Methods of deforming metals to large strains are reviewed and the process of equal channel angular extrusion is analysed in detail. The development of microstructure during large strain deformation is discussed, and it is concluded that the main criterion for the formation of a sub–micron grain structure is the generation of a sufficiently large fraction (> 0.7) of high–angle grain boundary during the deformation process. For aluminium alloys, it is found that a low–temperature anneal is required to convert the deformed microstructure into an equi–axed grain structure. The material, microstructural and processing factors that influence the formation of such fine–grain microstructures are discussed, and the stability of these microstructures at elevated temperatures is considered.

Developing stable fine-grain microstructures by large strain deformation - Discussion

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

Cyclic inelastic constitutive equations and their impact on the fatigue life predictions

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

Typically, for a fatigue strength assessment of a welded structure, the influence of welding residual stresses has to be considered. An easy applicable approach is given within the framework of the IIW recommendations; the design S-N curves can be upgraded in case of medium or low residual stresses. A relaxation or redistribution of tensile residual stresses, which is often accompanied by an increase in fatigue life, cannot be considered. To obtain a better understanding of the residual stresses and their influence on the fatigue strength, fatigue tests have been performed on longitudinal stiffeners in as-welded and stress relieved state with both stress ratios R = −1 and R = 0. Local strains and stresses were measured using strain gauges and X-ray diffraction technique to get insight into the local material’s response due to global loading close to the weld toe. The crack initiation and propagation phase was detected by a camera. FE models were set up to reproduce the observed material behavior in the notch for the crack initiation phase. By numerical analysis it could be shown that, due to sharp notches at the weld toe with an average toe radius of r = 0.05 mm, a stress redistribution occurs during the first load cycle. This leads to a fatigue strength, which is, particularly at higher load amplitudes, independent from mean as well as from initial residual stresses. A comparison of fatigue data derived on longitudinal stiffeners from literature confirms this effect. For lower load amplitudes instead, mean and residual stresses gain more influence. In this investigation a reliable assessment of the crack initiation period could be performed with strain-based concepts using damage parameter PSWT for a mean stress assessment. All fatigue tests could be transformed into a single damage parameter PSWT-N curve with a low scatter if residual stresses as well as the distortion of the specimens are regarded.

Influence of weld geometry and residual stresses on the fatigue strength of longitudinal stiffeners

Due to the new linear flow splitting forming process the production of bifurcated profiles from sheet metal without lamination of material becomes feasible. The continuous production of such structures takes place incrementally in a modified roll forming machine. By producing those complex parts process-related changes in the material properties occur as a result of the cold forming. Thus, the assumption of homogeneous properties in the processed part is not valid and a reliable analysis of structural durability is only possible by considering changes in material properties. Investigations on linear flow split profiles show large gradients in the microstructure and properties over the profile cross section. In the areas of low plastification the degree of deformation and the flow stress can be determined by microstructure and hardness measurements. In the severely deformed areas, like the upper flange surface, this determination becomes doubtable. Therefore the electron backscattered diffraction method was used to investigate, if an ultrafine-grained structure is occurring as it evolves in the processes of severe plastic deformation. Thus, the aim of this paper is the numerical analysis of the linear flow splitting process, the metallographic characterisation of bifurcated profiles and the numerical evaluation of the structural durability with consideration of the gained insights.

Thomas Bruder

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

Influence of weld geometry and residual stresses on the fatigue strength of longitudinal stiffeners

Severe plastic deformation by linear flow splitting