PSI - Issue 16

Leonid Lobanov et al. / Procedia Structural Integrity 16 (2019) 27–34 Leonid Lobanov, Nikolai Pashсhin / Structural Integrity Procedia 00 (2019) 000 – 000

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It can be seen in Fig. 7, where the results of fatigue tests are presented in coordinates 2  а – N of specimens in initial state and after EDT, that the cyclic fatigue life of N specimens of welded joints in the investigating range of 2  а is increased as a result of treatment of up to three times. Here, the fracture of both initial specimens and also treated ones occurs along the fusion line. It can be concluded on the basis of above-given results that EDT has a positive effect on increasing the fatigue life of welded joints of alloy AMg6, defined to a great extent by the reduction in level of the residual welding stresses. a b

Fig. 8. Fracture surfaces of specimens of alloy AMg6 after a low-cyclic loading, where df- size of facets: (a) initial specimen × 810; (b) after EDT × 810.

The fractographic analysis of a microrelief of surface of initial and treated specimens, fractured as a result of a low-cyclic loading (Fig. 8), showed that the treated metal is characterized by dispersing of structural elements, such as refining of size d f of facets by 3 times (Fig. 8b) as compared with initial state before EDT (Fig. 8a). This confirms the positive effect of electrodynamic effects on evolution of structure of the treated metal for improving its resistance to the fatigue fracture.

2.5. Results and discussions

The level of residual stresses determines the parameters of distortion of welded structures (Masubuchi (1980)). Investigations of effect of EDT on local deformations of “buckling” type, occurring in welding of load-carrying system in sheet hull structures, were carried out. Thus, the application of EDT in manufacture and repair of small-sized ships of alloy AMg6 allowed increasing the service characteristics of products. EDT-straightening of design elements gave an opportunity to improve significantly the hydrodynamic characteristics and habitability of hulls. Thus, straightening of lining buckling provides the decrease in local deflections in the zone of welded joints from 10 to 1.5 mm and the elimination of curvature of transverse beams of bottom reinforcement from 8 to 0.5 mm provided the guaranteed matching of bilge boards of sub-deck flooring. Moreover, the EDT of repair welded joints of lining and load-carrying system provided the decrease in residual stresses from 150 to 40 MPa, thus increasing the service life of hulls by up to 2 – 6 times. The design element of aircraft AN-74 is an intermediate casing of aircraft engine (ICAE), the purpose of which is the D-36 engine fastening to the wing. ICAE represents a large-sized hollow cast structure of magnesium alloy ML10, which consists of external and internal cylindrical shells, mated between themselves by stiffeners-supports, in internal cavities of which a cooling fluid is circulated. The typical damages of ICAE, which are removed by repair welding, are the fatigue cracks, violating the integrity of external and internal shells, as well as fractures on a facial surface of the external shell in the zone of reinforcement for flange of cooling pipeline. Here, the maximum tensile stresses in repair welds without heat treatment reach the values of 120 MPa, which corresponds to the yield strength of alloy ML10. Reduction in stresses with applying heat treatment is a rather expensive operation, which in some cases exceeding the cost of repair welding. The application of EDT allows changing the distribution of residual