Environmental Engineering Reference
In-Depth Information
If the loading conditions are such that the dimensions of critical cracks
in the HT mode are smaller than in the operating mode, HT ensures the
safety of subsequent operation. This situation for longitudinal and transverse
cracks is shown in Fig. 8.4 and 8.5. However, if the critical size of the
cracks in the HT mode - all cracks or some cracks from the entire possible
set of cracks (families of cracks), is equal to or greater than the critical
size of cracks in the operating mode of operation, HT does not provide in
this case complete safety of subsequent operation (safety according to the
fracture criterion), i.e. partial or complete failure of the structural member
can take place in this case.
Thus, for HT to ensure safety in subsequent operation, the pressure
should be chosen so that all possible critical size cracks during HT are
lower than all the possible critical size cracks in the operating mode (this
situation is indicated on Fig.8.4 and 8.5). In this case, the greater the
difference between the critical crack sizes in the operating mode and in
HT, the longer is the duration of safe operation.
At the same time, it is important to ensure that the vessel or pipeline
does not fracture in the HT mode. These successful HTs show that the tested
structure does not contain cracks whose size is greater than or equal to the
critical size of cracks in the HT mode. But at the same time, the structure
may contain smaller cracks, including cracks with the size close to the
critical size in the HT mode. However, these cracks are not dangerous for
the operating mode.
These cracks can grow during operation. However, whilst the cracks are
smaller than the critical size in the operating mode of operation, no failure
can take place. But once the cracks reach the critical size in the operating
mode of operation the structural member can fail.
Thus, the time during which the family of critical cracks in the HT
mode can grow to the critical size in the operating mode is the time of safe
operation. On this basis, the frequency of HT which provides full security
is chosen as the time of crack size growth of the family of critical cracks,
corresponding to the pressure in the HT mode, to the size of the family
of critical size cracks, corresponding to the operating mode (Fig. 8.7 and
8.8). The crack propagation time is determined by the well-known methods
of fracture mechanics, for example, Paris' formula for cyclic loading.
When selecting HT pressure, it is necessary to define the family of
critical size cracks, corresponding to the pressure in the operating mode
(using the fracture mechanics formulas), and this is followed by determining
the HT pressure which must exceed the pressure in the operating mode.
Subsequently, a family of critical size cracks is formed. If all the cracks for
the pressure in the HT mode are smaller than the critical crack size for the
pressure in the operating mode, this pressure can be achieved in practice. If
all the dimensions of critical size cracks, or parts thereof, for the pressure
in the HT mode are equal to or greater than the critical crack size for the
pressure in the operating mode, then the HT pressure is increased and the
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