| Under the pressure of environmental pollution,global warming and heavy haze,energy saving and emission reduction have become an issue of automobile industry.Car-body-lightweighting is considered to be an effective method of energy saving and emission reduction for automobiles.However,the High Strength Steel and Ultra High Strength Steel,which are widely used for car-body light-weighting currently,cannot achieve the target of next-generation light-weight car-body.Newly developed light-weighting materials,such as Aluminum Alloy,Magnizium Alloy and Composites materials,which have low density and high specific strength,are a good choice of the light-weighting materials for the next-generation light-weight car-body.From the perspectives of cost,processing technology and capacity,multi-material car-body refers to the mixed utilization of traditional steel-based materials and newly developed light-weight materials as the best choice for the next-generation car-body manufacture.In recent car-body manufacture,adhesive bonding is used to improve the stiffness of the car-body,prevent corrosion,dampen impact and isolate noise.However,influenced by the difference in thermal expansion coefficients and the pretension of the structure,huge distortion appears after the paint bake process,in which the structure is heated up to180~oC,and then the distortion causes adhesive fracture in the curing process,leading to larger area of defected adhesive layer before serving.Thus,there is an urgent necessity for the research of curing-induced distortion in dissimilar material bonding joints to understand the mechanism of curing-induced distortion and debonding as well as the influence of curing-induced debonding on joint performance.Moreover,establishing a set of criteria for curing-induced debond and meothods for reducing curing-induced distortion is also very necessary.In this research,the author started from measuring the transient distortion of aluminum-steel bonding joints with screw constraints using an experimental method and identified the fracture of adhesive caused by the large distortion of the substrates.Then,the mechanical performance of specimens with artificial defects following the defect patterns inuduced by curing distortion was examined.Next,theretical models such as kenetic model of curing reaction,viscoelastic constitutive model and cohesive zone fracture model were established to describe the behavior of adhesive in curing process.Based on these theretical models,a Finite-element Model was established to describe the curing-induced distortion and adhesive fracture in the curing proces.After that,the mechanism of curing inducecd distortion was summarized from the distortion evolution process and a set of criteria for curing-induced debond were established based on the FE-model.This model was also used to study the effect of process parameters on the reducetion of curing-inducecd distortion.Based on the results of this study,a method called single sided curing method was proposed and the effect of this method was validated by experiments.The findings from this research can provide guidance to other researchers and engneers.This dissertation consists of four parts:1)Experimental research of curing-induced distortion and debondingA distortion measurement apparatus was built to measure the dynamic distortion of al-steel bonding joints.Based on this appararus,the evolution of substrate distortion was measured and the curing-induced debonding was identified.Futher study was carried out to find the basical failure patterns of curing-induced debonding and their influence on joint performance.The experimental results suggested that the area of curing-induced debond should be less than 25%of the total bonding area;otherwise brittle failure will occur and the failure mode will change from cohesive failure to adhesive failure of bonding joints.2)Material model of adhesive in the curing proccessBased on reaction kenetics and rheology of polymers,experiments were carried out to measure the variation of adhesive properties in the curing process.From the experimental results,theoretical models were constructed to describe the bahaviour of adhesive in the curing process.These models include curing reaction kinetic model which describes the variation of cure degree and the Prony series which describes the viscoelastic behavior of adhesive in the curing process.These models can help to establish the FE-model of curing-induced distortion.3)Mechanism of curing induced distortion and debonding based on FE-modelAn FE-model was developed to describe the evolution of curing-induced distortion,focusing on thermal buckling,plastic deformation in substrates as well as the curing-induced debonding.The effects of substrates,constraints and adhesive layer on curing-induced distortion were discussed,and the distortion mechanisms of curing-induced distortion and debonding were summarised.4)Distortion reduction methods from the perspective of process parameterThe effects of process parameters,such as reducing curing temperature,changing substrate thickness and shortening the distance between the constraints,on curing-induced distortion were investigated.The results from the FE-model and experiments indicated that these paremeters had a limited influence on the difference of thermal expantion between substrates.To address this issue,a method called single-sided curing method was proposed and validated.The results indicated that this method is feasible and may provide a possible solution to the curing-induced distortion.In summary,this dissertation explored the curing-induced distortion of Al-steel bonding joints experimentally and theratically.An FE-model was established to estimate the curing-induced distortion and debonding.Additionally,a single-sided curing method was proposed.All this can facilitate the application of adhesive bonding in multi-material car-body manufacture. |
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