Sealing Analysis For Bolted Joint Systems Without Gaskets Based On Multi-scale Rough Contact Mechannics

Since mechanical equipments are expected to function under harsher conditions,roughness between contact interfaces plays a more and more vital role in the mechanical performance and working condition of these mechanical equipments(e.g.,sealing performance of bolted joint systems).However,rough contact analysis without adhesion is still a challenging problem,mainly owing to the multiscale and self-fractal characteristics of rough surfaces.The aim of this paper is,on a theoretical side,to propose a model that can quantitatively estimate the contact status of rough surfaces.On an engineering application side,the sealing status(or leakage prediction)of bolted joint system without gaskets is analyzed,and its lightweight optimal design is formulated.Up to now,theories for analyzing contact behavior between rough surfaces in literature can be generally categorized into two groups:the asperity-based Hertz contact models initiated by Greenwood and Williamson(G-W model),which is shown more accurate under small indentation distance,and the magnification-based pressure diffusion theory initiated by Persson(Persson model),which is shown to work well under full contact conditions.The aim of this paper is to propose a theoretical model that can effectively formulate the contact status of rough surfaces during the entire compression process.This is achieved by integrating the idea of magnification,or evolving resolution(proposed by Persson)into an asperity representation of rough surfaces(given by Greenwood-and Williamson),and a magnification-based multi-asperity model(MBMA model)is thus established where the multi-scale property of rough surfaces and the mutual interaction between asperities are well taken into account.In the derived model,the originally complex contact problem is decomposed into a family of sub-problems each defined on a morphologically simpler contact islands,and a number of explicit formulations from existing multi-asperity models can thus be used.The present model not only effectively reproduces the results of G-W models for short indentation distance,but is also shown that when indentation is sufficiently deep,the present model smoothly transits to the regime of elastic interaction between smooth surfaces governed by Hookean-law formulation.Compared to other G-W type models,the proposed method demonstrates its strength in the computation of the contact area.Moreover,about the relationship between the applied load and the average interfacial separation,the G-W and Persson models are found well connected by the proposed method.For its validation,the proposed model is well compared with existing molecular dynamics simulation and experimental results.Bolted joint systems without gaskets have generated many rough interface problems due to their full-face metal-to-metal contact characteristics,e.g.,sealing performance.It is found that most of the widely used standards(e.g.,ASME,Eurocode)for joint systems of this type are not compiled based on its sealing performance,and the current leakage prediction method is also not suitable for the multiple bolts systems.Under this circumstance,the present paper aims at introducing the above rough contact theories into the sealing analysis of bolted joint system without gaskets to solve its lightweight optimal design and leakage prediction problems.Firstly,two issues(i.e.,the relatively large number of mutually dependent geometric design-parameters and the complicated contact details between members)limiting the research of the sealing performance of bolted joint systems without gaskets are summarized.Then,bothABAQUS finite element(FE)method and theoretical model of bolted joint systems without gaskets are constructed to properly address the above two issues.After a detailed examination of the correlations between design parameters,the total number of free design variables is reduced to three:member thickness,bolt spacing and bolt specification,respectively.By systematically analyzing the contact status between the members,both the influencing factors on the pressure distribution and the size of the contact zone are quantitatively identified.Finally,on the basis of the above research,a lightweight optimal design model for the joint systems is derived and numerically implemented-in the present paper.Then,a more proper leakage prediction of bolted flange joints without gaskets is proposed based on rough contact percolation theory.The reasonable agreement between the theoretical predictions and three-dimensional FE analysis results verifies the validity and usefulness of the proposed theoretical model and the optimization design method in this paper.