Notch stress concepts for the fatigue assessment of welded joints – Background and applications

Probabilistic framework for fatigue life assessment of notched components under size effects

The notch stress approach for fatigue assessment of welded joints is based on the highest elastic stress at the weld toe or root. In order to avoid arbitrary or infinite stress results, a rounded shape with a reference radius, instead of the actual sharp toe or root, is usually assumed. Different proposals for reference radii exist, e.g. Radaj proposed a fictitious radius of 1 mm to consider micro-structural support effects for steel. The present guideline reviews different proposals for reference radii together with associated S-N curves. Detailed recommendations are given for the numerical analysis of the notch stress by the finite or boundary element method. Several aspects are discussed, such as the structural weakening by keyhole-shaped notches and the consideration of multiaxial stress states. Regarding the fatigue strength, appropriate S-N curves are presented for different materials. Finally, four examples illustrate the application of the approach, as well as the variety of structures that can be analysed and the scatter of results obtained from different models.

IIW recommendations for the fatigue assessment of welded structures by notch stress analysis: IIW-2006-09

Multiaxial fatigue analysis of notched components using combined critical plane and critical distance approach

At welded joints, fatigue cracks usually initiate either at the weld toe or the weld root. The latter may be influenced by small defects or other irregularities or even by non-fused root faces forming a slit which acts like a crack. Weld root failure has therefore to be checked for several weld types. Different approaches exist for fatigue assessment, including the rather simple nominal stress approach, local stress approaches and the crack propagation approach which are partly well suited for the assessment of weld root fatigue. However, it is not easy to keep the overview and to decide which approach should be applied in the case in question. For this purpose, the guideline has been established giving an overview of the different approaches with special emphasis on weld roots and discussing their suitability and the limitations. Six typical examples are described where different approaches are applied and in some cases compared with fatigue tests, thus giving insight into the practical application and allowing own judgement.

IIW guideline for the assessment of weld root fatigue

An assessment of thin welded structures with actual IIW design lines results in an overestimation of fatigue lives (strengths) at high load levels and a conservative estimation at low load levels, in many cases, independently of the applied fatigue assessment approach (nominal, structural or notch stress). This is mainly due to the slopes of the design S-N line k = 3.0 for normal and k = 5.0 for shear stresses, which are valid for thick and stiff structures. To overcome this inconsistency for welded thin and flexible structures, the slopes k = 5.0 for normal and k = 7.0 for shear stresses are suggested, keeping the already known FAT values derived for the notch stress concept variants with rref = 1.0, 0.3 or 0.05 mm, respectively. However, the slope is not only determined by plate thickness; it is the result of an interaction between thickness, local geometry (stress concentration), structural stiffness, loading mode and last but not least residual stresses. This complexity makes it difficult to identify the driving parameters and to predict the slope of the S-N line in many cases.

S-N Lines for Welded Thin Joints — Suggested Slopes and FAT Values for Applying the Notch Stress Concept with Various Reference Radii

Evaluation of nominal and local stress based approaches for the fatigue assessment of seam welds

For the application of the notch stress approach, local stresses at the notch root have to be calculated. This demands a fine discretization of the finite element model in the notch leading for complex structures to large finite element models which are difficult to handle and take a high amount of calculation time. This paper contains the detailed investigation of the required finite element mesh in the notch area for application of the notch stress approach. Based on small-scale specimens, various meshing rules are applied and compared to each other. As the most important factor for the efficiency of a finite element mesh, the element shape in the notch root was identified. It should be chosen according to the stress gradients: The higher the stress gradients are, the smaller the element edge length should be modeled. The shape of the subsequent elements inside the volume has no major influence on the accuracy of the calculation. Recommendations for practical use are given for various element types, including commonly used tetrahedron and hexahedron element with linear and quadratic shape function. Expected numerical errors for the chosen mesh configuration are indicated.

An efficient meshing approach for the calculation of notch stresses

Review and considerations on the fatigue assessment of welded joints using reference radii

This paper comprises a brief methodical analysis and practical application of both the stress averaging approach according to Neuber and the critical distance approach according to Taylor for a fatigue assessment on welded steel structures under axial loading. The stress averaging approach, as established theoretically by Neuber, is discussed and compared to the effective notch stress approach with a reference radius r
 ref = 1 mm and also to the r
 ref = 0.05 mm concept which is usually applied for thin-walled structures. Furthermore, both approaches are applied for an assessment of 38 individual test series. As a result of the evaluations, a microstructural length ρ* = 0.40 mm is recommended. For the critical distance approach, a distance a = 0.10 mm was derived. Fatigue design (FAT) values are recommended for the practical application. Compared to the notch stress approach with a radius r
 ref = 0.05 mm, it was possible to improve the assessment quality significantly. The non-local approaches should be preferred to the r
 ref = 0.05 mm concept, especially for welded components using thin sheets for which the radius of 1 mm cannot be applied.

Jörg Baumgartner

Papers

S-N Lines for Welded Thin Joints — Suggested Slopes and FAT Values for Applying the Notch Stress Concept with Various Reference Radii

Evaluation of nominal and local stress based approaches for the fatigue assessment of seam welds

An efficient meshing approach for the calculation of notch stresses

Review and considerations on the fatigue assessment of welded joints using reference radii

Fatigue assessment of welded joints using stress averaging and critical distance approaches