Issue 37

L. Susmel et alii, Frattura ed Integrità Strutturale, 37 (2016) 207-214; DOI: 10.3221/IGF-ESIS.37.27 211   8.10 3.4 k eff eff     (7)   9.38 2.22 eff eff fRe     MPa (8) As shown by the SN curves summarised in Tab. 1, the fatigue strength of these FS welded joints was seen to be sensitive to presence of non-zero mean stresses, and this holds true even though the specimens were tested in the as-welded condition. According to this experimental evidence, the mean stress sensitivity index, m, was taken equal to unity, with  lim being set equal to 1.3. B R R  N. of data k  A  A T  [°] [MPa] [MPa]  -1 - 9 6.5 33.5 - 1.58  0.1 - 10 4.4 18.6 - 1.82 0 -1 - 11 10.8 - 38.9 1.49 0 0 - 10 9.5 - 32.9 1.52 3 -1 0 8 5.3 26.1 15.1 1.55 3 0 0 7 4.2 17.2 9.9 1.73 3 -1 90 10 5.3 21.1 12.2 2.97 3 0 90 7 10.4 23.4 13.5 1.38 1 -1 0 7 5.4 23.2 23.2 2.12 1 0 0 7 3.2 12.8 12.8 2.00 1 -1 90 9 3.9 11.3 11.3 1.66 1 0 90 7 15.8 22.6 22.6 1.35 Table 1 : Summary of the generated experimental results. The experimental, N f , vs. estimated, N f,e , number of cycles to failure chart shown in Figure 4a summarises the overall accuracy which was obtained by using the MWCM in terms of nominal stresses to predict the lifetime of the FS welded tubular samples being tested. This graph makes clear that the use of the MWCM resulted in life estimates falling within the wider scatter band between the two that characterise the fully-reversed uniaxial and torsional fatigue curves used to calibrate the constants in the MWCM’s governing equations. Subsequently, the MWCM was applied in terms of notch stresses [14, 16]. The average notch root radius both on the retreating and the advancing side was measured to approach 0.5 mm (Fig. 2c). The required notch stresses were determined by solving axisymmetric linear-elastic finite element (FE) models done using commercial software ANSYS©. The stress analysis returned the following values for the gross stress concentration factors: K t,ax =2.4 (axial stress), K t,hs =0.48 (hoop stress), K t,t =1.7 (torsional stress). The fully-reversed uniaxial and torsional fatigue curves were used in terms of notch stresses to determine the constants in the MWCM’s calibration function, obtaining:   8.10 3.4 k eff eff     (9)   9.64 9.24 eff eff fRe     MPa (10) The uniaxial fatigue curve for a load ratio, R, equal to 0.1 was used to estimate both the mean stress sensitivity index and the limit value for  eff (i.e., m=1 and  lim =2). The error chart of Figure 4b confirms that the MWCM applied in terms of