PSI - Issue 19

Yukio Miyashita et al. / Procedia Structural Integrity 19 (2019) 604–609 Author name / Structural Integrity Procedia 00 (2019) 000–000

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According to results shown in above, a weld defect was observed as fracture origin in all weld specimens. Fracture mechanics approach might be effective to evaluate fatigue strength of weld part in non-combustible magnesium alloy. Therefore, a value of threshold stress intensity factor is important but possibly changes depending on welding condition. Welds were prepared with different TIG welding conditions in laboratory of the authors. Welding speed with 200 mm/min and flow rate of Ar shield gas was 10 l/min. Welding configuration, weld rod used and welding direction were the same with the above. Range of weld current applied was within 100A-130A in heat input; 726- 858 J/mm). This welding condition was obtained in pre-welding test and was applicable to obtain sound weld in stably. These appropriate welding conditions were the similar with that for SA and SB. Weld specimens obtained with 110 A (called T110A) and with 130 A (called T130A) were used for fatigue crack growth test. According to microstructure observation, average grain size at the weld region was 13.6 μ m in T110A and 13.0 μ m in T130A, namely was larger than average grain size of the base material. Weld defect was also observed in the weld region in T110A and T130A. Relationship between stress intensity factor range and fatigue crack growth rate is shown in Fig.7(a). Crack growth resistances for weld specimens of T110A and T130A were higher than that of the base materials. This trend is the similar with SA and SB shown in Fig.5(a). Value of threshold stress intensity factor range, Δ K th are 1.33 MPam 1/2 and 1.50 MPam 1/2 in T110A and T130A, respectively. This result means that Δ K th in weld obtained with higher heat input (T130A) is higher compared to weld obtained with lower heat input (T110A). Figure 7(b) shows fatigue crack growth curves arranged by effective stress intensity factor range, K eff . According to the figure, fatigue crack growth curves for welds and the base materials almost coincide and the similar value of effective threshold stress intensity factor range, K eff, th was obtained around 0.5 MPam 1/2 . This results suggests that deference in fatigue crack growth resistance was mainly caused by deference in crack closure behavior. Mechanical property at weld part changed depending on welding condition and resulted in that threshold stress intensity factor range, K th increases apparently with increase in heat input of welding process. Therefore, dependency of change in threshold stress intensity factor range, K th to heat input should be taken into account for fatigue design of weld part in non-combustible magnesium alloy.

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Fig.6 Relationship between stress intensity factor range, Δ K and number of cycles to failure, N f in SA and SB.