Safe Life Design

2.2.1 Prerequisites for safe life design

(1)P The predicted service history of the structure shall be available in terms of a loading sequence and frequency. Alternatively the stress response at all potential initiation sites shall be available in terms of stress histories.

(2) P The fatigue strength characteristics at all potential initiation sites shall be available in terms of fatigue strength curves.

(3)P The quality standards used in the manufacture of the components containing potential initiation sites shall be defined.

2.2.2 Calculation procedure for safe life design

(1) P Potential fatigue crack initiation sites in regions of the structure containing the highest stress fluctuations and/or the severest stress concentrations shall be checked first.

(2)P The basic procedure shall be as follows (see Figure 2.2.1):

a) An upper bound estimate of the service loading sequence for the structure's design life shall be obtained (see 3).

b) The resulting stress history at the potential initiation site being checked shall be estimated (see 4.1 and 4.2 or 4.3).

c) Where nominal stresses are being used, modify the stress history in any region of geometrical stress concentration which is not already included in the detail category, by applying an appropriate stress concentration factor (see 4.2).

d) The stress history shall be reduced to an equivalent number of cycles (n*) of different stress ranges Agj using a cycle counting technique (see 4.4).

e) The cycles shall be ranked in descending order of amplitude Acjj to form a stress-range spectrum, where i = 1,2,3 etc for the first, second, third band in the spectrum (see 4.5).

f) Categorise the detail in accordance with Tables 5.1.1 to 5.1.5. For the appropriate detail category and design stress range (Acij), find the permissible endurance (N„ etc.), from 5.2.1.

g) Calculate the total damage DL for all cycles using Miner's summation where and where the design life of TL has the same units as Ts i) If Ts is less than TL one or more of the following actions shall be taken:

- redesign the structure or member to reduce the stress levels

- change the detail to one with a higher category (see 5.1)

- use a damage tolerant design approach, where appropriate (see 2.3)

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