Ultrasonic | Testing Unlimited

BASIC APPLICATIONS OF ULTRASONICS

Ultrasonic testing methods are extremely versatile and can be used to test many ferrous and nonferrous materials for surface and subsurface discontinuities. There are a wide variety of applications and equipment to select from. Materials which can be tested include, but are not limited to, metallic and nonmetallic Forgings, Castings, Welds, Sheet, Tubing, Plastics, Rubber, Glass, and Composites.

Ultrasonic testing (UT) is essentially the mechanical introduction of a beam of sound energy into a material being examined. This causes the atoms to vibrate in the material. We then monitor the sound beam using an electronic display. Some of the characteristics of the sound beam which are of interest to us include;

The distance the sound beam has traveled,
Any interruption in the sound beam,
Any change of sound beam direction,
The strength of the return signal

The return signal is usually monitored for a reduction in magnitude/strength. The sound beam will always lose energy after it leaves the transmitter. Properties of a test material will tend to absorb the sound while abnormalities may scatter the sound beam or reflect the sound beam back to the sound transmitter (transducer).

The application of the method is relatively simple. However, the selection of the type test necessary to detect possible defects, the equipment to be used, and the interpretation and evaluation of the test results can be very complicated and requires extensive training and experience. The following pages discuss the industry specifications outlining the qualification and certification of technicians who will use the ultrasonic method.

Surface discontinuities can be detected but the UT method is limited due to the nature of how the sound is generated and how it enters the test material. It should be noted that no one single NDT method is best at detecting all discontinuities. When UT inspections are required, they may be supplemented with another NDT method to give complete detection capability. The Magnetic Particle method is typically used to detect surface defects in ferromagnetic materials, whereas, the Liquid Penetrant method is used to test non-ferromagnetic materials for surface defects.

Understanding the properties of sound and how they behave will provide essential information about the test and the material being tested. By measuring the amount of time it takes for the sound beam to travel through its path in the test material, we can also make a reasonable determination as to the soundness of a material and/or the location and size of a possible defect. UT is used extensively to make material thickness measurements and for detection of opposite wall corrosion.

Congruently, there are many industry specifications and codes governing the selection and use of the different ultrasonic methods. These specifications outline the test procedures, the minimum requirements and “Standard Practices” which must be adhered to when performing a UT examination. It is the responsibility of the customer or prime contractor to select and specify the applicable specifications, codes, and procedures which shall be used to examine the materials and product to be examined.

As with any examination, there must also be an “Acceptance Criteria” specified. This information is necessary to make an accept/reject decision based on the actual test results. These acceptance criteria are expressed as engineering judgments as to whether the material or part will meet its intended function or is serviceable.

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include;

It can detect surface, subsurface, and opposite wall discontinuities.

The depth of penetration for flaw detection or measurement is superior to other NDT methods.
Only single-sided access is needed when the pulse-echo technique is used.
It has high degree of accuracy in determining reflector position and estimating size and shape.
Minimal part preparation required.
Electronic equipment provides instantaneous results.
Detailed images can be produced with automated systems (“B” & “C” Scans) for permanent records.
It has other uses such as thickness measurements, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include;

Reference standards are required for both equipment calibration and characterization of flaws.
Linear defects oriented parallel to the sound beam may go undetected.
Cast iron, Stainless Steel, Inconel and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.
Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous can be difficult to inspect.
It normally requires a coupling medium to promote transfer of sound energy into test specimen.
Skill and training is more extensive than with other NDT methods.
Surface must be accessible to transmit ultrasound.