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 analysed 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.

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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 analyse 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 localised 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 (characterisation 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.

In depth: ultrasonic flaw detection – flaw detectors ↑ up

Ultrasonic flaw detection locates and sizes internal defects in a material — cracks, inclusions, lack of fusion in welds — non-destructively, by exploiting the reflection of ultrasound from discontinuities.

Non-destructive testing: internal defects without damaging the part
A-Scan display: echo amplitude as a function of sound path
Angle beam probes: inspection of welds and joints
Sizing toolkits: DAC, AWS, TCG, DGS
Detection: Z-Cross, Flank and Peak
Industries: welds, forgings, castings, aerospace
Also a thickness gauge: integrated thickness measurement

An internal defect (crack, porosity, inclusion, lamination) represents an acoustic discontinuity: when the ultrasonic pulse meets it, part of the energy is reflected and returns to the transducer. By analysing the amplitude and the position of this echo, the defect can be detected, located and sized.

The flaw detector displays the signal as an A-Scan trace: the horizontal axis represents the sound path (the depth), the vertical axis the echo amplitude. The operator interprets the trace to distinguish defect echoes from geometric ones.

How a defect is detected

The flaw detector generates high-voltage pulses and displays their echoes at a high refresh rate. Using angle beam probes, the beam enters the material at a known angle: this is the typical set-up for inspecting welds, where the defects (lack of fusion, cracks, slag inclusions) lie along the bead and cannot be reached with a straight beam.

The trigonometric function (TRIG) automatically calculates the beam path, the depth and the surface distance of the reflector, also taking the correction for curved surfaces into account. The Z-Cross, Flank and Peak detection modes define how the instrument locks onto the echo in order to measure its position and amplitude.

Sizing defects: DAC, AWS, TCG, DGS

Detecting a defect is not enough: it has to be sized in order to decide whether it is acceptable according to the specifications. Flaw detectors incorporate the main code-based toolkits:

DAC (Distance Amplitude Correction): reference curve built on reference holes, which compensates for attenuation with distance.
AWS: sizing of defects in welds according to American Welding Society practice.
TCG (Time Corrected Gain): gain that varies with time in order to equalise the response at different depths.
DGS/AVG: assessment of the defect against a calculated equivalent reflector.

The echo amplitude can be expressed in %DAC, in dB or in %FSH, depending on the criterion adopted.

Flaw detector and thickness gauge in a single instrument

Many modern instruments are hybrid: in addition to flaw detection they offer advanced thickness measurement (Pulse-Echo, through-coating measurement, temperature compensation). With a single device this allows both the search for defects and the check of the remaining thickness, reducing the equipment to be carried into the field.

Fields of application

UT flaw detection is used in the testing of welds on structures and pressure vessels, in the inspection of forgings and castings, in the search for fatigue cracks on axles and components, and in incoming inspection of semi-finished products. It is a technique recognised by the main non-destructive testing standards and requires qualified operators (e.g. UT levels according to ISO 9712).

The Dakota NDT instruments FX71-DL (handheld) and FX81-DL (top of the range, with a display readable in sunlight) distributed by RODER cover these needs: models and technical data on the linked catalogue pages.

Related products

RODER instruments for this topic. Go to the catalogue pages for models, technical data and documentation.