6 Screening Fatigue Assessment

6.1 Screening procedure

6.1.1 Assumptions

The screening fatigue procedure is based on:

Screening hot spot stress obtained by multiplying the stresses calculated from fine mesh analysis according to Ch 7, Sec 3 by the stress magnification factor of the considered structural detail.
Mean stress effect and thickness effect are used according to Ch 9, Sec 3, [3.2] and Ch 9, Sec 3, [3.3].

6.1.2 Procedure

The screening fatigue procedure includes the following three phases:

a) Phase 1: Calculation of fatigue stress.

Stresses are calculated at the stress read out point from the fine mesh element analysis with elements size of 50 × 50 mm, according to Ch 7, Sec 3 for all fatigue load cases defined in Ch 9, Sec 1, [7], for all loading conditions. Stresses to be used are element average membrane components stress defined in [6.2.3].
Hot- spot surface stress components are calculated for each load case ‘i1’ and ‘i2’ from the stresses multiplied by the stress magnification factor η, taken as:
- σ_{HS, i1(j)} = η σ_{S,
  i1(j)}
- σ_{HS, i2(j)} = η σ_{S,
  i2(j)}
Hot spot principal surface stress ranges are the difference of hot spot stress components obtained for each load case ‘i1’ and ‘i2’.
Fatigue stress ranges for welded joints are determined from hot spot principal surface stress ranges with correction factor for mean stress and thickness effect.

where:

σ_{S, i1(j)} : Stress calculated from the fine mesh analysis in load case ‘i1’ of loading condition (j) defined in [6.2].

σ_{S, i2(j)} : Stress calculated from the fine mesh analysis in load case ‘i2’ of loading condition (j) defined in [6.2].

η : Stress magnification factor given in Table 2.

b) Phase 2: Selection of S-N curve.

The S-N curve D defined in Ch 9, Sec 3, [4] is to be used with the fatigue stress range of weld toe in screening fatigue procedure.

c) Phase 3: Calculation of fatigue damage and fatigue life according to [6.1.3].

Table 2 : Stress magnification factor

Ship type	Structural detail category		Bulk hold	Stress magnification factor
Oil tanker	Toe of stringer		−	2.45
Oil tanker	Bracket toe of transverse web frame		−	1.65
Bulk carrier	Lower hopper welded knuckle		FA ⁽¹⁾	2.28
	Lower hopper welded knuckle		EA or C ⁽¹⁾	2.00
	Lower stool - Inner bottom	Non vertical (knuckle angle > 90°)	FA ⁽¹⁾	1.81
		Non vertical (knuckle angle > 90°)	EA or C ⁽¹⁾	1.47
		Vertical (knuckle angle = 90°)	FA ⁽¹⁾	2.09
		Vertical (knuckle angle = 90°)	EA or C ⁽¹⁾	2.75
⁽¹⁾ FA and EA mean "full cargo hold in alternate loading condition" and "empty cargo hold in alternate loading condition" respectively, C means “cargo hold of BC-B and BC-C bulk carriers”.

6.1.3 Screening fatigue criteria

The total fatigue damage and the fatigue life of screened details are to comply with the criteria given in Ch 9, Sec 3, [2].

Structural details that do not comply with the acceptance criteria are to be checked with respect to fatigue strength using a very fine mesh finite element analysis as described in Ch 9, Sec 5.

6.2 Stress read out procedure

6.2.1 Bracket toe

For bracket toe, the stress read out point is located at a 50 mm distance away from the bracket toe as shown in Figure 23.

Figure 23 : Stress read out point at bracket toe

6.2.2 Knuckle detail

For the lower hopper knuckle and for the connection between transverse bulkhead lower stool and inner bottom, the stress read out point is located at a 50 mm distance away from the knuckle line (i.e. model intersection line) as shown in Figure 24.

Figure 24 : Stress read out point of knuckle detail

6.2.3 Read out point stress

The average of membrane stress components at the centre of four elements, modelled with elements size of 50 × 50 mm connected to the stress read out point (or node) can be used as read out point stress.

When the element size is less than 50 x 50 mm, the stress of read out point can be derived using elements in an equivalent area as shown in Figure 25.

Figure 25 : Equivalent area for element size less than 50 × 50 mm