KARL DEUTSCH

Background

Non-visible and mostly hidden defects in almost all components can be detected non-destructively by means of ultrasound. Ultrasonic testing is particularly important for safety-relevant components. In addition, mere measurements are possible as well; the best known of these is the wall thickness measurement.

The main applications are:

• On-site inspection of welded joints
• Defect inspection and quality control of castings
• Automatic testing of mass products with simple geometries such as semi-finished products (round material, billets, profiles, sheets and tubes) made of steel, non-ferrous metals and plastics
• Wall thickness measurements on pipelines, vessels and chemical plants. Wall thickness measurement by means of ultrasound is useful wherever the measuring point is only accessible from one side and a caliper gauge cannot be used.

Advantages of Ultrasonic Testing

• Detection of surface and internal defects (hidden from the surface)
• All materials with good sound conductivity can be tested (up to 10 m if necessary)
• The process can be automated
• No special radiation protection regulations need to be followed
• Reliable detection of planar flaws (laminations, cracks, lack of fusion, …)

Principle

The definition of ultrasound covers sound components with a frequency above the human hearing threshold, i.e. more than 20,000 Hertz (= 20 kHz). The main frequency range for ultrasonic testing is 0.5 MHz to 10 MHz, which is well above the hearing threshold. For special applications the examination frequency can also take values above 10 MHz or below 0.5 MHz.

Testing in Detail

When an ultrasonic wave hits an interface (between medium 1 and medium 2) one part is reflected and another part is transmitted. Their ratio depends on the differences between the two adjacent media (e.g. with respect to sound velocity and density). At the transition point from steel to air, the difference is very large and results in a reflected ultrasonic wave of almost 100 % .

Defects in a component usually are made up of air inclusions (blowholes, pores, cracks, …). Therefore, the ultrasound wave is well reflected and, under favourable conditions, returns to the probe. To ensure that the air gap between the probe and the component does not interfere, a usually liquid coupling medium (water, oil, gel, etc.) is used. For automated testing, the entire component including probes often is immersed in water.

The sound velocity is a material constant and amounts to 330 m/s in air (at 0 °C; 344 m/s at 20 °C) and 5920 m/s in steel. If the sound velocity of the material to be tested is known, the depth position of the defect can be determined quite precisely from the transit time of the ultrasound. If the transit time to the opposite back wall is evaluated, the ultrasonic method can also be used for wall thickness measurement. Here a resolution down to the micrometer range can be achieved. The wall thickness gauges can be simplified and scaled down so that only the wall thickness value is displayed.

Less favorable is the determination of the defect size. Reliable evaluation methods unfortunately do not exist. Therefore, the amplitude of the reflected ultrasonic signal is usually compared with the reflected amplitudes of reference defects (circular disk-shaped reflectors, cylindrical reflectors, …). However, the prerequisite is always that the defect (and also the reference defect) is hit favorably by the sound.

The dimensions of those defects still detectable are in the ultrasonic wavelength range. Under favorable conditions, this range can start at a few tenths of a millimeter. In less favorable cases, defects can only be detected from millimeter sizes upwards.

Angle probes are used not only, but mainly, for weld seam inspection because coupling on the usually uneven welding bead with vertical probes does not allow reliable testing. The so-called angle of incidence depends on the defects to be detected (here, as well, the defects need to be insonificated from a advantageous direction).

Acoustic Material Properties

Information on the acoustic material properties of various materials, such as longitudinal and transverse sound velocity, density and acoustic impedance can be found in the following tables.

The values for sound velocity, density and acoustic impedance listed in the tables are valid for room temperature (20 °C to 23 °C). Variations due to material composition, crystal orientation, porosity and temperature are possible.

Source: “Ultraschallprüfung (translates to ‘Ultrasonic Testing’)” (Deutsch, Platte, Vogt), Springer Verlag 1997

The process of spot weld testing

Polished section of a spot weld lens

Electric resistance spot welds are used in automotive engineering to join bodywork parts. Among others, the welding spots are checked by means of ultrasound during production.

Two or more sheets are pressed together with two copper electrodes and and subjected to an electric current in the kA range for about 200 ms. The electrical resistance creates a melt which, after cooling, forms a welding spot in the form of a lens (nugget).

Common types of flaw:

• No lens (lack of fusion)
  
• Lens diameter too small
• Stick joint (adhesive bond), if only the zinc coating has been melted

The appropriate probe

The test is executed in pulse-echo technique with broadband 15 or 20 MHz probes. A rubber membrane and a water delay in the probe housing enable a flexible sound coupling to the pressed-in weld spot surfaces.

Different sheet thicknesses cause different lens diameters. Thus, in each case a probe suitable for the application is used, whose transducer diameter DS corresponds to the target diameter DL of the lens. The examination probe is guided manually.

Probe for the inspection of weld spots

The test image

A-scan of a good weld spot

The evaluation of the weld spot quality is performed automatically. A good spot weld with DL ≥ DS results in an echo sequence from the back wall of the last sheet of the spot weld.

The test example in the picture shows a weld spot of two sheets with a wall thickness of 1 mm each. Here, the nominal diameter of the lens is 4 mm.

The echo sequence is made up of clear back wall echoes. The measured sound attenuation of 2.9 dB/mm indicates the coarse-grained structure of the lens. During the melting phase, the welding spot was pressed in, which can be seen from the residual wall thickness of 1.87 mm.

Based on these criteria, the quality of the weld spot is rated “good”.

With faulty welding spots, the echo sequence shows significant deviations from that of a “good” spot. Lenses that are too small show intermediate echoes from the joining plane, glued joints produce a long sequence of backwall echoes because the sound attenuation is lower due to the lack of fusion. If there is no joint at all, there is a long sequence of echoes from the top sheet.

Other types of defect (pores, burnt spots, etc.) lead to further deviating echo sequences, for which the evaluation criteria can be adapted individually.