What is ultrasonic testing?
Ultrasonic testing (UT) is a non-destructive testing method that uses high-frequency sound waves to detect internal defects or measure the thickness of materials.
The sound waves are generated by a transducer (also called UT Probe), which is placed on the surface of the material being tested.
The transducer sends sound waves into the material and receives echoes of the sound waves that reflect back from internal features such as cracks, voids, or changes in thickness.
The reflected sound waves are then analyzed to determine the location, size, and nature of any defects or variations in material thickness.
Ultrasonic testing is used in a wide range of industries, including manufacturing, construction, aerospace, and oil and gas, to ensure the integrity of products and structures.
Ultrasonic testing principle
Ultrasonic testing (UT) is based on the principle of using high-frequency sound waves (in the range of MHz) to detect internal defects present in the material (Such as crack, lamination pores, etc.) or measure the thickness of materials (e.g., using UT gauging techniques).
The basic principle of ultrasonic testing is as follows:
- A transducer is placed on the surface of the material being tested.
- The transducer converts electrical energy into mechanical energy in the form of high-frequency sound waves.
- These sound waves are sent into the material through the use of a coupling medium such as water or oil (applied to have contact between probe & test material).
- The sound waves travel through the material and encounter internal features such as defects.
- These internal features cause a portion of the sound energy to be reflected back to the surface as shown in the below animation.
- These reflected sound waves are then received by the transducer.
- The transducer converts them back into electrical signals.
- These electrical signals are then analyzed by a computer (Inbuilt in the UT Machine) to determine the location, size, and nature of any defects or variations in material thickness.
Read more: How to carry out Ultrasonic Testing of Welds?
How Ultrasonic Testing works?
Ultrasonic testing (UT) is a non-destructive testing method that uses high-frequency sound waves to detect internal defects or measure the thickness of materials.
The basic steps of how ultrasonic testing works include:
- The surface of the material being tested is cleaned to remove any surface contaminants.
- A coupling medium, such as water or oil, is applied to the surface to enhance the transmission of sound waves.
- The transducer, which is the device that generates and receives the sound waves, is placed in contact with the material.
- The transducer sends high-frequency sound waves into the material.
- The sound waves travel through the material and encounter internal features such as defects or variations in thickness.
- The internal features cause a portion of the sound energy to be reflected back to the surface.
- The transducer receives the reflected sound waves and converts them into electrical signals.
- The electrical signals are then analyzed by a computer to determine the location, size, and nature of any defects or variations in material thickness.
Examples of ultrasonic testing include:
- Measuring the thickness of a metal pipe to determine if it has thinned due to corrosion
- Detecting internal cracks in a turbine blade to ensure it is safe to operate
- Inspecting a weld to ensure it is free of defects and meets the required specifications
- Determining the thickness of a composite material in an aircraft to ensure it meets safety standards
Ultrasonic testing methods
There are several different ultrasonic testing methods, including:
Pulse-echo
- This method sends a pulse of sound into the material and detects the echoes that are reflected back from internal defects or from the back surface of the material. This method is commonly used to detect surface and subsurface defects, as well as to measure material thickness.
Through-transmission
- This method sends sound waves through the material, and detects the waves that pass through any internal defects.
- This method is commonly used to detect subsurface defects, and can be used to inspect large areas quickly.
Phased array
- This method uses a phased array probe to send and receive sound waves.
- The probe can be adjusted to send and receive waves at different angles and frequencies, which allows for greater flexibility in detecting and locating defects.
Time-of-flight diffraction (TOFD)
- This method uses two probes to send and receive sound waves. It measures the time of flight of the sound waves to detect and locate defects.
Guided wave
- This method uses a probe to send a guided wave into the material, which travels along the surface of the material.
- The wave reflects off of any defects, and the reflection is detected by the probe.
- This method can be used to detect defects in hard-to-reach areas, such as the inside of pipes or tanks.
Advantages of ultrasonic testing
Ultrasonic testing is a non-destructive method that can detect internal defects, measure material thickness and has high accuracy and precision.
Read more: UT Thickness Gauging Method.
It can be used on a wide variety of materials, including metals, plastics, ceramics, and composites.
It is able to detect small defects that may be missed by other testing methods.
Additionally, it is portable, allowing for testing in the field and can be automated for high-volume testing. It can also be used for both in-process and final inspection.
Following are the main advantages of Ultrasonic Testing NDT Method:
| – High accuracy and precision. |
| – Can detect both surface and subsurface defects. |
| – Can be used to measure material thickness. |
| – Can be used on a wide variety of materials, including metals, plastics, ceramics, and composites. |
| – Able to detect small defects that may be missed by other testing methods. |
| – Portable, allowing for testing in the field. |
| – Can be automated for high-volume testing. |
| – Can be used for both in-process and final inspection. |
Limitations of ultrasonic testing
The main Limitations of ultrasonic testing are:
- Requires direct physical contact with the surface of the material being tested, which can be difficult or impossible to achieve in certain areas or on certain types of materials
- Requires a coupling medium, such as water or oil, to enhance the transmission of sound waves, which can be messy and time-consuming to apply
- Can be affected by surface roughness, surface finish and surface contamination which can result in poor signal-to-noise ratio and difficulty in interpretation
- Operator dependent, interpretation of the signals depends on the experience and skill of the operator
- Not suitable for testing materials that are too thick or too thin
- Can be affected by temperature changes and other environmental conditions
- Can be affected by temperature gradients in the material being tested
- Can be affected by the presence of air pockets, debris and other materials in the test area.
Types of Ultrasonic Testing Transducers or Probe
Ultrasonic testing transducers, also known as probes, are the devices that are used to generate and receive ultrasonic waves during testing.
There are several different types of ultrasonic testing transducers, each with its own unique characteristics and capabilities.
Some common types of ultrasonic testing transducers include:
Contact transducers
- These transducers are used for direct contact testing and are pressed directly against the surface of the material being tested.
- They typically feature a piezoelectric crystal element that generates and receives ultrasonic waves.
Immersion transducers
- These transducers are used for immersion testing and are placed in contact with the material being tested through a liquid medium, such as water or oil. They typically feature a piezoelectric crystal element that generates and receives ultrasonic waves.
Phased Array transducers
- These transducers use multiple elements to focus ultrasonic energy in a specific direction and can be used for both contact and immersion testing. They are used for advanced applications like weld inspections, thickness measurements, and material characterization.
TOFD (Time of flight diffraction) transducers
- These transducers are used for advanced applications such as weld inspections, thickness measurements, and material characterization.
- They use two transducers to send and receive ultrasonic waves and can detect small and large defects with high resolution.
Guided wave transducers
- These transducers are used for remote inspections and can inspect large areas with a single scan. They use a single transducer to send and receive ultrasonic waves along a waveguide, typically a pipe or a plate.
FAQS
What materials can be tested using ultrasonic testing?
It is commonly used to inspect castings, forgings, welds, and other metal components for defects.
What types of defects can be detected using ultrasonic testing?
It can also be used to measure material thickness and to detect corrosion and erosion.
