SVILUPPO, VALIDAZIONE E IMPLEMENTAZIONE DI APPROCCI LOCALI PER L'ANALISI DELLA DURABILITA' STRUTTURALE DI COSTRUZIONI SALDATE

Welding represents a widespread practice in the modern engineering framework, due to the constant development of the joining technologies and the possibility to combine simple components to create high-performance engineering structures. However, it is a well-known fact that the durability of welded structures strictly depends on the strength of the joints composing them, especially when time-variant multiaxial fatigue loadings are involved. The nominal stress-based approaches provided in International Standards and Recommendations represent the most employed method to address the challenges of the fatigue durability assessment of welded structures. However, Standards do not necessarily provide accurate fatigue design references when complex un-classified geometries and multiaxial loading conditions are involved. On the other hand, it has been proven in the Literature that local approaches enable to overcome the limitations of nominal stress-based approaches, since they adopt criteria based on local stress and strains acting at the welds to assess the fatigue strength of welded structures. The Peak Stress Method (PSM) is a rapid FE-based local approach for estimating the Notch Stress Intensity Factors (NSIFs) at the weld toes and weld roots, which are assumed as sharp V-notches. By adopting the averaged Strain Energy Density (SED) as a fatigue failure criterion, the PSM enables to define a design stress, the equivalent peak stress, which can be exploited to estimate the fatigue lifetime of welded structures made of structural steels or aluminium alloys, in compliance with PSM-based fatigue design curves. Even though being recognized as more reliable than traditional methods based on the calculation of nominal stresses, local approaches still suffer from limited spreading in the modern industrial design practice, due to the high computational FE requirements and the need for specific knowledge. In order to address the aforementioned challenge and promote the adoption of the PSM in the modern engineering practice, in this manuscript, the entire analysis workflow of the Peak Stress Method (PSM) has been implemented in the Ansys® Mechanical FE environment as a custom fatigue analysis toolbox, the so-called "PSM App". The aim of the research has been to fully automate the application of the PSM to generic welded structures in order to support the FE analyst in the fatigue design of complex engineering structures. In order to achieve this target, first, the theoretical background of the PSM has been extended to improve the automation degree of the PSM analysis workflow. In addition, the applicability of the PSM has been extended to more commerical FE codes. Then, the fatigue strength of complex tube-to-flange steel arc-welded joints with reinforcement ribs inspired by the amusement park structures by Antonio Zamperla S.p.a has been investigated experimentally by performing fatigue tests under constant and variable amplitude uniaxial and multiaxial loadings. In addition, more fatigue data relevant to V-notched specimens made of E355 structural steel and tested under constant amplitude uniaxial and multiaxial loadings have been recently collected. The obtained experimental fatigue life data have been re-evaluated in terms of the equivalent peak stress and compared with fatigue lifetime estimations performed by means of the PSM-based fatigue design curves with the aim to validate the proposed fatigue design approach. Eventually, the "PSM App" has been employed to analyse a large variety of laboratory welded joints coming from the Literature and complex welded structures by Antonio Zamperla S.p.a., made of structural steel and aluminium alloys and subjected to constant and variable uniaxial and multiaxial fatigue loadings. As a result, the "PSM App" enabled to achieve a remarkable reduction in time and effort required to assess the fatigue durability of complex welded structures.