Fatigue performance analysis of structures with casting defects

With the continuous development and progress of casting technology, the casting structure under the new technical conditions has many advantages such as high strength, high toughness and high wear resistance. Aluminum alloy castings are widely used in complex structures such as helicopter main reducer and engine shell. While casting technology brings great convenience to engineering manufacturing, casting defects bring great challenges to the use of structures. The solidification process of the casting usually shows paste solidification characteristics, and the eutectic solidification temperature range is wide, the solidification time is long, the number of eutectic groups is large, the rigidity of the mold wall is insufficient, and the expansion pressure of the casting is large during the solidification process. The expansion force easily leads to the deformation of the mold wall and the expansion displacement. In the solidification process, the early solidified solid cast iron is easy to divide the alloy liquid into many small closed areas, which will lead to the formation of a discontinuous molten pool in the casting where solid and liquid coexist. In addition, the structure of the casting is mostly complex, and the liquid metal forms a large temperature gradient in the solidification process. Casting defects often occur in castings due to the combined effects of various factors.

Casting defects often exist in the areas where the liquid metal is difficult to be fed and the internal final solidification areas in the complex casting structure. Only through advanced flaw detection technology can they be found, and it is difficult to repair. Casting defects are usually irregular in shape, uneven in structure, containing many large and small discontinuous shrinkage cavities, loose in material structure in local areas, and occupy a large proportion in the surface of defective castings (i.e., defect rate). The influence of casting defects on the mechanical properties of castings is mainly manifested in the uneven stress distribution and stress concentration around casting defects, and the poor crack resistance of casting defects shortens the nucleation life of cracks, which is one of the main threats to the fatigue resistance of structures.

The machining process of complex structures such as helicopter main reducer embodies very high labor value. If the impact of casting defects on the mechanical properties of the structure can not be accurately evaluated and the service life can not be predicted, the use safety of the equipment will be affected; On the contrary, if the assessment is too conservative, it will cause huge economic losses. Therefore, studying the influence of casting defects on the mechanical properties of structures and forming accurate judgment basis can not only ensure the safety and reliability of casting structure design, but also ensure the rational and effective utilization of resources.

Based on the shape, size, location, defect type and other characteristic parameters of casting defects, the influence of casting defects on the stress distribution characteristics of structures is explored, and further combined with the classical Paris erdogon formula, based on the stress intensity factor, the fatigue life prediction formula of structures with casting defects is deduced, which provides a reference for the evaluation of fatigue performance of commonly used casting structures.

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