PSI - Issue 5

Rui F. Martins et al. / Procedia Structural Integrity 5 (2017) 633–639 Diogo F. Almeida et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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flow rate of 13 litres/min. According to the welding parameters referred, the heat input was approximately 730J/mm, having been assumed a thermal efficiency factor equal to 0.6 due to the welding process used (TIG) (Pozo-Morejón et al. 2011). This factor takes into account the heat losses (incurred by convection and radiation) and the heat input, thus defining a measure of the energy transferred to the weld bead per unit length (Funderburk 1999). Besides, heat input value directly influences the rate of cooling, which in turn will affect the mechanical properties and the metallurgical structure of the weld bead and the heat affected zone (HAZ). Residual stresses can vary due to numerous factors, depending for instance on the physical and mechanical properties of the materials to be joined, the dimensions of the plates to be welded, the external constraints of the components to be welded, the heat input, the number of passes of the welding process, the welding sequence, the temperature preheating or the interpass temperature (Deng et al. 2008). However, the study of residual stresses is extremely important for evaluating the risk of nucleation and fatigue crack propagation. In fact, the residual stresses introduced by the welding process can greatly increase the net stress values applied beyond the design loads, causing fatigue crack propagation at higher crack growth rates or the spread of intergranular cracks due to stress corrosion cracking (Deng and Murakawa, 2006) or (Fitzpatrick and Edwards, 1998). So, to prevent these occurrences, it is quite common to carry out stress-relief heat treatments, with the aim of minimizing the effects of residual stresses; otherwise, the component / structure will tend to collapse when subjected to loading levels much lower than those used at the design stage. Currently, it is possible to make the measurement of residual stresses by various methods, and some of these techniques are of destructive type. The hole-drilling method is a semi-destructive method that is based on the measurement of strain relief values at the surface of a component caused by the drilling of a hole with small diameter, across the thickness of the component / welded joint. The tests in the study herein presented were carried out at Instituto de Soldadura e Qualidade (ISQ Lisbon) according to ASTM E 837-01 and a HBM strain rosette was placed approximately 1 mm from fusion line (Fig. 2) and at a distance of about 100 mm from the lateral edge of the plate (along the welding direction) (Fig. 1). The drilling system used was a Vishay RS200 model (Fig. 2) and during the test the incremental method was used, which meant drilling through thickness increments of 0.3 mm until a maximum depth of about 2.4 mm. In addition, a Vishay P3500 bridge coupled to a switching unit SB10 (Vishay) was used in order to make the reading of strain measured by strain gauge s at the specimen’s surface. The obtained results showed the presence of tensile stress values in the direction parallel to the weld bead and compression stresses in the direction perpendicular to the weld bead (Table 1). 2.2. Measurement of residual stresses

Fig. 2. Overall view of the equipments used for the measurement of residual stresses by the hole-drilling method.