It is often advantageous to design pressure containing plant, such as pipework, tubes, vessels, and boilers, on the basis of leak-before-break.   This means that partial failures which occur by sub-critical mechanisms (fatigue crack growth, stress corrosion cracking etc) are detected by loss of pressure in the plant before final catastrophic fracture occurs.  This requires a crack to grow in a stable manner through the wall of the component and cause a detectable leak and consequent loss of pressure.  This indication of a partial failure allows the plant to be shut down in a controlled manner and repairs/replacement carried out.

If it can be demonstrated that a leak-before-break situation exists, other useful benefits may accrue:

• supplementing the primary structural integrity safety case
• alleviating some of the responsibility of non-destructive testing for ensuring safety
• permitting a reduction in number of restraints engineered into a pipework system to control pipe whip on failure

The strategy in performing the analysis is as follows, and illustrated by reference to the figures below.  A surface (part-through) crack is assumed to initiate and grow by a sub-critical mechanism.  Generally, initiation will be from the inner surface of the pressurised container, as stresses are usually higher at this point and there may well be a corrosive environment present (Figure a).   However, industrial situations where cracking can occur from the external surface are relatively common.  A typical example might involve intergranular attack of reactor pipework at elevated temperatures.

1.    Calculate the length of through-thickness crack which will cause fracture, l_crit.

2.    Calculate the depth of part-through (or surface) crack which will cause fracture, a_crit.

3.    The value of a_crit must be > B, the wall thickness.  This allows the part-through crack to penetrate the wall (Figure b).

4.    Once wall penetration occurs, the part-through crack very quickly grows through the ligaments to become a through-thickness crack with a length l₁ = 2c, where 2c is the surface length of the part-through crack at wall penetration (Figure c).  Hence the aspect ratio of the part-through crack is an important parameter in a leak-before-break analysis.   Remember also that the length l of a through-thickness crack is defined as 2a when you are substituting for a in stress intensity equations.

5.    The value of l_crit must be > l_1.

6.    Calculate the time for the crack to grow from l₁ to l_crit.  If the leak rate of fluid is detectable in this time, then a leak-before-break design case is established.

                                           Figure a                                                                                                     Figure b             

    Figure c

An interesting example of an application of this philosophy is the design of the main boom brace tubes on a walking dragline.

If one of these should fracture during operation, the dragline would suffer severe damage, hence they are safety-critical.  Although the chance of such a fracture is (by design) very remote, a supplementary safety case is established by pressurising the booms (via nozzles) with an inert gas.  The internal boom pressure can be read on a gauge and any decrease over time immediately noted.  Non-destructive inspection of the boom would then be carried out to confirm whether a crack existed.

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