Basic Knowledge about Ultrasonic Testing

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

Metals

Metals	Sound Velocity (longitudinal wave) c_l [m/s]	Sound Velocity (transverse wave) c_l [m/s]	Density [10³ kg/m³]	Acoustic Impedance [10⁶ kg/m²s]
Aluminium	6200 – 6360	3100 – 3130	2.7	16.7 – 17.2
Beryllium	12720 – 12890	8330 – 8880	1.82 – 1.87	23.2 – 24.1
Brass (Ms58)	3830 – 4250	2050 – 2200	8.1	31.0 – 34.4
Cadmium	2665 – 3300	1500 – 1810	8.6 – 8.7	22.9 – 28.5
Carbide	6800 – 7300	4000 – 4700	11.0 – 15.0	74.8 – 109.5
Cast Iron	3500 – 5800	2200 – 3200	6.9 –7.3	24.2 – 42.3
Constantan	5240	2640	8.8	46.1
Copper	3666 – 4760	2260 – 2320	8.9	32.6 – 42.4
Gold	3240	1200	19.7	63.8
Grey Cast Iron	3500 – 5600	2200 – 3200	7.2	25.2 – 40.3
Iron	5950	3220 – 3240	7.9	47.0
Lead	2050 – 2400	700 – 710	11.3 – 11.7	23.2 – 28.1
Magnesium	4602 – 5900	3050 – 3280	1.70 – 1.75	7.8 – 10.3
Molybdenum	6250 – 6650	3350 – 3510	10.1 – 10.2	63.1 – 67.8
Nickel	4973 – 6040	2960 – 3219	8.8 – 8.9	43.8 – 53.8
Platinum	3960 – 4080	1670 – 1730	21.4	84.7 – 87.3
Silver	3600 – 3790	1590 – 1690	10.4	37.4 – 39.4
Tin	3210 – 3320	1530 – 1670	7.3	23.4 – 24.2
Titanium	5823 – 6260	2920 – 3215	4.5	26.2 – 28.2
Zinc	3890 – 4210	2290 – 2440	7.1	27.6 – 29.9

Non-Metals

Non-metals	Sound Velocity (longitudinal wave) c_l [m/s]	Sound Velocity (transverse wave) c_l [m/s]	Density [10³ kg/m³]	Acoustic Impedance [10⁶ kg/m²s]
Aluminium Oxide	9000 – 11000	5500 – 6500	3.6 – 3.95	32.4 – 43.5
Epoxy resin	2400 – 2900	1100	1.1 – 1.25	2.64 – 3.63
Glass (Quartz glass)	5570 – 5930	3415 – 3750	2.6	14.5 – 15.4
Glass (Window glass)	5770	3430	2.51	14.5
Graphite, pressed	1600 – 2500	1200 – 1500	1.7 – 2.3	2.72 – 5.8
Ice (H₂O at -4 °C)	3232 – 3980	1990	0.9	2.9 – 3.6
PA (Polyamide)	1800 – 2600	1100 – 1200	1.1 – 1.2	1.98 – 3.12
PE (Polyethylene)	1950 – 2000	540	0.9	1.76 – 1.80
PMMA (Plexiglass^®)	2670 – 2760	1120 – 1430	1.18	3.2 – 3.3
Polystyrene	2337 – 2350	1020 – 1150	1.05 – 1.06	2.45 – 2.49
Porcelain	5600 – 6200	3500 – 3700	2.4 – 2.5	13.4 – 15.5
PTFE (Teflon^®)	1350	550	2.2	2.97
PVDF (Polyvinylidene-fluoride)	2200	775	1.78	3.9
PVC (Polyvinyl chloride)	2180 – 2260	948	1.38 – 1.40	3.0 – 3.2
Quartz Crystal	5760	3840	2.65	15.2
Rubber, hard	1570 – 2300	–	1.2	1.88 – 2.76
Rubber, soft	1480 – 1550	–	0.90 – 0.95	1.33 – 1.47

Steel

Steel	Sound Velocity (longitudinal wave) c_l [m/s]	Sound Velocity (transverse wave) c_l [m/s]	Density [10³ kg/m³]	Acoustic Impedance [10⁶ kg/m²s]
Mild steel, unalloyed up to 0.2 % C, e. g. St 52-3 acc. to. DIN 54120
–annealed –annealed to 0.5 % C	5890 – 5950 5940 – 5960	3240 – 3270 3230 – 3245	7.85 7.8 – 7.85	46.2 – 46.7 46.3 – 46.8
Mild steel, alloyed (0,35 % C, 0,6 % Mn, 1 % Cr, 0,2 % Mo)
–annealed –remunerated –hardened	5950 5930 5900	3260 3240 3230	7.84 7.84 7.84	46.6 46.5 46.3
Mild steel, alloyed (0,3 % C, 0,4 % Mn, 2 % Cr, 2 % Ni, 0,2 % Mo
–annealed –remunerated –hardened	5930 5870 – 5880 5890	3220 3210 3210	7.85 7.85 7.85	46.6 46.1 – 46.2 46.2
Ball bearing steel (1 % C, 1,5 % Cr)	5990	3270	7.8	46.7
Stainless steel, austenitic
–(X 10 Cr Ni 18 8) annealed’ –(X 10 Cr Ni Nb 18 9) –(X 12 Cr Ni 18 8)	5530 5790 5660	2983 3100 3120	7.-9 7.8 – 7.9 7.8	43.7 45.2 – 45.7 44.1
Stainless steel, ferritic (0,15 % C, 17 % Cr)
–annealed	6010	3360	7.7 – 7.9	46.3 – 47.5
High Speed Steel (0,9 % C, 4 % Cr, 2,5 % Mo, 2,5 % V, 3 % W)
–annealed –hardened	6060 5880	3850 3190	–	–
Tool Steel (1 % C)
–annealed –hardened	5940 – 5960 5854	3220 – 3245 3150	7.8 – 7.84 7.84	46.3 – 46.7 45.9
Tool Steel (2 % C, 12 % Cr, 0,6 % W)
–annealed –hardened	6140 6010	3310 3220	7,75 – 7,8 7.75	47.6 – 47.9 46.6

Further Information

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Further, comprehensive information on ultrasonic testing can be found in our knowledge floater video “Ultrasonic Material Testing”.