Section 2 Fatigue strength improvement methods

2.1 General

2.1.1 In general, the presence of a weld or a bonded joint in a structural component represents a possible weakness with regard to both brittle fracture and fatigue life. The low fatigue life of welded details can be considered as a limiting factor for the design of more efficient structures, in particular, since the fatigue strength of steel and aluminium materials does not increase with the yield strength.

2.1.2 Upgrading of the fatigue life of a structural detail can be achieved in a number of ways as follows:

Design Stage
•	By adopting good detail design configuration, i.e. by the provision of soft connections, the geometrical stress concentration factor due to the geometrical discontinuity may be reduced to a satisfactory level.
•	By increasing the local scantlings, in particular those of the plate in which the potential crack sites are located, to reduce the local hot spot stresses.
•	By modifying the structural load path and/or reducing the structural load level by the provision of additional load carrying members.
Fabrication Stage
•	By improving the fatigue life of the detail by using an improvement method.

2.1.3 It is Lloyd”s Register's intention to promote the Design Stage method for fatigue strength improvement of a craft’s structural details. Fabrication stage improvement methods should only be considered as remedial measures, and subjected to strict quality control procedures. Where a fabrication stage improvement method is planned at the design stage, it is to be specially considered by LR to ensure that a satisfactory level of fatigue strength improvement is achieved.

2.2 Significant variables affecting fatigue strength improvement methods

2.2.1 The significant variables affecting the fatigue strength of a craft’s structural details are reviewed:

The geometrical notch at the weld toe region is normally the most fatigue critical area. Welded joints inherently contain a number of defects, most of which are so sharp that they start growing as fatigue cracks when the structure is subjected to dynamic loads, thus eliminating the crack initiation stage of the fatigue life.
The fatigue crack is most likely to initiate and propagate in the Heat Affected Zone (HAZ) region, since local metallurgical changes may affect the local fatigue properties of the material, and defects are usually concentrated in this area.
Residual stresses are set up in and near the weld due to the contraction of the weld metal during the cooling phase. These local residual stresses due to welding may reach yield stress magnitude, and affect the fatigue properties in a similar manner to externally imposed loads. Tensile residual stresses tend to reduce the fatigue strength, while compressive residual stresses may improve the fatigue strength. Attention to residual stresses is not only limited to the welding process, residual stresses may arise due to the restraints applied to the prefabricated units, the forcing of the prefabricated units during assembly, or uneven thermal expansion creating long range residual stresses acting over large areas. These long range residual stresses tend not to be relaxed by the occurrence of peak loads resulting in the so called shakedown process, or local treatment of the structural detail. However, they are generally of small magnitude compared to welding residual stresses.

2.3 Design stage fatigue strength improvement methods

2.3.1 It is clear that the most efficient method to improve the fatigue strength of welded structural details is at the design stage. To this effect, there are four factors which need to be specially considered to improve the fatigue strength of ship structural details as follows:

Nominal stress level.
Geometrical stress concentration due to the structural detail geometry.
Weld geometry and construction tolerances.
Residual stresses and construction procedure.

Each item outlined above is presented in the following Sections.

2.3.2 Nominal stress level

The most efficient way to improve fatigue strength is to increase the local scantling to reduce the nominal stress level, and hence the hot spot stress for a given structural detail. In general, structural details on higher tensile steel and aluminium require improvement in detail design over the mild steel and the base grade aluminium equivalent structural detail by virtue of the higher stress level and the constant fatigue strength for material of various yield strength.

The advantages and disadvantages of this fatigue strength improvement method are summarised in Table 2.2.1 Nominal stress level.

Table 2.2.1 Nominal stress level

Advantages		Disadvantages
•	Reduce stress level.	•	Increase structure weight.
•	Increase static strength.
•	Potential decrease in number of structural components and/or complexity required over that for a structural detail in higher tensile strength material.

2.3.3 Geometrical stress concentration

The adoption of a good detail design configuration by the provision of soft connections reduces the geometrical stress concentration factor due to the geometrical discontinuity to a satisfactory level. Typical detail design improvements for the critical areas are provided in Ch 4 Detail Design Improvement for Steel/Aluminium Construction and Ch 5 Detail Design Improvement for Composite Construction of these Guidance Notes for steel/aluminium alloy and composite construction respectively. These detail design improvements have been developed from the consolidation of service experience and finite element analysis.

The advantages and disadvantages of the subject fatigue improvement method are summarised in Table 2.2.2 Geometrical stress concentration.

Table 2.2.2 Geometrical stress concentration

Advantages		Disadvantages
•	Reduce hot spot stress level by reducing the local geometrical stress concentration.	•	May increase structural weight if additional pieces are required.
•	Most effective fatigue strength improvement technique.	•	Requires good workmanship where soft toe/heel are required.
•	May provide additional structural redundancy.

2.3.4 Weld geometry and construction tolerances

At the design stage, special attention may be given to achieving a favourable geometry and smooth transition at the weld toe, and minimising secondary stress concentration which may arise from the fit up and misalignment. Since the weld notch stress concentration is a direct function of the weld flank angle and the weld toe radius, critical structural details may be specified with an enhanced weld procedure and construction tolerances.

In view of the size and hull form of special service craft, additional considerations must, in general, be given to the accessibility for welding. This should include the selection of the depth, geometry and orientation of the stiffening members to provide the necessary access to carry out the required welding sequences, with the type and size of welding equipment available to the Builder.

The advantages and disadvantages of the subject fatigue improvement method are summarised in Table 2.2.3 Weld geometry and construction tolerances.

Table 2.2.3 Weld geometry and construction tolerances

Advantages		Disadvantages
•	The improvement may be introduced at the design stage.	•	Improvement can be subject to large scatter if not controlled under QA survey conditions i.e. Fatigue Control Plan.
•	The improvement is performed in the welding process itself.
•	Subject to well defined inspection plan and hence higher reliability.

2.3.5 Residual stresses, and construction procedures

The minimising of residual stresses through the adoption of appropriate welding procedures and sequences, the use of adequate unit size, and appropriate sequence of erection of the prefabrication unit do not constitute in themselves a fatigue strength improvement procedure. Nevertheless careful planning should be considered at the design stage to ensure that detrimental effects will not be introduced during the construction process.