PSI - Issue 7

F. Fomin et al. / Procedia Structural Integrity 7 (2017) 415–422 Fedor Fomin and Nikolai Kashaev / Structural Integrity Procedia 00 (2017) 000–000

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The results of the analysis are shown in Fig. 6(a). It can be seen that in the HCF range, the model shows satisfying agreement with the experimental S - N data. In general, the predicted lifetime is slightly underestimated; apparently, this is caused by the application of the simplified cyclic R-curve. Although the plain fatigue limit of coarse lamellar structure is lower; in the presence of porosity, the heat-treated joints exhibit superior fatigue properties compared to those of the as-welded conditions (Fig. 6(a)). Thus, a trade-off between the decreased smooth fatigue limit and increased fatigue threshold yielded a more favourable structure within the FZ in terms of HCF. The dashed lines in Fig. 6(a) represent the number of cycles that the crack is propagating within the ODA. In the region of HCF, up to 95% of the total life is spent in this region. Fig. 6(b) shows the predicted fatigue propagation behaviour of short cracks for different applied stresses. It can be seen that short cracks propagate faster than long ones at a given ∆ K , and they still grow in the region below ∆ K th,LC . At high SIF values, the growth rate of short cracks approaches the curve for long cracks. As shown in Fig. 6(b), when the applied stress is lower than some critical value, the crack can become arrested ( σ max = 470 MPa). This result is a sequence of the build-up of crack closure and the gradual crack retardation.

Fig. 6. (a) Predictions of the S-N curves of the laser beam welded Ti-6Al-4V butt joints in the as-welded machined and heat-treated machined conditions; (b) estimated FCP behaviour of physically short cracks. Although the simplified version of the cyclic R-curve was used in the analysis, fatigue limit can be derived from the condition of tangency of the crack driving force curve to the cyclic R-curve (Madia and Zerbst, 2016). As shown in Fig. 4, if the applied stress is low enough, then the crack driving force curve is tangential to the cyclic R-curve. In this case, the fatigue limit corresponds to the arrest of the initial crack by the gradual build-up of crack closure. This implies that arrested cracks are present at the notches in the laser beam welded seam even at the fatigue limit. To verify this interpretation of the fatigue limit, transverse cross sections of the specimens that endured 10 7 cycles were made, on which the areas in the vicinity of the pores were examined for the presence of cracks. Fig. (7) shows a transverse cross section of one of such specimens with the 45 µm-long crack that started from the pore. This result can be interpreted as additional evidence for the validity of the model; however, further investigations are required to prove whether these cracks are indeed the arrested ones. The estimated value of the fatigue limit for the as-welded and the heat-treated conditions were σ max = 471 and σ max = 503 MPa, respectively.

Fig. 7. Crack found at a pore in the specimen that endured 10 7 cycles. PWHT, � ��� = 550 MPa.