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After the brief review of Radiography in a previous article on Non-Destructive Examination (NDE), we will now turn our attention to the use of ultrasound as an inspection tool for the examination of welds. Ìý

Conventional Ultrasonic Testing

Ultrasonic sound waves are generated by applying electric pulses to crystals such as quartz or barium titanate which exhibit piezoelectric properties. These crystals vibrate when receiving electrical stimulus, and the electrical energy imparted is then converted into mechanical energy. This mechanical energy is transmitted through the object under test which, in our case is welds zones. These piezoelectric crystals are embedded in the inspection tool know as a transducer or probe.

Examination by the use of ultrasound utilizes sound frequencies between 20 kHz to10 MHz (10,000,000 cycles per second). For weld examinationÌý the frequency range is high, generally in the range of 2.5 to 5 MHz and these frequencies will be dictated by the fabrication and inspection standards in use.

Since the use of ultrasound is dependent upon its ability to be reflected from air gaps, a couplant of water, oil, grease or similar is applied between the face of the transducer and the test surface during a particular inspection. This couplant increases the amount of energy transmitted into the test material from the transducer by eliminating the air gap at the interface.Ìý

There are two main ultrasonic beam modes (or wave types) used in the testing of welds; these are:Ìý

• longitudinal waves , sometimes known as normal waves
• shear waves, sometimes known as angle waves

Longitudinal waves are propagated as pressure waves, that is, the particles of the material under test oscillate in the direction travelled by the ultrasonic waves as shown in left hand diagram in Figure 1. ÌýWhen shear waves are propagated through the material, the particle motion is transverse to the wave direction as illustrated in the right-hand diagram in Figure 1. The velocity at which the ultrasonic beam moves through a weld is constant for that specific material and for the wave mode (longitudinal wave or shear wave). For example, in carbon steel a longitudinal wave beam moves at a speed of 5.85 x 10³ m/s and a shear wave at a speed of 3.2 x 10³ ³¾/²õ.Ìý

Figure 1. Particle motion for Longitudinal and Shear Waves

When a change occurs in the material such as a crack, slag inclusion or a change from carbon to stainless steel in an overlay then an acoustic mismatch occurs. At this mismatch, all or some of the ultrasonic sound energy is reflected back to the piezoelectric crystal in the UT transducer.Ìý The transducer then converts the mechanical energy back to electrical energy which is subsequently displayed on the CRT screen. The complete cycle of this action, using longitudinal waves, is shown in Figure 2.Ìý

This phenomenon of reflection due to acoustic mismatch is the basis of all ultrasonic testing.

Figure 2. Cycle of transmit, receive and reflect using a Longitudinal Wave ProbeÌýand Representation of CRT Screen

In weld testing both wave modes are used and Figure 3Ìýillustrates the application of these two different modes.

Figure 3 Application of both Longitudinal and Shear Ultrasonic Waves

Ultrasonic testing is extremely sensitive and requires very precise procedures both for calibrating the test equipment and locating and evaluating discontinuities. Provided that the wavelength is short enough, the method can detect very small discontinuities.

Prior to examining a weld certain information will be required to determine the correct technique and interpretation of signals. This will include:Ìý

• The welding process, this is important as knowledge of the welding method is required when the operator is evaluating the type of defect. For example, slag inclusions would not occur in a weld deposited with the gas metal arc process. A partial joint penetration groove weld would be reported as incomplete penetration if it was inspected as a complete penetration joint. Tungsten inclusions would not be found in a weld made with the metal cored arc welding process.
• The weld joint design: The operator must know the angles of the weld preparation because one of the angleÌýbeams selected must strike the bevel as closely as possible to 90° to detect sidewall discontinuities. For example, if the weld bevel is 30°, one of the probes will be 60° (60° + 30° = 90°) and if the bevel is angle 35°, the probe selection would be 55° (See Figure 4).
• The type of material and condition of heat treatment: The operator must know the sound wave velocity of the material because it will affect the distance and angle calculations. Heat treatment in some materials affects sound wave velocity; this must be compensated for in the equipment calibration.Ìý

Figure 4. Probe Angles to hit Preparation at 90 degrees

Where the weld joint configuration is such that the ultrasonic beam cannot be manipulated so that it will interact with the bevel at 90°, alternate methods to the single transducer technique can be developed

Multi Probe Phased Array

Multi probe Phased Array Ultrasonic Testing (PAUT) is an advanced ultrasonic inspection technique. As the name suggests the technique uses a set of probes made up of numerous small elements. Each of these elements is pulsed individually and, with computer-control, creates phased aspect of the process.Ìý A phased array probe can replace several conventional ultrasonic probes, making complex procedures simpler and removing the need for setting up and calibrating multiple probes. This also allows for real-time imaging as depicted in Figure 5.

Figure 5 Illustration of PAUT and its Display

The advantages of phased arrays over conventional ultrasonic approaches include:

• improved portability,Ìý
• ³¦´Ç²Ô±¹±ð²Ô¾±±ð²Ô³¦±ð,Ìý
• inspection speed.Ìý
• provides a permanent record,Ìý

A phased array is more robust and easier to use than conventional single-element probes, capturing hundreds of signals at once and provides a permanent record.

Lastly, it must be noted that ultrasonic technicians in Canada must be certified under CAN/CGSB-48.9712, ISO 9712. To become certified in Phased Array Ultrasonic Testing (PAUT), you need to first hold a valid Ultrasonic Testing (UT) Level 2 certification and then complete additional training specific to phased array, essentially adding a "phased array" qualification to your existing UT certification. This is governed by the Canadian General Standards Board (CGSB) and administered by a third party, currently NRCan.Ìý

Interested readers can view for a background in ultrasonic testing.

Ìý

Mick J Pates IWE

President PPC and A