If you’ve ever been involved in the quality assurance or quality control aspect of construction or manufacturing, you’re likely familiar with non-destructive testing (NDT). Different methods of non-destructive testing have been developed for specific applications and industries.
Ultrasonic testing is helpful when a visual inspection on a job is not sufficient. UT transmits high-frequency sound waves through a material or a structure. As the sound waves penetrate the structure, they determine its thickness and detect defects.
The results gained from ultrasonic testing are highly reliable. This article will explain how ultrasonic testing works, the types of ultrasonic testing used, what materials it can be tested on, and the advantages and disadvantages of using UT.
How Ultrasonic Testing Works
Ultrasonic testing utilizes the same core technology as the ultrasound machines that you will find in the medical field. Sound waves travel in the same direction in an object until they come in contact with something else. At this point, they will reflect on their source. These reflections will allow a trained technician to measure an object’s thickness or determine if there are internal flaws.
During UT, the technician will apply a couplant to the structure. A couplant is a material such as a gel or oil that transmits the sound waves between the transducer and the test structure. They will then use a transducer connected to a diagnostic machine to inspect the structure.
Types of Ultrasonic Testing
There are two main types of UT, contact ultrasonic testing and immersion testing.
- Contact Ultrasonic Testing – Contact UT can be performed when a technician can reach only one side of the structure or if the test object is large or cannot be transported.
- Immersion Ultrasonic Testing – The test object is placed in a tank filled with water in immersion UT. The technician will move the ultrasound probe over the surface of the object. The ultrasound waves travel through the water into the object, detecting any flaws that may be present. The time it takes for the echo to return to the diagnostic machine and its amplitude informs the technician what flaws exist and it’s location.
Ultrasonic Testing Methods
The two methods of UT testing are pulse-echo (or reflection) mode and through-transmission (or attenuation) mode.
- Pulse-Echo Method – The transducer sends the pulse waves into the object and then receives the pulse waves as they are reflected back to the machine. The diagnostic machine then shows the results as a line plot with the amplitude on the y-axis, and the intensity, time, or distance of the reflections shown on the x-axis.
- Through-Transmission Testing – This method sends the ultrasound waves from an emitter on one surface to a receiver on the opposite side of the structure to receive the sound waves. If imperfections are present, the sound received by the second transmitter is reduced, allowing the technician to locate any flaws.
Why Choose Ultrasonic Testing?
Ultrasonic testing is often the chosen method of NDT because it can detect flaws in a structure without damaging the test object. Tank owners can also have UT performed periodically to check for corrosion and to see if identified defects have grown. Potentially, this can prevent the failure of the structure.
Materials That Can Be Tested
Ideally, UT is used for inspecting metal. However, it’s also possible to perform UT on plastic, composites, concrete, and ceramic, keeping in mind that these materials can cause reduced resolution due to the greater attenuation. Paper products and wood are the only materials not suited for UT using conventional equipment.
Advantages of Ultrasonic Testing
The most significant advantage of UT is that it is entirely non-destructive. The testing structure doesn’t have to be sectioned, cut, or exposed to chemicals that may damage it. Some other advantages of UT are:
- Access to only one side is required.
- UT is highly sensitive and can detect minimal flaws.
- Increased accuracy when compared to other NDT methods.
- Ability to estimate any detected defects’ shape, orientation, size, and nature.
- There are no health hazards associated with UT.
- Results are highly reliable when the test is set up correctly.
- Results are obtained immediately after testing.
Potential Disadvantages of Ultrasonic Testing
While there are many advantages of UT, there are also some disadvantages. These include the following:
- Experienced technicians are necessary to complete the inspection and analyze the data.
- False positives are possible.
- Any loose paint needs removing before the technician can perform the testing.
- Conventional UT requires the use of couplants.
- UT may be less sensitive for volumetric flaws such as metal inclusions than other testing methods.
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