Christoph Schaal

Dispersion of Lamb waves in a honeycomb composite sandwich panel

Composite materials are increasingly being used in advanced aircraft and aerospace structures. Despite their many advantages, composites are often susceptible to hidden damages that may occur during manufacturing and/or service of the structure. Therefore, safe operation of composite structures requires careful monitoring of the initiation and growth of such defects. Ultrasonic methods using guided waves offer a reliable and cost effective method for defects monitoring in advanced structures due to their long propagation range and their sensitivity to defects in their propagation path. In this paper, some of the useful properties of guided Lamb type waves are investigated, using analytical, numerical and experimental methods, in an effort to provide the knowledge base required for the development of viable structural health monitoring systems for composite structures. The laboratory experiments involve a pitch-catch method in which a pair of movable transducers is placed on the outside surface of the structure for generating and recording the wave signals. The specific cases considered include an aluminum plate, a woven composite laminate and an aluminum honeycomb sandwich panel. The agreement between experimental, numerical and theoretical results are shown to be excellent in certain frequency ranges, providing a guidance for the design of effective inspection systems.

Damage detection in multi-wire cables using guided ultrasonic waves

Structural Health Monitoring systems are developed to cost-efficiently prevent failure of mechanical and civil structures, and to predict the structure’s residual life. In this work, a damage detection algorithm based on the Hilbert transform of the recorded signals from induced guided ultrasonic waves is presented. By means of this algorithm, damage localization in multi-wire cables is performed through a time-of-flight analysis of the wave packets. The algorithm is fully automated and distinguishes between wave packets from different waves independently. Its applicability is analyzed for laboratory experiments on a single cylindrical wire and on multi-wire cables. As an additional damage indicator, second harmonic waves are evaluated. Furthermore, the possibility to perform damage identification by evaluating the waves’ amplitudes is analyzed. The amplitudes are compared with reference data from a novel hybrid finite-boundary element method.

An analytical study of the scattering of ultrasonic guided waves at a delamination-like discontinuity in a plate

Interface delaminations between individual plies in a composite, or disbonds of face sheets in honeycomb structures often remain undetected. Using guided ultrasonic waves (Rayleigh and Lamb waves) such hidden defects can be detected. In this work, an analytical framework that considers propagating, nonpropagating and evanescent waves to analyze the scattering of an incident ultrasonic wave at a delamination-like discontinuity is presented. Wave conversion at the interface of the damage is quantified in terms of the power flows of the individual waves. The analytical solutions are compared with results from numerical simulations. For an incident Lamb wave, excellent agreement is found. However, it is shown that the analytical solution for an incident Rayleigh wave has significant differences from the numerical results, due to the incomplete nature of the Rayleigh wave-field in the half-space. Even though this study is performed for isotropic waveguides, the method can be extended to transversely isotropic laminates by substituting the corresponding expressions for the dispersion equations, as well as displacement and stress fields.

The Influence of Stiffeners on the Propagation of Guided Ultrasonic Waves

Stiffeners are important structural components in modern composite and honey-comb structures. The safe operation of such composite structures, which are com-monly used in aeronautical applications, requires careful monitoring as hidden de-fects may compromise the structural safety. In order to improve the reliability of ultrasonic damage detection in stiffened plate structures, experimental and numeri-cal studies are conducted, in this paper, in an effort to understand the interaction of guided ultrasonic waves with simple models of the stiffener. Through a series of measurements in different positions, the amplitudes of scattered waves for various configurations of the stiffener are determined. Moreover, the group velocity of the waves in the stiffened structure is analyzed. The experimental findings are compared with results from numerical simulations. doi: 10.12783/SHM2015/248

Dispersion of guided waves in composite laminates and sandwich panels

In composite structures, damages are often invisible from the surface and can grow to reach a critical size, potentially causing catastrophic failure of the entire structure. Thus safe operation of these structures requires careful monitoring of the initiation and growth of such defects. Ultrasonic methods using guided waves offer a reliable and cost-effective method for structural health monitoring in advanced structures. Guided waves allow for long monitoring ranges and are very sensitive to defects within their propagation path. In this work, the relevant properties of guided Lamb waves for damage detection in composite structures are investigated. An efficient numerical approach is used to determine their dispersion characteristics, and these results are compared to those from laboratory experiments. The experiments are based on a pitch-catch method, in which a pair of movable transducers is placed on one surface of the structure to induce and detect guided Lamb waves. The specific cases considered include an aluminum plate and an aluminum honeycomb sandwich panel with woven composite face sheets. In addition, a disbond of the interface between one of the face sheets and the honeycomb core of the sandwich panel is also considered, and the dispersion characteristics of the two resultant waveguides are determined. Good agreement between numerical and experimental dispersion results is found, and suggestions on the applicability of the pitch-catch system for structural health monitoring are made.

Model Identification for a Modal State Estimator from Output-Only Data

Precise information of the current state of motion of a vibrating system is crucial for every effective active vibration control. Due to the limitation of the number of sensors in the final operational setup, a state estimator is needed to reconstruct the current state of vibration at important degrees of freedom from a small set of sensors. If the excitation of the structure (system input) cannot be measured directly, a sensor based estimator has to be used, which operates on response data only (system output). Core piece of this estimator is a reduced model of the vibrating system. If additionally some sensors are only capable to measure spatially distributed deflection instead of the deflection at single degrees of freedom (like piezo-electric foils), the reduced model must incorporate both those sensors as well as the important degrees of freedom as outputs. For structures which cannot be excited by a measurable excitation even in the development phase, the reduced model can still be identified by an output-only modal analysis. In this work, the identification technique is demonstrated for a curved plate submerged in water and exited by a surrounding sound field.

Laser Doppler velocimetry and PZT sensing for the study of guided waves in a stepped aluminum plate

Lamb waves propagating in thin plates and shells are being widely studied for their potential applications in nondestructive inspection of large-scale structures. These structures are generally characterized by the presence of geometrical discontinuities such as stiffeners, mechanical joints or variable thicknesses that affect the propagation characteristics of Lamb waves that can be very similar to those from defects occurring in service (delamination, disbond, etc.). Therefore, the knowledge of the effects of such discontinuities on the propagation of guided waves is essential to avoid their false identification as defects. In this work Lamb waves propagating in a metal plate with a downward step are studied through laboratory experiments. A single 10 mm piezoceramic disk (PZT) bonded to the host structure using cyanoacrylate gage adhesive is utilized for Lamb waves generation and the responses are measured at multiple locations, along a line crossing the step, using a scanning laser Doppler vibrometer (LDV). The interaction of the fundamental Lamb mode A0 with the geometrical discontinuity in the isotropic plate is investigated and discussed.

A semi-analytical method for calculating resonator energy loss into plate substrates

This paper develops a new semi-analytical technique for estimating the energy loss in a resonator mounted on a plate substrate. Resonator energy loss due to interaction with its substrate (so called “anchor loss”) has been widely studied, but most treatments have assumed a semi-infinite medium due to the small dimensions of the resonator compared to the substrate thickness. Recent high quality factor resonators, though, typically have resonant frequencies from a few kilohertz up to several hundred kilohertz and in these cases, due to the wavelengths involved, the substrate is more accurately treated as a plate. A procedure is developed whereby modal analysis is performed on a finite element model that is coupled to analytical expressions for the plate Lamb waves. This approach is distinguished from prior semi-analytical approaches by the absence of an applied load. Thus, a more fundamental treatment of dynamics of the coupled system is possible.

Analytical Investigation of the Interaction of Rayleigh and Lamb Waves at a Delamination-like Discontinuity in a Thick Plate

In this paper, an analytical framework to analyze the scattering of an incident ultrasonic Rayleigh wave at a delamination-like discontinuity located near the surface of a thick plate is developed. It is assumed that the waves are generated and received using a pitch-catch setup, in which a pair of movable transducers is placed on the surface of the plate. It can be shown that the incident Rayleigh wave is converted into Lamb waves propagating in the thin layer above the defect, and a Rayleigh wave propagating below it. The Rayleigh to Lamb wave conversion at the delamination is quantified in terms of the scattering coefficients. The results from the analytical solutions are compared with those from numerical simulations for a thick aluminum plate, revealing good agreement between them. doi: 10.12783/SHM2015/225