08-17-2017, 12:07 AM
ABSTRACT.
The ultrasonic testing of concrete structures has posed many problems. Conventional methods such as pulse echo and pitch catch are of limited use due to its composition, aggregates will cause scattering and multiple reflections. Alternative ultrasonic methods have been recently investigated that examine the shape of the waveform as it traverses through a complex material, the idea being not to located a single defect but to determine the overall mechanical properties within a certain region. In damaged materials, particularly ones that have micro-cracking, the stress-strain relationship does not obey Hooke s Law of elasticity, stress is not proportional to strain, it is not linear, resulting in distortions to a pure ultrasonic sine wave traversing through it. The degree of this distortion is measured by examining the spectral content of the waveform, second, third and higher harmonics will be present and are related to the degree of micro-structure damage. Additional practical advantages in detecting non- linearity may be achieved by transmitting the sum of two ultrasonic sine waves into a material from one transducer and examining the spectra for inter-modulation products. This paper details experiments on small samples of concrete using both harmonic and inter-modulation spectral analysis
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