Page 155 - Introduction to Naval Architecture

STRENGTH 14J

Unfortunately there is no simple physical test to which a material can
be subjected that will determine whether it is likely to be satisfactory in
terms of brittle fracture. This is because the behaviour of the structure
depends upon its geometry and method of loading. The choice is
between a simple test like the Charpy test and a more elaborate and
expensive test under more representative conditions such as the
Robertson crack arrest test. The Charpy test is still widely used for quality
control,
Since cracks will occur, it is necessary to use steels which have good
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crack arrest properties. It is recommended that one with a crack
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arrest toughness of 150 to 200MPa(m)°' is used. To provide a high
level of assurance that brittle fracture will not occur, a Charpy
crystailinity of less than 70 per cent at 0°C should be chosen. For good
crack arrest capability and virtually guaranteed fracture initiation
avoidance, the Charpy crystailinity at 0°C should be less than 50 per
cent. Special crack arrest strakes are provided in some designs. The
steel for these should show a completely fibrous Charpy fracture at
0°C.

Fatigue
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Fatigue is by far and away the most common cause of failure in
general engineering structures. It is of considerable importance in
ships which are usually expected to remain in service for 20 years or
more. Even when there is no initial defect present, repeated stressing of
a member causes a crack to form on the surface after a certain number
of cycles. This crack will propagate with continued stress repetitions.
Any initial crack-like defect will propagate with stress cycling. Crack
initiation and crack propagation are different in nature and need to be
considered separately.
Characteristically a fatigue failure, which can occur at stress levels
lower than yield, is smooth and usually stepped. If the applied stressing
is of constant amplitude the fracture can be expected to occur after a
defined number of cycles. Plotting the stress amplitude against the
number of reversals to failure gives the traditional S-N curve for the
material under test. The number of reversals is larger the lower the
applied stress until, for some materials including carbon steels, failure
does not occur no matter how many reversals are applied. There is
some evidence, however, that for steels under corrosive conditions
there is no lower limit. The lower level of stress is known as the fatigue
limit,
For steel it is found that a log—log plot of the S—N data yields two
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straight lines as in Figure 7.10. Further, laboratory tests of a range of