Research Of Hole Electromagnetic Strengthening Method Based On Two-stage Coils And Radially Driven

The statistical analysis suggested that fatigue of fastener holes has become the main factor of restricting the improvement of fatigue life of aircraft. Although cold expansion process can improve the fatigue life of holes in varying degrees, but there are some inherent problems of the process that the residual stress distribution through the thickness is nonuniform, the hole wall is easy to be damaged when cold expansion, end faces of cold expanded hole are uneven, and it is difficult to expand the special holes. These problems have leaded that it is difficult to further improve the whole fatigue life of aircraft. The hole electromagnetic strengthening method has the characteristics of non-contact, body load, flexible control, which has the potential of breaking the limit of cold expansion process. But the existing hole electromagnetic strengthening schemes are difficult to be realized in engineering, so it is still unable to reach the potential of the hole electromagnetic strengthening method until now.This paper proposed a new hole electromagnetic strengthening method based on two-stage coils and radially driven. In the new method, the two-stage coils are used together to generate radial pulsed electromagnetic force in the part with hole, then the force causes the hole’s edge to yield plastically and deform in a very short time, finally a residual compressive stress layer is introduced in the hole’s edge, which will improve the fatigue life of hole.First of all, this paper discussed in detail the basic principle and implementation of method of electromagnetic strengthening of hole based on two-stage coils and radially driven. Then the optimization method of hole electromagnetic strengthening system in space and time was presented. Based on the optimized results, a set of electromagnetic strengthening prototype system with double power supplies and two-stage coils was builded. The system’s coils have long service life and high structure strength, and delay dispersion of switch and time sequence trigger system is very small.Secondly, based on the theoretical model of cold expansion method, a theory model of hole electromagnetic strengthening method was builded based on the assumption of plane stress, the theory model can consider the effect of radial electromagnetic force distribution on the residual stress in the hole. This paper also builded two-dimensional and three-dimensional finite element models of the hole electromagnetic strengthening method, the models can consider the circuit, electromagnetic field, structure field, temperature field, which tightly coupled together in the hole electromagnetic strengthening process. By uni-lateralization of couple between the electromagnetic field and structure field, ignoring the temperature effect, and staticizing the dynamic process, the finite element model was simplified. The results showed that the model after simplified had improved the calculation efficiency and reduced the calculation time significantly. The effect of discharge parameters, electromagnetic force distribution, hole’s shape and position, and material mechanical and electromagnetic properties on the residual stress of hole was analyzed by the finite element models.In addition, from the aspects of the fatigue life, fracture morphology, crack, metallography and micro-hardness, the fatigue properties of electromagnetic strengthened holes were studied in detail. The paper also studied the effects of the re-strengthening hole, strengthening pre-fatigue hole, and strengthening hole before drilled on the fatigue life.Finally, electromagnetic strengthening of the thin sheet with hole, micro and small hole, and special-shaped hole were researched in detail.Theoretical and experimental results showed that the hole electromagnetic strengthening method based on two-stage coils and radially driven has a lot of advantages, such as high the fatigue life gain, perfect uniformity of residual stress, good hole wall surface integrity, insensitive to hole shape and hole size. The hole electromagnetic strengthening method presented is especially suitable for strengthening thin sheet with hole, micro-hole, and special-shaped hole.