Refraction and Mode Conversion. When a sound beam passes from one medium to another with a different
velocity at an angle not normal to the interface separating the two media, refraction occurs. The incident and refracted
angles follows Snell's law. Snell's law is written for use in ultrasonic Inspection as follows:
angle between the normal to the interface surface and the incident sound beam.
2 = angle between the normal to the interface surface and the refracted sound beam.
V1 = velocity of incident sound beam.
V2 = velocity of refracted sound beam.
Beam Divergence. The sound beam is not a straight-sided projection of the face of the search unit having
uniform intensity. The sound beam spreads out beyond the face of the search unit and varies in intensity.
6-8. Ultrasonic Inspection Equipment. The basic types of ultrasonic inspection equipment are explained in the
a. General. All ultrasonic pulse-echo instruments basically perform the functions of generating, receiving,
measuring the amplitude of, and determining the time of electrical pulses. Inspection information is displayed on the
cathode ray tube contained in all ultrasonic pulse-echo flaw detection inspection units.
Search Units. Search units are devices for generating and receiving ultrasonic energy. The units may contain
transducer elements or a microphone and coil.
Transducers. Transducers are electo-acoustical devices for converting electrical energy into acoustical energy
and vice versa. The most common piezoelectric substances used for transducer elements are:
Polarized ceramic materials-barium titanate, lead metaniobate, and lead zirconate titanate
Originally, ultrasonic inspections were performed almost exclusively with quartz transducer elements, but quartz is
used very little now because better elements are available. The inspection performance can be improved by selecting
different transducer elements for the transmitting transducer element and for the receiving transducer element.
Glycenne, silicones, and graphite greases shall not be used as couplants because they have an
unacceptable surface roughness.
Couplants. Couplants are substances used between the search unit and test part to permit or improve
transmission of ultrasonic energy into the test part. Typical couplant substances include water, oil, grease, and penetrant
6-9. Ultrasonic Inspection Techniques. Ultrasonic inspection can be separated into contact and immersion
inspection. Since immersion inspection is not applicable in the field, all information in the following paragraphs is for
contact inspection. Straight beam, angle beam, and surface wave methods of contact inspection are described in the
Straight Beam Method. The straight beam method uses longitudinal waves and is generally used on objects
1/2 inch thick or greater. The dead zone interferes with inspection of thinner objects. When required, application of
straight beam inspection to thinner objects can be extended by several different methods, such as:
Inspection of object from opposite sides. The dead zone which is not inspected from the first side is
covered when inspecting from the second side.
Use of dual search units.
Use of delay line search units.
Angle Beam Method. The angle beam method generally uses shear waves with refracted angles in the test part
from 30 to 70 degrees. This method is used extensively for field NDI, and can provide for inspection of areas with
complex geometry or limited access. This