Papers by Keyword: Lamb Wave

Abstract: The investigation was focused on Carbon Fibre Reinforced Polymers (CFRP). In the first part of research the aim was the characterization of CFRP surfaces. These surfaces were influenced by release agent, hydraulic fluid, moisture and overheating. In the second part of research adhesive bond quality was investigated. Three different cases of possible weak bonds were focused on. Weak bond caused by release agent contamination, moisture contamination and poor curing of adhesive. The characterization was conducted using laser vibrometry used as NDT tools. An active element in the form of piezoelectric transducer was used to excite the samples made out of CFRP material. Laser vibroemter was used to register the surface response. Combining the piezoelectric excitation with laser sensing a tool was obtained to measure precisely the propagating elastic waves. The excited waves were measured in defined points by the vibrometer obtaining the wavefield. In order to characterize the surface and bonding quality an indicator was proposed based on propagating wave parameters. The guided elastic wave velocity depends material properties (Young modulus, density, Poisson ratio) and thickness of the sample. It was assumed that comparison of the velocities can provide an information about the bond condition. All the investigated scenarios showed deviation from the reference case.

Abstract: In reported research piezoelectric sensors were used for damage identicitaion purposes. Piezoelectric sensor was used for specimen excitation. Two techniques were investigated. The Electromechanical impedance (EMI) technique and guided wave based technique. The principle of EMI technique is based on measurement and analysis of impedance of piezoelectric transducers bonded on or embedded in investigated structure. It is assumed that structural change should influence the impedance characteristics of the transducers. The guided wave based technique is based on the guided elastic wave propagation phenomena. This type of waves can be used in order to obtain information about structure condition and possibly damaged areas. In reported investigation piezoelectric sensor was used to excite guided waves in chosen structural element. Dispersive nature of guided waves results in changes of velocity with the wave frequency, therefore a narrowband signal was used to minimize the dispersion phenomenon. The generated signal was amplified before applying it to the transducer in order to ensure measurable amplitude of excited guided wave. Measurement of the wave field was realized using laser scanning vibrometer that registered the velocity responses at a points belonging to a defined mesh. This non-contact tool allowed to investigate phenomena related to wave propagation. For both techniques numerical signals processing tools were developed. These numerical tools were designed to extract damage relevant features from EMI measurements and guided wave propagation measurements. The damage index (DI) was introduced on the basis of the extracted features.

Abstract: One of the largest issues remaining on the way to in situ Structural Health Monitoring of composite structures using Lamb waves is the impact that non-damaging factors like temperature changes and humidity absorption have on most measurement strategies. While some of these tasks have been successfully conquered, others, especially related to slowly developing influences like humidity absorption or mechanical ageing, remain challenging. In this paper, a method to approach this problem for a Lamb-wave based passive impact detection system is presented. Passive approaches use the waves generated by the impact event itself to both localize said event and evaluate whether it was large enough to damage the structure. For this, the impacts energy has to be estimated from sensors detecting the Lamb waves. The problem provided by changing conditions within the material is that the locally measurable wave amplitude due to an impact event of a certain energy is altered if the material properties change. This might happen due to temperature changes, mechanical loads, humidity absorption, fluid loads and other factors. The main idea of the presented approach is to mix a passive and an active system. Piezoelectric elements are used to generate Lamb waves to obtain the attenuation coefficients of the material before and after hot/wet-conditioning. These coefficients are then used to estimate the impact energy from passive sensor responses. Both the approach and experimental validation performed with low velocity impacts from an impact hammer are presented to show the ability to correctly calculate impact forces after conditioning.

Abstract: This work constitutes a damage detection study of a glass plate using the statistical approach of outlier analysis, which is also referred to here as novelty detection. A glass plate instrumented with low-profile, surface-bonded transducers is used in the investigation. Ultrasonic Lamb waves are applied for detecting various crack length on the same plate. The study reveals a distinction between the damage and undamaged plate, and also assesses the severity of damage.

Abstract: Guided wave tomography is an attractive tool for the detection and monitoring of the critical area in a structure. Using signal difference coefficient (SDC) as the tomographic feature, RAPID (Reconstruction Algorithm for the Probabilistic Inspection of Damage) is an effective and flexible tomography algorithm. In this algorithm, signal changes are exclusively attributed to the structural variation. However, external environment factors like water loading or oil loading also change signals significantly. The presence of anti-symmetric mode with a predominant out of plane displacement makes it very sensitive to these interferences and leads to false alarms. In this paper, Lamb wave is excited in the low-frequency domain, where only the fundamental modes A0 and S0 exist. The significant difference in group velocity between the two modes makes it possible to separate them in time domain. A new method is proposed to extract pure S0 mode signal as valid measurement data to improve the algorithm in addressing false alarm caused by water loading. The results of the experiment demonstrate that the improved algorithm has the capability of providing accurate identification of damage in the presence of water loading.

Abstract: The use of Lamb waves for structural health monitoring (SHM) has complicated by its multi-mode character and dispersion effect, which impacts the damage positioning and high-resolution imaging. The group velocity dispersion curves of Lamb waves can be employed to warp the frequency axis, and then to establish warped frequency transform (WFT) to process Lamb waves. In this paper, received signals are directly compensated with warped frequency transform to suppress dispersion, and a new imaging method is proposed based on warped frequency transform. The propagation of Lamb waves in damaged aluminum plate is simulated by finite element software ABAQUS, results show that warped frequency transform can effectively compensate dispersive wave-packets, and high-resolution damage imaging can be obtained by the proposed method.

Abstract: In this paper, the propagation characteristics of Lamb waves activated and collected by piezoelectric sensors bonded on an aluminum panel were investigated both theoretically and experimentally on different bonding cases. The bonding layer is an intermedium of energy transmission between structure and sensor. The bonding quality heavily effects on the lamb propagation characteristics, including frequency dispersion, mode conversion, and signal amplitude. In particular, the variation of the surface strain with frequency for symmetric lamb mode was studied in different bonding quality. A sensor bonding quality evaluation method based on frequency response characteristics of the surface strain was developed to compensate the sensor network. The proposed approach can provide a useful reference for structural health monitoring strategy formulation.

Abstract: According to the characteristics of wave propagation in steel plate, choosing the suitable excite mode and frequency. Based on the wave structure of A0S0, obtaining its displacement discipline, then arriving at the sensitivity effect for different depth of defect in different frequency. Making different depth circular damage in the steel plate, calculating the law of lamb wave in different damage by using continuous wavelet transform.

Abstract: This paper reports on findings that extend previous work for the purpose of in-situ structural health monitoring of defects on the blind side of open holes using plate waves. A series of computational studies is presented to understand how and why the ultrasonic scattered wave field can be detected on the accessible surface. The uniqueness of these findings is that the length-scale of the defect and the incident waves are comparable. The combination of the experimental-computational-analytical approach gives rise to new insights and guidance for the quantification of defects located in hard-to-inspect regions of future unitised metallic and composite structures. The outcomes advance the knowledge base of inspection of hard-to-access regions with actuators and sensors placed in easily accessible locations.

Abstract: The incorporation of in situ structural health monitoring is currently an after-thought used to address critical areas identified in testing or service. This paper reports on a series of analytical/experimental work seeking to demonstrate the implementation of in situ structural health monitoring (iSHM) at the design stage of critical structures. This work is intended for the design of future generation aircraft. The work presented describes a systematic redesign scheme based on Lamb wave technology. The results demonstrate a strong possibility that such a system is effective and feasible and comes at a tolerable cost to the structure. To demonstrate the efficacy of this proposed design scheme, a series of experimental results will be presented using the fatigue critical location of structure representing the lower wing skin of an aircraft structure as a test case.