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