Fatigue Enhancing Theory And Simulation Study Of Interference Fitting In Light-weight Aircraft Structures

Lightweight and long service life required by aircraft structure accelerates the usage oflightweight materials, such as titanium alloy and composite materials, and development ofmechanical connection and strengthening technology, as well. Nevertheless, the strengtheningmechanism of interference fit is insufficiently recognized, and the compound strengtheningmethod of hole expansion plus interference fit deserves further study. Combined with theneeds of such projects as Ideal Interference Fit Technology, Titanium Alloy Interference Fit,and Small Edge-distance Hole Strengthening of Wing-Fuselage Connection, this thesiscenters on studying the strengthening mechanism of interference fit and puts forward thetheoretical basis of compound strengthening, which guides the application of relatedtechnology.The load transfer of stretched and compressed spring structure and elastically stretched andcompressed object is pre-analyzed, proving that the introduction of compressed componentchanges the transfer load amplitude of stretched parts. First, partition the pin and holed plateaccording to stress features, replace each partition with springs of approximately samestiffness or rigid bodies, then connect each spring properly and recombine them, and thereby aspring model for a pin interference fitted plate under tension loading can be established byreferring to the methodology of spring structure. Then, the relationship between the amplitudeof force transferred through the smallest corss section and that of external load is obtainined.It’s found that the maximum deviation is not more than20%by comparring the spring modelwith plentiful finite element (FE) analysis. The spring model reveals the strengtheningmechanism of interference fit and draws the following conclusions:(1) If the pin contacts tightly to the hole during loading, the reduction coefficient (the ratio oftransferred load amplitude to external load amplitude) of the smallest cross section’s (SCS)load amplitude is significantly decreased, between0and0.5. Besides, this reductioncoefficient is negatively related to the pin’s rigidity, positively related to the panel’s rigidity,but irrelevant to interference amount. If the pin and hole always situates in the largestseparation status, the load amplitude of the SCS is equal to the external load amplitude andirrelevant to the interference amount. If the contact status varies with the external load, theload amplitude reduction coefficient is between the above-mentioned two conditions and islessened by increasing interference amount. (2) The reciprocating fluctuations of contacting pressure and elastic compression of pin due toalternate external load reduce the load fluctuation on the SCS. The sum of the SCS’s variationamount and contacting pressure’s release amount on the loading side is the same as theexternal load. Though interference fit dramatically increases the SCS’s loading level, thereduction mechanism of load amplitude is achieved by the pin’s elastic deformation and theautomatic adjustment of contact, not the load rise on the SCS.(3) For specific structures and materials, particular external load should be taken intoconsideration in selecting interference amount. For structure’s fatigue life, the load rise inSCS is contradictory to the reduction in load amplitude. Principle of interference optimizing isto guarantee that the pin never separates the hole to reduce load amplitude when loading andthe interference amount should be minimized to decrease load level. The optimal interferenceis the critical value that keeps the pin contact the hole under the maximum external loading.Traditional theory holds that the strengthening mechanism of interference fit is actually thesupporting effect based on elastic deformation. However, this dissertation pointed out that, theautomatic adjusting effect generated when the fasteners deforming elastically and thecontacting pressure alternating with external load is proved to reduce the load transferamplitude. This reveals the dynamic feature of interference fit strengthening based on elasticdeformation, which is not contradictory to the static feature of cold expansion strengtheningbased on the residual compressive stress, thereby providing theoretic basis for the compoundstrengthening.In addition,3D FE simulation of interference fit, compound strengthening, and mechanicalresponse of strengthened structure is performed according to the interference fit theory, andfatigue assessment is also conducted, further validating the analytical results and relatedconclusions of the spring model. Through numerical simulation and lifetime estimation, theinfluence of interference, expansion amount, hole-edge distance, and load magnitude onresidual stress, stress redistribution, contacting stress, fatigue life, and failure location instrengthening structure are analyzed based on the research needs of related projects, andoptimal technological parameters are obtained, as well. Through the systematic analysis ofcompound strengthening, the residual stress of cold expansion and interference fit isunderstood in a new way. Cold extrusion can not only lower the stress level on the hole edge,but also decrease its stress amplitude to some degree due to its different influences on the loading and unloading stresses. Meanwhile, interference fit can both reduce the stressamplitude on the hole side, and change the stress distribution of elastic-plastic materialstructure along the transverse direction, hence greatly lessening the stress level on the holeedge. Therefore, as long as the technological parameters are properly selected, compoundstrengthening can gain longer fatigue life than any other single strengthening method.