Mechanical Characteristics Of The Hydraulic Forming Reinforced S-shaped Bellows

Reinforced S-shaped bellows is new kind of displacement compensation structure with high pressure.Compared with the bellows with the same specification parameters,they have been gradually favord by the aerospace and nuclear industry in the field of extreme high pressure working enviorment in recent years.To enhance the compensating ability of S-type bellows under extreme high pressure and large displacement,mechanical analysis involves many problems,such as material elastoplasticity,geometric nonlinearity and boundary condition nonlinearity.At present,the analysis and design methods for this type of structure are relatively scarce,and a systematic understanding has not been formed.Hydraulic forming technology is a typical processing method for reinforced S-shaped bellows.Hydraulic forming is a highly plastic yield deformation process.Most of the bellows are in a highly plastic yield state.The structural geometric parameters and material mechanical properties change in varying degrees,which directly affects the mechanical properties of the structure.The hydroforming of bellows mainly relies on the axial contraction of tubular product and the thinning of wall thickness,belonging to the category of axial compression and bulging.The hydroforming process is a severe plastic deformation process.Most area of the bellows is in the severe plastic yielding state,and changes can be found in mechanical properties of materials and structural thickness distribution,which directly affects the compressive strength and the swinging rigidity of bellows.The hydroforming process of reinforced S-shaped bellows was studied in the present study,while the variation mechanism of structure mechanical behavior is discussed.The principal purpose of this dissertation is to give a systemic and reliably analysis method.The main themes and contributions of the dissertation are stated as follows:1.A mathematical description method of the reinforced S-shaped bellows was introduced for the bellows and reinforced hoop based on the shell and beam theory.It can be used to the structural design and parametric analysis of the reinforced S-shaped bellows.2.Pressure bearing capability,axial tension,axial compression and bending characteristics of the reinforced S-shaped bellows were analyzed based on the theoretical and numerical simulation analysis.A systematic nonlinear mechanical analysis and evaluation method was established.3.Numerical simulation technology of hydroforming process was studied with regard to the reinforced S-shaped bellows to have access to the changes in the structural parameter through comparison with practical products.The hydroforming technology is a severe plastic deformation process.Most area of the bellows is in the state of highly plastic yielding.The control of hydraulic pressure is one of the key factors.According to the design parameters of the bellows and in combination with the numerical simulation of the forming process,the reinforced S-shaped bellows hydroforming process is accurately controlled.At the same time,the key factors that influence the residual plastic strain and wall thickness thinning are revealed.4.Material property and microstructure evolution of stainless steel were studied under the effect of hydroforming process.Based on the test and analysis results of different pre-strain materials,the mechanism of mechanical properties of materials is revealed.According to the relationship between deformation strengthening and martensite transformation in hydroforming process,the bilinear relationship between mechanical properties and pre-strain is established.5.The relationship between fatigue limit and pre-strain in hydroforming process is also bilinear.The essence of raising fatigue limit of pre-strained materials lies in the comprehensive effect of material transformation and plastic strengthening.It leads to the dislocation density in transition and non-transition increase significantly and hinders the further movement of dislocations.The reversible plastic damage is reduced and the fatigue limit of the material is increased.6.A fatigue life evaluation method,which considers the non-uniformity parameters,was introduced based on the modified Manson-coffin method.The non-uniformity parameter bellows fatigue life evaluation method reflects the structural mechanical characteristics more truly,and the fatigue life obtained by the analysis is slightly lower than that of the ideal structure analysis results.