PSI - Issue 37

Aleksandar Sedmak et al. / Procedia Structural Integrity 37 (2022) 263–268 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2.2. Tensile testing Tensile properties are tested in longitudinal and transvers direction according to standard EN 10002-1, [20], Tab. 3. In addition, specimens were taken out also from the perpendicular direction, to test contraction relevant for lamellar tearing. Contrary to the results obtained for standard testing, in this case contraction Z was only 2-15%, far below minimal values according to standard EN 10164 [21].

Table 3. Tensile properties of steel St. 3, standard testing in longitudinal and tranverse direction Direction R p0,2 [N/mm 2 ] R m [N/mm 2 ] A 5,65 [  ] Z [%] Longitudinal - surface 247 418 40.4 60.0 Longitudinal - midsection 222 400 35.7 60.9 Transverse - surface 251 414 32.2 60.9 Transverse - midsection 221 402 32.2 50.0

2.3. Fatigue crack growth testing ess analysis by Finite Element Method (FEM) Fatigue crack growth rate was determined according to Paris law, eq. 1, and standard ASTM E647 15e-1, [22], in perpendicular direction, being obviously critical one. Results are shown in Tab. 4, indicating high values of crack growth rate for amplitude loading corresponding to Δ K=10 MPa∙ √ m.

dN da =  

(1)

( ) m C K

Table 4. Fatigue crack growth parameters C and m, according to Paris law, in perpendicular direction Specimen ΔK th [ MPa∙ √ m] C m da/dN [m/cyc] 1 4.2 8.49∙10 -6 3.23 1.44∙10 -8 2 4.1 1.41∙10 -7 3.97 1.29∙10 -8 3 4.2 8.12∙10 -6 3.32 1.70∙10 -8 4 4.4 4.28∙10 -6 3.78 2.58∙10 -8

3. Structural integrity assessment of turbine cover Von Mises stress distribution, as obtained by the FEM, is shown in Fig. 5 for the regular regime and in Fig. 6 for the irregular one, indicating the value and location of the maximum stress. Having in mind locations of detected lamellar tearings, Figs. 3 and 4, relevant stress values are 45 MPa (contour 3, Fig. 5) and 70 MPa (contour 4, Fig. 6). Critical crack length values can now be calculated, if fracture toughness is taken as the minimum values, K Ic =30 MPa∙ √ m, [21], for the case of an edge crack, using common linear elastic fracture mechanics equation: (2) where Y=1.12. For regular regime one gets a c =354 mm, for irregular regime ac=146 mm. Since the maximum length of detected cracks is 25 mm, there is no danger of brittle fracture. In respect to fatigue crack growth, one should notice that the amplitude loading for high cycle fatigue in the regular regime is only 1.1-2.25 MPa, and 2.35-3.5 MPa in the irregular one, [23 ], producing ΔK far below values worth further analysis. In the case of low cycle fatigue, amplitude loading is 16.9 MPa, [23], producing ΔK≈5 MPa∙√m, still below testing value (10 MPa∙√m), and certainly not affecting the life of cover, since it happens only 2000 per year. a c =(K Ic /Yσ max ) 2