Page 101 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Non-destructive and Destructive Testing

removed during subsequent preparation. It is common for
macro samples to be varnished and mounted in plastic to
preserve them and avoid direct handling of the prepared
surface.
Micro samples
A micro sample is prepared to permit examination of the
grain structure under much higher magnifications than that
used for macros. Magnification is normally in excess of 100
and can be up to 2000+ using electron microscopes. The
sample is prepared in a similar manner to a macro but the
surface is finished with diamond paste to give a much more
polished finish of around 1–3 μm before etching with a nital
solution (see macro preparation). Micro samples are
normally used for research purposes or investigations into
the cause of defects or failures.
Fillet fracture test
Fillet fracture tests are used for welder approval testing using
fillet welded test coupons. A typical test coupon will range
from 150 mm (6 in) to 300 mm (12 in) long and will ideally
contain a stop/start in the centre. The end 25 mm (1 in) will
be removed either side and discarded (although it is a good
idea to make one or both cut ends into macros). The central
vertical section will be loaded in the direction shown in Fig.
6.13 until the specimen either fractures or bends flat. A
common method of applying loading is to strike it sharply
with a hammer in the loading direction. Fracture can be
assisted by cutting a groove 2 mm deep along the centre of
the weld to act as a fracture initiation point. The fracture
surfaces can now be inspected for any internal defects such as
porosity, slag inclusions or lack of fusion defects. The root
can also be inspected to ensure root penetration along the full
length has been achieved.
Butt weld fracture test (nick break)
Another welder approval test is the nick break test (Fig.
6.14), which is similar to the fillet fracture test but is used to

85

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200906QG Welding chap6.3d Page 85 of 87