Traditionally ultrasonic testing is used to estimate the extent of damage in a concrete structure. However Pulse-velocity
and amplitude attenuation methods are not very reliable, and are difficult to reveal early damage of concrete. In a
previous study, a new active modulation approach, Nonlinear Active Wave Modulation Spectroscopy, was developed
and found promising for early detection of damage in concrete. In this procedure, a probe wave is passed through the
system in a fashion similar to regular acoustic methods for inspection. Simultaneously, a second, low-frequency
modulating wave is applied to the system to effectively change the size and stiffness of flaws microscopically and
cyclically, thereby causing the frequency modulation to change cyclically as well. It has been also shown that it is
advantageous to apply the Hilbert-Huang transform to decompose nonlinear non-stationary time-domain responses of
plain concrete. Such procedure leads to improving the damage detection sensitivity of this modulation method in
concrete. In this paper, further investigation on mortar and fiber reinforced concrete will be presented and discussed.
Several nondestructive testing methods can be used to estimate the extent of damage in a concrete structure. Pulse-velocity
and amplitude attenuation methods are very common in nondestructive ultrasonic evaluation. Velocity of propagation is not very sensitive to the degree of damage unless a great deal of micro-damages have evolved into localized macro-damages. The amplitude attenuation method is potentially more sensitive to damage than the pulse-velocity
method. However, this method depends strongly on the coupling conditions between the transducers and the
concrete and hence is unreliable. In a previous study, a new active modulation approach, Nonlinear Active Wave
Modulation Spectroscopy, was developed and found promising for early detection of damage in concrete. In this
procedure, a probe wave is passed through the system in a fashion similar to regular acoustic methods for inspection.
Simultaneously, a second, low-frequency modulating wave is applied to the system to effectively change the size and
stiffness of flaws microscopically and cyclically, thereby causing the frequency modulation to change cyclically as well.
The resulting amplified modulations can be correlated to the extent of damage and quantification of small damage
becomes possible. In this paper, we present the use of Hilbert-Huang transform to significantly enhance the damage
detection sensitivity of this modulation method by performing time-frequency decomposition of nonlinear non-stationary
time-domain responses.
Non-linear nondestructive testing is different from linear acoustic in that it correlates the presence and characteristics of a defect with acoustical signals whose frequencies differ from the frequencies of the emitted probe signal. The difference in frequencies between the probe signal and the resulting frequencies is due to a nonlinear transformation of the probe signal as it passes through a defect. Under acoustic interrogation due to longitudinal waves, as the compression phase passes the defect the two sides of the interface are in direct contact and the contact area increases. Similarly, the tensile phase passes through the defect, the two sides separate and the contact area decreases, thereby modulating the signal amplitude. The contact area depends on the roughness of the surface and on the magnitude of the cohesive forces that arise from the small crack openings. Such cohesive forces may be attributed to aggregate interlock (in plain concrete), fiber bridging (in fiber reinforced concrete) or both. In this paper, the frequency shifts of the probe elastic wave will be analytically related to the roughness and varying cohesive forces of the crack-like defect.
Experimentation at Wayne State University has lead to the development of a quasi acousto-ultrasonic method that utilizes an acoustic perturbation in the test region to enhance the dispersion of energy in the frequency domain. Analytical modeling has been used to explain this energy redistribution on two crucial levels; harmonic frequency generation, and sideband generation through amplitude modulation. Further refinement in both testing and modeling is planned.
Advanced fiber-reinforced polymer composite (FRP) has been increasingly used in bridge deck to replace concrete or steel. A FRP bridge deck can be designed to meet AASHTO HS-25 load requirements. FRP decks have many advantages over the conventional reinforced concrete or steel decks owing to their lightweight, high strength and corrosion resistance. However, such new deck system requires extensive monitoring to ensure its designed performance before its widespread acceptance by the bridge community. For inspection and evaluation purpose, a proper monitoring system consisting of various kinds of sensors installed in the FRP deck is critical. This paper provides a framework for designing an efficient monitoring system. The strategic sensor locations are identified based on the stress analysis of the FRP deck.
Many recent studies have indicated that externally bonded Fiber Reinforced Plastic (FRP) composite sheets can be used effectively to strengthen concrete structures. Composite action suggests that the individual parts of a composite work together as one. Stresses must be transferred from one constituent to the other through some interface. If there is little or no bond at the interface then there is little or no transfer of stress and therefore poor composite action. In this study, we intend to develop a NDE tool for evaluating such interfacial bond conditions. An innovative active modulation approach, Nonlinear Active Wave Modulation Spectroscopy (NAWMS), is adopted in our study. In this procedure, a probe wave will be passed through the system. Simultaneously, a second, modulating wave will be applied to the system. Using three-inch thick concrete slabs we have prepared several FRP laminated samples with artificial flaws (one-inch square) designed into the interface. The sender is on one side of the flaw and the receiver on the other. Maintaining the separation of this pair of transducers, we progressively move them away from the flaw location. The resulting frequency modulations will be analyzed, and will be correlated to the presence of the flaws.
Several nondestructive testing methods can be used to estimate the extents of damage in a concrete structure. Pulse-velocity and amplitude attenuation, are very common in nondestructive ultrasonic evaluation. Velocity of propagation is not very sensitive to the degrees of damage unless a great deal of micro-damage having evolving into localized macro-damage. Amplitude attenuation is potentially more sensitive than pulse-velocity. However, this method depends strongly on the coupling conditions between transducers and concrete, hence unreliable. A new active modulation approach, Nonlinear Active Wave Modulation Spectroscopy, is adopted in our study. In this procedure, a probe wave will be passed through the system in a similar fashion to regular acoustics. Simultaneously, a second, low frequency modulating wave will be applied to the system to effectively change the size and stiffness of flaws microscopically and cyclically, thereby causing the frequency modulation to change cyclically as well. The resulting amplified modulations will be correlated to the extents of damage with the effect that even slight damage should become quantifiable. This study unveils the potential of nonlinear frequency analysis methods for micro-damage detection and evaluation using actively modulated acoustic signals. This method can interrogate materials exaggerating the nonlinearly that exists due to microcracking and deterioration.
Several nondestructive testing methods can be used to determine the damage in a concrete structure. Linear ultrasonic techniques, e.g. pulse-velocity and amplitude attenuation, are very common in nondestructive evaluation. Velocity of propagation is not very sensitive to the degrees of damage unless a great deal of micro-damage having evolving into localized macro-damage. This transition typically takes place around 80% of the ultimate compressive strength. Amplitude attenuation is potentially more sensitive than pulse-velocity. However, this method depends strongly on the coupling conditions between transducers and concrete, hence unreliable. A baseline test of the linear acoustics of several mortar samples was conducted. These mortar samples have been previously damaged to different levels. Several other testing methods were also performed on the same samples to form a comparison. The focus is in comparing the sensitivity of a new testing method (Non-linear Acoustic NDE) with several more traditional testing methods. Non-linearity of the material stiffness is expressed in non-linear acoustics as the effect that damage and flaws have on the modulation of a signal as it propagates through the material. Spectral (non-linear) analysis is much more sensitive to lower damage states and less dependent on the repeatability of the coupling of the transducers.
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