Fatigue assessment of welded joints using multiaxial fatigue space theory with nominal stress approach

Welded joints are widely employed in engineering field and they are always the starting points of fatigue damage. Because of the unfavorable material and geometry characters, as well as initial welding defects, the fatigue damage evaluation of welded joints is an important and troublesome issue for engineers. In this article, multiaxial fatigue space theory proposed by the first author for smooth specimens is extended for the fatigue damage assessment of welded joints, by adopting nominal stress approach. The fatigue test data with different materials, loading paths, and welded joints geometries are used to validate the capability of this theory. The result indicates a strong parallelization between predicted life and experimental life, with a favorable prediction error and beneficial error distribution. It can be concluded that multiaxial fatigue space theory is a useful method for fatigue damage assessment of welded joints with the help of nominal stress approach.

A survey on fatigue life analysis approaches for metallic notched components under multi-axial loading

In this paper, several fatigue failure approaches of metallic notched components under multi-axial loading in recent decades are reviewed in detail. They are classified into three categories according to their different fatigue physical mechanisms and hypotheses: nominal stress approach, local stress–strain approach and the theory of critical distance. The accuracy, applicable range and computing complexity of these three different fatigue failure theories of metallic notched specimen under multi-axial fatigue loading are given. It is concluded that theory of critical distance accords with experimental results under multi-axial fatigue loading and it gives unambiguous explanation for physical mechanism of fatigue damage. However, the computing process is complex, especially under non-proportional fatigue loading, and the key parameter of theory of critical distance is difficult to calculate especially in engineering. These difficulties limit the application of theory of critical distance.

Fatigue of steel bridges

Highway and railway bridges are exposed to cyclic stressing due to traffic loads and, therefore, have to be evaluated concerning fatigue. In most cases the fatigue evaluation is performed according to Eurocode 3 Part 1-9 on nominal stresses. To apply this nominal stress approach a detail catalogue is required classifying all relevant constructional details in terms of fatigue. Unfortunately, the existing detail catalogue of Eurocode 3 Part 1-9 reflects the state of the art of the 1990s and misses constructional details being important for today’s bridge design. As an example the derivation of a new detail, the so-called lamellae joint, is presented. Furthermore, for two new types of innovative steel bridges, where Eurocode 3 Part 1-9 does not yet specify rules able to evaluate the characteristics of these bridges, research results are shown. These are the thick-plate trough bridges and truss bridges made of thick-walled circular hollow sections (CHS). The paper starts with an overview on the recent Eurocode developments, addressing more specific the fatigue verification according to EN 1993-1-9 and the statistical analysis of fatigue test data. In the following, information is given on the outcome of some recent research projects striving to extend the application range of Eurocode 3 Part 1-9. The final conclusion, in spite of all differences, show a common tendency.

Criteria for Fatigue Failure of Materials: Application in Fatigue Assessment of Structures

Presently in rules for fatigue assessment of structures subjected to intensive alternating service loading the Stress-Life (S-N) criteria are recommended in versions of the Nominal stress approach, Hot-spot stress and Notch-stress approach based on using the stress range a representative of the current damage. The criteria and approaches provide assessment of fatigue properties of structures accompanied with a series of approximations and uncertainties. A physically and mechanically more correct procedures might be provided by the Strain-life and Inelastic strain energy criteria for fatigue failure and approaches, although specific with intrinsic sources of approximations. The nature of approximations in the approaches is briefly commented and feasible means of improvement the fatigue assessment procedures and applications are presented.

NUMERICAL METHODS FOR THE FATIGUE ASSESSMENT OF WELDED JOINTS: INFLUENCE OF MISALIGNMENT AND GEOMETRIC WELD IMPERFECTIONS

The fatigue design life of welded joints in steel structures is increasingly assessed by using numerical models and methods, such as the structural (hot-spot) stress method and the effective notch stress method. When compared to the classical design approach using nominal stress S-N design curves, these methods offer the advantage of flexibility and a wider scope of application. However, a number of questions arise when these methods are used to assess geometrically "imperfect" welded joints, such as joints with plate misalignments or excessive weld convexity or concavity. In these cases, the classical S-N curves are known to cover imperfections up to the common tolerance classes for fatigue-prone welded joints (e.g. in accordance with ISO 5817 class B). For the numerical methods, differing and conflicting recommendations exist on how to account for the geometric imperfections in the welded joints, with little or no background to these recommendations available. In this paper, a study is presented in which two standard welded joints (butt welds between plates of equal and unequal thickness; T-joints with fillet welds) are analysed with the help of the structural (hot-spot) stress and the effective notch stress approach, considering various levels of geometric imperfection up to the tolerance limits, and the resulting fatigue life predictions are compared to test results from the literature and the nominal stress approach predictions. Since the nominal stress approach curves are based on reliable statistical data and desired survival probabilities for these known, standard cases, this methodology allows one to determine the correct application of the numerical methods to cases with geometric imperfections. This information may be used for a pertinent refinement of design recommendations for these methods, as well as for cases where these methods are applied to fitness-for-purpose assessments - e.g. because the nominal stress approach is not applicable.

Assessment of Weld Fatigue by Effective Notch Stress Approach

Applicability of different approaches, i.e. nominal stress-, structural hot spot- and effective notch stress approach, for fatigue assessment of welded structures has been discussed. The effective notch stress approach was validated for a cruciform fillet welded and fully penetration welded joint, according to DNV guideline [1]. The same fillet welded joint was further investigated for varying weld sizes. The results show that with changing weld size, there are considerable differences between calculated stresses, and specially calculated fatigue lives, when comparing with nominal stress approach. Further investigations revealed that there is a nonlinear relation between effective notch stress and weld sizes, while comparison of the notch stress and nominal stress approaches indicate that a linear relation is to be expected. Based on the established methodology for the cruciform joint, another joint, i.e. a knee plate located in a horizontal brace of a drilling vessel, was assessed for weld root fatigue. Based on the proposed equations and the linear relation found between the nominal- and the notch stress S-N curves, a weld size providing a longer fatigue life at the weld root, rather than that at the toe, was proposed.