Applicability of Nominal Modified Nominal and Hot Spot Stresses Nominal stresses

(1)P Nominal stresses shall be used directly for the assessment of initiation sites in simple members and joints where the following conditions apply:

a) The details associated with the site are in reasonable agreement with the appropriate detail category requirements in Tables 5.1.1 to 5.1.5.

b) The detail category has been established by test in accordance with Annex C and where the results have been expressed in terms of the nominal stresses.

c) Gross geometrical effects such as those listed in 4.2.2 are not present in the vicinity of the initiation site.

d) The crack initiation site is located at the root of a fillet weld or a partial penetration butt weld.

4.2.2 Modified nominal stresses

(1)P Modified nominal stresses shall be used in place of nominal stresses where the initiation site is in the vicinity of one or more of the following gross geometrical stress concentrating effects (see Fig.4.2.1) provided that conditions 4.2.1(a) and (b) still apply:

a) gross changes in cross section shape, e.g. at cut-outs or re-entrant corners, b) gross changes in stiffness around the member cross-section at unstiffened angled junctions between open or hollow sections, c) changes in direction or alignment beyond those permitted in tables 5.1.1 to 5.1.5, d) shear lag and distortion in wide plated or hollow members, e) non-linear out-of-plane bending effects in slender components such as flat plates where the static stress is close to the elastic critical stress, e.g. tension- field in webs (see Part 1 of this Prestandard).

4.2.3 Hot spot stresses

(1)P Hot spot stresses shall only be used where the following conditions apply:

a) The initiation site is a weld toe in a joint with complex geometry where the nominal stress is not clearly defined, or b) A hot spot detail category has been established by test in accordance with Annex A and C where the results have been expressed in terms of the hot spot stress, for the appropriate loading mode.

c) Shell bending stresses are generated in flexible joints according to 4.1.2 (7).

4.3 Derivation of Stresses 4.3.1 Derivation of nominal stresses

4.3.1.1 Structural models using beam elements

(1)P The axial and shear stresses at the initiation site shall be calculated from the axial, bending, shear and torsional forces at the section concerned using linear elastic section properties.

(2)P The cross-sectional areas and section moduli shall take account of any specific requirements in tables 5.1.1 to 5.1.5.

4.3.1.2 Structural models using membrane, shell or solid elements

(1)P Where the axial stress distribution is linear across the member section about both axes, the stresses at the initiation point may be used directly.

(2)P Where the axial distribution is non-linear across the member section about either axis, the stresses across the section shall be integrated to obtain the axial force and bending moments. The latter shall be used in conjunction with the appropriate cross-sectional area and section moduli in accordance with tables 5.1.1 to

5.1.5 to obtain the nominal stresses.

y

1 i

<i i

Crack initiation site

Nominal stress range

Crack initiation site

(SDIB

Linear stress distribution assumed Weld toe stress concentration factor at z not calculated a) Local stress concentration (weld toe) Mean net stress, A a |<—>

plan view

Non-linear stress distribution

Modified nominal stress range at initiation site, X

Welded attachment

Opening

Welded attachment

Opening b) gross stress concentration (large opening)

modified nominal A a nominal Aakgt

Elastic Stress

c) hard point in a connection

Figure 4.2.1. Effect of stress concentrations on nominal and modified nominal stresses

4.3.2 Derivation of modified nominal stresses

4.3.2.1 Structural models using beam elements

(1)P The nominal stresses shall be multiplied by the appropriate elastic stress concentration factors Kgt according to the location of the initiation site and the type of stress field.

(2)P Kgt shall take into account all geometrical discontinuities except for those already incorporated within the detail category (see Tables 5.1.1 to 5.1.5).

(3)P Kgt shall be determined by one of the following methods:

a) standard solutions for stress concentration factors (see Annex A)

b) substructuring of the surrounding geometry using shell elements taking into account (2), and applying the nominal stresses to the boundaries.

c) Measurement of elastic strains on a physical model which incorporates the gross geometrical discontinuities, but excludes those features already incorporated within the detail categoiy (see (2)).

4.3.2.2 Structural models using membrane, shell or solid elements

(1)P Where the modified nominal stress is to be obtained from the global analysis in the region of the initiation site it shall be selected on the following basis:

a) local stress concentrations such as the classified detail and the weld profile already included in the detail categoiy shall be omitted.

b) the mesh in the region of the initiation site shall be fine enough to predict the general stress field around the site accurately (see Annex A) but without incorporating the effects in (a).

4.3.3. Derivation of hot spot stresses

(1)P The hot spot stress is the principal stress predominantly transverse to the weld toe line and shall be evaluated in general by numerical or experimental methods (see Annex A), except where standard solutions are available. For simple cases, as the one shown in Fig.4.2.1(c), the hot spot stress should be evaluated by multiplying the nominal stress for the geometrical stress concentration factor Kgt, defined as the theoretical stress concentration evaluated for linear elastic material omitting all the influences (local or geometric) already included in the design Aa-N curve of the classified detail considered as a reference.

(2)P In general, for structural configurations for which standard stress concentration factors are not applicable and which therefore require special analysis, the fatigue stress at the weld toe should omit the stress concentration effects due to the classified detail considered as a reference, i.e. the weld toe geometry.

4.3.4 Stress orientation

(1)P The principal stress range shall be greatest algebraic difference between the principal stresses acting in principal planes no more than 45° apart.

(2)P For the purposes of assessing whether a detail is normal or parallel to the axis of a weld if the direction of the principal tensile stress is less than 45° to the weld axis it shall be assumed to be parallel to it.

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