Ultrasonic Thickness Measurement
In the field of industrial ultrasonic testing, ultrasonic thickness measurement (UTM) is a method of performing non-destructive measurement (gauzing) of the local thickness of a solid element (typically made of metal, if using ultrasound testing for industrial purposes) basing on the time taken by the ultrasound wave to return to the surface. This type of measurement is typically performed with an ultrasonic thickness gauze.
Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. To illustrate the general inspection principle, a typical pulse/echo inspection configuration as illustrated below will be used. A typical UT inspection system consists of several functional units, such as the pulsar/receiver, transducer, and display devices. A pulsar/receiver is an electronic device that can produce high voltage electrical pulses.
Driven by the pulsar, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when an echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.
Ultrasonic Inspection is a very useful and versatile NDT method.
Some of the advantages of ultrasonic inspection that are often cited include:☛ Minimal part preparation is required.
☛ it is sensitive to both surface and subsurface discontinuities.
☛ The depth of penetration for flaw detection or measurement is superior to other NDT methods.
☛ Detailed images can be produced with automated systems.
☛ Only single-sided access is needed when the pulse-echo technique is used.
☛ It is highly accurate in determining reflector position and estimating size and shape.
☛ Electronic equipment provides instantaneous results.
☛ It has other uses, such as thickness measurement, in addition to flaw detection.
As with all NDT methods, ultrasonic inspection also has its limitations, which include :
☛ Surface must be accessible to transmit ultrasound.
☛ Skill and training is more extensive than with some other methods.
☛ It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.
☛ Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
☛ Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.
☛ Linear defects oriented parallel to the sound beam may go undetected.
☛ Reference standards are required for both equipment calibration and the characterization of flaws.
The above introduction provides a simplified introduction to the NDT method of ultrasonic testing. However, to effectively perform an inspection using ultrasonic, much more about the method needs to be known. The following pages present information on the science involved in ultrasonic inspection, the equipment that is commonly used, some of the measurement techniques used, as well as other information.
Ultrasonic thickness gages offer a variety of number of benefits over mechanical and optical measurement techniques in a variety of common manufacturing and in-service test applications, supporting quality control, reliability, and condition monitoring in a cost-effective and user-friendly way.
☛ Measurement from one side
Ultrasonic gages require access to only one side of pipes, tanks, tubing, containers, hollow castings, large metal or plastic sheets, and other test pieces where the inside surface is impossible or difficult to reach.
☛ Completely nondestructive
No cutting or sectioning of parts is required, saving scrap and labor costs.
☛ Highly reliable
Modern digital ultrasonic gages are highly accurate, repeatable, and reliable.
All common engineering materials can be measured with appropriate gage setups, including metals, plastics, composites, fiberglass, ceramics, and rubber. Most instruments can be pre-programmed with multiple application setups.
☛ Wide Measurement Range
Ultrasonic gages are available for measurement ranges as broad as 0.08 mm (0.003") minimum to 635 mm (25") maximum, depending on material and transducer selection. Resolution can be as fine as 0.001 mm or 0.0001".
☛ Easy to use
The vast majority of ultrasonic gaging applications use straightforward pre-programmed gage setups and require only a small amount of operator interaction. While this tutorial discusses a number of advanced techniques and challenging measurements, advanced training in ultrasonics is not required in most cases.
☛ Instant response
Measurements usually take only one or two seconds per point and are displayed as a digital readout.
☛ Compatible with data logging and statistical analysis programs
Most modern hand-held gages offer both on-board storage for measurement data, and USB or RS232 ports for transferring measurements to a computer for record-keeping and further analysis.