5 Fatigue Assessment of Porosity in Electron Beam Melted Ti-6Al-4V 39 Fig. 5.3 Feature identification 5.2 Results and Discussion An initial slice was identified for feature identification of the voids and can be seen in Fig. 5.3. The red circles in Fig. 5.3 indicate all the defects found in the image. The blue dot in the center of the image represents the center of the specimen. A border is also placed around the diameter of the specimen which is indicated by a red ring. Although it is best not to alter images to skew digital data, at times, brightness and contrast must be altered to obtain the data needed. However, the Feret diameter can be increased and decreased because of this. By taking the data from each slice and manipulating the data in MATLAB, the porosity within the entire specimen can be obtained. Figure 5.4 shows the result of a mapping of the porosity radially normal to the z-axis. Figure 5.4 is porosity relative to edge distance in a polar coordinate system from the z-plane of a single specimen. Data of the average porosity size found from ImageJ in Table 5.1 for all as-built specimens. Tammas-Williams proposed multiple rankings for stress intensity factors; a ranking of interest was selected from the multiple rankings. The failure location was estimated by using a simplistic version of the equation which includes the nonuniform stress due to the porosity position(σx(position)), the stress concentration in relation to the surface distance (Kt (surface)), the pore aspect ratio (Kt (AR)), proximity to other pores (Kt (proximity)), and the area of a defect normal to the applied stress ( 4√An) [4]. The full equation can be seen in Equation 5.1. KI(relative) =σx(position)Kt (surf ace)Kt (AR)Kt (proximity) 4 An (5.1) It is important to note that all the defects are assumed as round and spherical by shape; none are oblate. Because of this assumption, it can be shown by Biswal the aspect ratio will be 1.0 [5]. Therefore, the reduced equation is written as Equation 5.2 in the following. KI(relative) =Kt (surf ace)Kt (proximity) 4 An (5.2) There are two other methods proposed and evaluated in this study. Equation 5.3 uses portions of the previous values from Equation 5.1 but additionally includes the length of the pore in the z-direction (Kt (length)) and the radius (r) of the pore. Therefore, Equation 5.3 considers the extra dimension within the defect damage criterion. KI(relative) =Kt (surf ace)Kt (proximity)Kt (length)r (5.3)
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