PSI - Issue 18

12

Benjamin Möller et al. / Procedia Structural Integrity 18 (2019) 556–569 Author name / Structural Integrity Procedia 00 (2019) 000–000

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In Fig. 13, the fatigue test results under combined CAL and corrosive environment are compared to the results under CAL in ambient atmosphere (air). The Wöhler curve with respect to the corrosive impact is characterised by the knee point of F a,k ( N k = 2.5ꞏ10 6 ) = 0.81 kN, a slope k = 3.7 and a scatter T F = 1:1.21. The run-outs of this test series indicated that there might also be a knee point for the Wöhler curve under corrosion fatigue. However, this is not statistically proved. In the long life fatigue regime, a fatigue strength reduction of approximately 20 % is the consequence. At 2ꞏ10 6 cycles to failure, the fatigue strength in air of F a ( N k = 2ꞏ10 6 ) = 1.03 kN is decreased by17 % to F a ( N k = 2ꞏ10 6 ) = 0.86 kN under the corrosive environment. On the other hand, at F a = 2.5 kN, the fatigue life is reduced by about 50 %.

Fig. 14. Fatigue test results under CAL and corrosive environment as well as evaluated Wöhler curves of laser beam welded adapters V2

Compared to the Wöhler curve of lap joints under corrosion fatigue, a straight line Wöhler curve without a knee point is found for corrosion fatigue of laser beam welded adapters V2, see Fig. 14. The corrosion fatigue Wöhler curve is characterised by a force amplitude of F a ( N f = 2ꞏ10 6 ) = 3.21 kN, a slope k = 3.0 and a scatter T F = 1:1.33. Again, the fatigue test results are compared to the results under CAL in ambient atmosphere. At 2ꞏ10 6 cycles to failure and, hence, in the high cycle fatigue regime, the fatigue strength in air of F a ( N k = 2ꞏ10 6 ) = 3.63 kN is decreased by approximately 12 % under the corrosive environment. The corrosive environment does not have a major influence on the fatigue failure location. Fig. 15 shows that cracks start in most of the tests from the root of the first weld seam (with reference to the steel sheet) and propagate through the aluminium sheet, as was the case in air. Exceptions are specimen A05_05 (Fig. 15a), where the failure also occurs in the steel sheet, and A09_05 (Fig. 15b), where the rupture can be found in the aluminium sheet outside the welded section.

Fig. 15. Typical fatigue failure of laser beam welded adapters V2 under corrosion fatigue