Page 91 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

must be visible to give the correct sensitivity value for
different material thicknesses and different radiographic
techniques.

3. Geometric unsharpness (Ug), also known as penumbra, is
a measure of the ‘fuzziness’ of the radiographic image.
Geometric unsharpness (Ug) is calculated from the
formula
Ug = Fd/D
where
F = effective source size or focal spot
d = object to film distance
D = source to object distance
Acceptance is based on the relevant code requirement.
ASME V Section 2 gives recommended maximum limits
for Ug ranging from 0.020 in (0.51 mm) for material
thicknesses below 2 in to 0.070 in (1.78 mm) for materials
greater than 4 in.
4. Backscatter is stray radiation that can expose the film. A
lead letter ‘B’ is placed on the back of the film and if
excessive backscatter is present a light image ‘B’ will be
visible on the film, which should then be rejected. A
darker image, on the other hand, is not a cause for
rejection.
Advantages
. Gives a permanent record.
. Surface condition is not so critical.
. Lagging does not require removal.
Disadvantages
. Safety is an issue.
. May require production to be shut down.
. Personnel must be excluded from the vicinity.
. Will not find linear defects in all orientations.

76

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200906QG Welding chap6.3d Page 76 of 87