Info

Time

Time b) Stress history at x-x c) Cycle counting d) Stress-range spectrum

Time

Total cycles in design life

Ac (log scale)

Total cycles in design life e) Cycles to failure

Ac (log scale)

Ao2 Acr3 A04

Aa-N line for detail X-X

f) Damage summation (Palmgren-Miner rule)

Figure 2.2.1. Fatigue assessment procedure

(3)P The fatigue crack initiation sites determined in (2) shall be on or close to a surface which shall be readily accessible in service. The only exception shall be where safe alternative load paths are provided and details are designed to ensure that the cracks will be arrested without propagation beyond the first load path.

(4)P The procedure in 2.3.2 shall be applied to determine the inspection frequency and maximum permissible crack size before correction becomes necessary.

(5) Practical inspection methods shall be available which shall be capable of detecting the cracks and measuring their extent well before they have reached their fracture critical size.

(6) The maintenance manual shall specify the information listed in 1.8.4(1) for each crack location determined in (2).

2.3.2 Determination of inspection strategy for damage tolerant design

(1)P At each potential initiation site where the safe life Ts is less than the design life TL shall be calculated in accordance with 2.2.

(2)P The maintenance manual shall specify that the first inspection of each potential initiation site shall take place before the safe life has elapsed.

(3)P The maintenance manual shall specify that subsequent inspections shall take place at regular intervals Tj, where

Tj< 0.5 Tf where Tf is the calculated time for a crack, having initiated at the site being assessed, to grow from a detectable surface length ld to a fracture critical length lf (see Fig. 2.3.1).

(4)P The assumed minimum exposed length of surface crack shall take into consideration the accessibility, location, likely surface condition and method of inspection. Unless specific testing is undertaken to demonstrate that shorter lengths can be detected with a probability exceeding 90%, the assumed value of ld shall not be less than the value in Table 2.3.1 where the full crack length is accessible for inspection.

Table 23.1 Minimum safe values of detectable surface crack length in mm.

Method of Inspection

Crack location

Plain smooth surface

Rough surface, Weld cap

Sharp corner, Weld toe

visual, with magnifying aid

20

30

50

Liquid penetrant testing

5

10

15

NOTE: The above values assume close access, good lighting and removal of surface coatings.

(5)P Where any other permanent structural or non-structural part prevents full access to the crack, the obscured length of crack shall be added to the appropriate value in Table 2.3.1 to derive the value of Id for calculation purposes.

(6) Where heavy constructional thicknesses are used and where the initiation site is on an inaccessible surface, (e.g. the root of a single sided butt weld in a tubular member), it may be appropriate to plan an inspection strategy based on the use of ultrasonic testing to detect and measure cracks before they reach the accessible surface. Such a strategy should not be undertaken without prior testing and evaluation.

(7)P The value of If shall be such that the net section, taking into account the likely shape of the crack profile through the thickness, shall be able to sustain the maximum static tensile forces under the factored loading, calculated in accordance with Part 1.1 of this Prestandard, without unstable crack propagation.

(8)P Tf shall be estimated by means of calculation and/or by test, assuming factored loading (see 3.4), as follows:

a) The calculation method shall be based on fracture mechanics principles (see Annex B). An upper bound crack growth relationship shall be used. Alternatively specific crack growth data may be obtained from standard test specimens using the same material as in the crack propagation path. In which case the crack growth rate shall be factored in accordance with the fatigue test factor F (see Table C.l).

b) Where crack growth is obtained from structural or component tests simulating the correct materials, geometry and method of manufacture the relevant applied force pattern shall be applied to the test specimen (see Annex C). The crack growth rates recorded between the crack lengths ld and If shall be factored by the fatigue test factor F (see Table C.l).

(9)P The maintenance manual shall specify the actions to be taken in the event of discovery of a fatigue crack during a regular maintenance inspection, as follows:

a) If the measured crack length is less than Id no remedial action need be taken.

b) If the measured crack length is equal to or exceeds ld the component shall be assessed on a fitness-for-purpose basis with a view to determining how long the structure may safely be allowed to operate without rectification or replacement (see Annex B). In the event of continuation of operation consideration shall be given to increasing the frequency of inspection at the location in question.

c) If the measured crack length exceeds If the structure shall be immediately taken out of service.

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