Page 88 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Non-destructive and Destructive Testing

Disadvantages
. Surfaces must be smooth and free of all spatter and arc
strikes.
. Very dependent on skill of the operator.
. Normally limited to materials above 6 mm thick.
. Basic technique does not give a permanent record.
. Not so good on coarse grained materials.
Radiographic testing (RT)
Radiographic testing is used to detect internal defects in
welds. Although it can find planar (two-dimensional) defects
such as cracks or lack of fusion it will not find them in all
orientations. It will, however, more easily find volumetric
defects such as porosity or slag inclusions or shape defects
such as undercut or excess root penetration. It can also be
used for profile surveys of pipework and components to
check for loss of wall thickness caused by corrosion and/or
erosion. Figure 6.4 shows the basic technique.
Gamma rays (from a radioactive isotope) or X-rays (from
a machine) are passed through the material and strike a film
causing it to darken. The film gets darker the more radiation
that hits it so volumetric defects such as porosity that allow
more radiation through the material will show as areas
darker than the surrounding area. Conversely, areas such as
excess penetration, where more radiation is absorbed by the
material, will show as lighter than the surrounding area.
Gamma radiography is commonly used on site because it is
portable and does not require a power source. It is inherently
dangerous, though, because the radioactive isotope (source)
cannot be ‘turned off’ and is always radiating. Storage,
transportation and use of the source must therefore be
closely controlled to ensure the safety of the radiographers
and workplace personnel.
The type of isotope used will depend on the thickness of
material to be tested. The most common gamma isotope is
iridium 192 but thinner materials may use other isotopes

73

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200906QG Welding chap6.3d Page 73 of 87