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Wendelstein 7-X (W7-X), the world’s largest nuclear fusion experiment of modular stellarator type, started operation in 2015 and will be upgraded with a water cooled first wall for steady state operation in 2020. The first wall consists of a CFC armored island divertors, adjacent baffles, heat shields, and stainless steel wall panels. Baffle and heat shield segments consist of graphite tiles, bolted with low pre-stress onto heat sinks of CuCrZr that are in turn brazed onto water cooled steel pipes. Cracks were detected before installation in the baffles in the root of the brazed seam in over 100 locations. Such cracks are attributed to the imposed plastic deformation of the pipes to bring them into the final shape following the complex 3D geometry of the plasma vessel. This paper gives an overview of the experimental and numerical work using finite element method (FEM) and dual boundary element method (DBEM), including sub-modeling to assess the risk of a water leak during operation. Details of the numerical work is published in Giannella et al. (2017), Lepore et al. (2017) and Citarella et al. (2018). First fatigue crack growth experiments were carried out on pipe material and thermal-mechanical crack growth predictions were made with FEM and DBEM. It appeared that the Stress Intensity Factor (SIF) threshold of the ductile steel is only reached when large plastic strains occur, thus violating the field of application of linear elastic fracture mechanics to forecast crack growth. Afterwards, representative brazed pipe samples were manufactured and subjected to initial plastic deformation causing cracks in 11 out of 12 samples. Some samples were tested up to 60,000 bending load cycles. Two out of four samples failed after ∼35,000 cycles. Before and after the test, the shape of the cracks was measured using 3D computer tomography scans. Equivalence between thermal load in W7-X and the mechanical load in the cyclic test was determined with the numerical models to allow for a prediction of the fatigue life in W7X. Additional modeling showed that also plastic zones away from the cracks can limit the fatigue life.

During the last few years various experimental destructive and non-destructive methods were developed to evaluate residual stresses. However it is impossible to obtain a full residual stress distribution in welded structures by means of experimental methods. This disadvantage can be solved by means of computational analysis which allows to determine the whole stress and strain fields in complex structures. In this paper the temperature distribution and residual stresses were determined in a single-pass butt joint welded by GMAW (Gas Metal Arc Welding) process by finite element model (FEM). A 3D finite parametric element model has been carried out to analyze temperature distribution in butt weld joints and thermo-mechanical analyses were performed to evaluate resulting residual stresses. Temperature fields have been investigated by varying an initial preheating treatment. Moreover the technique of "element birth and death" was adopted to simulate the process of filler metal addition The high stresses were evaluated, with particular regard to fusion zone and heat affected zone. The influence of preheating and post-heating treatment on residual stresses was investigated. The residual stresses decrease when preheating temperature increases. The maximum value of longitudinal residual stresses without pre-heating can be reduced about 12% and 38% by using the preheating and post-heating process respectively.

This paper concerns the numerical and experimental characterization ofthe static and fatigue strength of a flat stiffened panel, designed as a fiber metal laminates (FML) and made of Aluminum alloy and Fiber Glass FRP. The panel is full scale and was tested under both static and fatigue biaxial loads, applied by means of an in house designed and built multi-axial fatigue machine. The static test is simulated by the Boundary Element Method (BEM) in a two-dimensional approach (only allowance for membrane stresses). The strain gauge outcomes are compared with corresponding numerical results, getting a satisfactory correlation. After the static test, an initial notch is created in the panel and the aforementioned biaxial fatigue load is applied, causing a crack initiation and propagation; the related experimental initiation times and crack growth rates are provided.

A parametric model is adopted and the technique of element “birth and death” is used to estimate the effect of thermal properties and weld efficiency on residual stresses in butt weld joints. Residual stresses and distortions on butt welded joints are numerically evaluated by means of finite element method. The FE analysis allows to highlight and evaluate the stress field and its gradient around the fusion zone of welded joints, higher than any other located in the surrounding area. The main conclusion is the significant effect of varying the value of the conductivity on residual stresses. Several experimental destructive and non destructive techniques for directly measuring residual stress have been developed. However, the application of these methods in practice is usually limited by either cost or accuracy. Numerical simulation based on finite element techniques, therefore, offers a comprehensive solution for the prediction of residual stress and strain as well as welding distortion in complex welded structures. In this study it is shown that the technique of element “birth and death” can be usefully applied to welding process in order to take in account the effect of the thermal properties of materials.