The velocity of ultrasound in any given material is a constant for that material and ultrasonic beams travel in straight lines in homogeneous materials. When ultrasonic waves pass from a given material with a given sound velocity to a second material with different velocity, refraction and a reflection of the sound beam will occur at the boundary between the two materials. The same laws of physics apply to ultrasonic waves as to light waves. Ultrasonic waves are refracted at a boundary between two materials having different acoustic properties so probes may be constructed which can beam sound into a material at (within certain limits) any given angle.

Because sound is reflected at a boundary between two materials having different acoustic properties ultrasound is a useful tool for the detection of weld defects.

Since velocity is a constant for any given material and sound travels in a straight line (with the right equipment) ultrasound can also be used to give accurate positional information about a given reflector.

Careful observation of the echo pattern of a given reflector and its behaviour as the ultrasonic probe is moved together with the positional information obtained above and knowledge of the component history enables the experienced ultrasonic operator to classify the reflector as slag, lack of fusion or a crack.

Advantages

   Limitations

Portable (no mains power) battery

       No permanent record

Direct location of defect (3 dimensional)

       Only ferritic materials (mainly)

Good for complex geometry

       High level of operator skill required

Safe operation (can be done next to someone)

       Calibration of equipment required

Instant results

       Special calibration blocks required

High penetrating capability

       No good for pin pointing porosity

Can be done from one side only

       Critical of surface conditions (clean smooth)

Good for finding planar defects

       Will not detect surface defects

      Material thickness >8mm due to