Artur Szewieczek

Characterization of Lamb wave attenuation mechanisms

Structural Health Monitoring (SHM) based on Lamb waves, a type of ultrasonic guided waves, is a promising technique for in-service inspection of composite structures. This study investigates the attenuation mechanisms of Lamb wave propagation fields. The attenuation of an anisotropic plate is experimental measured with air-coupled ultrasonic scanning techniques and analytical modeled using higher order plate theory. Based on the experimental and analytical data the various attenuation mechanisms are characterized for the fundamental Lamb wave modes.

Visualisation of guided wave propagation by ultrasonic imaging methods

Structural health monitoring (SHM) with Lamb waves (LWs) principally enables an in-service damage detection of large aircraft components without time-consuming scanning. Piezoelectric elements can be effectively used as senders and receivers for LWs. However, the complex interaction between defects and Lamb waves – especially for composites, the presence of two wave modes in minimum and its dispersive behaviour and reflections from all structure elements contribute to receiver signals which are very difficult to predict and to evaluate. Ulltrasonic imaging technique (UIT) can also be used for the visualisation of LW propagation in composites especially using air-coupled techniques. Out of the recorded full-wave data calculations of Lamb wave A-, B-, C- and D-scans and video animations can be carried out. Examples of visualisation in composites such as monolithic specimens, a stringer stiffened laminate and a sandwich tail boom which impressively show the Lamb wave propagation and also their interaction with defects.

Virtual Sensors for SHM using Isogeometric Piezoelectric Finite Elements

Purpose – A design of sensor networks for structural health monitoring (SHM) with guided waves poses a hard challenge. Therefore different approaches are possible. A known one is the usage of probability of detection (POD) criteria. Here, areas of potential impact sensitivity are calculated for every sensor which leads to a POD. The number of sensors is increased until a demanded POD is reached. However, these calculations are usually based on finite element methods and underlie different assumptions and approximations which can cause different inaccuracies. These limitations are avoided by using an experimental data basis for virtual sensors in this paper. The paper aims to discuss these issues. Design/methodology/approach – An air-coupled ultrasound scanning technique is used for guided wave investigations. Recorded displacements of a structure surface are used as stimulation of virtual sensors which can be designed by software and positioned within available data field. For the calculation of sensor si...

Structural Health Monitoring Based on Guided Waves

Structural Health Monitoring (SHM) based on ultrasonic guided waves, so-called Lamb waves, is a promising method for in-service inspection of composite structures without time consuming scanning like conventional ultrasonic techniques. Lamb waves are able to propagate over large distances and can be easily excited and received by a network of piezoelectric actuators and sensors. In principle different kinds of structural defects can be detected and located by analyzing the sensor signals. The chapter describes recent research activities at DLR on Structural Health Monitoring. The research are focused on the visualisation of Lamb wave propagation fields based on air-coupled ultrasonic technique, the simulation of virtual sensors, mode selective actuators as well as manufacturing of actuator and sensor networks. Additionally, the chapter present the development of a SHM system for impact detection in a helicopter tailboom (Eurocopter—EC 135).

Virtual Sensors for SHM

Guided waves can be used for Structural Health Monitoring (SHM) of modern composites. However, due to the complex wave propagation different scanning and imaging methods are used to achieve a better understanding of wave interaction with complex structures. The acquired data allows advanced applications. The following section presents a method for the design and optimization of sensors and sensor networks for SHM using guided wave propagation measurements.

Guided Wave Displacement Validation for SHM Applications using Air Coupled Ultrasonic Scanning Technique

Structural Health Monitoring with guided waves is a promising technique for online testing of metal or composite panels. However, wave propagation and interaction in complex structures poses a hard challenge for signal validation due to reflections, refractions or mode conversions. In the past different measuring and imaging techniques were developed which allow a deeper analysis and understanding of wave behavior. One known measuring technique for guided waves is the laser vibrometry. Its most important advantage is the possibility of a three dimensional scanning of surface deformations using different scanning directions. However, this method requires a reflection alignment with films or painting and a high averaging rate due to a low signal-noise ratio. These disadvantages increase measurement costs and can be avoided using an ultrasonic scanning technique. Here, compression waves emitted by leaky waves are measured with a contactless ultrasonic sensor. The necessary measurement equipment is comparatively cheaper and scanning cycles are faster. The wave emission of leaky waves depends on the specimen, the fluid between the specimen and a sensor, the leaky wave mode, its in-plane propagation direction and frequency. All these parameters can influence a wave emission decisively. They have to be deducted from the air coupled ultrasonic scanning data in order to reconstruct the three dimensional displacements of a specimen surface. This paper presents all these necessary alignments for isotropic and anisotropic specimens. Based on the derived methods a validation of wave behavior in any homogeneous specimen is possible without undesirable emission influences. All used proceedings are based on analytical calculations and numerically determined dispersion curves and do not require finite element simulations. Based on the computed deformation information different kinds of application are possible. One of them is the design and optimization of virtual sensors for Structural Health Monitoring. A further possibility is the usage of guided waves as a Non Destructive Testing method for impact detection in situations, where no transmission or impulse-echo investigations are possible.

Characterization of mode selective actuator and sensor systems for Lamb wave excitation

Structural Health Monitoring (SHM) based on Lamb waves, a type of ultrasonic guided waves, is a promising technique for in-service inspection of composite structures. This study presents the development of mode selective actuator and sensor systems based on interdigital transducer (IDT) design. Various parameters such as wavelengths, number and apodization of electrodes as well as eigenfrequencies of the transducer are characterized. Therefore, an analytical model based on the theory of surface acoustic wave (SAW) filters is investigated in order to evaluate the acoustic response of the transducer. Furthermore, experimental tests on composite plates are performed.