In this non-destructive testing method, the high frequency (0.1-20 MHZ) produced by the inspection probe is propagated in the test material and reflected back into the probe after it hits a discontinuity and thus it is detected by the probe in order to detect discontinuities in the material desired to be examined. The structure of a typical ultrasonic probe is shown in Figure 1. Waves perceived by the probe (with piezoelectric effect) are converted into electrical signals and are displayed as echoes (echoes), which are the precursors of the internal structure of the cathode rays tube screen. The locations and amplitudes of the echoes observed on the screen give information about the location and dimensions of the discontinuity (Figures 2 and 3).

Figure 1. Structure of a typical ultrasonic probe

Figure 2. Views of modern ultrasonic examination devices

Figure 3. Working principle of ultrasonic inspection method

It can be used to detect expected volumetric errors and crack type surface errors in metallic or non-metallic materials. Discontinuities are best perceived as they are perpendicular to the ultrasonic beam, and ultrasonic method is difficult to apply for coarse-grained structures, especially for austenitic materials. The high frequency sound waves sent into the material are reflected in the event of an impact on the sound path. Depending on the angle of impact, the reflected signal may or may not come to the receiving probe (Figure 4). The reflected signal reaching the receiving probe creates an echo indication on the screen of the ultrasonic inspection device. The coordinates of the reflector within the inspection part can be calculated according to the position of the echo. In addition, the height of the echo gives an idea of the size of the reflector. It may also be possible to make a comment about the type of reflector by looking at the shape of the echo signal.

Figure 4. Reflection patterns according to the probe position on the examined part.

Accurate evaluation becomes difficult if the sound velocity and sound attenuation characteristics of the inspection part show strong regional changes. In materials where sound attenuation is too great due to coarse grain structure or absorption, inspection may sometimes be impossible. A sufficiently large surface should be prepared for inspection. Surface condition directly affects inspection parameters. Inspection of thin parts is relatively difficult. It is not possible to detect planar discontinuities positioned parallel to the axis of the sound beam. Generally, reference standard blocks are needed. These blocks are collectively seen in Figure 5. High frequency sound waves are produced by a piezoelectric crystal in a piece called the probe. The frequency range used in the ultrasonic examination of metallic materials can be between 500 kHz and 10 MHz. The appropriate frequency is determined according to the microstructure characteristics of the inspection part. When the probe is in contact with the inspection surface, a suitable contact fluid (oil, grease, water, etc.) must be used in order for the sound waves to penetrate into the material (senses cannot spread in the space). By analyzing the probe on the inspection surface, the positions and heights of the echoes arising from the part geometry are evaluated and error analysis is performed. The most commonly used wave types for ultrasonic inspection are longitudinal (pressure) and transverse (shear) waves. When working with probes with zero degrees of input angle, called normal probes, the waves that travel through the material are longitudinal waves. Angled probes send transverse waves into the material, usually at 45 °, 60 ° and 70 ° (these values ??are for steel material).

Figure 5. Various calibration blocks used in ultrasonic inspection

Three basic ultrasonic techniques are widely used:

1. Pulse-echo and transmission method

• In the pulse-echo test, a probe sends an energy pulse and receives the reflected energy, also known as the same or a second probe echo. Impact reverberation is especially effective when only one side of the material is accessible.
• The transmission method is carried out using two transducers on opposite sides of the sample. One acts as a donor and the other acts as a receiver. The transmission method is useful for detecting discontinuities when the reflectors are not good and the signal strength is weak.
  • Normal / Angled Beam - The normal beam test uses a beam applied at a 90 degree angle to the surface, while the angled beam uses a beam applied to the sample at a different angle from 90 degrees. The choice between the two is made depending on:
• Harmonization of the interest feature so that the sound can produce the largest reflection from the feature
• Obstacles to avoid on the sample surface
  • Contact and Immersion - In order to obtain useful levels of sound energy in the material, the air between the probe and the sample must be removed. This is called the coupling. Two types of couplings are used:
• In the contact test, a contact substance such as water, oil or gel is applied between the probe and the sample.
• In the immersion test, the sample and probe are placed in a water bath. This allows the probe to move better while providing continuous coupling.

Some of the most common Ultrasonic applications:

• Error detection (cracks, slags, pores, layering, etc.)
• Erosion / Corrosion thickness measurement
• Evaluation of adhesion integrity
• Grain size estimation in metals
• Gap content estimation in composites and plastics

Information from the ultrasonic examination can be presented in a number of formats:

• A-Scan shows the amount of ultrasonic energy received as a function of time
• B-Scan shows profile view (cross section) of a sample
• C-Scan shows a top view of samples and discontinuities
• Hybrid / Stitch shows a top view of C-Scan with A and / or B Scan views with C-Scan views woven together to show a clearer picture of damaged areas of a sample. Said stitch appearances are used for larger samples and surface areas.

Some of the main advantages of ultrasonic inspection:

• Detects surface and subsurface defects
• Penetration depth is superior to other test methods
• Only allows one-sided access with the pulse-echo technique
• High accuracy regarding estimation of discontinuity size and shape
• Minimum sample preparation is required
• Instant results obtained using electronic equipment
• Detailed images can be obtained with automatic systems.