Terminologies You Need to Know for Ultrasonic Testing

Non-Destructive Testing (NDT) is a crucial tool for evaluating the integrity of materials and structures without damaging them. Ultrasonic Testing (UT) is one of the most widely used NDT methods that uses high-frequency sound waves to detect flaws or defects in materials. This article covers some essential terminology used in NDT Ultrasonic Testing, including the principles, techniques, and instruments involved

Ultrasonic Waves

Ultrasonic waves are high-frequency sound waves (above 20 kHz) that are used to penetrate through materials to detect internal flaws. They are generated by a transducer that converts electrical energy into mechanical vibrations. The frequency of the ultrasonic wave determines its penetration depth and resolution. The higher the frequency, the shallower the penetration and the higher the resolution.

Transducer

The transducer is a critical component in UT that converts electrical energy into mechanical vibrations and vice versa. It is used to produce and receive ultrasonic waves. The transducer consists of a piezoelectric crystal that vibrates when a voltage is applied to it. The crystal emits ultrasonic waves into the material being inspected, and when the waves encounter a flaw, they are reflected back to the transducer, which detects the returning echo.

Pulse-Echo Technique

The pulse-echo technique is a UT method where a short burst of ultrasonic waves is sent into the material, and the returning echo is received and analyzed to detect flaws. The time between the emission of the pulse and the reception of the echo is measured to determine the distance to the flaw. The pulse-echo technique is the most commonly used UT method and can detect both surface and subsurface flaws.

Attenuation

Attenuation is the loss of energy of the ultrasonic wave as it travels through a material. Attenuation is caused by absorption, scattering, and reflection. Attenuation is an essential factor in UT as it determines the maximum depth of penetration and the sensitivity to small flaws.

Amplitude

Amplitude is the maximum height of the ultrasonic wave, which is used to measure the size and depth of a flaw. The amplitude of the returning echo is compared to the amplitude of the initial pulse to determine the size and depth of the flaw.

Time-of-Flight

The time-of-flight is the time it takes for an ultrasonic wave to travel through a material and reflect back to the transducer. It is used to measure the distance to the flaw. The time-of-flight is affected by the velocity of the ultrasonic wave in the material, which is influenced by the material's density and elastic properties.

Sound Velocity

Sound velocity is the speed at which ultrasonic waves travel through a material. The sound velocity is influenced by the material's density and elastic properties. Accurate measurement of the sound velocity is crucial in UT to determine the correct distance to the flaw.

Gain

Gain is the amplification of the received signal to enhance the detection of small flaws. Gain is used to adjust the sensitivity of the instrument to the material being inspected. The gain is increased for materials with high attenuation and decreased for materials with low attenuation.

A-Scan

The A-scan is a graph of the amplitude of the received signal as a function of time. The A-scan is used to detect the presence and location of flaws. The amplitude and time-of-flight of the returning echo are displayed on the A-scan.

B-Scan

The B-scan is a two-dimensional image of the material's internal structure generated by scanning the transducer over the surface and recording the echoes at different positions. The B-scan is used to visualize the shape, size, and location of flaws within the material. The B-scan is similar to a medical ultrasound image and is commonly used in the inspection of welds and composite materials.

C-Scan

A C-scan is a two-dimensional image of the material's internal structure, generated by scanning the transducer over the surface and recording the echoes at different positions. The C-scan is used to produce a visual map of the material's internal structure and the location of flaws. The C-scan is commonly used in the inspection of large areas or complex shapes.

DAC/TVG

Distance Amplitude Correction (DAC) and Time Variable Gain (TVG) are techniques used to adjust the amplitude of the received signal based on the distance and depth of the flaw. The DAC is used to correct the amplitude of the signal for the distance traveled by the wave. The TVG is used to adjust the gain of the signal based on the depth of the flaw. DAC/TVG is commonly used in the inspection of thick materials.

Refraction

Refraction is the bending of ultrasonic waves when they pass through a material with a different acoustic impedance, causing them to deviate from their original path. Refraction is essential in UT as it allows for the detection of flaws at different angles and depths within the material. Refraction is governed by Snell's law.

Snell's Law

Snell's law is a law in physics that describes the refraction of waves at an interface between two materials with different acoustic impedances. Snell's law is used to determine the angle and direction of the refracted wave and is crucial in UT for detecting flaws at different depths and angles within the material.

Back Wall Echo

The back wall echo is the echo received from the back surface of the material, used as a reference signal to measure the size and depth of a flaw. The back wall echo is also used to determine the sound velocity in the material.

Dead Zone

The dead zone is the region near the transducer where the ultrasonic waves are not detectable due to the initial ringing of the transducer. The dead zone is a crucial factor in UT as it determines the minimum distance from the surface where flaws can be detected.

Near Field

The near field is the region close to the transducer where the ultrasonic waves are not fully developed and have a non-uniform amplitude and frequency distribution. The near field is a challenging region to inspect as the waves are not fully developed and can lead to inaccurate results.

Far Field

The far field is the region far from the transducer where the ultrasonic waves are fully developed and have a uniform amplitude and frequency distribution. The far field is the ideal region for flaw detection as the waves are fully developed and can accurately detect flaws within the material.

Velocity Calibration

Velocity calibration is the process of calibrating the ultrasonic instrument to measure the correct sound velocity in the material being inspected. Accurate velocity calibration is crucial in UT as it determines the correct distance to the flaw.

Thickness Measurement

Thickness measurement is a UT technique used to measure the thickness of a material by sending ultrasonic waves through it and measuring the time-of-flight of the echo from the back surface. Thickness measurement is commonly used in the inspection of pipes, tanks, and other structures to ensure they meet design specifications.

Ultrasonic Testing is a powerful NDT method used to evaluate the integrity of materials and structures without damaging them. The terminology covered in this article is essential for understanding the principles, techniques, and instruments involved in UT. By understanding these terms, inspectors can effectively use UT to detect flaws and ensure the safety and reliability of critical structures and equipment.