Page 225 - Welding of Aluminium and its Alloys

Weld defects and quality control 205

static or dynamic loading, temperature and corrosive conditions and the
consequences of failure.

11.3 Non-destructive testing methods
NDE may be used to reveal defects that would be difficult or impossible to
detect by visual examination. The techniques are used during manufacture
as a quality control tool to determine the quality of the work. The extent
of NDE depends upon the application and the criticality of the joint and is
generally specified in the relevant application standards or contract speci-
fication. It is important for NDE to be included in the planning of the
fabrication process as it can require substantial time and resources. Full
account of this must be taken if disruption of production and delays to the
programme are to be avoided.
The requirement to perform NDE must also be taken into account during
the design phase. As with welding, access for NDE must be planned into
the component. The implication of this is that both welding engineers and
designers must be conversant with the techniques and their limitations if
the processes are to be used effectively.

11.3.1 Penetrant examination

This is a technique that is capable of detecting surface breaking defects only.
It relies upon a coloured or fluorescent dye, sprayed upon the surface,
penetrating these defects. After cleaning the excess from the surface, the
dye within the defect is drawn to the surface by spraying on a developer in
the case of the colour contrast dye or by exposing the surface to ultra-violet
light. The defect is revealed by the dye staining the developer or by
fluorescing (Fig. 11.9).
Figure 11.10 is a photograph of a typical penetrant examination indica-
tion. The fluorescent dye gives greater sensitivity than the colour contrast
dye and does not require the use of a colour contrast developer but does
require the use of an ultra-violet light and preferably a darkened room.The
cleaners, penetrant dyes and developers can all be obtained in aerosol cans,
making the process extremely portable and ideal for site use.
The dye used as a penetrant must be capable of penetrating narrow
cracks but must not be removed from more open defects during the clean-
ing operation carried out before the application of the developer. The dye
must have a high contrast with the developer. It is important that the test
piece is thoroughly pre-cleaned – any dirt, oil or water in the crack may
prevent the penetrant from entering. Degreasing should be carried out by
swabbing or immersing the item in one of the proprietary cleaners, acetone
or methanol. Immersion in an ultrasonic cleaning bath is probably the best