13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- In the present work, mixed mode (I+II) FCG experiments were performed on an in-situ stage equipped in SEM by using CTS (Compact-Tension-Shear) specimens developed by Richard [18]. The effect of external loading angle on the crack growth rate and crack growth direction was analyzed in terms of local microstructure gradient in the HAZ. The crack growth direction was finally predicted on the basis of the existing models. 2. Materials and experiments 2.1. Materials and specimens The material studied was the HAZ of a welded joint. The base material of the welding joint was a NiCrMoV type steel. It was undergone quenching and tempering heat treatment, and the resultant tensile strength and yield strength were 835.5 and 938 MPa, respectively. The weldment was formed by submerged arc welding (SAW) technique using filler metal of the 2.5%Ni low alloy steel. The HAZ, which had a width of about 3 mm, included three micro-zones, i.e., fully quenched-tempered zone (FQTZ), partially quenched-tempered zone (PQTZ) and tempered zone (TZ) [19]. The microstructures mainly included tempered martensites and bainites with martensite/bainite constituents altered in the HAZ. In order to prepare CTS specimen for in-situ observation, standard three-point bending samples with a/W ratio of 0.5 and the thickness B of 16 mm were firstly machined from the welded joint. The initial notch in the sample was located in the FQTZ of the HAZ, with a distance of 120 μm to the fusion line. The samples were then pre-cracked under pure mode I loading at a stress ratio of 0.1 with a sinusoidal waveform to form a crack length of about 2 mm. Subsequently, relatively smaller specimens with a thickness of about 0.5 mm were prepared for in-situ tests. Fig. 1 shows the shape and dimensions of the CTS specimens. Figure 1. The shape and dimensions of the CTS specimens (a) and the loading device for mixed mode tests (b) 2.2. Fatigue experiments Before the in-situ fatigue experiments, the CTS specimens were grounded, polished and finally
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