ToFDtekniikka

ToFDtekniikka, officially Time of Flight Diffraction (TOFD), is a nondestructive testing (NDT) method used to detect and size flaws in welds and other metallic components. It relies on ultrasonic waves and the diffraction from crack tips to locate and characterize defects.

Principle and operation: TOFD uses two high-frequency ultrasonic transducers placed on opposite sides of the test object, typically aligned with a weld. Each transducer emits ultrasound and also receives the wave that returns from the opposite side. Flaws produce diffracted waves at their tips, which arrive at the receiving transducer at characteristic times. By measuring these times and the known probe geometry, the position and size of flaws can be calculated. The resulting data are displayed as two-dimensional images (A-scans combined into B- or C-scans) that show defect depth and position along the weld.

Equipment and procedure: A TOFD setup includes a UT instrument, paired transducers with wedges, scanning hardware, and reference standards for calibration. Proper alignment, wedge angle, and coupling are essential. Operators collect pulse-echo data from both sides and process it with specialized software to generate defect maps and sizing estimates. Calibration blocks with known defect sizes are used to validate measurements.

Advantages and limitations: TOFD offers accurate defect sizing, high signal-to-noise ratio, and good detection of flaws beneath complex weld geometries. It is relatively insensitive to surface roughness and provides deep penetration. Limitations include sensitivity to geometry and alignment, reduced effectiveness for defects not lying within the beam paths, and the need for experienced interpretation.

Applications: Commonly used in weld inspection for pipelines, pressure vessels, shipbuilding, and structural steel, as well as other critical components where precise flaw sizing is required. History: TOFD was developed in the 1960s–1970s and became widely adopted in the following decades, evolving with digital processing and software tools.