Issue 48

R. Baptista et alii, Frattura ed Integrità Strutturale, 48 (2019) 257-268; DOI: 10.3221/IGF-ESIS.48.27 258 I NTRODUCTION s the demand for high strength steels (HSS) increases, so does the necessity to assess the integrity of the various mechanical components produced with these materials. The two major goals are: increase the components fatigue life expectancy, and their safety. Previously, the study and analyses were constantly made from laboratory experiments, which were lengthy, expensive and sometimes difficult to implement, due to several rules. Today, there are new materials and mechanical components appearing to a daily rhythm, which must be tested and certified to be available for the consumers. Inevitably, the engineers presented with systematic solutions, with higher precision, such as the Finite Element Method, making the whole process effective and efficient. However, some mechanical phenomena have always been difficult to model with the FEM, amongst which the study of cracks, stationary and especially its propagation. This particular study is necessary in order to predict the mechanical behavior of the equipment but also in order to increase its life expectancy. When considering welded materials, the problem is even more complex, as Maier et al. [1], showed that the fatigue crack propagations properties are influenced by the material microstructure of the heat affected zone (HAZ). Qiang et al. [2] also showed that the mixed mode fatigue crack propagation of welded HSS is not clear, as the welded material actually has a better fatigue behavior justified by the more favorable microstructure. This study intends to study the fatigue behavior of HSS welded specimens using a newly developed algorithm for fatigue crack propagation. This algorithm can use any type of Finite Element Method (FEM) model to automatically calculate the Stress Intensity Factor (SIF) on the crack front and use the Paris Law to predict the elapsed number of cycles for a constant crack increment. The crack propagation direction is predicted using the Maximum Tangential Stress criterion, using the same methodology as Ayatollahi et al. [3]. The algorithm also allows to use or determine the direct Paris Law material parameters, unlike the ABAQUS implementation of eXtended Finite Element Method (XFEM), which forces parameters conversions and is also a very computer intensive solution, with an advantage of not requiring the remeshing of the model, Singh et al. [4]. M ATERIAL AND METHODS (J OÃO ) Materials he high strength TMCP steel was supplied under the requirements of S700MC in EN 10149-2 specification [5]. Tab. 1 and 2 present the mechanical properties and chemical composition of the said steel, respectively. σ y (MPa) σ UTS (MPa) A(%) 785 867 13.4 Table 1: Mechanical properties of the tested S700MC steel. C (max %) Si (max %) Mn (max %) Al (min%) S (max %) P (max %) Nb* (max %) Ti* (max %) V* (max%) 0.12 0.25 2.10 0.015 0.010 0.020 0.09 0.15 0.20 *Sum of Nb, Ti and V ≤ 0.22 wt-%. Table 2: Chemical composition of the tested S700MC steel. Welding Two steel plates (1000 mm long, 200 mm width, 8 mm thick) were welded together by MAG welding (Kemppi Promig 530 automatized system). The electrode was a filler wire that presented a approx. the same composition; shield gas was Mison 8 (Ar+8%-CO2+0.03%-NO). The weld groove was V-shaped with a 50º angle and two weld passes were executed. The Interpass temperature for each weld was T room  23 ºC. An interpass time of at least 40 min was required for complete cooling. Fig. 1 displays a schematic representation of the joint design and of welding sequence. A T