Fiber-reinforced polymer matrix composites have excellent in-plane stiffness and strength properties, and are therefore ideal for usage in panels of aircraft wings or fuselage as well as launch vehicle case segments. Those thin plates or shell structures are often stiffened with many locally increased thickness regions, or beams of various cross-sectional shapes such as flat or T-shaped. Small defects in any of those stiffened regions would greatly reduce the structural performance as a whole. Locating such defects is time consuming because of the large extend of the panels as well as the number of stiffeners. Guided ultrasonic wave-based techniques could be applied for damage detection in large areas. However, the scattering characteristics of stiffeners are complex. In particular, when multiple stiffeners are present, the incident Lamb wave signal is altered with the passing of each stiffener. Thus, the goal of this work is to efficiently model Lamb wave propagation when multiple stiffeners are present, with and without defects, in an effort to identify useful signal features for damage detection. To this end, the so-called global-local method is used for Lamb wave modeling. The global functions are used to represent the nominal composite region – parameters are obtained by means of waveguide finite element (WFE) method – and the stiffened region is represented by finite element discretization. With a recently developed coupling technique, a source problem, representing a surface-mounted transducer is coupled with multiple stiffener-scattering models to examine the transmission characteristics. The global-local model is validated by laboratory waveform measurements on a stiffened composite plate. The results from global-local method can then be used to efficiently determine the maximum number of stiffeners before the transmitted Lamb waves become too weak to identify defects.
Guided ultrasonic wave-based methods are promising for structural health monitoring of isotropic and composite materials and structures. The technology has seen a lot of attention in the research community over the past decades, and many analytical and numerical methods have been developed to describe different aspects of guided wave propagation and scattering phenomena as well as damage detection. However, very little research was geared towards the influence of the uncertainty in the material properties for the calculation of the dispersion curves. The lack of knowledge of the exact material properties together with manufacturing tolerances could lead to erroneous conclusions. Hence, in this study, an uncertainty analysis for the material properties of fiber-reinforced composites is conducted to quantify the effect of uncertain material constants on the dispersion curves. A fuzzy arithmetical approach based on the Transformation Method is used to generate the dispersion curves with uncertain parameters in conjunction with a root-finding algorithm. The uncertain parameters are modeled as linear fuzzy numbers. Using triangular membership functions, both the nominal value and the worst-case interval are adequately combined into one fuzzy number. Furthermore, it is shown that the measure of influence for the uncertain material parameters on the group velocities of the considered Lamb waves is not equally weighted. These findings might allow for the development of efficient, nondestructive material characterization techniques in the future.
In recent years, many researchers have explored the use of guided ultrasonic waves for nondestructive testing (NDT). When guided waves are transmitted into a structure, any geometric and material discontinuities in the waves’ path modify these waves. Using appropriate signal processing methods for the waves received at a sensor, information about these features can be extracted. However, little research has been conducted to locate the features and automatically generate maps without using a priori knowledge. For NDT of large-scale structures, such as the wings of an airplane, many (automated) measurements need to be conducted, and localization of identified features on a map is crucial for successful damage detection. Hence, in this work, methods to detect edges are investigated in an effort to generate a map of the structure using Lamb waves. Measurements are conducted with contact and air-coupled ultrasound transducers in laboratory experiments. While the used contact transducers do not exhibit any directional sensitivity, air-coupled transducers are only sensitive to incoming waves from one direction. Therefore, different data processing methods have to be applied, depending on the applied actuator or sensor technology. Even though the experiments are conducted for a pristine aluminum plate, an outlook for composite plates is given as well. In addition, it is explored whether guided-wave based methods also allow for the detection of other structural features, such as stiffeners. The accuracy of the applied identification methods is validated against the structures’ true dimensions. Even though substantial assumptions have to be made, the investigated methods show promise for successful application in real scenarios.
While high-intensity focused ultrasound (HIFU) is already being used for the ablation of tissue near the skin, such as in the case of prostate cancer, targeting tissue deeper inside the body remains challenging due to the increased obstruction and scattering of ultrasonic waves. In this work, the partial and complete obstruction of the ultrasonic beam path from a HIFU transducer operating at 670 kHz by bone phantom is imaged in laboratory experiments to visualize wave transmission and reflection at solid-fluid interfaces. Ultrasonic wave-scattering under such conditions has scarcely been the focus of previous ultrasound visualization studies. Thus, this work provides a qualitative visual reference for focused waves scattering at water-bone interfaces. A diffraction-based shadowgraph technique is used for the ultrasound visualization. The ultrasonic waves are imaged in water with no obstruction, with varying partial obstruction, and with complete obstruction by a thin fiber-filled epoxy plate mimicking bone tissue. Experimental findings are compared to those obtained through finite element simulations, showing good agreement. Furthermore, it is found that in certain partial obstruction cases, the waves scatter in such a way that the destructive interference between the transmitted waves lead to a significantly reduced maximum pressure at the focal point. Overall, the results of this study can provide a visual framework for future research in the field of therapeutic ultrasound.
For the purpose of nondestructive testing (NDT), guided waves can be transmitted into a structure, and any defects or anomalies in the waves’ path modify the measured waves. Signal processing methods can be used to extract information about these features. In this work, an NDT method is demonstrated based on laboratory experiments for the case of a flat, rectangular, aluminum plate, which has a stiffener mounted underneath along the middle axis, such that the stiffener cannot be seen from the upper “outside” surface. Piezoelectric transducers are set up in a pitch-catch arrangement on this surface with the assumption that the location of the stiffener is unknown. When guided waves are induced in the plate by one of the transducers, the waves that are received by the other carry the information of the stiffener, as well as any defects in or boundaries of the structure. By transmitting from different points on a grid on the plate, the location and size of any geometry or material discontinuities can be identified. Hence, the developed algorithm reverse engineers the plate by mapping its edges and identifying the region of the stiffener.
Damping in miniature resonators is a consequence of many factors, one of which is due to interaction with the substrate to which the resonator is mounted. It is common practice to create a model of the resonator that includes a small segment of the substrate plate with a finite element (FE) software in conjunction with absorbing boundary elements. As an alternative to implementing absorbing boundary elements, semi-analytical methods have been developed in which such elements are replaced by analytical expressions for Lamb waves. This approach requires the specification of a harmonic load and the determination of the subsequent harmonic response at a point on the resonator. The modal frequency and damping can then be estimated from the computation of the frequency response function on a frequency grid. In this paper, the approach is demonstrated for single and double cantilever configurations on a plate in the case of plain strain. The influence of the number of selected Lamb modes, mesh density and the size of the modeled plate segment is investigated through parametric studies. Moreover, it is shown that the semi-analytical results are in good agreement with those from conventional transient finite element simulations.
Due to their excellent strength-to-weight ratio, honeycomb sandwich panels are being increasingly used in lightweight structures, in particular in aircraft and aerospace industry. Delaminations of individual plies in the composite skins or disbonds of a layer in the multi-layer plate structures often remain undetected during visual inspection. Using guided ultrasonic waves, such hidden defects can be detected. For the successful application of ultrasonic nondestructive testing methods, however, wave propagation characteristics have to be well-understood. Recently developed semi-analytical techniques allow for the calculation of dispersion characteristics for many materials. However, the elastic material behavior is often simplified for these calculations. For example, woven composite laminates are modeled as a homogeneous, transversely isotropic plate. While these simplifications only lead to minor errors, the modeling of aluminum honeycomb core sandwich panels with homogeneous, transversely isotropic layers has yet to be validated. In this paper, an efficient numerical approach is used to determine the dispersion characteristics of a honeycomb core layer with and without simplified material behavior. A full 3D-model, including the honeycomb cells, of a small representative volume element of the material is generated using finite elements, and the resulting dispersion curves are compared to the ones obtained from simplified models. In addition to dispersion curves, the displacement fields of the waves are also analyzed.
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.
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.