Ultrasonic Testing Terminology

Ultrasonic Flaw Detector: An ultrasonic flaw detector is a portable industrial non-destructive testing instrument. It enables rapid, convenient, non-destructive, and precise detection, location, evaluation, and diagnosis of various internal defects (such as cracks, inclusions, folds, porosity, blowholes, etc.) within workpieces. It can be used both in laboratories and on engineering sites. It is an indispensable testing instrument in the non-destructive testing industry.

Ultrasonic non-destructive testing involves numerous specific terms. Commonly used ones include:

A-scan display: The standard ultrasonic waveform display showing signal amplitude vs. time/depth.

Transmit pulse: The initial electrical pulse sent to the transducer.

Pulse amplitude: The height or strength of the ultrasonic pulse.

Pulse width: The duration of the ultrasonic pulse.

Echo: The reflected ultrasonic wave received by the transducer.

Back-wall echo / Back reflection: The echo received from the opposite surface of the test piece.

Flaw echo / Defect echo: An echo originating from a discontinuity or defect within the material.

Interference echo： refer collectively to all echoes that hinder the assessment of flaw detection results. This includes false echoes, internal reverberation echoes, as well as echoes caused by instrument noise, external interference, and similar factors.

Delayed echo: Delayed echo:
An echo originating from the same reflector that arrives later due to either:
(a) traveling a different sound path, or
(b) undergoing wave mode conversion during propagation.

Ghost echo: A non-genuine echo (ghost echo) produced in materials with high acoustic transparency due to excessively high repeat frequency of the ultrasonic flaw detector.

Decibel (dB): The unit used to measure signal amplitude ratios (gain, attenuation).

Acoustic impedance: Ratio of sound pressure to particle velocity in a sound wave.

Attenuation: The phenomenon where sound pressure gradually weakens as ultrasonic waves propagate through a medium with increasing distance.

Flaw / Defect / Discontinuity: An imperfection or interruption in the material structure.

Time base line: The horizontal axis on the A-scan display representing time or distance.

Sweep: The horizontal movement of the trace across the display.

Sweep range: The total time or distance represented across the full width of the display.

Sweep velocity: The rate at which the trace moves horizontally (time/distance per division).

Horizontal linearity: The accuracy of the time/distance representation on the horizontal axis.

Vertical linearity (of time base): The degree to which the displayed signal height on the ultrasonic flaw detector's screen (along the time or distance axis) is proportional to the amplitude of the input signal to the receiver.

Dynamic range: The ratio (in dB) between the largest and smallest signals the instrument can display without distortion.

Pulse repetition frequency (PRF): The number of times per second that the probe crystal is excited by pulses from the pulse generator.

Test frequency / Inspection frequency: The ultrasonic frequency used during testing, typically ranging from 0.5 to 15 MHz.

Echo frequency: The actual frequency content of a received ultrasonic echo.

Sensitivity: The instrument's ability to detect small reflectors.

Sensitivity index: The gain setting used as a baseline for evaluation.

Sensitivity margin / Excess gain: In an ultrasonic flaw detection system, the difference between the standardized defect detection sensitivity (expressed at a specific signal level) and the maximum detectable sensitivity.

Resolution: The ability to distinguish between closely spaced reflectors (axial and lateral).

Gate: An electronic window on the A-scan used to monitor a specific depth range.

Attenuator: A control that reduces the amplitude of received signals by a known amount (dB).

Signal-to-noise ratio (SNR): The ratio of the amplitude of a relevant signal to the amplitude of background noise.

Gain: The amplification applied to the received signal (measured in dB).

Distance Amplitude Correction (DAC) curve: A set of curves plotted using three parameters - the distance of known reference reflectors that produce echoes, the gain setting of the flaw detector, and the size of the reflectors - under specified conditions. During actual inspection, the equivalent flaw size can be determined from this curve using the measured defect distance and gain value.

Coupling: The process of transmitting ultrasonic energy from the transducer into the test piece, usually using a couplant (liquid or gel).

Test block: A reference block with known reflectors used for calibrating and evaluating the ultrasonic system.

Standard test block: A block manufactured to a specific standard (e.g., IIW block, DSC block).

Reference block / Calibration block: A block used specifically for calibrating the equipment and establishing sensitivity levels.

Probe / Transducer /: The device that converts electrical energy into ultrasonic energy and vice versa.

Piezoelectric material / Crystal: The element within the probe that generates and receives ultrasound. Materials exhibiting piezoelectric properties, such as quartz, barium titanate, and lead zirconate titanate (PZT).

Backing / Damping block: Material behind the crystal to absorb backward energy and shorten pulse duration.

Straight beam probe / Normal probe: A probe designed to transmit sound waves perpendicular to the test surface.

Angle beam probe / Shear wave probe / T/R probe: A probe designed to transmit sound waves at an angle to the test surface.

Probe index point: The point on the wearing surface of an angle beam probe or surface wave probe through which the central axis of the emitted sound beam passes..

Probe angle / Refraction angle: The angle (relative to normal) at which ultrasound enters the test piece (e.g., 45°, 60°, 70°).

Probe front edge / Near surface resolution distance: The distance from the front face of the probe to the index point.

Understanding the exact meaning represented by these terms is essential for effectively utilizing the instrument for flaw detection.