1,721,170 research outputs found
Mode-converted Lamb wave sensitivity prediction for part-thickness crack-like defects
Get PDFFatigue crack growth is one of the most common damage types in aluminum components, widely used in aircraft
structures. Detection of fatigue cracks at an early stage is important to guarantee aircraft safety. Efficient non-destructive
evaluation (NDE) and structural health monitoring (SHM) can be achieved by employing low frequency guided
ultrasonic waves, as they can propagate long distances along plate structures. SHM systems using distributed guided
waves sensors have been proposed for efficient monitoring, but have limitations due to environmental influences such as
the temperature stability of the conventional baseline subtraction method. The scattering and mode conversion of guided
waves at part-thickness defects was investigated to quantify the sensitivity for defect detection and the potential for the
development of a baseline-free SHM methodology employing mode converted guided waves. Baseline-free SHM
methodology employing mode conversion is expected to overcome some of the limitations caused by environmental
factors and to improve sensitivity and stability by employing new or modified signal processing algorithms. A three
dimensional (3D) Finite Element (FE) model was developed to predict the mode conversion of the fundamental guided
wave modes. The influence of defect length and depth on detection results were investigated numerically. The detection
sensitivity for part-thickness defects in a plate is quantified
Guided Wave Energy Focusing and Steering in Composite Laminates
Get PDFLightweight, carbon fiber reinforced composites are often selected for aerospace components but are prone to barely visible
impact damage, caused by low velocity impacts, during service. Guided-wave-based structural health monitoring (SHM)
techniques can efficiently detect impact damage impact in composite structures. However, wave propagation is influenced
by material anisotropy resulting in a number of effects. The phase and group velocity of propagating wave modes depend
on the wave launching direction, with increased wave speeds in the high stiffness (fiber) directions. Wave energy tends to
be focused along the fiber directions, resulting in beam steering or skewing away from the initial wave launching direction.
These anisotropic effects, if unaccounted for, could lead to inaccurate localization of damage, and potential regions of the
structure where guided waves cannot propagate with sufficient amplitude, reducing damage sensitivity. Wave propagation
in an undamaged unidirectional carbon fiber reinforced polymer (CFRP) panel was investigated for the A0 mode for
multiple wave launching directions. Finite Element (FE) modelling was carried out using homogenized anisotropic
material properties to investigate the directional dependency of velocity. Point and line sources were modelled to
investigate the influence of the excitation source on the guided wave evaluation and signal processing. Wave skewing
behavior was visualized for the line source, and wave skew angles and beam spread angles were calculated for a range of
propagation angles. Experimental non-contact guided wave measurements were obtained using a laser vibrometer. A PZT
strip transducer was developed in order to measure wave skew angles. Experimental and numerical velocities and skew
angles were compared with theoretical predictions and good agreement was observed
On ultrasound propagation in composite laminates: advances in numerical simulation
Get PDFThe growing use of composite materials for aerospace applications has resulted in the need for quantitative methods to analyze composite components. Ultrasonic guided waves constitute the physical approach for nondestructive evaluation (NDE) and structural health monitoring (SHM) of solid composite materials, such as carbon fiber reinforced polymer (CFRP) laminates. Ultrasound based NDE methods are commonly used in the aerospace field, but ultrasonic wave behavior can be complicated by the presence of material anisotropy, complex geometries (e.g., highly curved parts, stiffeners, and joints) and complex geometry defects. Common defects occurring in aerospace composites include delaminations, porosity, and microcracking. Computational models of ultrasonic wave propagation in CFRP composites can be extremely valuable in designing practical NDE and SHM hardware, software, and methodologies that accomplish the desired accuracy, reliability, efficiency, and coverage. Physics based simulation tools that model ultrasonic wave propagation can aid in the development of optimized inspection methods and in the interpretation of NDE data. This paper presents a review of numerical methodologies for ultrasound and guided wave simulation in fiber reinforced composite laminates summarizing the relevant works to date, different methods, and their respective applications
Guided wave scattering at a delamination in a quasi-isotropic composite laminate: Experiment and simulation
Get PDFCarbon fibre composite laminates are increasingly being used for aerospace structures due to their low weight and improved mechanical performance. Impact damage can cause delaminations below the visible surface of the structure due to limited interlaminar strength. Guided ultrasonic waves can detect and characterize delaminations in composite laminates. The scattering of the A0 Lamb wave mode at an artificial delamination, located at an asymmetric depth in a quasi-isotropic laminate, was investigated. Full field non-contact laser measurements were used to visualise wave trapping and scattered waves. A three-dimensional finite element model was developed and validated against the experiments. The influence of delamination shape and depth on guided wave scattering were studied. Small variations in delamination shape significantly affected the interference pattern on top of the delamination, but had limited effect on the scattered wave outside the delamination. Delamination depth was found to strongly influence the angular direction and amplitude of scattered waves. Implications for structural health monitoring were discussed
Extended Properties of Magnetic Spins of Zinc Ferrite Nanoparticles in the THz Frequency Range
No full textGuided wave scattering analysis around a circular delamination in a quasi-isotropic fiber-composite laminate
Get PDFCarbon fiber reinforced composites are widely used in the aerospace industry, but barely visible impact damage can lead to delamination and compromise the structural integrity. The scattering of the fundamental anti-symmetric guided wave mode (A0 Lamb mode) at an artificial circular delamination in a quasi-isotropic laminate was investigated experimentally. A 5 cycle Hanning windowed wave pulse was used as the excitation signal for the experiments. Fast Fourier Transform was employed to identify the guided wave amplitude of the scattered field along various directions. The experimental wavefield was captured using a laser Doppler vibrometer. Experimental results are presented for the scattering pattern and scattering amplitude as a function of distance from the damage. The results of this study can help to improve delamination detection techniques using guided waves and to gain physical insights into the scattering of guided waves at a delamination
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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