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Articles

Locations of strain gauges for fatigue analysis of welded joints (2)

Pages 655-664 | Published online: 27 Oct 2011
 

Notes

1. Formerly: Sumitomo light Metal Ind. Ltd.

2. Expression of the design hotspot SN curve is the same as for nominal stress. Since, in all standards, it is lowered by two standard deviations only from the mean SN curve found from the deviations in experimental points, it is linear on both the logarithmic scales and the geometric stress range Δσ ( = σs.max − σs.min), a major factor influencing fatigue life, is expressed as follows:

Here, N is the number of iterations, K s is a constant, and m is the inverse gradient of the SN curve.

The constant amplitude fatigue limit is N = 5 × 106 and when this is subjected to variable amplitude, it ranges from N = 5 × 106 to m 2 = m 1+2, with a cut-off at N = 108. If K s is found from a fatigue classification FAT (N/mm2) of N = 2 × 106 and m 1, it is possible to calculate Δσs for the desired number of iterations N.

3. In this paper14, the positions of the strain gauges in Figure (b) are 0.4, 0.5, and 0.8t.

4. When geometric stress is found by FEM, the calculations usually assume an ideal welded joint, with no geometric irregularities or other misalignments, but it is necessary to incorporate those geometric irregularities that may be encountered. As for the components, the stress and stress gradient must be found at locations corresponding to the extrapolation points used for strain gauge measurements with adequate precision near the hotspots.

A short description of these is given as ‘Calculation of geometric stress’ in Section 2.2.3.2 of the IIW recommendations1, and Ref. 4, by Niemi, is recommended as a guide to these.

5. Since in a flat sheet welded structure for maritime use, the welded structural joints used in pipe assembly show little variation in SCF associated with the changes in the dimensions of the structure, there is some doubt as to whether the method of defining hotspot stress is logical. At the 202nd Conference of the Japan Shipbuilding Research Association17, there was a report of a study in which large experimental fatigue model specimens of 30 mm bracket doubler pad and stiffener assembled from TMCP steel sheet with main sheet thickness of T = 15 mm and with a yield stress of 36 kgf/mm2 for the studies in which the first and second strain gauges were used at six different combinations of locations at differing distances from the weld toe.

Since the results, including the gradient of the hotspot SN curve, showed a mean − 2σ− − : standard deviation) line, which is closest to the T diagram of the UK DEn (in which the sheet thickness and toe angle are corrected), the following flat sheet welded structure is proposed. In this, the positions of the strain gauges are such that the first is 1.57√T 3 and the second is 4.9√T 3 from the weld toe and the hotspot SN curve is as follows:

Here, Δσ: geometric stress range (kgf/mm2) N f, number of iterations of fracture

When a Δσ of N = 2 × 106 is found from Equation (Equation6), it has an average of 106 N/mm2 and − 2σ−  is 78 N/mm2, findings which are similar to the values in Table (EC 3) and Table (IIW).

6. The characteristic curve (or characteristic line), which is shown in Figure , is usually very similar to two standard deviations below the mean SN line. This is based on a probability of survival of 95% calculated on the basis of a mean figure found from an interval estimate with a 75% confidence level. See Section 3.7 ‘How to find fatigue resistance by testing’ in Ref. 1.

7. For tubular pipe welded joints, European standard EC 32 has specifications for butt, K, and N joints of tubular pipes and square pipes, and there are also IIW specifications1 of fatigue classification by nominal stress for butt joints. Here, I describe only the tendency for parametric equations.

8. When the brace wall thickness is t 1 < 8 mm, it has a butt weld with leg lengths of t 1 √2, and when t l>8 mm, it has a 45°single bevel groove and a root gap of 2 mm. When β = 1.0, the bead is ground flat so that the brace has a single bevel groove and the corner of the brace coincides with the chord surface. According to the European standard 25–72, steel pipes are to be Fe430D and Fe430B hot-rolled square pipes and welding is to be performed using rutile electrodes (ISO 2560), with three passes for fillet welding and four passes for butt welding. For the symbols used, see Table .

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