic fibers are studied. Several kinds of optic fibers are used. Girder form structures with embedded optic fibers are braided through four-step method and processed through Mold Pressing. Some special measures are taken to prevent damage of the optic fibers. Measurement of some performance parameters of optic fibers shows little changes of those parameters before, after optic fibers being braided into the structures, and after the molding process. A polarimetric type of optic fiber sensors is designed for measuring strain in braided testpieces.
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