Papers by Keyword: Time Reversal

Abstract: This research work presents a hierarchical method able to reconstruct the time history of the impact force on a composite wing stringer-skin panel by using the structural responses measured by a set of surface bonded ultrasonic transducers. Time reversal method was used to identify the impact location by the knowledge of structural responses recorded from a set of excitation points arbitrarily chosen on the plane of the structure. Radial basis function interpolation approach was then used to calculate the transfer function at the impact point and reconstruct the impact force history. Experimental results showed the high level of accuracy of the proposed impact force reconstruction method for a number of low-velocity impact sources and energies.

Abstract: A majority of the research in Structural Health Monitoring focuses on detection of damage. This paper presents a method of imaging crack damage in an isotropic material using the Time Reversal imaging algorithm. Inputs for the algorithm are obtained via computational simulation of the propagation field of a crack in a medium under tone-burst excitation. The approach is similar to existing techniques such as Diffraction Tomography which makes use of the multi-static data matrix constructed using scatter field measurements from the computational simulation. Results indicate excellent reconstruction quality and accurate estimation of damage size.

Abstract: Existing non-destructive test methods are usually ineffective in detection of surface breaking cracks with large depths in concrete structure. This paper introduces a method suitable for detection of deep cracks based on surface wave time reversal theory. A numerical simulation based on the finite element model is performed to investigate surface cracks detection. A damage index was defined based on the correlation coefficient between the actuated and the reconstructed wave signals. The results demonstrated that the presence of crack had a significant influence on the propagation characteristics of surface waves along concrete. Cracks in different sizes were introduced and correlated with the damage index. Enlarging the crack depth resulted in an increase in the distortion of reconstructed signals, and a higher damage index was obtained. The results illustrated the effectiveness of the surface wave time-reversal process in identifying cracks in concrete structures.

Abstract: According to the insufficient ability of the existing ultrasonic guided wave detection technique, proposed a method for small defects in pipes based on time reversal (TR) ultrasonic guided wave. The L(0,2) mode guided waves was encouraged in the simulation research experiment. Results show that the guided wave detection signal has linear superposition properties, realized the detection energy focus on the location of the defects, and improved the ability of detection for small defects. The experiment lay a foundation for the miniaturization of the detection equipment.

Abstract: Because there are many mode transformations when the ultrasonic guided waves run into defects in the pipeline, the reflected signals got by means of the traditional ultrasonic guided wave technique are complex and the amplitudes are small. The time reversal method is a way to intercept the reflected signals in the initial results with a certain bandwidth and excite the time reversal guided waves on the corresponding nodes. In this way, energies of the guided wave are focused in time and space. By comparing the accuracy of defects identification in the straight pipes and the bent pipes with the traditional ultrasonic guided wave method and the time reversal method, this paper proves that the time reversal method has many advantages over the tradition ultrasonic guided wave method. The time reversal method overcomes disadvantages of the traditional guided wave technique and improves the identification degree and accuracy of defects effectively. It lays the foundation for the final defect identification.

Abstract: This paper investigates the potential of single transducer pair guided waves time reversal to detect damage in composite laminates. According to dynamic reciprocity of Lamb waves propagation in linear media, the time reversal process should reconstruct the original signal. The similarity of original and reconstructed time signals is calculated for different damage types using numerical and experimental studies with the aim to investigate, if the interaction of the wave pulse with inhomogeneities introduces any nonlinearity that time reversibility breaks down and single transducer pair time reversal could be used as damage diagnostics tool. 3D explicit finite element analysis is used for the numerical simulation and laser Doppler vibrometry is used to capture out-of-plane displacement time histories excited by an adhesively bonded piezoceramic transducer disc in the experimental time reversal process. In the case of an undamaged composite laminate the similarity index used to quantify the similarity of the original and reconstructed wave pulses is better than 95%. The similarity index is smaller for laminates with artificial damages including embedded fluoro polymer films to simulate delamination damage, through holes and bonded mass inhomogeneities. Although numerical and experimental similarity indices are smaller at higher frequencies, there is no clear evidence that single transducer pair time reversibility breaks down and represents a reliable damage diagnostics tool.

Abstract: Damage detection using guided waves in the inspection of tapered sandwich structures with high density foam core (Dyvinicell HP100) is investigated. Characterisation of the fundamental symmetric and anti-symmetric Lamb wave modes is carried out in terms of their velocity and magnitude variation as they encounter a change in the thickness of a composite sandwich plate, aiming at optimising the mode selection to improve the capability and increase the sensitivity of guided waves in inspection of tapered sandwich structures. In addition, an imaging algorithm based on time reversal is developed to detect multiple debonding and artificial damage in tapered sandwich panels based guided waves from an active sensor network. The correlation coefficients between the original and reconstructed time reversal signals are calculated to define a damage index for individual sensing paths, which are used later in the fusion process, identifying the presence of damage in the monitoring area enclosed by the active sensor network. The results confirm that the incident wave signals and their reconstructed time-reversed counterparts can be used to accurately detect the debonding/damage in tapered sandwich structures.

Abstract: In order to realize defect imaging for guided waves inspection in a pipe, a time reversal method based on baseline subtraction method was introduced. In this method, baseline signals were prepared by detection on undamaged pipe. After that residual signals were obtained by executing subtraction between inspection signals and baseline signals. At last, without using the rectangular window for signal interception, time reversals signals are produced by time reversing the whole residual signals. Experiment and numerical simulation results show that this method simplifies the existing time reversal defect imaging method for guided wave inspection in pipes. Especially, a relatively whole defect’s passive wave field captured by this method will help us to achieve better imaging effect.

Abstract: Signal enhancement of ultrasonic guided waves is studied using time reversal technique. The defect in pipe is detected by activating single mode guided waves L(0,2) at the center frequency of 70 kHz. Time-reversal technique is used to further enhance the defect detection capability. A method to identify the defect is described on a finite element model. Numerical results show that the time reversal technique has the ability to evidently enhance the amplitude of damage reflected wave and improve the detection efficiency of defect.

Abstract: This research work presents an in-situ 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 time reversal approach applied to a number of waveforms stored into a database containing the experimental Green’s function of the medium. The present method exploits the benefits of multiple scattering, mode conversion and boundaries reflections to achieve the focusing of the source with high resolution. The optimal re-focusing of the back propagated wave field at the impact point is accomplished through a “virtual” imaging process, which does not require any iterative algorithms and a priori knowledge of the mechanical properties of the structure. The robustness of the time reversal method is experimentally demonstrated on a stiffened composite panel and the source position can be retrieved with a high level of accuracy (error less than 3%). The simple configuration, minimal processing requirements and computational time (less than 1 sec) make this method a valid alternative to the conventional imaging structural health monitoring systems for the acoustic emission source localization.