The ultrasonic field generated by a Micro Intereferometric Acoustic Lens used for high precision Rayleigh wave velocity
measurements is modeled by the recently developed mesh-free technique called Distributed Point Source Method
(DPSM). The field generated by the three individual ultrasonic transducer elements forming the micro intereferometric
acoustic lens are computed and compared with experimental measurements. Qualitative agreement between the
theoretical and experimental results is observed; both results show converging beams up to the focal point and then the
beams diverge. However, some of the minute detailed features in the generated ultrasonic field could only be observed in
the computed results. Effects of non-uniform surface of the transducer and its contribution to the non-uniform ultrasonic
source strength are investigated to understand and optimize the acoustic lens for localized quantitative elastic property
measurements.
This paper explores the feasibility of detecting and quantifying corrosion and delamination (separation) at the interface
between reinforcing steel bars and concrete using ultrasonic guided waves. The problem of corrosion of the reinforcing
steel in structures has increased significantly with time. Concrete is strengthened by the inclusion of the reinforcement
steel such as deformed or corrugated steel bars. Bonding between the two materials plays a vital role in maximizing
performance capacity of the structural members. Corrosion of reinforcing steel has led to premature deterioration of
many concrete members before their design life is attained. It is therefore, important to be able to detect and measure the
level of corrosion in reinforcing steel or delamination at the interface. The development and implementation of damage
detection strategies, and the continuous health assessment of concrete structures then become a matter of utmost
importance. The ultimate goal is to develop a nondestructive testing technique to quantify the amount of corrosion in the
reinforcing steel. The guided mechanical wave approach has been explored towards the development of such
methodology. The ultrasonic waves, specifically cylindrical guided waves, can propagate a long distance along the
reinforcing steel bars and have been found to be sensitive to the interface conditions between steel bars and concrete.
Ultrasonic transducers are used to launch and detect cylindrical guided waves along the steel bar.
Several investigators have modeled ultrasonic fields in front of finite sized transducers. Most of these models are based
on Huygens principle. Following Huygens-Fresnel superposition principle one can assume that the total field of a finite
size transducer is obtained by simply superimposing the contributions of a number of point sources uniformly distributed
on the transducer face. If the point source solution, also known as the Green's function, is known then integrating that
point source solution over the transducer face one can obtain the total ultrasonic field generated by a finite transducer.
This integral is known as Rayleigh-Sommerfield integral. It is investigated here how the ultrasonic field in front of the
transducer varies for different interface conditions at the transducer face-fluid interface such as 1) when only the normal
component of the transducer velocity is assumed to be uniform on the transducer face and continuous across the fluid-solid
interface, or 2) when all three components of velocity are assumed to be uniform on the transducer face and
continuous across the interface, 3) when the pressure instead of velocity is assumed to be uniform on the transducer face
and continuous across the interface. All these different boundary and interface conditions can be modeled by the newly
developed Distributed Point Source Method (DPSM). These results are compared with the Rayleigh-Sommerfield
integral representation that gives the fluid pressure in front of the transducer when the transducer-fluid interface is
subjected to uniform normal velocity.
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