refl & refraction
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Reflection and refraction, mode conversion
<Gibson 2005>
<www.ndt-ed.org>
When an obliquely incident wave encounters the boundary with
another material, reflection and refraction become dependant on ϑi
(Snell’s law). Conversion to other wave modes also occurs.
Beam divergenceThe principles of wave interference and superposition control the
directivity of the generated pressure field. A given transducer may
primarily generate P-wave energy in some directivity field, although
some S-wave and Rayleigh wave energy, may also generated in solid media
<www.ndt-ed.org>
1.2sin
D
λα =
The beam divergence
angle α of a given transducer can be
estimated:α
Beam divergence: point source of waves
<Richard et al. 1970>
Solid material
Snapshot of wave fields (stress) in material owing to transient point load
at some time “t” after wave excitation
Point sources of waves have poor directivity and generate P-waves, S-
waves and Rayleigh waves
ScatteringThe reflection of ultrasonic energy away from the original direction of
propagation; caused by reflection, refraction and mode conversion from internal
inclusions. Causes signal loss, signal dispersion and scattering “noise”
Detected back-
scattered signal
<Oelze 2007>
Absorption and attenuation
<http://www.greerindustries.com>
Wave absorption is the conversion of ultrasonic wave energy to other
forms of energy (heat). A
significant source of wave energy
loss for asphalt concrete
Wave attenuation is the overall loss of wave energy with propagation,
caused by
* beam divergence (geometric) * scattering
* absorption
<www.ndt-ed.org>
Implications: transducer contact (coupling)
<M. Schickert and MSIA Spectrum>
<www.ndt-ed.org>
To eliminate significant wave reflection at the transducer-test material interface, must use a
substance to displace air and ensure good
contact: oil, gel, grease, solid
Problematic for rough or uneven surfaces
Dry point contact transducers obviate the need for couplant
material. Each point transducer
needs vertical pressure to
ensure wave energy transfer
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