Page 109 - Failure Analysis Case Studies II

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29400 x 1.35
bH = = 3.4 MN m-2
0.01 17
Knowing the mean failure stress of weld sample under the controlled conditions of a simple
mechanical test (Table l), it was now possible to determine the stress concentration factor, K,,
which was present at the side of the critical pinhole. Thus,
failure stress under test conditions of weld
Kt = (3)
actual stress at origin
Hence,

K 20.4 -6
t- 3.4
It is possible to test this conclusion in a very simple way by evaluating the stress concentration
factor for the specific dimensions measured on the pinhole directly. The cross-section of the
pinhole was ellipsoidal in shape with axes roughly 0.6 x 0.3 mm. Using Inglis’s evaluation of this
configuration, as given by Peterson [63, then the parameter b/a = 0.6/0.3 = 2. Interpolation on
Peterson’s figure gives a value of
Kt = 5

The agreement between theory and practice is good, bearing in mind the errors of measurement
from the fracture surface as well as the errors associated with experimental tensile analysis,
sampling error, and so on.
The above analysis is the simplest possible for this situation, and there are some known deviations
from the simple assumptions underlying the various calculations given above. For example, creep
had occurred in the exposed lower single panel where the fracture happened. Figure 3 shows the
bulging caused by the pressure from the caustic soda contents, a fact confirmed by direct measure-
ment of the circumference at two points on the tank, when a creep strain of about 0.2% was
recorded. It may also be borne in mind that the tank by the time of investigation and measurement
had been empty for about a month, so that some considerable strain recovery will have occurred.
Such bulging will of course have added a small but not insignificant bending moment to the critical
weld, adding an extra tensile component to the stress system acting on the pinhole.
A more serious deviation is that posed by the bending stress imposed by the need to form the
final weld. Although the details of analysis of the problem are reserved for Part 11, it is very evident
that a tensile stress in the outer surface of the panels will enhance the possibility of failure from
weak zones such as welds. Although there will be some stress relaxation after welding, there will
be a substantial contribution to the gross stress acting on the weld defects. In addition, panels cut
from the tank will tend to relax back to flatness. This was confirmed by re-measuring the dimensions
of the portions of cut panel still retained after sampling. Three substantial samples (chord length
ca 76 cm) were measured for their radii of curvature:
sample 1 (single sheet from buttress near good weld), R = 2.0 m
sample 2 (double sheet near good weld), R = 1.6 m
sample 3 (double sheet near poor weld), R = 1.64 m