Traditional Ultrasonic inspection uses high frequency sound energy to conduct examinations and perform measurements. Considerable information may be gathered during ultrasonic testing such as the presence of discontinuities, material or coating thickness. The detection and location of discontinuities is enabled by the interpretation of ultrasonic wave reflections generated by a transducer. These waves are introduced into a material and travel in a straight line and at a constant speed until they encounter a surface. The surface interface causes some of the wave energy to be reflected and the rest of it to be transmitted. The amount of reflected vs. transmitted energy is detected and provides information on the size of the reflector, therefore the discontinuity encountered. Three basic ultrasonic techniques are commonly used:
1. Pulse-echo and through transmission
In pulse-echo testing a transducer sends out a pulse of energy and the same or a second transducer listens for reflected energy, also known as an echo. Pulse echo is especially effective when only one side of a material is accessible.
Through transmission is performed using two transducers on opposing sides of the specimen. One acts as a transmitter and the other as a receiver. Through transmission is useful detecting discontinuities that are not good reflectors when signal strength is weak.
2. Normal/Angle Beam
Normal beam testing uses a sound beam that is introduced at 90 degrees to the surface, while angle beam utilizes a beam that is introduced into the specimen at some angle other than 90 degrees. The choice between the two is made based on:
The orientation of the feature of interest so that the sound may produce the largest reflection from the feature.
Obstructions on the surface of the specimen that must be avoided.
3. Contact and Immersion
To get useful levels of sound energy into the material, the air between the transducer and the specimen must be removed. This is referred to as coupling. Two types of coupling are utlizied:
In contact testing, a couplant such as water, oil or a gel is applied between the transducer and the specimen.
In immersion testing, the specimen and the transducer are placed in a water bath. This allows better movement of the transducer while maintaining consistent coupling.
Some of the most common Ultrasonic applications are:
Flaw detection (cracks, inclusions, porosity, delaminations etc.)
Erosion/Corrosion thickness gauging
Assessment of bond integrity
Estimation of grain size in metals
Estimation of void content in composites and plastics
Info from ultrasonic inspection can be presented in a number of formats:
A-Scan displays the amount of received ultrasonic energy as a function of time.
B-Scan displays a profile view (cross-sectional) of a specimen.
C-Scan displays a plan type view of the specimen and discontinuities.
Hybrid/Stitched displays a C-Scan plan view with A and/or B Scan views along with C-Scan views that have been woven together to illustrate a clearer picture of the damaged areas of a specimen. The stitched views are used for larger specimens and surface areas.
Some of the major advantages of ultrasonic testing are:
Detects surface and subsurface defects.
Depth of penetration vs. other test methods is superior.
Only single sided access is required with a pulse-echo technique.
High accuracy regarding estimating discontinuity size and shape.
Minimal specimen preparation is required.
Instantaneous results produced by using electronic equipment.
Detailed images can be produced with automated systems.
Major limitations of ultrasonic testing are:
Surface must be accessible.
Skill training is more extensive that with some other methods.
Normally requires couplant to promote sound transfer.
Surface roughness, complex geometries, small parts or exceptionally thin materials are difficult to inspect.
Coarse grained materials i.e. cast iron are difficult to inspect due to low sound transmission and high signal noise.
Linear defects oriented parallel to the sound beam go undetected
Reference standards are required for equipment calibration