Ultrasonic flaw detectors (UT)

Ultrasonic flaw detectors (UT) are non-destructive testing (NDT) instruments dedicated to locating cracks, lack of fusion, inclusions, porosity and internal defects in welds, castings, forgings, rolled products and other metal products. They use ultrasonic pulses (frequencies of 0.5-15 MHz) that travel through the material; the reflections from internal discontinuities are picked up by the same probe or by a second one and analyzed to determine the position, size and type of the defect. They are used in the testing of welded structures (bridges, pressure vessels, gas pipelines), in the acceptance inspection of forgings and castings, in the periodic inspection of industrial plant and in NDT personnel qualification.

The UT principle

The principle rests on the propagation of high-frequency elastic waves (ultrasound) through the material. A piezoelectric transducer (usually PZT) generates very short ultrasonic pulses (tenths of a µs) that enter the material through a couplant (gel, water or oil, to ensure acoustic transmission between probe and workpiece). The waves travel at a known velocity (for example 5920 m/s for longitudinal waves in steel) and are reflected at every internal discontinuity (cracks, inclusions) or at the opposite surface. The same transducer (in pulse-echo mode) or a second transducer (pitch-catch mode) detects the reflections, and the electronics analyze the signals in the time domain (A-scan).

Types of flaw detector

Modern flaw detectors are compact portable instruments with a real-time A-scan, gates for automatic detection of defect echoes, calibration of vertical (amplitude) and horizontal (time/distance) linearity, DAC (Distance Amplitude Correction) functions for evaluating defects at different distances from the probe, TVG (Time Varied Gain) to compensate for material attenuation, and AVG/DGS functions for defect sizing. The most advanced models incorporate Phased Array UT (with multi-element transducers and electronic beam steering), TOFD (Time-Of-Flight Diffraction), and B-scan and C-scan for defect mapping.

Transducers and probes

UT transducers differ in: frequency (0.5-1 MHz for thick, attenuating parts such as cast iron; 2-5 MHz for standard applications on steel; 10-15 MHz for high-precision measurements on thin materials); geometry (direct contact – single crystal, dual element, twin-line; angle beam – 45°, 60°, 70° for weld inspection; immersion); size (from a few mm to tens of mm in diameter); and coupling method (contact with couplant; water immersion; air-coupled for special applications). The choice of transducer follows from the material, the thickness, the type of defect expected and the inspection technique.

Applications

The industrial applications are varied: weld inspection (checking for cracks, lack of fusion, slag inclusions and porosity – to sector standards such as EN 17640 for UT on welds); acceptance of forgings and castings (dealing with central porosity and segregation); in-service inspection of piping, pressure vessels and tanks (to find fatigue cracks and localized corrosion); inspection of railway wheels and axles to prevent catastrophic failures; aeronautical inspections (testing structural components in light alloy); and R&D in the materials sector.

Reference standards

The main standards are: ISO 9712 (NDT personnel qualification, levels 1-2-3); ISO 17640 (UT of welds); ASTM E164 (contact UT); EN 12668 (characterization of UT equipment); EN 583 (parts 1-6, UT test procedure). Specific personnel qualifications for aerospace applications follow NAS 410. UT instruments are calibrated with V1 reference blocks (for linearity and basic calibration) and V2 blocks (for small-angle applications). Transducers are verified with dedicated blocks for amplitude, frequency and pulse width.

Selection criteria

The choice of flaw detector depends on: the application (plain UT versus Phased Array versus TOFD); the range of thicknesses and materials to be inspected; portability (compact units for on-site service, benchtop units for the laboratory); an A-scan display alone or also B/C-scan and 3D imaging; compliance with sector standards (for aerospace, power generation, nuclear); software for analysis and for generating ASNT/EN 17637 reports; and the supplier’s support and instrument qualification service.