Issue 43

P. Corigliano et alii, Frattura ed Integrità Strutturale, 43 (2018) 171-181; DOI: 10.3221/IGF-ESIS.43.13 172 I NTRODUCTION itanium alloys, in general, are employed in various applications, ranging from aerospace sector (turbine disks, blades of the compressors, structural elements), passing from the naval sector, to medical and surgical devices [1]. In particular, the Ti-6Al-4V alloy is one of the most widely used, thanks to an excellent combination of low density, high specific strength and corrosion resistance. Potential applications include also shafts and pressure housings, subsea wellhead and riser components. The first application of titanium alloys in dynamic offshore riser systems was a Taper stress joint (TSJ) [2]. TSJs are specially designed tubular joint sections with a taper over length that permits large deflections of risers on floating production systems, while limiting the bending load transmitted to wellheads or other adjoining structures. The natural flexibility of titanium alloy tubulars derived from a combination of high strength, low elastic modulus (50% of steel), and exceptional air and seawater fatigue resistance and, makes titanium a natural choice for TSJs. Titanium alloys are also reasonable candidates for low-pressure drilling risers in very deep waters, high pressure drilling risers, and deep water completion and intervention risers. They cost less, and are much more robust, durable, and able to be inspected than composite risers under development. For deep water intervention, titanium alloy risers are sufficiently light to enable small mono-hull vessels (e.g. diving support vessels) to be used for well servicing, offering much lower subsea well maintenance costs. A Ti–6Al–4V (Ti Gr. 5) alloy drill pipe offers half the modulus of steel, which means twice the flexibility, a high (827 MPa) minimum yield strength, an elevated fatigue strength which is relatively unaffected by the drilling environment and resistance to corrosion and erosion in drill muds and sweet/sour well fluids. However, given the complex workability of the material with the common techniques, as well as the higher specific cost compared to the most common metallic alloys, the development and tuning of joining techniques suitable to this type of alloys it is undoubtedly one of the factors that most influence the possible dissemination to industrial sectors with lower value added. In this context, the laser weld is considered as an alternative to the traditional techniques to operate the joining of plates of titanium alloys. The main advantages of this process are an increased penetration depth and a reduction of possible welding cracks, and the size of the molten zone with respect to a TIG welding or arc, thus entailing an increase of the mechanical resistance of the welded structures. Innovative methods of welding such as the electron beam have the disadvantage of having to operate in vacuum, in addition, this processing involves the emission of X-rays. However, the welding process induces variations caused also by microstructural factors; in fact, the local mechanical properties of three different areas (base material, BM, heat affected zone, HAZ, welded zone, WZ) will be different [3, 4]. The literature on fatigue analysis of welded joints was reviewed by Fricke [4]. The Recommendations reported in the current Codes, such as International Institute of Welding [5] and Eurocode [6, 7] and also accepted by some of the major Classification Societies of ships and offshore structures, are very conservative. The assessment becomes more complex in presence of a multiaxial stress state [8-11]. The goal of this research activity was to study the fatigue behavior of laser welded joints of titanium alloy, in which the welding was performed using a laser source and in the absence of filler material, by means of unconventional full field techniques: the digital image correlation (DIC, Digital Image Correlation) and the Infrared Thermography (IRT, Infrared Thermography), in order to analyze the behavior in the surrounding areas of geometric discontinuities and the different material properties. In particular, T-joints were analyzed, obtained from titanium sheets with a thickness of 3 mm and 5 mm, welded by a laser source YB: YAG laser with a maximum power of 10 kW. The fatigue tests were led using loading systems developed ad-hoc and systematic analysis of the results obtained by the DIC and IRT techniques has allowed to better understand the mechanisms of evolution of the local damage within the joints during the application of cyclic loading. Full-field techniques were already applied by some of the authors have already applied full-field for the assessment of different materials: AA6082 aluminum alloy [12], S355 and high strength steels [13, 14], AISI4140 steel in very high cycle fatigue regime [15], Iroko wood under static loading [16, 17] and shape memory alloys [18]. M ATERIALS AND METHODS Welded joints preparation he Ti-6Al-4V alloy investigated is of α + β type and its chemical composition is shown in Tab. 1, in which it is observed the presence of aluminum that operates as alpha stabilizer and vanadium with the function of beta stabilizer. T T