Non-destructive tests of weld commonly used in industries are summarised in Table 10.1. They include Visual examination, Dye-penetrant inspection, Magnetic-particle inspection. Radiography and ultrasonics. The last three tests are more common and will be described in the following paragraphs.

10.3.1 Magnetic Particle Inspection

Magnetic particle inspection, as the name implies, requires the use of a magnetic field. The work to be checked must be able to accept magnetism. This process is therefore limited to magnetic metals. It is also limited to surface or near-surface faults. Steel castings, forgings,

and sections that have been welded are the most common parts to be inspected by the mag­

netic particle process. There are several variations of this process.

Longitudinal Magnetization

By using a coil it is possible to include a magnetic field in a part that has the lines of

force running through the length of the shaft as seen in Fig. 10.20.

Fig. 10.20 Longitudinal magnetic inspection

10.3.2 Radiographic Inspection

Radiography uses X-rays or gamma rays, which have the ability to penetrate materials that absorb or reflect ordinary light. X-rays are created under controlled conditions by bombarding a specific area with a flow of electrons. Gamma rays are produced by radioactive isotopes. These isotopes never stop giving off radiation; therefore, they must be stored in special shielded containers.

The ability of a material to absorb radiation is dependent upon its density and the wavelength of radiation being used. Lead absorbs more radiation than iron and iron absorbs more than aluminium. This absorption of radiation also varies with the thickness of a piece of material. A thinner piece of material will absorb less radiation as the rays pass through the object; therefore, more radiation will escape through the object. A film placed behind the object to be inspected will be affected more in thin sections than thick sections. Defects in the part being examined will allow more radiation to pass through it and the defect will then be visible on the film.

A radiograph is the recorded image produced on a photographic plate by X-ray. A sim­plified version of the process is shown in Fig. 10.22. The flaw in the specimen will not absorb as much radiation as does the rest of the part. Therefore, a darker image is present on the film where the flaw exists.

One of the most important facts to remember when working in the area where X-ray or gamma ray equipment is being used is that this process is very dangerous. If excessive radia­tion is absorbed by the body, sickness and even death can be the result.

Fig. 10.23 shows a simplified version of an X-ray tube. X-ray tubes used in industry consist of two electrodes located in a vacuumed glass tube.

The X-ray inspection process has become a very common method of inspection in industry today. Aircraft inspection of major sections of the aircraft are successfully accomplished by X - ray. The pipeline industry is very dependent upon the X-ray process to ensure that each weld on the pipe is sound.

The pipeline industry uses X-ray units that will swing completely around the circumference of a weldment on the pipe. On completion of the travel around the pipe, complete picture of that entire weld is presented on the radiogram (X-ray film). The films are maintained as a permanent record of the inspection. They are numbered to identify each weld on an entire pipeline and may be referred to at a later date if a breakdown of the pipe occurs.

10.3.3 Ultrasonic Inspection

Ultrasonic Inspection makes use of the science of acoustics in frequencies above the upper audible limit of approximately 15,000 cycles per second.

The basic operation of ultrasonic inspection is the conversion of pulsating electronic waves into ultrasonic sound. These sound waves are introduced into the material to be tested through a quartz crystal. The crystal is set into a special search unit that not only sends out the sound but also acts as a receiver to accept reflections of that sound on its return. If the signal sent out runs into a defect in the material, a return signal comes back to the receiver in less time than it would have had it travelled the full distance to the other side of the part and

back.

A cathode ray tube (CRT) is incorporated in the ultrasonic equipment to provide a visual indication on the screen of the initial signal and reflected signals. Fig 10.24 shows a diagram of the CRT screen with pips of the initial pulse, discontinuity, and back surface reflection. Fig. 10.25 shows the basic cathode ray tube construction.

the points in the material they represent. For example, a pip representing a defect close to the back surface reflection indicates a defect that is close to the far edge of the part being inspected.

As with all electronic non-destructive testing methods, a considerable amount of skill is required to operate the ultrasonic inspection unit. As is the case with many skilled tasks, technique, practice, and experience determine the efficiency with which the inspection is completed. This inspection method is becoming more useful in the welding industry as new techniques for scanning welds are being perfected.