Multiscale Models Of Stiffness And Strength Of Chopped Carbon Fiber Sheet Molding Compound

To meet the increasingly strict vehicle emission regulations,vehicle lightweight is a possible solution.Carbon fiber/resin composite is widely applied due to its superior property(high stiffness/density ratio,high strength/density ratio and high corrosion resistance.However,the application of continuous carbon fiber composite on the vehicle is limited due to its high production cost and cycle and low formability.Therefore,chopped carbon fiber sheet molding compound(SMC)provides an alternative option for vehicle light weighting material due to its lower production cost and repaid production cycle and easier to form compare to the continuous carbon fiber composite.Due to the complex flow pattern of the fiber chips during the compression molding,the properties of the material vary spatially.The property of the SMC material is closely related to three impact factors: the first one is the fiber orientation tensor,which varies spatially due to the compression molding process;the second one is the material mesostructure.SMC material has its special mesostructure,which needs to be described carefully;the third one is the stress state.The material will have different stress state under different loading conditions,which will lead to different failure strength.Therefore,take the three impact factors into account and accurately predict the stiffness and strength of the material is of great importance.To accurately predict the stiffness and strength of the material and take the three impact factors into account,the thesis conduct researches on stiffness prediction,ultimate failure strength prediction,failure criteria of the material and strength evaluation of the material components.The contents and conclusion of the study are summarized as follows:(1)For the problem to predict the stiffness of the SMC material,Voronoi diagram based and chip packing-based reconstruction algorithms are proposed to reconstruct the mesostructure of the SMC material,and the influence of the fiber chip orientation distribution is also considered in these two algorithms.Then,the RVE models of different fiber orientation tensor of the SMC material are established.The proposed methods can generate the models with high fiber chip volume fraction and different fiber orientation tensor,which cannot be taken into account by existing models.The accuracy of the stiffness prediction results of the SMC material are validated by the experimental results.(2)For the problem to predict the ultimate failure strength of the SMC material of different fiber orientation tensor,a new chip material model is proposed,and the characteristic of asymmetry of tension and compression,anisotropic as well as transversely nonlinearity of the chip are considered in the model.The proposed model can overcome the problems that the existing chip material model cannot capture the nonlinear and failure behavior of the chip under complex stress condition and accurately describe the behavior of the chip.A conforming mesh based RVE model of SMC material is proposed to overcome the problem that the voxel mesh based RVE model will give inaccurate stress prediction due to the stepped mesh.The model can predict the strength of SMC of different fiber orientation tensor under different loading conditions and clarify the crack initiation and propagation process of the material under complex stress state.The correctness of the model is validated by the experiments.(3)For the problem that it lacks the macroscale strength evaluation criterion for SMC material,an RVE based macroscale strength evaluation method is proposed.The method can help to propose a strength evaluation criterion to predict the strength of the material under complex stress state.The method is featured with two new development: firstly,the method factorizes the fiber orientation tensor to the principal fiber orientation tensor,which greatly decompose the tensor and reduce the needed RVE models to propose the strength evaluation criteria;secondly,several new strength evaluation criterions are proposed by modifying the existing failure criterions for anisotropic materials to take the fiber orientation tensor into account.These criterions and strength evaluation method based on micromechanics method are then compared with the RVE results,and the one which can best described the failure surfaces are obtained.The obtained failure surface can evaluate the strength of SMC material under complex stress state.(4)For the problem to predict the elastic property and strength of different locations of a chopped carbon fiber SMC component,a multiscale modeling framework including manufacture simulation,metamodel and macroscopic strength evaluation criteria as well as the macroscale performance of the component is proposed in this study,which can take the influence of manufacture parameters and computational efficiency into account.The multiscale modeling framework integrates the proposed stiffness and strength prediction models based on mesoscale RVE reconstruction algorithm and the developed macroscopic strength evaluation criteria to quickly predict the stiffness and strength of different locations of the SMC component.The uni-axial tensile,compression and V-notch coupon models are also established using the framework.And the predicted results are valided by comparing with the experimental results.