Christoph Heinze

A Minimal Model for Fast Approximation of Lamb Wave Propagation in Complex Aircraft Parts

For future integrated structural health monitoring (SHM) systems the complexity of the used algorithms is limited due to the restricted computational capacities of the onboard CPU. Therefore, an efficient method to approximate the wave propagation in complex fiber reinforced polymer structures is proposed. Model properties are reduced to phase velocities in individual areas and interaction characteristics at their connecting spots. A ray tracing algorithm enables a fast identification of possible wave paths from an actuator to a sensor. With this information, signals at the sensor position can be calculated analytically. The accuracy of this method is checked on an aluminum plate with and without a cutout. The present chapter is concluded with a discussion of advantages and limitations of the proposed method and an outlook to its use for anisotropic materials.

A Minimal Model-Based Approach for the Fast Approximation of Wave Propagation in Complex Structures

Guided wave–based structural health monitoring is a promising technology to continuously monitor aircraft and to identify damage. The simulation of such structures is still a numerically challenging task due to their size and complexity. In this article, a minimal model approach is proposed to allow an approximate but very fast calculation of the wave propagation. The properties being relevant to the propagation of Lamb waves or in general guided waves are determined as part of the preprocessing. These include phase velocities as well as interaction parameters of all discontinuities. The minimal model consists of two steps to calculate a time signal at specific locations. First, a ray tracing algorithm determines suitable wave paths between two points in the reduced model of the structure. Second, wave packets of every incident ray are superimposed to synthesize a time signal. The obtained results are compared with both finite element simulations and experimental 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.