NDT Controls (UT)

Thickness Measurements and Defect Detection Investigations

Ultrasonic testing is an advanced technique used for thickness measurements and defect detection investigations on a wide range of materials and semi-finished products, including plates, castings, forgings, welds, and composites. This technology relies on the propagation of sound through vibrations in the form of elastic waves, characterized by a specific frequency measured in Hertz (Hz). It is known that ultrasonic frequencies are not audible to the human ear.

Ultrasonic waves are distinguished by their wavelength, defined as the distance between two successive peaks, and their frequency, which represents the number of vibrations per second. The vibrations can generate different types of waves, including longitudinal, transverse, and surface waves.

Like light waves, ultrasound can be reflected, refracted, and focused. Reflection and refraction occur when the ultrasonic beam passes between two materials with different acoustic properties. The reflection of the beam at discontinuities allows for the characterization and location of defects within the material.

The propagation speed of the ultrasonic beam is constant in a given material. For example, in a material like steel, longitudinal waves propagate at a speed of 5900 m/s, while transverse waves travel at 3200 m/s.

This innovative technology is essential for ensuring the quality and integrity of materials used in many industrial sectors.

Generation of Ultrasound: How It Works and Testing Techniques

Ultrasonic testing is a non-destructive inspection technique that uses ultrasound to examine the integrity of materials. In this article, we will explore how ultrasound is generated and the various testing techniques used to identify defects in materials.

Generation of Ultrasound

Ultrasound is generated by a transducer, commonly known as a probe. A piezoelectric transducer has the ability to convert an electrical impulse into a mechanical vibration and vice versa, functioning as both a transmitter and receiver. This process is fundamental for creating the ultrasonic beam necessary for inspections.

Principles of Ultrasonic Testing

Ultrasonic testing relies on some fundamental principles:

• The ultrasonic beam, once introduced into the material, propagates in a straight line at a constant speed until it reaches the nearest reflector.
• Near the reflector, part of the beam is transmitted and part is reflected.
• The reflected portion of the beam provides information about the characteristics of the reflector.
• The “time of flight” of the beam allows for determining the position of the reflector, given the known propagation speed of the beam.

Testing Techniques

Ultrasonic testing is highly versatile and can be applied using multiple techniques. Here are some key aspects:

Pulse-Echo Technique

The pulse-echo technique is one of the most common methods. In this method, a probe emits an ultrasonic signal that is received by the same probe or a second transducer. The amount of reflected energy is plotted as a function of time, providing information about the size and position of the reflector.

Flat and Angle Probes

• Flat Probes: Typically, the ultrasonic beam strikes the surface of the part at a right angle. This is effective for most inspections where defects are perpendicular to the beam.
• Angle Probes: For inspecting welds or defects not perpendicular to the beam, angles less than 90° must be used. The choice of angle depends on the probable orientation of the reflector and the need to overcome obstacles along the path of the beam.

Contact Techniques

In contact techniques, angled ultrasonic beams, often called oblique beams, are generated using special transducers equipped with a plexiglass backing to exploit the phenomenon of refraction. An important consideration is that with angled beams, there is no background echo, thus lacking crucial information about the adequacy of acoustic coupling between the transducer and the material.

Practical Applications of Angle Probes

To effectively inspect a welding joint, the transducer must be moved back and forth while also being transversely translated to cover the entire volume of the weld bead. In some cases, to eliminate disturbances caused by the swelling of the outer surface, it is recommended to grind the outer surface, especially in high-quality joints.

Location of Discontinuities in Ultrasound

The location of discontinuities in materials is a crucial aspect of ultrasonic testing. In this section, we will explore techniques to determine the depth of discontinuities, the use of DAC curves, and the presentation of the results through various scan types.

Location of Discontinuities

To locate the depth of a discontinuity in the weld bead, the horizontal distance scale (time base) can be calibrated directly. However, it is also possible to calculate the depth using simple trigonometric relations.

The operator must position the transducer to optimize the amplitude of the discontinuity echo, having positioned the ultrasonic beam at the first reflection. The distance LLL is then measured directly on the workpiece from the zero point of the transducer to the symmetry axis of the weld bead. This technique allows the operator to measure the distance directly on the workpiece, without needing to take other measurements on the device’s screen, except for observing when the maximum reflection from the discontinuity is reached.

DAC (Distance Amplitude Correction) Curves

Using sample blocks from the distance-amplitude series, it is possible to determine the relationship between the distance traveled by the ultrasonic beam and the amplitude of the echo relative to the bottom of the hole. Even though the number of sample blocks can be large, it is not necessary to have the entire series. The distance-amplitude relationship can be obtained with a limited number of blocks, interpolating the results.

Presentation of Results

The information obtained from ultrasonic inspections can be represented in various forms. The most common are:

• A-Scan: Represents the reflected signal as a function of time. The relative size of the reflector is estimated by comparing it with known reflectors, and the position can be evaluated by observing the echo on the time axis.
• B-Scan: Provides a representation of the cross-section. It is possible to obtain only the depth of the reflector considered in that section. Any discontinuities near the surface can compromise the examination.
• C-Scan: Provides a true mapping of the discontinuity, thanks to an automated immersion scan.