Ultrasonic Inspection | Aircraft Ultrasonic Inspection (NDT)

Ultrasonic Inspection

Ultrasonic Inspection

Ultrasonic inspection is an Non Destructive Inspection (NDI) technique that uses sound energy moving through the test specimen to detect flaws. The cathode ray tube (CRT), liquid crystal display (LCD), computer data programme, or video/camera medium displays the sound energy that is travelling through the specimen. Vertical signals on the CRT screen or nodes of data in the computer test software serve as indicators of the front and rear surface as well as internal and exterior circumstances. The three different display patterns are "A" scan, "B" scan, and "C" scan. Every scan offers a distinct image or perspective of the test specimen.

Locating inspection is made feasible by ultrasonic detecting equipment. All sorts of material faults must be accessible to an ultrasonic test device. When utilised with either straight line or angle beam testing procedures, ultrasonic can be used to find minute fractures, checks, and voids that are only visible on one surface of the material being tested and are too small to be seen by x-ray.

Ultrasonic inspection is done using two fundamental techniques. Immersion testing is the earliest of these techniques. This kind of inspection involves totally submerging the component being examined and the search unit in a liquid couplant, such as water or other appropriate fluids.

Contact testing is the name of the second technique. It is the technique covered in this chapter and is easily adaptable to field use. In this procedure, the search unit and the component being examined are joined with a viscous substance, liquid, or paste that wets the search unit's face as well as the part being examined. Pulse echo, through transmission, and resonance are the three fundamental types of ultrasonic inspection techniques.

Pulse Echo

By monitoring the amplitude of signals reflected and the amount of time needed for these signals to travel between particular surfaces and the discontinuities, flaws can be found.

Each transmission pulse simultaneously triggers the time base, which causes a spot to move across the CRT or LCD screen. If high-speed automated scanning is needed, the spot can sweep around the scope's face 50 to 5,000 times per second or more. The image on the oscilloscope appears to be stationary because of how quickly the send and receive cycles go back and forth.

After the sweep starts, the pulser is electrically excited by the rate generator for a brief period of time before emitting an electrical pulse. This pulse is transformed into a brief sequence of ultrasonic sound waves by the transducer. The ultrasound is reflected back to the transducer when it reaches the internal fault and the opposite surface of the specimen if the interfaces of the transducer and the specimen are positioned correctly. The amount of time between the initial impulse's transmission and the specimen's internal signals being received is

by the timing circuits, which measure. The reflected pulse that the transducer picked up is amplified, sent to the instrument, and shown on the screen. The defect and the pulse are exhibited with respect to the front and back surfaces of the specimen in the same relationship.

By using the angle beam testing technique, pulse-echo instruments can also be utilised to find faults that are not directly underneath the probe. Only the way the ultrasonic waves pass through the material being evaluated makes angle beam testing different from straight beam testing. According to the illustration in, a crystal with an angled cut that is mounted in plastic projects the beam into the material at an acute angle to the surface. The beam, or a portion of it, successively reflects from the material's surfaces or any other discontinuity, including the piece's edge. In contrast to angle beam testing, where this distance between the searching unit and the piece's opposite edge represents the width of the material, straight beam testing uses the horizontal distance on the screen between the initial pulse and the first back reflection to indicate the thickness of the test object.

Through Transmission

Two transducers are used in transmission inspection; one is used to generate the pulse and the other is used to receive it. A fault is indicated by a break in the sound stream, which is shown on the instrument screen. Compared to the pulse-echo approach, via transmission is less susceptible to minor imperfections.


The pulse approach is not applicable to this system since the transmission frequency can be continuously changed. When the two sides of the material being examined are smooth and parallel and the backside is unavailable, the resonance method is typically employed to estimate thickness. The thickness is determined by the frequency at which the resonance point of the material under test is met. Accurate knowledge of the ultrasonic wave frequency corresponding to a specific dial setting is essential. To check for potential frequency drift, standard test blocks are used.

The reflected wave returns to the transducer in the same phase as the original transmission if the fundamental frequency of an ultrasonic wave is such that its wavelength is twice the thickness of a specimen. This causes the signal to be amplified. This is represented on the indicator screen as a high amplitude value and arises from constructive interference or a resonance. The reflected signal returns entirely out of phase with the transmitted signal and cancels when the frequency is raised until three times the wavelength equals four times the thickness. A further increase in frequency results in the wavelength once more equaling the thickness, a reflected signal that is in phase with the transmitted signal, and a new resonance. The sequential cancellations and resonances can be noticed, and the readings utilised to verify the fundamental frequency reading, by beginning at the fundamental frequency and gradually raising the frequency.

Some instruments have a motor-driven capacitor in the oscillator circuit that adjusts the oscillator's frequency. Other instruments use electronic methods to adjust the frequency. The frequency change coincides with a CRT's horizontal sweep. A frequency range is shown by the horizontal axis. If resonances exist in the frequency range, the circuitry is set up to display them vertically. After that, the tube is placed in front of calibrated transparent scales, which allow the thickness to be measured immediately. The instruments typically work in four or five bands between 0.25 millicycle (mc) and 10 mc.

The thickness of several metals, including steel, cast iron, brass, nickel, copper, silver, lead, aluminium, and magnesium, can be measured using the resonance thickness instrument. Additionally, it is possible to identify and assess regions of corrosion or wear on tanks, tubing, aeroplane wing skins, and other structures or goods. There are direct reading dial-operated systems that can measure thickness with an accuracy of better than 1% between 0.025 inch and 3 inch thickness. An experienced operator who is familiar with the equipment being used as well as the inspection technique to be employed for the various parts being checked is needed for ultrasonic inspection.

Ultrasonic Instruments

A field inspection of the structure of an aeroplane is conducted using a mobile, battery-operated ultrasonic equipment. The device emits an ultrasonic pulse, picks up the echo that returns, amplifies it, and displays the amplified signal on a CRT or similar display. The most typical wave types for aircraft structural assessment are longitudinal or shear waves, which are produced by piezoelectric transducers.

Reference Standards

The ultrasonic instrument is calibrated using reference standards. Reference standards are used to establish the appropriate inspection sensitivity as well as to offer an ultrasonic response pattern relating to the part being inspected. The reference standard configuration must be identical to the test structure's configuration or it must produce an ultrasonic response pattern that is representative of the test structure in order to get a representative response pattern. A simulated flaw (notch) that is placed in the reference standard to give a calibration signal corresponding to the anticipated defect is present. The size of the notch is selected to determine inspection sensitivity (response to the expected defect size). The required reference standard is described in detail in the inspection procedure.


Ultrasonic inspection is only feasible on parts that are in direct contact with the transducer. Due to the fact that ultrasonic radiation cannot pass through air, a layer of couplant must be used to link the transducer to the test object. Water, glycerin, motor oils, and grease are some examples of commonly used couplants.

Inspection of Bonded Structures 

In aircraft bond construction and maintenance, ultrasonic examination is becoming more and more prevalent. In aircraft, bonded structures come in a wide variety of forms and types. The use of ultrasonic inspections is made more challenging by all of these variances. It's possible that an inspection technique that works well on one part or one area of a component won't work on other parts or portions of the same part. The following are some of the factors that affect the different bonded structure types:

  • Various materials and thicknesses are used to make the top skin material.
  • Adhesives of various sorts and strengths are utilised in bonded structures.
  • A bonded structure's top only or top and bottom skin may be accessible.
  • Underlying structures differ in their core material, cell size, thickness, height, back skin material and thickness, doublers (material and thickness), closure member attachments, foam adhesive, steps in skins, internal ribs, and laminates.

Types of Defects

To reflect the various regions of bonded and laminate constructions, defects can be divided into five broad types as follows:

  • Disbonds or cavities in the outer skin-to-adhesive interface are Type I defects.
  • Type II—disbonds or voids at the interface of the adhesive and the core.
  • Type III: Holes in a laminate's layers.
  • Type IV—Variations between the adhesive and a closure member at joints connecting the core to the closure member, such as cavities in the foam adhesive.
  • Water in the core is Type V.

Acoustic Emission Inspection

An aircraft structure is loaded or stressed while acoustic emission sensors are placed at various locations throughout it as part of an NDI technique. The materials produce ultrasonic pulses of sound and stress waves. The strained airframe structure emits sound waves that are detected by the sensors in areas of corrosion and cracks. These acoustic emission bursts can be used to identify defects and assess how quickly they spread in response to applied stress. In comparison to other NDI techniques, acoustic emission testing has the advantage of being able to identify and find every activated fault in a building in a single test. Application of acoustic emission testing to aircraft has necessitated a new level of sophistication in testing technique and data interpretation due to the complexity of aircraft structures.

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