Issue 36

I. Camagic et alii, Frattura ed Integrità Strutturale, 36 (2016) 1-7; DOI: 10.3221/IGF-ESIS.36.01 7 The largest values of fatigue threshold,  K th , are obtianed in WM, from 6.8 MPa  m (20  C) to 5.6 MPa  m (540  C), Tab. 7. Fatigue crack growth rate, da/dN, increases with temperature, being in the range 5.09  10 -9 m/cycle for new PM (20  C) to 1.33  10 -7 m/cycle (540  C), Tab. 5. Exploition period additionally increases fatigue crack growth rate, da/dN, from 2.70  10 -8 m/cycle (20  C) to 1.03  10 -6 m/cycle (540  C), Tab. 6. The same holds for HAZ, where values of fatigue crack growth rate, da/dN, are in the range 7.68  10 -9 m/cycle (20  C) to 4.04  10 -7 m/cycle (540  C) for new material, i.e. in the range 7.09  10 -8 m/cycle (20  C), to 1.14  10 -6 m/cycle (540  C) for exploited material, Tabs. 8 and 9, respectivley. One should notice significantly higher values for fatigue crack growth rate in HAZ as compared to PM. The values for WM are in between, in the range 7.72  10 -9 m/cycle (20  C) to 4.32  10 -7 m/cycle (540  C), Tab. 7. C ONCLUSION ased on the presented results, one can conclude the following:  Influence of material heterogeneity, as well as temperature and exploation effects, on fatigue threshold, da/dN, and crack growth rate, da/dN, is significant.  Fatigue threshold values are the lowest for WM, and lowest for HAZ, whereas crack growth rate values are highest for HAZ and lowest for PM. Therefore, generally speaking, the lowest fatigue crack resistance is in HAZ.  Higher temperature and longer exploitation peroids increase crack growth rates and decreases fatigue thresholds for both new and exploited materials in all regions of welded joint (PM, WM, HAZ). These effects are due to microstructural changes such as carbide formation and growth at grain boundaries and inside grains. R EFERENCES [1] Čamagić, I., Investigation of the effects of exploitation conditions on the structural life and integrity assessment of pressure vessels for high temperatures (in Serbian), doctoral thesis, University of Pristina, Faculty of Technical Sciences with the seat in Kosovska Mitrovica, (2013). [2] Čamagić, I., Vasić, N., Jović, S., Burzić, Z., Sedmak, A., Influence of temperature and exploitation time on tensile properties and microstructure of specific welded joint zones, In: 5 th International Congress of Serbian Society of Mechanics Arandjelovac, Serbia, (2015). [3] ASTM E399-89, Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials, Annual Book of ASTM Standards, 03.01 (1986) 522. [4] BS 7448-Part 1, Fracture mechanics toughness tests-Method for determination of K Ic critical CTOD and critical J values of metallic materials, BSI, (1991). B