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The Peak Stress Method (PSM) is an engineering, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints, which takes advantage of the singular linear elastic peak stresses from FE analyses with coarse meshes. Originally, the PSM was calibrated by using 3D, eight-node brick elements, taking advantage of the submodeling technique. Recently, the PSM has been calibrated by using ten-node tetra elements, which are able to directly discretize complex 3D geometries without the need for submodels. The PSM was validated for pure axial or bending loadings as well as pure torsion loadings; recently it has been extended to multiaxial loadings by adopting a design stress, the so-called equivalent peak stress, in conjunction with a reference design fatigue curve. After having briefly recalled the calibration of the PSM with tetra elements, the paper presents some applications of the PSM relevant to steel plate-to-tube welded details of industrial interest under in-phase bending-torsion fatigue loadings. Experimental data have been re-analysed using the PSM based on tetra elements. Eventually a good agreement between experimental and theoretical results has been obtained in terms of fatigue crack initiation location as well as total fatigue life.

In this paper, a two-dimensional model to describe the non-planar features of crack morphologies is presented. The model accounts for frictional tractions along the crack surfaces by considering an elastic-plastic-like constitutive interface law. Dilatancy effect due to crack roughness is described by the model, leading to a Mode I/II coupling between displacements and tractions along the crack surfaces. The non-linear solution of the rough and frictional crack under general remote scenarios is obtained using the Distributed Dislocation Technique (DDT). By considering a linear piecewise periodic profile of the interface crack, the influence of roughness and friction of interface cracks is examined in relation to both the resulting near-tip stress field and the fracture resistance under monotonic mixed-mode loading. The present model is able to quantify the increase of the fracture resistance due to roughnessand friction-induced crack tip shielding and to correlate it with a dimensionless crack size parameter.

Nickel-Titanium alloys (Nitinol) are widely used for biomedical applications. Peripheral stents are almost exclusively composed of Nitinol, as its superelasticity is suited for minimally-invasive insertion and durable effect. After crimping and deployment stents undergo cyclic multi-axial loads imposed by vascular and lower-limb motion (e.g. axial compression, bending, and torsion). This complex mechanical environment could lead to metal fatigue and device fracture, with possible severe consequences (e.g. in-stent restenosis). Standard regulations require experimental verification of stent fatigue behaviour for preclinical assessment, but no exact indications are provided to direct the load combination. Moreover, different fatigue criteria were developed for common metals to predict fatigue endurance, but no criteria were specifically defined for the unique thermo-mechanical properties of Nitinol. This study investigated the role of cyclic multi-axial loading conditions on different stent geometries, looking at how they affect the stress/strain distribution along the device and how different criteria may affect the fatigue prediction (e.g. the standard Von Mises alternate approach and other critical plane approaches). Then, a preliminary experimental fatigue campaign was performed in agreement with the numerical simulations in order to compare the numerical predictions with the experimental results. The result suggest that the critical plane approaches are more reliable than the standard Von Mises criterion.