Time-of-Flight Diffraction (TOFD) is an advanced ultrasonic testing technique used to detect and size defects, primarily in welds. Unlike conventional ultrasonic testing, which relies on the amplitude of reflected waves, TOFD uses the time of arrival of diffracted waves to identify and characterize flaws.
How does TOFD work?
In a typical TOFD setup, two ultrasonic transducers are placed on opposite sides of the weld or test piece.
- One transducer emits an ultrasonic pulse.
- The other transducer receives the ultrasonic waves.
In a defect-free material, the received signals consist of waves that travel along the surface (lateral wave) and waves that reflect from the far wall (back wall reflection).
When an ultrasonic wave encounters a defect, such as a crack, the wave energy is diffracted from the tips of the defect. These diffracted waves are received by the second transducer. By precisely measuring the time it takes for these diffracted waves to reach the receiver, the location and size (especially the height) of the defect can be accurately determined using triangulation principles.
Advantages of TOFD
- Accurate defect sizing: TOFD is particularly accurate in determining the height or through-wall dimension of defects, which is crucial for assessing the severity of a flaw.
- High probability of detection: TOFD is highly sensitive to various types of defects, regardless of their orientation.
- Fast scanning: TOFD can rapidly scan large areas, making it a time-efficient inspection method.
- Digital data and imaging: TOFD produces digital data and images that can be stored, analyzed, and used for future reference.
- Versatility: TOFD can be used to inspect a wide range of materials and thicknesses.
Applications of TOFD
TOFD is commonly used in various industries, including:
- Weld inspection: Inspecting welds in pipelines, pressure vessels, and structural components.
- Pipeline inspection: Detecting and sizing defects in pipelines to ensure their integrity.
- Pressure vessel inspection: Examining pressure vessels for flaws that could lead to failure.
- Storage tank inspection: Assessing the condition of storage tanks and detecting any potential defects.
- Aerospace: Inspecting critical components for cracks and other defects.
Nuclear industry: Ensuring the safety and reliability of nuclear power plant components.