KEYWORDS: Signal attenuation, Composites, Semiconducting wafers, Signal detection, Acoustic emission, Structural health monitoring, Sensors, Wave plates, Wave propagation, Signal to noise ratio
In structural health monitoring (SHM) of composite members, the choice of sensor positions, frequency band and signal interpretations are directly affected by the attenuation levels. Hence, it is important to consider the influence of attenuation for quantitative interpretation of signals in SHM applications. In this paper, attenuation of the two fundamentals Lamb wave modes, namely the symmetric mode S0 and antisymmetric A0 were experimentally measured in Carbon Fiber Reinforced Polymer (CFRP) laminates. The stress waves were launched using piezoelectric wafers bonded to the center of the laminate. Symmetric and antisymmetric modes were excitation through appropriate combination of two piezoelectric wafers bonded on opposite surfaces of the plate at the same location. The out of plane displacements corresponding to both modes were measured using a scanning laser vibrometer (SLV), along different orientations starting from 00 to 900 at 150 increments. Attenuation coefficients were obtained from 100 kHz to 350 kHz for both cross-ply as well as quasi-isotropic laminates. The attenuation coefficients ranged from about 5 nepers/m to 40 nepers/m. The experimental results as obtained by the scanning laser vibrometer compared favorably with results available in the literature. The antisymmetric mode was found to undergo significantly higher attenuation compared to the symmetric mode. The attenuation of the two modes further depends on the direction of propagation and their frequency components. Because of the dependence of the attenuation level on the Lamb wave mode, frequency, and direction of propagation, a wide frequency content signal such as acoustic emission will undergo substantial changes before being detected by sensors. Successful acoustic emission monitoring will have to take such attenuation into account to get reliable results.
Acoustic emission (AE) based structural health monitoring relies on detection and analysis of stress waves released by damage growth. The features of AE waveforms such as amplitude, frequency, energy, and rise time could be used characterize the relationship between acoustic emission signals received at a particular instant and the nature of damage growth, and several studies have examined such relationships. Acoustic emission generated by fatigue cracks propagate as a combination of a number of Lamb wave modes in plates, and hence by identifying the modal components it is potentially possible to learn about the conditions under which the crack is propagating. This paper examines the modal features of the acoustic emission signals generated at different stages of fatigue crack growth. A key requirement for this analysis is high fidelity required from the sensor recording AE signals, so that the different modes can be identified. Fatigue crack in a 6061 aluminum plate was monitored using a wideband sensor. The fatigue crack grew over a length of about 3 inches during 74,000 cycles and resulted in over 100,000 waveforms. The waveforms were examined in detail and the features were evaluated. As expected, the waveform’s peak amplitude and energy content were indicative of the rate of fatigue crack growth. More importantly, the modal features of the waveform were found to be indicative of the nature of crack growth. Mode 1 crack growth that resulted in signals that contained almost exclusively the fundamental symmetric mode S0. When the crack is propagating in shear mode, the waveforms contained dominant A0 mode. Hence, acoustic emission waveform can potentially identify the transition of a Mode 1 crack to shear crack.
Relative motion between surfaces of mechanical parts causes surface wear and damage. The degradation to the surfaces has been monitored using vibration characteristics as well as acoustic emissions generated during the relative surface movements. In particular, acoustic emission signals were found to be sensitive to some of the microscopic processes occurring at the frictional interface. In this study, friction between two surfaces was monitored experimentally under controlled conditions. Relative velocity, contact pressure, and surface roughness values were varied in the experiments. Friction related acoustic emission signals were recorded and analyzed to understand the relationship between the signals generated and the physical processes giving rise to these signals. Information related to the stick-slip movements during cyclic motion, in the experiments, was observed from the signals. Features of the waveforms were found to reveal the conditions existing at the friction interface. In particular, the changes in the surface roughness and contact pressure were readily observed from the acoustic emission signals.
Composite materials are profuse emitters of acoustic emissions (AE) during the occurrence of damage. In some cases, the signals measured are due to growth of noncritical damage. To have a more accurate SHM technique, distinguishing between critical and non-critical damage is necessary. This work focuses on monitoring damage growth and relating AE signals of critical growth to the reduction in structural integrity. Specifically, AE signals were experimentally measured in carbon fiber reinforced polymers (CFRP) specimens. Once the specimens were prepared, they were placed under quasi-static and fatigue loading until failure. Attenuation of AE signals were studied and mode-I delamination tests were conducted as well.
Machine parts often contain components which experience relative motion during service. Relative motion between
surfaces causes fatigue crack, wear and eventual surface deterioration. Acoustic emission based machinery condition
monitoring is a method which can potentially be used to monitor the state of damage present on surfaces during service.
This research deals with changes that occur in the characteristics of acoustic emission signals due to altering surface
roughness and texture caused by friction loading. A test fixture was used to simulate friction between surfaces of
comparable surface finish and obtain acoustic emission signals generated in the process. The different characteristics of
signal waveforms obtained at different instances during the test were examined. It was shown that some features like
amplitude and duration of the waveforms are sensitive to surface wear.
Mating parts often experience repetitive relative motion termed fretting which results in friction, wear, as well as
acoustic emission signals. Acoustic emission signals have the potential for monitoring the condition of the surfaces
participating in the frictional process. In structural health monitoring studies, where the focus is on quantifying crack
growth related acoustic emission signals, the signals generated by other mechanisms give rise to undesirable false
positives. A major source of such false positives are fretting related signals. The present paper describes an experimental
approach for characterizing the friction related acoustic emission signals. A test fixture is developed to obtain fretting
related signals under controlled conditions. The waveforms are analyzed to extract features common to these signals. A
comparison of acoustic emission signals related to fretting and crack growth is provided.
Among different failure modes observed in structures, loss of stability due to buckling is a major concern. Buckling may
be induced because of overload or as a consequence of other types of failures in the structure. This paper examines two
techniques, namely, vibration based analysis, and stress wave propagation analysis for detecting this onset of instability.
The responses of a bar and a plate are used to illustrate the effectiveness of the two approaches. These analyses were
performed through finite element simulations and limited experiments. Changes in vibration frequencies and mode shapes are found to provide good indications of the impending failure as well as its progress. Changes in the wave propagation characteristics showed some limited success in detecting the incipient buckling.
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