Issue 36

M. Arsic et alii, Frattura ed Integrità Strutturale, 36 (2016) 27-35; DOI: 10.3221/IGF-ESIS.36.03 29  cracks with length which is comparable with dimensions of microstructural grain size,  cracks which dimensions are proportional to local plastic zone, i.e. short crack with dimensions from 0.1 up to 1.0 mm for structural steels [2]. I NFLUENCE OF STRESS CONCENTRATION ON FATIGUE STRENGTH OF WELDED JOINTS elded joints in real constructions are areas of high stress concentration level, especially in the places with imperfections, such as incomplete root penetration, undercuts or cracks. Ratio of the maximal main stress σ max , which is established in the minimal cross-section area and nominal stress σ represents stress concentration factor k t :    max t k (1) Factor k t for the same element shape depends on loading type and it is maximal in the case of tension, somewhat smaller in the case of bending, and minimal in the case of torsion. In the case of variable load, notch reduces fatigue strength in all metals. Nevertheless, reduction of fatigue strength is not as significant as influence of k t , which is thus not used for variable loads, but only for static loads. Fatigue notch factor k f is used to characterize influence of notch to fatigue strength, and it represents the ratio of smooth specimen fatigue strength S f and notched specimen fatigue strength S z .  f f z S k S (2) Factor k f is determined by experiment. Its magnitude, for uniaxial stress state, depends on welded joint shape and notch dimensions, material, part dimensions and magnitude of variable load. Relation between k t and k f factors is defined by stress concentration sensitivity factor q, which is given by the expression: q k k f t    1 1 (3) Steel sensitivity to stress concentration is increased with the increase of tensile strength, yield stress and hardness. Stress concentration factors are minimal for butt welds, depending on plate thickness, joint shape and welding procedure, k f = 1.1 – 3.0, for fillet welds k f = 2 - 8, and for overlapping joints k f = 2 – 7. Significant fluctuation of notch factor for one type of welded joint, which is predetermined by the joint shape and joint imperfections, sometimes lead to rapid decrease of fatigue strength. Yield stress R eH [MPa] Tensile strength R m [MPa] E longation A5 [%] Base metal S355J2+N 368 512 26 Electrode "Garant" E 43 4B 110 20 (H) 480 550 35 Table 1 : Mechanical properties of base metal and electrode. F ATIGUE EXAMINATION OF K- GROOVED WELDED JOINTS rogram of experimental examinations of characteristic welded joints is based on research plan aimed to establish relationship between fatigue strength of smooth specimens and notched specimens, as well as to investigate mechanism of crack appearance and crack propagation in welded joints. W P